yo2001 Posted October 9, 2002 Share Posted October 9, 2002 I was wandering what kind of psi are you getting from your high comp motor? I just need a reference point. I'm pushing 220psi 10.3:1 Quote Link to comment Share on other sites More sharing options...
Bob_H Posted October 9, 2002 Share Posted October 9, 2002 Don't forget what cam you have impacts that rather significantly. A stock cam with high compression will have high psi. A large aftermarket cam,(read long duration), will have a low psi reading for a given CR. My ~9.7:1 with a 270/280 .460 lift cam is about 175-180 psi. You must have a fairly small cam would be my guess. -Bob Quote Link to comment Share on other sites More sharing options...
yo2001 Posted October 9, 2002 Author Share Posted October 9, 2002 No Bob, 274 with .488" lift. That's why I was wandering. I think your cam is the one that's one size bigger than mine. So, I was little concern Quote Link to comment Share on other sites More sharing options...
Lockjaw Posted October 9, 2002 Share Posted October 9, 2002 I got 170 with a 288(234 @ 50) duration .488 lift. Bob I got into a disagreement with the guy I talked with you privately about. He says camshaft has no bearing on the cranking compression reading of an engine. See why I think he is full of **** ? I asked very specifically to make sure what he was saying. A stock cam or big cam won't make any difference. You know what is scary? This guy works on expensive cars, and races ITS and SCCA. So I guess stupidity is not limited to just ricers. Yo's cam is 234 @ 50 according to the Schneider website. Quote Link to comment Share on other sites More sharing options...
johnc Posted October 9, 2002 Share Posted October 9, 2002 Well... Let me be the Devil's advocate. While I agree that overlap will have some effect on a static compression test how do other aspects of a cam design effect static compression tests at low cranking speeds? Quote Link to comment Share on other sites More sharing options...
yo2001 Posted October 10, 2002 Author Share Posted October 10, 2002 I think cam timing got something to do with psi #. but I can't be sure. My car took more than 5-7 times to get the cylinders to fill up and I think that was because my car is set little retarded. I was only getting 185psi at 5 turns. That kind of explains why retard cam do better on top but not enough on low. In low rpm, there is not enough piston speed to pull the air in without loosing the cylinder pressure (late intake valve opening, early exhuast valve opening) This could be complete BS as I'm just speculating and come up with a thoery. lol BTW, where is Norm? Anyway, if anyone of you guys know your compression ratio, compresion psi, and your cam profile, post it here please. I like to know where my engine sits and also how cam can effect the compression test result. It'll be good for everybody as a reference. Quote Link to comment Share on other sites More sharing options...
Lockjaw Posted October 10, 2002 Share Posted October 10, 2002 I think it is overlap that has the major influence, although I might argue that lift might have an effect too. More lift is more flow to a point, and if the valves are open wider and longer, that could have an effect. I have not read the article yet, but LSA is supposed to help compression as it narrows according to the latest issue of Car Craft. I have some issue's based upon what I have been told. It just seems to me you would have a very hard time coming up with 3 cams with everything identical except lsa. The difference was only 12 psi, so I might be inclined to think that valve lash could have an effect too. Quote Link to comment Share on other sites More sharing options...
Bob_H Posted October 10, 2002 Share Posted October 10, 2002 Ok Devil: how do other aspects of a cam design effect static compression tests at low cranking speeds? At very low engine speeds,(say cranking engine speed of 2-300 rpm), there is not much velocity in the intake charge. Therefore, the overlap and the intake valve closure point have a larger effect. Once the engine is running, there is some momentum to the intake charge, which allows some of that "pressure" to stay in the combustion chamber when the intake valve is still open after the piston starts rising from BDC. At cranking speed, as soon as the piston begins rising from BDC, some of the charge is pushed back out the intake valve until it closes,(which by some areas I have read, is effective at about .050 lift). Late (radical) intake valve closure causes some mixture to escape even at lower speeds. So to answer the Devils questions, the intake valve event timing,(i.e. when it closes), has a large impact on pressures read during cranking. A cam that was designed for really high rpm will have an intake event timing that might not close the valve until the piston is almost halfway to TDC,(that is exagerating a bit for example). So at cranking speeds, you are only compressing about half the volume... But on cams that are similar, the difference shouldn't be too much. And Yo2001, your cam specs are smaller in regard to exhaust timing, but not intake timing or overall lift. And what is not known is when your intake valve actually closes,(can be derived from your cam spec card). I dont think your readings are low or abnormal. I think Norms are high. -Bob Quote Link to comment Share on other sites More sharing options...
yo2001 Posted October 10, 2002 Author Share Posted October 10, 2002 Originally posted by Bob H:And Yo2001, your cam specs are smaller in regard to exhaust timing, but not intake timing or overall lift. And what is not known is when your intake valve actually closes,(can be derived from your cam spec card). I dont think your readings are low or abnormal. I think Norms are high. -Bob Bob, I lost the cam card It was in my another car that I had but it got towed. So the cam card got towed with it. My cam LSA is 107 accoring to the manufacture. I was not concern about how low it was. I thought 220 psi was pretty high with the cam that I'm running. But who knows, that's why I popped the question. I figured that norm's car would be a good measuring stick to see if my compression was too high or not. Anyway, my car is crazy fast now. Well, at least for me. And haven't exploded in last 250 miles of HARD driving so I guess I'm okay. for now Quote Link to comment Share on other sites More sharing options...
Lockjaw Posted October 10, 2002 Share Posted October 10, 2002 I want 220 psi too. Quote Link to comment Share on other sites More sharing options...
johnc Posted October 11, 2002 Share Posted October 11, 2002 Devil here... > Therefore, the overlap and the intake valve > closure point have a larger effect. Once the > engine is running, there is some momentum to > the intake charge, which allows some of > that "pressure" to stay in the combustion > chamber when the intake valve is still open > after the piston starts rising from BDC. At > cranking speed, as soon as the piston begins > rising from BDC, some of the charge is pushed > back out the intake valve until it closes, > (which by some areas I have read, is effective > at about .050 lift). But, conversly, your are not exhausting the cylinder as effectively so some residual "charge" remains that would normally be scavanged by the header and exhaust system. Does this offset what's lost by leaving the intake valve open longer? Maybe to some degree but I don't think anyone has the numbers. For those interested in the numbers, sticking a 'tater in the exhaust tip would pretty much eliminate the cam from the static compression test. This thread does help to point out that static compression numbers are useful for comparision over time for your particular engine (a quick check of wear) but appear to be useless for comparison with dissimilar engines. Quote Link to comment Share on other sites More sharing options...
Guest norm[T12SDSUD] Posted October 12, 2002 Share Posted October 12, 2002 Hey yo, when I was busy breaking my pistons last year at 11.6:1 CR I was cranking at 225psi. It took about one month on each rebuild to break pistons three different times at this CR. At 11.3:1 CR I was cranking around 218 psi. It took about three months to break 4 pistons on one drag run at this compression ratio. I am now back at 10.6:1 CR and my cranking pressure is at 195 psi. The last time I ran this CR it took two and half years and an intentional overev to 7600 rpm to break two pistons. I'd say at 220 psi you may be a future candidate for a rebuild!! Watch your fuel mixture and don't run too much timing advance.I think my high CR motors would have survived with cast pistons had I been more conservative with the ignition timing and had run a richer mixture. Run a 180 or cooler thermostat to hold off detonation since as the carbon builds in the cylinders your compression ratio will go even higher and higher engine temps will cause detonation even sooner. BTW, not only will the cam specs cause different cranking pressure,s but your valve lash adjustment can cause a 10 psi plus or minus difference. Later,norm Quote Link to comment Share on other sites More sharing options...
yo2001 Posted October 12, 2002 Author Share Posted October 12, 2002 Thanks, Norm. That's what I needed to know. I'm running my car rich, and 19-20 deg at idle with no vacuum adv. I read your post on zcar.com and I notice you have your vacuum adv. hooked on your intake vacuum rather than ported intake because you said you you get less timing under throttle and more in idle and WOT. I need to get my timing and fuel straight it. Thankd Norm again. I guess I better not turn my motor up to 7.5 k rpm Quote Link to comment Share on other sites More sharing options...
Guest norm[T12SDSUD] Posted October 12, 2002 Share Posted October 12, 2002 Yo, when you are at wide open throttle you do not get any vacumm advance. A lot of people think that vacumm advance is for heavy acceleration but that's just not the case.It's purpose is to improve the idle and gas milage when under crusing conditions. If you take a 6 foot vacumm line and hook a gauge on the end and route it into the car you can watch the vacumm fall to almost zero under heavy acceleration. If you are under very light acceleration you will still get a small amount of vacuum advance though. Later,norm Quote Link to comment Share on other sites More sharing options...
Lockjaw Posted October 12, 2002 Share Posted October 12, 2002 Norm, what should we be looking for total advance wise with an NA engine? Mine is beyond 40 if my dial back is right. That has to be to much right? Quote Link to comment Share on other sites More sharing options...
yo2001 Posted October 13, 2002 Author Share Posted October 13, 2002 Originally posted by norm[T12SDSUD]:Yo, when you are at wide open throttle you do not get any vacumm advance. A lot of people think that vacumm advance is for heavy acceleration but that's just not the case.It's purpose is to improve the idle and gas milage when under crusing conditions. If you take a 6 foot vacumm line and hook a gauge on the end and route it into the car you can watch the vacumm fall to almost zero under heavy acceleration. If you are under very light acceleration you will still get a small amount of vacuum advance though. Later,norm I knew that about no vacuum on WOT. but on L28E the vacuum is hooked on ported vacuum so the vacuum increace as the throttle opens up? I least thought that's how is is on the Z car. Where is your vacuum advance hooked up to? on the intake or in front of the throttle plate? the GM vaccum advance is hooked up to intake vaccum to save gas on interstate. Quote Link to comment Share on other sites More sharing options...
yo2001 Posted October 13, 2002 Author Share Posted October 13, 2002 Originally posted by Lockjaw:beyond 40 if my dial back is right. I don't think that's right. Quote Link to comment Share on other sites More sharing options...
Guest norm[T12SDSUD] Posted October 13, 2002 Share Posted October 13, 2002 Yeah 40 is probably pushin it a bit however if your CR is below 10:1 you should be okay. Some engines need more ignition timing than others in order to run correctly. Hemi motors generally need around 45 degrees advance to run their best. Hey Yo, I have always run my vac advance connected to the balance tube and not the front carb. Later,norm Quote Link to comment Share on other sites More sharing options...
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