LT-1 compression ratios
Thread Starter
Le Mans Master
Joined: Jan 2000
Posts: 5,072
Likes: 1,556
From: Poway CA
2025 C1 of the Year - Unmodified Finalist
2024 C1 of the Year Finalist - Unmodified
2023 C1 of the Year Finalist - Unmodified
2022 C1 of the Year Finalist - Unmodified
2019 C1 of Year Finalist (stock)
2019 C1 of Year Finalist (stock)
2016 C1 of Year Finalist
LT-1 compression ratios
Greetings C3 brothers!
I have read a LOT on the LT-1, including this thread: http://forums.corvetteforum.com/c3-t...out-lt1-6.html
But I still have this question: Was the reduction in the CR from 1970 (11:1) to 1971/2 (9:1) accomplished with a change to the pistons, or a change to the heads? Or both?
The reason I ask: The motor in my 1961 is based on a LT-1 short block. In 1972, after 100K miles on the original 283, my father (the original owner) purchased a 1972 shortblock from the local Chevy dealer (for $375!), and rebuilt the motor using parts from the original 283 (heads, intake, carb, pan, etc.). I remember helping him do the work in the garage (I was 7).
I believe the original 230hp/283 heads are 64cc.
So what is my compression ratio?
P.S. The motor runs very strong and I'm having to pull timing a bit to run on California pump premium (91 octane).
Fred
I have read a LOT on the LT-1, including this thread: http://forums.corvetteforum.com/c3-t...out-lt1-6.html
But I still have this question: Was the reduction in the CR from 1970 (11:1) to 1971/2 (9:1) accomplished with a change to the pistons, or a change to the heads? Or both?
The reason I ask: The motor in my 1961 is based on a LT-1 short block. In 1972, after 100K miles on the original 283, my father (the original owner) purchased a 1972 shortblock from the local Chevy dealer (for $375!), and rebuilt the motor using parts from the original 283 (heads, intake, carb, pan, etc.). I remember helping him do the work in the garage (I was 7).
I believe the original 230hp/283 heads are 64cc.
So what is my compression ratio?
P.S. The motor runs very strong and I'm having to pull timing a bit to run on California pump premium (91 octane).
Fred
Last edited by SDVette; Jun 5, 2014 at 06:32 PM.
Le Mans Master
Joined: Mar 2008
Posts: 5,948
Likes: 72
From: Oxford MA-----You just lost the game!!!!
According to the info on this page:
http://www.jimsperformance.com/headchart.html
the '70 LT-1 used a 64cc head, and the '71-'72 LT-1 used a 75cc head. I am not sure about the pistons, but I don't think 11ccs is enough to drop compression 2 full points, so it is very possible that the '71-'72 LT-1 used a different piston, maybe a flat-top? Hope this helps,
Scott
http://www.jimsperformance.com/headchart.html
the '70 LT-1 used a 64cc head, and the '71-'72 LT-1 used a 75cc head. I am not sure about the pistons, but I don't think 11ccs is enough to drop compression 2 full points, so it is very possible that the '71-'72 LT-1 used a different piston, maybe a flat-top? Hope this helps,
Scott
Melting Slicks
Joined: Oct 2001
Posts: 2,064
Likes: 1,382
From: Halifax, Nova Scotia, Canada
According to the info on this page:
http://www.jimsperformance.com/headchart.html
the '70 LT-1 used a 64cc head, and the '71-'72 LT-1 used a 75cc head. I am not sure about the pistons, but I don't think 11ccs is enough to drop compression 2 full points, so it is very possible that the '71-'72 LT-1 used a different piston, maybe a flat-top? Hope this helps,
Scott
http://www.jimsperformance.com/headchart.html
the '70 LT-1 used a 64cc head, and the '71-'72 LT-1 used a 75cc head. I am not sure about the pistons, but I don't think 11ccs is enough to drop compression 2 full points, so it is very possible that the '71-'72 LT-1 used a different piston, maybe a flat-top? Hope this helps,
Scott
1970 LT-1 pistons were "dome", forged, TRW
1971-72 pistons were flat top, I assume forged, TRW.
Thread Starter
Le Mans Master
Joined: Jan 2000
Posts: 5,072
Likes: 1,556
From: Poway CA
2025 C1 of the Year - Unmodified Finalist
2024 C1 of the Year Finalist - Unmodified
2023 C1 of the Year Finalist - Unmodified
2022 C1 of the Year Finalist - Unmodified
2019 C1 of Year Finalist (stock)
2019 C1 of Year Finalist (stock)
2016 C1 of Year Finalist
Great info - thanks guys.
Does anyone happen to know the size (in cc) of the dome on the 1970 pistons?
Even though it was purchased in 1972, the P/N (3966921) indicates a "Engine, partial, 370hp, 11.1:1cr", according to this:
http://www.gmpartswiki.com/getpage?pageid=88803
So I guess my cr is 11.1:1....
Does anyone happen to know the size (in cc) of the dome on the 1970 pistons?
Even though it was purchased in 1972, the P/N (3966921) indicates a "Engine, partial, 370hp, 11.1:1cr", according to this:
http://www.gmpartswiki.com/getpage?pageid=88803
So I guess my cr is 11.1:1....
Last edited by SDVette; Jun 6, 2014 at 01:21 AM.
Racer
Joined: Feb 2010
Posts: 425
Likes: 2
I believe the factory compression ratios are a bit optimistic, as 11:1 would be the blueprinted compression ratio (zero deck clearance on block, heads right at 64cc)...the reality is that it's probably more like 10.5:1, which is still too much for pump premium today. Are you running lead additive in the gas ? You'll destroy those old 283 heads without it (non-hardened valve seats).
A 71-72 is more like 8.5:1, as an un-decked block with a 76cc head and a flattop piston will not get 9:1 . And 76cc heads run more like 78cc unless they're shaved and the typical TRW 4 valve relief pistons add at least 4 more cc to the combustion chamber size when figuring compression. So 78cc, .025 down the hole, .040 gasket thickness, and at least 4cc of valve relief in the pistons = less than 9:1 .
The way to make your old girl happy would be a rebuild or a different 350 with a 6" long connecting rod, a modern cam, vortec heads, zero deck, and a piston with just 2 valve reliefs. Add a vortec specific intake, a bigger Edelbrock carb (if you still have a Carter), Brezininski ported cast iron exhaust manifolds, and a 2.5" X pipe dual exhaust.
A 71-72 is more like 8.5:1, as an un-decked block with a 76cc head and a flattop piston will not get 9:1 . And 76cc heads run more like 78cc unless they're shaved and the typical TRW 4 valve relief pistons add at least 4 more cc to the combustion chamber size when figuring compression. So 78cc, .025 down the hole, .040 gasket thickness, and at least 4cc of valve relief in the pistons = less than 9:1 .
The way to make your old girl happy would be a rebuild or a different 350 with a 6" long connecting rod, a modern cam, vortec heads, zero deck, and a piston with just 2 valve reliefs. Add a vortec specific intake, a bigger Edelbrock carb (if you still have a Carter), Brezininski ported cast iron exhaust manifolds, and a 2.5" X pipe dual exhaust.
Last edited by c6silver; Jun 6, 2014 at 06:27 AM.
Melting Slicks
Joined: Jan 2004
Posts: 3,247
Likes: 210
From: Detroit MI
Great info - thanks guys.
Does anyone happen to know the size (in cc) of the dome on the 1970 pistons?
Even though it was purchased in 1972, the P/N (3966921) indicates a "Engine, partial, 370hp, 11.1:1cr", according to this:
http://www.gmpartswiki.com/getpage?pageid=88803
So I guess my cr is 11.1:1....
Does anyone happen to know the size (in cc) of the dome on the 1970 pistons?
Even though it was purchased in 1972, the P/N (3966921) indicates a "Engine, partial, 370hp, 11.1:1cr", according to this:
http://www.gmpartswiki.com/getpage?pageid=88803
So I guess my cr is 11.1:1....
Corvette Stories
The Best of Corvette for Corvette Enthusiasts
150 hp to 1,250 hp: Every Corvette Generation Compared by the Specs That Matter
Joe Kucinski
8 Coolest Corvette Pace Cars (and Replicas) of All Time
Verdad Gallardo
Top 10 Corvette Engines RANKED by Peak Torque (70+ Years of Muscle!)
Joe Kucinski
Corvette ZR1X Will Be Pacing the Indy 500, And Could Probably Race, Too!
Verdad Gallardo
Top 10 Corvettes Coming to Mecum Indy 2026!
Brett Foote
Top 10 C9 Corvette MUST-HAVES to Fix These C8 Generation Flaws!
Michael S. Palmer
10 Revolutionary 'Corvette Firsts' Most People Don't Know
Joe Kucinski
5 Reasons to Upgrade to an LS6-Powered Corvette; 5 Reasons to Stay LT2
Michael S. Palmer
2027 Corvette vs The World: Every C8 vs Its Closest Competitor
Joe Kucinski
Thread Starter
Le Mans Master
Joined: Jan 2000
Posts: 5,072
Likes: 1,556
From: Poway CA
2025 C1 of the Year - Unmodified Finalist
2024 C1 of the Year Finalist - Unmodified
2023 C1 of the Year Finalist - Unmodified
2022 C1 of the Year Finalist - Unmodified
2019 C1 of Year Finalist (stock)
2019 C1 of Year Finalist (stock)
2016 C1 of Year Finalist
Yes, it's running the original Carter WCFB. No, I'm not running any fuel additives.. I do understand the concern.. but after doing a deep dive into facts and opinions on the valveseat/lead issue, I decided to just run pump gas. I'm not saying I know for sure there will be no issues - rather I'm taking a calculated risk, knowing that a new motor is in my future, sooner or later, anyway.
Perhaps a crate 350?... perhaps a period "correct" 283?... but I've been so busy working on EVERYTHING ELSE on the car that I'm just glad that it seems to be running fine for now. I've put a few hundred miles on it since it came out of a 30-year hibernation last year, and as I said, other than pulling the timing a few degrees, it runs very stong (no trouble lighting up those skinny tires at all!)
Perhaps a crate 350?... perhaps a period "correct" 283?... but I've been so busy working on EVERYTHING ELSE on the car that I'm just glad that it seems to be running fine for now. I've put a few hundred miles on it since it came out of a 30-year hibernation last year, and as I said, other than pulling the timing a few degrees, it runs very stong (no trouble lighting up those skinny tires at all!)
Racer
Joined: Feb 2010
Posts: 425
Likes: 2
I would put in a 400 or 406 or a 420 small block, but that's just me...Glad it runs good for you. Usually oxygenated pump gas with ethanol is hard on old cast iron heads without hardened stellite seats, but maybe your old 283 heads have "work hardened" over the years.
Race Director
Joined: Feb 2001
Posts: 11,195
Likes: 2,038
From: Grapevine Tx
2018 C3 of Year Finalist
2016 C3 of the Year Finalist
Yes, it's running the original Carter WCFB. No, I'm not running any fuel additives.. I do understand the concern.. but after doing a deep dive into facts and opinions on the valveseat/lead issue, I decided to just run pump gas. I'm not saying I know for sure there will be no issues - rather I'm taking a calculated risk, knowing that a new motor is in my future, sooner or later, anyway.
Perhaps a crate 350?... perhaps a period "correct" 283?... but I've been so busy working on EVERYTHING ELSE on the car that I'm just glad that it seems to be running fine for now. I've put a few hundred miles on it since it came out of a 30-year hibernation last year, and as I said, other than pulling the timing a few degrees, it runs very stong (no trouble lighting up those skinny tires at all!)
Perhaps a crate 350?... perhaps a period "correct" 283?... but I've been so busy working on EVERYTHING ELSE on the car that I'm just glad that it seems to be running fine for now. I've put a few hundred miles on it since it came out of a 30-year hibernation last year, and as I said, other than pulling the timing a few degrees, it runs very stong (no trouble lighting up those skinny tires at all!)
If you ever have your heads off, then you can check and replace it you want.