[rememberfireball#22]

I have been reading a lot of posts on here about CR and I would like to ask a question. I bought my 68 GTO about a year ago. The engine is the numbers matching 400 HO WS engine. The engine had been rebuilt about a year before I got it,but the seller knew nothing about the parts or combinations of parts. I know the engine runs great,no noises, no valve train noises, uses no oil,cranks very easy, has great power, and basically has not presented any problems. I use the higest octane gas I can find around here{usually 91]. I have never heard it spark knock or ping at all. Every once in a while I will add a can of octang boost from the local NAPA store,but it doesn't seem to make a difference in the way it runs. My question is , is there any way to tell the CR without pulling the heads? The way the engine runs, I would never go into the engine just to find this out. It runs to good to take a chance on messing something up. The fact that it has never spark knocked makes me wonder if the CR was lowered on the rebuild.

[68gtoMN]

Hey man! Remember (no pun) advertised, or theoretical, compression ratio is just one of many determining factors when you look at cylinder pressures. Cylinder pressure is a truer measure of what is going on and reflects mechanical compression vs. advertised (theoretical) compression.

Other contributing factors are:

• Camshaft specifications, including ramp speed, lift, duration, lobe separation angles, how installed (advanced, retarded, straight up), rocker ratio.. really affect the CR.
• In addition, your ignition timing (total timing, initial, advance curve) as well as point dwell / condition (if you have points) and coil strength are major factors for power also.

Without pulling your engine apart to determine the actual parts, as installed, and conduct measurements, you can simply use a compression testing device to determine cylinder pressure in PSI. That will give you a ball park measurement on where the engine's CR lies.

I notice that you mention no pinging/knocking (GREAT) and little/no effect from octane booster. I would then follow with a few questions:

• Is your timing set to maximize performance?
• Is your advance curve set for same?

You may want to experiment with advancing your timing 2 degrees to see how it runs.

• If no detonation, continue advancing timing in 1 degree increments until you can first detect detonation, then back off the throttle when you hear it. You can then back down the timing a degree or so until detonation disappears.
• Another way to achieve this result, perhaps faster, is to set the timing to highest vacuum signal, then drive and listen for detonation, backing down immediately upon detection.

[Craig Hendrickson]

Originally Posted by rememberfireball#22

(snip) My question is , is there any way to tell the CR without pulling the heads? (snip)

Simple answer to that single question: NO, not with the equipment available in the average garage. If you had a displacement measuring apparatus, you could, in theory, measure the volume at BDC with valve closed compared to the volume at TDC with valve closed and take that ratio, which would be CR.

See 68gtoMN's answer above for more info.

[rememberfireball#22]

Thanks for the replies and advice. I presently have the timing set on 14 degrees. I am running an EFI dist. I have a timing light with a dial back guage, but I'm not sure how it works. I just purchased a new dist and sent it to Suntuned to set up. I'm sure when he sends it back the curve will be where it needs to be. I also sent a Qjet to Cliff to rework. I know I should be grateful it does not ping or run hot and runs as good as it does. I see a lot of guys on here are having a hard time getting rid of spark knock. I see so much on here concerning CR and cams, I was just curious where mine falls,I guess I'll never know if it means going into the engine. I'm getting the car ready for a cruise in tonight,so I am just going to drive her and not worry about the CR as long as she runs like it does now.

[68gtoMN]

Originally Posted by rememberfireball#22

Thanks for the replies and advice. I presently have the timing set on 14 degrees. I am running an EFI dist. I have a timing light with a dial back guage, but I'm not sure how it works. I just purchased a new dist and sent it to Suntuned to set up. I'm sure when he sends it back the curve will be where it needs to be. I also sent a Qjet to Cliff to rework. I know I should be grateful it does not ping or run hot and runs as good as it does. I see a lot of guys on here are having a hard time getting rid of spark knock. I see so much on here concerning CR and cams, I was just curious where mine falls,I guess I'll never know if it means going into the engine. I'm getting the car ready for a cruise in tonight,so I am just going to drive her and not worry about the CR as long as she runs like it does now.

When you say, EFI distributor, I think you mean HEI (high energy ignition, right?) Your adjustable timing light is easy to work and especially for changing base timing. You say you're at 14 now with your vacuum advance hose disconnected from distributor and plugged for this, right? OK, let's say you want it to be 16 degrees initial (base) timing, then set your dial on the timing light to 16, and turn the distributor until the timing mark on the balancer aligns with 0 on the timing cover. Simple! Another great use is for checking total timing. That is, when you rev the car to, oh, 3000 RPM (hold it) turn the knob on the timing light until the mark aligns again to zero. That number will be your total timing (base + Advance).

Have fun at the cruise! Talk to you later!

Kurt

[rememberfireball#22]

Thats easy enought. I bought timing light at a swap meet and didn't get any directions. Yes, I meant HEI and I did have vaccum line off when I set time to 14. I could give her some throttle by hand while I had light pointed at balancer and the mark would travel on down toward bottom, so I know advance is working. Thanks for your help. Suntuned talked like he would have my dist back to me in a couple of weeks. When I recieve it and bolt it in I think my timming issues will be over and I can focus on another area.