03-31-2014 | #1 |
Join Date: Nov 2011
Location: Salt Lake City, Utah
Posts: 169
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Compression Question
I just changes spark plugs and did a compression test while I was there. My compression ranged from 165 to 175. Is that OK for a non-modified engine. It seems a little low to me.
Thanks Chris
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91 ZR-1 Red on Red #922 |
03-31-2014 | #2 |
Join Date: Feb 2011
Location: San Marcos CA
Posts: 1,802
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Re: Compression Question
I'd expect 205-210, but it does make a difference on how many times you crank it. It may also make a difference if the engine hasn't run recently- put a little heat in it.
On most engines, you will see 90 psi on the very first compression stroke/ needle swing. On the LT5, it should be even more. I'd say warm it up a little, pull the plugs, and then crank it 4-5 times watching the needle rise on the gauge. All cylinders should behave the same- especially on the first rise of the needle. watching and comparing the initial compression rise is like a poor man's leakdown test- easy to see the weak cylinder if there is one. |
03-31-2014 | #3 |
Join Date: Mar 2007
Location: Crystal Lake, IL
Posts: 7,191
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Re: Compression Question
Seems low...could be the tester? The #s are consistent...does it seem down on power? Most run 190+ I believe, I think my 90 rebuilt was 225 or so
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LGAFF 90 #966-150K miles-sold 92 #234-sold 1987 Callaway TT #17 1991 ZR-1 #1359 Last edited by LGAFF; 03-31-2014 at 10:55 PM. |
03-31-2014 | #4 |
Join Date: Nov 2011
Location: Salt Lake City, Utah
Posts: 169
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Re: Compression Question
I did the test last year when I changed plugs and got the same result. The engine was cold when I have done this. It's had this much power since I bought it 3 years ago. I am not sure, but I think it's a little week. I have only drove my one ZR-1 so I have nothing to compare it to.
Chris
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91 ZR-1 Red on Red #922 |
03-31-2014 | #5 |
Join Date: Mar 2007
Location: Crystal Lake, IL
Posts: 7,191
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Re: Compression Question
Might want to do a leakdown test
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LGAFF 90 #966-150K miles-sold 92 #234-sold 1987 Callaway TT #17 1991 ZR-1 #1359 |
03-31-2014 | #6 |
Join Date: May 2013
Location: Beaver, PA
Posts: 516
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Re: Compression Question
You may not get the kind of compression the guys at sea level get if you are at altitude like me. I'm 5500 feet above sea level and the compression is lower. What is your altitude there in SLC?
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03-31-2014 | #7 | |
Join Date: Aug 2009
Location: South Dakota/California
Posts: 3,788
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Altitude Correction Factor for Compression Test
All spark plugs removed, fully charged battery.........with starter hold in start position for at least three Compression strokes on the cylinder being tested.....(keep three compression strokes as the test basis for all 8 cylinders).
The following Compression tests where accomplished with a cold engine and foot off throttle. Got 210-215 lbs on 90' with 68K miles. Got 230 lbs on a 90' with 38K miles (just last week). Of significance is that the compression of each cylinder does not vary more than plus or minus 5 lbs. Quote:
Alt Factor 0500 0.987 1500 0.960 2500 0.933 3500 0.907 4500 0.880 5500 0.853 6500 0.826 7500 0.800 8500 0.773 At 4500 feet a compression test of 215 lbs (sea level) would be .88 x 215 or 189 lbs. At 5500 feet a compression test of 210 lbs (sea level) would be .85 x 210 or 178 lbs. Which is close to your top compression.
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Left Clickable links -Solutions- LT5 Modifications/Rebuild Tricks Low Mileage ZR-1 Restoration 1990 Corvette (L98) Modifications LT5 Eliminated Systems LT5 Added Systems LT5/ZR-1 Fluids 1995 LT5 SPECIFIC TOP END REBUILD TRICKS Last edited by Dynomite; 04-01-2014 at 07:48 AM. |
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04-01-2014 | #8 |
Join Date: Dec 2012
Location: Ponoka, Alberta, Canada
Posts: 158
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Re: Compression Question
" I have only drove my one ZR-1 so I have nothing to compare it to."
Well, therein lye's the problem, buy another Z to compare to...... Pretty simple to figure that problem out......lol....
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91 ZR-1 Arctic White/Gray 01 C-5 Magnetic Red/Black 85 FJ 1100 (Big Bore kit, Hooligan Bike) 05 FJR 1300 Yamaha (Long Distance, Sport Touring) And a whole whack of RC Planes |
04-01-2014 | #9 |
Join Date: Nov 2011
Location: Salt Lake City, Utah
Posts: 169
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Re: Compression Question
My altitude is about 5000 feet. I keep hoping I find a rich relative that has left me a ton of $$$. so far, not luck. I guess I am not that far off the mark. I will go to the auto parts store and rent one of the compression testers and see what I get on a few of the easy to get to cylinders. If its close I guess it is what it is. Time to save lunch money to give to Marc.
Thanks for all the help! Chris
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91 ZR-1 Red on Red #922 |
04-01-2014 | #10 |
Join Date: Jul 2010
Location: Woodstock, Ontario, Canada
Posts: 1,275
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Re: Compression Question
Ive always held the throttle at WOT., keeps the injectors from firing on most ECM/PCM's.
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peace Paul ZR-1 Net Registry Member #1494 |
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