Pistons specs question for 340 build

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70Hardtop

70Hardtop

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Hi all - can someone tell me what the -cc value refers to in the specs for the D.S.S Racing forged pistons? I am building a 1971 340 and wanting around 9.8 to 10.2:1 CR (any more than that, even with 98 octane, our fuel is sh** apparently in this country I've been told by foreigners). The two DSS FX1 pistons I'm looking at, one says -5cc and the other -16cc and gives the CR at some specified comb chamber volumes. Are these just referring to different compression heights? The minus should mean reducing the Comb Chamber volume but it seems to be making the CR lower (adding to the volume?).

I only have 360 J heads (1.88) so will need to get them milled for my desired CR (I'm very thankful to see the milling size chart for this in the Sticky section)

I have used KB Hypers several times before but would like to try the forged DSS ones, hoping they are lighter. I am very surprised that I can not see the actual weight of the piston in the specs. Why is that? If that was given then it would be easy to see at a glance the differences in weights between piston brands to help make an informed choice.

PS, I can't afford the SRP or Wiseco or JR pistons - around A$2500+
 
I would not mill too much off the heads because then you have intake fitting problems to deal with.
 
-cc is the positive pop up (EDIT: negative) portion most likely (seen 'em written both ways)
 
The -5cc pistons are likely flattops with a five ccdeep valve relief. The -16cc are likely dished. You will need to have the compression height of the pisons (measurement from center of piston pin to the flat part of the piston head) to have a vague idea of c.r.
There are a bunch of ACTUAL measurements that need to be done to get an accurate c.r.
 
brian6pac said:
I would not mill too much off the heads because then you have intake fitting problems to deal with.
Click to expand...

Just mill the intake side of the head .95 x block side mill. Or for an example, .020 from the block side of the head, and .019 from the intake side of the head.
 
brian6pac said:
I would not mill too much off the heads because then you have intake fitting problems to deal with.
Click to expand...
I will talk to my machine shop about machining the intake side, it's a better idea than milling the actual intake. But I have had some open chamber 273 heads milled a whole 60thou to increase the CR and don't need to mill the other surface. Just put a rat tail file through the intake holes and slightly open them up a bit and use RTV Gasket Maker instead of the front and rear intake wave seals. (It is a low 8.4:1 engine and using the original in-the-hole stock pistons) Which I'm glad about because it cost me A$200 each head to do that. But it was a low mile engine, never been pulled down so everything was almost like new inside. Just needed a hone and refit the original pistons
 
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If a piston has a negative piston head volume figure, that means whatever that volume is REMOVES total chamber volume. That's how domes are measured, with NEGATIVE volume. That's why when looking at a domed piston, or a piston whose head has a positive deck height its figure is written as a negative number. It should be figured into a compression calculator as a negative number, since it REMOVES total chamber volume through the displacement of the piston head into the chamber. When looking at flat tops with valve reliefs, or dishes, they will have a positive number, since their piston heads ADD TO the total chamber volume.
 
33IMP said:
The -5cc pistons are likely flattops with a five ccdeep valve relief. The -16cc are likely dished. You will need to have the compression height of the pisons (measurement from center of piston pin to the flat part of the piston head) to have a vague idea of c.r.
There are a bunch of ACTUAL measurements that need to be done to get an accurate c.r.
Click to expand...

Thanks, you are right, I looked at the DSS website, One is dished (must be for turbo & supercharged applications?) and the other is flat top which would have those valve reliefs. Summit Racing has it listed as -3cc but I assume that's a typo, DSS site says -5cc.

Does anyone know if the DSS FX1 forged are lighter than the KB HyperEu's? I wish they would list the weights and I'm also disappointed that they don't offer a domed piston to Chrysler people. That would be much easier way to get up the CR than go through the milling process.

I suppose you have to get custom pistons made up at expense if you want things like that, 'cos for Chrysler 340 there is only limited off-the-shelf stuff for pistons.
 
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66fs said:
Just mill the intake side of the head .95 x block side mill. Or for an example, .020 from the block side of the head, and .019 from the intake side of the head.
Click to expand...
Thanks for the numbers!
 
33IMP said:
The -5cc pistons are likely flattops with a five ccdeep valve relief. The -16cc are likely dished. You will need to have the compression height of the pisons (measurement from center of piston pin to the flat part of the piston head) to have a vague idea of c.r.
There are a bunch of ACTUAL measurements that need to be done to get an accurate c.r.
Click to expand...

This is incorrect. You have this backwards, as does "wherever" you found it. Let me give you these two examples off the top of my head. 351C ford.

KB148 shows a POSITIVE volume number since its dish ADDS TO the total chamber volume.

KB Automotive Hyper - Ford 351 / 5.8 - KB148

KB149 on the other hand shows a NEGATIVE volume number since it has a dome and REMOVES total chamber volume.

KB Automotive Hyper - Ford 351 / 5.8 - KB149

Care needs to be taken when talking about this stuff, because BIG mistakes can be made. lol
 
RustyRatRod said:
Another example using Chrysler pistons.

KB167 Flat top with valve reliefs. Has a POSITIVE head volume. ADDS TO the total chamber volume.

KB Automotive Hyper - Chrysler/Dodge 360 / 5.9 - KB107

KB191 Dome with valve reliefs. Has a NEGATIVE head volume. REMOVES from total chamber volume.

KB Automotive Hyper - Chrysler/Dodge 360 / 5.9 - KB191
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The guy shopping for pistons on DSS site says i'm right. I think the problem is semantics, depending on the compression ratio calculator. Wallace wants a negative number for a dome piston, a positive number for a dish or valve reliefs. Any piston that has around five cc's, TO ME, means a flat top with reliefs, whether its listed as (-) or not, so i assumed the -5 and -16 were both minus'ing chamber volume.
 
like I stated before, seen it both ways but the DSS website says the dish is -16..Seems most state a dish as a negative cc
"...-16cc Small Block Mopar Dish Top..."

Probe states a 360 dished is -16
Ross piston states 340 Forged Dome volume is +5.5 cc

Chrysler Dodge Mopar 340 Piston Sets
 
RustyRatRod said:
Another example using Chrysler pistons.

KB167 Flat top with valve reliefs. Has a POSITIVE head volume. ADDS TO the total chamber volume.

KB Automotive Hyper - Chrysler/Dodge 360 / 5.9 - KB107

KB191 Dome with valve reliefs. Has a NEGATIVE head volume. REMOVES from total chamber volume.

KB Automotive Hyper - Chrysler/Dodge 360 / 5.9 - KB191
Click to expand...
I think that "CROWN CC" is a rarely used measurement of the volume of the eyebrows, not the piston material. in the 4595 specs, the volume is actually 5 cc's of material REMOVED from the piston face... anyone else would state that's a -5cc ...That's whack!
 
33IMP said:
The guy shopping for pistons on DSS site says i'm right. I think the problem is semantics, depending on the compression ratio calculator. Wallace wants a negative number for a dome piston, a positive number for a dish or valve reliefs. Any piston that has around five cc's, TO ME, means a flat top with reliefs, whether its listed as (-) or not, so i assumed the -5 and -16 were both minus'ing chamber volume.
Click to expand...

He's wrong. It's nothing about semantics. It's called physics. The examples I gave are how pistons have been measured for 100 years. Just because you've found someone whose suddenly found a "better way" doesn't make him right. IT makes him more wrong, and you too if you follow him over the cliff.

Whatever you do, good luck with it.
 
pishta said:
I think that "CROWN CC" is a rarely used measurement of the volume of the eyebrows, not the piston material. in the 4595 specs, the volume is actually 5 cc's of material REMOVED from the piston face... anyone else would state that's a -5cc ...That's whack!
Click to expand...


Look at ANY online compression ratio calculator. The examples I gave from the UEM web site are how they ALL calculate compression. Why anyone would go against the math and risk calculating compression WRONG is beyond me, but again, good luck.
 
pishta said:
like I stated before, seen it both ways but the DSS website says the dish is -16..Seems most state a dish as a negative cc
"...-16cc Small Block Mopar Dish Top..."

Probe states a 360 dished is -16
Ross piston states 340 Forged Dome volume is +5.5 cc

Chrysler Dodge Mopar 340 Piston Sets
Click to expand...

Ross has it right. Any amount of piston head that REMOVES volume from the overall chamber is stated as a NEGATIVE NUMBER. It will not come out correctly when calculated for compression otherwise. Don't take my word for it, DO THE MATH!!!!! Use a positive number there instead and you'll quickly see IT AIN'T RIGHT! lol
 
Read this from Summit. It clearly states how to measure a dish and dome. Dish=positive number dome=negative number.

Piston Head Volume

It also states that those figures are not standard across the industry. Neither is intelligence.

That means to get the math RIGHT you MUST USE a positive number for any dishes or valve reliefs and a negative number for any domes or protrusions into the chamber, or your math WILL BE WRONG.

Just because someone uses one particular way to state a measurement doesn't make it right, I don't give a damn who it is.

So be damned careful when you measure volume, because it's easy to get wrong, especially with stupid people giving incorrect measurements for their products.
 
I say its a dished piston, the op says its a dished piston , DSS says its a dished piston , but you say we're all wrong.




Okay.
 
33IMP said:
I say its a dished piston, the op says its a dished piston , DSS says its a dished piston , but you say we're all wrong.




Okay.
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No. That's not what I said. AT ALL.
 
What I said is, if someone uses a NEGATIVE NUMBER in calculating a dished piston, THAT is incorrect.
 
RustyRatRod said:
What I said is, if someone uses a NEGATIVE NUMBER in calculating a dished piston, THAT is incorrect.
Click to expand...
DIRECT quote from Wallace racing compression ratio calculator:
Input a positive number for valve reliefs.
Input a negative number if it has a dome.
Maybe we can agree that following instructions for the particular ratio calculator you use is a good idea?
Edit: and yeah, if my figures tell me my dished piston is 12.8 to one, i can tell i did something wrong., lol.
 
33IMP said:
DIRECT quote from Wallace racing compression ratio calculator:
Input a positive number for valve reliefs.
Input a negative number if it has a dome.
Maybe we can agree that following instructions for the particular ratio calculator you use is a good idea?
Click to expand...

Evidently they calculate it backwards. Go to the UEM site and use theirs. I'll wait right here. Did you read the Summit link? It's not standard across the board.

I don't have a dog in the hunt other than to say MAKE DAMN SURE your math is right, because since the methods are not standardized, it's easy to make a mistake.
 
RustyRatRod said:
Evidently they calculate it backwards. Go to the UEM site and use theirs. I'll wait right here. Did you read the Summit link? It's not standard across the board.

I don't have a dog in the hunt other than to say MAKE DAMN SURE your math is right, because since the methods are not standardized, it's easy to make a mistake.
Click to expand...
Now THAT i can agree with! I did say earlier that actual MEASUREMENTS will be required if you want the calculations to be right. He cant just plug in numbers.
 
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