52mph WOT! What gives 25' Others get 70+
 

I have a larson Senza Spectre with a 515hp (fresh build) 454 Mercruiser. Has a 4 blade 23p stainless prop Bravo I believe. WOT is 4400rpms. GPS is 52 at most. People are getting 70+ out os this 25' boat. Even Factory specs say 65-68 mph. What is my issue here haha. Prop>?

did you actually dyno the motor or are you guessing its 515? if so at what rpm did make 515? I'm guessing it was not 4400 rpm? If you actually have 515 you should be able to pull that prop much higher than 4400 rpm. you drive height is probably pretty deep and will likely be faster with a 3 blade but you still seem to be way off.

4400 is way too low. In order to get those speeds you need to be up around 5500 more than likely. Try a lower pitch prop and see what it gives you.

drive ratio??

rev limiter ?

I do not know the rev limiter rpms. The gear ratio i believe is 1.50:1 Bravo one. The engine shop built it well, bored 40 over, stroked, blue printed etc with full roller cam lifters and rockers. Huge heads. I should see more than 4400rpms, stock tach by the way.

I may need a 24p prop?

NO, you may need less prop. Your RPM is the problem. Motor either doesn't make the power you think it does OR your are over propped.

Was it dyno'd??? How do you know its 515hp???
What is the timing set at???

+1 with making sure how much power you have, and if you have 515hp then check all the motor i.e. spark plug cables distributor fuel pressure etc

The guy has been building big blocks for 30 years, he builds race car motors mostly. He said mine has roughly 515 to 530hp. It could be the timing, what should it be set at? I have no clue. 8 atdc?

You don't want to do that.


try 8-10 Btdc

Sounds like you need a 3 blade prop - BUT the prop is not the problem.
You are not producing the HP you think you are.
With 500 HP you should easily turn that prop to over 5000 RPM.
Either you have the wrong cam for a boat engine or something is really wrong inside the engine.
With 500 HP you should be running at least a 25 mirage + to 5100 RPM.

Race car guys are great at car motors. There are a lot that can not build a good boat motor.

Get us the motor specs on the motor. There are a lot on here that will be able to tell if the specs add up to a good boat motor.

My guess is wrong cam selection.

When you upgraded your engine did you upgrade your exhaust.
What do you have for exhaust???
With 500 HP you need aftermarket through transom exhaust.

good thinking it might be 515 with car headers but not with crap boat manifolds that are just an open chamber. my guess is that its basically a stock 454 with a cam/ intake manifold

The timing should be set around 34* at full advance. Using base timing to set it without knowing the amount of advance in the ignition system will get you no where.

Do you have anything resembling a build sheet?

Was it dynod? (pretty sure I know the answer to this)

Who tuned it?

If you cross connected the spark plug wires you wont be able to pull any RPM.

The guys so far are all correct-car engines typically get great exhausts and run way more duration and valve overlap than a boat engine will.

Uncle Dave

A standard merc ingnition rev limter is 4350 , put in a msd dist and ingntion and see what happens
if you have the hp you say you are on the limiter and thats why you are stuck at 4400 and you should then go with a bigger prop after you fix rpm problem . seen this problem before on a friends build , gave him my extra crane box ( fire wire merc ) and it took care of the problem , You can but a dist from msd wiith a rev limiter in it have fun

Did the boat pull more than 4400rpm before the new motor? If so then rev-limiter is not the problem.

If that's not the problem that much HP should pull a 23-24 Bravo past 5000rpm. If you have stock exhaust than the HP number is down under 500 more likely 450-475 in the boat.

My mild 468 on the dyno pulled 407hp but is probably around 365hp in the boat since I still got stock exhaust. My boat is 24ft and I can pull a 22 Bravo to 4900rpm and a 26 Bravo to 4300rpm. I'm still looking for a 24 Bravo to try out.

No merc BBC engines have a 4350 rev limiter. The lowest one is 4750 rpms.

stock 454 ( 330 hp ) , that is what i was told , if i iam wrong i am sorry . I was just trying to help a guy out

4,600 max RPM for stock 330 HP 7.4's with the rev limiter set at 4,750.

Was it a 330 with peanut port heads?

Do as others have said, get all the specs on this engine, with that info we can help you out. There was a 25 Senza on my lake when they were new with a 454 mag and yes they are a fast boat. One thing lots of guys have found out is car guys usually do not build great boat engines. What works in a car typically doesn't work in a boat, especially a larger single engine boat like yours. In order to get 515 real hp out of a marine 454 would need good aftermarket heads or heavily worked iron heads. IMO a good 454 with stock heads could make around 460 hp, if done right with aftermarket exhaust. If you're using stock exhaust drop that number to 400 hp. Even at that number you should be at 70 mph in that boat.

I went and ran some prop slip numbers and guessing the slip on that hull would be around 12-13%. With a 502 and 70 mph at 5000 rpms you would be spinning a 25 pitch mirage or a 23 bravo. The fact that you are only able to spin at 4400 rpms tells me that your engine was build wrong for a boat and is not making anywhere near 515 hp. That boat with a 330 454 would run low 60's.

There is a guy on the swap shop that has a hp 500 dist and thunderbolt 4 ing for sale , $150 .00 buy it and try it

I would first get us all the engine specs. I feel replacing the distributor at this point would just be throwing good money after bad. One question for you, is the bottom painted?

Sounds like your running that boat with an engine similar to that of a Fred sanfords truck, it just does knot have any power. Maybe you are running it on 6 cylinders.

haha, you guys are great. The boat has an edelbrock 750. I was told by the shop that I am starving the motor of fuel. Secondly, it has a Corsa thru hull exhaust. The heads are the huge rectangular port heads with roller cam, roller lifters, and roller rockers. Bored 40 over with forged sem-flat top pistons. The valves are upgraded to as big as they fit. THe ignition is a stock prestolite i believe. I dont think it i am running on 6 cylinders haha. I should have more rpms thats for sure. Maybe throttle doesnt open all the way idk. I thought it was when i put motor in.

We need more info! What cam grind, intake, compression ratio. Biggest valves doesn't mean anything- what are the actual sizes. Yes the 750 is a bit small but your not going to lose 20 mph from being 50 cfm to small. There are bigger issues than the carb size. Also Corsa through-hull doesn't mean much either, what are the actual exhaust manifolds? If you don't know, chances are they are just the stock restrictive manifolds.

We can't help You if You won't help us with the info we ask for.

You also need to make sure your carb is opening all the way. This should have been one of the first things that you did with a fresh engine install. This takes but two minutes to do, so go do it and report back.

I over looked that in my first post. That has to be part of the problem. 50cfm is not the big problem its the fuel. I have that carb on my mild 468 with a 110gph fuel pump. Out of the box that carb ran lean on my engine and we had to put in the biggest jets that Edelbrock makes so it would not run lean. With those jets it was just enough fuel to run the engine safely and not lean. I'm sure his engine is more thirsty than mine.

A merc 502 with 425 hp runs the same carb and so does the 454 and both push the boat well over 52 mph. There is a report posted on this boat in the general question section and with a stock 454 Mag the boat would radar at 60 MPH. There was one guy that claimed his boat hit 72 on the GPS but I don't believe it, I'm going to believe the magazine article. So with that said 52 mph would be right around 270 hp. The stock prop was a 23 laser II, so a 23 4 blade is two much. He needs to either run a 21 blade or a 23 3 blade and then see where he is at. Then yes- he needs a holley 800 cfm carb. Seeing that a car guy built this, I'm betting we are going to see that it has a cam that produces power past 6000 rpms. This would also add to the problem. Then on top of that I'm willing to bet its running the stock exhaust. The exhaust, prop, and carb might be good for an 80+ hp drop but I think the biggest problem is going to be the cam that was installed. However these are all guesses until we get the engine specs. With 525 hp this boat should be right around 70-72 mph.

Good point on the cam rpm, really need those cam specs.

Sound's like 515 auto trader HP !!!!!!!!!! I hope your buddy gave you a kiss because it sounds to me like you were just on the receiving end of a sexual experience .

My senza spectre always ran best with a 3 blade but i was also running 425 hp small block and a alpha when it would hold together boat ran mid 70s

adding my 2 cents worth, had a senza back in the day, 454 525 hp, bravo 1, 1.5 gears, labbed 26 mirage, 4800 at 78 mph gps, a little dicey on the turns that big step would really hook but it flewww.

And zero slip?

riiiight........

doesn't quite compute does it?

UD