 compression ratio, I forgot |
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| TimT |
 Mar 17 2007, 04:07 PM
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#1
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retired  Group: Members Posts: 4,033 Joined: 18-February 03 From: Wantagh, NY Member No.: 313  |
I did a compression check on an engine I built years ago.. I cant find the paperwork or build info for this engine... so anyway
All six cylinders are 175 psi!!! I like is that really 12:1 compression? I did another compression check.. and came up with the same results 175 psi |
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| r_towle |
Mar 17 2007, 04:32 PM
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#2
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Custom Member Group: Members Posts: 24,574 Joined: 9-January 03 From: Taxachusetts Member No.: 124 Region Association: North East States 
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bored today?
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| TimT |
Mar 17 2007, 04:35 PM
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#3
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retired Group: Members Posts: 4,033 Joined: 18-February 03 From: Wantagh, NY Member No.: 313 |
QUOTE bored today? nah... got a big ole pot of ragu bubblin and just finished making some meatballs Ive forgotten if just dividing cylinder pressure by atmospheric pressure give you compression ratio... |
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| Dave_Darling |
Mar 17 2007, 05:52 PM
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#4
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914 Idiot Group: Members Posts: 14,986 Joined: 9-January 03 From: Silicon Valley / Kailua-Kona Member No.: 121 Region Association: Northern California
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Leave it forgotten--it doesn't work.
The relationship of compression pressure to compression ratio is a complex one, and rather non-linear. Things like the state of charge of the battery, the electrical losses to and from the starter, friction losses inside the engine, and so on add up. So do things like the cam selection--a milder cam will keep more of the pressure "inside" the chamber than a wilder one with lots of overlap. Finally, the pressure-to-volume relationship is not a constant one--what's inside the chamber is very much not an "ideal gas", so you cannot use the Ideal Gas Law on it. A tight stock 2.0 motor can easily make 150 PSI compression. That does not mean it has 10.3:1 compression... --DD |
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| TimT |
Mar 17 2007, 09:22 PM
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#5
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retired Group: Members Posts: 4,033 Joined: 18-February 03 From: Wantagh, NY Member No.: 313 |
Now this is a funny anecdote:
Our GT3RS lost power in one cylinder a few weeks ago.... so we did some forensics turns out the GT3RS is making 175 psi when we check compression, also our RS just had a bad injector...interesting it has 175 psi on a cold cranking compression test I know the compression ratio of the RS....built by Porsche Motorsports.. then refined by a rebuild from Peter Dawe. QUOTE The relationship of compression pressure to compression ratio is a complex one, and rather non-linear Now thats not really scary at all..Im a math geek.. MS in engineering etc.. The mothballed engine I just did a compression check on has the same cylinder pressures as a fresh GT3RS engine.. I must just be having a pre senior moment.... |
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| Twystd1 |
Mar 17 2007, 09:57 PM
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#6
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You don't want to know... really..... Group: Members Posts: 2,514 Joined: 12-September 04 From: Newport Beach, California Member No.: 2,743 |
175 PSI is normal for EVERY engine I have ever built...
Then again... My gauge has been stuck on 175 for 40 years. So all my engines have great compression....!!!! On another note. I think DD said it best.... And unless you know exactly what cam you have and what the leakdown numbers are on a HOT engine. All it tells ya is that you probably have good rings and a solid valve/seat interface. What are the compression numbers for each individual cylinder? Regards, Clayton |
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| Aaron Cox |
Mar 17 2007, 09:59 PM
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#7
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Professional Lawn Dart Group: Retired Admin Posts: 24,541 Joined: 1-February 03 From: OC Member No.: 219 Region Association: Southern California |
here is an example....
a 2L 906 motor, may only have 40 psi of compression..... on a test. but it makes 230 hp.... cams make a huge difference.... john rogers had a nice reply to this question once. |
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| Dave_Darling |
Mar 17 2007, 10:06 PM
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#8
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914 Idiot Group: Members Posts: 14,986 Joined: 9-January 03 From: Silicon Valley / Kailua-Kona Member No.: 121 Region Association: Northern California
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QUOTE(TimT @ Mar 17 2007, 07:22 PM)  QUOTE The relationship of compression pressure to compression ratio is a complex one, and rather non-linear Now thats not really scary at all..Im a math geek.. MS in engineering etc.. OK, then start doing some modelling... Figure out all of the factors that go into piston speed (battery, electrical path to and from the starter, etc.), figure all of the places you're going to lose pressure (cam overlap, valve-to-seat interface, rings, etc.). Then dig out the chem textbooks to look up adiabatic reactions, and grind through what sort of pressure increase you can expect with the volume reduction at a given rate minus the escaping gas at a given rate. Then remember that the air will heat up as you compress it, which changes the pressure... (Oh, and don't forget to take water vapor into account!) That's about ten steps past where I get a four-alarm headache and say, "f--k this!" If you have friends who design auto engines for a living, you may be able to get a decent model out of them. Otherwise, it's probably best to just say "high numbers good, even numbers better, low numbers bad". Which even a programmer like me can remember!! (IMG:style_emoticons/default/lol2.gif) --DD |
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| TimT |
Mar 17 2007, 10:20 PM
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#9
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retired Group: Members Posts: 4,033 Joined: 18-February 03 From: Wantagh, NY Member No.: 313 |
QUOTE What are the compression numbers for each individual cylinder? heheh... I would never believe something like this myself if someone came to me with the same story... 175psi for five... 180 for the sixth |
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| Twystd1 |
Mar 17 2007, 11:26 PM
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#10
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You don't want to know... really..... Group: Members Posts: 2,514 Joined: 12-September 04 From: Newport Beach, California Member No.: 2,743 |
Them thar numbers probably mean...... Good things...
Keep dem fingers crossed and fire the bitch up...!!! Good luck Tim..!!!! Clayton |
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| Aaron Cox |
Mar 18 2007, 12:00 AM
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#11
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Professional Lawn Dart Group: Retired Admin Posts: 24,541 Joined: 1-February 03 From: OC Member No.: 219 Region Association: Southern California |
adiabats..... worst section of physics LOL
heat engines etc.... |
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