Compression on a hot cammed motor
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Staging Lane
Joined: Feb 2002
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From: Knoxville TN
Compression on a hot cammed motor what should a compression check on a good hotcammed motor yeild? only had time to do 5 cylinders but they all were 170-175 psi. is this normal, low, or high? i am thinking that the overlap of the cam has some effect on it
Re: Compression on a hot cammed motor The cam only has an effect on the dynamic ccompression ratio, not the static. What you are checking is static, so the psi should be the same as stock. Gotta look that up in a helms though.
bret
bret
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From: the moon
Re: Compression on a hot cammed motor He was doing a cranking compression check, which is a dynamic compression check at cranking speeds. Static is just the amount the total volume is compressed in the cylinders.
Cranking compression tests are greatly effected by cam timing. Especially the intake closing point.
Most stock cammed ls1's show around 170psi cranking compression that I have seen.
The important thing with a compression check is that all the cyls be fairly close to the same reading.
No more than 10% apart for a good running motor.
I have seen gauges read as much as 20-30psi different, seems the cheap gauges can vary due to the shrader valve "cracking" spring tension.
Steve
Cranking compression tests are greatly effected by cam timing. Especially the intake closing point.
Most stock cammed ls1's show around 170psi cranking compression that I have seen.
The important thing with a compression check is that all the cyls be fairly close to the same reading.
No more than 10% apart for a good running motor.
I have seen gauges read as much as 20-30psi different, seems the cheap gauges can vary due to the shrader valve "cracking" spring tension.
Steve
Thread Starter
Staging Lane
Joined: Feb 2002
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From: Knoxville TN
Re: Compression on a hot cammed motor Thanks for the info Steve... i believe the gauge is working fine...seemed to correct on other motors. the reason i asked was b/c i have a problem with low vacuum at idle... it is only about 13in/hg. what was your vac at idle with the hot cam? i thought it should have been higher like 18-20,but not sure how much the hot cam will kill it... the vac is very steady and doesn't move around at all during idle. i can't find any leaks anywhere... i believe the low vac is what is causing my oil burn.consumtion problems as there is alot of oil in the intake track.... have completely worked over the pcv mechanics. the motor does't appear to have any kind of miss to it...is some hesitation at around 1500 rpm and below,kinda like a small dead spot. did not have oil probs before i did the hotcam install... i wonder if i may have it out of time alittle bit...could the motor still run smooth if it was out of time? it is not making the power i expected....really haven't seen any gains at the track from when i have installed hot cam and mac headers vs before the work. <img border="0" title="" alt="[Sad]" src="gr_sad.gif" />
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From: the moon
Re: Compression on a hot cammed motor 13-15" idle vacuum is normal for a hotcam.
Mine used to idle at 13-14" at 850rpm.
Less intake vacuum actually reduces your oil being pulled into the intake.
1500rpm and below is a dead spot for that cam, it will not be like a stock cam at that low of a rpm range.
I had real good gains from that cam. I am one of the few I guess. I got 4mph with it.But it made all of it's gains from 6000-6600 rpm over the stocker. If I did not rev it that high it did very little over a stock cam at the track.
I installed it on a 111 ICL.
Mine used to idle at 13-14" at 850rpm.
Less intake vacuum actually reduces your oil being pulled into the intake.
1500rpm and below is a dead spot for that cam, it will not be like a stock cam at that low of a rpm range.
I had real good gains from that cam. I am one of the few I guess. I got 4mph with it.But it made all of it's gains from 6000-6600 rpm over the stocker. If I did not rev it that high it did very little over a stock cam at the track.
I installed it on a 111 ICL.
