Fletchmo72

So, I have a '72 CS with the dreaded low compression 350. The engine, 4-bl carb and trans were rebuilt around 100k miles by the PO, but it's not clear what upgrades they did, other than an aluminum intake and mild cam. It runs and shifts great, but it's annoying that my minivan could blow the doors off my Cutlass. I have upgraded to an HEI distributor with adj. vac advance and optimized the timing. Also installed a 3.23 rear end w/posi. I'm considering rebuilding the motor this winter to install higher compression pistons and have the heads redone. My question is how much will this cost? Do I have to replace the rods? What else can stay/must go? How much horsepower will the TH350 trans be able to handle? Thanks for any help/advice. -Jack

Fletchmo72

Found this on Hotrodders.com. Can I just change my pistons and do nothing else? Or would I then have to rebalance the crank? Other things?

The first problem is, where are you starting from? Prior to 1973, all small block Olds motors used the same size chambers in the heads (about 64 CC). Compression ratio was varied by changing the volume of the dish in the piston. The W-31 motors used flattop pistons and got a 10.5:1 CR with the stock steel shim head gaskets. The lesser "high compression" engines used a small dish and got about 10.25:1 CR. The low compression engines used a large dish and got 9:1, which dropped to about 8.5:1 for 1971. If you have the 64 cc heads (casting numbers 1 through 7 and including 7a - though not the 7A heads used on the 307 motors) then you need new pistons with the correct dish volume. If you have later heads with the 80cc chambers, you can swap the early heads and raise compression without changing the pistons.
One other consideration is that the stock head gaskets had a compressed thickness of around 0.014". The more common FelPro blue head gaskets seal better but have a compressed thickness of around 0.040". This results in about a 1/4 to 1/2 point drop in compression.

coppercutlass

here is a link to my build. I built my th350 and its living behind a low 12 sec 355 olds. gonna try to get an 11 sec e/t this fall. https://classicoldsmobile.com/forums...-i-do-now.html

70sgeek

Think there are too many unknowns about your motor to predict upgrade cost and power gain outcomes.


others here can no doubt cite chapter and verse on the subject to a much higher degree than myself but a methodic approach will definitely yield best results available for whatever your budget is.


I estimate having approx. $5k in my original '72 350, including parts and machine labor. I gutted the block and built it with all new parts but for crank, rods and caps. Good name-brand performance components and careful matching of parts and install specs to ensure (as much as possible) everything would play well together and perform well above original levels.


It now runs flat-top pistons, roughly 9.5:1 comp and a Howards performance cam, E-brock intake, HEI and 750 Qjet, amongst other upgrades.


Basically I didn't want to repeat things or risk a failure by something I didn't replace ruining much or all of what I did install.


Access permitting, you could probably also build a mild 455 for that kind of money (or less depending on what you start with) and see instant performance gains just because of it being a big block.


I didn't have one and was happy to be able to simply drop my original 350 back in the car when done without other mods being required for a big-block retrofit. Very happy with the performance improvements.

Fletchmo72

Thanks. Wondering if I can simply install flat top pistons and put it back together. That alone, I think, would give me the boost I'm looking for.

coppercutlass

installing pistons alone would boost compression but you need to know the specs of the cam otherwise you could end up with too much cylinder pressure .

Fletchmo72

Thanks. The PO installed the cam so I have no idea. But it would be easy to swap in whatever cam would be best with those pistons. Would you bother putting larger valves in the heads or just go with what's there?

I should add that I never plan to race this car. I just want it to be faster than my minivan (for crissakes!)

cutlassefi

9.5:1 or so, larger valves, a well tuned 750 carb and a mild cam is the combination you should shoot for.

Fletchmo72

Thanks. Any recommendations on pistons and where to get them?

coppercutlass

Its not just about racing a well thought out combo will both be reliable and have power. My set up is fully streetable , and regularly drive it to cruise nights and the 120 mile round trip to the track often. just food for thought. But cutlassefi said it simply and i agree with what he said. Ultimately you gotta have usage goals for the kind of build and power you want.

Part of that means that i drive arounf with 3.90 gears and a 3000 stall converter. I could put 3.42 gears and less converter if i wanted but since i race it those 2 items help with my time slips. thats the trade off. I dont mind crusing down the highway at 3500 + rpms.

cutlassefi

Speed pro L2321F if you want price. CP if you want a quality modern piston and ring pack.

Inline

Fully streetable depends person to person. Fully streetable, for me, means i can run whole tank empty @ 75mph,
day to day without getting bored to it.