Technical Are valve reliefs on pistons actually needed?

I notice the volume of both 4 valve relief and 2 valve relief pistons are right around 7cc’s. (Looking at pistons on Summit) so seems the 4 valve relief pistons have a less volume per relief than two valve relief pistons do.
Do the valve relief propagate a better air flow or something?

I know the old low compression engines (Lheads and Flatheads) had no reliefs. Maybe wouldn’t matter at 6-7:1 static CR?

I used to think they were in pistons for the use of high lift cams. But after looking at specs it makes me wonder when running 9-11 :1 CR.
Thanks.

 

No need for reliefs in a piston on a flatly. The valves aren't over the pistons.

The higher the cam lift and longer duration the more the reliefs are necessary but only on overhead valve engines.

 

Is it for possible valve to piston contact then? Made me wonder why 7cc reliefs are fine on a 2 relief piston, but shallower reliefs on a 4 valve relief piston are fine as well?

Thanks.

 

It is absolutely possible. Part of the blue printing process when building a high performance engine with a high lift, long duration camshaft is to check the valve to piston clearance. We usually went for a minimum of .100 on the intake valve and .080 on the exhaust valve on a SBC.

Pistons with 4 valve reliefs were designed to be used on either bank of a V8 while pistons with 2 reliefs were usually higher performance replacements. That is why the reliefs were deeper for use with performance cams.

 

1955-56 Chevy 265s didn't have any valve reliefs. A lot of guys found out the hard way that you don't put a Duntov cam in one of those engines without either changing the pistons or cutting some reliefs in the existing ones.

 

• If you could ask someone that drove one of the original 265’s back in 1955 they would answer with a resounding yes. The first SBC pistons were flat top and when driven hard the valves were providing their own reliefs. The answer from GM was immediate, new pistons were fitted and they had four reliefs.

 

Granny's 307 2bbl that never saw the other side of 2500 on a tach didn't need em, but when her grandson got the car and floated the valves a time or 20 he was glad they were there...

 

When piston buyers see valve reliefs it makes them think "high-performance super pistons". It's like they just expect them to be there for the kind of money that they're spending. So from a marketing standpoint, they can add some sizzle to the steak and avoid a lot of questions. But it's also true that valve reliefs give a margin of safety for both the buyers and the piston makers for the times when somebody stuffs in a crazy/stupid cam.

 

Valve reliefs or no depend on the engine. Buick Nailheads, to overcome the small ports, used a longer duration cam. Add any higher lift with that duration, and the reliefs are a necessity.

 

y blocks however never had reliefs as far as i know

 

Its simply there to insure that there is space existing below a fully open overhead valve. When a higher lift cam is installed, or a rocker arm with a higher lift ratio (1.5/1.6/1.7/1.8) is used, the valve will open farther than it originally did. You don't want to lower the whole top of a piston to clear the valve, because it lowers your compression ratio and loses the power you gained with the cam or rocker arms.
On the other hand, if someone is building a supercharged engine, its normal to lower the compression by using a piston with a lower top..........and it may also incorporate valve reliefs if needed to clear the valves . Its kind of a juggling exercise to get the valves to not hit the pistons and still get the compression ratio you want/need.
Normally when installing a higher lift cam or higher lift rocker arms (or both), you put the piston at about 10 degrees before Top Dead Center, and then 10 degrees After Top Dead Center and have some clay on top of the piston. Then the open valve will leave a depression in the clay. Measure that and you know how much clearance there is between your valve and your piston. More is better..........

The reason you go 10 degrees before and then 10 degrees after is to check both the intake and exhaust valves, as they will be moving differently and you have to check both of them.

 

So why wouldn’t you check valve relief when installing a crazy/stupid cam (my favorite type) or any cam not original to the engine. I know you can install a cam without removing the head/heads but you’d only be saving the cost of gaskets also should degree the cam as cam timing can change the valve to piston clearance. Milling the head or decking the block will also reduce piston to valve clearance. Personally I never got close to minimum piston to valve clearance with any cam I’ve installed but I’ve never used a super high lift cam either. I don’t know much about flatheads but I would think that with enough lift the valve might smack the head.
Dan

 

keep in mind that advancing or retarding the camshaft makes a major difference.
what works at straight up my not work when the openings and closings are changed.

 

If you have ever floated the valves on a motor, those little factory notches in the tops of the pistons give you a different perspective.

The combustion chamber shape and the angles the valves are in relation to the tops of the pistons also makes a pretty big difference on the need for valve notches or not.

 

So circling back some. I mentioned it before, but why are 4 valve notches shallower than two valve?
Both (looking at .030 SBC pistons Weisco, Summit and KB seem to all be 7cc of volume 4 and 2 reliefs.
That’s what kinda struck me odd a bit.
Id ***ume say 7 cc for 2 and 14 cc for 4.
So I guess the reason for the notches have been explained, but valves (if floating) wouldn’t float less on a 4 valve relief piston.
Hope that makes some sense.

 

I don't know the actual answer so this is a guess. Given the discussion above and if two valve relief pistons are deeper then it may be that a two valve relief piston can accommodate a higher lift cam than a four valve relief piston. Also the depth of a four valve relief is probably limited so you don't loose to much volume and therefore compression ratio.

 

As has been mentioned, the valve position on overlap is where it's important. Performance pistons for higher compression will have deeper reliefs to compensate for dome height. If you were ordering custom pistons, the designer will ask what cam you are using. If it's an unknown, they will ask you for the depth of pocket needed. I had a 426 head with a chamber cut in half so I could see where I was at. The dome on a hemi is so tall that every dimension is critical to get compression properly. A flat top piston with a real small combustion chamber will have deeper valve reliefs to make up for the lack of space. In response to why aftermarket pistons have deeper reliefs, most builders are using different cams with more lift.

 

4 relief pistons can be on either side 2 reliefs pistons are rights and lefts

 

When buying somewhat generic pistons even tho they have a well known name on them, basically they sell thousands of these pistons, and people install them with no problem. They are doing mild or stock builds and these pistons are reasonably cheap, so they buy them. They usually work just fine.......and having reliefs on both sides makes them interchangable for both sides. Putting four deeper reliefs in a piston can lower the comp ratio more than desired/realized by a novice builder, so they put some shallower ones in to keep comp up...........but its still up to that builder, novice or not.......to insure there is piston clearance. If there is insufficient clearance, its the builders problem.

The two relief pistons provide more room for additional valve lift. By just using 2, they can cut deeper and still not affect comp too much. Sometimes, even those reliefs are not deep enough. Then the builder must make them even deeper by machining them with a special tool. Some pistons are designed with the the wristpin off center slightly. Those pistons should only be installed one way. There are lots of little things going on, so as someone starts building higher performance into an engine they consider these things when buying their pistons.

The more generic pistons are usually used in lower dollar builds and basically made to work with mild cam upgrades and be trouble free for most mild builds.

The two relief pistons will also work in mild builds but are usually a little more costly and allow someone to have room to increase the cam lift later on.........room for improvement so to speak.

Now this is just a general explanation, so I'm not saying its a perfect answer........

 

If the four-relief piston was machined (deeper) the same as the two-relief item, it wouldn't have a volume of 7cc, it would be larger and affect the compression ratio similarly. I would imagine if you look hard enough you'll find a four-relief piston with deeper reliefs, but the volume might not work.

 

Please , take the time to research the architecture of a flat head engine .

 

Yuppers the simple freaking answer is that those reliefs are clearance so the valve doesn't hit the piston.
I can remember that back in the 60's more than one guy I knew that had a tri 5 Chevy bought a cam and lifters for his car and he and his buds did a Saturday cam swap only to bend the valves when they fired it up because the stock two barrel pistons didn't have valve reliefs. I ground the seats on the heads of one of those engines in High school auto shop and put the new valves in.

 

Hello 2OLD2FAST I do understand that on a flathead the valve can’t hit the piston but can a valve hit the head with enough lift? I do need to study up on flat heads I think they are a**** the coolest looking engines produced I would love to have a flat head powered hot rod maybe someday.

 

Not enough info on the internet for you ?
Oh yeah , well Duh !

 

This is a case of OP taking a single point (valve relief volume spec) and not understanding the huge number of variables that went into the design decision. Do you ask why cast VS forged weights are close and which weight is better?
Valve reliefs are so that the valves do not hit the piston. This allows different cam lifts and durations while still providing decent compression. Engine builders pro and amateur will mix and match parts, including rods, valves, rocker arms and cams that all affect this clearance. Then there is the common surface machining of the heads and block. Piston makers will choose to make a single part that will cover the most widely used combos without requiring custom fit. A 'one size fits all' off the shelf part. The setup for machining a valve relief is not simple and also requires more material on the underside, making the piston heavier. For those builders who want an exact custom piston, they can spend much more.
The 2 VS 4 size question could have a dozen answers. One is that pistons may have an offset pin. Having 4 allows this to be selected for thrust/slap. Read up on Piston Pin Offset. It may be that the design team found this volume was the best compromise and used it for both. It could be that someone found or decided that the customers who chose 2 relief pistons often chose more valve motion and needed a bit more clearance or that this piston has some other dimension that made it need it.

 

A picture is worth a thousand words. this should show you how easily a big cam can make a valve hit the head (or spark plug).


This is why there are valve reliefs in a flathead, to allow clearance.

 

Don't always take the info you see online as 100% accurate. Yes, it should be, but, you can always call the manufacturer and get a spec sheet or have them verify the specs for you. Any company worth dealing with will do this for you.

It is entirely possible the 4-relief pistons' notch volume is identical to the 2-relief pistons' notch volume, but as noted by others, the 4-relief pistons' reliefs would need to be shallower, smaller in diameter, etc. to take up half as much volume.

As mentioned, some piston with four valve reliefs are are meant to be used on both the left and right sides of a typical V-type engine we are familiar with. Below is a such a piston, for an Olds 215 V8. with four notches in the piston's head, though this is only a two valve per cylinder engine. Note the angles of the relief floors are mirrored about the vertical center plane of the piston, again so that the angle of the relief floor will be correct for use on either the left or right side of the engine.



The small v-notch at the edge of the piston's head indicates that side of the piston faces forward when installed. It's difficult for me to tell on the above piston if the valve relief are in fact all identical, but if there were only two relief instead of four, the notches would be facing forward on one side of the block and rearward on the other. Not sure how much that would affect the dynamic balance once everything is together and rotating, but the cost of casting one set of pistons which can be used on both right and left sides is likely cheaper than creating unique left and right side pistons for every V-type engine the company sells pistons for.

Here are two videos showing the use of clay to measure valve-to-piston clearance on a V8:

 

more info with pics
https://www.enginelabs.com/engine-t...piston-design-trends-and-technology-for-2017/

 

I'm not taking advice from a girl! And why do my Flathead pistons have four valve reliefs?!

 

Because it is very likely that those same pistons also fit in an in head valve V8 motor.
If the bore, the ring spacing and ring groove width, and the piston pin height also match pistons from an already existing motor, you can bet the piston manufacturer is not going to make specific flathead pistons. You can probably blame your over bore and higher compression.

As far as not taking advice from a girl, didn't you ever take advice from your mother? Back in our time, she was a girl.