perfboat

What are some of the first modifications people make to their 502 for more power. So far, I've had the prop lab finished and I am looking at imco's exhaust and the MSD ignition. Will I see any gain in performance or better driveability.

Dave1972

hello perfboat,

This subject has been discussed several times on the board. If you do a search for 502 mods, you'll find countless posts. Is your 502EFI or carb? The exhaust will not really net you a huge increase other than the weight savings and clean up the transom (typically makes the engine run a little less rich). You can add an aftermarket fuel pressure regulator (Arizona Speed and Marine) and have the ECM reprogrammed. Other's have changed cams and done some minor head work and bumped their 502 to upwards of 500 horsepower. OF course there's always a supercharger (whipple gets my vote) for ~$6,000 ish and you'll increase your horsepower much more that way dollar for dollar. Some questions to ask yourself....how much do you want to spend, how fast do you want to go, how important is turn-key reliability. Have fun and welcome to the board!

Sea Ya

I beg to differ about the exhaust change. I changed to Gil Big Guns and high flow intakes. I started hitting the rev limiters and changed 28 pitch props to 30p. A 72/73mph boat is now 80/81mph boat. 33' AVS Scarab otherwise stock.

Dave1972

Sea Ya,

Just an exhaust change on a 502 will not give you 8-9mph. As much as we'd like to believe it it ain't gonna happen. Typically it takes 15 horsepower for a 1 MPH increase. You mentioned that you changed intakes as well....I would believe the combination of both gave you the increase you mentioned. That's a great improvement!!!! My guess is that you are running carbed 502's? Which intake did you change to?

tomcat

I agree with Dave. The exhaust change will do very little by itself and the MSD will do nothing. You have to bring the whole intake and exhaust pathway up to the next level for big gains. If you don't want to go into the engine just leave it alone.

If you do want to go into the engine, keep the stock EFI intake, it is a great piece. Remove the heads, do a valve job and some minor porting, and reinstall them with the best head gaskets you can buy. Change to the HP500EFI cam, add new springs and roller rockers, add CMI headers, adjustable fuel pressure regulator and reprogram the computer. All of this will get you to 500 HP.

You will then need a new prop so the money you spent on lab finishing is gone. Welcome to performance boating.

We just added our supercharger system to a pair of 502s that were converted as described above. We figure 700 HP @ 6 psi boost, 35 Fountain goes 100 mph now.

We didn't have to change props because the boat got XR drives with 1.26 ratio at the same time. Did I mention that you need to get stronger drives for this much HP? Welcome to performance boating.

Tom

alter ego

Nice job tomcat,

I have a 502mpi in my 28 equalizer. I have a lot of the internals done and worked my prop for cruise speed. My top end is in the mid 60s now. How is reliability and maintenance with the supercharger? I am thinking of making that my next change but I don't want to ruin the reliability of what I have now.

Gary Anderson

Had a friend with a carbed 38 scarab w/502 (410hp). He swapped out the carb (Holley), intake (Dart single plane), & exhaust (CMI) for the HP500 parts. But he wouldnt change the cam or anything internal, because he couldn't "see" the change.
The boat ran exactly the same speed after spending $8000.
I'm definitely with Tomcat on this one.
Gary

tomcat

alter ego:

There is no reason to give up any reliability with a conservative setup and slightly more frequent maintenance. Keep the boost to 6 psi and lots of water to a big intercooler. The computer will keep timing at around 30 degrees. Use a slightly colder plug and change your oil more frequently. The reason for the oil change is all of these 502s have some blowby and pressurizing increases the amount. That means some fuel will always find its way into the oil, eventually thinning it. Listen to what guys on this board tell you about using the best head gaskets when you freshen the heads.

The number one problem when supercharging is detonation. Too much boost, not enough cooling and head gaskets will fail. It's all down hill after that.

There is a very good article in one of the car magazines this month about centrifugal supercharging. They used a Procharger blower and an air to air intercooler on a dyno engine. Played with different pulleys and measured temperature and pressure at different points. Then they removed the intercooler and repeated the tests. Some of the numbers reported are a bit suspect but the main finding was that the engine blew up after they removed the intercooler. Head gasket failed. Why? Heat and detonation.

alter ego

Tomcat,

Thanks for the insight. I know about detonation....My boat got serviced at the marina and they thought it was running too rich. They turned my regulator down and sent me off to run, As soon as I got to about 4000rpm i would get some really bad knocking. My computer was recurved at ASM for my current setup and my fuel pressure is up at 42psi now so I don't get detonation. They had it set to 20psi!!! Like you said though head gaskets are the key. My engine has a lot done to it already. Whats funny is you can spend a lot on all kinds of parts and not get the gains in speed like Gary was saying. My boat is a good example of that right now. Dingler Racing Engines built my motor and somehow even after all the modifications it isn't quite right yet. I'll be reworking my prop again with Darin and next year will be forced induction most likely. I'll probably switch to a totally different prop for that. Speed is not cheap.....

jspeeddemon

SeaYa is that a personal best racing those Baja's the other week?
That certainly is a heck of an increase for the Big Guns, are you sure that's not nitrous assisted? You know what happen last time you sniffed nitrous!