djdaveofkc

Hey guys, I'm Dave from Kansas. I have a olds 455. I had the motor rebuilt when i was still in college early 90's and have a compression question. I don't really need to have the motor rebuilt at this time (next winter maybe) and am thinking my mechanic put in standard dish pistons 20 years ago which are more than likely 8:5:1 compression. It's been to long ago to know exactly. I only read 150 psi on each cylinder. If i assume i have the standard .40 head gasket, and i replace my 80cc heads with 77cc and go with a .27 gasket will i increase my compression maybe half a point?

olds 307 and 403

Use this.
https://www.uempistons.com/index.php...6df33ff2f21568

djdaveofkc

I've tried those before but their is too many questions i'm not sure of. I don't know deck clearance or the piston head volume on my pistons etc. Any info i had Burnt up in a shop fire years ago. I was kinda hoping the .40 to .27 and the 80cc head to 77cc would maybe give a ballpark estimate

wr1970

Where are you at in Kansas.

Fun71

I ran some numbers though a CR calculator making a bunch of assumptions to get 8.5:1 CR starting point. If you really have 80cc heads, then changing to 77 cc heads and .027" thick gaskets would give 0.39 increase in compression ratio (8.89:1)

djdaveofkc

That's kinda what i was thinking, at least i'd be close to 9:1. I'm from Gardner ks. how far do you think i could have the heads milled down without affecting the intake to get closer to 9:5:1. It's a eldebrock performer intake with soon to be put on eldebrock heads.

Fun71

Well, note that above I kinda questioned if your heads were really 80cc. The BBO heads I have are larger than that at around 84cc.

Without knowing the combustion chamber volume, piston-to-deck clearance, and piston dish volume it's all just guessing.

djdaveofkc

The current heads I have on there are 69 c heads which ive read are 80cc correcct?

Fun71

Yes, I am sure you have read that they are 80cc, but in reality they could be as much as 85cc.

It's similar to the SBO 5, 6, 7, and 7a heads that are claimed to be 64cc but are actually anywhere from 64 to 70cc, with most being on the larger end of the range. That's why you have to actually measure to verify what you have.

wr1970

^^^^^^^this^^^^^^

djdaveofkc

good enough, thanks!

brownbomber77

Things to ponder on Olds engines...deck clearance. My 403 was .030 in the hole. Using early heads with a .011 gasket even still only yielded 8.8:1 compression.

olds 307 and 403

Most later blocks supposedly have pistons around .020" in the hole. Most early blocks have around .030" clearance but your 403 is definitely possible. What did the heads CC at? Bad news for the poster, even with a clean up mill on the deck and heads, cast replacement pistons sit way in hole, probably .040" or more and his heads will be 80+CC almost guaranteed. Of course that means more of a gain with 77 cc heads but means he is probably starting at 8 to 1 vs 8.5 to 1. I figured 8.9 to 1 at 64 measured CC, one head is 62cc, so 9 to 1, .028" head gaskets and .030" in the hole on my 350, I need to measure that.

djdaveofkc

Ouch, hope im not starting that low. So the 150 psi doesnt really calculate into anything useful when figuring a guestimation of compression? Im kinda planning on tearing the block down late this year and doing a low end rebuild. I might wait on the head swap if my compressions really that low and do it all at once.

Fun71

Not really. The intake valve closing will affect the amount of air that is "trapped" in the cylinder and then compressed. This means in an engine with the same static compression ratio, a cam with an early intake closing event will trap more air and give a higher pressure reading than a cam with a late intake closing event that doesn't trap as much air.