Looking for a set of performance heads that I can find in a junk yard, it seems that my heads are very low compression and make no power. What cars should I look for? The engine is a 350 from a 78 nova.

 

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You have '624 heads on that Nova? I think that was stock. LM1 engine?

Pretty lame, eh? I'm in the same boat with an '81 Corvette. Heads are coming off next week (after some delays) to help the dished pistons make a little more compression. If my data is correct, you have a similar set of problems, with 76cc heads loosely bolted to a 4" bore filled with 12cc dished pistons. What the Hell were they thinking?

The 'Y' car has 8.2:1 compression as stock, and will have 9.6:1 when the '416 heads go on. What's really sad is that the runners on those '624 heads are snaller than the 305 heads ('416s) from later years. 'F' cars got far better heads just a few years after the decade turned. That would be a good starting point.

So, what block casting numbers do you have? Based on that, we can probably suggest some decent heads to compliment your displacement. If my guess is correct, you'll be searching for '84-86 Camaro and Firebird L69 or TPI heads.

 

Its a LM1, the casting numbers are 366245.

 

I was looking at the specs on the heads you suggested, and was wondering if they would make more horse than the 350 L98 heads seeing how they are slightly higher compression?

 

If you were thinking of the 14101128 heads, they still have 58cc chambers but a little better valve area and port flow.

One caveat of using smaller chamber heads on the LM1 is that the engine has dished pistons, so there will naturally be no quench are between the piston top and flat deck of the head. This can contribute to detonation problems if the head temperature is not controlled and the pressure gets too high. For that reason, I'm limiting the compression to about 9.6:1 on the test car. It's still way better than 8.2:1 and 165# of cranking compression.

 

I'm a little confused now, should I be going with the L69 heads or the 14101128 heads?

 

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It's your call. The LM1 has dished pistons (about a 12cc void in the top), heads with 76cc chambers, and a 4.166" diameter head gasket that is .039" thick. Typical deck height on these old smogger engines is about .025-.030". Put that all together, and the factory rewarded you with a whopping 8.22:1 static compression ratio. I've got the same thing in an '81 Corvette project. Cranking compression is a meager 165 PSI on an engine that has about 20,000 miles on it.

I'm planning to use 14014416 heads on it WITH the dished pistons. The valves in the '416s were enlarged to 1.94 on the intake side from the standard 1.84". Valve bowls were ported, valve faces unshrouded in the chambers, and port runners were resized, matched, and polished on the exhaust side. Of course I installed screwedm studs and 1.4" valve springs while there, but that isn't necessary for this engine and mild cam. Even with unshrouding, I'm still holding just under 59cc of combustion chamber volume because of the intrusion of the larger valves. With a 1004 head gasket, I should net about 9.5:1.

That's about all I'm comfortable doing with the dished pistons and lack of a quench area in the chamber. I'm already going to loose a little "squish" to help scavenge the exhaust out, so that may help control detonation a bit. Not the best overall scenario, but without removing pistons, it's about as good as it will get.

That doesn't mean what I do is right. I'll be able to tell you how it works out in a week or so, when I can get back to the car. If you're doing this right away, it's your call. The 128s are aluminum, and already have larger valves without the shrouding problem, but still have 58cc chambers. The aluminum will theoretically allow a little more compression due to the better cooling. They are going to be a little better choice is you are concerned about detonation. If you already have the heads, it's a no-brainer. If you are shopping for heads, you'll spend a whole lot less on the '416s, but make up some of that in valves and machining.

Like I said, it's your call.

 

Let me know how it works for you, I'm not going to do this until later this summer when I get everything running right on it. I'd really like to get a new set of heads that won't require any machining to get them to work. I'm on an extremely low budget untill next year or maybe even the year after. Right now I'm just seeing the cheapest way to make power, and I figure a set of heads out of the junk yard will be cheap enough so thats why I'm asking about them. I know a guy who has a dozen or more 3rd gens sitting around so I should be able to find the heads you suggested.