2.7 Rebuild Question

[jsm911E]

According to the literature that I have seen, these P&C's create between 9.3 to 9.5:1 and were "OK" for use with today's 93 octane fuels.

What's the story on these new Mahle P&C's?

I know the octane debate has been discussed here many times before and that the consensus is that anything over 9.5 should entertain twin plugs.

I certainly want to keep in single-plug range.

thanks, John

[ttweed]

Originally posted by jsm911E
"OK" for use with today's 93 octane fuels.

I WISH we could get 93 octane here in CA, all we have now is an anemic 91 octane at most stations.

I know the octane debate has been discussed here many times before and that the consensus is that anything over 9.5 should entertain twin plugs.

I certainly want to keep in single-plug range.

My stock '67S motor was listed by the factory as 9.8:1 compression, and it has gotten by OK for many years on crap CA unleaded premium gas with single plug ignition. When I'm going to run it hard on a hot day, I will throw in a couple gallons of 110 leaded race gas just for peace of mind.

TT

[Chuck Miller]

According to the literature that I have seen, these P&C's create between 9.3 to 9.5:1 and were "OK" for use with today's 93 octane fuels.

>> Not sure I know which P&C's your talking about ...

What's the story on these new Mahle P&C's?

>> Well, I think Mahle started making 10.5 90mm RS racing piston sets as soon as they started making 8.5's stockers… I have a few period pics of narrow bodied RS's racing in Europe in other then the 'stock' touring group against some pretty interesting (fast) competition. I think the 9.5's are something Andail ordered somewhat recently for there customers who wanted a little more punch and still be streetable and normal-gassable.

I know the octane debate has been discussed here many times before and that the consensus is that anything over 9.5 should entertain twin plugs.

I certainly want to keep in single-plug range.

>> Single plug is a good idea … twin plugs can be a royal PITA… and at best they're still the cost of a small condo in Palo Alto ;-)

>> While we're on this… knowing you can safely squeeze a little more compression out of the 8.5 set (if your lucky, about 9 - 9.1) AND the 8.5 set is less then half the price of the 9.5's… AND, who knows what's going to happen with this war and the price and availability of over 90 octane gas… I'm starting to have second thoughts about using the Mahle 9.5's …

>> Anybody want to jump in on this ???

>> Thanks,

[studeb]

Originally posted by ttweed
I WISH we could get 93 octane here in CA, all we have now is an anemic 91 octane at most stations.


My stock '67S motor was listed by the factory as 9.8:1 compression, and it has gotten by OK for many years on crap CA unleaded premium gas with single plug ignition. When I'm going to run it hard on a hot day, I will throw in a couple gallons of 110 leaded race gas just for peace of mind.

TT

Tom,

i think the 9.8 is the calculated compression ratio. With the S cam valve timing the real compression ratio is going to be less. Have you ever measured the compression on that car? i saw a 67 for sale listed with compression numbers less than 140lbs, which is not that high.

[ttweed]

Originally posted by studeb
i think the 9.8 is the calculated compression ratio. With the S cam valve timing the real compression ratio is going to be less. Have you ever measured the compression on that car? i saw a 67 for sale listed with compression numbers less than 140lbs, which is not that high.

Yes, I think the larger overlap on the "S" cams makes for lower combustion chamber pressures in actual practice than a cam which has the valves completely closed for more of the compression stroke- no doubt. My measured psi was close to 140 across the board last time it was done, IIRC, but I was always taught to look at the relative difference between cylinders and not the absolute value of a compression test, since instruments and procedures can vary widely.

TT