Who has that perfect balance?

[Rinkrat456]

I've been doing my homework looking for the combination of parts to build a stroked 4.0L for my Gf's '96 XJ. The XJ is her DD currently with 15x,xxx miles on the stock 4.0L and a 3.5" lift with 31's is in the VERY near future. After the lift and tires, the engine is next up on the mods list.

Through my reading, I have found the use of the 5.875" rods leaves the piston too far down the hole for my liking. I am wondering how you guys have gotten around the need to deck the block a significant amount, which therefore raises the compression ratio to above 10:1 simply to keep the quench height in check. Is there a piston that sits higher on the rod, lessening the amount needed to be decked? The 58cc Jeep head and short AMC 258 rods really limit a lot, I'm used to 60-72cc Chevy heads.

I'm using:
'97 4.0L block (bore to be determined but piston scoring requires at least a .011" overbore) casting # 53020569
'89 AMC 258 3.985" crank casting # 3727
'89 AMC 258 5.875" rods
'97 HO head casting # 0630


Engine will be used as a daily driver for at least the next 3 years. Lots of highway travel and in-town driving. MANY hills = low RPM chugging. Weekend warrior at state off-road parks. Looking for 9.2-9.5 compression with a dynamic compression below 8.5:1 using the Comp Cams 68-235-4 Xtreme 4x4 camshaft (210/218, 254/262, lift .477/.493 that I'm having custom ground on a 114 LSA and 112 ICL). I'm looking for that perfect combination of parts to build a 1hp/ci 4.X liter with a very tall and fat torque curve centered around idle-5000. I can do my own PCM tuning. I've built many SBC's, mostly Gen III and IV motors 4.8-7.0L's newer than 99up like the LS1, ect ect, but this is my first dive into a Jeep motor.

Any bits of advice while I keep looking for this "fix all" piston? This is on a budget because she wont let me pay for any parts and doesn't make a whole lot of money, otherwise this engine would be done already.

[Plechtan]

You have a typo on the stroke of the crank, it should be 3.895" not 3.985. The PCM tuning is not easy, very few people can actually change thae fuel and timing tables. Unichip is a more accessable solution.

Sombody on the board here ( i'm sure sombody will post his name) will make the dish deeper on your pistons lowering the CR. Decking the block and a deeper dish is the cheapest way to go.

[SilverXJ]

Oleshot is the one whom does the piston dishing. You also have the option of the KB944 pistons and 4.0L rods, which with a .030 over and .045" quench will get you right around 9.5:1 static compression.

As far as the PCM tuning, the Jeep PCM isn't controllable like some GM ones are. Piggy backs are one option, [email protected] can send the PCM out for a custom tune as well. Unichip is one piggy back, but there are a host of others as well.

[Cheromaniac]

My stroker is built with a '00 4.0L block bored +0.020", home-ported '92 head, '72-'80 12cwt 258 crank, 258 rods, 677P +0.020" pistons, and stock cam. Static CR is 9.25:1 and quench is 0.088", but I don't have any problem running 89 octane in the cooler months. You could build your stroker similarly to mine but with the Comp Cams 68-235-4 cam and Mopar Performance 5249464 valve springs/retainers/locks. You won't quite have 1.0hp/ci but you won't be far off with ~265hp.

[dwg86]

My stroker is a short rod(258), .040 over with 9.25:1scr, 7.75 dcr, zero deck. Engle cam [email protected] 501 lift 112 lobe seperation 3 deg advance. Mopar performance Head gasket(.043). I run 87 octane without pinging. I was shooting for 9.5:1 SCR, but after the shop installed the SBC valves, my head cc's increased 2.4 cc's.

My next stroker I am going to run a long rod(4.0), KB 944's with zero deck, same cam.

[gradon]

The majority of us that went with the shorter 4.2 rod would use a 4.0 rod and custom piston for the next build. Those pistons are 2-3x more in price, but you could save $40-80 for not having to deck the block and if you didn't have to spend $100-120 for the recon'd 4.2 rods cause you already had 4.0 rods, then you're looking at a difference of $100-200 more for the custom piston setup(well worth it). With the right tune, your 1hp/cu. in. is possible, especially if you do the 9.5-10:1 scr and 8.0-8.5dcr high comp build that uses 93 octane.

[Rinkrat456]

Definitely typo' on the stroke length. Good catch.

$600+ dollar pistons are out of her budget. I'd spring for the best non-forged hypereutectic pistons in a heart beat but I'd still probably go with the shorter rod for this one.

I have been extensively reading into the short rod vs long rod debate and for her motor I am sticking with the short rods this go around. Reasons: 1) Simply put, I believe the shorter rod will create a greater vacuum behind the intake valve on the intake stroke and 2) will reach max torque at 90* on the compression stroke sooner than the long rod version which will in theory bring peak torque down to a lower RPM. By how many RPM is up to mathematicians and men smarter than I. I'm sure some of my old engineering text books could help dive into this theory but I'm not going to go to that extent. I do not see any benefit this motor could gain by having a lighter rod and piston combo when RPM's are kept more or less stock after a well thought out and tight rebuild. This motor will never see above 6000k RPM's and if it ever did, it would be for a very short burst, less than a few seconds until the auto trans catches up with it's shifting. The torque converter will remain stock, as well as the ring and pinion set in the D30/D35. I do not believe 245/75R16's warrant a gear swap, especially with a 4.5-4.7L I-6. If I dive into the rear end at all it will be to tear out that awful open differential that keeps getting her stuck.

About the concern over tuning. I have the newest version of EFI Live and have already linked up to her OBDII port to clear SES codes a few times (she has a bad cat converter I plan to ex-nay during the build since Minnesota requires no emissions testing) and I have uploaded her PCM's entire tune to my laptop. I am able to make changes and save the tune on my laptop but have not actually paid for the credits yet to upload the custom tune to her PCM. Are you guys saying I am not able to save data to the '96 Cherokee PCM? I haven't tried yet and wont buy the EFI Live credits until the date of swap, most likely. If the '96 PCM is incapable of tuning, will another Jeep PCM work? I have tuned my buddy's 99 1500 Ram after his Magnum 6.7L swap and the physical PCM casing looks identical from his truck to her Jeep. Any thoughts?

I'd like to favor a small bore over a hogged-out design. Reducing piston rocking is a priority high on the list. A smaller bore will also be more efficient in terms of the speed the fuel burns at and since this motor will be daily driven, MPG is a thought I'm keeping in mind. Right now her Jeep gets ~19mpg combined 20% city/ 75% highway/ 5% fun. After I'm done, 17mpg would be a target to shoot for considering the larger tires, larger displacement and being less aerodynamic. As a mechanical engineer, these are just the simple things I take into account before a build like this which is why I have a goal of 1hp/ci. I've never completed a motor that made less than that at the flywheel when adding 12% to rwhp through an automatic.

What about building a square motor 3.895x3.895 (.020" overbore) with these pistons and have my machine shop increase the dish to 24-26cc? Too much?
http://www.summitracing.com/parts/STL-H802CP20/

To be honest I've never installed a dished piston before. My SBC's all run at 9.8:1 or higher compression, hell, my work truck and weekend warrior runs 10.3:1 compression (5.7L iron block LS1) with 325/70R17's and I just put a 7.0L SBC Silverado away for the winter running 11.6:1. These Jeeps have been a learning curve for me to say the least, which is why I'm here asking questions.

[Rinkrat456]

Oh I forgot to add, my machinist charges a flat rate to do anything to the block I can think of. When I have him machine the block for me, the same rate is charged whether I simply install cam bearings or completely blueprint the block so every rebuild gets an (almost) NASCAR quality finished product.

[lafrad]

I'm looking at the EFI live site... they don't show ANYTHING about mopar... am I missing something?