engine and drivetrain compression test results

[Hans]

What would be the ideal/ acceptable range of results for 9.3 CR motor?

[TobyB]

100- 180.

 

Because it's driven in very large part by your cam overlap.

 

Among other things.

 

Realistically, you probably want to see +- 2% across all 4.

When the valves are lashed up carefully.

 

From memory, a few personal examples:

Stock, tired street motor, stock cam- 140

Stock, fresh street motor, stock cam- 155

"race" 9.5:1, total seal rings, something between a 292 and 304 lift cam with short duration- 180

Same engine, higher lift cam- 150

Race 11.5:1, forged pistons, '314' ish cam- 120

 

all with the same compression tester and similar methods.

 

So it's all over the map.  You can lash loose and get higher compression,

lash tight and get lower, blah de blah.  

 

Compression's not a BAD test, it's just a relative test.

 

fwiw,

 

t

 

[Mike Self]

If you really want a good reading on cylinder condition, do a leakdown test.

 

 A compression test is best used to determine if all cylinders fall within a fairly tight range, not one significantly lower than the other three, or two adjacent cylinders both lower than the other two (possible blown head gasket).

 

mike

[cda951]

It is difficult to quantify a specific universal compression number for any given engine configuration due to variances in compression tester gauges, battery/starter circuit condition, atmospheric pressure, camshaft specs, whether the throttle is open or not, etc. The general rule of thumb is that the values for each cylinder should be within about 10% of each other, regardless of the actual numbers.

 

The merits of compression vs leakdown tests are often debated, but neither test is "better" than the other; rather, each tests a different aspect of the engine's health, and each is limited in its own right. A compression test provides a glimpse of an engine's breathing and sealing capabilities, but only while being cranked at starter speed (~200-400 RPM in most cases). A leakdown test shows a cylinder's ultimate sealing capability while static. Therefore, neither provides a 100% accurate assessment of the health of an engine, but when viewed as complimentary tests and properly performed at the same time, they provide a decent picture of what is going on.

 

When performing a compression test, it is critical that the car's battery is strong to ensure even cranking speeds for all cylinders. Fully charge the battery before the test, and hook up a portable booster pack or jumper battery for extra insurance.

 

When compression testing a carbureted engine, it is imperative to hold the throttle fully open while cranking the engine to ensure its full breathing capability. If the throttle is closed, only the idle air circuit is being utilized, which will lower the results (as it turns out, I have proven idle circuit issues in customer cars by testing compression closed vs open throttle). In any fuel-injected engine with a stepper motor idle control valve (think M20/M42/S14, etc), a properly functioning ICV provides more than enough air at starter cranking speeds, but holding the throttle open is still a good idea in case of ICV issues . . . .

 

The ignition system of any engine should be disabled while performing a compression test for obvious safety reasons. In the case of a 2002tii or other fuel injected model, do whatever is necessary to disable the fuel pump during the compression test (remove the fuel pump fuse or relay). Otherwise, the constantly spraying fuel during cranking will wash down the cylinder walls and lower the compression figures of each subsequent cylinder that is tested.

 

As Toby stated, a compression test can sniff out insufficient valve clearances (especially on the intake side), while a leakdown test may not. I have also pinpointed worn camshaft lobes via a compression test, while a leakdown test would not have revealed such a condition (though this is rather unusual). 

 

My latest project, a junkyard Alfa Romeo GTV6 that had sat for ~15 years (I know, I must be a masochist) demonstrated yet another wrinkle of the compression/leakdown test balance. The initial leakdown test revealed 3 cylinders that were higher than normal (over 20% past the piston rings), which was worrisome, but not necessarily life-threatening considering how long the car had sat without the engine being run. However, after I got the engine running, it was clear that cylinder #2 wasn't very happy, and subsequent tests revealed that the compression had halved from ~100 to 50 PSI and leakdown was now over 80%. The culprit ended up being a slightly bent intake valve on cylinder #2; as with most other production engines, that gap between the keepers of each valve are meant to allow free rotation of said valve, and once the engine actually ran, its condition was revealed (I have since performed a valve job, and it should run this weekend if all goes to plan)!

 

Sorry if I muddied the waters a bit, but I hope that someone finds this info useful!

 

Chris

 

 

[solo 2]

I just performed a CT on my 73, approx 160K, assuming stock, sat for a long time and have known service records up to 100K. I have done nothing to the engine as of yet, haven't checked valve clearances. 1-4 are 145-155-145-142. I'm planning on a 292 cam and dual 40's, daily driver. Any advice, I'm trying to keep the costs down till I can get it road worthy, may keep the 32/36 on till then. I already own 40's from other projects BTW. TIA

[Hans]

I seem to recall 180 as ideal for a 9.3, all things being equal.

[avoirdupois]

Not all compression gauges are calibrated, let alone, calibrated the same.  So testing of the same subject with different testing equipment may yield dramatically different results.  cda951 did a nice job of pointing this out.  

 

Because exceptions abound regarding dynamic compression numbers, even with the obvious differences in testing equipment, most text book compression numbers based on static compression ratios are broad generalities.  Here are some general numbers posted on the back side of a compression tester from the '70s.