How can one determine what the clearance is BEFORE installing a higher lift cam? I don't know of a way. I'd hate to buy new cam only to find out after installing it that it won't work!! Is there a way?

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Willie

Supercharged 1987 305 IROC-Z, Daily-Driver, Emissions-Legal.
Former Paxton (6-psig) with 50-hp nitrous: 12.043 @ 112.86 mph.
ATI D1SC (10-psig): 12.056 @ 116.62 mph.
All stats are altitude corrected for 3,100 feet using NHRA's Altitude Correction Table.

http://willie.camaro-firebird.org/

1987 "20th Anniversary Commemorative Edition" Z28 Convertible -- Super Chevy Show Class Winner, 1998.

 

Well, just thinking about it, you could move a piston to TDC, measure the installed heigth..remove the springs and lower the valve until it contacts the piston, then measure the difference.

To be safe, you'd want measure each cylinder.

Then factor in the clearance spec, and do the math for the valve lift of the new cam.

Or just search for "tried and true" combinations

 

8Mike9 about covered it....

GM SBC heads don't have any hint of a clearance problem with any sort of cam that one might run on the street until around .600" of lift; and even then, it's not usually an issue unless the cam is installed "advanced" (intake valve already significantly open by the time the piston makes it to TDC) and/or small values of lobe separation (exhaust still open at that point).

What setup are you considering, maybe more specific information would help figure out what you need to know?

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"So many Mustangs, so little time..."
ICON Motorsports

 

i've checked a few with the head on by installing a set of checker type valve springs. then bringing the engine around to max lift. setting up and indicator on the valve and setting it to .7 or so then pressing the valve down with my thumb and seeing how much is taken off the indicator. i doubt you have much to worry about till .6 or so like rb said.

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ICON Motorsports
1st & 3rd
MM Black Diamond 538 F&AM

 

The cam isn't at max lift when the piston is at TDC. It's actually a bit advanced. The proper way to check it is by removing the head a few times. Put some modeling clay on the piston, reassemble the head and turn the engine over 2 revolutions. Pull the head and check how much the valves pushed into the clay. This "must" be done with solid lifters for that cylinder. You will never get an accurate reading with hydraulics.

Checking clearance without pulling the head or pulling the intake to use solid lifters will only be a guess. Having a valve hit the piston after a cam swap costs more than a few gaskets required to check it properly.

Duration also affects the clearance. Having a cam that just offers enough lift (Like mine) may work fine but it you change it to keep the same lift but use more duration, you may smack a valve into a piston.

So taking a reading from one cam doesn't mean much if you use a cam with more lift AND duration since the clearances will all be different.

Buy the cam you want to use then modify the engine to accept it. This usually means having the pistons flycut for extra clearance.

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87 IROC bracket car, 91 454SS daily driver, 95 Homebuilt Harley

 

Checking the lift on your current cam and taking the difference of your new cam and subtracting it from your reading wont work unless:

Both cams have the same duration and both cams are installed in the same position.

 

Like stated above, the easiest way is using clay. It can be done mathmatically, but you need the actual cam profile (along with timing and stroke info) and is not a straight forward problem for most. You might also be surprised to find that the exhaust (at least on my big block) was the problem, not the intake.

Miles

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88 427 Camaro
12.18 @ 113mph 1.75 60ft
Currently installing 3.73's into my 9 bolt (YES, it is a 9 bolt & YES they are 3.73's, not 3.70. I currently have 4.10 gear sets and new series 3 carriers in stock for 9 bolts )
www.koolmeister.com

 

Mine is the intake. Allowing a minimum of .100 clearance, my intake is .091. The exhaust is .139. I could put 1.8 rockers on the exhaust and max it out but I have no clearance left on the intake without flycutting the pistons some more or putting in some real race pistons like JE or SRP that have very deep valve pockets.

So until I'm ready to build another engine or bore this one out to .060 over, I live with the cam that's currently in the engine.

 

Im having the same dillema with the TFS Twisted wedge heads and moving to 1.6 ratio rockers.Builder has to flycut the pistons to clear the offset valves..I made mention to the builder to give enough clearance for 1.6 ratio rockers in the future but I dunno how to check for sure with the heads on.He did say that theres at least .200 valve to piston clearance

my cam specs
Comp Cams 08-305-8
.510/.510
220/230 duration at .050
114 LSA

Daz

 

Those TFS heads, especially the older ones (before the G2s) are famous for that problem. That's the specific example I had in mind when I qualified my earlier comment by saying "GM heads".

The max lift number by itself doesn't tell the whole story though. Max lift occurs with the piston roughly halfway down the bore, on both the intake and exhaust strokes. But as far as clearance, it's alot more important what the valve happens to be doing as the piston reaches TDC, at which point the exhaust is closing and the intake is opening. So, if your cam has high .050" duration and very fast ramps, either of which could result in one or both valves been a good way open as the piston passes through TDC, you could be in trouble even with low max lift values and low ratio rockers. On the other hand, if you have short .050" duration and slow ramps, you could have lots of clearance even with high ratio rockers and high max lift.

Altering the cam timing has an effect too. Retarding the cam means that the exhaust will close later than it otherwise would, i.e. it may still be significantly open when the piston reaches TDC; advancing the cam will similarly bring the intake valve closer to the piston.

This all adds up to why the only way to be sure you have adequate clearance is to assemble the motor far enough to directly measure it with the exact combination of parts you intend to use. There is no valid rule of thumb or any other way to predict this because there are too many variables.

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"So many Mustangs, so little time..."
ICON Motorsports

 

based on my experience, the only time piston to valve clearance comes into play is the overlap cycle. That is basically the only time when both valves are in the process of opening or closing and the piston is close to TDC.

You can sort of look at the valve reliefs in the pistons and get a general idea. Most aftermarket pistons have a trough that is about .26 inch or larger. Most factory pistons are barely .1 deep.



[This message has been edited by jcb999 (edited October 26, 2001).]

 

Does anybody have any experience with valve clearance putting Vortec L31 heads on a stock pistoned LO3 305? Will the larger intake clear with an LT4 cam with a lift of .480 with 1.6's?

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91 RS 5.0 TBI....LT4 cam....Edelbrock headers....3"Dynomax exhaust....5spd.... 3.08.....Ultimate tbi....afpr...ZR 255-50's...Koni's