Piston-2-Valce Clearance Question..

Wasim05 · Aug 2, 2004

I have some Edelbrock heads, that have been milled to 51cc for more compression. They will be going on a stock shortblock, so stock pistons also. Question is, since they are milled that much, how big of a cam can I use? I wanted to run the Crower 15512 cam, with 1.6's but I think that might be too much lift and I'll run into PVC problems. The total lift for the cam is .496"/.512". Too big? If not, what other cams would you suggest?

1Quik85GT · Aug 2, 2004

Wasim05 said:
I have some Edelbrock heads, that have been milled to 51cc for more compression. They will be going on a stock shortblock, so stock pistons also. Question is, since they are milled that much, how big of a cam can I use? I wanted to run the Crower 15512 cam, with 1.6's but I think that might be too much lift and I'll run into PVC problems. The total lift for the cam is .496"/.512". Too big? If not, what other cams would you suggest?

if they are 2.02 valves you will not have enough PVC.....if they are 1.90 they will work..

Wasim05 · Aug 2, 2004

yeah they are 1.90" valves... what about with 1.7 roller rockers?

Michael Yount · Aug 2, 2004

No one else can tell you if you're gonna have enough p to v clearance -- every combo is unique. THe only way to know is to measure; there's no way around it. Any other approach is just hoping/guessing -- ESPECIALLY with a big cut on the heads. Also, peak lift has absolutely nothing to do with p to v clearance - so you have a fundamental misunderstanding of what the dynamic issues are. You'd be wise to clear that up before you proceed lest you cause a bigger problem. At peak lift the pistons are well down the cylinder. P to v is small during overlap when the piston is passing through tdc and the intake is just opening, exhaust is just closing. See the link for more info. As a result, it's more duration/more overlap/lower LSA's that are likely to get you into clearance trouble, not more peak lift. However, with the lift you're talking about, you'd best check springs for bind, and for pressures to be sure they'll control float. http://www.diyporting.com/PVC.html

Did you cut your intake or oval the bolt holes so that it will fit with your milled heads? Did you check pushrod length with the shorter heads? Do you have shims to raise the rockers - also likely necessary with the cut heads?

Lots of stuff going on here - be careful.

Rick 91GT · Aug 2, 2004

Any head milled that much will definitly require the manifold to be milled on all 4 sides to even have a chance at matching up.

That is a lot to take off a E-brock head IMO. You want .080 minimum on intake and .100 on exh.

Michael Yount · Aug 2, 2004

Rick - I'd have him work closely with a reputable machine shop - I don't think the manifold to head interface will have to be machined; just the bottom of the intake to keep it from binding against the top (horizontal surface) of the block when being tightened. Best case - it won't seal against the heads because it can't go low enough - vacuum and water leaks. Worse case - it breaks the manifold when tightenting. But with that much cut, I doubt he'll be able to get the bolts in -- the holes in the intake will likely have to be "ovaled" out towards the inside (middle of the motor) so the bolts can enter the hole in the head. Milling the head moves the head lower and towards the center of the block. The cut on the bottom of the intake is usually about 70% of the cut on the heads. And even with that, a port match may need to be done to get the port tops and bottoms to line up properly.

Rick 91GT · Aug 2, 2004

Michael Yount said:
Rick - I'd have him work closely with a reputable machine shop - I don't think the manifold to head interface will have to be machined; just the bottom of the intake to keep it from binding against the top (horizontal surface) of the block when being tightened. Best case - it won't seal against the heads because it can't go low enough - vacuum and water leaks. Worse case - it breaks the manifold when tightenting. But with that much cut, I doubt he'll be able to get the bolts in -- the holes in the intake will likely have to be "ovaled" out towards the inside (middle of the motor) so the bolts can enter the hole in the head. Milling the head moves the head lower and towards the center of the block. The cut on the bottom of the intake is usually about 70% of the cut on the heads. And even with that, a port match may need to be done to get the port tops and bottoms to line up properly.

I agree 100%, work with a local machine shop that has expierence.

Michael, I know on a Victor 50 we had to mill all the sides when we took over .040 off the head, I guess it all depends on the manifold. It really helped on the port match, basically there was no rail in the front or rear of the intake any more.

Michael Yount · Aug 2, 2004

I'm with ya Rick - probably a good reason to work closely with a good machinist - they can measure/calculate and decide what needs cutting. I've even seen them conclude the manifold doesn't need to be cut when a simple port match would solve the problem. I guess one 'cut' doesn't fit all!

Rick 91GT · Aug 2, 2004

yeah...if it were only that easy

Wasim05 · Aug 2, 2004

hey guys, the intake is matched up to the heads. its a systemax II. i bought the setup from a friend of a friend. it came off a 95 gt, with a stock cam, and stock 3:08's and the car ran 13.3's at 109mph.. yeah 109 on a stock cam and gears.. trapped real good. thats a full weight car too. i know the setup will bolt on and all.. so thats not an issue. they are matched to work with each other... but back to my original question, will the crower cam work? with 1.7's it brings the total lift to .512/??? and according to a friend, that SHOULD work. but i need to know for sure. thanks for all the input guys.

Mhypertext · Aug 2, 2004

clay the heads and see

Wasim05 · Aug 2, 2004

dunno how to do that, whats involved???and how do you do it?

Michael Yount · Aug 2, 2004

Wasim - back to my original response....

"No one else can tell you if you're gonna have enough p to v clearance -- every combo is unique. THe only way to know is to measure; there's no way around it. Any other approach is just hoping/guessing -- ESPECIALLY with a big cut on the heads. Also, peak lift has absolutely nothing to do with p to v clearance - so you have a fundamental misunderstanding of what the dynamic issues are. You'd be wise to clear that up before you proceed lest you cause a bigger problem. At peak lift the pistons are well down the cylinder. P to v is small during overlap when the piston is passing through tdc and the intake is just opening, exhaust is just closing. See the link for more info. As a result, it's more duration/more overlap/lower LSA's that are likely to get you into clearance trouble, not more peak lift. However, with the lift you're talking about, you'd best check springs for bind, and for pressures to be sure they'll control float. http://www.diyporting.com/PVC.html"

Wasim05 · Aug 2, 2004

yeah that website is real useful... i dont have the came yet, but when i do ill go ahead and check on it. thanks for all you guys input.

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