CP piston diameter

[miberns]

Greetings All,

Does anybody know why the CP piston is a few thou bigger than the Ford one? It seems that the idea was to exactly meet the rule. The added displacement is less than 1cc, but it ssems to require boring a block with perfectly good new sleves that were bored for Ford / Hepolite pistions. If a person wanted the max. diaplacement, then they can use the .005 over ones. Is this another example of SCCA reducing costs?

Mike Bernstein

Other SCCA cost saving rules for FF:

Using a distributor - distributors now cost 2X a crank fire conversion
Allowing expensive fuels at regional or national events, but not the runoffs
Not allowing an alternate block

[John Merriman]

The standard size CP forged piston is within a thou or so of the original Ford Grade E piston which was the piston used most often in 'upper level' National engines when they were available. In 99% of cases where new pistons are fitted, the OD helps with achieving the proper piston/wall clearance. In the rare instance when sleeved bores are sized to fit the slightly smaller AE piston - and then the far better CP piston option is selected - it is very likely that just honing will get you the right sized bores. The size was determined by very experienced engine builders based on 20 years of experience with this engine. The SCCA wisely approved the piston, including its size and weight, which resulted in a piston with probably five times the life of the old cast piece and thus a huge 'net' cost reduction was achieved. As regards the distributor and the block, those are simply 'facts of life' as this is Formula Ford 1600 and those are the 'specs' of the class. Exotic and expensive fuel? Well, you got me there. All I can suggest is that you don't buy it.

[miberns]

Thank you John,

My motor had the AE pistons. So I'll need to hone it, and will run yet another race on my other very tired motor.

Mike

[DrJay911]

Perhaps John or someone else knows the answer to this question: If the CP pistons are installed in freshly honed cylinders of say 3.192-3.1993 dia., or thereabouts, and the ring gaps are correct, what is a typical compression pressure reading if the engine is warmed up to say 140F and the throttle is held wide open during the compression test?

Thanks,
Jay H.

[John Merriman]

Well, theoretically, under ideal circumstances - good ring and valve seal and the correct combustion chamber CCs - it should be the CR of 9.3 x atmospheric pressure of, what, about 14.3 psi? That would be about 130 psi, no? But something tells me there's another factor involved there. Anybody know? I recall higher numbers for some reason but, hey, maybe my compression tester was wacky!

[Larry]

...at sea level on a standard day with a temp of 59F or 15C...approximately...

Larry Oliver

[sc94]

Compressing the air should increase it's temperature also. This will cause a proportional increase in pressure. Not sure if the increase in temperature would be enough to account for the perceived pressure difference.

[Jon Jeffords]

Compression readings do not follow the theoretical value due to the fact that the intake valve closes AFTER bottom dead center.

To better calculate the theoretical compression value, you must know the combustion chamber volume at the point in which the intake valve closes.

[DrJay911]

Surely someone must have just put a pressure gauge into the spark plug hole of a fresh FF engine and cranked it over. That used to be standard procedure before things got sophisticated with leak down test, etc. I can check my friend's national motor that makes good dyno HP, but I figured lots of folks would know the number that one should expect.

[bob darcey]

Back to Mike's original question, Jay Ivey posted this info a couple of years ago:

The pistons are ready to go when you get them no prep needed. CP pistons recomends .003" clearance. Piston size is 3.187 They pour the same volume as the 20552 AE piston. So .018"-.020" deck will get you there. I have been getting 12 sets about every 2 weeks. That's the batches they make at a time. I sell the pistons by the same pricing that they sell at. Thanks Jay

The CP pistons I've seen are 3.187", as Jay states above, and same as the Hepolites.

I'm curious as to what piston-to-bore clearance peop;e are running after some experience.

[Brian]

I did a compression check on my motor with aluminium head. When cold, it cranked about 60 - 80 pounds - low enough to cause a minor heart attack! I instantly fired it - sounded normal, warmed it up a bit, did a leak-down test. The leakdown was a bit tricky because it seemed hard to get the rings to be on a compression stroke and sitting at TDC, but I got readings of 2.5% to 3% which I consider good for a new, broken in engine (which is what this was less the two over-revs that I did to it...

Heart attack over, I figure the low compression test readings are due to the extremely small valve clearance you run on aluminium head engines at dead cold. They have to be fully warmed up (oil and water, not just water) to get their spec valve clearance, and until then you won't get anything like a true compression test reading.

Since I figure a compression test is just a fast way to tell me bad news before I get out the leakdown tester, and because I hate working on fully hot engines and I don't have a way of warming them up at home anyway, I just use the compression tester on street cars to see if the head gasket really is blown, or the exhaust valves really are burnt...

Brian