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Compression test question

1K views 3 replies 2 participants last post by  eschaider 
#1 ·
I've been a long time lurker but don't really post much. I've always considered the people here the experts on these engines, especially Ed.

So I decided to do a compression test on my 04 Cobra, production date was Dec 2003. No notable issues, just curious. The car runs and idles perfect. I did it on the engine cold, all spark plugs removed, and the fuel pump fuse removed. Cranked it over 10-12 times with the throttle wide open. The engine or heads have never been opened. About 28k miles on the car. It is modified with a Kenne Bell with supporting mods. I've run E85 for the past several years.

Here are the numbers:
1-4: 190-195
5: 180
6: 175
7: 172
8: 180

To me these seem great. However can someone explain why the passenger side is 10-15 psi higher? My only guess would be cam timing differences between the banks? Any insight is appreciated.

Thanks,
Jon
 
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#2 ·
The OEM timing components leave a lot to be desired for a performance engine, Jon. With as many chains and sprockets as Ford uses it is almost impossible to get all four cams phased identically. As a result one bank is usually retarded with respect to the other. Same perspective, different reference point one bank is usually advanced with respect to the other. Which ever bank's cams are leading the other bank will typically show increased cylinder pressure during a compression test and in service will produce more power that the other bank that is, relatively speaking, retarded.

The compression differences you are seeing would suggest the cams are close but slightly different. The higher compression bank would be the bank with the advanced cams. The challenge is getting the necessary components to correct the cam phasing. Obviously you will need Cloyes, or similar, adjustable secondary gears. Most people only get two, one for each intake cam. Depending on how consistent you want to be bank to bank you will most probably need four.

The issue with the 2 vs 4 sprockets arises from the geometry of the secondary sprockets. With 20 teeth, each tooth is 18˚ cam degrees or 36 crank degrees different from its neighbors. A typical Cloyes secondary gear will provide +/- 4 crank degrees. With 36 crank degrees tooth to tooth on the secondary sprockets that means you have a 28˚ window between each pair of teeth which is a no mans land that you can not get to for cam phasing purposes by hook or by crook. If your desired cam phasing requires access to this timing window you are simply out of luck.

This can be mitigated, somewhat, by running a Cloyes 9-Way on both the intake and the exhaust cams. A Cloyes 9-Way on the exhaust cam will not move the exhaust cam. The only thing that will move the exhaust cam is the primary chain. The Cloyes 9-Way on the exhaust moves the secondary drive chain an additional +/- 8˚ of crank rotation to use in phasing the intake cams. You now have a cam phasing adjustment window of +/- a total of 16 crank degrees with a no-mans land timing window that has shrunk down to 20˚ — not ideal by any stretch of the imagination but an improvement over the alternative.

BTW do not forget to replace the passenger secondary tensioner with one of the aftermarket units that properly positions the secondary tensioner on the slack side of the drive chain. The OEM tensioner will tension the chain on the wrong side of the drive mechanism which plays havoc with the intake cam phasing for that bank.

In the FWIW bucket, for a street driven car, you're probably OK. For a race car I would want to correct the timing to precisely what I wanted.



Ed
 
#3 ·
Thanks for the detailed response Ed! I was generally aware that the stock timing components were not spot on. This car is just a weekend cruiser and the heads/timing components have never been touched since original assembly. I take it I have nothing to worry about then. Looking to the future if I ever do have the timing components off I'd definitely dial it in with those parts. I appreciate the response.
 
#4 · (Edited)
if you ever intend to "dial it in " in the future, Jon, you should keep your eyes open for the Cloyes stuff now because Cloyes quit making it which means it is going to get progressively harder and more expensive to find.


Ed
 
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