I am in the process of switching to E-85 for my motor.
(520 RB, 7.1 Rod, 10.7 deck, .005 down, Ported TF 240 heads 360 cfm)

The highest CR I am finding for these pistons is Icons, which will come in at a little over 12:1.
What company would get me to about 14:1 without having to have custom pistons? I think someone had said Diamond will do 2 changes for like $40.

Is the jump from 12-1 to 14-1 worth it?

My cam is 260/266 @50.

Wallace says I will have this for cranking pressure with 12:1 which seems reasonable-
CID is 520.16 CID
Effective CR is 10.47
Cranking Pressure is 248.60 PSI
Gauge Pressure is 233.75 PSI
Atmos. Pressure is 14.85 PSI

with 14:1 it jumps to this

CID is 520.16 CID
Effective CR is 11.99
Cranking Pressure is 292.57 PSI
Gauge Pressure is 277.72 PSI
Atmos. Pressure is 14.85 PSI

My car is mostly strip with some local drive around, 4 speed, 4:10 gears.

Is 233 lbs to high anyhow with the 12:1 , my present cam, and E85?

If i jump to 14:1, what would be a reasonable duration to go to on the cam?


Thanks in advance.

Cn't answer your question, but 14.85 is below sea level.

IMHO using duration to modify the cam is a mistake.

Have pistons made to fit the combo. If you don't it may bug you forever, like it does me at times. If you get by for less than a couple hundred bucks it is cheap hp gained for a lot of builds.

14.7 is seal level..

Use 4.25 pistons then cut the top a little, only looking at .060

I'm using the Icon 837+.033 in my 511. 4.250x4.375, 67cc chamber and 270@50SFT. intake opens at 29* closes at 61* and my cranking compression is 200# with 12.5:1 calculated compression.

My current bracket E85 motor is right at 15.00 to 1 due to the used heads I bought to use having 60 CC combustion chambers instead of 76.00 that the Indy 440-1 come with, my mistake for believing the seller and not verifying the actual combustion chambers sizes before ordering the custom Diamond pistons
I wouldn't worry about the compression ratio as long as the fuel you buy is always E85 or higher, not like the winter mix is here at E75
Any compression ratio under 13.5 to 1 true mechanical ratio should run fine on E75 also

ISA weather used for flying by the FAA , Instrument Standard Atomsphere, is 59F at mean sea level (halfway between high and low tide) at 14.92 Barometric pressure with zero humidity
ASE weather is the same except it is calculated at 60 F with everything else exactly the same

I don't know about all that. But i live at sea level within a hundred feet. The computer and two different Map sensors I have read right at 14.7 here.

Thank you for all the suggestions.
As for the sea level question, when you enter the barometric pressure in Tampa @ 30.24, that is what the Wallace calculator spits out.

I will contact the engine machinist about skimming off 4.25 pistons, that sounds like a good plan.

Was kinda hoping to reuse the cam/valvetrain that are there, but, much more than 12:1 sounds like a lot of cranking pressure. Don't want to raise the RPM range of the internal oiled engine too much past the 6400 rpm peak it has now.
I really don't know which way to go, I don't want to redo everything, that's why I am asking.

I am using a stock 76/77 block, so it really isn't a max effort anyhow, at some point I will get an aftermarket block, that's a whole new thread.

What is the high end of cranking pressure with E85? Or maybe another question would be, at what point will a stock mini starter not handle the compression?

Thanks again!!

Was trying to figure out your combo
These would put you .011 above deck

Thanks for looking.

The Icon pistons have valve pockets that don't match Trickflow exhaust valve location, and are the only shelf piston I could find with a 1.423 compression height.

They would be my choice if the pockets work, but, since there will be money spent for custom pistons, just gonna go big.

As one of the earlier posters said, rather than muck around with a compromise, just going to order Diamond pistons hopefully around 13= 13.5-1 CR.

Also upgrading to a roller cam.

Thanks again for your help.

I’m pretty sure those pistons will work just fine
You’ll be fine with 12.25-1, your block will thank you also.

Since swapping cylinder heads can be a costly and time-consuming process, Trick Flow® designed the PowerPort® 240 heads to accomodate all factory-style big block Mopar pistons. They are also manufactured to accept aftermarket roller rocker arms, intake manifolds, and headers. “As per TrickFlow”

These are the Icon step dish pistons in the block now.
They came with the 440 Source kit. I missed a shift big once, left witness marks from the exhaust valves.
I contacted both Source and Icon to see if they could tell me if the current pistons in their kits, would work with TF heads, neither company knows or would make the effort to find out.
Photo attached.

That’s very odd.

“Usually” when the piston has symmetrical valve pockets(which appears to be the case in the pic you posted), the exhaust valve is very close to the outer edge of the pocket......basically the opposite side from where your witness mark is.

I’d pull a couple springs off and use some clay in the pockets to see exactly what the static radial clearance situation is.

Generally, pistons that have “correct” intake & exhaust pockets will have more P/V clearance on the exhaust.

If you end up going custom, specify you don’t want the symmetrical valve pockets.

Edit- looking at my 2016 Icon catalog, piston number IC843(step dish for 4.375 stroke) shows it does not have symmetrical valve pockets. Intake pocket diameter is 2.300/exhaust pocket is 1.880.

Perhaps there has been a design change to that piston, or maybe 440Source has them made to their specs with symmetrical valve pockets, so there is only one piston instead of having “lefts” and “rights”.

Looking at the pic of your piston more, it’s hard to tell if it has symmetrical valve pockets or not.
It’s easier to tell in a pic when there is no dish involved.
The non-symmetrical pocket looks like this:

Pistons mixed up, in backwards ???

I had one customer who had a set of custom made pistons that had 5 lefts and 3 rights, he didn't catch that until he over revved it

The mark does look like what you’d see from an intake valve hitting an exhaust pocket.

barometric pressure in Tampa @ 30.24
Tampa is below sea level?

W/r/t P-V clearance: unless the valves are (nearly) vertical, the valve moves sideways as it opens based on its stem angle (a Gen-2 hemi is really bad).
A relief pocket that is round and concentric to the static valve position (seated) will close up on one side, how much depends on lift near TDC and rod geo (short rods move the piston more). Remember if the pin is offset, the position on opposite sides of TDC will be different.
The shape of the smallest pocket that combines minimal chamber volume increase and contact protection is called a "stadium" (not an ellipse or oval, since the longer sides are parallel). This extra length represents the valve's sideways motion.





Some very bright people believe that those sharp edges shown above cause mixture leaving the valve curtain close to the reliefs to "trip", change direction, and lose some velocity (droplet fallout).
If you radius the machined edges (some even join the intake and exhaust reliefs laterally into a trench) this will also promote flow-through on overlap.
If the cam is small it may help, if it's too big that's bad.

Thanks for all the help.
At work now, but, here's another picture of piston location.
Will give the proper response when able.