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Old 05-06-2011, 11:28:23 PM   #1
my71bmf
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cranking pressue too high

I posted an issue to engine vibration last year, and I just now had a chance to figure it out. My cranking psi is 220 lbs on the low side and 230 lbs on the high side. I'm willing to bet on detonation. So I called Comp cams and they recommended retarding the cam 4*, since they grind them at +4*. I put that -4* figure in my Desktop Dyno and it actually showed an increase in power and torque. How much psi should I expect to loose? There guy said around 25 to 30 psi, which would put me back in pump gas range. Any suggestions??????
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388 ci, 10.5:1 compression, sportsman II heads, comp xr 274 cam, ported, performer rpm, holley 830 dp, 700r4 with 4l65 internals, 3.90 on lock rite, south side machine lift bars, sub frame connectors, and MT drag radials kinda keep things planted.
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Old 05-07-2011, 03:32:32 AM   #2
AJ_72
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IIRC, 180-190 cold cranking cylinder pressure is OK for pump gas (91 octane).

Should get you close. Only one way to find out.

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Old 05-07-2011, 01:48:23 PM   #3
Rich Schmidt
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It really depends on the combination. My first car was a67 Olds Cutlass with the factory 10:1 330 in it. With the very mild luxury cam it had 220 psi of cranking compression even with rings so bad that it smoked a little. It ran just fine on the cheap 87 octane gas which was all my 17 year old broke azz could afford. If it does detonate then the recommendation to move the cam back 4 degrees is a good one. Keeping it cool and keeping the timing at or below about 30 degrees total and using cold spark plugs will help too.
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Old 05-08-2011, 12:10:20 AM   #4
rustbucket79
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You could try retarding the cam but other than reducing the compression test numbers it won't fix the issue of excessive compression ratio. The best fix would be to put larger chamber heads on the engine.
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Old 05-08-2011, 12:53:58 AM   #5
73 Z
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whats the lobe center of that cam
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Old 05-08-2011, 05:19:02 AM   #6
hhott71
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Quote:
How much psi should I expect to loose?
I don't understand the loose... Loose psi? Is that when an air hose fitting breaks?

Try the Comp recommendation. They should know their products best.
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Old 05-08-2011, 10:42:39 AM   #7
1980RS
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Do what Joe Shrerman does add more duration to lower the cranking pressure and make more power. I read one time that he did this on a customers engines wirh 265psi. changed cams the engines psi fell to 215psi. and the thing picked up around 75hp. That guy knows what to do when he see stuff like this. Joe said the engine was fighting itself.

When I have had an engine like this I always use a 114° LCA cam and have never run into problems. I don't think retarding you're cam is going to help other than giving you more drivability troubles and less low end tq.

On a side note, I retarded a 292 CC 9° one time, The engine was a pig N/A but on the spray OMG the 350 was a monster. Even with some tuning issues it was still 11 flat that day.
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Old 05-24-2011, 12:31:03 PM   #8
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The lobe separation is 110. I will probably get to it this weekend. On a side note, I would have posted sooner but I didn't get the notifications. The intake is 106.
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Old 05-24-2011, 01:22:33 PM   #9
Mike-78 Z-28
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I don't think your going to see a vibration fix by lowering the compression
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Old 05-25-2011, 10:05:51 AM   #10
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I know that an engine that is detonating wil not run as smooth as one that isn't. Also, the guys at comp don't think my combo should have cranking compression that high. I'm still learning, so I just try to talk to people to limit trial and error.
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388 ci, 10.5:1 compression, sportsman II heads, comp xr 274 cam, ported, performer rpm, holley 830 dp, 700r4 with 4l65 internals, 3.90 on lock rite, south side machine lift bars, sub frame connectors, and MT drag radials kinda keep things planted.
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Old 05-29-2011, 04:33:02 PM   #11
Mike-78 Z-28
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Quote:
Originally Posted by my71bmf
I know that an engine that is detonating wil not run as smooth as one that isn't. Also, the guys at comp don't think my combo should have cranking compression that high. I'm still learning, so I just try to talk to people to limit trial and error.
Oh i thought it was vibrating all the time not just when it's pounding out the rod bearings from detonation.Can you hear the detonation or do you hear spark knock, do you have specs on your spark plugs? If so thats melted piston from detonation also.
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Old 05-30-2011, 10:42:31 AM   #12
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Sounds like a perfect combo for E85. I'd leave the cam alone and switch over. You'll pick up more than you would by retarding the camshaft.

Another option would be to put a bigger cam in it.
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Old 05-30-2011, 11:15:45 AM   #13
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When you've got too much compression it's relatively easy to drop 20-25 PSI off the cranking compression by retarding the cam. When it's already low (say, 150 or less) the same change will have a much smaller effect. I doubt you'll get it below 200, but the change should be noticable given the modest work and low cost of doing it. Certainly worth a shot before you swap cams or spring for a different set of heads.

Oh, and by the way, don't be afraid of retarding it even further. I've gone as far as 8* retard at the chain since the frst 4 is only bringing it back to "straight up", another 4 is only 4* retarded from there in actuality.

Low end torque need not suffer noticably. With the lower cylinder pressure comes increased ability to add spark advance. It usually makes up most of the difference, sometimes it MORE than makes up the difference, especially if you had to keep the spark advance modest to prevent going into detonation before.
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Old 05-30-2011, 11:19:09 AM   #14
my71bmf
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If e85 was more available I would consider changing over. And yes, I could hear the detonation. I never hear any pre ignition. I got the cam retarded yesterday. Should fire up today. The cranking pressure now is at 180psi. I have the accel 576s plugs in right now, and I have a set of 574s on the shelf if I should need a cooler plug.
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388 ci, 10.5:1 compression, sportsman II heads, comp xr 274 cam, ported, performer rpm, holley 830 dp, 700r4 with 4l65 internals, 3.90 on lock rite, south side machine lift bars, sub frame connectors, and MT drag radials kinda keep things planted.

Last edited by my71bmf : 05-30-2011 at 11:23:59 AM.
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Old 05-30-2011, 12:26:43 PM   #15
tom3
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I have the same comp. pressures on my 70. Run 93 octane with no problems and the engine runs very smooth. I'd suspect something else is causing the problem. A flywheel or pressure plate will sure give a vibration at about 2500 rpm, or maybe worn valve guides, could be a lot of things.
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