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AU 6 Cylinder Compression 

 

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 Post subject: AU 6 Cylinder Compression
Posted: Wed May 04, 2011 2:13 pm 
Getting Side Ways
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I have a AU 6 - it has a "high comp" rebuild according to the previous owner.
I did a compression test, and all cylinders read around the 210psi, 215psi.

What the best way to work out my compression ratio without removing the head.

Thanks
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 Post subject: Re: AU 6 Cylinder Compression
Posted: Wed May 04, 2011 6:28 pm 
Getting Side Ways
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You can't work out the ratio with out removing the head.

but at 210psi, it is higher then stock

 

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 Post subject: Re: AU 6 Cylinder Compression
Posted: Wed May 04, 2011 11:26 pm 
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i dont think its high at all...

My VCT engine is at around 220 - 230.

180 - 200 is usually the norm on standard engines.

I don't know why the VCT's have higher compression.
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 Post subject: Re: AU 6 Cylinder Compression
Posted: Wed May 04, 2011 11:29 pm 
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your looking at between 10:1 to 10:5.1 roughly
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 Post subject: Re: AU 6 Cylinder Compression
Posted: Thu May 05, 2011 10:14 am 
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Ride: EF XR6 Wagon, AU2 XR6 VCT

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I'm no engineer, but as I understand it static compression and dynamic compression are two different things.

Static compression or mechanical compression is the ratio of space above the piston at the bottom of the stroke to space at the top of the stroke. This is the quoted compression ratio.

Dynamic compression is what you measure with your compression gauge. It measures the pressure actually created in the cylinder with the engine turning over on the starter. It is affected not only by the compression ratio, but also by the valves opening and closing, letting gases into and out of the cylinder during the stroke. So your cam lobe profiles, overlap, lobe separation, and cam timing will all affect the pressure the guage will show.

A 9:1 engine with a conservative, stock cam might show a higher dynamic compression (at the low rpms the starter will turn it) than a 10.5:1 race engine with a 320 degree race cam, as the race cam is capturing less gases on the compression stroke and allowing more of the gases out of the cylinder on the power stroke. If you could measure the dynamic compression at 5000rpm the story would be the other way around due to the race cam's higher volumetric efficiency at that speed. It's getting more cylinder filling from the gas velocity while the stock engine is being restricted by the valve timing.

It makes sense that a VCT would have higher measured compression at starter rpms than a non-VCT, as it adjusts the cam timing to act like a more conservative cam at low revs and like a hotter cam at high revs.

In short, you can't determine the compression ratio from the compression test with any accuracy unless you take the valve events (cam timing) into account. But if you leave your cam unchanged and your compression goes up, then you do have a higher compression ratio. What you are normally looking for in a compression test is consistency across the cylinders, not absolute numbers. Hope this helps.

 

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 Post subject: Re: AU 6 Cylinder Compression
Posted: Thu May 05, 2011 11:15 am 
Getting Side Ways
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Thanks for the reply guys
was pretty sure that i would have to take the head off to check - which wont be happening.
here is the cam spec sheet
Image

so my comp could be higher than expected due to this cam ?
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 Post subject: Re: AU 6 Cylinder Compression
Posted: Thu May 05, 2011 11:22 am 
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Ride: ford falcon au3 xr8

Location: a shit suburb in sydney
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in std form both std au, xr6 and vct are rated at 9.65:1.
my old std au engine would push 200psi.. the vct engine i have now has 195psi, it also comes down to the cam being used aswell...
not to mention the quality of the compression gauge you use, ive used 3 different gauges with 3 different results, all were+ or minus 10 psi of each other...

which one do you beleive???
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