Doh! My CR is 1 full point higher than I thought!

[jerry S]

Rattus fornicari!!!!!

I have been laboring under the mistaken idea that I have 10.5:1 CR when in fact it is 11.5:1 CR. Time to back the advance down a few degrees.

 

[2nd Mustang]

I guess it's true, size does matter.

 

[Cannoball888]

This brings up a question I've been meaning to ask. How can one measure compression ratio in a second hand engine with no build sheet?

 

[brianj5600]

There have been some articles in mags lately stating that dynamic CR is what is important. Also stating that cranking cylinder IIRC is around 180 psi. 200 is possible with a well thought out combo. I would look more into cranking pressure vs CR since you can not measure it w/o disassembling the motor.

 

[Max Power]

This brings up a question I've been meaning to ask. How can one measure compression ratio in a second hand engine with no build sheet?

That is an excellent question....

 

[jerry S]

That is an excellent question....

In the absence of a build sheet, there is no way to tell short of disassembling the engine.

 

[NasaGT]

Given that tidbit, what has suddenly led you to beleive that your ratio is the new number?

 

[jerry S]

Given that tidbit, what has suddenly led you to beleive that your ratio is the new number?

The new number isn't quite as bad as originally thought in that it is 11:1 and not 11.5:1. I originally was told I had 10.5:1 by the shop that did the short block. I took that info on face value and tuned my car accordingly with respect to heat range for the plugs, the gap, and the total advance. Later, it occurred to me that I should have part numbers for everything in my car and I called back to the shop to get the PN on the pistons. These are SRP PN 18731. I went to the JE website and looked these pistons up and almost had a heart attack. With 58cc combustion chambers, these are 11.8 CR pistons and with 64cc combustion chambers, these are 11.1 CR pistons. Gulp. I have 60cc combustion chambers, which meant my CR was about 11.5, which is more than I thought and definitely more than I wanted.

I called the shop to rhetorically ask WTF!!!!!???
They said that the SRP calculation is based on a deck height of -.020 whereas this shop decks everything out at ZERO. I re-ran the numbers and got 11:1 CR, which is still a half point higher than I wanted but still manageable. I will increase the combustion chamber size to 64cc when I send the heads to California get ported and this will drop the CR back to 10.5 CR. In the mean time, it will be no hot days and no heavy loads.

 

[NasaGT]

Damn, your experience with that shop keeps getting better and better. Sorry to hear about your woes.

 

[jerry S]

Damn, your experience with that shop keeps getting better and better. Sorry to hear about your woes.

Actually, this was the machine shop in PA. They are straight shooters. They originally planned to put Trick Flow CNC High Ports on there with 70cc combustion chambers but the mechanic doing the overall build ditched these heads for the Edelbrocks. This is the same mechanic who billed me for Flowmaster 40s but who installed summit mufflers and who billed me $300 for a Be-Cool radiator but who installed a $189 Griffin universal (just but 2 examples).

In addition to the Beaver Falls, PA mechanic who done me ugly, the Amsterdam shop of Moritztuning.nl also screwed me. I paid EUR 723 for the dyno tuning. When all was said and done, they had the wrong metering blocks/jets on my carb, bolted open the vacuum secondaries, and tuned my car for peak rwhp without regard to the A/F ratio. I was running so rich I would have been due to replace the rings in the very near future. I found a new guy who is an honest joe and I trust him.