Crystal Cranks

Can someone explain to the uneducated the meaning of the results of these tests, my history shows compression results after a top end rebuild as,

Cylinder 1: 225psi
Cylinder 2: 222 psi
Cylinder 3: 220 psi
Cylinder 4: 215 psi
Cylinder 5: 215 psi
Cylinder 6: 220 psi

What is the difference between a leakdown and compressionn test?

Thanks,

MisterRisky

Those aren't leak down numbers... (edit: you said they were compression test numbers in your post but I missed that... sorry) or if they are you need to know what the PSI is at 0 leakage.

The idea of a leakdown test is that they pressurize the cylinder at TDC and record what % of the pressure is held in the cylinder. So if they have 100 PSI and hook it up to the cylinder and the cylinder lets 10% out, the pressure would read 90 PSI and you would call that 10% lossage. Here's a good link on building your own leakdown tester and how to use it. Its worth reading even if you are not going to DIY:

http://www.xs11.com/tips/misc/misc3.shtml

A compression test is when you turn the engine over with the starter and measure the pressure at the spark plug hole. In a leak down test the pressure is created by an external compressor and you can determine when the air is going by listening/feeling if it is coming out the intake (intake valves), exhaust (exhaust valves), or crank case (rings). With a compression test you really don't know where the pressure is going, you can only read how much pressure is produced at that cylinder.

darth

Compression results look good as far as mean value. Rule of thumb take the average reading (i.e. 220 psi) and calculate 10% (i.e. 22 psi). The readings should not deviate more than this value therefore maximum value should be 225 + 22 = 247 psi and the minimum should be 225 - 22 = 203 psi.

A leakdown test is performed by injecting compressed air from a compressor (i.e around 100psi) in the sparkplug hole via. a leakdown test guage which measures the percentage of air that escapes. Single digit percentages are exceptable in an engine such as this and below 5% is good - the lower the better.

Bill

ginch

Ummmm. Ok this thread is freaking me out a bit. I got a compression test done on my 93 prior to purchase and the results were an even 170 psi across the board. I was told they were good numbers and didn't question them. Now I'm seeing your test at 225 psi. What the hell???

Derek

Indycam

170 is good .

ginch

Ok thanks man! Kinda scared me for a bit. Whats good and whats bad as far as psi numbers go?
Derek

ginch

Ok thanks man! Kinda scared me for a bit. Whats good and whats bad as far as psi numbers go?
Derek

darth

170 is good as long that is the average and you don't have any other readings that differ more than 10% of that value (i.e. 10% of 170 = 17 --> 170 - 17 = 153psi or 170 + 17 = 187psi). The lower reading than Crystal can be attributed to many variables (i.e. all compression guages are not the same, the person doing the test may not have had the throtle opened up etc.) or you may in fact have a low reading due to wear but it's still good enough to create good combustion.

Indycam

Bad is one cylinder at zero psi .
Bad is one cylinder lower than the others , that tells you something is going south on that cylinder .
Good is even / high numbers .
You have even and high numbers , so , the valves and rings are good all around .

I had an old mb 220 with a zero reading in one cylinder ...

Crystal Cranks

As usual thanks for the great replys, I think I understand this alot more now,
Indycam I think you sum up the compression test in a nutshell " Good is even / high numbers ."
The average numbers are as important as the high numbers.
Misterisky, thanks for the clear explanation on the differance between the two.