Help Modifying OMC 31.8 cid

[Emustard]

Modifying a 1990 20hp 31.8 cid

I have already changed the Carb and done some porting. I am looking for more hp, what are the best trick/tips to achieve this and does any body have any advice on the tuner (how should it be modify).

[Fast Fred]

:cool:hey, some how i'm seein a crossflow motor, but that may not be right, got a shot of the powerhead

[Emustard]

Ya your correct its a crossflow engine. I will try to get some pictures up tonight.

[Smokin' Joe]

Crossflow 35 hp motor with the carb from the 35. Mike Petty

bmh20h@att.net

is the expert. Here's a photo of one of his 31.8 ci OMCs.
He runs Mod C against Yamahas.




Modifying a 1990 20hp 31.8 cid

I have already changed the Carb and done some porting. I am looking for more hp, what are the best trick/tips to achieve this and does any body have any advice on the tuner (how should it be modify).

[LakeRacer99]

Emustard...what boat are you running? Got any pics of your mods? What about raising the compression?

[Emustard]

I am running this engine on a 14 foot aluminum princecraft. I am currently in the Process of porting so I do not have any pictures. I am looking for a high compression head at the moment but can't find one anywhere, so will probably end up making one.

[LakeRacer99]

I am running this engine on a 14 foot aluminum princecraft. I am currently in the Process of porting so I do not have any pictures. I am looking for a high compression head at the moment but can't find one anywhere, so will probably end up making one.

Good luck finding one. they do exist, but i've only seen one online in the last many years I think.
What does your rig weigh and what speeds? What prop? Do you have porting specs?

[Emustard]

Ya I have been looking for one for a while and have not seen one. My rig weighs 275-300 lb boat alone with engine and gas tank 450 lb. 13x10 pitch aluminum OMC prop 32mph @ 6200RPM ( I can put 5 people in the boat and we slow down to 29mph) with a 15x10 pitch stainless solas prop 33mph @ 5500 by my self (two people in the boat 32mph @ 5300). I do not have porting specs.

[LakeRacer99]

That's cool, post pics if you can.
Look up JohnsonM50 and SmokinJoe on here, they've been modding up the 32ci OMCs a bunch.
I have an old 14ft alum runabout (250#rigged) with a stock '57 18hp (22ci) spinning a 12p 2 blade that runs 28-29 with just me in the boat. but add another person and speed falls off to 24-25ish. I have a couple 32ci OMCs that I wanna play with and some 60s/70s 22ci that I want to modify also.
Raising the compression, reeds and bigger carb is where I planned to start, just too many projects goin.

[Smokin' Joe]

Ya I have been looking for one for a while and have not seen one. My rig weighs 275-300 lb boat alone with engine and gas tank 450 lb. 13x10 pitch aluminum OMC prop 32mph @ 6200RPM ( I can put 5 people in the boat and we slow down to 29mph) with a 15x10 pitch stainless solas prop 33mph @ 5500 by my self (two people in the boat 32mph @ 5300). I do not have porting specs.

Porting is not the answer, prop work is. I run a stock 1983 35 with .060" milled head on a 350 lb Allison
pad-v fishing boat, tiller handle. I run a Stiletto 10x14 ss prop(actually measures 10x13) 6150 RPM,
41.7 mph 2 way GPS avg. A stock Stiletto ran 37 mph. It's tricky to get it right. I have a spare new
Stiletto, can rework it and sell it to you. Forget al props. You need to turn 6000 RPM too. How high is
the motor on the transom. I.e., what's the transom height from boat bottom to top of transom. The
Allison has a 7" offset so I run 19-19.5" high. With a normal transom you'll
need to run a shortshaft at least 18" high to get any speed. The stock motor will turn 7000 RPM
then hits a wall because of the flat reed plates. You would need v-block reeds and better exhaust tuning
to do better. Porting will not make much difference, and do not change the port timing. I run the milled head (150 psi)
and 33 degree spark advance but have never risked pump gas. I run a mix of Sun 110 octane and pump 93 at
1/5 (98 octane avg.) with Evinrude XD-100 oil@37:1 to avoid sticking a piston.

[JohnsonM50]

I agree with Joe on porting not making as much difference as I'd hoped, My 1st attempt was to square the tops of all the ports & I raised the exhaust a little. I re ran the re-asemble as it was before & it was slightly faster, alot louder & pretty darn thirsty. I figured the intakes were good as were so the next one I did about the same to the exhaust only & thats a little better yet but by no means great. The tuner that you saw curved is mostly to fit but also is in a shortened tower. I did a little sheetmetal work to protect the waterpump in that too. I got lucky & found a 2nd effort head on a motor I traded some junque for. The catch is this motor was so badly blown its surprising it got that bad w/o stopping lomg before. [musta been wail-n hot]. Gotta wonder if the head was a contribting factor for some reason like timing or jetting.
Good & bad news.. On a reed experiment I foung a motor to run significantly faster till a reed failed & that is probably the best reason to used composite reeds. I tried Boyesen 2 stage & decided to take them out before I broke em. No great improvement noticed. I have a single stage set from Carlson to try but haven't yet. Reeds are where some potential is & Ive been thinking about how to v reed one for some time as this would decrease crank case volume as well as flow better.

[Smokin' Joe]

I agree with Joe on porting not making as much difference as I'd hoped, My 1st attempt was to square the tops of all the ports & I raised the exhaust a little. I re ran the re-asemble as it was before & it was slightly faster, alot louder & pretty darn thirsty. I figured the intakes were good as were so the next one I did about the same to the exhaust only & thats a little better yet but by no means great. The tuner that you saw curved is mostly to fit but also is in a shortened tower. I did a little sheetmetal work to protect the waterpump in that too. I got lucky & found a 2nd effort head on a motor I traded some junque for. The catch is this motor was so badly blown its surprising it got that bad w/o stopping lomg before. [musta been wail-n hot]. Gotta wonder if the head was a contribting factor for some reason like timing or jetting.
Good & bad news.. On a reed experiment I foung a motor to run significantly faster till a reed failed & that is probably the best reason to used composite reeds. I tried Boyesen 2 stage & decided to take them out before I broke em. No great improvement noticed. I have a single stage set from Carlson to try but haven't yet. Reeds are where some potential is & Ive been thinking about how to v reed one for some time as this would decrease crank case volume as well as flow better.

Jim Nerstrom told me that the only hope with that (fishing) motor is reeds and exhaust tuning. Carlton Callahan once fabricated a
plastic manifold for v-block reeds on a river racer, and used a 2 barrel carb from an old 85. I've never experienced a broken reed on any motor. In SST45 and SST60
we replace plastic reeds with steel ones because the plastic reeds leak and kill the acceleration. We're turning 7500-8000 RPM with steel reeds. And we do accelerate.

[JohnsonM50]

The rig: In many cases someone will put more pitch on or a bigger motor to go faster to find the motor could harly spin the prop & the bigger motor mostly only accelerated better. So having not done that yet its a good time to look at what you do have. Is the boat bottom straight? meaning no 'hook' or 'rocker' condition. Is it smooth? aluminum can drag pretty hard so these things as well as good paint or epoxy type coating can help. The weight sometimes is not as important as its distribution. Moving the gas & battery [if used] are usually all that is needed if not optimal already.
Set up.. as important as thr prop [almost] The prop should be as near to the surface as it can be & still work & the boat handle adequetly. The tilt should be so it has a lazy porpoise slow & almost goes away at speed. Pounding is of course no good for several reasons. If your using the standart pin tilt you can sleeve the pin for finer adjustments as the difference between adjust holes can be too much.
The GT pros last I saw were about to break 50 & are not super light so the potential is there. The motor Mike Petty built is to go Yamato hunting.. Hope he does well, theres alot of work in it but it is a stock block aside from the open exhaust. It should get well into the 70's. My fastest yet did 65. its above water exhaust but is in the midsection so is closed. The motor is pretty modified but a little on the cheep & back-yardish. I'm going to spend a bit on making it a more professinal build soon.

[Smokin' Joe]

The only good head is from 1982, the others have only a 180 degree squish band.
A stock 1982 head might be better than a milled head of too small squish band.
I had a powerhead with light rods and pistons and 1982 stock head, it
was slightly faster than my 1983 powerhead with heavy rods and pistons and
.060" milled head. Milling the head really picks up the acceleration. also, be aware that the
only good gearcase is 1980-1983. 1976-1979 is not streamlined and will not run. 1984 and later has a lower than
1:2 gear ratio. 1980-1983 is streamlined and has a .53 gear ratio.

[Smokin' Joe]

The rig: In many cases someone will put more pitch on or a bigger motor to go faster to find the motor could harly spin the prop & the bigger motor mostly only accelerated better. So having not done that yet its a good time to look at what you do have. Is the boat bottom straight? meaning no 'hook' or 'rocker' condition. Is it smooth? aluminum can drag pretty hard so these things as well as good paint or epoxy type coating can help. The weight sometimes is not as important as its distribution. Moving the gas & battery [if used] are usually all that is needed if not optimal already.
Set up.. as important as thr prop [almost] The prop should be as near to the surface as it can be & still work & the boat handle adequetly. The tilt should be so it has a lazy porpoise slow & almost goes away at speed. Pounding is of course no good for several reasons. If your using the standart pin tilt you can sleeve the pin for finer adjustments as the difference between adjust holes can be too much.
The GT pros last I saw were about to break 50 & are not super light so the potential is there. The motor Mike Petty built is to go Yamato hunting.. Hope he does well, theres alot of work in it but it is a stock block aside from the open exhaust. It should get well into the 70's. My fastest yet did 65. its above water exhaust but is in the midsection so is closed. The motor is pretty modified but a little on the cheep & back-yardish. I'm going to spend a bit on making it a more professinal build soon.

Minor correction. I built the (chopper) props for the fastest two GT-Pros and they haven't broken 48 mph yet to my knowledge. I
think they may be running the post-1983 gearcases. Them that has a 1980-83 gearcase ain't telling,
but I informed them of the difference ca. a year ago.

[JohnsonM50]

Joe, here's the head I got:
http://i35.photobucket.com/albums/d151/mes355/DSCN1084.jpg
http://i35.photobucket.com/albums/d151/mes355/DSCN1078.jpg
Its the one on the left. I really admire the work done in GT Pro, & what's been learned about how to 'go' bone stock. Its nowhere near here tho, so I'm just playing with these for fun, no rules. The 1st intent was to have a practice motor to put time on instead of wear & tear on the Yamato. It came down to this:
http://i35.photobucket.com/albums/d151/mes355/DSCN0655.jpg
Same transome height, 80#'s 1:1's. A very similar ride to my 102 in the same place. I think the OMC has more pull & slightly less top end. I do intend to break 70 with this or the next 31.8 RaT.

[Smokin' Joe]

Joe, here's the head I got:
http://i35.photobucket.com/albums/d151/mes355/DSCN1084.jpg
http://i35.photobucket.com/albums/d151/mes355/DSCN1078.jpg
Its the one on the left. I really admire the work done in GT Pro, & what's been learned about how to 'go' bone stock. Its nowhere near here tho, so I'm just playing with these for fun, no rules. The 1st intent was to have a practice motor to put time on instead of wear & tear on the Yamato. It came down to this:
http://i35.photobucket.com/albums/d151/mes355/DSCN0655.jpg
Same transome height, 80#'s 1:1's. A very similar ride to my 102 in the same place. I think the OMC has more pull & slightly less top end. I do intend to break 70 with this or the next 31.8 RaT.

On the left looks like nearly 180 deg. squish band. That would be 1982 according to my limited experience. Amazing how much
has been gotten out of that motor in spite of the flat reed plate. I assume you're running the light 1976-79 rods and pistons.

[Emustard]

JohnsonM50 what kind of compression do you get with that performance head?

The aluminum sticks a little to the water but not bad. I have played around with the weight in the boat and trimming it and still think I need it up higher but will have to wait till the ice goes away. I use this engine almost every weekend for fishing so would like to keep the stock lower unit.

So I replace the OMC steel reed with boyesen reeds do you think I should switch back to the steel reeds? I heard some one say you should take out your water pump and you will gain 3-7 hp what are your thoughts on this, is it true if I can still cool the engine.

I’m just looking to get into the 40s with her and need help getting there.

[Emustard]

Smokin' Joe i tryed to PM you but said your message box was full or somthing so.
I would be interested in the stiletto prop how much $$ are you looking to get for it and is it for the 14 splined shaft . Have you ever ran an OMC prop or something else to compare your prop, if so what speeds did you see with the other props.
The transom is about half an inch below the bottom of the boat and the motor is a short shaft. How did you modify or exhaust pipe?

[Smokin' Joe]

Smokin' Joe i tryed to PM you but said your message box was full or somthing so.
I would be interested in the stiletto prop how much $$ are you looking to get for it and is it for the 14 splined shaft . Have you ever ran an OMC prop or something else to compare your prop, if so what speeds did you see with the other props.
The transom is about half an inch below the bottom of the boat and the motor is a short shaft. How did you modify or exhaust pipe?

outboard_services@mccauleyandson.com

[Smokin' Joe]

JohnsonM50 what kind of compression do you get with that performance head?

The aluminum sticks a little to the water but not bad. I have played around with the weight in the boat and trimming it and still think I need it up higher but will have to wait till the ice goes away. I use this engine almost every weekend for fishing so would like to keep the stock lower unit.

So I replace the OMC steel reed with boyesen reeds do you think I should switch back to the steel reeds? I heard some one say you should take out your water pump and you will gain 3-7 hp what are your thoughts on this, is it true if I can still cool the engine.

I’m just looking to get into the 40s with her and need help getting there.

I left out the fundamentals. You can't jack the motor up and prop it unless the
bottom is smooth and straight to within a few thousandths of an inch. Any hook or rocker and
you're dead. Smooth al won't drag more than any other smooth surface (water sticks to all surfaces and
drags the first monolayer, which drags the 2nd and so on), and by smooth I mean gelcoat
or gray primer on gel coat. Trailering picks up oil from the road, the bottom must be cleaned
regularly as well if you trailer it. There are a lot of details that have to be right before even a fast
boat can be made to run up to its potential.

[LakeRacer99]

Lots of good suggestion here on this thread. Pics would help us see what you are working with also. :cool:

[JohnsonM50]

Thanks Joe, its fast but then again the Bezoats its on is always fast. It weighs in at about 430# including me. The Yamato foot is les than 2" wide & 3/4 deep at prop shaft center. A good running stock used powerhead would make about 58-59 on that tower so 65 is realy only up about 6mph. Thing is I know theres more. Whether to spin more prop or more R's is the Q. Im guessing it to be about 7200 to 7500 now.
Eric.. When you look at various 2 strokes theres motorcycle motors with ridiculous power, so why not outboards? They need the torque & have to trade off some hp to get it. Water resists almost 800 times air. A set of wheels on pavement rolling on bearings is nothing compared. An outboard has to be tuff-r too for the rigors of being run like its always going uphill. If you took a really hot MC motor removed the 5 speed gearbox & attached it to a prop it would not be likely to get the boat over the hump & on plane. That being said the best thing is for it to be right, piston clearaces, ring gaps, tuning the list goes on. Check your 20 over for built in restriction, I run time at about 29 30, never saw much difference as long as its 30/35. the motor I mentioned with the wiseco's & 3 cyl rods has 150psi with a regular head cut a bit.
When I got my Airborn done I started out with a 22ci 25hp OMC. had it running nice but was stock. It did about 33 tops. Then I got a 31.8 [big block]. It's very similar, like a big cuz. That made 36 1st try & the torque was a seat buster LoLs Keep in mind thats a flat bottom racing runabout at about 475 total. I stared messing with carbs & set up & props, tweek-d it up to about 39, & actually got a 40 once. [It was a conditions permitting moment] Then I got my hands on this 13.5 P 2 blade righty speed prop. I adapted it to an old club foot
http://i35.photobucket.com/albums/d151/mes355/DSCN0493.jpg
Yes, its modified a bit but is still 12:21 That made 44 1st try then 46.1 soon after. [Some said 'that'll never go'] Its on my pals hydro now & has made 52gps at about 500# @ well over 7G's.
That's about where I stopped working with stock gearcases & never had a thru hub 31.8. for all the work done I have to say besides putting a racing gear case on & running it on a race boat, the most speed Ive ever seen gained was from a prop & subsequent set up change. Theres 2 ways to approach boat motor performance.. you either turn more R's with the same prop or you spin more prop on more hp. if your 31.8 is open & flowing like a 35 it already has the capability to just a matter of the right prop. over-rev Check also into whether or not you have a rev limiter built into your power pack. thats an easy change.
Joe, I tried to PM you long ago but didn't hear back.. had some Q's

[JohnsonM50]

Hey Jason, how are you? Did you say pics? How bout a RaT's history?..but 1st a radar run. clik the pic, blow it up & crank it up. Had a little carb burp at 1st but nursed it up to speed:
http://i35.photobucket.com/albums/d151/mes355/th_DSCN0035_zpsf37c4a10.jpg (http://i35.photobucket.com/albums/d151/mes355/DSCN0035_zpsf37c4a10.mp4)
I made a trade for this nice Yamato Pro case.. Stared at it for awhile & started chopping up parts.
http://i35.photobucket.com/albums/d151/mes355/DSCN0481.jpg
& fitting them up.
http://i35.photobucket.com/albums/d151/mes355/DSCN0625.jpg
It took a little work to eliminate the 'clamshell' lower mounts.
http://i35.photobucket.com/albums/d151/mes355/DSCN0613-1.jpg
& fit the exhaust.
http://i35.photobucket.com/albums/d151/mes355/DSCN0561.jpg
But in the end its turned out to be quite a motor. It has a waterpump for the 'slo-zone's so is really a good play motor.

[Smokin' Joe]

Thanks Joe, its fast but then again the Bezoats its on is always fast. It weighs in at about 430# including me. The Yamato foot is les than 2" wide & 3/4 deep at prop shaft center. A good running stock used powerhead would make about 58-59 on that tower so 65 is realy only up about 6mph. Thing is I know theres more. Whether to spin more prop or more R's is the Q. Im guessing it to be about 7200 to 7500 now.
Eric.. When you look at various 2 strokes theres motorcycle motors with ridiculous power, so why not outboards? They need the torque & have to trade off some hp to get it. Water resists almost 800 times air. A set of wheels on pavement rolling on bearings is nothing compared. An outboard has to be tuff-r too for the rigors of being run like its always going uphill. If you took a really hot MC motor removed the 5 speed gearbox & attached it to a prop it would not be likely to get the boat over the hump & on plane. That being said the best thing is for it to be right, piston clearaces, ring gaps, tuning the list goes on. Check your 20 over for built in restriction, I run time at about 29 30, never saw much difference as long as its 30/35. the motor I mentioned with the wiseco's & 3 cyl rods has 150psi with a regular head cut a bit.
When I got my Airborn done I started out with a 22ci 25hp OMC. had it running nice but was stock. It did about 33 tops. Then I got a 31.8 [big block]. It's very similar, like a big cuz. That made 36 1st try & the torque was a seat buster LoLs Keep in mind thats a flat bottom racing runabout at about 475 total. I stared messing with carbs & set up & props, tweek-d it up to about 39, & actually got a 40 once. [It was a conditions permitting moment] Then I got my hands on this 13.5 P 2 blade righty speed prop. I adapted it to an old club foot
http://i35.photobucket.com/albums/d151/mes355/DSCN0493.jpg
Yes, its modified a bit but is still 12:21 That made 44 1st try then 46.1 soon after. [Some said 'that'll never go'] Its on my pals hydro now & has made 52gps at about 500# @ well over 7G's.
That's about where I stopped working with stock gearcases & never had a thru hub 31.8. for all the work done I have to say besides putting a racing gear case on & running it on a race boat, the most speed Ive ever seen gained was from a prop & subsequent set up change. Theres 2 ways to approach boat motor performance.. you either turn more R's with the same prop or you spin more prop on more hp. if your 31.8 is open & flowing like a 35 it already has the capability to just a matter of the right prop. over-rev Check also into whether or not you have a rev limiter built into your power pack. thats an easy change.
Joe, I tried to PM you long ago but didn't hear back.. had some Q's

Try

outboard_services@mccauleyandson.com


I wouldn't think that small fishing gearcase would work well when jacked
up to 21" tr. hgt. or better, too little rudder/skeg to correct against
prop walk. Even at 19.5" tr. hgt. the 1976-79 35 hp straight skeg
model gearcase failed completely because the bullet and 'rudder' above the
bullet are too blunt. I could only eliminate blowout by straightening the
motor, and then the boat was in a sharp left turn at 40 mph. The more streamlined
1980-83 gearcase runs 41.7 mph.

[Emustard]

Do you guys have a relief cut in the side of your cylinder wall or is that just something they did on the 20hp or is it on all the 31.8cid 20,25,30,35hp.

[JohnsonM50]

I've read of that but never saw one, most of my stuff is early 80's or older. Best reason I think noted was easier starting & that it had no effect on performance. If anything raising the exhaust port time slightly would be typically for more top end. The rings & compression stroke 'cc's are the key to compression. It can vari from motor to motor about 120-135 w/o any significance in speed. 150 from what Ive seen takes milling. 2 of mine are to the point where the pistons will hit the head w/o the head gasket. I don't know how much can be milled but be advised they are already close.

[Smokin' Joe]

Do you guys have a relief cut in the side of your cylinder wall or is that just something they did on the 20hp or is it on all the 31.8cid 20,25,30,35hp.

Where, for what?

[LakeRacer99]

Hey Jason, how are you? Did you say pics? How bout a RaT's history?..but 1st a radar run. clik the pic, blow it up & crank it up. Had a little carb burp at 1st but nursed it up to speed:

But in the end its turned out to be quite a motor. It has a waterpump for the 'slo-zone's so is really a good play motor.

Very nice work, I admire your initiative. love the pics and video...did he say 65mph? That's awesome.
I don't wanna derail EMustards thread, but what waterpump did you use with the yamato case? I have a Konig lower that I have wanted to graft on my 31.8, but I wont do it without a waterpump.

[LakeRacer99]

emust...I still go back to the rigging set up and prop work, then carbs, compression, exhaust- squeezing every last mph out of the setup before i go grinding on the ports. Probably because I wouldn't know where to start there also. But I have seen many good running motors sacrifice low end and midrange post grind over the years. pop up a few pics of your rig if you can.

[JohnsonM50]

Yes it was 65. I also have a Konig I want to fit to another on the 'someday' list. Good news is after doing a Yamato 102 case with 2 center bolts the 4 bolt'ers are easy. I cut the OMC foot off just above the anticav plate. Make up an adapt plate to have welded on. That saves the OMC pump, & needle bearing just under it. Below that the OMC part is bored just enough for the coupler & is just hight enough. I'll dig up more pics. I used a sgug socket to push in & hols it in line for the weld process. Then re-align it all afterwards. I do try to get it exact but of course the heat takes its toll.

[Emustard]

On the exhaust side on the middle port there is a cut about a 1/4 inch above the exhaust port and 1/8 inch deep. They call it a relief cut and I think JohnsonM50 is right it's for easy starting.

[JohnsonM50]

On the exhaust side on the middle port there is a cut about a 1/4 inch above the exhaust port and 1/8 inch deep. They call it a relief cut and I think JohnsonM50 is right it's for easy starting.
Something I read here on BRF.

[JohnsonM50]

To adapt a gear case & save the pump. [I think this answers & asks a question] Luck has it the the OMC pump plenum happens to be right where the Yamato & I beleive Konig too water ports come up.
http://i35.photobucket.com/albums/d151/mes355/DSCN0227.jpg
http://i35.photobucket.com/albums/d151/mes355/DSCN0233.jpg
http://i35.photobucket.com/albums/d151/mes355/DSCN0236.jpg
So the Q being since there is a bearing below the pump then the top driveshaft bearing of the gearcase within a couple inches, robing potential speed. I decided to err on the side if security there & so far they hold up. To lube the isolated pump bearing I put an upper & lower hole to squeeze lube in.

[JohnsonM50]

As far as gearfeet go you can get the most out of a motor with one that is a good match. I'd be afraid to adapt an Merc AB or C to a 31.8.. could be too much hammering at 3" bore A 'D' maybe but other units can be had cheeper usually.
V reeds are something I went as far as to buy from a bigger motor, stare at for awhile & decide they were 1. Too big, 2.would be too small & still not fit well if one petal station were cut off. so I traded them off to someone building a pulse jet.
I'm convinced that 'ReaL' power gain can be found in some reed improvement but that leaves to question why? Its obvious why the reed wall doesn't quite do it but is the answer more volume, more velocity or just better open/shut capacity? On this particular motor you would almost be better off re-designing the crankcase front.
http://i35.photobucket.com/albums/d151/mes355/DSCN0108.jpg
[Eric, the pic above is one of the lower crank case opening clearanced to be like the upper].
The sort of funnel area behind the reeds is alot of C.C. volume for one thing & since its roundish theres no good way I see to get V reeds on/in w/o increasing C.C. volume as much as to possibly counter any advantage from using an add on reed compartment.
Ive done way more work on the 31.8's than I have results to show for it. One guy who I admire here is Tim K. He's done the same kind of V reed adaption to a Merc. & its awesome. If he were to take on a 31.8 I'd bet there'd be a work of art that works... meanwhile this idea has me thinking. As you see in the pic the rods swing pretty close to the inner surface of the case so shaving it back is limited & presents a mount stud issue. Filling it in some comes to mind but getting it right so it doesn't become a blockage is one concern & I'd hate to think of what is 'breaking loose' would look like.

[JohnsonM50]

Raising the motor, With the club foot application I think in a way I was lucky. The skeg is small for the 50mph neigborhood & the prop pretty big compared to the usual race boat situation. That could result in poor control & blow out. The cone is all epoxy just stuck on the assembled foot & shaped. When it was time for sevice I carefully hacksawed it at the seam line & re-assembled it with silicone. Well over 5 y.o. & its still there??, yes somehow. I'm thinking it didn't do much for recordable speed but may be why it works given the 9 or so inch prop thats on it. It is run submerged as it won't grab much higher but is right at the surface, [probably explains how the skeg works]
When you do go up, whether the prop will work is one thing but cooling w/o a low water pick up is another. Ive heard of plugging the top hole on a thru hub but have not done it. I've come up with an auxillery pick up:
http://i35.photobucket.com/albums/d151/mes355/DSCN0357.jpg
http://i35.photobucket.com/albums/d151/mes355/019.jpg
This is a Yamato 102 adapt to an OMC clubfoot type case with a revese gear water inlet so it was easy to make a threaded copy to induce more water. It was a nightmare plumbing the 102 pickup to the OMC & it sorta worked but I didn't trust it. This works as just an add on w/o changing prop thrust pickup, just puts more water into a questionable situation.
On a thru hub you would have to drill & thread a hole at a good location to do this & then decide whether or not it works along with the side inlets. I'm thinking leaving some of it [bottom hole?] open but don't know. Would guess it to depend on how high your prop can go as well as water level at speed. Keep in mind that when the water goes by the rear bottom edge of the boat it begins immediatly to seek its own level so back at the foot it will be higher than the boats bottom.

[LakeRacer99]

Thanks Mike for the info on the waterpump, I wasn't thinking about chopping an omc case off for the waterpump. Although the Konig case has a cav plate and what appears to be an exhaust outlet above it, but it does give me some ideas.

[JohnsonM50]

The one I've got is similar. So, on the stub goes the coupler. I have thought about somehow getting a pump in there but not very feasable. The Bass units with pumps have them below the coupler but up into the tower. Once going the pickup would force feed enough but getting going is tuff on a 31.8, they'll heat pretty quick. I've heard of electric pumping but have not done it. Ive wondered if the altitude of the pump makes that much difference? Could moving one up higher in the tower work?

[calvin]

So the head to look for is an 82.....does it have pads on both sides? I have a couple of heads but all have pads on one side? Is there a serial # on the head to go by?

[LakeRacer99]

The one I've got is similar. So, on the stub goes the coupler. I have thought about somehow getting a pump in there but not very feasable. The Bass units with pumps have them below the coupler but up into the tower. Once going the pickup would force feed enough but getting going is tuff on a 31.8, they'll heat pretty quick. I've heard of electric pumping but have not done it. Ive wondered if the altitude of the pump makes that much difference? Could moving one up higher in the tower work?
I don't think a few inches in altitude would matter as long as it got primed. I'm not interested in electric water feeding this right now. I guess I could weld some length on the shaft if I had to, but I haven't had this unit apart yet. This is a project for down the road I guess.

[LakeRacer99]

Heads...Joe says 1982 is the good one. Is it all 31.8 1982 heads or just 35hp? Why just 1982-What happened in 1983?

Joe/mike/anyone? have you made a chart with the part numbers and differences over the years for the 31.8 OMCs? I saw one for the 49.7 OMCs and wanted to make a similar one for the 31.8. I started one somewhere.

[LakeRacer99]

Does anyone have any pics of a 25/35 with a Second Effort case installed?

52161

[Smokin' Joe]

Heads...Joe says 1982 is the good one. Is it all 31.8 1982 heads or just 35hp? Why just 1982-What happened in 1983?

Joe/mike/anyone? have you made a chart with the part numbers and differences over the years for the 31.8 OMCs? I saw one for the 49.7 OMCs and wanted to make a similar one for the 31.8. I started one somewhere.

Only the 31.8 c.i. powerhead. In 1982 they had a nearly 360 degree squish band, in other
years no more than 180 degrees. Why they produced it and then dropped it, no idea.

[JohnsonM50]

The ones I have done have the pump on the driveshaft above the couple square. It could be done that way with or without the top part of a gaercase. I got a few with bad inner's to do this with. You would have to create a base & plenum for the pump.. why I went that way.

[JohnsonM50]

Does anyone have any pics of a 25/35 with a Second Effort case installed?

52161 That gearcase? I WISH!! LoLs, Funny I was just looking for that page pic, The head is what the one I have looks like. Calvin I'll see about a # when I can get to it. Maybe thats what the 82's had? The pad on this is different on the intake side.
http://i35.photobucket.com/albums/d151/mes355/DSCN1084.jpg
http://i35.photobucket.com/albums/d151/mes355/DSCN1083.jpg

[JohnsonM50]

I haven't but wish I had noted all iIve looked up [duh] A good place to start is BRP parts catalogues [on line]. There is broken down all the parts & part #s. No 2nd effeot stuff tho.. Sea Way Marine is the best bet to call & ask. Ive found 2nd effort solid mounts & this head I thought was.. or might be?, but never saw one of those neat gearcases.

[LakeRacer99]

...but never saw one of those neat gearcases.

There was one for sale last year on craigslist of all places, it was in phoenix or out in the southwest somewhere. The guy wanted $600 for it, I could not justify it. But that is the only one I ever saw.

[calvin]

The gear case to look for is also a 1982?....How do you verify that piece...serial # or length maybe?

[JohnsonM50]

The gear case to look for is also a 1982?....How do you verify that piece...serial # or length maybe? Somewhere in this thread Joe mentioned which year lower units were doing best in GT PRO, don't think it was the 82, that year I've heard twice now had the strongest running power head.

[calvin]

Was the 1982 motor..a 35 or 30 hp motor?

[Smokin' Joe]

Was the 1982 motor..a 35 or 30 hp motor?

35 at the crank, 30 at the propshaft. They started calling it a 30 in 1984. The 35 is listed in
the parts book through 2001 but no one has ever seen one. Maybe was still labeled 35 in some other part of the
world outside U.S. and Europe.

[LakeRacer99]

35 at the crank, 30 at the propshaft. They started calling it a 30 in 1984. The 35 is listed in
the parts book through 2001 but no one has ever seen one. Maybe was still labeled 35 in some other part of the
world outside U.S. and Europe.

I have a 82 25hp, which I am pretty sure is 31.8. I wonder if it has the 360Deg head on it. I should go out and check the PN on it. It had the pin drive lower.

[calvin]

The 1982 gearcase with the .53 ratio.....does it have a part # ....tell by looking..or do you have to spin it over to find the ratio?

[JohnsonM50]

I have a 82 25hp, which I am pretty sure is 31.8. I wonder if it has the 360Deg head on it. I should go out and check the PN on it. It had the pin drive lower.

Far as I know the pin drive was last made for a 31.8 in in 84 & on the 25's not 35's, the thru hub[s] I think were always on the 35's. However the 31.8 powerheads as far as I have seen will all interchange at the block/tower location. Your '82 is a 31.8 The # I found on the water jacket of the head I pictured is: 327673 D2 I
1
Its a raised #, not stamped & I coudn't find anything on the actual head. [under the jacket?] That head or not? perhaps you could tell thru thr spark plug hole?.

[JohnsonM50]

The 1982 gearcase with the .53 ratio.....does it have a part # ....tell by looking..or do you have to spin it over to find the ratio?
BRP Parts cataloge has expandable exploded views, you could ID the type for sure. As for ratio, spin the propshaft once & count flywheel spins is the best bet. From what Ive read the gearcase difference is more about aquadynamics than ratio since prop choice can improvise to some extent ratio difference. From what Ive read about the 2nd Effort case, stock parts of whatever years applicable are used in it.

[Smokin' Joe]

BRP Parts cataloge has expandable exploded views, you could ID the type for sure. As for ratio, spin the propshaft once & count flywheel spins is the best bet. From what Ive read the gearcase difference is more about aquadynamics than ratio since prop choice can improvise to some extent ratio difference. From what Ive read about the 2nd Effort case, stock parts of whatever years applicable are used in it.

I think 1980-83 vs later gearcases is only about gear ratio. I'm not Ware of a different gearcase shape
or size there.

[JohnsonM50]

I think 1980-83 vs later gearcases is only about gear ratio. I'm not Ware of a different gearcase shape
or size there. The pin drives are .571:1, never worked with thru hubs on this motor.

[LakeRacer99]

I haven't but wish I had noted all iIve looked up [duh] A good place to start is BRP parts catalogues [on line]. There is broken down all the parts & part #s. No 2nd effeot stuff tho.. Sea Way Marine is the best bet to call & ask. Ive found 2nd effort solid mounts & this head I thought was.. or might be?, but never saw one of those neat gearcases.

This is what I started last year when we were talking about the 25/30/35 carb differences. This is nothing more than me writing down the numbers on 3 of the 31.8 motors I have. I do plan to add more columns for additional parts. If anyone is interested, I could add their motors and part numbers to this.

52349

[calvin]

Was there ever a carb with adjustable high speed jets?

[Tim Weber]

You guys seem to be expert on this motor.

I have a 1993, 25 hp on my Lund fishing boat. To bump it up to 30 or 35 I've read to change the carb and the intake manifold. Is that it?

Tim

[JohnsonM50]

Yes & no, I never saw one but took the low plug out, pipe threaded it for the smallest compression fitting home depo had, epoxied it in then threaded the inside of it to align with the fixed jet at the low speed needle. size I blunted a needle & ran it, It works & I have 3 but I can't call it better or at least have no way of gaining precise performance data if it is.
http://i35.photobucket.com/albums/d151/mes355/DSCN0705.jpg

[JohnsonM50]

You guys seem to be expert on this motor.

I have a 1993, 25 hp on my Lund fishing boat. To bump it up to 30 or 35 I've read to change the carb and the intake manifold. Is that it?

Tim It might be, check the timing spec too. It might have a different crank but that shouldn't hold it back if aspirated as a 30. Also look at the power pack, might be rev limited. I'll look & get back If I see or think of anything else.

[JohnsonM50]

It might be, check the timing spec too. It might have a different crank but that shouldn't hold it back if aspirated as a 30. Also look at the power pack, might be rev limited. I'll look & get back If I see or think of anything else.
I just spent a few on BRP parts cat. it does appear to have a different block-crank assembly as well as a different intake. Theres alot of choices that year..20, 25, 28, 30 & 35 all of the 31.8ci. Even tho most look alike on the outside there's quite a few internal variations thru the years & models. Id look at the things I mentioned before, be sure its not restricted, rev limited & timed as a 35, might help some.

[Chairman]

For GT Pro we simply upgrade the carb and don't bother with the intake (unless you are upgrading a 20hp). One racer did compare intakes and got slightly better results with the 25hp intake.

One internal difference you will find over the years -- some (most) powerheads have drilled ports, but some have oval ports. The oval port motors seem to have come on some 30s and 35s. I've raced both types and have found no difference between them.

[JohnsonM50]

For GT Pro we simply upgrade the carb and don't bother with the intake (unless you are upgrading a 20hp). One racer did compare intakes and got slightly better results with the 25hp intake.

One internal difference you will find over the years -- some (most) powerheads have drilled ports, but some have oval ports. The oval port motors seem to have come on some 30s and 35s. I've raced both types and have found no difference between them.
I agree that most intakes are the same but since 18's & 20's are made of this motor, a few have a markedly restricted hole where the carb mounts &/or some carbs have a plastic insert venturi that resticts some.

[Chairman]

That's right, which is why I said "unless you are upgrading a 20hp." The 20 intake is very restricted.