Compression Ratio Confusion... help!

Trying to see if Im doing this correctly...

Bore, 3.895"
Stroke, 3.41"
Head, 57cc
Dish, 17cc
Deck Clearance, .078"
Gasket Thickness, .045

= 7.79:1???

Using Summit Racing compression ratio calculator. I know stock specs of the 4.0 (242) should be 8.8:1. My numbers should be stock, what am I doing wrong?

Stroke 3.41" / 2 = 1.705"

Piston comp height = 1.592"

Rod length = 6.125"

Total 9.42" Subtract DH 9.5" = .078" Compression Distance.

Dish = 17cc

Head = 57cc

Gasket = .045"

Thanks in advance for any comments

The reason Im asking, I was hoping to toss a mopar performance gasket on it, trying to confirm my compression ratios. Summit told me the gasket I wanted to use was not compatible with my block since Im a 3.895" bore. What do you guys think?

Doing the math on paper I come up with 8.66:1 using your numbers. That's assuming I did my math right. What gasket are you trying to go to?

What am I calculating wrong? I was looking at using the Mopar P4529242.

Trust me, buy Victor reinz hs-5713z.

Its what Mopar resells and is fine at your bore.

Looks like part of your problem is the numbers your using.

I'm assuming your basing some of your numbers off a 4.0 engine
Standard bore is 3.875
Stroke is 3.876
A Felpro head gasket should be around .051, and the Mopar/Victor Reinz is .043

Hope that clears up some of the confusion.

Well the master has spoken.

Russ Pottenger said:

Looks like part of your problem is the numbers your using.

I'm assuming your basing some of your numbers off a 4.0 engine
Standard bore is 3.875
Stroke is 3.876
A Felpro head gasket should be around .051, and the Mopar/Victor Reinz is .043

Hope that clears up some of the confusion.

Click to expand...

I thought the stock stroke of my 4.0 was 3.41"?

On another note, I just remeasured my deck height again, tried to be more precise. I now have 9-7/16" or 9.4375"

With that new number I still don't get 8.8:1 on the calculator.

Russ I don't have a 232 crank, I have the 242, and my block is bored .020" over.

What does this mean? "Recessed piston design is 0.116 inch deep." and how does it factor into my measurements? Ive been looking all over to find out what the stock CC's are on the piston dish, none listed anywhere Ive looked.

The only pistons Ive found that list dish CCs are ICON forged pistons, and its either 11cc or 17cc. Those are WAY out of my price range, $500+ per set!

Im seeing a lot of these recessed piston design... I even found one that says its .100 inch deep.

I just measured the dish on my stock pistons, and got 15cc. I used the plexiglass and a CC dropper.

Did the same to my head chambers, and got 58cc average. 57, 58, and 59...

My Bad.Your correct on your stroke number of 3.411
Your compression ratio of 8.80:1 seems to be in the ball park.

I have to run out the door now, but when I return I'll try to answer some of your questions and go over some choices in pistons.

Russ Pottenger said:

My Bad.Your correct on your stroke number of 3.411
Your compression ratio of 8.80:1 seems to be in the ball park.

I have to run out the door now, but when I return I'll try to answer some of your questions and go over some choices in pistons.

Click to expand...

Lol! I was like... wait... doesn't the mini stroker have a 3.5" stroke? Hows my stock one 3.8"?

Thanks Russ! I will look forward to talking about pistons with you!

I would love to throw in flat tops, but I cant seem to find any that will fit my setup

svshaaw82, Sorry about givin ya some bogus numbers for the 242 (4.0).
Looking back now I've only done 1 stock 4.0 engine and between having
a long day and having incorrect information from two different sources I didn't catch
it.

Going back to some of your questions.
The recess in the piston dish, or cup has a volume that has to be measured and calculated the same way as you would the cylinder head combustion chambers, including the volume of the head gasket.

Your correct that your .116 dish is going to be about 15cc and compressed .043
gasket with 4.00 diameter will give you another 8.85cc

ICON as you referenced, is making a piston formally made under their parent company Kieth Black. To my knowledge they make only two variations of it's forged piston for the inline 6 cylinder Jeep engines.

The IC944 has a cup volume of 21cc. The other piston carries a part number IC945 and
has a smaller volume of 10.8cc.
Both of these pistons are meant to be used for 4.6 to 4.7 stroker combinations using a 258 crankshaft with a stroke of 3.895. With a C/H of 1.353 you can use the longer 6.125 4.0 connection rod that came in all 242 engines.

The way I see it is you have three cost choices with pistons for your 4.0
Silvolite makes a piston with a part# 2229. It's a cast piston with a .100 recess D-Cup with a volume of about 11.5 cc. with a C/D of 1.581
Seal Power has 2 piston options.
677AP is a cast piston with a .116 deep recess measuring 15cc and a C/D of 1.585
H825CP is a cast hypereutectic piston that has a teflon coated skirt.
It also has a .116 recess, but has a 1.592 C/D

The other obvious choice is you can machine the C-cup lower in any of these pistons to get more volume. For example. I'll trace the pocket to a total depth of .200 and increase the total volume to 26cc.

There are several ways and methods to come up with a desired compression ratio to run on a specific octane fuel. It is my opinion that in a normally aspirated gasoline or alcohol fueled engine you want the piston ATDC to come as close to hitting the cylinder head as possible without doing so. All things being equal, this will give you the best combustion/burn possible.

Hope this helps and gives you something to think about.

HI Russ! That was a lot of great advice! Thank you so much!

I have been doing a lot of research into the 232 crank in my 4.0, and for the costs I don't see an advantage for me. So Ive decided to just look into bumping my compression a little with a piston swap, since I have to replace them anyway.

Do you have any piston recommendations that can bump my compression some? I was looking at the 1.592" C/D pistons. Unless you have another idea? Was just hoping to get it close to 9:1.

If you use the H825CP piston you should be at 8.93:1

That's assuming the piston is .016 down below the deck.
9.438 DH
.043 Mopar gasket
57cc head

The 232 combo can work if you use the 2229 piston, Felpro gasket, and you rebuild your connecting rods (and hopefully replace the rod bolts w/ARP's) that would shorten the rods by about .002
That's a good combination and works really well.

Russ Pottenger said:

If you use the H825CP piston you should be at 8.93:1

That's assuming the piston is .016 down below the deck.
9.438 DH
.043 Mopar gasket
57cc head

The 232 combo can work if you use the 2229 piston, Felpro gasket, and you rebuild your connecting rods (and hopefully replace the rod bolts w/ARP's) that would shorten the rods by about .002
That's a good combination and works really well.

Click to expand...

Thanks Russ. I started reading more into the 232 combo, and I dont think its for me. I was told I need to upgrade my injectors, and get an adjustable map sensor. Then about concerns about pinging and smog.

Basically its seems out of my budget range. With having to do pistons, injectors, map sensor, etc.

As where now I just need some pistons, and I dont have to worry about pinging or smog. Trying to keep my rebuild on some kind of budget. Currently its taken me over a year to get this far. (Kinda ridiculous) But family and bills come first.

Injectors are cheap. You don't need to do a map or anything else.

cal said:

Injectors are cheap. You don't need to do a map or anything else.

Click to expand...

I was told I needed Ford Blue 24lbs injector. Then a adjustable MAP sensor to curb the extra fuel to pass smog.

I already have the 4 hole orange Ford injectors for a previous upgrade, and the blue ones Ive seen at $150.

Im starting to get out of my confort zone on this, and I dont want all my hard work to blow up on the first start up. That would be devastating. The wife has already told me if it breaks down again after this rebuild, were getting rid of the jeep.

Russ Pottenger said:

If you use the H825CP piston you should be at 8.93:1

That's assuming the piston is .016 down below the deck.
9.438 DH
.043 Mopar gasket
57cc head

The 232 combo can work if you use the 2229 piston, Felpro gasket, and you rebuild your connecting rods (and hopefully replace the rod bolts w/ARP's) that would shorten the rods by about .002
That's a good combination and works really well.

Click to expand...

Russ... can I use Sealed Power H802CP20 to achieve the same thing?