jeff32

Dear doctor engine...

Could you please tell me if, a used or even tired 454, could reach his maximum RPM, (5000) but could not reach the maximum horsepower it was rated for when new?(thus making the boat going slower than it should go) Let's say a 454-420 hp, could it only produce let's say, 350 HP ?

Vinny P

Speed is a mathematical equation. It is not effected by horsepower. What is effected is the size and type of prop you can run to a given rpm. That is what will slow you down. In other words, if you can spin a 24" prop to 5000 rpm, your speed will be the same whether you have 1000 hp or 500 hp. The difference will be, that with 1000 hp, you can spin a larger prop to 5000 rpm, or higher, thereby increasing speed.

The formula for speed is
(( pitch x rpm) / drive ratio ) / 1056 = theoretical speed
Then you have to figure in your slip #..

SeaRay Jim

Ok, back to the other post.... LOL

I'm not really sure I understand the question. That may be the reason you're not getting responses.

I'll say this, any motor has a 10% (I think it is) range it has to be in form the rated HP. Every production motor will be different. If a motor is set up as a 420HP motor with a MAX RPM range of 4800-5000 RPM, that's where peak will be whether it's really at 400 HP or 450 HP.

350 HP vs. 420 HP at peak is a problem, not a motor rating issue. That's assuming someone hasn't changed the cam or done something else along the way erronously thinking that was going to get more HP.

jeff32

i RUN 21's OR EVEN A 23's AT 5000 RPM, with almost no speed difference besides 2 or 3 mp/h. Came from factory with 21's, claimed speed is 61 mph, and I run only 50 on gauge and 47 - 48 gps...that's is really slow. 3000 rpm is about 30 mph and 5000 rpm is about 50 mph... looking for another 10 mph. humidity is between 10 and 30 except transom 50% but that not enough to add weight. But I have interprotect bottom.

gene stokes

Got some seaweed in my lower pick up on my bravo drive. Will it flush out?

FOUNTAINATLAST

Did it run that speed new or has it slowed over time???? A tired motor loses compression over time. The timing chain will get looser. Those are just a few things that will cause a power lose, but this usually happens gradually.

jeff32

I bought it this year 89 f 35-7 original motors. Don't know how it was before, guy had it stored for 2 years. since still original motors, that's why I was asking about the horsepower engines were putting out, if this could be my problem. Besides that... slippage ? hook in hull? what else ?

GOODT

power,speed,rpms have alot of variables and yes a tired mtr may still turn the rpms, the amount of fuel in the boat could cost you a couple of mph, you get the Idea....the best way to tell the condition of the mtrs is to do a commpressin test and a leak down test

jeff32

I have to do this this week, just did not have time to do it up to now. What are the spec to know if the leak down test is good or if it is bad. I have the tool but never used it!

oldandtired

I don't know what rpm peak horsepower occurs on your engines, but you should definitely see an rpm change if you are only changing pitch. If the props are different types, then that may be why you aren't seeing an rpm change. Your slip seems to be high. Assuming that you are using 1.5 drives, and using 51 mph at 5000 rpm, 23 pitch prop, that is 30% slip which is way unacceptable. Are you sure that your tach is right? How are you driving the boat trim wise? If you are driving it trimmed up with the proper attitude, then you really need to start with the prop (assuming your tach is right). Hook is always a possibility and is easy enough to check, so you may want to get a straight edge out and look at it, but 30% slip is ALOT.

Rene