Computing piston to valve clearance from lift at TDC values?

Bumps440 · Post by **Bumps440** » Tue Dec 21, 2004 8:18 pm

I am trying to maximize compresion ratio in a smaller inch engine and am trying to compute the valve depths necessary for adequate P/V clearance. I was wondering how close you come to getting the right amount of P/V clearance from Valve lift specs supplied by cam manufacturers. Is this an accurate way to determine valve notch depth needed? Or is this just a rough estimation? Any help will be appreciated. Thanks
Bob

Post by **maxracesoftware** » Tue Dec 21, 2004 10:55 pm

for "BallPark estimation" then you would =>

measure the "Total-Valve-to-Piston Clearance"
at TDC and approx 10 deg BTDC or ATDC

Total-Valve-to-Piston-Clearance = this is measured by placing a 1" travel dial indicator zeroed ontop of valve stem...then with light spring or no spring..just by hand push down on valve until valve hits or rests ontop of piston at TDC and again at 10 deg ...this is your "Total-Valve-to-Piston-Clearance"

you then would lookup Cam's lobe lift at published @ TDC according to various CenterLines,
then pick a Lobe Lift @ TDC close to what you will be using...multiply your Rocker Ratio times the Lobe Lift at TDC, then subtract that from Total-Valve-to-Piston-Clearance....and after all that you will have an approximation....but it won't be accurate enough if you are trying to maximize your Comp Ratio

To maximize CR, you'll have to check Valve-to-Piston-Clearance with real valve springs installed ...and all valves lashed...and check each valve/cylinder out ....don't take this for granted if you are maximizing CR

you should be keeping records of the various Engines you build with Cam specs and things like;
Total-Valve-to-Piston-Clearance
Valve depths in Heads
Piston Valve notch locations, depths, Flycutter radius used, etc.

Bumps440 · Post by **Bumps440** » Tue Dec 21, 2004 11:48 pm

Maxrace.... thanks for the reply, and some new Ideas to work with. Maybe more info might be helpful, but I guess it will still be an estimation till I can actually mock it up with a piston I have laying around.

Intake valve depth above deck..... .085
Exhaust " " "......... .100
Total Deck clearance .040
Lobe lift at TDC .161 At 106*
.144 at 110*
Rocker ratio. .................... 1.6

LCA 108* installed intake centerline 106*

My calculations are that I need a .217 valve notch on the intake to allow .085 P/V clearance. Would this be true? Has anyone done this reliably?

If this is true, I dont know how to handle the exhaust side.

Thanks for the help

Bob

Post by **maxracesoftware** » Wed Dec 22, 2004 12:27 am

Total-Valve-to-Piston-Clearance

is "not" the same thing as Valve-to-Piston Clearance

at the very least for a better approximation,
you need to actually check your Engine's
"Total Valve-to-Piston -Clearance"

you need to atleast do this, because valves are at different angles to Bore axis

1.60 * .161" Lift = .2576" Lift at 106 CL
you would need .3426" Total Valve to Piston Clearance

1.60 * .144" Lift = .2304" Lift at 110 CL
you would need .3154" Total Valve to Piston Clearance

you would check Total Valve to Piston Clearance "without" any Rockers installed....checking only the actual valve travel...since its traveling at a different angle relative to the Bore axis, than just using how far Valve is above decks can cause inaccuracies ?

all this will be just an approximation at best of the real clearance, unless you have records and actual mockup experience of how much TVPC is needed with how much Lift at TDC with certain Cam CenterLines

Post by **maxracesoftware** » Wed Dec 22, 2004 12:33 am

if you have a junk Piston laying around with the correct Bore size and CH
..even if its a Flat-top...or cast piston...don't be afraid to "TIG-Weld" a dome on it if you like...
then custom fit it to your Chambers and valve notch depths/locations/angles

if you mess up a little..just re-TIG weld it up again

that way you have a permanent record of your specifc combo

you could UPS that junk piston to a manufacturer to digitize
better than a Mold sometimes !

Guest · Post by **Guest** » Wed Dec 22, 2004 8:05 am

If you are interested in getting a 'true' minimum PtoV, then I would recommend you do it the old fashioned way, and that's go at it a little at a time with BOTH clay and a dial indicator, and sneak up on the clearance that is acceptable to you.

If indeed you need an absolute minimum clearance, then a formula is never going to get you there. There are just too many dynamics involved in the ramp of the cam, the geometry of the rocker arms, and then all said and done, ultimately the real springs affect VtoP as well due to spring pressure affecting all the valve train parts.

Bumps440 · Post by **Bumps440** » Wed Dec 22, 2004 6:21 pm

Maxrace............ Larry, thanks for all the info you have given me, I understand all the theory behind it, Im not too concerned with the dome aspect of this, just the P/V clearance issue, Well anyway, I found an old Std bore flattop laying around so I will mock up this assy with the cam degreed and see EXACTLY what I will need to have for valve notches. The slight angular difference betwen the valve angle (15*) and bore shouldnt make that much difference in the mock up. I am concerned with the depths, not the valve centerline locations at this time.

I will record everything about this mock up to learn exact relationship between all factors involved. Again thanks for all your help.

Enjoy the Holidays !!!

Bob