Mikey_mick

hey, guys. I bought a 76 cutlass this summer that has a rebuilt 350 Olds in it. The guy said he only drove it 25 miles then parked it because the entire brake system needed repaired and he didn’t have the money and then he got married.
I also have to assume that he was disappointed in the performance of the car because it appears to be a low compression build. It has the #8 heads and a .030 over cast pistons with the 10ish cc dish. It sounds like it has a performance cam in it. When I drive it it seems very slow. I would be surprised if it has more than 250hp.

I have a set of #5 heads that I recently had rebuilt with larger valves and .020 shaved off the bottom. With the approx 10cc dished piston and felpro gasket this still would only be around 9.0-1 compression, right?

I was hoping for around 9.5-1 compression. I know 10-1 would be best for performance but we only have 87 and 91 octane around here and I’m worried about detonation.

So in order to achieve my desired compression could I just swap out the regular cast pistons for a set of sealed power hypereutectic small dish or flat top pistons? I’m mainly wondering if The bore size that the regular pistons are using will be suitable for the hypereutectics.

thanks for any advise you can give me.

olds 307 and 403

You should be around 9 to 1 with those #5 heads. You will need see how far down they sit in the bore at tdc.

olds 307 and 403

Plus if they are cast replacements you need to figure put if they are the 6cc or 14cc cast replacement pistons. Most available now have the 24cc dish, dreadful. The 76 motor had factory 14cc dish pistons.

cutlassefi

I'd like to know who makes a hypereutectic piston for a 350 Olds?

olds 307 and 403

Exactly as Mark said, no hyper pistons for the olds 350 and no 10cc replacement that I know of. The .030" Speddpro flat top or 5.8cc dish forged can be pressed on in place of the cast pistons and will sit about .025" down in the hole like factory. You may need to hone the block for more clearance, you need .004 minimum clearance. Figure out what cam is in it and use a compression calculator like the KB piston or Summit racing one.

Mikey_mick

i thought speed pro made hypereutectic pistons?

cutlassefi

They do, lots of them. But not for a 350 Olds.

Mikey_mick

I stuck a bore scope camera into the spark plug hole and the pistons appear to be federal mogul units. The part number stamped on the top suggests that they have The .215" deep x 2.920" diameter dish. Is that the big dish?

cutlassefi

Yep.

Fun71

24cc dish pistons with around .020" short compression height, .025" factory deck, and 80cc #8 heads calculates to right at 6.9:1 compression ratio. That "performance" camshaft will reduce cylinder pressure even more, which explains why your engine has no power.

olds 307 and 403

Either wait a month for the Mahle piston Cutlassefi brought to the market, best idea or the old school Speedpro LF2321 5.8cc dish with your #5 heads. The Speedpro pistons will give up 10+ hp over the very modern ring pack and much lighter Mahl pistons. You will need a slight overbore for the 4.100" bore version and either should be rebalanced. No good way to get good compression with those 24cc dish pistons.

oldsmobiledave

Talk to Mark & wait for the Mahle pistons. Also get some cam advice from him while you are at it.


Good luck with your build.

Mikey_mick

Is there any idea of how much those Mahles will cost?

Mikey_mick

Thanks. I bought the car hoping that they would have done something right with the engine but apparently they didn’t have any idea what they were doing. The guy said that the shop dyno tested it but he couldn’t find the paperwork.🙄 I would like to see the papers to see just to see how bad it was.

cutlassefi

Pm me.

Fun71

It may be better that you didn't see the dyno numbers.

AZ520

You should do a compression test on it, if its over 135 psi in each cylinder it should run pretty good if its jetted and tuned properly.