Ptv clearance spec

Notthenow

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I am degreeing in Comp 262AH cams in pi heads on an 1998 NPI. What clearance spec should I shoot for on the intake valve? I have seen .035 mentioned, but not sure if that is a good number. Installed thus far straight up, my left bank is 109.5 and .027, and right is 107 and .021. I am using Trick Flow adjustable crank gears. Next step will be to retard right 4 degrees and left 2 degrees. I will loosen cam bolt on left to see if I can lose the 1/2 degree so both will be 111 and hopefully see better clearance without needing to retard further as was hoping for 110.
Thanks for any insight.
 

Notthenow

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I emailed Jordan at MHS and asked what a safe spec for ptv clearance on the 2v intake was in his opinion. He said minimum .040, optimal .050 for an aftermarket cam. Just passing the info on for anyone looking for the spec in the future.
 

lwarrior1016

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Sorry, I’m a little late to the show. Jordan got you squared away, but just for more info, I’ve run them at .030” up to 8500 rpm and didn’t have an issue.
 

Notthenow

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Thanks, I had read Nick say .035 years ago, but wanted to make sure. This is on an npi short block with 160k so not new parts and wanted to be careful. I assume yours were new builds?
 

lwarrior1016

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Thanks, I had read Nick say .035 years ago, but wanted to make sure. This is on an npi short block with 160k so not new parts and wanted to be careful. I assume yours were new builds?
Yes, they were new builds. Funny though, I also remember reading Nick say that same thing. I’d trust that man’s word any day when it comes to modulars. He was a little before my time, but his info is still around.
 

Notthenow

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Yeah, he was one of the pioneers with Johnny Langton RIP. They both shared a lot of info they came by trial and error.
 

96blak54

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Some sharp guys mentioned here ^

Be sure to use an adjustable lasher when degreeing. Some people make their own by gutting a lasher, welding a nut on it and use a rounded head screw to keep the followers roller on the cam lobe. An adjustable or make your own isnt the only way to achieve this but are the simplest.

A normal lasher will bleed out and is spongy and youll get a false reading.

You know if you send that cam back to Comp, tell them to make the exhaust opening at 34* BBDC....that thing will scream!

We here for you when you need us.
 
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Notthenow

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Thanks Blak, I am using the Trick Flow adjustable lash adjuster and setting at 0 lash plus .001. Tonight I will re-check my clearance at 111 and since the clearance closes quick, I hope it opens quick as well with retarding the opening point.
I had read that the exhaust port on the pi head was pretty active and the aftermarket ground the exhaust lobe to keep it more mellow. What does the change to 34* do?
They would be surprised to see my cams, I bought them in 2014...
 

Notthenow

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Finished timing the cams. Got both sides to 112 with the right side on #1 having .048 at 10* ATDC the closest and the left side on #6 at .043 at 10*ATDC.
 

96blak54

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Thanks Blak, I am using the Trick Flow adjustable lash adjuster and setting at 0 lash plus .001. Tonight I will re-check my clearance at 111 and since the clearance closes quick, I hope it opens quick as well with retarding the opening point.
I had read that the exhaust port on the pi head was pretty active and the aftermarket ground the exhaust lobe to keep it more mellow. What does the change to 34* do?
They would be surprised to see my cams, I bought them in 2014...
From what ive heard, comp receives old and odd cams for a regrind all the time. They the big dogs of the cam business.

Cams that are claimed " off the shelf ", like what you have, basically are base model cams of desired intake valve duration. When a special grind is desired, the cam closest to desired intake valve duration is pulled off the shelf and reground to desired duration and at most times just regrinding lobe seperation by altering the exhaust opening. Delaying exhaust opening adds that much more of combustion pressure on the pistons top.

Remember valve opening and closing point cut in half relative to the other lobe opening and closing cut in half is lobe seperation angle.
And when grinding the exhaust to a delayed opening 20*, that makes the lobe seperation angle 10* less. In example a 115* lsa camshaft ,regrind the exhaust 20* will make lsa 105*. The lsa refers to the camshaft geometry while the exhaust duration refers to the valve action relative to crankshaft degree.

The pi big exhaust valve and port can handle some serious evacuation while keeping pumping losses to a minimum. Meaning the piston speeds get so great that it begins to push the exhaust out on the exhaust stroke. Given the 4.6l BDC piston dwell time, a very delayed exhaust opening is possible.
 

Notthenow

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I had heard they would custom grind on request for a new order, didnt know they would do that on already finished product. Wasn't Bullet also doing some special grinds designed for MHS? Have you seen anyone run that spec? Duration and lift are easy concepts, the timing events get complicated quickly.
 

joe65

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Be sure to post that motor running when you're all finished up. I'd like to hear it.
 

96blak54

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The timing events do get complicated quickly! Add in other valve train quirks at higher rpm's. For example if you hard core drag race in a very strict class such as a stock engine class, you find the valve float and valve bounce and utilize it to stretch stock cam duration events.
 

Notthenow

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Sure will Joe, it's been down since October with suspension, exhaust, and engine work. A lot of work was similiar to past projects, but never this far into heads and cams so it's a fingers crossed moment when the key gets turned the first time.
 

96blak54

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Ehh...youll be fine. Youve lined out the timing. I have faith in you.

What head gasket did you end up with?
 

Notthenow

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I used the Mahle that crush to .043. My squish ranged from .045 to .051 so plus or minus .003 off stock (.012 in the hole plus .036 gasket). Both heads were at 42cc when I finished them. My first time porting, installing guides, and recutting seats, so a new area of engine work for me. Otherwise, the timing is basic in performing once you do it a few times (I recheck stuff multiple times until I trust it) even if the timing events are hard to visualize sometimes.

I checked the seal on all the valves after lapping and assembly by back blowing compressed air while the chamber had water in it so they should be good. All new valves, seals, springs, rockers. Lash adjusters.
 

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