Safe rpm at WOT with a stock Evinrude 75hp

[tunnelvision07]

What's the most rpm's I should run with a stock 75hp Evinrude so I don't blow it up? The only mods are drilled lower unit and hustler exhaust. It's on a 12' tunnel hull

[Mark75H]

Don't go beyond stock rpm, power goes down as rpm increases if you have not done a bunch of other mods.

If you want to go fast and faster, prop a std powerhead for std rpm

[tunnelvision07]

Thanks for the help. I appreciate it!

[zul8tr]

Don't go beyond stock rpm, power goes down as rpm increases if you have not done a bunch of other mods.

If you want to go fast and faster, prop a std powerhead for std rpm

Sam

I know he prefaced his inquiry with "so I don't blow it up" but if that wasn't the issue......

What about stock Mercury Mark 25 and 28, 2 cyl powerheads that are rated at 5500 there abouts and being used on quiki units without internal edits and consistently run at 7000 with no problems?

Another example is the Merc 200 series fish engine that was used in the first edition 25ss that runs at 7000 with the same internal bearings etc. as the base Merc 200.

Then there are the old 44ci Merc blocks that I have seen with no mods put on a quicki and run at 7000 with no issues.

Are the Mercs designed better than the Rudes to be able to run at the higher rpms?

Can't he just run extra oil and go at the higher rpms and be aware of the risks? :eek:

[Mark75H]

Thanks. My first post does ignore the "blowing it up question" in favor of going faster (which is not posted, but inferred)

The second line in my post refers directly to going faster with the triple.

The for power, 2 cylinder motors are not fighting inherent exhaust tuning found in the triples.


That I know of, there are no problems with running either of these motors into the 7,000+ range.

[Yellowjacket]

Then there are the old 44ci Merc blocks that I have seen with no mods put on a quicki and run at 7000 with no issues.

Are the Mercs designed better than the Rudes to be able to run at the higher rpms?

Can't he just run extra oil and go at the higher rpms and be aware of the risks? :eek:

Of course old Merc's are better than Rudes... so what's your point???.

I'm told by reliable folks, that you absolutely can run a stock 44 merc to 7,000, but it won't last very long doing that. The reason being that you will burn the big end rod bearings out of it in pretty short order. The real limit is 6,000 rpm without eventually causing the rods and crank to be damaged. A few zings up there won't harm it, but if you stay above 6k for a few minutes at a time you're going to ruin it.

If you open up the big end clearancs, you can spin the wee wee out of it, and that's all you need to do... And no, putting extra oil in a 44 merc won't get you there either. If it was that easy everybody would just lube it up and go......

I'm about to tear into my Merc 500 for just this reason.

[Mark75H]

No problem that I have seen with running the 44's at 7,000+


In fact, they usually run 8,400. I have not seen a bearing failure

[MN1]

Didn't the factory Mod 50s run 9,000 rpms or higher?
Mark N

[zul8tr]

Of course old Merc's are better than Rudes... so what's your point???.

I'm told by reliable folks, that you absolutely can run a stock 44 merc to 7,000, ........


Yellowjacket
No point just asking a question.:)

[Powerabout]

old engine bad design pistons or are we talking all new spec kit?
That would be 1000rpm difference at least

[Yellowjacket]

No problem that I have seen with running the 44's at 7,000+


In fact, they usually run 8,400. I have not seen a bearing failure
I guess I should have qualified as to what "old mercs" we are taking about here. If old means 30 years old, engines from the 70's and 80's then we are talking about these engines. If old means the earlier Mark 55h and the 44 cu in 500's of the 60's, that's a different story. The Merc 44's from the 70's and 80's had tighter rod clearances than the earlier motors and if you spin them too fast you will damage the rod bearings and the crankpin journals. The change was made to make the engine more quiet, but the result is that you can't spin the later classic engines as fast without opening up the rod clearances. This includes the 44xs and any of the "classic 50" powerheads from the 70's and 80's.

[zul8tr]

I guess I should have qualified as to what "old mercs" we are taking about here. If old means 30 years old, engines from the 70's and 80's then we are talking about these engines. If old means the earlier Mark 55h and the 44 cu in 500's of the 60's, that's a different story. The Merc 44's from the 70's and 80's had tighter rod clearances than the earlier motors and if you spin them too fast you will damage the rod bearings and the crankpin journals. The change was made to make the engine more quiet, but the result is that you can't spin the later classic engines as fast without opening up the rod clearances. This includes the 44xs and any of the "classic 50" powerheads from the 70's and 80's.

I have a 1973 25ss with the deflector pistons and the factory info tag that it came with recommended 6600 rpm max but when I was racing it in APBA in the 1970's I consistently ran it at 7000-7100 with no issues. Still have it and run it the same at local AOMCI meets. I have had it apart several times thru the years and it looks OK at the rod bearings and cylindere bores. Is it possible that Mercury clearanced the Merc 200 rods for the first series 25ss engine?

[Yellowjacket]

Is it possible that Mercury clearanced the Merc 200 rods for the first series 25ss engine?

Don't know how the 25's were built. The change in rod clearances on the four cylinder engines coincided with a change in crankshafts and other changes to the top of the motor. Not sure if they ever tightened up the clearances on the two cylinder motors. The change was made to make the 4 cyl engine more quiet. Probably more important on the bigger motor and not such a big deal for a smaller motor. At that point they weren't racing the 44 and it was probably more important to have the customer feel that he had a quiet and smooth engine as opposed to making a few racers happy.

Just a thought, I really don't know, you would have to ask someone who is building the 2 cyl motors to find out. There is also a reference to this problem in Harry Brinkman's notes on building the 44 for and he states that you need to open the rod clearance on the stock 44xs as well as engines running higher rpms in modified racing so I am sure it is real. I was corresponding with Jerry Wienandt (Trident Racing) and he is the one who recommended that I use an RPM limit of 6,000 for my 1977 stock Merc 500 until I get the rods opened up. Jerry is an expert, when experts talk I listen.

[zul8tr]

Don't know how the 25's were built. The change in rod clearances on the four cylinder engines coincided with a change in crankshafts and other changes to the top of the motor. Not sure if they ever tightened up the clearances on the two cylinder motors. The change was made to make the 4 cyl engine more quiet. Probably more important on the bigger motor and not such a big deal for a smaller motor. At that point they weren't racing the 44 and it was probably more important to have the customer feel that he had a quiet and smooth engine as opposed to making a few racers happy.

Just a thought, I really don't know, you would have to ask someone who is building the 2 cyl motors to find out. There is also a reference to this problem in Harry Brinkman's notes on building the 44 for and he states that you need to open the rod clearance on the stock 44xs as well as engines running higher rpms in modified racing so I am sure it is real. I was corresponding with Jerry Wienandt (Trident Racing) and he is the one who recommended that I use an RPM limit of 6,000 for my 1977 stock Merc 500 until I get the rods opened up. Jerry is an expert, when experts talk I listen.

I know Jerry and have received emails on specifics. I wll ask him.

Thanks

[A/B Speedliner]

We have run the 4 carb 44 CI engine that Doug Kay built with all of the Brinkman Mod's plus a few of his own and have consistently turn over 9000RPM
David