Check My PR Calculations Please
Check My PR Calculations Please Ok so after much reading and researching heres what I got. Im installing the MS4 with a set of PRC LS6 heads. They are unmilled and Im using GM head gaskets. I currently have 7.400 PRs from my previous cam. I used the calculator from this thread https://ls1tech.com/forums/generation-iii-internal-engine/327734-cam-guide.html. In the calculator I put:
Base Circle (MS4) : 1.409 (measured with digital micrometer)
Cylinder Head Milling : 0
Headgasket Thickness : .052
Total distance closer to cam : .052
The calculator gave me an answer of 7.4. I just want to make sure that I put all the variables in correctly. I also did on paper.
Stock base 1.55 - (your measurement)= Difference
Now difference / 2 = how much longer your p-rods should be
7.400-(how much longer)= p-rods needed
1.55 - 1.409 = 0.141
0.141 / 2 = .0705
7.400 + .0705 = 7.3295
Now to take head gasket and milling (if any) into account.
7.3295 + .052 (GM head gasket) = 7.3815
7.3815 +. 015(longer aftermarket PR) = 7.3965
Does that sound right? Thank you for any input and schooling.
Base Circle (MS4) : 1.409 (measured with digital micrometer)
Cylinder Head Milling : 0
Headgasket Thickness : .052
Total distance closer to cam : .052
The calculator gave me an answer of 7.4. I just want to make sure that I put all the variables in correctly. I also did on paper.
Stock base 1.55 - (your measurement)= Difference
Now difference / 2 = how much longer your p-rods should be
7.400-(how much longer)= p-rods needed
1.55 - 1.409 = 0.141
0.141 / 2 = .0705
7.400 + .0705 = 7.3295
Now to take head gasket and milling (if any) into account.
7.3295 + .052 (GM head gasket) = 7.3815
7.3815 +. 015(longer aftermarket PR) = 7.3965
Does that sound right? Thank you for any input and schooling.
Last edited by Oh4GTO; Oct 21, 2006 at 08:13 PM.
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Joined: Mar 2004
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From: BFE
1.55 (Stock base)-1.409 (MS4 base)= .141 (Base difference)
.141 / 2 = .0705 (effective base radius difference)
7.400+.0705 = 7.4705 would put you back to stock preload on lifters.
You can run 7.450 safely and at least 7.425's
If you run 7.400, you are at least .06x short from stock preload, too light IMO.
you measured intake lobe right? that is a LSK
Now measure exhaust
Do your calc again but this time add the effective base radius difference>
7.400+ (+ base diff/2) = p-rod
what do you find?
I think you'll see something interesting
.141 / 2 = .0705 (effective base radius difference)
7.400+.0705 = 7.4705 would put you back to stock preload on lifters.
You can run 7.450 safely and at least 7.425's
If you run 7.400, you are at least .06x short from stock preload, too light IMO.
you measured intake lobe right? that is a LSK
Now measure exhaust
Do your calc again but this time add the effective base radius difference>
7.400+ (+ base diff/2) = p-rod
what do you find?
I think you'll see something interesting
Last edited by PREDATOR-Z; Oct 22, 2006 at 03:26 AM.
Ok. The 1.409 measurement that I got was for the exhaust lobe. The intake lobe is 1.360 
TSP told me that I would be fine with 7.400 pushrods. Thats crap.
So:
1.55 - 1.36 = 0.19
0.19 / 2 = 0.095
7.40 + 0.095 = 7.495
You mentioned that 7.4705 would put me back to stock preload on lifters. What dis you mean by that? Im going to order new PRs so I want to make sure I get the right ones. Dont want it to be too loose or make too much noise. Thanks.
TSP told me that I would be fine with 7.400 pushrods. Thats crap. So:
1.55 - 1.36 = 0.19
0.19 / 2 = 0.095
7.40 + 0.095 = 7.495
You mentioned that 7.4705 would put me back to stock preload on lifters. What dis you mean by that? Im going to order new PRs so I want to make sure I get the right ones. Dont want it to be too loose or make too much noise. Thanks.
Last edited by Oh4GTO; Oct 22, 2006 at 12:33 PM.
Also take into account those PRC ls6 heads have stock ls6 valves which are slightly longer stems to account for the smaller base circle of the ls6 cam while still using 7.4 pushrods that all LS1/2/6 cars used oem.
Hense people using 7.42 pushrods when just installing an LS6 cam on LS1 heads.
Hense people using 7.42 pushrods when just installing an LS6 cam on LS1 heads.
TECH Senior Member
Joined: Mar 2004
Posts: 14,620
Likes: 17
From: BFE
Originally Posted by Oh4GTO
Ok. The 1.409 measurement that I got was for the exhaust lobe. The intake lobe is 1.360 TSP told me that I would be fine with 7.400 pushrods. Thats crap.
So:
1.55 - 1.36 = 0.19
0.19 / 2 = 0.095
7.40 + 0.095 = 7.495
You mentioned that 7.4705 would put me back to stock preload on lifters. What dis you mean by that? Im going to order new PRs so I want to make sure I get the right ones. Dont want it to be too loose or make too much noise. Thanks.
TSP told me that I would be fine with 7.400 pushrods. Thats crap. So:
1.55 - 1.36 = 0.19
0.19 / 2 = 0.095
7.40 + 0.095 = 7.495
You mentioned that 7.4705 would put me back to stock preload on lifters. What dis you mean by that? Im going to order new PRs so I want to make sure I get the right ones. Dont want it to be too loose or make too much noise. Thanks.
If you use stock length with smaller base circles, then the difference will taken away from stock preload. (~.100)
I like to keep at least .060 cold preload (or .080 hot) on my setups. Much quieter this way.
that is why i said check both lobes
ideally this cam needs 2 types of p-rods (int/exh)
7.425 exh and 7.450 int
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Originally Posted by PREDATOR-Z
Our system is non adjustable rocker one. So lifter preload is adjusted via p-rod length.
If you use stock length with smaller base circles, then the difference will taken away from stock preload. (~.100)
I like to keep at least .060 cold preload (or .080 hot) on my setups. Much quieter this way.
that is why i said check both lobes
ideally this cam needs 2 types of p-rods (int/exh)
7.425 exh and 7.450 int
If you use stock length with smaller base circles, then the difference will taken away from stock preload. (~.100)
I like to keep at least .060 cold preload (or .080 hot) on my setups. Much quieter this way.
that is why i said check both lobes
ideally this cam needs 2 types of p-rods (int/exh)
7.425 exh and 7.450 int
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Verdad Gallardo Just a quick update if anyone is interested. Did a wipe test tonight and the 7.450 PRs on the intake were too long and the 7.425 PRs on the exhaust as well. Looks like the longer LS6 stems are the cause. I did a wipe test with 7.425 PRs on the intake and 7.400 on the exhaust and they were in the center.
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From: Bergen County, NJ
Did you also measure preload, or at least include the difference between the valve stems in your calculation? The swipe test checks the geometry but unless I am mistaken does not necessarily confirm preload.
For the "wipy" test I colored the top of the vavle with a marker. Bolted up the rockers and turned the engine 720 degrees. Took the rockers off and checked the wipe. It was right on the center. Now I do realize that this is on a set of lifters that are not pumped up. So Im guessing when they fo pump up the actual wipe will move over further outward. I think. As for the base circle I measured it with a digital micrometer. I twisted it around the smallest part off the lobe and found the lowest number. Im still pretty new at this so I may be wrong on everything I just said.
I Was Told Another Way To Tell If Your At The Base Circle If For Example You Turn The Motor Over By By Hand And When You See The Exhuast Valve Fully Open And Then Start To Close Thats When Your Intake Side Is At The Base And Visa Versa.. Thats What I Was Told And Thats How I Measure My Ps Length.
