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  #1  
Old 02-13-2010, 05:40 PM
t walgamuth's Avatar
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compression in an 03 Miata?

I am looking at an 03 Miata to buy. I asked the seller to do a compression test. His results were 120 on three and 135 on the fourth. He says that is normal for an engine with the variable cam. Supposedly the cam advance since it works off oil pressure does not advance the cam enough to avoid overlap of open valves resulting in a false low reading.

Anybody have knowledge of this? I have been searching for a specified compression reading but no luck as yet.

I have also posted on the Miata forum but nobody there seems to know as yet.

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..I also have a 427 Cobra replica with an aluminum chassis.
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  #2  
Old 02-13-2010, 05:52 PM
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give me a few days I can look it up for you
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  #3  
Old 02-13-2010, 06:21 PM
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I think the variable timing would give good compression numbers at cranking speed.
You could send an e-mail directly to Keith at Flying Miata for a very informed answer.
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  #4  
Old 02-13-2010, 06:29 PM
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Thanks! I'll try that.
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[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.
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  #5  
Old 02-14-2010, 10:03 AM
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Quote:
Originally Posted by Oracle12345 View Post
give me a few days I can look it up for you
I am still searching for info so if you can find anything I would be most grateful!
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[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.
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  #6  
Old 02-14-2010, 10:44 AM
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If the cranking RPM was consistent for all compression measurements, then wouldn't the variable cam activity be consistent as well? As suggested above, I would be surprised that any variable timing activity would be occuring sub-idle RPM.
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  #7  
Old 02-14-2010, 10:47 AM
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Can't speak with firsthand knowledge on the concept of it being impossible to get a valid compression test done on an 03 Miata - first I've heard of such an anomaly. My first Miata was a 2001 - one of the first things I did on it was a compression test (just for my own education - no issues). For the life of me, I can't remember the numbers (~ 6-7 years ago) but I do remember that I had no trouble testing dry or wet and that all four came up within a spread of less than 10 psi.

A little bit of poking about on miata.net and I've found one thread where one person says VVT makes no difference in a compression test. Makes sense to me - pretty sure the variable valve timing comes into play at RPM's well above starter motor cranking.....

A few things to ask about that may matter....

1) Was the compression test done with the throttle wide open? Not doing so will make for wacky results.

2) Was the engine warm when tested?

3) Are those numbers wet or dry? Assuming dry, what are the readings wet?

4) Any chance of getting a leak down test done?

Found three separate references to the following numbers for NB engines:

Quote:
Standard is 209-psi, Min. 146-psi. Max acceptable difference between cylinders is 28-psi.
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  #8  
Old 02-14-2010, 12:02 PM
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variable valve timing wouldnt have any effect on the compression. variable valve timing is to improve performance out of small motors like 4 clyinders, improve emissions and fuel mileage. It has no affect on the ability of the motor to hold air in closed container, the combustion chamber. an engine in a mechincal sense is a huge air pump.

When doing a compression test make sure you cut off the fuel supply and count the same number times for each clyinder. i have access to alldata pro.
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  #9  
Old 02-14-2010, 12:24 PM
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Cam timing has no effect on compression ratio, but has a dramatic effect on cylinder pressure at a given RPM.

At 0 RPM, maximum pressure will be generated if the valves open and close exactly at TDC and BDC. Cranking speed is effectively about 0. At higher RPM, the valves must close after BDC/TDC to get maximum volumetric efficiency.

If the camshaft timing is moving around during the test, because, for example, engine oil is running out of the adjusting mechanism and back to the pan, intra-cylinder comparisons are pretty much impossible to determine.
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  #10  
Old 02-14-2010, 12:25 PM
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Valve timing, variable or not, plays a large part in compression numbers. The closing of the intake, usually after BDC, determines the amount of air trapped in the cylinder.
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  #11  
Old 02-14-2010, 01:51 PM
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Yes. If either intake or exhaust valve is open for any amount of the compression stroke the captured air would be that much less. The compression numbers he gave me were so low that it seems questionable that it would run at all. 120 psi?

He cranked five strokes per with wot and dry cylinders (afik).

He is not willing to run a leak down test for me.
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..I also have a 427 Cobra replica with an aluminum chassis.
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  #12  
Old 02-14-2010, 04:07 PM
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Run.
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  #13  
Old 02-14-2010, 10:40 PM
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Quote:
Originally Posted by t walgamuth View Post
Yes. If either intake or exhaust valve is open for any amount of the compression stroke the captured air would be that much less. The compression numbers he gave me were so low that it seems questionable that it would run at all. 120 psi?

He cranked five strokes per with wot and dry cylinders (afik).

He is not willing to run a leak down test for me.
ok hes an idiot...

you dont do a compression test at wot...

Have him give you the numbers for the dry test. the wot will give you false numbers.

For those who think variable valve timing has effect on compression test numbers go read about vvt. vvt takes affect only when the vehicle is running and will have no affect on a simple compression test...
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1983 300D(300k)
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  #14  
Old 02-14-2010, 10:50 PM
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Quote:
Originally Posted by Oracle12345 View Post
ok hes an idiot...

you dont do a compression test at wot...

Have him give you the numbers for the dry test. the wot will give you false numbers.

For those who think variable valve timing has effect on compression test numbers go read about vvt. vvt takes affect only when the vehicle is running and will have no affect on a simple compression test...
A true compression test is ALWAYS done at wot. I suspect the seller in this case did not; hence the low numbers.
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  #15  
Old 02-15-2010, 06:06 AM
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I agree that it should be done at WOT (wide open throttle).

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[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.
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