My engine is approximately 9-9.5 to 1. Closer to 9.5 I believe. My cranking compression is 195 lbs. In reading and speaking w a friend. He asked about my vacuum and detonation. I have low vacuum no more than 12” at idle around 900-1000. Car has detonated I believe. he said to retard cam to get 150-160 pounds. Car will not lose bottom end but will pick up tremendous on top. Have more vacuum and drivability. It does sound bad Azz now and when warm drives just fine. he thinks car will be faster. Comp extreme energy. 230/236@.050 501/501
Cranking compression pressure is related to compression ratio, but there's two dozen other variables that affect the results, among them cranking speed, altitude, cam profile, etc. Was the cam "degreed" when it was installed? 9.5:1 compression ratio and 195 psi seems...unlikely...if that cam is installed "straight up". That or you're way below sea level. Around here, depending on overlap, I'd expect more like 130 psi. I would test the compression gauge to see if it's accurate. Maybe it's reading higher-than-true pressure. Most compression testers with quick-couplers use the very-common "Industrial Interchange" "Milton M" style coupler (there are exceptions) and they can be plugged into an air compressor tank fairly easily if there's a "Universal" or "Industrial Interchange" coupler on the hose. See if the compression gauge reads the same as the gauge on the regulator. Or just use a different gauge to test compression, and see if both gauges read the same.
How do you know if it's detonated? Is it audible? If so I would be inclined to retard timing a degree or 2, instead of the cam. With 195 cranking psi you are right at the limit of pump gas. Could always mix in a little E85 or racing gas. Those comp XE lobe profiles have short seat timing for their 0.050 durations. That plus any advanced intake center line would account for the higher cranking psi (intake valve closing early.)
Sounds to me like you aren't sure what your static compression is. Do you know what the cam was degreed to? That cam has a 110* LSA, so the cam card probably said to degree the intake lobe to 106*. If your static compression was 10:1, the DCR would be getting close to Race Gas Territory.
Yes I do not know exact static compressions I do know my cranking is 195 verified. With second tester. Both snap on. I have speed pros .035 approx in hole. 73 heads milled .0200. .027 head gasket. Cam appears to be degreed advanced. so intKe closing early. Which makes sense high crank compression. Need to pull timing cover and check it.
An easy first step would be to retard timing a degree or 2 and see if you are satisfied with that, then go from there.
Was told it will pick up on top. And not hurt bottom. Which I hope not. Goes consistent 1.70s 60 ft. Want to start whittling down to high 11s.
I think Scott meant ignition timing since you mentioned it was detonating. When you retard the cam, you close the intake valve later reducing DCR. You do transfer some low end to top end. I'm not sure you are going to pick up 1/2 a second like that though. The most important part of a 1/4 mile pass happens in the first 60'. A BBB has some low end to spare though.
Thanks Larry. I know I’m on cusp of detonation. I have heard it. Put Holley on for now and don’t hear it. Seems to run nice. I need to contact cliff eventually for parts for qjets. I know it won’t get me 1/2 second. Just a first step. Maybe 373 and a nice converter from Jim. Would love to get heads but other projects come first. One thing I did miss when intake and heads were off the port size of intake. And you showed some pics of a plenum opened a bit. Lots of great info here.
When you heard the audible detonation, did you try reducing your distributor ignition timing, you know by turning your distributor housing and use a timing light?
