Min Piston to valve.

Gary 540 · Post by **Gary 540** » Sun Oct 17, 2004 6:42 am

Hi all.
I should know this, but I am not sure how tight I can go.
What is the minimum piston to valve, Int ? Exh. ? I can get away with.?
The springs will be checked before and after the race meeting, and will be set up to comp cams specifications but engine has to survive 10 - 12 quarter mile passes without checking.
400 Chev, 4340 steel crank, H beam steel rod, forged SRP piston, cast iron dart heads, 2.100" intake , 1.600" exhaust, Stainless valves.
The cam is a solid roller 4-7 swap 268 - 274 106 lsa .668 lift using 1.6 rockers.
Also how thin can I machine piston tops when cutting vallve pockets without making them too thin, This engine is 14-1 and runs on race gas. Max ever Rpm 7000 and no more,
Thankyou,

tk · Post by tk » Sun Oct 17, 2004 10:11 am

Gary 540 wrote:Hi all.
I should know this, but I am not sure how tight I can go.
What is the minimum piston to valve, Int ? Exh. ? I can get away with.?
The springs will be checked before and after the race meeting, and will be set up to comp cams specifications but engine has to survive 10 - 12 quarter mile passes without checking.
400 Chev, 4340 steel crank, H beam steel rod, forged SRP piston, cast iron dart heads, 2.100" intake , 1.600" exhaust, Stainless valves.
The cam is a solid roller 4-7 swap 268 - 274 106 lsa .668 lift using 1.6 rockers.
Also how thin can I machine piston tops when cutting vallve pockets without making them too thin, This engine is 14-1 and runs on race gas. Max ever Rpm 7000 and no more,
Thankyou,

On the Hemi's we never ran less than .050" intake and .070" exhaust.
For the piston for N/A never thinner than .200" piston thickness.

Post by **maxracesoftware** » Sun Oct 17, 2004 2:06 pm

the absolute "minimum" on the Intake side = is usually the .005 to .010" more than your total Deck Clearance, in other words, the piston should be capable of hitting the decks harder than the intake valve .

the Intake valve will push down on and chase the Piston downwards,
so with the Pistons hitting the Decks harder, it will be very hard to bend the Intake valve.

of course, you don't want the Pistons hitting the Decks at all, or maybe very very lightly in some maxed out effort type applications ?

On the Exhaust side, like TK stated, its usually the very minimum of .065 to .070" if the valvetrain dynamics are really stable, and up to .090 to .100" if you really want to be safe !

Always check valve-to-piston clearance with the "Real" valve springs and every valve lashed on a fully assembled engine like you are going to Start the engine

Check every Cylinder

and when Flycutting, usually allow 1/2 to 1 degree difference in Flycutter angle -VS- piston notch angle for "Rocking" , depending on Piston-to-wall clearances....and add .050" to setting up Flycutter radius for wedges, and .060+" to radius for Canted valve.

for Hemi or canted valve, or all non-wedge heads, stake piston valve CenterLines "without" headgasket on block, with Piston at TDC, to allow
for stretch effects moving these CenterLines in a different location

always check piston's thickness "before" Flycutting in those areas

Guest · Post by **Guest** » Sun Oct 17, 2004 4:11 pm

Thanks TK & Maxrace, Im stressing out now !
I was going to run .030" deck height, and now may have to open that out
to .040" to help with piston to valve...

speedtalk · Post by **speedtalk** » Sun Oct 17, 2004 5:01 pm

You've got two of the best helping you so I don't know that I can help any. I have survived with .035/.050, but with less rpm.

Gary 540 · Post by **Gary 540** » Mon Oct 18, 2004 3:56 am

Thanks Don, I guess the next step is to measure the thickness of the dome top SRP pistons, cut valve reliefs & make sure there is still .200" left, and the rest will have to be made up in deck height.
I wonder if I can go a little lower than .200" and ceramic coat the piston tops
to compensate ?

la360 · Post by **la360** » Mon Oct 18, 2004 7:41 am

I know of some 5.0 Ford and Chev engine's in our V8 Supercar racing over here that have run as low as 0.030"/0.050" , but these were for short races only, and these engines are limited to 7500 Rpm.
AL...

Gary 540 · Post by **Gary 540** » Mon Oct 18, 2004 9:26 am

Thanks Al. I am going to take pistons in and get the reliefs cut so there is
.200" left and try and run a min of .040" and .060" dangerously close I know
but I cant run the deck ( quench) out at above .040" so I will just have to drop redline to maybe 6800, and be very watchful of valve springs.
Maybe even make the backwards step of going back to 1.5 rocker ratio.

Post by **maxracesoftware** » Mon Oct 18, 2004 2:19 pm

i've had Pistons with "Dime-size" area as thin as .090" and live ,as long as the Engine is perfectly tuned, no chance of detonation (no Nitrous)

but usually .140 to .160" thick is safe enough (no Nitrous)

.200" is more than enough thickness

airflowdevelop · Post by **airflowdevelop** » Mon Oct 18, 2004 2:25 pm

gary,
Reminder, keep an eye on piston to cylinder wall clearance, keeping an eye on piston rock. This can make or break you at these clearances.

I am also a bit confused as to why you need to run your quench so tight?

Dennis

Gary 540 · Post by **Gary 540** » Mon Oct 18, 2004 5:56 pm

Thanks Larry, now with those dimentions I have plenty of piston to valve.
I will also ceramic coat the piston tops to help.
Engine is no nitrous, and will be tuned using lambda approx 12.9 a/f in high gear.
Piston to bore is .0045".
Great forum this. I have been so stressed, but happy now.
Thanks.
As for tight Quench, the Engines I build, which are just for struggling friends always end up making far more HP than the parts in em suggest.
I dont know why but I honestly believe it must be combustion efficiency ???
All of my mates build with piston "0" to -.005" down the hole with a .041" felpro, where I always put piston + .010" out with a .039" felpro.
My engines need less timing than similar engines also.
Be interesting what others think ???

Gary.

70MC · Post by **70MC** » Tue Oct 19, 2004 12:09 am

I have built 3 race engines, all sbc's with roller cams. I have noticed when i use the soft checking spings to check valve to piston clearances and double checking with clay i get about .030 more clearance with clay. Also when i check my cam lift with a soft spring i see the same thing about .030 more lift than my cam card reads, but when i check my lift with a real spring i get the correct lift figures. I have seen this in the 3 engines i have built but don't have an explanation, has anyone else seen this? If someone else has seen it why does this happen, and if Gary is using the soft springs to check his clearance may he have to consider this also?

speedtalk · Post by **speedtalk** » Tue Oct 19, 2004 12:19 am

airflowdevelop wrote:keep an eye on piston to cylinder wall clearance, This can make or break you at these clearances.

Good point Dennis - exactly right.

Gary 540 · Post by **Gary 540** » Tue Oct 19, 2004 5:55 am

Hi 70mc, yes I am using very soft checking springs to check.
Maybe I have more than I think ??
I am very worried about winding posi's down till valve hits piston with 200+ lbs seat pressure, I have read this is the correct way to determine actual piston to valve, but I am worried I might bend a valve using this method ??
How do you do it Don ?

Gary.

70MC · Post by **70MC** » Tue Oct 19, 2004 1:07 pm

Test to see if you get more lift with the checking springs compared to the real springs. If you do get more lift that will also affect your valve to piston clearance. When i use clay i first lay aluminum foil on the piston then clay then foil again, that keeps the clay from sticking to everything (oil don't work). Then when you slice the clay for measuring, the foil also keeps your calipers from sinking into the clay. Then subtract the thickness of the foil from your measurment.

Don Terrill’s Speed-Talk

Min Piston to valve.

Min Piston to valve.

Re: Min Piston to valve.