How do you work out you engines compression ratio??
I have used a pressure gauge to get the PSI pressure, how now do i get it to a ratio??
Compression ratios
Started by
_mike_nofx_
, Nov 24 2005 08:30 PM
4 replies to this topic
#2
_draglc_
_draglc_
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Posted 24 November 2005 - 10:09 PM
not as easy as that mate. u gotta find out how big your combustion chambers are, then that fits in with what pistons u have, whether your block has been decked, bored etc.. it is an exact science 
someone might know a roundabout way to guess, but i dont!
Andrew
someone might know a roundabout way to guess, but i dont!
Andrew
#3
_devilsadvocate_
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Posted 24 November 2005 - 11:16 PM
Compression ratio is related to the readings obtained in a pressure test.
However, two different engines can get the same pressure readings but have different compression ratios.
Theoretically 9:1 compression ratio should only read an increase of 9 times atmospheric pressure when piston is at tdc if done at constant temperature. ie 9 x 15(atm press) =135psi, but typically pressures are higher than this when done dynamically as heat cant be released quickly from the compression gas.
Variations also exist due to cam timing the intake valve may not close at bottom of the stroke - reducing readings in performance cam setups.
For a holden six in good nic 165psi is about what youd expect with a 9:1 compression ratio being cranked over on the starter.
However, two different engines can get the same pressure readings but have different compression ratios.
Theoretically 9:1 compression ratio should only read an increase of 9 times atmospheric pressure when piston is at tdc if done at constant temperature. ie 9 x 15(atm press) =135psi, but typically pressures are higher than this when done dynamically as heat cant be released quickly from the compression gas.
Variations also exist due to cam timing the intake valve may not close at bottom of the stroke - reducing readings in performance cam setups.
For a holden six in good nic 165psi is about what youd expect with a 9:1 compression ratio being cranked over on the starter.
#4
_mike_nofx_
_mike_nofx_
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Posted 26 November 2005 - 09:12 AM
The readings i got were: 160, 170, 160, 160, 165, 160. The gauge i used was about 30 yrs old and has been nocked around a bit, so not sure how accurate this is. (should buy a new 1)
This seems a little high as my car is a turbo. what yous reckon?
Also, because i bought the car already as a turbo, i cant be sure exactly what type of pistons etc it has.
Im running 6 psi, should i be thinking about lowering compression?
This seems a little high as my car is a turbo. what yous reckon?
Also, because i bought the car already as a turbo, i cant be sure exactly what type of pistons etc it has.
Im running 6 psi, should i be thinking about lowering compression?
#5
Litre8
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Thrillseeker
- Name:Howard
- Location:Melbourne, Victoria
- Car:1976 LX SLR8000
- Joined: 05-February 07
Posted 26 November 2005 - 09:21 AM
The same mechanical compression ratio in two engines will yield different effective/cranking compression (as measured by a guage) if the two engines have different cams/cam timing. An engine with a mechanical comp of 12:1 may only have an effective/cranking compression of 150lbs if it has a big overlap cam which effectively bleeds of low rpm compression.
With a turbo cam in my old 202, and about 7:1 mechanical compression, I observed a cranking compression of about 130lbs.
If you arent experiencing any detonation/pre-ignition with 6psi, and are still able to run a reasonable ignition advance curve, then I wouldnt worry about it.
With a turbo cam in my old 202, and about 7:1 mechanical compression, I observed a cranking compression of about 130lbs.
If you arent experiencing any detonation/pre-ignition with 6psi, and are still able to run a reasonable ignition advance curve, then I wouldnt worry about it.
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