11 replies to this topic

[jonny d]

HI all I did a compression test the other day and got no.1 205 no.2 212 no.3 212 no.4 212 but I didn't open the throttle would this have given me
higher or lower figures? thanks in advance. oh and why have you got to open the throttle anyway.

[998dave]

I really wish people would put units on these statements, saves us guessing.
Without the throttle open there's no air for the engine to pull in properly.

As long as all four are similar, which they are in this case, you should be okay,



I'm a little dubious however as they look a little higher then I'd normally expect. Do you know what compression ratio this engine is running?
Air pressure at sea level is 14.96 PSI, with a 10.5:1 Compression Ratio I'd expect to see a maximum in cylinder pressure of about 170 PSI.

[jonny d]

HI 998dave thanks for replying no I don't know what the compression ratio is didn't burrette the head will the figure drop when I open the throttle
can't crank it and hold the carb open,( on my own at the moment) anyone able to work out my compression on your formula if it is 212psi

Edited by jonny d, 04 November 2008 - 08:43 PM.

[998dave]

i worked it out approximately, figures depend on your engine, what is it?

D

[icklemini]

Logically the compression gauge pressure with the piston down at the bottom of the bore (BDC) would be 0psi. In other words it is at one atmosphere, or 14.7 psi.

with a 10:1 compression ratio, (and in a perfect world) this air will be squeezed to 1/10th of its volume. That means the pressure will be 10 times greater than 14.7 or abut 147psi.

Measuring with a compression gauge means the engine will be turning at cranking speed. Pretty slow. Intake valves aren't closed at BDC so some of the gas is pushed out as the piston starts moving up up so the PSI at TDC is a little lower.

The gas heats due to compression so the pressure goes up.

Piston rings and valves seats, even when new, don't provide a perfect seal, so the reading goes down.

Dynamic vs static compression ratios!

In a nutshell, i think your readings are quite high - but this could well be down to the pressure gauge you're using..

[jonny d]

998DAVE engine is 1330cc

Icklemini thanks for your reply also

[998dave]

I'm with Icklemini, that's roughly how I'd worked it out, except i added another 14.7 to my calculations.

Dave

[jonny d]

Does that mean that my compression ratio is way over 10.1 then?

[icklemini]

not necesserilly - could be that your pressure gauge is inaccurate...

the *only* way of working out your static CR is by measuring it rather than trying to guess with a compression test..

[jonny d]

I only wanted a very rough quess really Thanks for the replies guys

[jif2006]

If you were to re-test the compression test with the throttle open and rather than trying to hold the carb sprink open and crank perhaps crank and put ur foot flat to the floor on ur accelerator

Also if you were to achieve around the 170psi figure that sounds like a good figure .

You would be looking for figures within a 10% tolerance as a rule of thumb off the top of my head any one else verify?

So for arguments sake;
1-165psi
2-173psi
3-165
4-154psi

You would be within tolerence either way but if your figures showed

1-149psi
2-139psi
3-172psi
4-165psi

This woudl indicate a fault as the tolerence has been exceeded and links ot CH faults etc.

So in short the tolerence is better for compression testing rather than attemting to estimate you ratio.

Bradley

[jonny d]

Thanks for your help Bradley