Compression Math?

[Infidel Edition]

OK. So the Stage II Stroker Kit says it'll get me 8.8:1 CR
But that's will stock spec's. Pending on how much the block is decked and the gasket, the range (per their reply) is 8.8:1 to 9.0:1

So I was doing the math and am stuck on the Chamber Volume (CV). I got all the #'s to work using the stated stock CV of 66.
The CV should go up if the cylinders are bored to 0.030 over and the stroke is .455 longer... right?
:bang:
And the piston dish is -27.5cc vs. the stock is 8cc which would also add volume?

How do I calculate the new CV with the bigger stroke and 0.030 overbore?

Here's a screen shot of the spreadsheet I made (using -27.5 makes the CR 16.2, so I kept that as a + vs. -):

9774

[Infidel Edition]

* the "cc" between the stock and bored displacement is for the 63.58 below it... not the displacement. That would be cubic inches

[4.3LXJ]

Yes, if you have a dished piston you have more combustion chamber volume

[Infidel Edition]

Yes, if you have a dished piston you have more combustion chamber volume

That's what I was thinking too... But the #'s from Clegg are that the final compression will be 8.8:1 or higher.
I've gotta be missing something here.
The formula that I didn't get to work was the Chamber Volume. For the stock #, I just used the book reference for the XJ.

[4.3LXJ]

Check to see if their numbers involve a different head or a shaved head

[Infidel Edition]

Their #'s are based on a fully stock engine with the stroked crank and the -27.5 cc pistons
I'm thinking the increases with the bigger stroke are offset with the -27.5
But I can't get the math to work.

I shot them am email for clarification.

[4.3LXJ]

27 cc is a big jump. Also lowers the quench a little, which will affect milage slightly

[Infidel Edition]

Stroking it to 4.6
...mileage wasn't a goal:D

[4.3LXJ]

OK, just saying

[Infidel Edition]

Was meant with a smile...
Didn't know that had any effect on MPG.
I mean I know stroking doesn't help MPG, but i didn't know that the shape of the head and size of the quench had an effect other than total volume...

[4.3LXJ]

The quench, is not so much related to size, but shape. When you dish the piston, the gasses are not forced out of the flat area of the head at the same velocity, hence not as good a mixing going on. There is a lot of tech in combustion chamber design now, with heart shape the shape that gets better milage and horsepower

[Infidel Edition]

I was goofing around with a "demo" engine dyno program.
It calculates the compression as the same for the stock and for the bored and stroked.
~ the increased volumes and other changes all balance out (according to the simulator).
* though I cant find a simulator that does Jeep engines.

[4.3LXJ]

It should be the same for any engine

[denverd1]

Chamber is likely calcd at TDC so your crank shouldn't play into it are my armchair quarterback thoughts

Sent from my SAMSUNG-SM-G930A using Tapatalk

[Infidel Edition]

Chamber is likely calcd at TDC so your crank shouldn't play into it are my armchair quarterback thoughts

Sent from my SAMSUNG-SM-G930A using Tapatalk

Yes, Chamber is TDC. But to calculate CR, you need to take BDC / TDC and that's where the stroke comes in.
Sometimes my brain works faster than my fingers type...
...and I get 1/2 of the beginning thought tied to 1/2 of the objective purpose.