are people ordering forged pistons with increased valve relief?

[acannell]

just curious. havent done the measurement yet, not sure if the higher lift cams would require additional valve relief and if so how much. I would think so otherwise that would mean the stock pistons have too much relief wouldn't it?

[Shadow]

Only for Very high lift cams AFAIK. (above .550 lift)

[4 l-bodies]

Asa.
It depends on what exactly your talking about. Piston to valve interference in running motor, or piston to valve if belt breaks and valve hangs wide open? At .500 valve lift, I've seen many engines that are interference. Depends on how far in or out of hole pistons are, size of valves, headgasket thinkness, etc. If having custom pistons made, what size valve reliefs are being cut into pistons, stock, +1mm, big valve, etc?
Todd

[acannell]

Asa.
It depends on what exactly your talking about. Piston to valve interference in running motor, or piston to valve if belt breaks and valve hangs wide open? At .500 valve lift, I've seen many engines that are interference. Depends on how far in or out of hole pistons are, size of valves, headgasket thinkness, etc. If having custom pistons made, what size valve reliefs are being cut into pistons, stock, +1mm, big valve, etc?
Todd

Todd, do you think there is any consequence to increasing valve relief say 0.050" besides a very small increase in combustion chamber volume and maybe some minor effect on chamber behavior?

I dont have a specific build in mind, I'm only asking generically. When I order mine I'll have to measure what I'm putting together as you describe.

One question would be, are people unknowingly creating an interference engine when they buy one of the ~ 0.5" lift cams and forged pistons?

[83scamp]

I would think the stock valve reliefs would be fine. When I rebuilt my N/A 2.2, I cut the block .030", cut the head .015", I'm running the MP "767" cam with .460" lift, and I'm running the stock flat top pistons with no reliefs. When I measured my valve clearance using clay and a dummy solid lash adjuster, I still had plenty of clearance. My pistons stick out of the hole about .012". This is on a non-common turbo block with a G head...

[OmniLuvr]

G head...

this is the kicker, what head are you going to run?

and as stated above, a lot of factors...

headgasket thickness, material removed from head and/or block, style of pistons, type of head, lift on camshaft.

and again, are you worried about interference with a "running" engine (will valves touch piston because of the large amount of lift during the normal 4 stroke cycle with correct valve timing) or an interference with the piston to valves during a timing belt breaking...

[Warren Stramer]

With .605 valve lift and 310 deg. seat duration, and a 1.731 dia. int. valve, I still have .395 Int.valve to piston clearance and .471 exhaust V/P clearance. (G head)
If my timing belt breaks piston will just barely kiss the valves. My valve reliefs are 1.815 diameter x .085 deep @ 7.5 degree tilt from bore center.
The cam duration has as much influence on V/P clearance as lift. (assuming it is close to being in phase)
Put a valve in a head, lift the valve off the seat about .585,(or what ever you wish your max lift to be) measure valve stick-out from head deck surface, that is the minimum interference distance for that amount of valve lift, and valve diameter. Hope that makes sense.

[acannell]

With .605 valve lift and 310 deg. seat duration, and a 1.731 dia. int. valve, I still have .395 Int.valve to piston clearance and .471 exhaust V/P clearance. (G head)
If my timing belt breaks piston will just barely kiss the valves. My valve reliefs are 1.815 diameter x .085 deep @ 7.5 degree tilt from bore center.
The cam duration has as much influence on V/P clearance as lift. (assuming it is close to being in phase)
Put a valve in a head, lift the valve off the seat about .585,(or what ever you wish your max lift to be) measure valve stick-out from head deck surface, that is the minimum interference distance for that amount of valve lift, and valve diameter. Hope that makes sense.

thanks for the edit, I was frantically searching for where this universal constant "0.585" was coming from lol

curious why you didnt make the reliefs just a little deeper so the valve piston kiss would be avoided if the belt broke?

[Warren Stramer]

thanks for the edit, I was frantically searching for where this universal constant "0.585" was coming from lol

curious why you didnt make the reliefs just a little deeper so the valve piston kiss would be avoided if the belt broke?

Everything in an engine is a juggling act, change one thing it effects five others it seems. The piston blank I used would get thin in the ring land area, and crown if I had gone deeper., also I used an asymmetrical skirt design with offset pin and the piston was already light on the thrust side even with the pin offset toward thrust. I also did not want the extra volume of the deeper reliefs. I have never seen a timing belt break that was not very old or rotten so I took a chance on an interference engine. You have to compromise sometimes.

[4 l-bodies]

Todd, do you think there is any consequence to increasing valve relief say 0.050" besides a very small increase in combustion chamber volume and maybe some minor effect on chamber behavior?

I dont have a specific build in mind, I'm only asking generically. When I order mine I'll have to measure what I'm putting together as you describe.

One question would be, are people unknowingly creating an interference engine when they buy one of the ~ 0.5" lift cams and forged pistons?
Asa,
I wouldn't worry much about volume increase caused by deeper reliefs. Wouldn't be more than a cc or two. That is the beauty of custom piston orders, you can get what you want. One look at a Masi piston and you would never worry about combustion chamber flame propagation issues again with your 8V pistons (lol). minnows could swim in the dish and valve reliefs! As Warren stated, cutting too deep will weaken ring land and crown of piston might get to thin.
Oh I'm sure there are few interference 2.2 swirl head motors running around. Owners may or may not know that depending on whether they assembled the engine or bothered to check.

[4 l-bodies]

I have never seen a timing belt break that was not very old or rotten so I took a chance on an interference engine.
I lost a timing belt coming back from SDAC 2-3 years ago. Driving along interstate just outside Eau Claire, WI when belt sheared at least 5/8 of all it's teeth off! Outside of belt looked like new, inside of belt not so much. Belt had maybe a couple/few thousand miles on it. I chalked it up to maybe a NOS belt that was made long, long, ago. Pushed vehicle off the interstate and changed the timing belt in a restaurant parking lot.

[black86glhs]

I ordered my last set of pistons with Alka Seltzer. That way I would have all the relief I need!!!!:banana-mario:

[acannell]

I lost a timing belt coming back from SDAC 2-3 years ago. Driving along interstate just outside Eau Claire, WI when belt sheared at least 5/8 of all it's teeth off! Outside of belt looked like new, inside of belt not so much. Belt had maybe a couple/few thousand miles on it. I chalked it up to maybe a NOS belt that was made long, long, ago. Pushed vehicle off the interstate and changed the timing belt in a restaurant parking lot.

You know that little clamp that holds the metal fuel lines of the fuel rail and holds them to the valve cover bolt?

And you know that little screw that pinches it together?

Well I dropped it by accident. I looked for it for probably 10 hours and couldn't find it. I tried EVERYTHING to find it..lights, mirrors, magnets...by inductive reasoning I eventually concluded that it was almost certainly in the little trough underneath the crank sprocket where I can neither see it nor retrieve it without taking the entire engine apart to get that aluminum retainer off. In hindsight I probably could have somehow attached a sponge or something to the timing belt and somehow pushed it out that way, perhaps.

Anyways, since big time engine disassembly was my only option, I started it up.

Guess what happened a few miles down the road.

EDIT: Actually maybe I could have just taken the crank sprocket off. I forget. Whatever it was, I really did not want to do it for some reason.

- - - Updated - - -


I ordered my last set of pistons with Alka Seltzer. That way I would have all the relief I need!!!!:banana-mario:

Those work great for HEAD aches too dont they??

[OmniLuvr]

so warren, what "style" piston top did you use? dished, flat or something in between?

[black86glhs]

Those work great for HEAD aches too dont they??Yes, but don't swing the piston too hard or it will cause more pain when it makes contact with your head.:thumb:

[Warren Stramer]

so warren, what "style" piston top did you use? dished, flat or something in between?

They are a very shallow -7.1cc hemispherical dish.

[OmniLuvr]

so almost like the stock g-head style dish (but maybe shallower)?

[Ondonti]

When I got my set of forged venolias they were made without the valves existing and with a larger lift cam and +2mm valves, had slight interference. I had the machine shop flycut my pistons because I was paranoid back then. Now I brazenly run flat top pistons at higher rpms and don't care. How many brand new Neons bent their valves vs 100-200k mile motors that didn't get serviced? With longer warranties you would think they would be more paranoid but that is not happening.

As Warren said, you have to consider crown thickness and ringlands, and then balance/combustion etc of the piston if you are really getting that picky.

[5DIGITS]

There's several items of consideration.



Has the head been milled?
Has the block been decked?
What head gasket is being used on the application?
Are the pistons being run out of the hole/above the deck (taller compression height)?
Are larger valves being used that would require larger diameter relief cuts?
The already stated concern of what lift does the cam support?
What is the installed cam center-line?
Are a multiple or all of the above being used requiring a significant stack-up relief cut in the piston?
Will the above combined result require a unique piston?