What's a good compression ratio?
Thread Starter
Drifting
Joined: Jul 1999
Posts: 1,337
Likes: 0
From: Brighton, MI
What's a good compression ratio?
With my car, I got a 350 shortblock, a 383 shortblock and a set of ported heads and intake. I had the heads checked, and they measured 56cc. The machine shop told me the 383 block has been decked, but this has not been measured. He assumes it was set to 0 deck with the pistons. He seems to think this will put me into the high 11.x:1 range with the BRC flattop pistons I currently have for that engine. He says this would not be a pump fuel setup. If I put the heads on the stock 350 shortblock, he says I will be closer to 10:1 which is where I want to be. Just looking for some forum feedback. I have no plans for NOS or supercharging. I just want a good street setup (pump gas). I'm leaning toward a comp 305 cam or LT4 hotcam, or something close to those.
Drifting
Joined: Mar 1999
Posts: 1,931
Likes: 1
From: Norcross GA
Re: What's a good compression ratio? (Jeff Hickman)
11:1 is close to the edge of pump gas. If you want a streetable machine, you will be happier w/ the 10:1 setup.
The 11:1 combo can be done for the street, but you have to know your stuff. Cam selection and setup is big.
There is no big output gain going from 10-11. Make it easy on yourself and do the 10. (9.5 is even better)
The 11:1 combo can be done for the street, but you have to know your stuff. Cam selection and setup is big.
There is no big output gain going from 10-11. Make it easy on yourself and do the 10. (9.5 is even better)
Team Owner
Joined: Sep 2001
Posts: 62,161
Likes: 1,733
From: Athens AL
C7 of the Year - Unmodified Finalist 2021
C4 of Year Finalist (performance mods) 2019
Re: What's a good compression ratio? (Jeff Hickman)
My goal for my 383 is approx 10-10.5:1.
Any further than 11.5 or so, and you wont be able to run pump gas without an octane booster.
Any further than 11.5 or so, and you wont be able to run pump gas without an octane booster.
Le Mans Master
Joined: Oct 1999
Posts: 9,713
Likes: 3
From: Sachse Texas
Cruise-In I Veteran
Cruise-In II Veteran
Re: What's a good compression ratio? (Jeff Hickman)
With an LT1 you can run it up to about 11.5:1 on premium pump gas without any issues. 11:1 is ideal for an LT1 setup IMO, and around 10.2-10.3 is stock.
I would say get the exact CR of what it is before deciding. High 11.x is not really what you want though.
I think a lot of engine builders use the same gauge for LT1 compression ratio limitations as they do with standard 350's which is entirely incorrect. Heck, a stock LT4 is 10.8:1 compression.
I would say get the exact CR of what it is before deciding. High 11.x is not really what you want though.
I think a lot of engine builders use the same gauge for LT1 compression ratio limitations as they do with standard 350's which is entirely incorrect. Heck, a stock LT4 is 10.8:1 compression.
Thread Starter
Drifting
Joined: Jul 1999
Posts: 1,337
Likes: 0
From: Brighton, MI
Re: What's a good compression ratio? (Vette92)
Thanks for the info. I think the machinist was ballparking for the most part. He estimated 11.7:1 or so, saying that the book showed my pistons at 0 deck to be 11:1 with 64cc head chamber. My heads are 56cc. I believe the stock ratio on the LT1 was 10.4:1. If I stick these heads on the stock shortblock, I should end up with a higher ratio because these heads have been milled. I'm not sure what cc the heads were stock. All mods were done by the previous owner.
Le Mans Master
Joined: Oct 1999
Posts: 9,713
Likes: 3
From: Sachse Texas
Cruise-In I Veteran
Cruise-In II Veteran
Re: What's a good compression ratio? (Jeff Hickman)
Stock LT1 heads will range in between 56 and 58cc combustion chambers depending on manufacturing discrepencies. Milling will not drop the chamber cc down much (maybe 1cc). Not sure how much decking the block will effect it overall as it depends how much they remove.
Just for reference, my block has been 0 decked, I am running around 54cc heads because they have been milled 3-4 times, and am using dished pistons to get to 11:1 compression with standard LT1 head gaskets. If I swapped to the Impala head gaskets it would bump me up to about 11.5:1 from what I figure from the calculators.
Just for reference, my block has been 0 decked, I am running around 54cc heads because they have been milled 3-4 times, and am using dished pistons to get to 11:1 compression with standard LT1 head gaskets. If I swapped to the Impala head gaskets it would bump me up to about 11.5:1 from what I figure from the calculators.
