Trying to decide which pistons for 440 build

[33 IMP]

The following quote is from the UEM catalog that BSB67 mentioned proves his point, and he is correct that Icon expresses a piston with a dish as a positive value making this a dished piston. I concede that this would not be considered a domed piston as I previously mentioned

• "Piston Head Volume and Deck Clearance. Values for Piston Head Volume and Deck Clearance can be entered as both positive or negative values. Negative numbers indicate a reduction in cylinder head volume. Pistons with pop-ups or pistons sticking out above the deck will reduce chamber volume and will usually require a negative number. Pistons with a dish or pistons below deck increase chamber volume and receive a positive value. Located above each picture in the KB Catalog is the value for the piston head volume shown in positive or negative numbers."

While I was wrong in this instance, the below quote from Summitt.com has a note at the end stating some manufacturers use the "+" and "-" values opposite of how Icon does. My mistake was assuming that all piston manufacturers used the same language in describing their products.


"Pistons are available with different head volumes. This is the size of the top of the piston. It is listed in cubic centimeters (cc).

We list the head volume in the way it affects the size of the combustion chamber. This helps when calculating the compression ratio. For example,

• A dished piston will increase the size of the chamber. So it has a positive volume (+).
• A domed piston will decrease the size of the chamber. So it has a negative volume (-).
• A flat top piston has zero volume (0).
• • A flat top piston with valve reliefs may list a small positive volume.

NOTE: This method of listing head volume is NOT standard in the industry. Some manufacturers may list a negative value for a dish and positive value for a dome."

The CR calculator I use (Wallace) uses a + number for a dish or valve reliefs, a - number for a dome.
Tricky is when a piston has a D-dish for reduced compression, and a quench dome on the other side , for quench with an open chamber head.

 

[Bigsbetter]

How much cam are you planning?
That's more compression than I would run on pump 91, without at least a 250° at .050 cam.
For 225-230°, I'd shoot for 9.5.

I’ve ordered the Comp 23-701-9 roller cam. 242/248 @.050. I’m hoping it has enough overlap to work out. I’m not real familiar with intake and valve timing to know if it will, but I do have a lot of options on advancing and retarding the cam to help. I know it’s going to be close.

 

[Iroc78]

I agree 33 Imp thats also how any compression calculator I've used works. I am just having a hard time squaring how a flat top piston with valve reliefs is given a "+" volume on a spec sheet, but I guess that's how some manufacturers do it. I do however understand why a dished piston would be given a "+" before it's value in the compression calculator. At least I learned something new today. I also know better than to get involved in the cam discussion !

 

[RemCharger]

The + is to add those cc's to the chamber volume. A dome would require a " minus" cc' s ..

 

[33 IMP]

I agree 33 Imp thats also how any compression calculator I've used works. I am just having a hard time squaring how a flat top piston with valve reliefs is given a "+" volume on a spec sheet, but I guess that's how some manufacturers do it. I do however understand why a dished piston would be given a "+" before it's value in the compression calculator. At least I learned something new today. I also know better than to get involved in the cam discussion !

Easy. The chamber volume above the piston INCREASES with valve reliefs, or a dish. Therefore, a positive result for the calculator.

 

[Splangj]

Easy. The chamber volume above the piston INCREASES with valve reliefs, or a dish. Therefore, a positive result for the calculator.

Exactly correct. Just like going from a smaller closed chamber head, to a larger chamber head, you INCREASE volume in the entire combustion chamber which lowers the compression ratio. If going for anything over 10:1, even with aluminum heads, try to get a good quench clearance, even if it takes more expensive pistons, you won’t regret it!

 

[Randy Wilson]

Stealth heads are 80cc

I would like to know if anyone ever had a set of stealths that checked in at 80cc -mine were 84 and I’ve heard others say they had 82-84

 

[Christopher]

9947 - Not a dome, it’s the volume of the valve relief.

Maybe the Speed Pro 2266 would work for you

The 2266 pistons are in the hole about .080. Avoid them. I literally removed a new set off an engine never run and replaced them with 2355's.

 

[BSB67]

The 2266 pistons are in the hole about .080.

Yep.

I literally removed a new set off an engine never run and replaced them with 2355's.

His compression will be on the edge with the 2355. IMO, the OP in this thread probably should not run a 10:4 CR pump gas motor. The 9.7 is better.

 

[33 IMP]

Yep.



His compression will be on the edge with the 2355. IMO, the OP in this thread probably should not run a 10:4 CR pump gas motor. The 9.7 is better.

Nothing wrong with a pump gas iron head 10.5 compression ratio...... as long as he's running a 250-260 degree at .050 cam. (Sarcasm)
I'd try to build a 9-9.5, at the most, with quench if possible. That would be a zero deck D-dish piston, and a closed chamber head.
Hard to do with open chamber iron!

 

[lewtot184]

Using a performance camshaft with more duration to solve a cylinder pressure problem isn't absolute. In fact i think it's not a good idea. The only way i can think of to achieve meaningful pressure relief is with a mechanical cam with asyemetrical lobes. With that kind of lobe you can make noticeable changes to the intake valve closing event by playing with lash. Lobe centers are also a factor but there aren't any exact results without experimentation. I think the bottom line is use the correct piston/ combustion chamber the first time instead of trying to chase the foggy bottom tail end with valve timing. If you have to use valve timing to solve cylinder pressure problems you've probably already lost the game. I've found that edelbrock 84cc chamber and an icon 836 piston is a very good pump gas friendly combo.

 

[dvw]

Easiest way not to make a mistake when calcuting relief/dome is put it the number in both ways. Once positive, once negative. The lowest answer will be with reliefs, the highest with a dome. Not every calculator formula does it the same way (though they should).
Doug

 

[lewtot184]

Easiest way not to make a mistake when calcuting relief/dome is put it the number in both ways. Once positive, once negative. The lowest answer will be with reliefs, the highest with a dome. Not every calculator formula does it the same way (though they should).
Doug

I think the most accurate way is to cc head and check down fill. Anyhow, that's what i've done in the past.

 

[Christopher]

Yep.



His compression will be on the edge with the 2355. IMO, the OP in this thread probably should not run a 10:4 CR pump gas motor. The 9.7 is better.

After decking, the 2355's were .017 in the hole, only took .005 off head 346 head and used a felpro .040 gasket. All the math shows around 9.4:1 CR.

 

[BSB67]

With a 75cc head? I doubt it. You probably have a 90cc head.

The OP has 75 cc heads, and you recommend the 2355 piston.

 

[Christopher]

With a 75cc head? I doubt it. You probably have a 90cc head.

The OP has 75 cc heads, and you recommend the 2355 piston.

Correct! It's simple math.

If its stock bore, 4.32, stock stroke 3.75, stock rod length of 6.76, and stock deck height 10.725, ...that leaves 2.090 compression distance. 2355's are 2.061 & have a 7cc vr's, leaving a .029 deck clearance. Using a basic compression calculator and the OP's 75 cc head data, a .040 gasket, thats 10.13:1. If he opted for a .020 gasket, which may be tough to find with an aluminum head, thats 10.6:1. Which I believe is in the vicinity of what he was looking for. Adjust accordingly with some Cometic gaskets. Should definitely double check the head CC's.

 

[BSB67]

Thanks for confirming I was correct.