Looking for Info / Specs on Obsolete Mopar Piston

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1970 TA

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I'm an E-Body owner and have spent a lot of time on this Forum and have been impressed by the number of knowledgeable people on small block engine builds, what works, what doesn't.
So here's my problem.
Working on a 1970 340. Currently out of the car for a refresh. I'm trying to calculate the actual static compression of the current configuration.
Previous owner had the motor built with the Mopar Performance pistons P3690825 (made by TRW), advertised compression of 11.5:1. These pistons are obsolete and I can't find any published data on them. Motor has "O" heads, 72cc open chambers. Cam is the purple shaft, P4452761, duration @ 0.050 = 228/231, lift = 0.450I/ 0.455E.
I emailed TRW and they have no archived data on these pistons.
I'm looking for the dome volume and piston compression height. Anyone have those specs?
I tried measuring the dome volume, i.e., put the piston down a fixed distance in the hole, calculate the volume of the "perfect" cylinder at that depth, measure the volume with the burette and take the difference of the two readings. To calculate the "perfect" cylinder, the reference surfaces I used are the block deck and the quench flat of the piston. So the volume of fluid needed to fill the cylinder (after taking the difference in readings) would be the "net" volume of the piston dome.
Using this technique gives me a net volume of around 19.5 cc.
Does my technique makes sense? Anyone have data for comparison?
deck ht_1.JPG

piston_2.JPG
 
It makes sense if I understand it. If the reference surface (the quench flat) is x" down, then you computed the 'perfect' cylinder volume as if the piston was a flat top. Then you measured the actual open volume with the dome in place, and the difference was the dome volume of 19.5 cc.

One more measurement is needed, which can be done in one of 2 ways:
Method A is to measure from the center of the pin to your reference surface.
Method B is to measure the distance of the reference relative to the deck when the piston is at TDC.

Method B is preferred, as it will compensate for the block having been decked, or for any rod length changes made. Method B should be easy if the lower end is still assembled.

With this last piece of info, it will be easy to run an SCR computation for you.
 
It makes sense if I understand it. If the reference surface (the quench flat) is x" down, then you computed the 'perfect' cylinder volume as if the piston was a flat top. Then you measured the actual open volume with the dome in place, and the difference was the dome volume of 19.5 cc.

One more measurement is needed, which can be done in one of 2 ways:
Method A is to measure from the center of the pin to your reference surface.
Method B is to measure the distance of the reference relative to the deck when the piston is at TDC.

Method B is preferred, as it will compensate for the block having been decked, or for any rod length changes made. Method B should be easy if the lower end is still assembled.

With this last piece of info, it will be easy to run an SCR computation for you.
Thanks for your response.
I went over my data and discovered a math error.
Here is the data I'm using:
Head gasket:Felpro 1008: dia=4.174 inches, measured thickness=0.044 inches, Vol=0.61 cu in=9.87cc
"Perfect" cylinder is 0.425 inch below block deck. (block deck to quench flat)
Vol=5.53 cu in=90.62cc
Burette measured 80.4cc to fill chamber
Dome volume = 90.62 - 80.4 = 10.22cc
Quench flat on piston is 0.010 inches above deck.
Cylinder bore = 4.070 inches
Cam data: intake closes 62 degrees ABDC.​
I plugged these numbers into the Diamond Racing Compression Calculator and it gives me a CR = 11.15.
The Wallace Dynamic Compression Calculator gives me a DC = 9.00:1
Dynamic cranking pressure = 186 psi.
I did a compression test before pulling the motor. Readings were 185 to 200 psi.
I'd like to get the CR / DC down at least one full point to be pump gas friendly (91 to 93).
A local engine builder said they may be able to machine the pistons down.
Given this combination of parts, how would you proceed?
This motor has 3500 miles on this build. Still has the crosshatch pattern in the bores.
piston_4.JPG
 
just looked in my mopar race manual, they show, @ .500 down, with 63 cc head, .038 gasket, CR is 13.5. if your piston has solid dome, you can cut the dome off to a flat style piston. I did one , and still leaves lots of material. the piston, is a trw style.
 
Right, I was going to recommend the .500 down method, but you beat me to it. lol
 
OK good numbers. BTW, the OP has 72 cc head chambers, not 63 cc head chambers.

With the quench surface .010 ABOVE the deck, then I get 11.16. Yes kinda high for that cam for pure street use. (Tho' AJ would try to run it on liquified mothballs LOL!)

Any reason to stick with that cam? Is this just for the street and cruising?

I personally would hate to waste a classic set of pistons; I'd check the bores and put in a set of KB flat tops if I could. But as these are probably forged, the piston-to-bore clearance would likely be too large for a good fit with KB hypers.

Then there is the use of a larger cam, retarding it a bit, and putting in thicker head gaskets.
 
Usually around 8.5 DCR/185 psi is at the "safe" limit for iron heads on pump 91. When I say safe I mean you can be off a little on timing. Or carb tuning. Or get a bad tank. And it won't blow itself up.

But you can get away with more as long as you stay on top of your tune. Just gotta be careful.

If you have 93 available there. And it's good gas. You should be able to run 185-200 psi/higher than 8.5 DCR . Dunno how much of a problem it would be for for you to stick to only filling up at that station. But if you do it. You should be able to keep your current setup and the power/throttle response that comes from it. While the total power loss from stepping your compression down a bit is probably 5% or less. You will notice that the throttle response and "crisp"ness that comes with will be less. Up to you which way you go. Might not even be able to use those pistons if it needs a bore anyways.
 
How far is the quench pad out of the hole? .017 is what TRW said but that is pretty low. I'm .054 out and that made me 11.08:1 with 72 cc chambers and a .054 gasket I hate.
 
OK good numbers. BTW, the OP has 72 cc head chambers, not 63 cc head chambers.

With the quench surface .010 ABOVE the deck, then I get 11.16. Yes kinda high for that cam for pure street use. (Tho' AJ would try to run it on liquified mothballs LOL!)

Any reason to stick with that cam? Is this just for the street and cruising?

I personally would hate to waste a classic set of pistons; I'd check the bores and put in a set of KB flat tops if I could. But as these are probably forged, the piston-to-bore clearance would likely be too large for a good fit with KB hypers.

Then there is the use of a larger cam, retarding it a bit, and putting in thicker head gaskets.
hi, I know that he has bigger chambers, I was quoting mopar book!!!!! back off!!!
 
From internet forums (may not be accurate):

The old forged pistons from TRW required a pretty good amount of clearance. I think on the order of .004"-.006". Seen as high as .008" and RUN as high as .012". LMAO But that joker didnt burn a lick of oil because I file fit the rings. It was noisy when cold but got better when warm. Those old forgings are heavy and TOUGH. They can get by with some clearance.

Are the top rings .062" , or .078" ... if the top rings are .062" then part number is L2322F (P3690825 ?) a nice 662 gram piston with a 10.7cc dome volume, gives 12.1/1 with a 68cc head.
 
BTW, the OP is better off to stick the piston out of the bore and mill the dome down a bit to keep the CR where he wants it.
 
Using the Eagle calculator I come up with a 11.77 to 1 compression ratio (more than 11.5 to 1 because of the .030 overbore) using 9.599" deck height, 4.07" cylinder bore, 10.7 cc dome volume, 72 cc combustion chamber volume, .039" head gasket, 3.31" crankshaft stroke, 6.123" connecting rod length, and 1.84 piston compression height. It also shows that with above specs the flat part of the piston to be above deck by .019". (it reads -0.019" on Eagle software)

P.S. With a 68 cc combustion chamber/cylinder head volume Eagle shows a 12.47 to 1 compression ratio.

I am using these same pistons (TRW L2322F + .030") in my Mopar 305 CID built motor using a 2.96" stroke, a 6.298" connecting rod and 72 cc CC volume open chamber heads where the Eagle calculator shows only a 10.63 to 1 compression ratio, which is good for me because i want to run pump gas. By de-stroking the 340, it is more difficult to maintain higher compression ratios.
 
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Using the Eagle calculator I come up with a 11.77 to 1 compression ratio (more that 11.5 to 1 because of the .030 overbore) using 9.599" deck height, 4.07" cylinder bore, 10.7 cc dome volume, 72 cc combustion chamber volume, .039" head gasket, 3.31" crankshaft stroke, 6.123" connecting rod length, and 1.84 piston compression height. It also shows that with above specs the flat part of the piston to be above deck by .019". (it reads -0.019" on Eagle software)

P.S. With a 68 cc combustion chamber/cylinder head volume Eagle shows a 12.47 to 1 compression ratio.

I am using these same pistons (TRW L2322F + .030") in my Mopar 305 CID built motor using a 2.96" stroke, a 6.298" connecting rod and 72 cc CC volume open chamber heads where the Eagle calculator shows only a 10.63 to 1 compression ratio, which is good for me because i want to run pump gas. By de-stroking the 340, it is more difficult to maintain higher compression ratios.


The problem is the valve notches in that piston are big. Way big. I'd have to find my math but I think to get a 10 cc dome the piston has to be out of the deck a ways.
 
That's correct, the 11.5 to 1 compression ratio with these pistons is calculated/assumed with the flat part of the piston .019" above deck, which puts the dome right up there against the combustion chamber, necessitating the substantial valve notches. But the notches in the other TRW "obsolete" 12.5 to 1 pistons are even bigger!
 
With the OP gasket thickness of .044" and the piston flat being .010" above the block deck like he says, the Eagle calculator shows 11.3 to 1 compression ratio.
 
With the OP gasket thickness of .044" and the piston flat being .010" above the block deck like he says, the Eagle calculator shows 11.3 to 1 compression ratio.


That sounds correct. But because I have "issues" and I have most of the tools here at the shop, I stuck the piston out .050 and milled the dome down to get 11.08:1.

As of now, my quench is a bit loose, so if I run into detonation I'll pull the heads, use an .039 gasket and make it 11.25:1 and close the quench up a bit.
 
Thanks for all the responses, I need to read through your responses again to absorb all the info.
My car is a real 1970 Challenger TA 340 - 6 with 727 Torqueflite, Turbo Action torque converter, 2500 rpm stall, Mopar Performance Electronic Distributor with Orange Box. The car doesn't see any track time, just basically a nice cruise night car.
The original motor was gone when I bought it around 1992. The current motor is a 340 block cast in July 1970 with the "O" heads, 2.02 intakes. Has the original 6 Pack setup.
3500 miles on the build. Bored 0.030 over, still has the crosshatch pattern in the cylinders.
When I bought the car, the motor was fresh but the heads were off the motor, i.e., needed some final assembly. The cam in the motor was the purple cam P4120233, 0.508 lift.
I thought this cam was too radical for a Torqueflite equipped street cruiser, so based on recommendations from Mopar Performance, I installed the P4452761, 0.455 lift cam.
This was before I knew anything about the concept of "Dynamic Compression".
The "233" cam has 76 degrees overlap vs 50 degrees in my "761".
I'd like to switch to a retrofit roller from Howards (711915-12, 221/225 @ 0,050, 52 degrees overlap, lift = 0.475 I / 0.500 E), but the block has the chamferred lifter bores.
I know Hughes makes a retrofit roller that is supposed to work with the chamferred bores.
Still sounds like a roll of the dice if it works and you still might need to get the lifter bores bushed. ($700 ?)
If I went with the Howards roller, I'd probably just have the pistons machined to get the compression in the range of 9.5 to 10.0. Not sure what kind of power it would produce with the open chamber heads. There's no quench in the open chamber heads. The quench pocket in the heads runs about 0.080 / 0.090 inches depth. I've been looking for a forged "step-dish" design piston that would improve the quench with these heads, but haven't found anything suitable.
More homework to do.
TA_1.JPG

lh head_061217.JPG
 
Since the engine is apart,sorta, I would just machine the piston tops off to where I needed it to be. This is done all the time in the motorcycle world, no big deal. Of course you could always get one of those thumper cams. They do a good job of killing off some compression. And of course you are set up perfectly for some open chamber aluminum Eddies.
Yeah, I am a big fan of compression. It makes a snappy bottom end and a strong midrange, and it makes for a much more efficient part throttle. Sometimes,I like to look at it from a dollars to mile perspective. Sometimes burning hi-test in a hi-c engine, gets to be nearly as cheap as burning 87 in a low-C, in dollars per mile. And the Hi-c engine has so many benefits.
But above all, the engine has to survive full-load WOT operation.
There are some tricks you can use on a hi-c or too-hi hi-c engine, to help it survive. They are temperature,load,and timing
You can;
1) increase the chamber turbulence,increase the chamber efficiency, or reduce the chamber temperature,and/or control the engine temperature over a very narrow operating range.
3) run fresh cold air into the carb; this is a really big deal
4) reduce the load; either with gearing, or chassis lightening,or stall speed, or a smaller carb, or driving style.
5) retard the full-load WOT timing, and/or delay the all-in timing to something like 3000/3600 rpm, and/or slow the rate of advance from the stall rpm to the all-in rpm
6) And of course,reduce the compression

In my 360, I was able to run 11.3Scr using all the tricks, except #6, and while running 87E10.And then I put the 750DP back on.

But, for your application, considering all factors, I think #6 is probably the best option. You are probably 1/2 point too high on Dcr. With iron heads 165psi is usually a nice safe bet. For low-rpm useage as a cruiser, you will not even notice the slight power loss; you will just drive a little deeper into the carb from time to time. And if you get your throttle pressure set just right, you will be able downshift the tranny using the gas pedal. Besides that cam is a real torque maker, I mean compared to the 292/508.
If that TC stalled at 2500 with the big cam, it should stall a little higher with the little one;hopefully. If not I really enjoyed a 2800.
 
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"Besides that cam is a real torque maker, I mean compared to the 292/508."
Are you referring to the '761' cam that is in it now, or the Howard's roller I'm considering?
 
That's why I said to deck the block and put the piston out of the hole. If the head is .080 recessed and you stick the flat out .050 that will give you .030 quench. A Fel-Pro 1008 is .039 so you'd be at .070 and if that's the case, you can mill another .030 of the head. Then machine the dome to get the desired compression ratio.

I'd have to do a bit of checking because I have that piston out in the shop but I think *I THINK* the rings are low enough you can stick the piston .060-.070 out of the hole if you have to.
 
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