Compression Test

High 190's to low 200's.

 

I've seen a lot of numbers in that range. My understanding is that its more important that the numbers are grouped in a tight range rather than focusing on the absolute values.

 

That looks really good to me!

-Peter (veteran of taking hundreds of compression tests)

 

(veteran of taking hundreds of compression tests)
Outboard motors don't count!!!!!!!

 

damm i feel so lucky my car was giving 220-230 in the compression test!

 

I was in the 220 range too

 

Don't feel so lucky......I was told as long as the numbers are in a close range +/-5 psi things are good. When performing the test with different equipment different operators could use more or less air when testing compression and yield lower or higher numbers for the same vehicle but the ratio of compression/leakdown between the cylinders will still be the same.

 

I would expect near to or at 220psi on a fresh new engine rebuild.......ABSOLUTELY NO MORE THAN 220psi on a 11 to 1 compression ratio engine(F355) at sea level(14.7) and 59 degrees F.(check service manual for the altitude and ambient air temp. used by Ferrari to calculate the 355 engine compression ratio).

If you have 10,000 miles on your engine and the compression ratio is near or OVER 220psi you got problems.

Some of the compression check readings I've seen on PPI's are about as valuable as used toilet paper.

 

Does anyone have the compression test numbers from a newly rebuilt engine(one that has been done right), and which has the cam timing set PROPERLY(I suspect that there are alot of 355's running around out there with cam timing off...may have even come from the factory that way)?

This would be the base line for future comparison on that specific engine. IF, I were able to reach the maximum cylinder pressure using the starter motor on a F355 engine, it should be near or at 220psi(11 x 20=220) at 14.7bar and air temp of 59 deg. F...engine at operating temperature.
Because of the radical cam(overlap) the starter motor may not turn the engine fast enough to reach its' max. cyl. pressure, that is why you need to do a compression check when the engine is freshly rebuilt....the numbers obtained will be for reference to compare with future test. It is very important that you use the same identical testing procedures/techniques and guage.
I use the old 2-4-6 method for race engines...2 rev's 66%...4 rev's 88%...6 rev's 100%

IF, I tested a new engine(with proper cam timing-very important) and got 220psi readings and then did another test 10,000 miles later and got readings in the 170's, guess what? I have gone from an 11:1 compression down to under 9:1.....I don't care if cylinder to cylinder psi numbers are within 10%(this is good for minimizing wear)....heck, they can be identical.....your engine has gone from high performance to GEO Metro. More importantly does this effects the burn rate of the fuel?
If the AFR and ignition timing are set for a 11:1 compression ratio, how does the ecu adapt to a 9:1 compression ratio when the O2 sensors go offline(open loop).

These are just some thoughts I'm sharing.......anyone have any ideas about this??

Again, for those of you who have some mileage on your engine and you are near or over 220psi...YOU GOT PROBLEMS

 

If the throttles are not wide open one will get lower numbers. As was said before it is more important to have them grouped tightly.... !

 

AMEN!

 

As has been repeated a few times.... the actual numbers dont mean that much AS LONG as they are grouped together. The general rule of thumb is no greater than 10% variance from one cylinder to the other. A compression test should only be used to identify a difference in compression between the cylinders...from there.. if one cylinder or two next two each other show a problem....then a leakdown test is needed. The leakdown can determine WHAT the problem is....generally.

 

If people don't know to do it with throttles wide open AND air filters removed they don't need to be doing a compression test......this is a rule learned in engine diagnostics 101. Let's not forget about disconnecting the ignition and fuel.

The most important thing is that all of your compression testing for that engine be done using the same method/procedure.

Another item of importance is how quickly I achieve maximum cylinder pressure....anything more than 6 revolutions is a big concern to me...and I want all cylinders to come up to max. pressure with the same number of revolutions.

 

Why? Carbon buildup?

 

Yes,that would be my first consideration.

 

And dont forget....engine needs to be at operating temp!

 

I stated that in my post numer #12

 

Sorry to disagree, but the actual numbers mean alot; when, and only when you have a reference point. That reference point is usually in the service manual.......the absolute best reference point is to take the readings when the engine is freshly rebuilt/new(these are the ONLY ways I do it because I don't trust manufacturers, ESPECIALLY Ferrari). I cannot stress the importance of having the cam timing spot-on to get an accurate/true reading.

Secondly, as I stated above, I want to see maximum cylinder pressure by 6 revolutions, and that all cylinders reach max. press. within those 6 revolutions.

Thirdly, that 10% variance thingy has been around FOREVER, and is a rule of thumb for everything from a tractor to a Bel Air station wagon.....with a high output, high revving, border- line full race engine, would YOU consider this acceptable?

Fourthly, if you know your engine, the compression test can tell you some of the "WHATS", if done both dry and wet. IF NOT, then you can proceed to a leak down. I always do both regardless. When I do a compression test I am also looking real hard at the spark plugs for any indicators.

Fifthly: In the hypothetical scenerio I posted earlier, I used an F355 11:1 compression engine for determining, thru a mathmatical formula, what a starting point should be for max. cylinder pressures. This is a very crude method, but atleast we have something to guage our readings too.

At sea level(14.7 bar), 59deg. F, engine at operating temp.,throttles wide open(air filters removed), fuel disconnected, fully charged battery and good starter, I now compute.

11 x 17 to 20(I always use 20)=187-220psi(NOTE:If this engine is truly 11:1, I should not have a reading higher than 220psi). Now, because the starter motor is unable to spin the engine fast enough to overcome the pressure loss from the cam overlap, I must subtract psi from my totals: I do not have the exact numbers, so I will assume a 10-15psi loss, which now brings the max. cylinder pressures to the 172-210psi range. This gives us now, a 38psi differential. To me that is way to big of a spread, and is the reason why it is imperative to get an accurate reference point from a freshly rebuilt or new engine.

Moving right along with this scenerio, if I had a 210psi reading on my newly rebuilt engine and later on, pulled the engine and got readings of 175psi....this is a 35psi pressure drop...which means I have gone from an 11:1 comp. ratio down to approx. 9.3:1....that my friend is a HUGE drop. Assuming this drop is due to cylinder or ring wear(as a matter of fact for any reason),I am not only loosing alot of hp/torque, but more than likely I am putting more oil into the combustion chamber which is bad news for the cats,O2 sensors,valves,and on and on and on. In other words, this is a MESS waiting for a repair shop visit.

I would be even more of an idiot than I already am, if I designed an engine, using an 11:1 comp. ratio figure to build my fuel and ignition systems, program my fuel maps and timing curve, specified a slow burn high octane fuel, and then just used heads or pistons for only a 9.3:1 comp. ratio. That is rediculous!!!!


Close grouping(the 10% thingy) of cylinder to cylinder compression is OK for most...it simply means that the wear and operation of the engine is somewhat balanced(if you can call 10% balanced).....even my GEO Metro is within 10%. If you don't think the compression numbers mean anything, then so be it. As for me, the Marque deserves to always be at its best...whether on the street or track...and I aint talkin here about a fresh wax job and Armor All on the tires...lol

 

Fantastic post, Bruce. I'm interested to learn all I can on this topic.

 

The absolute single most import thing, if you don't have a "baseline" you will spin in the wind forever.








Bruce I challange your Geo with my Toyota Matrix, compression leakdown number that is LOL