Low Cranking Compression L98?????
Thread Starter
Drifting
Joined: Jun 2001
Posts: 1,704
Likes: 459
From: Mass Mass
Low Cranking Compression L98?????
I was changing the spark plugs and thought I would check the cranking pressure while the plugs are out.
Surprised at how low it came out?
Is this normal, or is my engine on the way out?
1-125psi 2-120psi
3-120psi 3-115psi
5-115psi 6-125psi
7-120psi 8-120psi
Engine had been off for a few hours (water temp still 100F)
All plugs out, throttle wedged open, reading after about four strokes.
1988 L98 - Stock cam, block and heads.
50,000 miles mostly supercharged (6 to 9psi)
1.6 roller rockers exhaust and intake
Just pasts emissions test, and no visible smoke, don't burn much oil between 3000 mile changes.
Other than normal deposits, old plugs looked clean (no oil)
Compression jumps almost to the full value on the first stroke.
Service manual says lowest 75% of highest and none below 100psi.
That's not much help.
I've never tested a late model smog motor, years ago I remember my 1972 455 Buick was in the 220psi range.
Surprised at how low it came out?
Is this normal, or is my engine on the way out?
1-125psi 2-120psi
3-120psi 3-115psi
5-115psi 6-125psi
7-120psi 8-120psi
Engine had been off for a few hours (water temp still 100F)
All plugs out, throttle wedged open, reading after about four strokes.
1988 L98 - Stock cam, block and heads.
50,000 miles mostly supercharged (6 to 9psi)
1.6 roller rockers exhaust and intake
Just pasts emissions test, and no visible smoke, don't burn much oil between 3000 mile changes.
Other than normal deposits, old plugs looked clean (no oil)
Compression jumps almost to the full value on the first stroke.
Service manual says lowest 75% of highest and none below 100psi.
That's not much help.
I've never tested a late model smog motor, years ago I remember my 1972 455 Buick was in the 220psi range.
Team Owner
Joined: Dec 2000
Posts: 90,675
Likes: 304
From: SE NY
Cruise-In II Veteran
Re: Low Cranking Compression L98????? (SuperL98)
Those numbers look like my 1st stroke values (105-125psi); my final values were 162-175psi.
You might want to repeat after squirting a copla drops of oil into a cylinder or two to see if it makes a drastic diff.
Did you see any excessive leak down?
You might want to repeat after squirting a copla drops of oil into a cylinder or two to see if it makes a drastic diff.
Did you see any excessive leak down?
Le Mans Master
Joined: Feb 1999
Posts: 6,172
Likes: 11
From: somers, ny
Re: Low Cranking Compression L98????? (65Z01)
was that engine purpose build for the supercharger? If so, it's probably low comprssion in the 8-8.5:1 range. That might explain the low psi. when stock, my 9:1 engine used to have about 150psi. Also try another guage is possible, i have found some big variations.
Le Mans Master
Joined: Mar 1999
Posts: 9,662
Likes: 1,680
From: Fort Worth Texas
2024 Corvette of the Year Finalist - Unmodified
2023 C8 of the Year Winner - Unmodified
2020 Corvette of the Year Finalist (stock)
2019 C7 of Year Winner (stock)
Former Moderator
Cruise-In I-II-III Veteran
Re: Low Cranking Compression L98????? (ralph)
was that engine purpose build for the supercharger? If so, it's probably low comprssion in the 8-8.5:1 range. That might explain the low psi. when stock, my 9:1 engine used to have about 150psi. Also try another guage is possible, i have found some big variations.
That's what I'm thinking..... probably lower compression motor for the supercharger. My '85 was around 170psi on all cylinders on my new motor and my '90 has about 210-220 after doing the heads in May.
Thread Starter
Drifting
Joined: Jun 2001
Posts: 1,704
Likes: 459
From: Mass Mass
Re: Low Cranking Compression L98????? (SuperL98)
Totally stock block,heads, and cam, never been opened.
Should be the stock compression.
I loaned out my leakdown tester, long ago, and never got it back, so I haven't done a test. But I let the compression tester sit for a while, after the five cranks, and it stayed pretty much at full pressure.
Tried again today, engine full cold, but this time I sat in the car with my foot full throttle (I had just proped it about half open last time) and the numbers picked up about 15 psi. Still low!
Then I tried a little oil in number 3 & 6 (easy to reach) and they jumped up to 170psi :(
You would think this points to bad rings, but
the car runs high 12's at 112mph (with an auto and 2.59 axle!!)
how sick can it be?
Put the plugs back in for now
Have to think about what to do next....
Should be the stock compression.
I loaned out my leakdown tester, long ago, and never got it back, so I haven't done a test. But I let the compression tester sit for a while, after the five cranks, and it stayed pretty much at full pressure.
Tried again today, engine full cold, but this time I sat in the car with my foot full throttle (I had just proped it about half open last time) and the numbers picked up about 15 psi. Still low!
Then I tried a little oil in number 3 & 6 (easy to reach) and they jumped up to 170psi :(
You would think this points to bad rings, but
the car runs high 12's at 112mph (with an auto and 2.59 axle!!)
how sick can it be?
Put the plugs back in for now
Have to think about what to do next....
Thread Starter
Drifting
Joined: Jun 2001
Posts: 1,704
Likes: 459
From: Mass Mass
Re: Low Cranking Compression L98????? (Sharky Guam)
just ordered a leak down tester from Summit ($65)
see how that works out......
maybe it's time for that 383 i always wanted :confused:
see how that works out......
maybe it's time for that 383 i always wanted :confused:
