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Kirkham Motorsports

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Old 03-11-2017, 07:26 AM
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Default Low compression ?

I think my cranking compression readings are low.
Basics: 454 c.i. FE, 10:1 comp, cam specd for webers,
Just looking for ball-park range pressures.
Mine are below 200 psi.
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Old 03-11-2017, 07:38 AM
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wet? dry? are all cylinders relatively the same?
if all are about 200 i wouldn't be worried
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Old 03-11-2017, 08:12 AM
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If you have a "big" cam with a late intake valve closing point, your numbers will be a smidgen low. Even a cam spec'ed for Webers.
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Old 03-11-2017, 10:39 AM
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I would be expecting around 175 psi, but I'm no expert.
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Old 03-11-2017, 01:23 PM
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My 1965 Ford Service Manual calls for 160-200psi for the 352, 390, and 427. For the 390(4v) it calls for 170-210psi. The allowable tolerance between cylinders is 20psi.
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Old 03-11-2017, 05:56 PM
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Mine are betwen 175 and 190, not what I expected, but I guess ok for now3. Engine performs well, no smoke, or strange noises. I'll be pulling it soon anyway to deal with oil leaks and a general inspection of parts for abnormal wear. Probably will put the Webers back on.
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Old 03-11-2017, 09:36 PM
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Cranking compression pressure is the direct result of dynamic compression ratio, not static compression ratio.

Hence a 9:1 engine with a baby 210 @.050 cam can have better cranking compression than a 10:1 engine with a 240 @. 050 cam.

You have 10:1 static compression, what camshaft have you got?

Are your compression pressures within 10%, preferably within 5%?

Did you do a dry test with ALL spark plugs out, and a good healthy battery?

The inlet closing point determines the start of the compression stroke, and hence when the cylinder starts to build pressure as observed on your compression gauge.

Gary

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Old 03-12-2017, 04:15 AM
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I see nothing wrong with those at all. I certainly wouldn't pull it apart looking for problems.

I've had some pretty healthy 482-496 inch engines that crank right around there. All the guys are right....several things go into it: compression, camshaft, even how quickly your starter will wheel one over.

If they're all around the same, I'd not be concerned in the least bit.
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Old 03-13-2017, 06:18 AM
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Gary: Cam is from Comp, spec'd by Barry R, hyd roller , intake dur 234 @050, exh 240.
lobe sep 112.
Numbers are within 10 %
All plugs out, charger on batts, carb wide open
Done while engine still warm.
Should have squirted a little oil in cyls and done over.
When I pull engine I'll be pulling heads for some machine work, so I'll be able to look at cyl walls.

Brent: Was hoping you'd chime in ! If I wasn't pulling the engine anyway, I'd leave it alone.
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Old 03-19-2017, 05:51 PM
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Are we going for a camshaft change/engine refresh at the same time?

Your figures are fine if they are even for all 8 with a cam like that.

Gary
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Old 03-21-2017, 06:11 AM
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Gary: they are not even, from 173 to 190. But even so, it still puts out more power than I can use on the street, and haven't been able to open track it (yet).
No cam change, but probably will put the webers back on, and anything else that needs attention.
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