Determining comp ratio of new pistons

[70RTSE383]

I am looking at KB pistons from Summit. The only referance to what com they are is-5cc. What the heck does that mean?
I only understand ratio with a given combustion chamber size, in my case 88cc.

[HP_Cuda]

From your friends at Summit!!!

https://www.summitracing.com/newsandevents/calcsandtools/compression-calculator

[70RTSE383]

Thanks HP Cuda but I only need to estimate the compression to about 1/2 point. I cant use the tool you sent me as I don't have some of the info. I only know the bore size (standard 440) but it will be bored over by some amount. Head gasket I don't have them yet but could go up or down a little to adjustand no idea what the distance the piston will be down in the bore. I'm just trying to see if these pistons will give me between 9 and 9.5 to one. I would think there is a way to estimate this without the unknown details. Any help with experience or estimation would be much appreciated.

[GY3R/T]

     Subtle Changes will make a big difference in compression ratio. Especially on a 4.32 bore.

[1 Wild R/T]

Ok, unfortunately there' no easy way to do it... But it's worth getting it right... 88cc means your probably planning to use 906 heads.... Which in reality rarely measure 88cc unless they've been milled a little... 

And they are hard to make useable quench with.... Which means your throwing away free HP & tolerance of marginal fuel...

You can make it work but you'll need to do twice as much machine work as opposed to using a set of close chamber heads.....

KB offers "quench dome" pistons for the 440 look at the kb236...   88cc makes 9.9 but your heads are probably 90-92cc so figure 9.6-9.7....   Head gasket is a variable as is any machining that needs to be done to the dome....

Personally I'd look at 915 heads if you must stay iron or Stealths if your willing to have it look stock till someone really stops & looks....  Now piston selection improves, quench & C/R is easier to achieve....  More HP, less trouble... Win.Win...

What pistons are you looking at & are you planning to use 906's?

[70RTSE383]

Thanks R/T, I have "fresh" 906 heads on my 383. They came with the car. The 383 had been rebuilt with low comp pistons so has no real potential for significant upgradea. Right now I am trying to work the cost/benefit of two different directions to take with. 71' 440 I recently purchased. One is replace pistons rings and bearings using the forged stock crank. The other is a new rotating assy. Using the new assy I can get it with pistons that will work with my 906's for now at a functional cr then go to aftermarket aluminum heads and bumping the cr to about 10:1 . I need to find and cost out parts and machining to compare these. It seems like a waste to not use good heads, crank rods etc. i am open to both possibilities. I am not looking for a track car rather a very nice cruiser that can hold its own.

[1 Wild R/T]

At least the 906's work allot better on a 440 than a 383..  Not because of flow but because those huge combustion chambers make it hard to build compression on a short stroke engine..

Ok, so you know the stroke is 3.75, You know the rods should be 6.768....  So half of 3.75 = 1.875  plus the connecting rod  6.768 plus the compression height of the piston you select  I like ones in the 2.067 range.....  So

1.875
2.067
6.768
--------------

10.71

A 440 has a deck height spec of 10.725 so in theory your pistons will be .015 in the hole... In reality 440's are rarely 10.725...  More likely 10.735-10.740...   So with 90cc heads & pistons .025-.030 in the hole......Add a .041 head gasket with a gasket bore of 4.4....


The KB236 with a step head which with some work will give you Quench and a C/R of 9.37

Or the KB237 with a flat top which won't offer quench but doesn't need extra machine work will give you a C/R of 9.13

Now since your using iron heads you could dump the composite head gasket & use a steel shim like the stock Chrysler gaskets & C/R goes up to

KB326     10.77             KB237     C/R 10.5


Be aware these are only guesses as far as head cc's & deck height....  You need to measure to be sure.. When you get close you can use the head gaskets to tune...

[70RTSE383]

Thanks W R/T that is very helpful. I am just a hobbiest so don't know the nuances of these engines particularly when stepping outside the bone stock arena. I rely on those of you who really know this stiff. Thanks so much for all your inputs.🤗

[1 Wild R/T]

Happy to help, cause back before I learned it myself I had to trust my machinist to pick parts & I rarely got what I was hoping for....

[HP_Cuda]

I was going down the road that Wild went but I didn't have time.

As Wild said you can vary the thickness of the head gasket to give or take on the compression ratio although slightly.

Here is kinda a cool site to checkout:
http://fme-cat.com/PerfApplication.aspx?brand=FEL-PRO

(Not pushing FEL-PRO at all)

[1 Wild R/T]

And now a correction.... I screwed up...   those last C/R numbers are off by one full point.....  The correct numbers are blue


The KB236 with a step head which with some work will give you Quench and a C/R of 9.37

Or the KB237 with a flat top which won't offer quench but doesn't need extra machine work will give you a C/R of 9.13

Now since your using iron heads you could dump the composite head gasket & use a steel shim like the stock Chrysler gaskets & C/R goes up to

KB326     10.77       9.77      KB237     C/R 10.5          9.5


Be aware these are only guesses as far as head cc's & deck height....  You need to measure to be sure.. When you get close you can use the head gaskets to tune...
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[70RTSE383]

Thanks again W R/T, those numbers are even better. 9.5 with Iron is where I want to be. Now I have something to work with if I just go with new pistons and keep the rest of the reciprocating assy.