heavy D

I just built my first Oldsmobile engine. I am new to olds specs. I have always built sbc engines. I was wondering if someone could tell me my compression ratio of my engine. I know it is very low,that is why I kept my cam on the small side. My engine is a 69 350 , stock bore and pistons and I am running stock #8 heads. Lol yeah the number 5 heads didn't come with the engine. I would be thankful for any help.

DENT

Welcome to the forum.

MDchanic

So, if you've built SBC engines, then you know how to calculate the CR, right?

And if you've just built this engine (past tense, I notice), then you MUST have calculated the compression ratio before selecting the components, right?

Are the "stock" 350 pistons new 350 pistons, or did they come from the GM plant inside the block?
If they are 1969 pistons, then they should be either 6cc or 14cc.
If they are "stock" aftermarket pistons, they could be anywhere from 0cc to 24cc, and should be labeled as such.
Didn't you measure them when you were building it?

We may be able to help if you provide more information.

Welcome to ClassicOlds.

- Eric

heavy D

Thanks for the reply Eric. I didn't CC the heads and pistons. Just wanted a rough guess on compression ratio. It is the factory 1969 GM pistons. With the open chamber #8 heads. The bottom end just needed freshened up. I wasn't building the engine to expect big power. The 69 350 was rated at 10.2 comperssion. With the #5 heads. I was thinking about 8-8.5 with the smog heads

coppercutlass

I doubt your engine was the 10.25 to 1 engine. Probably more likely it was the 9 to 1 with 14 cc pistons. #8 heads 79cc's , bore 4.057 , stroke 3.385 , head gasket probably .040 , deck height probably .025 appx.
here ya go.


http://www.rbracing-rsr.com/compstaticcalc.html

heavy D

Thanks again for the help

MDchanic

The 1969 350s were either 4bbl 10.25:1, or 2bbl 9:1.

#5 heads are nominally 64cc. #8 heads are nominally 80cc

You can use these numbers, along with your deck height and gasket volume to calculate your CR.

- Eric

oops - Copper, you beat me!

captjim

Chances are the pistons are 14cc, though they could be 6cc. Like Eric alluded to, if you have engine building experience, why would you not measure everything while it was apart? In any event, #8 heads with a cheap gasket and 14 cc pistons puts you in the high 7.xx range.

captjim

Eric, #8 heads are 79cc, not 69cc. I know you know that and I'm sure it was just a typo.

heavy D

The 2 barrel engines must have used lower comperssion pistons correct. The engine is out of a 69 cutlass with the 2 barrel carb. Thanks for the info! The #8 heads looked to me more open chambered than 69cc. Lol. I guess I am used to looking at sbc heads

coppercutlass

Eric on the #8's did you mean 79 ? Not trying to be a DI** but I think everything I have looked at say's 79 +/- and 10 cc's seems like alot for a variable. J-Chicago cc'd some heads and show me how to do it and we cc'd a set of #8's and they where 79 cc's.


Edit : damnit eric you edited before I could ask lol.

captjim

Yes, they will have the 14cc dished pistons.

MDchanic

Yes, I meant 79cc (should have just typed 80). They always put that darned "6" key right next to the "7".

coppercutlass

Yeah don't worry I got a serious case of fat finger syndrome specialy on my phone.