Driving it off the cliff

[ike84]

Originally Posted by blue62

that formula for compression does not work

My 1969 Jaguar E-type XK engine has a 9:1 CR factory spec.for compression is 175 PSI

My 1960 Austin healey BT7 engine has 9:1 CR factory spec. for compression is also 175 PSI

How is that possible? Not questioning your experience, but the math there doesnt add (or multiply) up lol.

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[Stl-986]

Originally Posted by ike84

How is that possible? Not questioning your experience, but the math there doesnt add (or multiply) up lol.

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off topic, but, I know what you are saying here, but.....his numbers are right. I thought the same about the multiplication you mentioned. Even our cars the math doesn't work cause these are 11.1 compression rate which means they should top out at 163.17. Not the case though. This isn't my video but one I found while going through the process today: https://www.youtube.com/watch?v=cznw9idwpkU & this: http://986forum.com/forums/boxster-general-discussions/35231-compression-test-results.html

in my own words....no clue how it's all calculated. Too much work to figure it out.

[blue62]

Originally Posted by ike84

How is that possible? Not questioning your experience, but the math there doesnt add (or multiply) up lol.

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Well you can start with things like valve overlap.
Valve timing. air temp.
The simple math formula of multiplying atmospheric air pressure by CR assumes things like.
Intake valve wide open exactly at BDC and closed at exactly TDC with no exhaust valve overlap.
Piston cranking speed is another example. Watch the needle on the vacuum gauge in the video's in this thread.
As RPM increases vacuum increases. = increased cylinder fill.

So the simple math formula of multiplying atmospheric air pressure by CR will not work to give you cranking PSI.
Look up any engine specs. you can find that show CR and cranking compression PSI
That simple math formula will not give you stated cranking PSI

As for leak down tests being more helpful than compression tests:
A leak down test is a "static test" of the cylinders ability to seal. (hold pressure)

A compression test is more of an operational test of volumetric efficiency (cylinder air filling ability) which is directly related to vacuum
and the ability to "build pressure"
Very closely related but you can get a little different info from each test.
So it depends on what info your after on how helpful a given test method is.

[blue62]

STL-986

When you get time graph your O2 sensor "voltage signals" for all 4 O2 sensors at idle.
and RPM. Then post the graph so I can have a look see.

They are in actual values under the engine module.
Before you do the graphing go in and click on each O2 sensor "voltage signal" with key on but engine off.
That should show you the "BIAS" voltage on the signal wire.
It should be 0.45V That will test the signal wiring between the DME/ECU and the O2sensor.
Then click on the O2 sensor heater signal again with key on but engine off. This will test the wiring and voltage to the O2 sensor heater circuit. I think the spec. is 4.5V or close to that.

Then get the car up to operating temp. and graph the 4 O2 sensor voltage signals and the RPM.
Just let it idle during the graphing