why does an oval port have an advantage over a bridgeport motor for holeshot

[filthy phill]

What is it that makes a bridgeport omc 56 slower out the hole than the same size motor having oval port exhaust port ?
not talking about super modded motors, talking straight out the crate fishing motors so they are both on a level playing field.



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[Hounddog]

Oval port has higher compression and denser fuel mixture.

[filthy phill]

I am not understanding what your saying about DENSER FUEL MIXTURE ?
do you mean a richer mixture ?

[Hounddog]

I am not understanding what your saying about DENSER FUEL MIXTURE ?
do you mean a richer mixture ?

Yes, because of the smaller carbs. You will get more fuel, less air at the start. The stock oval port has approximately 145 compression.

I fellow over here did an interest test. He had both a 1994 oval port 70 hp on a family runabout and a 1986 bridge port 70 hp on another family runabout . He switched the carbs. The only change he did was to replace the 69 jet in the 1986 carbs with a 73 jet. The bridge port boat ran basically the same as before. The oval port boat ran 3 miles an hour faster with the larger carbs. Just one test and I wonder if others get the same result.

[filthy phill]

1994 oval port carbs are not smaller at all. they are as big as the sst60 and all the bridgeport 70hp carbs in the venturi and the bore.
the carbs that are smaller are the 60hp oval port and the realy tiny ones in the 1995 50hp models.
the only difference between a 60hp and 70hp in the oval port motors is the size of the carbs. everything else is exact the same.

As for testing i have done this with my boat,
I switched from the 60hp to 70 hp carbs on my 1995 oval port and gained about 2mph top end, bit of a disappointed to be honest, but i got a massive difference in mid-range and
a little bit more from zero to plane.
Zero to plane time dropped by about 1/2 second, but the plane to max rpm is almost instant, that was a massive improvement that I was not expecting.
I was hoping for about 5mph extra when going up to the bigger carbs so a bit disappointed with just 2mph extra, about 200 rpm.

the 60hp oval port black top carbs comes with 45D on the bottom jet and 35D on the top.
the 70hp black top carbs have 53D bottom and 55D top for years 93 and 94 but for 1995 onwards it 52D bottom 60D top.

i wonder what would be best to run on mine the 53D and 55D that are in them at the moment or the later 52D & 60D ? would this make any difference to top end ?.

[Hounddog]

i wonder what would be best to run on mine the 53D and 55D that are in them at the moment or the later 52D & 60D ? would this make any difference to top end ?.

Your still testing similar style carbs. Try the older SST 60 style carbs.
If the oval port 70 hp carbs are the same as the older carbs, then you should be able to run a 69D jet.

[filthy phill]

I see what your saying about the jet size. if putting a 69D in the bottom, what would be a size for the top jet ?

in 1989 on the 70hp bridgeports there was 2 different carbs ,early production and late production, both look similar in that they are the older style carbs.
early production had 65D at the bottom and 33 at the top.
later production had 52D at the bottom and 35D at the top . what one of those motors ran the best ? I would think the later motor as it seems strange for OMC to
change the jet size to make the motor slower or less power.

as anyone tried the newer black plastic top carbs on the older brigeports to see if they improved any with them ?

[filthy phill]

I will have to have a good read up about jets ,

[Hounddog]

I will have to have a good read up about jets ,

As you study here are a few things to find out about.
- which is richer 100 to 1 or 50 to 1
- most commercial two stokes are premix, have lower compression and larger jets...why?
- if you have a direct oil injected 2 stoke and remove the oil injection what premix ratio should you use and should you change the jets.
- is it good that your two stroke starts with a puff of smoke?
- if you have too large a main jet what happens?
- does the idle change when you change to a larger main jet?
- should you change to a smaller main jet if the plugs are black?
- if you run race fuel do you run a smaller main jet?
- what causes the fuel lines to get hard?
- if you have a light white color plug reading with 50 to 1 premix what do you do to get it a bit brownish?

[filthy phill]

well here is my effort at answering the 10 questions above.

1/ 100 to 1 or 50 to 1 has nothing to do with how rich the mixture is, rich/lean as I understand it is the ratio of fuel to air (that is how we see rich/lean in England).
so no difference between the two different oil ratios., but then you could be talking about a richer/leaner oil mixture / then 50-1 is richer.
so it realy depends on what you are talking about, but I presume you mean fuel / air.

2/ commercial motors are sold mainly for commercial businesses and will more than likely be abused more by the users, thus are normally a detuned version to save on wear and tear.
also some commercial engines that are sold with premix to military and RNLI can run on different grade fuels and also kerosene, so it depends on engine specific. but being premix
allows the commercial operator to adjust it to suit the environment its being used in and by whom if rented out. (sort of extra fail safe/idiot proof idea)

3/ direct fuel inject ...if swapped out for premix should be 50 to 1 or maybe 40 to 1, depends entirely on specific engine . as for jet change ? would try and read plugs and go by them ?

4/ yes its good to see a puff of smoke when starting a 2 stroke. shows lubrication is there straight away.

5/ too large a main jet will flood the motor and cause it to be far too rich and not give best performance.

6/ idle would change ,it would drop a bit until air screw was altered to compensate for more fuel .

7/ plugs being black could be too rich ,could also be the ignition not giving off enough spark to fully ignite the fuel, stale fuel not burning correctly,or could
be wrong oil causing the fuel mix not to burn.

8/ race fuel I have no idea about at all.

9/ ethanol and alcohol makes fuel line hard.

10/ change rating of plug or richen up the fuel mixture by either changing jets or adjust mixture screws if available ?


could be shot down for most if this ha ha

[Hounddog]

Phill, there is nothing wrong with your answers. Pleasure boaters have different needs and don't think the same way as racers and racers don't always agree with each other.

[filthy phill]

How would you of answered each of those questions differently ?
Then I might be able to u derstand it from Your side.

[Hounddog]

As you study here are a few things to find out about.
- which is richer 100 to 1 or 50 to 1
- most commercial two stokes are premix, have lower compression and larger jets...why?
- if you have a direct oil injected 2 stoke and remove the oil injection what premix ratio should you use and should you change the jets.
- is it good that your two stroke starts with a puff of smoke?
- if you have too large a main jet what happens?
- does the idle change when you change to a larger main jet?
- should you change to a smaller main jet if the plugs are black?
- if you run race fuel do you run a smaller main jet?
- what causes the fuel lines to get hard?
- if you have a light white color plug reading with 50 to 1 premix what do you do to get it a bit brownish?

How would you of answered each of those questions differently ?
Then I might be able to u derstand it from Your side.

Racers are in competition with each other. They have a set of rules they must follow and a record book that shows best in their class. They all what to win and they are always looking for an edge over the competition. They spend hours working on getting the best performance and they are always rebuilding their motors. The motors have a very short life span. They don't care how it idles and how much smoke it makes. In circle and drag they only care about how quick an ET they can run. Competition is so tight at the top of the classes that it takes many small changes to be keep up with the top runners.
- to me 100 to 1 is richer. It has more gasoline in the mix
- commercial motors have to be more durable and have less parts that could fail. They run a lot of below normal octane fuel. Many racers prefer year old fuel in their low compression motors because they think that fuel gives bigger bang.
- on a direct crankcase oil injected motor, the amount of oil supplied by the system is far less than a premix, pure gas runs thru the carb, adding oil to the mix reduces the amount of gas i so I would initially run a larger jet.
- our motors smoke most of the time, at start up and on early acceleration.
- too large a carb can cause motor to bog off the line and have poor mid-range power
- idle goes up, doesn't matter , 1000 to 1500 idle is common and we do not shift.
- black plug colour is common because of idle to the pits.
- race fuel burns hotter, av gas burns cooler, many classes have spec fuel. Race gas sometimes works better with a smaller jet.
- I agree with you on the hard gas lines. ethanol and alcohol makes fuel line hard
- sometimes I will go 1 jet size larger or change the mix from 25 to 1 to 40 to one

[filthy phill]

there is no racers in Europe that would go anywhere near year old fuel, the gas is a joke here if a year old unless its sealed aviation fuel or sealed race fuel, but they would not do it by choice.
I understand the no shift due to a lot of racers running solid hub props.
I understand your black plugs because a lot of the race motors use a lot more oil than most recreation motors.

I was thinking of switching to 100LL to get fuel that is more consistent, but the price of it right now is a joke, its been heavily taxed and retails around £2.50 per litre, thats about
4 Canadian dollars a litre tat todays exchange rate...
might have to settle for shell vmax or Tesco 99. that will give me about 84 to 95 octane although here its called 99 octane ?????.

it would be a rise from say 89 octane to 95 octane , would this make any more power worth noticing ?

Like I say I am not after a full race motor that lasts a weekend, just chasing those few more revs. and trying to work out why some of these motors can rev to certain rpm without changing
out the electrics and raising the ports.

is there a variance in the oval ports actual exhaust port height and width and shape of different years or batches etc , mine is not a proper oval, its longer at the top than the bottom.

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