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Discussion Starter #1
Got a stock Gen 6 420HP 502 Chev with 8.5:1 compression. Wondering if I could bolt on a set of Mark 4 closed chambered heads to bump up compression thus more horsepower. Got a set of Edelbrock AL open chambered but that won't help anything. I don't think a cam swap would help me either with stock engine. Looking for some cheap horsepower and need some help.
 

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Boat Nut
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5,131 Posts
Widow Maker was runing a stock bottom 502, making close to 750 HP
 

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steelcomp was here
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26,526 Posts
Heads and cam swap will both help.
Bolt the alum heads on with the right cam and you'll make plenty more power. You didn't mention this in the Dyno section. Even just a cam swap with your heads would make a big difference.
 

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Progressive Auto/Marine
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Heads

Oval or rect.port? If I remember correctly the 420's are rect. I have replaced them b4 with oval GM casting # 353049's which make awesome marine power. Pocket port them and gasket match for an extra 70 hp. The larger valves help out a bunch too. I went from rect. to oval due to the power characteristics. These heads are 113 cc. and came stock with 207, 172 valves.
If you want to bump your compression a bit, you can straight mill these heads without any problems. What I did however was used a .020 shim gasket from Mr.Gasket and punched in additional cooling passenges to match the existing Mercruiser gaskets.
The Comp Cams 11-236-4 cam set has awesome numbers and has a nice idle with noticable lope.
This makes a nice combo.
Do you know your existing head casting #'s?
Go to www.compcams.com Download their Camquest 6 software.[Free] There you will be able to custom tailer your heads, compression, carb, intake and what have you to build the ultimate powerplant and the cam that works best for your needs. It will basically pick the correct cam for your application. Cool tool to keep in mind.
 

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E-7 Sheepdog (ret)
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6,834 Posts
Oval or rect.port? If I remember correctly the 420's are rect. I have replaced them b4 with oval GM casting # 353049's which make awesome marine power. Pocket port them and gasket match for an extra 70 hp. The larger valves help out a bunch too. I went from rect. to oval due to the power characteristics. These heads are 113 cc. and came stock with 207, 172 valves.
Did (had my stock '049 heads ported and the larger valves installed) that winter of '07-'08, and made a bunch more power, and great throttle response all over the rpm range.
No money to hit a dyno (as always) but pulled 400RPM more against my Aggressor B, zero other engine changes (cam, intake and headers were not stock tho so the heads were the "choke point")
 

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steelcomp was here
Joined
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26,526 Posts
Did (had my stock '049 heads ported and the larger valves installed) that winter of '07-'08, and made a bunch more power, and great throttle response all over the rpm range.
No money to hit a dyno (as always) but pulled 400RPM more against my Aggressor B, zero other engine changes (cam, intake and headers were not stock tho so the heads were the "choke point")
I have another set of those done with big valves, bowl blend, and flow sheets...fuly assembled, if any one is interested.;)
 

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E-7 Sheepdog (ret)
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6,834 Posts
I have another set of those done with big valves, bowl blend, and flow sheets...fuly assembled, if any one is interested.;)
If I needed another set, I'd be interested. :)
 
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