Hi there
I know the drill for cc'ing heads, determining rocker geometry and adjusting the cylinders with base shims to raise or lower CR. My question is not about any of this, rather it is about the stock, out-of-the-box CR that NPR, (or those who may remember because you installed and checked them), said was to be expected from stock "A" or "B" heads and stock crank and cam for these same years on a normal configuration. In other words, what did the spec sheet for these indicate would be an expected CR with a "bolt-on" application?
I have an engine with these and have serious doubts about the PO's expertise in this never-run build. It's one of many challenges on this engine.
Thanks.
Best regards,

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Best regards,

FE Meek

NPRs, with their 30 degree piston angle, were not a bolt-on with early heads.

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"In other words, what did the spec sheet for these indicate would be an expected CR with a "bolt-on" application?"

Whatever the spec sheet says can be ignored, assuming it says anything in detail. The piston dome is only one value of at least three I can think of right now in establishing CR.

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Hi Frank,
As Vic said, the NPR pistons are 30* and the early, pre 1964, heads are 22*. In order to make these a "bolt on" many people simply shim the cylinders so that the pistons do not make contact with the head at the small diameter portion of the squish. Doing so, shimming, gaurantees an engine with piss poor performance. This is because it will have a lower compression ratio and, more importantly, it will have very poor combustion. Without a tight and well controlled squish, you will lose a lot of power. You also lose fuel mileage and run a high risk of detonation because quite a bit of mixture is trapped in a void that is not "visible" to the spark plug. I bought a S90 with a newly rebuilt engine with this flaw. The difference after I corrected the mistake is shown in the graph below.

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Cliff Murray

 

'55 Cabriolet Racer

'57 Speedster

'59 Sunroof

'63 Coupe Pushrod GS 2133

'67 911 S

Frank-I have a copy of a double sided 8.5 X 11 spec sheet that came with NPR's. There is no mention of compression ratio or anything close, like head angle. I could scan them and put them up here but it's waste of time due to the none info. Curt

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Hi Frank
The important no is the dome volume. If you have all the other variables you can calculate the CR. typical Cr are about 8-8.5 but it is mandatory you reshape the head or the pistons to use them.
cheers
jacques

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Back in the day when we had NPR's to sell, we told customers the CR was about 10:1 and you need 1,00 copper cyl shims to keep 9:1 or less.

Saint Harry was prone to prose about NPR's. Read ABC's, pages 156 (Rev 3.5) thru 158 for "the Truth".

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Hi there
Thanks to all for the information. I will be investigating this in the next weeks and, if there is anything worth reporting, I will post same. This is yet another twist in a curiouser engine.
Best regards,

_________________

Best regards,

FE Meek