olie05 Posted June 27, 2006 Share Posted June 27, 2006 I think i made a post like this a while back, but i have searched for a while and can not find it! The question is... Does the dynamic CR (as read from a compression tester) Help to determine if you car will be able to run on pump gas? for example: (reason for asking!) My engine, gets 215ish psi, and that is too much for the stock head design and 93 octane. the stock z's came with an acceptable 164-178 psi, which will run on 87 octane once I get a bigger cam should I be in the sub 200 psi range to be able to floor the car under load without expecting detonation? obviously with other engines and efficient head/piston combinations and crossflow, hemi style head available, results may vary... ... but is there a general rule of thumb? Quote Link to comment Share on other sites More sharing options...
2126 Posted June 27, 2006 Share Posted June 27, 2006 I can't provide you an answer but I think you are talking about static compression, not dynamic. Quote Link to comment Share on other sites More sharing options...
olie05 Posted June 27, 2006 Author Share Posted June 27, 2006 I can't provide you an answer but I think you are talking about static compression, not dynamic. no, I am talking about dynamic. If i were talking about static I would be throwing around numbers like 10:1 and the like. anyone else? Quote Link to comment Share on other sites More sharing options...
jt1 Posted June 28, 2006 Share Posted June 28, 2006 Whar you're reading off a compression tester is cranking psi, not dynamic compression, although the two are related. Dynamic compression is a calculated value depending on when the intake valve closes and compression actually begins. There are a LOT of variables here- chamber design, quench, gas quality, altitude, static and dynamic compression, plugs, timing, advance curve, A/F mixture, engine temp, air temp, so YMMV..... But a general rule is most engines do OK on 93 at 180 cranking psi or less. 190 -200 is getting pretty borderline, works in some situations but not others. John Quote Link to comment Share on other sites More sharing options...
olie05 Posted June 28, 2006 Author Share Posted June 28, 2006 Thanks John. I shall henceforth call compression readings cranking psi, instead of dynamic cr. Your response was exactly what I was looking for. I figured an engine with a pent roof or hemi crossflow head would do much better with >200 "cranking psi" than a 2 valve non-crossflow L-engine head. I am curious what my cranking compression will be when I install a cam. Quote Link to comment Share on other sites More sharing options...
jt1 Posted June 29, 2006 Share Posted June 29, 2006 Too many variables to predict, you'll just have to cut and try. If it's too high, retarding the cam a few degrees can help, also retarding the timing, or slowing down the advance curve may help. FWIW, I had a 10.5:1 355 with a med cam, 230ish @50, cranking pressure 190-195, and it ran fine on pump 93 until the ambient temp hit about 90deg, then it would spark knock some. Like I said, YMMV! John Quote Link to comment Share on other sites More sharing options...
2126 Posted June 29, 2006 Share Posted June 29, 2006 no' date=' I am talking about dynamic. If i were talking about static I would be throwing around numbers like 10:1 and the like. anyone else?[/quote'] Some students are a bit haisty in their responce. When I was your age I knew everything too!!! Sorry for the slam, but you needed it! Quote Link to comment Share on other sites More sharing options...
deMideon Posted June 29, 2006 Share Posted June 29, 2006 Afew important things to consider when trying to figure out if you will be able to run on pump gas or not is quench and dynamic compression. Do a search for quench and take a look at this thread as well... http://forums.hybridz.org/showthread.php?t=111782 Quote Link to comment Share on other sites More sharing options...
grumpyvette Posted June 29, 2006 Share Posted June 29, 2006 http://forums.hybridz.org/showthread.php?t=110913&highlight=dynamic+compression Quote Link to comment Share on other sites More sharing options...
olie05 Posted June 29, 2006 Author Share Posted June 29, 2006 Thanks grumpy! Thats the thread I was looking for. It appears my question has evolved a bit from the original one though i.e. we are no longer taking into account temperatures. Quote Link to comment Share on other sites More sharing options...
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