Posts: 1936
Threads: 209
Registered: October 29th, 2007
Member Is Offline
Location: Sydney - Liverpool area
Theme: UltimaBB Pro Blue
posted on August 5th, 2010 at 09:31 PM
Engine compression to ratio guide
Hi all,
Just wondering if there is a rough way of telling from what engine cylinder compression you have to what the compression ratio would be.
I know that this will not be an exact science on working out compression ratio, but it must give you an approximate ratio idea.
My way of thinking is, the higher the compression the higher the compression ratio eg 10:1 and if you have a lower compression (excluding any faults)
that the ratio must be lower eg 8:1.
Is anyone able to tell me factually there compression ratio and also there results from a compression test.
Would be good to get a list of results of all differing ratios and they compression result.
Thanks in advance, Kev.
I thought rebuilding a VW would be cheap - Shit I was wrong, just don't let the wife know !!!!
A.k.a.: Chris Leete
23 Windows of Awesome
The international telephone dialing code for Antarctica is 672.
Posts: 6661
Threads: 534
Registered: April 8th, 2005
Member Is Offline
Location: Tanah Merah, SE-QLD
Theme: UltimaBB Pro Blue ( Default )
Mood: A bee bit ma' bottom, now ma' bottom's big!
posted on August 6th, 2010 at 08:49 AM
Yea low comp = low comp ratio. (ratio of initial volume inside combustion chamber to final volume at full stroke, as its simply a measure of how much
the air is 'compressed' compared to outside. 8:1 suggests the air is compressed to one 8th the volume it originally was)
You can get a pretty good approximate of compression ratio to a few decimal points by measuring the ratios of pressure with a compression gauge.
Measure the compression, and with the gauge remember its not going to be 'absolute' pressure as it doesn't take atmospheric pressure into
account.
So! with your reading from the gauge, ad atmospheric pressure to this value, if ur at sea level, its 101.3kPa, 1.013bar, 1 atm... depends what units
your using. This will be P2, let P1 will be the value of your atmospheric pressure. Might as well assume the pressures at sea level being yours, it
doesn't change much unless your at top of some huge mountain.
Now, the relationship between CR (comp ratio) and PR(pressure ratio is the following)
Where gamma (the upper case power symbol) is the ratio of specific heats, and for air it is usually around 1.4. Note that the ratio of specific heats
changes with temperature, but for say ur avg day, this will do for an estimate.
So plug in your values for gamma, P1, and P2. and you will then have a value for (V1/V2). Go 1 / (v1/v2) to obtain your compression ratio of
(v2/v1)...
A.k.a.: Chris Leete
23 Windows of Awesome
The international telephone dialing code for Antarctica is 672.
Posts: 6661
Threads: 534
Registered: April 8th, 2005
Member Is Offline
Location: Tanah Merah, SE-QLD
Theme: UltimaBB Pro Blue ( Default )
Mood: A bee bit ma' bottom, now ma' bottom's big!
posted on August 6th, 2010 at 09:00 AM
haha. ur fine mate, tell me what ur getting, and whether you live on a massive hill or not....
and i will throw ya an estimate :P
(ô_!_/ô) (ô_!_/ô)
69' baja: kombi box, thing spindles, irs, disc front, type 3 rear drums, 2 inch lift kit, 31x10 rears.
New engine in process: 94mm p&bs, 74mm C/w chomol Crank, 35.5x39 SP heads, turbo. Wierd combo, hopeful torque monsta!
donn
Wolfsburg Elder
Posts: 3515
Threads: 428
Registered: November 30th, 2005
Member Is Offline
Location: NEWCASTLE NSW
Theme: UltimaBB Pro Blue
Mood: LAID BACK AND CONFUSED
posted on August 6th, 2010 at 10:35 AM
Thanks Chris, I also been wondering about this for some time, now it's clear I'll go out and clean up my shed.
I dream of a day when a chicken can cross a road without being asked for it's motive!
Phil74Camper
Son of Jim - Creator of Good
Head in the Clouds
Posts: 2703
Threads: 193
Registered: August 28th, 2002
Member Is Offline
Location: Sydney
Theme: UltimaBB Pro Blue ( Default )
posted on August 6th, 2010 at 03:07 PM
Hi Kev,
I think you are thinking of two different things.
Compression Ratio - this is the ratio of the total swept volume of the cylinder and combustion chamber, at bottom dead centre, compared to at top dead
centre. For an engine with 8.0:1 compression, the total volume at BDC is 8 times bigger than at TDC. The only way to determine it is to measure it
during the assembly process. It depends on the bore and stroke, the combustion chamber volume, any deck height volume, and any volume added by dished
pistons, or removed by dome pistons, etc etc.
Compression - this is the air pressure the rising piston generates in the combustion chamber. You measure it by taking out a spark plug and inserting
a pressure gauge.
They are related in a general way - a higher compression ratio is likely to give you a high compression reading on the gauge. BUT they are not exactly
related. You might have a high-compression ratio engine with, say, stuffed piston rings or burned valves. It still has the same Compression Ratio but
it would give a low reading on the gauge. At the other extreme you might have a fresh 36hp engine with a low CR (7:1 or less), yet it might give a
good reading on the gauge because you have good rings and valves.
vw54
Super Administrator
Always Waiting 4 Friday
Posts: 16723
Threads: 378
Registered: August 26th, 2002
Member Is Offline
Location: Sunny Sydney
Theme: UltimaBB Pro Blue ( Default )
Mood: Oval resto ongoing
posted on August 6th, 2010 at 04:36 PM
yep as per above theres no comparision it must be measured when apart
A.k.a.: Chris Leete
23 Windows of Awesome
The international telephone dialing code for Antarctica is 672.
Posts: 6661
Threads: 534
Registered: April 8th, 2005
Member Is Offline
Location: Tanah Merah, SE-QLD
Theme: UltimaBB Pro Blue ( Default )
Mood: A bee bit ma' bottom, now ma' bottom's big!
posted on August 7th, 2010 at 10:32 AM
the above formulation i added is only a estimate, but is published and can be used for a 'rough' figure.
its certainly not as 'certain' and exact as it is by physically measuring. thats for sure , but will certainly do the job if its a curiosity thing that might be out by +/- a decimal-ish...
Using compression, i can get the values for pressure, and in turn change it to a compression ratio. Compression itself of course is NOT the ratio
(ô_!_/ô) (ô_!_/ô)
69' baja: kombi box, thing spindles, irs, disc front, type 3 rear drums, 2 inch lift kit, 31x10 rears.
New engine in process: 94mm p&bs, 74mm C/w chomol Crank, 35.5x39 SP heads, turbo. Wierd combo, hopeful torque monsta!
Snap Crackle Bang
Seriously Crusin Dubber
Posts: 146
Threads: 9
Registered: August 30th, 2009
Member Is Offline
Location: Fernvale
Theme: UltimaBB Pro Blue ( Default )
posted on August 7th, 2010 at 10:36 PM
And just to make it more complicated you then also factor in the valve timing.
If you have a wild cam the inlet valve will still be open for a significant part of the "compression" stroke.
That means a high performance engine can have, on paper at least, a higher compression ratio than would normally be considered safe for the fuel used,
if the cam is chosen to suit.
