Valve to Piston Clearance

[Schober Motorsports]

Hey guys,

Well here it goes. I have been looking out trying to find some info on how to measure piston to valve clearance WITHOUT having the cam. I have not been able to find a single way except in the Mopar High Performance Book where it mentions something about "Tappet at TDC".

Piston is going to be a .010 in the hole. Is there any way to measure the piston to valve clearance prior to buying the cam? Or at least give me an idea on what I can and can't run? Pistons have valve relief cuts .215 on Intake, .199 on the Exhaust.

Any advice and ideas are welcome, plus what do you guys run? This is for a bracket racing application, not so much for a street car. Thanks.

 

[lewtot184]

no way. due to different lobe profiles, ramp speeds, rocker ratios, valve sizes and even rod ratio the only way to do it is with some computer generated program.

 

[IQ52]

So are we to understand that the engine design has switched from a standard stroke 440 with dome top pistons to a 4.25" stroker with flat top pistons?

The ICON 828 has a generous fly cut and will accommodate a fairly large cam. How big a cam were you thinking of? What heads are you using? Have they been milled? Valve diameters?

 

[mopar 3 B]

no way. due to different lobe profiles, ramp speeds, rocker ratios, valve sizes and even rod ratio the only way to do it is with some computer generated program.

Why would lobe profile and ramp speed have any affect on max valve opening?
No need for a computers generated program a complexed spread sheet maybe.
Getting the correct measurements for the procedure would be the biggest problem.
What would it cost to blue print the cylinder and head cross section? Any machine work would throw factory figures off.

 

[dvw]

Intake opening point and exhaust closing point are the tight spots. Roughly 10 BTDC and 10 ATDC. Look at a cam card and you'll see. Also advancing the cam timing tightens the P/V @ int, retarding the cam tightens P/V @ exh. Deck height and head milling come into play as well. There's a difference between engine building and engine assembling. Remember if you have to much clearance only you will know, Not enough and it'll be front page news.
Doug

 

[lewtot184]

Why would lobe profile and ramp speed have any affect on max valve opening?
No need for a computers generated program a complexed spread sheet maybe.
Getting the correct measurements for the procedure would be the biggest problem.
What would it cost to blue print the cylinder and head cross section? Any machine work would throw factory figures off.

think about it. a faster opening ramp will let the intake valve chase the piston. a longer closing ramp will let the piston chase the exhaust valve. there's no way this can be guessed at. timing events, lobe separation, duration, rod ratio; to many variables.

 

[Schober Motorsports]

Haha I should of been a bit more descriptive.

505 CI
4.250 Stroke
7.100 Rod
1.485 Compression Height on Piston
Pistons are going to come to about .010-.000

Heads are going to Iron 452's. Aluminum heads aren't in the budget this year. Compression ratio with an estimate of 88cc combustion chamber is going to be 10.7-1. I also noticed that the IC828's have a real nice cut in em for the bigger valves that the trick flow has, another reason why I went with em.

As for the cam, I had something in mind around 260-270 duration at .050 with a lift 640-650.

I know the heads fall off at about .550 if you stare at flow charts all day... Thinking for the future when I upgrade the heads.

 

[383man]

Why even worry about it because even if you could figure it out you should still check it when the eng is being put together and if you find out its not enough you will deal with it then. Even if you had a way and it comes out saying it will be fine your still going to check it when your putting it together to be sure it has enough clearance and if it was figured wrong you gotta fix it then. If the piston has a good valve relief cut in them then at least you know it may come out ok. And as most have said its to many varibels to know for sure anyway. Just go with what you think will be good and then hope it works out when you check it on assembly. Ron

 

[mopar 3 B]

think about it. a faster opening ramp will let the intake valve chase the piston. a longer closing ramp will let the piston chase the exhaust valve. there's no way this can be guessed at. timing events, lobe separation, duration, rod ratio; to many variables.

Thought about it and finally makes sense.

 

[dieseldazzle]

I'm sure you could get into theory all day but nobody in their right mind isn't goin to measure once they finally have the cam so I don't understand why you would worry about it right now. If you see trouble on the horizon then beg borrow or steal a cam close to what you are gonna shoot for and try it

 

[Schober Motorsports]

This is my first "race" motor that I've put together, so I am just trying to do it right. I've done everything on this car so far with overkill on the planning, and its been pretty straight forward so far. Thanks for the help guys.

 

[66Satellite47]

No way to "guess" at piston to valve. You need to measure it. All cams are different. You need to use the dial indicator & see what you have.

 

[69 GTX]

I guess for a reference you could put the piston at TDC, measure the height of the valve. Then remove the valve spring and measure again. This would give you the interference # @ TDC and you could use that just as a reference # till you assemble it.....

 

[66Satellite47]

FYI, piston to valve is not at TDC. Depending on the cam profile, you to track it up & down both intake & exhaust. You need to measure it up & down on both sides.

 

[69 GTX]

I know that! I was merely pointing out what the min clearance would be. Hence interference @ TDC.

 

[66Satellite47]

69 GTX, the minimum piston to valve is NOT at TDC. At TDC it does not give any kind of reference. You need to follow the lobe & see what it shows. There is a BIG difference between TDC numbers & the actual minimum.

 

[69 GTX]

ok, I should say I was referring to max lift at TDC

 

[moper]

One probably could calculate it, plus or minus probably .030" due to slack and tolerance stacking... Not me because I suck at most higher math, but someone who's really good with trig and algebra. However you have to know the exact lift at the exact points the valves are closest. That is not typically found anywhere - you have to profile the cam lobes in which case it would be easier to just stick a rod/piston in the block and measure as part of your mock up.
BTW I think you'll be fine but you have to measure it.

 

[66Satellite47]

You need to profile the cam lobe. The piston to valve clearance varies with the cam profile. You need to check it!

 

[67 coronet]

your whole problem is not being able to get the exact specs you need to figure out when to measure the clearance.there is always diff thickness head gaskets if you are a smidge off.you can try to measure the valve in the head when extended to the lift you desire.then use a strait edge and check clearance to the deck of the head.compare that to the deck height of the piston valve relief when fully topped out.make sure to add the gasket thickness.