Compression Ratio Question.......
Drifting
Joined: Feb 2003
Posts: 1,978
Likes: 2
From: San Antonio Texas
Safety Car
Joined: May 2006
Posts: 4,290
Likes: 140
From: Salem NH
If you'd read and think about WTF we all just posted in the last 2 pages...
I'll spell it out one more time for you.
They upped the CR by .75:1 for 1990. If they had done that on an 89, they wouldve gotten more than 250hp, but they decided they wanted it to stay where it was, and so made the cam smaller. This is likely because the LT1 was in testing, and they didnt know how much it would make on a production car, and wanted to make sure the LT1 made a fair bit more hp than the L98.
They then made small muffler changes to affect the other 5hp.
I'll spell it out one more time for you.
They upped the CR by .75:1 for 1990. If they had done that on an 89, they wouldve gotten more than 250hp, but they decided they wanted it to stay where it was, and so made the cam smaller. This is likely because the LT1 was in testing, and they didnt know how much it would make on a production car, and wanted to make sure the LT1 made a fair bit more hp than the L98.
They then made small muffler changes to affect the other 5hp.
Then they switched to an entirely different engine management system,
speed density utilizing a weatherpack ECM, integrated knock circuitry,
and an updated knock sensor.
Then they changed the cam, not to utilize cylinder pressure and expel exhaust charges out from the higher compression ratio - but to dumb the engine down, to make the LT1 'look better.
Sounds logical to me.
-- Joe
Melting Slicks
Joined: May 2007
Posts: 2,382
Likes: 2
From: Kathleen FL
If the 90-91 Vettes used the 89 cam, GM would be forced to rate them something like 270 hp, 370 tq. But maybe that's not as underrated as most Chevys seem to be. But it would still be considered quite underrated in my book.
If my stock automatic 89 TPI 2 years ago dynoed at 224/344, assuming a 17% drivetrain loss through the slushbox, then the real figures GM would have to rate the car would be along the lines of 260/400 to the crank as most cars are rated.
If a friend's 6 speed 90 dynoed at 227/339, then geeeez, talk about dumbing down the cam...
ps: both dynos were done on premium gas.
If my stock automatic 89 TPI 2 years ago dynoed at 224/344, assuming a 17% drivetrain loss through the slushbox, then the real figures GM would have to rate the car would be along the lines of 260/400 to the crank as most cars are rated.
If a friend's 6 speed 90 dynoed at 227/339, then geeeez, talk about dumbing down the cam...
ps: both dynos were done on premium gas.
Thread Starter
Team Owner
Joined: Oct 2004
Posts: 35,065
Likes: 0
From: Bergen County, NJ Democrats, doing for the country what they did for Michigan
So let's see. They changed the piston which gained a slightly higher compression ratio, and made the engine more efficient. (by utilizing better quench, not from the higher ratio).
Then they switched to an entirely different engine management system,
speed density utilizing a weatherpack ECM, integrated knock circuitry,
and an updated knock sensor.
Then they changed the cam, not to utilize cylinder pressure and expel exhaust charges out from the higher compression ratio - but to dumb the engine down, to make the LT1 'look better.
Sounds logical to me.
-- Joe
Then they switched to an entirely different engine management system,
speed density utilizing a weatherpack ECM, integrated knock circuitry,
and an updated knock sensor.
Then they changed the cam, not to utilize cylinder pressure and expel exhaust charges out from the higher compression ratio - but to dumb the engine down, to make the LT1 'look better.
Sounds logical to me.
-- Joe
Safety Car
Joined: May 2006
Posts: 4,290
Likes: 140
From: Salem NH
I don't know but..
With every model I do using:
CAM 88-89 L98 10066049 In/Exh Dur@.050" 207'/213' Lift (w/1.5) .415"/.430" LobSep 117'
CAM 90-91 L98 10111773 In/Exh Dur@.050" 202'/207' Lift (w/1.5) .413"/.428" LobSep 114.5'
And everything else the same, I get a little more torque with the 2nd cam, and a little more HP around 4000 RPM.
I don't know what the ICA is of either cam installed, I have to assume 'straight up' for now unless someone can chime in.
If I up the compression by .75 on the '90-91' cam combo, I get about 12hp increase, and about 20 foot lbs.
I think it's safe to say that, the results are consistent with the published data. GM says 5hp, which may be true. I could go and post the spark advance tables of the '89 tune using the $6E mask and the 90-91 tune using $8D, but I think you can see what I'm getting at. Most guys who "do their own chips" pick up at least the difference in my numbers.
The cam with the tighter lobe seperation does make more power than the previous camshaft in every model I do. Simply stating that it's lesser cam because the slightly less duration is missleading.
-- Joe
With every model I do using:
CAM 88-89 L98 10066049 In/Exh Dur@.050" 207'/213' Lift (w/1.5) .415"/.430" LobSep 117'
CAM 90-91 L98 10111773 In/Exh Dur@.050" 202'/207' Lift (w/1.5) .413"/.428" LobSep 114.5'
And everything else the same, I get a little more torque with the 2nd cam, and a little more HP around 4000 RPM.
I don't know what the ICA is of either cam installed, I have to assume 'straight up' for now unless someone can chime in.
If I up the compression by .75 on the '90-91' cam combo, I get about 12hp increase, and about 20 foot lbs.
I think it's safe to say that, the results are consistent with the published data. GM says 5hp, which may be true. I could go and post the spark advance tables of the '89 tune using the $6E mask and the 90-91 tune using $8D, but I think you can see what I'm getting at. Most guys who "do their own chips" pick up at least the difference in my numbers.
The cam with the tighter lobe seperation does make more power than the previous camshaft in every model I do. Simply stating that it's lesser cam because the slightly less duration is missleading.
-- Joe
Corvette Stories
The Best of Corvette for Corvette Enthusiasts
Top 10 Most Expensive Corvettes Ever Sold on Bring A Trailer
Brett Foote
10 Things Every Corvette Owner Needs (2026 Edition)
Michael S. Palmer
8 Most "Only Corvette Owners Understand" Quirks and Problems
Pouria Savadkouei
10 Reasons the C6 Z06 is Still A Performance Benchmark After 20 Years
Joe Kucinski
How Much Horsepower Every Corvette Engine "LOST" in 1972
Joe Kucinski
Top 10 DOs and DON'Ts for Protecting Your Convertible Top!
Michael S. Palmer
Top 10 Most Explosive Corvettes Ever Made: Power-to-Weight Ratio Ranked!
Joe Kucinski
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
Safety Car
Joined: May 2006
Posts: 4,290
Likes: 140
From: Salem NH
I gotta look at the timing events and overlap again (i'm at work not home), but I'm guessing the tighter lobe seperation is to help bleed off a little of the pressure from the higher compression ratio to keep the thing out of detonation. A safe power building combo. If I recall, '89 had the most aggressive spark advance out of all the years. (not counting ARAP). With some more advance, ramped in a little quicker, a cooler thermostat and lower fan enable temps it should be possible to tune another 20hp+ into the thing.
-- Joe
Thread Starter
Team Owner
Joined: Oct 2004
Posts: 35,065
Likes: 0
From: Bergen County, NJ Democrats, doing for the country what they did for Michigan
Le Mans Master
Joined: Feb 2005
Posts: 5,146
Likes: 3
From: The Motor City
Tighter lobe seperation be my guess. It's really not much less duration.
I gotta look at the timing events and overlap again (i'm at work not home), but I'm guessing the tighter lobe seperation is to help bleed off a little of the pressure from the higher compression ratio to keep the thing out of detonation. A safe power building combo. If I recall, '89 had the most aggressive spark advance out of all the years. (not counting ARAP). Joe
I gotta look at the timing events and overlap again (i'm at work not home), but I'm guessing the tighter lobe seperation is to help bleed off a little of the pressure from the higher compression ratio to keep the thing out of detonation. A safe power building combo. If I recall, '89 had the most aggressive spark advance out of all the years. (not counting ARAP). Joe
GM attests their power gain was in part tdue to the mufflers, as the verts of 90-91 had the 240 rating (coupes were the only ones w/5 gain...according to GM)
All the 165/prom, that I have seen has some real aggressive S.A.
Safety Car
Joined: May 2006
Posts: 4,290
Likes: 140
From: Salem NH
Me too, but that's illogical (I know I know, gm logic) again because the ESC/KNOCK circuitry and code is soo much more superior in the '730/$8D stuff. But GM would rather have a car 5-10hp slower, than fix a bunch of motors under warranty because the owners blew them up.
Though, looking at the way some of the tables are setup, I can't help but wonder if GM knew customers would run cooler thermostats,
advance timing a hair, lower fan enable switches, etc. 5 degrees more advance makes an L98 kick a lot harder. Keeping it
away from 220-240f temps helps too.
I can't recall. If you look at some of the fueling and spark tables based on air temp, cooling things down can really help. (which is why I wonder why soo many "experts" are against running cooler thermostats and lower fan enable temps).
-- Joe
Though, looking at the way some of the tables are setup, I can't help but wonder if GM knew customers would run cooler thermostats,
advance timing a hair, lower fan enable switches, etc. 5 degrees more advance makes an L98 kick a lot harder. Keeping it
away from 220-240f temps helps too.
I can't recall. If you look at some of the fueling and spark tables based on air temp, cooling things down can really help. (which is why I wonder why soo many "experts" are against running cooler thermostats and lower fan enable temps).
-- Joe
Last edited by anesthes; Feb 25, 2008 at 09:23 AM.
Melting Slicks
Joined: May 2007
Posts: 2,382
Likes: 2
From: Kathleen FL
About the computer ish, sometimes I would get frustrated when the tune doesn't go right. I was incapable of tuning the higher compression 383 without it always detonating so that's yet is another reason I went with a carb. The air/fuel might not be as precise as the EFI, but it does its job well even with a rather small carb for this application. I can maintain decent around town mileage as long as I just push the pedal far enough without engaging the secondaries. But damn its a wonderful SOTP feeling as soon as they hit. It hits like a TPI with LS3 top end power and more!
Safety Car
Joined: Sep 2003
Posts: 3,599
Likes: 24
From: Orange County NY
St. Jude Donor '10-'11-'12-'13
Le Mans Master
Joined: Feb 2005
Posts: 5,146
Likes: 3
From: The Motor City
I really was taliking about the earlier stuff, long before your 383.....however, if it is working for you I guess that 's all that matters.
Melting Slicks
Joined: May 2007
Posts: 2,382
Likes: 2
From: Kathleen FL
I guess my tune was so off that I just started having problems like that. I didn't think an 11:1 compression 383 was that hard to tune, but yet failed. It was good at full throttle when it wouldn't ping, but once heated up and part throttle given, it pinged like crazy...
Melting Slicks
Joined: May 2007
Posts: 2,382
Likes: 2
From: Kathleen FL
I literally threw a party once I removed the old wiring harness and saw a lot of exposed copper. Could have been part of the problem. In fact, I think it was the problem the whole time. Various sensors that I'm not gonna go into detail here were shorting out to the ground and/or being grounded with other sensor inputs, no wonder why the car sometimes drove like such a piece of crap a year and a half ago. But ever since I went old school, I think that now is the time to learn what the old schoolers know and at least engage in an intelligent conversation with them without straining my brain like I'm trying to understand a West German dialect.
Last edited by hexane; Feb 26, 2008 at 03:10 PM.
