CR calculation, deck height/effective dome volume?

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fshd4it

fshd4it

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I'm trying to figure the compression ratio on my 340, and since this is the first time I've worked with pistons that stick up above the deck I want to make sure I'm figuring the volumes correctly. The pistons are .030+ speed pro L2316F, flat top w/ 2 valve reliefs. They are out of the hole by .036" at TDC, so I used the .250" down formula which gave me a volume of 55.5 cc. Area (13.01") X .250" gives me 53.3 cc, so I ended up with a volume of +2.2 cc ( I think the eyebrows are large enough to more than make up for the height above deck).The formula I have uses both DHV and EDV, but am I right in thinking I can just use the 2.2 as the DHV and run the numbers? Or do I need to go back and measure both, which I'm not sure how to do? Thanks in advance, Karlin.
 
I do it at 1/2 inch but either way it comes out the same.

Don't know what formula your using, but I think you are a couple of cc's off. You have to account for the amount of piston out of the hole.

So, .036 x diameter of piston etc and add to your dome volume.

The easiest math is in the Chrysler book.
 
Just working out the volume above deck for the .036" above deck, I get 7.7 cc's above deck with no eyebrows, and since the eyebrow volume is likely in the 5 to 6 cc range, then your 2.2 cc number looks about right. If you want to use this number in a formula, then treat it like a domed piston with deck height = .000" and a dome volume of 2.2 cc's.

However, there are easy tool to compute the SCR without all of this rigamarole. You can just directly enter numbers like the +.036" deck height and the eyebrow volume, etc.
See here for the one I like: Dynamic CR

BTW, .036" deck height is not normal for a 340 piston. If it is a stock-like replacement (which I would expect your Speed Pro pistons to be), then the deck height would be around +.018". Are you sure you measured this right? Or has the block been decked?
 
YR, thanks for the reply. The formula I've always used is CR= SV+CCV+HGV+DHV+EDV divided by CCV+HGV+DHV+EDV. Since I've never had domed or above deck pistons, i've just ignored the EDV. The only Chrysler engine book I have is the old DC one, and it doesn't go into great detail about formulas, but it does refer to the 1/2 or 1" down method. I'm just not understanding how pushing the piston down .250", filling the cylinder, and subtracting a volume equal to the mathematical volume of area x 250" (Larry Shepard compares it to a "disc") isn't correct. Which lets me ignore the negative EDV number, at least in my case. Thankyou for your time and patience.
 
Yep, she's ben decked. I'm thinking .020" from my piston measurement (and I measured all 4 corners to make sure the block was square), but since this motor came my way with most of the machine work already done, that's just my guess. With a static CR I've figured at 10.4-1 (my X-heads measure 63.6 cc) I'm wondering about ways to lower it now.
 
In your calculations, set DHV (deck height volume) = 0 cc, and SUBTRACT EHV = 2.2 cc Since you have positive net piston head volume sticking above deck, it will subtract from the chamber (CCV) + head gasket volume (HGV) when the piston is at the top of the stroke. (EDV? Does E stand for extra, external, extrapolated, extraterrestrial, egg-shaped? LOL)

Using my preferred calculator, and a .051" head gasket with a 4.17" hole (standard Felpro type), I am getting 10.6 SCR using your measured numbers for the piston 'dome'. So we are close.

OK on the decking.. that all makes sense. Are your 'X heads' the Indy-X types or Mopar X heads? If the latter, it looks they have been milled around .050-.060" to get to a 63-64 cc chamber volume.....yes? no? If they are Indy-X heads, then a close look needs to be taken at the head gasket thickness to insure adequate piston-to-head clearance.

Is there a reason to lower the SCR? What is the use for the engine? Street? Cruise? Pure drag race? Any info to share on the cam? Depending on this info, maybe the SCR needs to be kept as is...
 
nm9stheham said:
Just working out the volume above deck for the .036" above deck, I get 7.7 cc's above deck with no eyebrows, and since the eyebrow volume is likely in the 5 to 6 cc range, then your 2.2 cc number looks about right. If you want to use this number in a formula, then treat it like a domed piston with deck height = .000" and a dome volume of 2.2 cc's.

However, there are easy tool to compute the SCR without all of this rigamarole. You can just directly enter numbers like the +.036" deck height and the eyebrow volume, etc.
See here for the one I like: Dynamic CR

BTW, .036" deck height is not normal for a 340 piston. If it is a stock-like replacement (which I would expect your Speed Pro pistons to be), then the deck height would be around +.018". Are you sure you measured this right? Or has the block been decked?
Click to expand...


Actually, the .017-.018 positive deck height was where the factory wanted it. Most were way lower than that.

If you had heads that had chambers on the low side of volume, rods on the long side of tolerance, Pistons with the pins low on the tolerance, then, if you were lucky and the decks we cut so you had .018 positive, you could get close to the 10.5:1 CR the 340 was advertised at. If you were real lucky, you get everything on the end of tolerance for max CR, you could end up at about 10.75:1...if you were lucky.

The most I'd stick one out is .075 and that would be with .055 gasket. If the gasket is thinner, you can move the piston down accordingly.

Right now I'm .045 out on mine, so his isn't unheard of.
 
fshd4it said:
YR, thanks for the reply. The formula I've always used is CR= SV+CCV+HGV+DHV+EDV divided by CCV+HGV+DHV+EDV. Since I've never had domed or above deck pistons, i've just ignored the EDV. The only Chrysler engine book I have is the old DC one, and it doesn't go into great detail about formulas, but it does refer to the 1/2 or 1" down method. I'm just not understanding how pushing the piston down .250", filling the cylinder, and subtracting a volume equal to the mathematical volume of area x 250" (Larry Shepard compares it to a "disc") isn't correct. Which lets me ignore the negative EDV number, at least in my case. Thankyou for your time and patience.
Click to expand...


Your math looks hard to me. It's really vbdc/vtdc. So, add up all the stuff you have, like chamber volume, head gasket and dome/dish volume. Then determine the volume of your cylinder. Then add that to your other volume an and device the two.

I'm pretty simple, so I like simple math, also here is what I'd say it looks like, in easy math.

The total of your volume is 100 cc's for the chamber. The total cylinder volume is 900 cc. Add the two together for 1000 cc's
Now it's simple math...1000/100 equals 10:1

As to the disc part of the Shepard book, you have to account for the flat part of the piston that sticks above the bore. Since my piston is .045 out, once you do the downfill, you have to calculate the flat of the piston.

So the math would be bore squared times 12.87 times the stroke, or thickness. It's the same math for figuring gasket volume.


So, bore squared is 4.04 times 4.04 equals 16.3216 times 12.87 equals 210.05899, then times the stroke, or thickness, or positive deck, times .036 equals 7.5621236 cc's for just the .036 that is out of the bore.


Now that I have totally screwed up your day with math, I'll leave you to your calculations!!!

Really, the math is much more simple in the Chrysler book. You just need to account for the "disc" that is the piston out of the hole. You calculate it like gasket thickness.
 
nm9stheham said:
In your calculations, set DHV (deck height volume) = 0 cc, and SUBTRACT EHV = 2.2 cc Since you have positive net piston head volume sticking above deck, it will subtract from the chamber (CCV) + head gasket volume (HGV) when the piston is at the top of the stroke. (EDV? Does E stand for extra, external, extrapolated, extraterrestrial, egg-shaped? LOL)

Using my preferred calculator, and a .051" head gasket with a 4.17" hole (standard Felpro type), I am getting 10.6 SCR using your measured numbers for the piston 'dome'. So we are close.

OK on the decking.. that all makes sense. Are your 'X heads' the Indy-X types or Mopar X heads? If the latter, it looks they have been milled around .050-.060" to get to a 63-64 cc chamber volume.....yes? no? If they are Indy-X heads, then a close look needs to be taken at the head gasket thickness to insure adequate piston-to-head clearance.

Is there a reason to lower the SCR? What is the use for the engine? Street? Cruise? Pure drag race? Any info to share on the cam? Depending on this info, maybe the SCR needs to be kept as is...
Click to expand...
EDV is the Effective Dome Volume, and as I understand it, normally becomes a negative number for the part of the piston that sticks above the deck. I used United Engine & Machine's on-line calculator with the numbers I generated, as well as .036 EDV and a valve relief of 7.5cc ( a figure I found for these pistons) and it produced 10.2+ and 10.3 respectively. I used a 4.180 x .040 gasket, at 8.9cc every time. My heads are the factory type, with Ferrea nail head valves and hardened seats installed. At first I thought I had confused my start amount in my burette (20 vs. 25), but after checking 3 more chambers, they are all within .3cc. I used the blueprint pictured when measuring the chambers, and they came out at .706 to.711. It looks like the heads have only had a cleanup? I haven't picked a cam yet, was thinking a pump gas friendly DCR (low 8's?) would play a big part in that. The motor will be for a street-only car (if I keep it).

FABO001_zpsc596184d-2.jpg
 
fshd4it said:
EDV is the Effective Dome Volume, and as I understand it, normally becomes a negative number for the part of the piston that sticks above the deck. I used United Engine & Machine's on-line calculator with the numbers I generated, as well as .036 EDV and a valve relief of 7.5cc ( a figure I found for these pistons) and it produced 10.2+ and 10.3 respectively. I used a 4.180 x .040 gasket, at 8.9cc every time. My heads are the factory type, with Ferrea nail head valves and hardened seats installed. At first I thought I had confused my start amount in my burette (20 vs. 25), but after checking 3 more chambers, they are all within .3cc. I used the blueprint pictured when measuring the chambers, and they came out at .706 to.711. It looks like the heads have only had a cleanup? I haven't picked a cam yet, was thinking a pump gas friendly DCR (low 8's?) would play a big part in that. The motor will be for a street-only car (if I keep it).

View attachment 1714991802
Click to expand...


Is the EDV calculating the positive deck height? You have to account for that, and the downfill won't do it.
 
Sorry, I should have said that I entered -.036" for what they are calling Deck Clearance and I used 7.5cc for the Piston Head Volume. I'm thinking the 7.5 would be if the piston was theoretically flush with the deck?
 
I don't like the online calculators because 99.9999999% of everything built is zero deck, or negative deck volume.

I just do the math. I think about it like a Chevy. A domed piston on a Chevy, with a zero deck doing the downfill method will actually calculate the dome minus valve notches. If it has a positive deck, you have to account for the actual diameter of the piston that is out of the deck, however far that is.
 
Looking at the UEM caclulator, the Piston Head Volume is the volume of a dish into the piston top, or the valve relief volume in a flat top. It does not vary regardless of where the top edge of the piston ends up relative to the deck.

And you are right to use -.036" as the deck height in that calculator with the piston top above the deck. This would be related to the compression height listed for a given piston.

Using your numbers of:
- 63.6 cc chamber volume (Cylinder Head Volume per UEM)
- Valve reliefs of 7.5 cc (Piston Head Volume per UEM)
- Top of piston .036" above deck (-.036" Deck Clearance per UEM)
- Gasket of .040" thick and 4.18" bore
- 4.07" cylinder bore diameter
- 3.31" stroke
With UEM calculator, I get 10.745
With the Pat Kelly calculator, I get 10.744

At this point, I can't explain why you get lower numbers with the same inputs plugged into the UEM calculator. Can you list all of the inputs that you put in the UEM calculator in the same way I listed above, so we can compare numbers directly?

OK on the street use, and I would be lowering the SCR too, to around 10, depending on the exact cam, and shooting for the range level of DCR.

You look to have gotten a lot of chamber volume out with the valves (and maybe higher seats? I dunno), and lost probably about 2 cc's with an approximate .010" mill.
 
nm9stheham said:
Looking at the UEM caclulator, the Piston Head Volume is the volume of a dish into the piston top, or the valve relief volume in a flat top. It does not vary regardless of where the top edge of the piston ends up relative to the deck.

And you are right to use -.036" as the deck height in that calculator with the piston top above the deck. This would be related to the compression height listed for a given piston.

Using your numbers of:
- 63.6 cc chamber volume (Cylinder Head Volume per UEM)
- Valve reliefs of 7.5 cc (Piston Head Volume per UEM)
- Top of piston .036" above deck (-.036" Deck Clearance per UEM)
- Gasket of .040" thick and 4.18" bore
- 4.07" cylinder bore diameter
- 3.31" stroke
With UEM calculator, I get 10.745
With the Pat Kelly calculator, I get 10.744

At this point, I can't explain why you get lower numbers with the same inputs plugged into the UEM calculator. Can you list all of the inputs that you put in the UEM calculator in the same way I listed above, so we can compare numbers directly?

OK on the street use, and I would be lowering the SCR too, to around 10, depending on the exact cam, and shooting for the range level of DCR.

You look to have gotten a lot of chamber volume out with the valves (and maybe higher seats? I dunno), and lost probably about 2 cc's with an approximate .010" mill.
Click to expand...
I think i'm starting to get my head around this, finally. 4.070 area x .036" = -7.675cc for my EDV. With the piston head volume at 7.5cc, I'm basically at zero. The 2.2cc mistake threw me off a bit. This time I used the exact numbers you did, which also put my CR right at 10.745 w/ UEM's calculator. The more I think about it, I see how my measuring came out wrong. I have a basically new set of Milodon(?) tulip-head valves that i'll swap in to see how they affect the chamber volume, or I have a rebuilt set of 2.02 J heads (also with tulips) that are a whisker under 70cc. Amazing what a shave and some valves will do.
 
OK, well at least we now know that the laws of mathematics still work the same on both sides of the country LOL

I usually would guess not more than 3 cc's with the tulip valves, maybe 4 cc's tops? But I have not looked at each and every one....Or just touch up the chambers a bit. I took some 11.2 pistons to 10.3 by just judicious chamber grinding.

You could also easily go to a Felpro standard head gaskets at .051" thick. With that gasket and 3.4 extra cc's in the valve heads, I am coming up with 10.0. That'll get you down in the low 8 SCR range with a 268-ish type cam with 4-6 degrees of cam advance and a 112 LSA.
 
nm9stheham said:
OK, well at least we now know that the laws of mathematics still work the same on both sides of the country LOL

I usually would guess not more than 3 cc's with the tulip valves, maybe 4 cc's tops? But I have not looked at each and every one....Or just touch up the chambers a bit. I took some 11.2 pistons to 10.3 by just judicious chamber grinding.

You could also easily go to a Felpro standard head gaskets at .051" thick. With that gasket and 3.4 extra cc's in the valve heads, I am coming up with 10.0. That'll get you down in the low 8 SCR range with a 268-ish type cam with 4-6 degrees of cam advance and a 112 LSA.
Click to expand...


That's the only Fel-Pro blue gasket I can find anymore...that .051 thick one. I'd like to find the ones that were .039 and with the 4.100 bore.
 
Thanks guys, I truly appreciate your help with figuring this out. The irony is, of course, I'm normally thinking about ways to increase the CR in an engine. My '72 340, which has the low compression Speed Pro (TRW actually) Forged units, is getting either some EQ heads or a new set of hypers, or both. The cam suggestion is helpful also, since I'm only now starting to learn about how important all the different aspects of them are. All I used to focus on were the lift numbers. I'd blame it on the ignorance of youth, but I'm not all that young...
 
yellow rose said:
That's the only Fel-Pro blue gasket I can find anymore...that .051 thick one. I'd like to find the ones that were .039 and with the 4.100 bore.
Click to expand...
This is the one I was thinking the OP was originally using.... I assume not 4.100 bore though: Felpro 1008.
 
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