160 cranking psi; advance the cam or run it?

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Baxter61

Baxter61

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.040 over 360 block with a 4" arm, 9:1 compression, MP .557 cam, 596 heads, Eddy air-gap intake, in a 3550 lb duster with 3.91s and a 3500 converter. had the cam in at 108 and it had 145 psi cranking. i advanced it to 104 and it now has 160. mind you i take my readings with the motor cold, all plugs in, and carb at rest. do ya'll think i should advance it 2 more degrees or run it as is?
 
Advance till peak.Good info. You are smart ,to use a compression gauge.
 
You need to warm the motor to 170*-190*. Remove all the spark plugs. Block the throttle wide open. Then redo the cranking compression test. This is the correct procedure. http://www.gregsengine.com/cylinder-compression-testing.html
I bet your compression comes up when done this way. Cranking compression testing is very under utilized in testing the outcome of various valve timing mods.
 
You can keep advancing the cam until you're blue in the face, doesn't mean your motor will RUN better.

What are the full specs on the cam?
 
Solid mopar .557 solid. Still go with the comp gauge. If you have a clue.
 
Advancing the cam 6 degrees will increase cylinder pressure at 300RPM just fine

but at 5000RPM. torque and horsepower will take a big hit.

Which is more useful to you?

There are an almost unlimited number of variables in a motor. A motor that makes great cranking pressure doesn't necessarily make great power or drive well around town.
 
To a intellectual engine builder,a good tuning gauge. If it doesn't work for you,accept it.
 
cranking pressure is a good tool for fault diagnosis as you can compare one cylinder's pressure to the others and look for inconsistencies.

BUT Any cylinder pressure lost or gained at one extreme of the operating range will assuredly have the opposite effect on the other extreme end of the operating range.

Since cranking pressure isn't even taken IN the operating rpm range I would consider it to be a seriously unappealing specification for tuning.

Cranking pressure is a bi-product of an engine combination and it's tuning, not the other way around.
 
You have good points, Frosty . Done the 408 gig(wanna go back!). This is a 408.:When you break in a big cam let me know....
 
The laws of physics don't magically change just because it's a 408 with a big cam.
 
I read somewhere in here that advancing the timing increases compression and gives you more low end - won't rev as high, decreasing it gives you more RPM and better
top end. With a 3500 stall, wouldn't you want more RPMs? Guess the best way would be to change it and take it to the track and see if it helps or hurts...
 
A quick calculation on engine analyzer pro determines that advancing from 108 to 104 ICL will be an even trade off of low end and high end torque output.

However with a 102 ICL the power drops off across the entire RPM range from 850RPM up to 7200RPM with nothing gained in return, not even Idle vacuum. (approximately 7 HP lost on average)

your cam is 252 @ 0.050" and 110 lobe separation. you really should consider running more like 10:1 ~10.5:1 static compression ratio to get the most out of that cam, especially when driving around town.

If you can get the compression ratio up, I would recommend installing the cam at 108 as per the manufacturer specs.
 
"MY" rule of thumb using a compression gauge coupled with degreeing a cam (and I use one on all that I do) is that if I can get the cranking compression to fall off at no more than 6* advanced, I go with the compression gauge, if not, I don't. Not every combination will peak and fall off within that range. Take for instance a 508 cam in a 9:1 motor. We all know that is WAY over cammed. As such, you can probably advance that cam in that motor 10 or more degrees before cranking compression peaks and falls off. That certainly doesn't mean we run it that far advanced. Using the compression gauge is an old tuner's trick that was taught to me years ago. I was shown that you don't just keep advancing if it doesn't fall off. You have to have a feel for it for sure. The way some of this stuff is being machined nowadays <insert eyeroll here> you may have to change cam timing by 10* before you get to where the cam card says to install it and THEN start compression gauge tuning. That's the main reason I like incorporating the gauge. It just gives you another tool to degree by.
 
frosty_the_punk said:
your cam is 252 @ 0.050"......
Click to expand...

Not accordin to Herb McCandless it ain't. He's mapped them all and found none of them were right......but that's opening up a nuther whole can of worms. lol
 
Run it as is. If you find you need to try a different position then change it. cranking numbers do not make HP or predict how quick it will run down a track.
 
well i left it at 104. i had it at 108 last sunday when i ran it so ill see if it picks up at all on friday when i go back. although my rings arent real sealed up so not sure if itll even make a difference : (
 
Baxter61 said:
well i left it at 104. i had it at 108 last sunday when i ran it so ill see if it picks up at all on friday when i go back. although my rings arent real sealed up so not sure if itll even make a difference : (
Click to expand...
Rings should seat on the starter if everything is right. With other issues changing the cam centerline is not a good way to test. Hope things work out. Did you ever run the Comp TL grind?
 
Yea, this wont be the most accurate testing to say the least. Drove it yesterday, but dyno didnt tell me much, feels the same, find out friday.

and no i havent ran the cam yet, still sitting here waiting for a build.
 
My brother in law had installed a MP .509 in a 9.7:1 340 @ 110* (retarded from get go, 108* recommended) biggest dog down low it was crazy only 110lbs cranking pressure. Having no $ and wanting to use what he had I re-degreed the cam to 104* and cylinder pressure jumped to 150lbs. Total diff motor. He is happy as heck and he was ready to scrap it. Car pulls well up to 6000 RPM now where before it was so doggy it would not even power brake the tires. Now she will spin upshifting into 2nd. Cam position can make all the difference in the world.
 
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