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502 bolt-ons

Old 01-10-2002, 10:40 AM
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Mr. Chiong
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Post 502 bolt-ons

I would like opinions on which of the following strictly bolt on components would optimize my stock carburated 502:

1. carb (someone stole my Q-jet)
2. exhaust (I have Corsa diverters I would like to keep)
3. intake manifold
4. cams
5. heads

I tried to do many searches, but the topics are too spread out and few involve a stock setup. I am strictly a recreational boater who likes to "get on it" and also pull wakeboards and inflatables.

I am looking for some extra punch while keeping reliability. Could you please include ballpark prices and of coarse specific manufacturers.

Thanks!
 
Old 01-10-2002, 10:49 AM
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1. carb 850 Holley
2. I bought lighting headers around $1400
3. Dart intake used I paid 150
4. cam full roller I don't remember how much I paid I bought mine from CP Performance which was a comp cam which I would never buy again I waited 6 weeks for the cam and the roller lifters that cam with the kit where wrong.
5. Heads I bought Merlin Marine heads new for around 1200.
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Old 01-10-2002, 11:05 AM
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The Edelbrock Airgap rpm intake makes good power on this engine.

Good Luck
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Old 01-10-2002, 11:25 AM
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I would think that the biggest change would be with the cam. Don't go real nuts, otherwise you'll have reversion problems with your exhaust. If you do want exhaust, search the ads on the board for some good used manifolds. They'll be adequate for your application and easier on the wallet. I honestly don't think there would be a whole lot of improvement with the intake on your application. I think that there are a bunch of possiblities and gains to be made with the heads, but the ones you have now will work. Besides, it's alot of work to change out the heads. Not really a bolt on option.
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Old 01-10-2002, 11:34 AM
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I'll rate them on what I feel will affect your performance the most to least..

1) Cam (Not too radical, I always use Crane but that's just me)
2) Intake (Dual plane for your application because it builds more torque - I always used Edlebrock Performers and they have a marine version)
3) Heads
4) Exhaust

You have to buy a carb, so go ahead and get a Holley 850.
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Old 01-10-2002, 11:35 AM
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Welcome to the Board. Lots of little options on how to do this, depending on the desired HP level. But it sounds like you want "Stage 1" - the Best Bang for the Buck without losing low RPM torque and reliability. I am assuming that you have an early 502 Magnum with GM dual plane intake (390 HP). Why don't you try to duplicate Mercury's HP465, which made 445 HP?

Holley 800 Marine carb
2" square bore adaptor on your GM dual plane intake
Mild porting job on your heads - especially exhaust side, do a valve job while you're there
Bigger hydraulic cam - ~226/236 @ .050"
Roller rockers
Better exhaust - expensive, but the rest of the parts won't breathe if you don't have it.

This is going to add 55 HP to your stock (390) HP. If that's not enough, you can duplicate an HP500 and add 80 HP by changing to a Dart single plane intake and converting to hydraulic roller cam to make 470 HP. The roller cam and lifter conversion is expensive.

The good thing about duplicating Mercury's proven combinations is you can get the exact carb jetting and ignition timing specs out of their Service Manual.

Don't forget that adding a lot more power requires a prop change to get the increased speed. This will drop your RPM when towing wakeboards and inflatables, so don't get carried away; you can't afford to give up any low RPM torque. Good luck.
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Old 01-10-2002, 12:19 PM
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Thanks for the input everyone. I am leaning towards the Merlin heads. Should I just have my heads ported? Should I just trust a local machine shop or send them to a recommended shop?

Anyone heard of the Westech Top Thunder Kit? It includes a Demon 750cfm carb, Edelbrock Victor Jr. intake, double roller timing chain and a Comp Cams camshaft w/ valvetrain. Total cost is around $1100. They claim 25-60hp. This plus a set of Merlin heads would have been my choice of components prior to finding this website.

What do you think?
 
Old 01-10-2002, 01:15 PM
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Aftermarket heads like the Merlins do have better exhaust ports out of the box, but they achieve this by raising the exhaust port in the head about 1/2". This can cause alignment problems if your current exhaust system is already tight. I had to stick with GM heads for the stock port location because my silencers are bolted to the transom and the exhaust tailpipes had to fit them exactly! If you are hooked up with rubber hose you may have enough slop to handle the higher port/exhaust manifold/pipe position. Just check it first. Unless your heads are junk, I would just work with them and apply the money saved to the purchase of better exhaust manifolds.

A dyno-tested package like Westech's eliminates some of the guesswork, but the single plane manifold often seems to give up low RPM torque for questionable gains in high RPM HP (in the up to 500 HP range). My own experience with single planes is a rougher idle and less torque. Others on the Board have reported much smoother idle and no loss of top end when switching from a single plane to an Edelbrock RPM Air Gap dual plane. They said that the improvement in low RPM torque was like a cam change!

If you look at the Westech dyno curves carefully you can see that they stopped plotting the stock HP and torque curves below 3200 RPM, so you really can't tell how much you're giving up. I have single plane intakes on my 502s because they just look so damn good and I never run the boat less than 3700 RPM. But I'm not pulling wakeboarders; I can't even plane at wakeboard speed.

Hope this helps.
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Old 01-10-2002, 06:51 PM
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Small blower, low boost,comletely reliable.Most bang for buck.I have a friend that installed one on a 91 Baja Sport 270,502 Mag,5 Lbs boost,, picked up 9 mph,stock exhaust.
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Old 01-11-2002, 11:09 AM
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I had a carbed 454 Mag that I began modifying. First I swapped cams to an Ultradyne hydraulic flat tappet,added Stainless Marine headers and switched the Weber carb to a holley 750. I gained about 2 MPH and lost bottom end! I then rebuilt the engine. Raised compression to 9.4, added ported Merlin heads and a hydraulic roller cam. The result was an additional 8 MPH gain, for a total of 10! The trick is to have a complete package. Just changing 1 or 2 items usually doesn't give too good of results. The heads are where the HP is.
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