Correcting the compression ratio on a ZZ502

[davepl]

So years ago I put a ZZ502 in my 69 Camaro. Didn't everyone? Photo below.

As you can see, I tried to keep room on the driver's side for an F1R or F2 supercharger. Frankly, the car is fast enough for what I need without one - I have faster cars that are safer to drive anyway.

The problem is that when I built it I used Trick Flow 340 heads with a larger chamber to bring the compression down to 8:1 or so (in anticipation of the supercharger). So as you can see this is really a case of "10 years ago I picked out a terrible parts combination".

I would like to bring the motor up to 9.5:1 (or whatever you folks in know are recommending for 93 octane in a ZZ502 these days). But here are my problems:

a) The heads were expensive. I'll toss or sell at loss if I need to, just would prefer not to.
b) That expensive Hogan sheet metal intake is port-matched to the heads. Also not a showstopper, but another reason to want to keep the same heads

However, those heads will never have any velocity at 340cc I imagine. So I think these are my options below; I'd like to get your input on which is the best approach or if there's a completely different one that I've overlooked:

1) Replace the heads with a rectangular port set that have smaller chambers and more velocity
2) Plane or otherwise modify the heads to bring up the compression (a gasket change won't do it)
3) Rebuild the motor and use domed pistons to bring the compression up

I'd also like it to idle like a Cadillac. Did I mention that? In other words, I probably want a smaller cam. I have a stock ZZ502 in a different car and its still a little lumpy for my purposes. I would imagine bringing up the compression would improve the idle quality too, just by itself.

So, I want to turn my "was going to be a monster because I was young and wild" into a "drives nicely because I'm old and sedate". Not quite, but you get the point.

Thanks for any advice!

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After Wheel Repair.jpg

[1989TransAm]

Pistons. That way you keep everything else the same for alignment purposes. I would run it with the increase in compression before I changed the camshaft. By the way what camshaft is in it now?

[user-612937456]

You will have to change heads, cam, intake, and milder tune if you want it to idle and respond like a Cadillac. Do a tune, cam, and Pistons as a compromise. Just speculating on the cam, ZZ Gm cam may be fine but if I wanted your goals I would consider a torque monster grind and lower stall if you have an automatic.

Look at your $ spend out and what you can sell the 502 combo for compared to an LS 7 swap and sideways with your current fuel injection

[davepl]

I'd love to sell it and put an LS3 in it but if I can clean up this combo, I actually prefer a Mark IV block just for nostalgic reasons.

So do people think that with increased compression it can still be made driveable with the giant 340cc intake ports? ie: If I did the pistons and wound up with good compression but still giant intake runners, I'm afraid the port velocity would be really low.

I can stick Edelbrock Performer RPM 454 heads on it for about $2300, but that's not cheap. No idea what I could get out of my old heads.

[user-612937456]

Unless all you want is general suggestions you need to fill in some more blanks like weight, gear, tires, transmission, stall spd if its an auto,and anything else you think is relevant. Are you willing to compromise in how mild you want your Cadillac?
A 496 vortec injection and heads may meet your goals with a tune and a little more aggressive cam.

[gnicholson]

A smaller cam and compression increase would go a long ways in covering up for the to large intake runners

[rfoll]

$2300 for heads makes pistons look cheap.

[Rizzle]

I'd love to sell it and put an LS3 in it but if I can clean up this combo, I actually prefer a Mark IV block just for nostalgic reasons.

So do people think that with increased compression it can still be made driveable with the giant 340cc intake ports? ie: If I did the pistons and wound up with good compression but still giant intake runners, I'm afraid the port velocity would be really low.

I can stick Edelbrock Performer RPM 454 heads on it for about $2300, but that's not cheap. No idea what I could get out of my old heads.

Epoxy can fix large intake ports. You'll probably end up with a nicer set of heads getting those filled in and ported vs new smaller castings. Thats not accounting for the necessary piston change $ to bump compression up, but theres probably some frictional improvements in the the ring pack to justify those on top.

[MadBill]

Just a smaller cam (depending on what you have now) will pick up the low RPM cylinder pressure and give as smooth an idle as you like. It should still be plenty stout and any low speed torque loss from the big heads and less than maximum possible CR will only help in hooking up those great looking but less than Pro Street-sized wheels & tires.

[In-Tech]

Forgive me but I'm not understanding why this 340cc head is too big for a 502 with EFI? A smaller cam(don't know what you have) and increased compression sounds like a recipe for detonation. I'm intrigued. It's EFI, you can do what you want

[F-BIRD'88]

If you are going to re cam it for a caddy idle you don't
Need to change the cr.
If you wanted a caddy motor idle why not get a caddy
Motor? 1970 472 or 500 cid.

[F-BIRD'88]

Instead of fubaring up this cool 502, why not
Start with a motor that matches the purpose.
Get a old stock 396/325hp motor. Rebuild it stock.
These are a dime a dozen. You can probabily find
A real good one that is already rebuilt ready to run
For peanuts. Now upgrade that.
Say modern bbc vortec heads. Efi, retro fit hyd roller cam. ( small stock like).
It will idle and run like a caddy. For peanuts.
Lots of caddy like torque, better on gas.
Use the 502 in something else.Instead of fubaring it.

Note: the good old stock 66-69 396/325hp motor
Is 10.25:1 cr. The small factory dome on the stock 396
Piston may need some hand reshaping to clear the
Modern vortec bbc head. But the old stock closed chamber
396 oval port head is not bad at all. 215 063 etc

All you are going to achieve by modding that 502 is creating a worthless dog motor
That does not do anything well at all.

You can do this 396 up for peanuts compared to what you plan on the 502.
And you will have the motor combo you wanted in the first place. And a cool
Valuable not fubared 502 for another car. Another day...

[Keith Morganstein]

Not sure if I get some of the responses on this?

Anyways, always set the comp on a BBC with the piston dome (or dish)...don't mill the heads.

Contact Mike Jones for cam that will have good manners, but still make excellent power.

[user-612937456]

Or why not just get a 496 Vortec use your current injection with an aftermarket bolt on intake I bet you still do it all for less 2000 use a mild comp cams XFI TORQUE cam. There's a Cadillac I'd be proud to drive. Not being sarcastic either about the Cadillac. Im 51 yo and thats my style now too. I like mild yet wild or a sleeper that sounds like MaMa's Caprice in 1977

[MDT]

Looks to me like the quick easy and cheapest would be pistons and a cam. ... I think upping the compression 2 points may Smooth out the idle a touch as well.