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Old 06-24-2008, 02:46 PM
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Rebuild and port old GM sbc low comp heads for blower setup

Hi,
I will rebuild and port a pair of old GM sbc low comp heads for a supercharged 350. Right now I have two different castings to work on:

3998991 72-73 307/350 75cc 1.71"/1.5"
3997993 72-73 307/350 75cc 1.94"/1.5"

I plan on using a kit from Competition Products, #851326, and manganese bronze valve guides to rebuild the heads. The kit includes 2.02"/1.6" stainless steel valves, 115@1.750"/340@1.250" 0.570"max lift valve springs, 7deg CrMo retainers and valve locks and umbrella valve seals.

Witch of the castings should I use? Or is their better flowing low comp castings to look fore?

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Old 06-24-2008, 03:02 PM
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I wouldn;t bother with either.

If you're going to mess with smog castings at least start with a better set.
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Old 06-24-2008, 03:09 PM
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If you want to do a cheap blower motor that is cost effective get a set of WP 200cc Sportsman II 72cc heads and work the chambers and ports on them.
You'll be a full 100hp ahead of any modded smog head for few just more bucks. Bullet proof castings.
Some areas of a build are just not worth chinzing out on.
The engine with the better heads (both in terms of casting and ports/chamber) will make way way more power with less boost and last land last and last.
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Old 06-24-2008, 05:00 PM
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Thanks for the casting suggestions, I will look them up more closely.

I can really see your point in aftermarket heads F-BIRD'88, and I agree. But if you do all the work for yourself, a set of smog heads with new parts will cost $200 in total. Thatīs 4 - 5 times less than aftermarket heads. Even a pair of Vortecs is 2.5 - 3 times more. Besides, in this project I am bound to old castings and a tight budget..
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Old 06-24-2008, 06:36 PM
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Quote:
Originally Posted by burkgul
Hi,
I will rebuild and port a pair of old GM sbc low comp heads for a supercharged 350. Right now I have two different castings to work on:

3998991 72-73 307/350 75cc 1.71"/1.5"
3997993 72-73 307/350 75cc 1.94"/1.5"

I plan on using a kit from Competition Products, #851326, and manganese bronze valve guides to rebuild the heads. The kit includes 2.02"/1.6" stainless steel valves, 115@1.750"/340@1.250" 0.570"max lift valve springs, 7deg CrMo retainers and valve locks and umbrella valve seals.

Witch of the castings should I use? Or is their better flowing low comp castings to look fore?
These are early thin wall castings, I rather doubt they'll stand up to blower pressures for long. The problem everybody has with blowers (Ford, Chrysler, GM, the imports) is that the combustion chamber flexes and finally breaks with a rush of coolant into the cylinder. True blower heads usually include some extra support thru additional material thickness and some posts cast in to connect the chamber roof to the rocker box deck to spread the loads out over more structure.

My experience says you're being "penny wise and pound foolish".

Bogie
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Old 06-25-2008, 09:58 AM
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These heads should be heavy casting. Donīt know the thickness of material in the combustion chambers but the deck is really thick. From 1975 and onward GM switched to light casting and double heat riser passage witch made the heads prone to warp and crack, especially at the induction hardened exhaust seats.
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