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| jsup | Compression Ratio Question....... | ||
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Elite Guru
 
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Ok, last annoying question as to my build.......
The stock CR was 10.25:1. The only thing I changed was the gasket. I went from .050 to .040. What is my CR now. I know that there are CR calculators out there. I don't have all the info such as deck height, which would be stock, and piston volume, again, stock..... Any help would be appreciated. Anyone think I picked up 1/4 point? 1/2? |
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Posted on: 2008/2/20 2:42
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| bogus | RE:Compression Ratio Question....... | ||
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Grand Imperial Pooh-Bah
 
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where did you get the info that your stock CR was 10.25:1? I thought all the L98s were 9.5:1 motors. Someone correct me.
Taking a stab, I would suspect that your CR is now about 10.25:1. |
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Posted on: 2008/2/20 2:49
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| jsup | RE:Compression Ratio Question....... | ||
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Elite Guru
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Quote:
where did you get the info that your stock CR was 10.25:1? I thought all the L98s were 9.5:1 motors. Someone correct me. I am positive it was 10.25. I spent a shitload of money on the stupid FSM, and that's what it says. |
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Posted on: 2008/2/20 3:07
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| MK 82 | RE:Compression Ratio Question....... | ||
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Senior Guru
 
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What year is the engine?
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Posted on: 2008/2/20 3:09
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| jsup | RE:Compression Ratio Question....... | ||
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Elite Guru
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1990
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Posted on: 2008/2/20 3:15
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| MK 82 | RE:Compression Ratio Question....... | ||
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Posted on: 2008/2/20 3:43
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| jsup | RE:Compression Ratio Question....... | ||
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Elite Guru
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positive. 10.25
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Posted on: 2008/2/20 12:14
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| dan0617 | RE:Compression Ratio Question....... | ||
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Senior Guru
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Deck height = (.025)
Pistons are dished = (-13CC) Bore = (4.00) Stroke - (3.5) Stock Head Gasket = (.051) Stock Head Combustion Chamber= (58CC) Most all calculators come out to about 9.23 or 9.25. I'm not sure of the stock head gasket bore diameter, but it only changes the final static compression ratio very little. I'm pretty sure you are thinking of an LT1 or LT4 engine. I'm not sure what their stock ratios are but I think they are in the 10.XX range. I don't know what the final CC of your heads are, but I know stock with 113 aluminum heads your compression ratio is around 9.25, but GM advertised it as 9.5:1. Below is the web address of a good calculator. On my engine build I used the stock bottom end, .012 shaved off of 113 heads, and a .024 gasket with a .060 bore diameter. Since I don't know the new CC of the heads I just use the 58 CC as stock, then subtract the .012 off of the deck height. That will give me the same result. My CR now is 10.15:1. Try it out and let us know what yours is. http://members.cisdi.com/~anesthes/comp.html |
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Posted on: 2008/2/20 12:35
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´89 Vert, 383, 230/236 cam, AFR 195's, LT Headers, HSR intake, 2800 stall, Zex 200 shot, ET Street Radials, tune by me. Runs were with D36 3.07's. On spray, 10.55 @ 132.78, 1.55 60 ft. On motor, 12.08 @ 113.15, 1.66 60 ft. |
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| jsup | RE:Compression Ratio Question....... | ||
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Elite Guru
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Quote:
Deck height = (.025) The FSM and Corvette Black Book put it at 10.25. I'll scan in the FSM page later today. 1990/91 were 10.25. Seems someone got it right. But there is a lot of misinformation out there. http://www.corvetteactioncenter.com/specs/c4/1990/90specs.html |
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Posted on: 2008/2/20 12:40
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| MK 82 | RE:Compression Ratio Question....... | ||
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Senior Guru
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I honestly don't know. You can find sources that state both numbers. LT1 is 10.4. LT4 is 10.8.
What grade fuel did you use in it before the build? http://www.corvetteactioncenter.com/forums/showthread.php?t=99829 |
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Posted on: 2008/2/20 14:20
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| dan0617 | RE:Compression Ratio Question....... | ||
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Senior Guru
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If the compression ratio changed for '90 and up L98 vettes, I'd like to know how. How, as in what did they do to change it. All my resources and info point to it having the same dished pistons, same 58cc 113 casting aluminum heads, same .025 deck height, and even the same .051 factory gasket. If static compression ratio changed, and the displacement is still 350, then one of these factors had to change, and pretty significantly to get from roughly 9.25:1 to 10.25:1. If anyone knows for sure that it changed or that it didn't for the '90 and up L98's please inform me.
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Posted on: 2008/2/20 16:17
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´89 Vert, 383, 230/236 cam, AFR 195's, LT Headers, HSR intake, 2800 stall, Zex 200 shot, ET Street Radials, tune by me. Runs were with D36 3.07's. On spray, 10.55 @ 132.78, 1.55 60 ft. On motor, 12.08 @ 113.15, 1.66 60 ft. |
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| MK 82 | RE:Compression Ratio Question....... | ||
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I too would be curious to know how they did it.
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Posted on: 2008/2/20 19:24
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| jsup | RE:Compression Ratio Question....... | ||
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Elite Guru
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Quote:
Deck height = (.025) I measured the stock head gasket, it was .050 The new gasket has a spec of .039 |
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Posted on: 2008/2/20 22:05
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| bogus | RE:Compression Ratio Question....... | ||
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Grand Imperial Pooh-Bah
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you measured a used - re: compressed - head gasket.
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Posted on: 2008/2/21 3:35
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| dan0617 | RE:Compression Ratio Question....... | ||
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Senior Guru
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Quote:
Your measurement of .050 is close enough to the spec I found of my '89 of .051 to tell me that they didn't change the headgasket. They would have had to change the pistons or the heads. I don't even think a deck height change would yield the difference from 9.25 (which GM calls 9.5) up to 10.25. Nobody seems to know if the 10.25 is a misprint or if it is real and different pistons/heads were used to make the change. |
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Posted on: 2008/2/21 15:04
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´89 Vert, 383, 230/236 cam, AFR 195's, LT Headers, HSR intake, 2800 stall, Zex 200 shot, ET Street Radials, tune by me. Runs were with D36 3.07's. On spray, 10.55 @ 132.78, 1.55 60 ft. On motor, 12.08 @ 113.15, 1.66 60 ft. |
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| toptechx6 | RE:Compression Ratio Question....... | ||
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Senior Guru
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My parts reference shows different numbers for both the head gasket and piston.
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Posted on: 2008/2/21 15:28
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| jsup | RE:Compression Ratio Question....... | ||
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Elite Guru
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Quote:
My parts reference shows different numbers for both the head gasket and piston. I think 89 was a dished piston, 90 was flat. That would do it, wouldn't it? |
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Posted on: 2008/2/21 15:54
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| MK 82 | RE:Compression Ratio Question....... | ||
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Senior Guru
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My 86 had flat top pistons with 4 small valve reliefs and it was 9.5:1. I would love to see a photo of a dished stock piston from a Corvette L98.
I would be willing to chip in on a call to Gordon. |
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Posted on: 2008/2/21 16:14
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| jsup | RE:Compression Ratio Question....... | ||
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Elite Guru
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Quote:
My 86 had flat top pistons with 4 small valve reliefs and it was 9.5:1. I would love to see a photo of a dished stock piston from a Corvette L98. Would the dish be dramatic enough to see it? |
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Posted on: 2008/2/21 16:48
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| dan0617 | RE:Compression Ratio Question....... | ||
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Senior Guru
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Wish I would have taken pics of mine but I didn't. There are 4 valve reliefs per piston, and a slight dish. The dish is noticeable if you are looking for it.
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Posted on: 2008/2/21 18:08
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´89 Vert, 383, 230/236 cam, AFR 195's, LT Headers, HSR intake, 2800 stall, Zex 200 shot, ET Street Radials, tune by me. Runs were with D36 3.07's. On spray, 10.55 @ 132.78, 1.55 60 ft. On motor, 12.08 @ 113.15, 1.66 60 ft. |
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