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Dynamic Compression 

 

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 Post subject: Dynamic Compression
Posted: Mon Jul 10, 2006 10:50 pm 
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Just wondering what is a good reading for Dynamic Compression, did a comression test and got between 210 to 215 psi across the six cylinders.

 

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4.0L EF Fairmont Station Wagon. EL GT Snorkel. K&N Pod in air box. 3" Intake piping.
Home port job. 1636 Wade cam. Wildcat Extractors, 2 1/2" cat and exhaust.
2200/2400rpm Dominator More Stall. Stage two mechanical shift kit. 3.73 4 pinion LSD. 15.078 @ 92mph.

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Posted: Mon Jul 10, 2006 11:07 pm 
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Seems really high. Dynamic compression (compression check with engine running) should be around 50% of static compression at idle and 80% of static compression with a WOT burst.

Seeing that the minimum static compression should be 146psi and around 160-180 is excellent, 210 would be very high for a dynamic check. Unless your engine is very worked with massive compression.

I would think that your compression guage is not calibrated.

cheers
Scott

 

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Posted: Mon Jul 10, 2006 11:52 pm 
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Ride: EF Fairmont Station Wagon.

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Yeah Scott, just followed the Gregorys Cylinder Comression Test procedure, this was done at cranking speed with the plugs out and crank angle sensor disconnected. No major work done, new head thats been cleaned up and fitted with a AU head gasket. Ill see if I can borrow another comression gauge and check the results.

 

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4.0L EF Fairmont Station Wagon. EL GT Snorkel. K&N Pod in air box. 3" Intake piping.
Home port job. 1636 Wade cam. Wildcat Extractors, 2 1/2" cat and exhaust.
2200/2400rpm Dominator More Stall. Stage two mechanical shift kit. 3.73 4 pinion LSD. 15.078 @ 92mph.

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Posted: Tue Jul 11, 2006 1:17 am 
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Dima's car did around that when we comp tested his on the std cam, and with the profile that your running now it will have similar characteristics to the std cam with regards to comp testing and vac testing. Have you done a vac test on it? Hows your lifter tick going?

Cheers
Jay

 

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Posted: Tue Jul 11, 2006 9:56 am 
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Gday Jaysen, Havent tried to advance the cam any more since I spoke to you last, will be playing around with that this weekend. The vernier cam gear has improved power a fair bit, power is still a bit to high in the rev range though. At 3800/4000 rpm when the bbm switches over there is a definate boost in power, I cable tied the bbm switch so its in the short runner position and I can feel the power coming on at around 3500rpm, cant say Ive noticed a big drop in power by not using the long runners below the 3800rpm switch point. Still think I need to advance another couple of degrees but will run into the problem of the HLA's/Lifters becoming noisier as I advance the cam. This problem of the HLA's becoming noisier as the cam is advanced has got me stummped, there is no reason why it should happen beacause advancing the cam doesnt change the valve train geometry, I need to figure out why this is happening. Anyone.

 

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4.0L EF Fairmont Station Wagon. EL GT Snorkel. K&N Pod in air box. 3" Intake piping.
Home port job. 1636 Wade cam. Wildcat Extractors, 2 1/2" cat and exhaust.
2200/2400rpm Dominator More Stall. Stage two mechanical shift kit. 3.73 4 pinion LSD. 15.078 @ 92mph.

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Posted: Tue Jul 11, 2006 10:22 am 
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noiser with an advanced cam..??? you're not getting close to kissing pistons are you?

 

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Posted: Tue Jul 11, 2006 10:13 pm 
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What you described is static compression check. You simply cranked the engine and held the accelerator to WOT. 210psi is very good.

Dynamic is when you actually start the engine and do the same tests, frequently venting the gauge. At idle it should be around 50% of 210psi in your case and with a very fast WOT blip (so that the revs don't actually rise) it should read 80% of 210psi. The dynamic test can tell if you have an intake or exhaust blockage, either by valves not opening fully of blocked exhaust/intake.

cheers
Scott

 

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Posted: Tue Jul 11, 2006 10:45 pm 
Getting Side Ways
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Ride: EF Fairmont Station Wagon.

Power: 150 rwkw

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WA, Australia

My Mistake Scott, thought dynamic was with the engine rotating not actually running.
Walkinshaw I dont think it is piston/valve contact. Its a new head not shaved with AU head gasket, standard bottom end and a stage one RPD cam, should have plenty of clearance. Ive only advanced the cam 3 degrees from where it was with the standard cam gear and the power of the cam is still to high in the rev range. Can advancing the cam to far cause piston/valve contact.

 

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4.0L EF Fairmont Station Wagon. EL GT Snorkel. K&N Pod in air box. 3" Intake piping.
Home port job. 1636 Wade cam. Wildcat Extractors, 2 1/2" cat and exhaust.
2200/2400rpm Dominator More Stall. Stage two mechanical shift kit. 3.73 4 pinion LSD. 15.078 @ 92mph.

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