KB944 pistons

[Reelm n Rubi]

I have to get my son's Cherokee going first. We assembled most of his 4.0 today. My block is headed to the machine shop tomarrow. My cam ships out tomarrow also and has about a 1 week turnaround. So hopefully I'll be assembling within 10-14 days.

[Flash]

Flash the 4.0 deck height is 9.450 to 9.456 inch or for those metric inclined 240.03 to 240.18 mm. Data source Mopar Jeep Engines book 3rd. Edition.The deck height for the 4.2 motor is 9.487 to 9.493 inch or 240.97 to 241.12mm. And implied/inferred doesn't get it in performance. They need to spec. clearly what they are selling . Or hell, should I run out and buy one of those evil bay electric superchargers?
John

Thanks, Mopar Jeep Engines book 3rd. Edition........Good enough


So using the deck height of 9.450
9.1= 8.72 W/.052 head gasket and a quench of .0885
10.1=9.64 " "
With out any mods this is what we get..........less compression for similar quench.
Is the price of this piston worth it?

#1 How much more is a custom piston compared to this one?

#2 How much will it cost for the deck to be cut .031" esh, then you need different push rod Because of the decking for this new piston?

#3 How much would it cost to buy the silvo's, Have it dished to proper cc, and then have it decked to get a good .045-.040 quench height......... and push rod would need to be figured here also.

If your goal is "X" compression with .040 quench...........would the custom piston be cheaper, in the end????????

And i guess there is option 4

#4 Eagle rods with a the 6.150 length and the new KB piston.......................Hmmmm, Well lookie looky, piston to deck is .0095" or .010" if you round it up.......Just what KB said it was.
So this combo would not require new push rods, no shaving of the block,
With a .043 head gasket would make a [9.1 KB] and actual 9.32:1 compression ratio W .0525 Quench!!!!!!!
9.16 with a .052" thick head gasket!!!

So which route would be the cheapest including machine work??????


Flash

I pulled this from the first page because it has all the numbers that i had figured out previously.

John came up with some Vary interesting info that when in into the Block FAQ section...........that i felt as worthy of discussion out here.


Deck height is 9.450 to 9.456 inch or for those metric inclined 240.03 to 240.18 mm. Data source Mopar Jeep Engines book 3rd. Edition & 1999 Cherokee service manual, HO engines.

But if we go back to the early 4.0 engines, From Mopar Jeep Engines 3rd Edition & the 1989 Cherokee Factory Service Manual.
Deck height 9.429 to 9.435 inch or 239.49 to 239.64mm.

The first part you have seen before, but the lower is new info that John has just discovered.

Now on the KB site, they state the deck hight is 9.429 for the 10.1 and 9.433 for the 9.1 pistons. pretty Dang close to the 89 jeep service manual.
Now 9.450 minus 9.429=.021" thats .021" these pistons would set, higher in a Renix block (1987-90). in other words the quench would go from .0885, as i had stated at the top. To .0675" in the Renix block. And thats with a .052" head gasket!

I remember some one saying that they thought there was a cc difference between renix and the H0 head, which it is not. How deck hight is different, could cause a difference in compression as well as quench, for the same stroker parts.

What say you

Flash

[dwg86]

Eagle rods use a .912 pin, so the kb pistons would not work.

[dwg86]

I am disapointed in the kb pistons. If you use the kb pistons, you are still dealing with a bad quench. If you deck the block to get a good quench, now you are dealing with higher compression and possible detonation or using higher octane/ more expensive gas. Also they computed the comp ratio using a 60cc head. As far as I know the heads are around 57cc. So the comp ratio is even higher than what is stated on thier web site. I opened up a 7120 head to 61.4cc. It took a lot of work and time to do that, and then get all the combustion chambers to match. I did that because I was going to use the 258 rods and the 1.592 compression distance pistons, and deck the block to get a better quench. I am still undecided what piston and rod combo I am going to use, but if my budget allows me to buy forged pistons I am not sure I will use the kb's. For a little more money I can get any comp ratio and quench that I want.

[Flash]

I'm only kind, sticking up for KB as i to am disappointed at what look like, not the idea stroker piston. BUT
with all the calculation, and going at it from several different directions,.................
well, any and all stock 4.0L piston will have .5 to .7 more compression for the same quench.....could be even on full point of compression higher.........which is good !?
The quench is better on the stroker piston(KB) just not near as good as we would all like to see.

Once all the fact have been proven........we might be surprised at the end number

I agree tho, with a custom piston, sounds like you could have your cake and eat it too for just a little bit more cash.


Flash

[John]

Well I can tell we didn't get what we might consider ideal, but I still don't know for sure what we got. A couple of builders have pistons in hand, so if we get some hard measurements we will know a lot more. With the scarcity of 4.0 piston choices in off shelf items we have now one more. That is good.
John

[Alex22]

How thick is the piston where it has already been machined out for the dish, is there enough to make the dish larger?

~Alepx

[John]

For illustration.

148_0312_stroke_02_z.jpg

Deck height is the distance between the crankshaft centerline and the deck surface of the block.

1981 4.2 engine
Deck height = 9.487 - 9.493 inch or 240.97 - 241.12 mm
.0149 inch or .376 mm below deck
CR 8.0:1
Pin bore CL to top of piston 1.651 - 1.655 in or41.74 - 42.04 MM

1989 4.0 Engine
Deck height = 9.429 - 9.435 inch or 239.49 - 239.64 mm
.0215 inch or .546 mm below deck
CR ? Different normally reliable source 9.45:1
Pin bore CL to top of piston 1.651 - 1.655 inch or 41.94 - 42.04 MM

1999 4.0 engine
Deck height = 9.450 - 9.456 inch or 240.03 - 240.18 mm
.0215 inch or .546 mm below deck
8.8:1 CR
Pin bore CL to top of piston 1.599 - 1.603 inch or 40.61 - 40.72 MM

All above data from Mopar Jeep engines 3rd. Edition

Then I go to the 1988 FSM and find for both the 4.2 and 4.0 CR listed as 9.2:1

Anyone else find this interesting.
John

[Flash]

For illustration.

0412em_14_z+rotating_assembly+.jpg

Was that a link? it didn't work........at least it didn't work for me?

[John]

Likely just me editing the last post too much info I kept adding from my notepad.
John

[gradon]

I can't access my Jeep engines or any of my files for that matter due to Linux missing some startup sequence(ehem, Nick). I am using the Ubuntu startup disc though just to get by. Anyhow how did the renixes and/or the 4.2s, have a higher CR if their heads did indeed have a larger mid-60cc head volume? Did their pistons have less of a dish?

[John]

I am still trying to find out myself. I don't sleep well so reading passes too many nights.
John

[gradon]

So I need to chime in again since apparently people are paying $110 a piston for these now. Originally I thought that they'd be $45-65 a piston because most other KB forged pistons that Summit sells costs that much(some v8 sets cost $330!). I talked with KB techs and they explained that those v8 sets were more in demand, so that was why they were cheaper. I understand this to an extent but I looked at 20-30 different v8 sets and I find it hard to believe that every one of those sets would sell better than a similarly priced Jeep set. I was willing(begrudgingly) to buy them@$80 a piston, but they screwed up the first batch, so they weren't ready in time for my build. So summit quoted me $478 before they were released and now they want $200 more on top of that? Please tell me that they include rings at least(a $70 value at best).

[dwg86]

You can buy a set of Ross pistons any comp ratio/comp distance you want for $620.00. Check out their web site. I don't know how much JE or diamond cost. You can buy Ross pistons on ebay right now "buy it now" for $675.00 and that includes pins. I don't see the cost savings that was expressed by KB.
I think I'll stick to my original "budget stroker" plans.

[dwg86]

My mistake, the Ross pistons on ebay are for the jeep 4.0