compressin ratio?
Racer
Joined: May 2000
Posts: 386
Likes: 0
From: Niedernhausen, Germany
Re: compressin ratio? (ykp53)
3 Alternatives:
1) If you are sure, that your engine is stock in every bit (block, heads, no rebuilts), than you can check in the appropriate books which show the specs of your modelyear and engine
2) If it is a crate-engine (like GM-Goodwrench std. target master that has 8,2:1 or ZZ4 that has 9,5:1 as far as I remember...) without any mods after crate, then you can check with a reputable parts store or ask specifically in the forum
3) If you do not know anything about the engine, than the only way is to disassemble it, measure combustion-chamber volume, bore, stroke, deck-heigth, gasket bore and thicknes and calculate your CR-ratio.
Regards,
Thomas
1) If you are sure, that your engine is stock in every bit (block, heads, no rebuilts), than you can check in the appropriate books which show the specs of your modelyear and engine
2) If it is a crate-engine (like GM-Goodwrench std. target master that has 8,2:1 or ZZ4 that has 9,5:1 as far as I remember...) without any mods after crate, then you can check with a reputable parts store or ask specifically in the forum
3) If you do not know anything about the engine, than the only way is to disassemble it, measure combustion-chamber volume, bore, stroke, deck-heigth, gasket bore and thicknes and calculate your CR-ratio.
Regards,
Thomas
Le Mans Master
Joined: Apr 2002
Posts: 7,281
Likes: 5
From: Boston
Re: compressin ratio? (V8yunkie)
3) If you do not know anything about the engine, than the only way is to disassemble it, measure combustion-chamber volume, bore, stroke, deck-heigth, gasket bore and thicknes and calculate your CR-ratio.
Here's a hint
:cheers:
Racer
Joined: May 2000
Posts: 386
Likes: 0
From: Niedernhausen, Germany
Re: compressin ratio? (inferno-vette)
Inferno-Vette: Sorry, but as BB Wowbagger stated, these two values are totally different:
The compression ratio is ratio between volume with piston at TDC and volume with piston at BDC.
The compression that you messure with the gauge is the pressure that the piston builds up in the combustion chamber within the compression-stroke - it is measured in psi or bar. This pressure is dependent BOTH on the CR-Ratio AS WELL AS from condition of piston rings AND much more important, dependent on the cam-shaft.
The camshaft determine, how long the valves are closed during the compression stroke: If they are closed all stroke long, you will have maximum compression, if they start closing later, compression will be lower than maximum. I.e. with the proper cam, you can obtain extremely high compression values even on engines with 8,5:1 compression ratio (this is the case with cam's that produce hight torque figures on low rpm).
Unfortunately, a lot of people (even "professional" mechanics) tend to mix up these two figures a lot.
AGAIN: CR can only be determined by measuring all the involved geometrical values. If you do not believe me, read the appropriate books (e.g. Lingenfelter on modifying Chevy SB etc.).
The compression ratio is ratio between volume with piston at TDC and volume with piston at BDC.
The compression that you messure with the gauge is the pressure that the piston builds up in the combustion chamber within the compression-stroke - it is measured in psi or bar. This pressure is dependent BOTH on the CR-Ratio AS WELL AS from condition of piston rings AND much more important, dependent on the cam-shaft.
The camshaft determine, how long the valves are closed during the compression stroke: If they are closed all stroke long, you will have maximum compression, if they start closing later, compression will be lower than maximum. I.e. with the proper cam, you can obtain extremely high compression values even on engines with 8,5:1 compression ratio (this is the case with cam's that produce hight torque figures on low rpm).
Unfortunately, a lot of people (even "professional" mechanics) tend to mix up these two figures a lot.
AGAIN: CR can only be determined by measuring all the involved geometrical values. If you do not believe me, read the appropriate books (e.g. Lingenfelter on modifying Chevy SB etc.).
Le Mans Master
Joined: Jun 2002
Posts: 8,339
Likes: 24
From: I can walk to MA
Re: compressin ratio? (V8yunkie)
This is correct - NO WAY can you accurately measure CR with a compression
meter. Leaky valves/rings and cam geometry will DEFINITELY play greatly
on the final #(PSI) on your guage.
You gotta tear her down and take the measurements, unless you know what's
in there.
If you can answer all the following, then you can calculate your CR.
Have pistons been oversized ? Stroked crank ? Head combustion
chamber volume ? Head gasket thickness ? Block deck height modified ?
:seeya
meter. Leaky valves/rings and cam geometry will DEFINITELY play greatly
on the final #(PSI) on your guage.
You gotta tear her down and take the measurements, unless you know what's
in there.
If you can answer all the following, then you can calculate your CR.
Have pistons been oversized ? Stroked crank ? Head combustion
chamber volume ? Head gasket thickness ? Block deck height modified ?
:seeya
Race Director
Joined: Jan 2000
Posts: 12,399
Likes: 247
From: San Jose CA
Re: compressin ratio? (V8yunkie)
Not sure on the ZZ4, but a guy at Sallee-Chevrolet told me that LM-1s(the Targetmaster / GM GW350) have a tendency to vary very much in their production tolerances. They're listed as 8.5:1 now, but it seems to vary from the high 7's(!!!) to that number. When Chevy High Performance tested their LM-1(back when it was rated at 8.2:1), they actually found a ratio of 7.9:1.
-Steve
-Steve
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Race Director
Joined: Mar 2001
Posts: 11,322
Likes: 1
From: Living in the Hartland
Re: compressin ratio? (Pacin'California)
What if you took one of your rocker arms off so you only had one valve opening and closing? If you know your cam event timings seems you could come up with a pretty close approx using a comp tester. Never done it but thought about it. If you check all you cylinders and choose the high comp tested one you could factor out some ring leakage too, on a fresh engine it should be pretty close to nill.
Pacin'California:
What do you think is the cause of the variance? Head volumes maybe? Seems the crank, pistons and rods would be pretty uniform but the combustion chambers would be the area of greatest variance. I am hoping with a 64cc head to get somewhere in the low to mid 9's. BTW I ran the hell out of LM1 this weekend and it rean great. Drove from Ann Arbor to Toronto at about 3300 to 3500 the whole way with lots of hard driving in between.
Pacin'California:
What do you think is the cause of the variance? Head volumes maybe? Seems the crank, pistons and rods would be pretty uniform but the combustion chambers would be the area of greatest variance. I am hoping with a 64cc head to get somewhere in the low to mid 9's. BTW I ran the hell out of LM1 this weekend and it rean great. Drove from Ann Arbor to Toronto at about 3300 to 3500 the whole way with lots of hard driving in between.