I think I'm stuck with my KB424C

[RubiMikeTX]

I've been gathering parts for years in order to build my dream 4.6L motor for my 04 Rubicon. I'm harvesting the crank out of an old 80's 4.2L, have a TUPY head, and a 2000's 4.0 block. Early on, I got a set of .030 Keith Black KB424C pistons without realizing the consequences of their large 27.5CC dish, but found that the Mopar cam would result in the desired SCR and DCR; yet it has been discontinued. Not only that, I can't seem to find a cam even close...anywhere.

I really don't want to lay out a large chunk of change on new "custom" pistons and am thinking of just using .30 over 4.0L stock pistons with the 4.2 rods instead in order to salvage this build. The question is...am I just creating more problems than I'm solving?

[Cheromaniac]

You could try selling those pistons as they'd be useful to someone who wants to build a low compression stroker with a view to adding forced induction.

[jsawduste]

Dino, I`m not sure a hyper piston would be in the best interest of forced induction.

[Cheromaniac]

Yes you're right John. I forgot that they're hypereutectic. They might be OK for a mildly boosted application up to ~8psi but beyond that, definitely need forged pistons.

[Cummins90]

I havent personally used hypereutectic pistions but I'm running a stock engine on more than 8psi with a Mitsubishi 20g. They cant be more brittle than stock pistons?

[RubiMikeTX]

Thanks for the feedback. Looks like I'm going with a Compcams 68-231-4, zero decked, .043 gasket. SR9.1 DCR 7.95.
Either it will work or it won't.

[Cheromaniac]

It'll work. You may not get the maximum amount of torque out of it but at least you'll have a higher threshold for detonation.

[XJOverland]

RubiMike, what kind of compression are you aiming for?

I ask because I'm trying to put together my parts list and I thought those pistons would be the right choice for an 87 octane poor-mans build?

It begs the question, who exactly are the KB424C pistons for? Are they for the poor-man builds without any decking performed? Or is the 27.5 cc dish too much even then?

Not to hijack your thread but I don't want to be "stuck" with KB424C pistons either, but if they were the right piston for my build I might offer to take them off your hands. So I'm asking the same questions you are/did in my build thread and if there is a lesson to be learned from you I'd like to learn it build thread: viewtopic.php?f=8&t=6250

[RubiMikeTX]

It'll work. You may not get the maximum amount of torque out of it but at least you'll have a higher threshold for detonation.

Thank you, Sir!

[RubiMikeTX]

RubiMike, what kind of compression are you aiming for?

I ask because I'm trying to put together my parts list and I thought those pistons would be the right choice for an 87 octane poor-mans build?

It begs the question, who exactly are the KB424C pistons for? Are they for the poor-man builds without any decking performed? Or is the 27.5 cc dish too much even then?

Not to hijack your thread but I don't want to be "stuck" with KB424C pistons either, but if they were the right piston for my build I might offer to take them off your hands. So I'm asking the same questions you are/did in my build thread and if there is a lesson to be learned from you I'd like to learn it build thread: viewtopic.php?f=8&t=6250

Hi Overland,

I was hoping to get a Dynamic Compression between 8.0 and 8.25 and a CR under 10 so I can still run pump gas.

As for the application of the KB424C's, I have the same question. However, the only way I can make sense of them with such a deep dish, as a laymen, is that they were intended to go with a zeroed deck. Yet even then, they seem to conflict with pretty much any cam option out there.

Perhaps in the distant future, there may be a supercharger possibility for me...

[Russ Pottenger]

The problem with any of the KB pistons is how they spec out the dish volume and compression height.

They’re forcing you to build a stroker around their piston rather than designing a piston correct in the first place, and allow you more choices in dish volumes.

For example with it’s compression height of 1.353 you need to pay your engine machinist to surface the block .025 to .030 off the deck to make that piston work properly. That’s foolish on many levels in my opinion.

I make my pistons with a 1.895 CH allowing you the choice of not having to surface the deck of the block if you choose. The cost savings of the extra and unnecessary machine work could be applied towards the price difference of approximately $225.00 of the KB Hypereutectic piston and my custom forged piston with Dish volume choices from 12cc to 28cc in 2cc Increments.

Bottom line is the correct piston will eliminate a lot of the unnecessary compromises.
Feel free to hit me up I’ll be happy to send out information and pricing.

Russ Pottenger
Bishop-Buehl Racing Engines
531 N. Lyall Avenue
West Covina, California 91790
Work (626) 967-1000
Cell (626) 673-2203
Email: [email protected]

[wjk052s]

I was just looking at these pistons.....

I currently have a 1994 4.0 at my machine shop in Southern MO. I am building a mild street stroker motor for an 80 CJ-5. Nothing race. My question is about pistons. The KB 944 ICON set is pricey, I saw the KB 424c set, but you mention in the forum they are not right for compression...."For example with it’s compression height of 1.353 you need to pay your engine machinist to surface the block .025 to .030 off the deck to make that piston work properly. That’s foolish on many levels in my opinion."
What pistons do you reccomend?

Crank is from an 80 4.2
I have both the 4.0 and 4.2 rods (forum prefers the 4.0 rods)
Running the 7120 head.
I bought the comp 68-232-4 cam and lifter (not the springs).

Thanks,
William

[Russ Pottenger]

I was just looking at these pistons.....

I currently have a 1994 4.0 at my machine shop in Southern MO. I am building a mild street stroker motor for an 80 CJ-5. Nothing race. My question is about pistons. The KB 944 ICON set is pricey, I saw the KB 424c set, but you mention in the forum they are not right for compression...."For example with it’s compression height of 1.353 you need to pay your engine machinist to surface the block .025 to .030 off the deck to make that piston work properly. That’s foolish on many levels in my opinion."
What pistons do you reccomend?

Crank is from an 80 4.2
I have both the 4.0 and 4.2 rods (forum prefers the 4.0 rods)
Running the 7120 head.
I bought the comp 68-232-4 cam and lifter (not the springs).

Thanks,
William

Hi William,

In my previous post I laid out my reason similar as yours for coming up with a new piston spec addressing what you laid out.

If budget is a concern one should consider the machine shop cost savings of not having to surface the deck of the block .030
That helps to bridge the cost between a production KB944 piston and my custom spec piston.

Just something worth considering,
Russ

[TJBUD]

This a good thread worth going deep.
Russ, is your compression height a typo?—I think it’s 1.385 (not 1.895)? If so 1.385 is .032 taller than the KB424s 1.353, which explains your good argument on machining. With 1.385 CH you get near zero deck clearance with little to no deck surfacing.

The other problem with the KB424 piston is the dish is round and nearly the full diameter of the piston leaving only a small ridge around the outside at full compression height. So even though the stroker compression calculator says you have good quench you really don’t. Quench is deck clearance + gasket thickness. Russ’ piston has a D shaped dish which is the same shape as the combustion chamber leaving a flat area (quench pad) at full compression height to “squish” the air/fuel into the combustion chamber and dish as the piston approaches TDC for a good clean burn. This quench helps the engine tolerate higher compression on pump gas. I’m running >9.5 CR in my build from last summer with these pistons just fine on pump gas. After research I was trying for .040-.050 of quench with .040 being better. Engine labs has a good article on quench:
https://www.enginelabs.com/engine-tech/ ... -and-head/

The Icon 944 forged piston has a better d-dish but it still has the same 1.353 CH problem and the 21cc dish size isn’t optimized. With .060 over pistons If you machine to .015 of deck clearance you hit about 9.6 CR but .055 quench. Increase the clearance to lower the compression and the quench gets larger (less effective). In this example you wish the dish was 24 cc so you could dial in to lower CR or better quench. This why Russ has pistons with 2cc dish size increments.

Comp cams doesn’t recommend the 231 cam for use in computer controlled engines because it has too much overlap (32 degrees). Apparently the Jeep computer likes the idle vacuum generated with 30 or less degrees overlap. So the 231 is borderline-some guys run it just fine. Comp recommends the 232, 201 and 301 cams because they have a wider lobe separation angle (LSA) and less overlap. LSA is spec’d in cam degrees and overlap is spec’d in crank degrees so every additional 1 degree in LSA is 2 degrees less overlap. The 201 and 301 have a 113 LSA and 4 degrees less overlap than the 231 with 111 LSA, even though they all have the same duration. I used a custom comp cam Russ designed with even more duration but 114 LSA and only 29 degrees overlap. While I wait for a custom tune the engine is running great without fail codes on the factory tune.
The Comp cam specs are in https://www.angelfire.com/my/fan/Jeep4.0Camshafts.htm

Sorry for so much detail, but a lot folks building this stroker struggle with optimizing piston and cam selection. I certainly did.
TJBUD

[Russ Pottenger]

Thanks for catching my mistake. 1.385 compression height Is the optimum number of giving you a choice on whether to do a minimum cut of the deck or not surface the block at all.