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Biggest cam with flat-tops-no reliefs


madkaw

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You here it all the time-"SEARCH", but sometimes that takess a lot of time and reading.

My recent search was regarding deck height and cams. I am currently running p-30 with an early e-88. .005 shaved of the deck with stock .040 over pistons and the head shave .012. At the time of the rebuild I was very new to these engines and relied on my machinist, with the help of 'How to Modify' book in his hands. Now I am doubling back to see where my engine is as far as quench and other data. My build sheet never specified actual deck height , so I can only figure it out by what I know and what I have read on these forums.

What I have found is that average deck height is .019 with stock flat-top pistons, minus my .005 cut and a .047 Nismo

head gasket puts me at .022 head to piston clearance. From what I read, my machinist nailed it, by luck maybe, not sure.

 

The reason for all this search was more for finding the max cam for flat-tops and still have valve clearance. I know that measuring actual clearance is prudent, but I was looking for ballpark examples. After jumping from one link to another in different threads I found the ideal piston/head clearance and this post

 

1 fast z

 

 

HybridZ Supporter

 

 

 

 

Group:Donating Members

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LocationPhoenix, Arizona

 

 

Posted 25 July 2007 - 06:02 PM

 

Ill ad some imput on piston valve relifes. I run a .585, 286 duration cam, on my NA car, with shaved P90, yada, yada. I measured piston to valve clearence on my last install, which was with a 270, 280 duration .475 lift cam, and had .215" EX valve clearnce, and .115" intake. I had then installed the BIG .585 cam about a year later and had .120" on the EX, and .050" on the intake, and have run it this way for about 5 months now. 8000RPM plus and no tapping. I do run titanium retainers, light valves, etc. Just some imput. This is on a DIRECT flat top, NO dish, and .022" piston to head clearnce also.

 

 

bandmzcars.com

 

This pretty exciting info for me because I want to move up from my "wussie cam", as Jon calls it, to something that takes advantage of my triples.

 

Not saying this is the definite answer, but it shows you there is a wealth of info in this threads if you pursue it.

 

Love to hear from others that have run a BIG cam with there flat-tops with no reliefs.

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  • 2 years later...

Kris is right. Duration and when the valve is open is the killer, not lift. Years ago BRAAP showed me a tool to fly cut the pistons without removing them. It was a tool that fit into the valve guides and you would just hook a drill motor to it and then cut the piston right there. Tape over the piston and bore area, and then just vacuum/blow out the aluminum bits. Still have to take the head off, of course. I seem to recall you can make your own tool by simply notching the rim of an old valve and just use that.

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You also need to take into consideration the dwell time of the piston near TDC. This changes with rod/stroke ratio, so it's not always the same, either.  The area of concern is basically the overlap period of the cam, and how that coincides with the travel of the piston as it approaches and leaves TDC.  The piston is tracing out what essentially looks like a sine wave (it's not exactly a sine wave but close enough for illustration purposes), and the "flatness" of the top of that wave is determined by rod/stroke ratio.  The longer the rod, the flatter the top of the curve, the longer the piston hangs around at the top of the bore, the more chance you have to crash into it...

 

So essentially this means that there is no good way to say what cam will definitely work and what won't without knowing a lot of information about your particular engine.  This is why most people just figure this part out empirically.

Edited by TimZ
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Ok well,what if the bottom end is fully stock, 0 miles and the only mod to block was a set of flat top pistons sitting say .17 thou above deck.n47 shaved .10thou 480 lift cam,lots of over lap. Considering all of this, would the over lap be the determining factor as to P2V clearance?

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No.

 

You need to physically measure the piston-to-valve clearance as it will vary according to the cam timing, the lift doesn't matter so much. A stock cam will have the valves hit the pistons if it's not timed correctly...

 

The only way to know how much piston-to-valve clearance you have, is to do a shit-ton of somewhat advance trigonometry, or measure it in the assembled engine.

 

Use a set of checking springs, and roll the engine through a complete valve cycle, using a dial indicator and pushing the valve down with your fingertip to feel for piston contact is a viable method if you are NOT going to tear the engine down. It's not exact but it is "good enough"

 

If you aren't opposed to pulling the head, claying the piston dome, bolting the head back on with a used headgasket and rolling it through the cycle will give you exact measurements.

Edited by Xnke
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The only way to know how much piston-to-valve clearance you have, is to do a shit-ton of somewhat advance trigonometry, or measure it in the assembled engine.

 

Exactly - it's also worth noting that in order to perform the shit-ton of advanced trig that you need in order to calculate the clearance, you would need to disassemble the engine and take a lot of precise measurements, since you have absolutely no other way of knowing exactly what machining has been done on your motor in the last 35 or more years with suitable precision.

 

Again, THIS IS WHY YOU JUST HAVE TO MEASURE IT EMPIRICALLY.

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