Upgraded 260hp to ~300hp, wot rpm less

[Lee Jolly]

Hello,

I've checked the FAQ's but I can't seem to think of good search terms for this. I have a 1987 21' Reinell with a 350 and OMC Cobra stern drive. The stock engine was rated for 260hp, but I had to replace that engine recently.

The engine builder was selling a 330hp package that ran on 89 octane, but I asked them to build me one that would run on 87 and they (I'm assuming a bit here) lowered the compression a bit and said it should be putting out somewhere around 300hp. At the same time, I had my carb rebuilt, I'm not sure of the jetting, but I did tell them its going on a newer slightly higher HP engine.

With the stock engine, at WOT with a 15x17 prop I would max out at around 4500 RPM and I would hit about 47mph. With the new, supposedly higher HP engine, with a 14.5x19 prop, WOT rpm is ~4000 and I am going 40mph or so. I understand that every inch of pitch should change the rpm about 200, so if I put the 15x17 back on that would still get me to slightly less rpm than stock (I'm still going to try this out, but I have to get my 15x17 repaired first, had a little incident with it )

I guess my question is basically, why isn't it faster? 40hp is not totally insignificant. And shouldn't the operating RPM be a little higher than stock with a higher HP engine?

Thanks in advance!! I hope this is not too poorly worded, I tried to give as much info as possible to make the diagnosis easier.

 

[JustJason]

Re: Upgraded 260hp to ~300hp, wot rpm less

who was the engine builder and does he/it have a good reputation.

 

[gcboat]

Re: Upgraded 260hp to ~300hp, wot rpm less

Confusing question but are you out slamming a fresh motor at WOT just out of the box?

 

[Lee Jolly]

Re: Upgraded 260hp to ~300hp, wot rpm less

It was Diablo Engine and Machine and at the time they had a good reputation with Chevy guys. I did some research and a lot of people liked their work for street engines.

As far as being reputable, I thought they were. This is all past tense because they are now out of business. Haha...I know instantly that makes people think they were not reputable, but coming from someone who ran a small business unsuccessfully, not all business failures are due to lack of quality, we can find lots of ways to mess up!

 

[Lee Jolly]

Re: Upgraded 260hp to ~300hp, wot rpm less

Confusing question but are you out slamming a fresh motor at WOT just out of the box?

No, its been a while now with the new engine and it's been properly broken in.

 

[chiefalen]

Re: Upgraded 260hp to ~300hp, wot rpm less

Welcome aboard !

In theory, when you put a lower pitch blade on, you will turn MORE rpms.

And if with everything you posted, assuming the motor is running correctly,thru the power curve and, the carb is opening all the way and running correctly you should be doing another maybe 10 mph faster at wot.

Maybe that carb isn't right?

The larger danger lies in your drive.

I hope your gears don't get eatin up from that extra 40 hp.

You didn't mention a gear change.

 

[SuperNova]

Re: Upgraded 260hp to ~300hp, wot rpm less

Simple answer: Cause it's NOT a 300 hp engine. A lot of supposed experts are really just good at self-promotion and can easily fool a lot of people who really don't know what they are looking at. Are you qualified to judge whether their work was any good or not? Or do you just go by "looks good". Most people who would know what to look for are capable of building their own engines and do so. So they could tell you or anybody else whatever sounds good. I work in the industry and I see it all the time.

If it is truly what they claim, then it is possible for you to have carb issues (secondaries opening?) or timing issues (damper slipped? or correct one?) or a simple error during the build such as the timing chain off by a tooth could easily cause your problem. This engine may require more fuel than your pump and lines can keep it fed with. There are literally tons of possibilities.

 

[Lee Jolly]

Re: Upgraded 260hp to ~300hp, wot rpm less

Welcome aboard !

In theory, when you put a lower pitch blade on, you will turn MORE rpms.

And if with everything you posted, assuming the motor is running correctly,thru the power curve and, the carb is opening all the way and running correctly you should be doing another maybe 10 mph faster at wot.

Maybe that carb isn't right?

The larger danger lies in your drive.

I hope your gears don't get eatin up from that extra 40 hp.

You didn't mention a gear change.

Thanks!

About the gears, no I haven't upgraded them. I try to take it easy though, no WOT out of the hole, that sort of thing. I know I should upgrade that, but money issues you know.

Regarding the carb, it could be I guess. It runs well when warm, but when cold it doesnt quite idle as well as I would like (maybe choke issue). And it seems to load up a little. I suppose I've had enough problems with carbs in the past to assume it will never be absolutely perfect in that dept!

Timing may be off a tad. It was gone through by a pro, but you know how that goes sometimes. Timing chain was installed correctly for sure.

I was looking at the FAQ about fuel pressure and making sure the fuel delivery system was in good condition and I want to do all those tests. It does have a new fuel pump as well. It's just seems SO close to being ideal, like stock, I just expected more from the upgrade.

 

[Bondo]

Re: Upgraded 260hp to ~300hp, wot rpm less

With the stock engine, at WOT with a 15x17 prop I would max out at around 4500 RPM and I would hit about 47mph. With the new, supposedly higher HP engine, with a 14.5x19 prop, WOT rpm is ~4000 and I am going 40mph or so.

Ayuh,...

Seeing as We're all Guessing,.... Here's Mine...

Put the 17" prop back on,+ Try it....
I'm Guessing you'll see 4800rpms,+ just under 50mph....

At 4000rpms,.. You're Lugging that brand new motor....
You've got to get the rpms up so it can Breathe....

My other Guess is,... If that motor were able to pull 5150rpms,...
You just mght see that 300hp....

 

[a70eliminator]

Re: Upgraded 260hp to ~300hp, wot rpm less

Exactly my thoughts too, an rpm power band closer to that of a street engine rather than for a boat, find out what cam you have and list the numbers here.

 

[f_inscreenname]

Re: Upgraded 260hp to ~300hp, wot rpm less

That?s what I was going to say, what cam? And the next question begs to be asked. Just how did they get it down to 87 oct? A 330hp 350ci is a bit of a hopped up motor. Sounds like the they took it back to stock.

 

[Don S]

Re: Upgraded 260hp to ~300hp, wot rpm less

An auto guy building an auto engine, then you sticking it in a boat may not be what you want. Boats don't run in the same power band as an automotive engine, and what works on a street machine could be a worthless peice of metal in a boat. HP is secondary in boats, you need torque, and in all the right places.

 

[Tail_Gunner]

Re: Upgraded 260hp to ~300hp, wot rpm less

Heres a WAG....Was your former prop a black worn out old alum prop ...replaced by a nice shiny new stainless steel prop...

 

[Lee Jolly]

Re: Upgraded 260hp to ~300hp, wot rpm less

An auto guy building an auto engine, then you sticking it in a boat may not be what you want. Boats don't run in the same power band as an automotive engine, and what works on a street machine could be a worthless peice of metal in a boat. HP is secondary in boats, you need torque, and in all the right places.

Well since the builder is no longer around I cant really find out what cam he used. But I did specify a marine engine, and he did some things like brass freeze plugs, and I think a different cam.

 

[Lee Jolly]

Re: Upgraded 260hp to ~300hp, wot rpm less

Heres a WAG....Was your former prop a black worn out old alum prop ...replaced by a nice shiny new stainless steel prop...

Nah, both props are aluminum. The 15x17 just has a big chip on one of the blades. I really need to get it fixed and give it a shot...Hopefully Bond-o is right!

Thanks all for your help...after I get it fixed I'll try to find this post and update in case you may be curious how it turns out. Thanks again for all your ideas and assistance!

 

[newport dave]

Re: Upgraded 260hp to ~300hp, wot rpm less

Not sure if someone already pointed this out, but keep in mind that Mercruiser rates thier engines using the Propshaft HP, your engine guy was probably rating the engine using crankshft HP.

 

[Bondo]

Re: Upgraded 260hp to ~300hp, wot rpm less

I'll try to find this post and update in case you may be curious how it turns out.

Ayuh,...

Just click on your user name,+ follow the path,...
You'll be able to bring up Every post you ever posted....

 

[180shabah]

Re: Upgraded 260hp to ~300hp, wot rpm less

Without knowing what parts are in your engine and exactly what power to expect from it then all you will get are a bunch of guesses. There is a huge difference between an engine that makes 300hp at 6000rpm and one that does it at 4800rpm.

Tune the carb, fix your choke problem and set/verify the timing both at idle and full advance then prop to match. Your top speed will be what it is.

 

[Silvertip]

Re: Upgraded 260hp to ~300hp, wot rpm less

And although you said this guy was great with street engines, are you sure he didn't build you a street engine instead of a marine engine? There is a difference you know. Building you a truck engine would have been a better choice of words. Unless you see a dyno printout, you have no idea what you have.