HP500 Buildup questions
 

I am in the process of upgrading the original Mercury Racing HP500's in my 1999 Formula 353 Fastech and need some advice.
The boat is running 79 MPH on GPS and our goal is 90 MPH which I believe will require about 650 HP output from each motor.
We are running Bravo 1.5 drives (I know - another weak point but that's a whole different conversation) mounted on factory installed extension boxes spinning 30P Bravo 4 blade props.

We did a pre teardown dyno run and although the motors were tired, we were surprised they ran 530 HP on the stand.
My engine builder has recommended we stay with the original 850 carbs and Dart intakes and will port match them to the new Merlin heads we are installing.
Internally, we are basically replacing everthing except the crank. The BBC pistons and rods with Manley pieces, Inconel valves and hydraulic roller valvetrain components for ease of maintenance.
The 502 blocks were decked, bored and honed so they are now 516 CI. Due to durability and cost, we want to avoid blowers and run on pump gas.
We are stuck on whether to reuse the exising camshafts and exhaust with these upgrades in order to acheive the 650 HP goal.
The motors came from the factory with GIL exhaust manifolds and silencer tips with rubber flappers.
This is a major concern since I have read numerous articles on water reversion issues with high overlap cams in this application.
If we move to headers we will tack on 7-10k to our rebuild costs which is not in the budget plus they typically are not a reliable as the GILs.
I would welcome any comments and recommendations on the pieces we have chosen especially in the camshaft and exhaust area.
Thanks!

Going to need more than 650 hp to get 90 out of it. :eek:

We're only 10 mph short of this on tired motors. Can you offer any suggestions?

The boat should run 82mph with 500's and Stellings boxes. That is what my friends ran with low hour 500efi's. 79-80mph is what a 353 runs without boxes.

If the engines were tired then no way they were making 530hp. I suspect the correction factors on the dyno were not set up correctly. You are down about 3 mph from what the boat should run. That equals about 40hp per side.

When you say..only 10mph????? That is a lot to gain. It will take all of real 650hp engines to gain that, with correct dyno correction factors.

The heads are going to need to be extensively ported and then you will need a matched cam to make the 650hp number, and I still doubt you will.

The Gils will start choking the engines and will cost you hp as well. Plenty of Gil manifolds crack as well. My 8 year old Gils had cracks. My 13 year old CMI's are fine.

Thanks for the suggestions. I agree that the dyno numbers look high but the boat did run close to what it did when new. The other thing to note was that only one motor was dynoed. When we did the cylinder drop and leakdown tests during the survey one was in worse shape than the other. Unfortunately, with all the excitement that night, I didn't note which one was brought to the test facility. I'll have to check back with my mechanic to see if he remembers. We also had some carb issues last year that I'm sure were contributing to the 3 mph loss. Now my GILS only have about 2 seasons on them so dumping them will be painful. What model CMI's do you have? I've heard great things about their power gains but understand they only come with a 1 year warranty which is not inspiring.

The HP 500's are solid engines, but to get 650HP you will need more cubes or a blower. Not only that but other things will need to be upgraded like the fuel system, lines, pickup, pump, ect... When you say only 10MPH I almost fell off my chair, 10MPH is hard to get in a boat. You will be going through Bravo drives, better keep a few spares.

Oh yea, call RMbuilder for your cam, he will give you a education on what you can actually do. He can get other parts for you other than the cam, so wait till you talk to him to buy any more parts. Good luck with your project.

I have the older style E Tops on my AT. I replaced the cracked Gils with CMI Unitops on my Pantera which are basically a one piece E Top.

Sorry if anyone is getting hurt falling off their chairs :) Guess I'm an optimist asking for 10 MPH. I heard the Stainless Marine III or Kieth Eickert T&L cast manifolds are good - any thoughts on replacing the GIL's with these?

I caught myself before I did any damage :drink: so all is good.

But really give Bob RMbuilder a call, he can custom grind you a cam to meet your needs. He gives great advice and will point you in the right direction for matching components.

Im not an expert and not looking to be a d@#%, but i cant see trying to go for 650hp without putting on a good set of headers. Just my 2 cents.