Comp Cams 239-4 on my stroker

[acer44]

Hi Guys,

First post - been asking around on other forums but can't get an answer, hoping you guys can help me figure this out.

My mechanic built up a 4.0L stroker in my 2000 XJ. Not sure exactly which pistons were used, or if they were dished; not sure how much that matters though. The head is from a 1993 XJ, and I have a 63mm TB. I have the stock 2.25" exhaust still.

He put a Comp Cam 239-4 in there because he said it would let me run 87 octane so I didn't have to run 93 octane. The engine runs really rough at idle, and my gas mileage averages less than 10mpg. I'm thinking that if I were running a Comp Cam 232-4, even with 93 octane, I'd be getting a lot better gas mileage.

I see that Comp Cam says the 239-4 shouldn't be run with an EFI engine, and my understanding is the stock ECU can't read the aggressive cam, so Comp Cam advises against it.

I want to avoid having to pull the 239 and put a new 232 in because of the costs involved... but I realize that may be my only option.

I've read some people advance the 239 2* to 4* to get the ECU to respond correctly, but I've never advanced or retarded a cam so I don't know if that's something I can do... or even it it would work.

I'm also wondering if a UniChip or Jet Stage 2 would help the ECU read better, fixing this idle issue and hopefully the mpg issue, or if that's just a lost cause.

Basically, I'm open to advice and would love to get some perspective.

Thanks!

[superstingray77]

Im running a large solid cam with .525 lift and 236 deg duration with my 4.7 in my 99 WJ. With a MAP adjuster it will run fine with a stock program you just have to pull some voltage out to get the fuel to lean up a bit, it will want to richen due to the lower vac signal on the MAP sensor. Mine is a daily driver WJ 4x4 with 32" Kevlar Mudders and 4" lift.

Now if you want it to run correctly vs. a band-aid you will need to ping FLYINRYAN on this forum and spend a few bucks getting a custom tune for your PCM using an SCT flashpack. I have done so and the difference is nothing short of amazing.

My advice keep the larger cam so you can fill the cylinders properly on the larger motor and tune the PCM to match.

[FlyinRyan]

Welcome to the forum.

That stuff does matter...pistons affect compression ratio, and that affects the octane demands of the engine (among other things).

With the right kind of datalogging, I can fix these issues and improve performance. I can tune the stock ECU through re-programming, addressing issues that not even the most skilled MAP adjuster can touch. I provide not a bandaid fix but a true tuning solution. You can PM me or call me at 732 539 9614 (best time to reach me is 6pm-midnight East Coast time) . Since I too am new here (but not to Jeeps), I would be happy to refer you to past/present clients that can vouch for my work.

Hi Guys,

First post - been asking around on other forums but can't get an answer, hoping you guys can help me figure this out.

My mechanic built up a 4.0L stroker in my 2000 XJ. Not sure exactly which pistons were used, or if they were dished; not sure how much that matters though. The head is from a 1993 XJ, and I have a 63mm TB. I have the stock 2.25" exhaust still.

He put a Comp Cam 239-4 in there because he said it would let me run 87 octane so I didn't have to run 93 octane. The engine runs really rough at idle, and my gas mileage averages less than 10mpg. I'm thinking that if I were running a Comp Cam 232-4, even with 93 octane, I'd be getting a lot better gas mileage.

I see that Comp Cam says the 239-4 shouldn't be run with an EFI engine, and my understanding is the stock ECU can't read the aggressive cam, so Comp Cam advises against it.

I want to avoid having to pull the 239 and put a new 232 in because of the costs involved... but I realize that may be my only option.

I've read some people advance the 239 2* to 4* to get the ECU to respond correctly, but I've never advanced or retarded a cam so I don't know if that's something I can do... or even it it would work.

I'm also wondering if a UniChip or Jet Stage 2 would help the ECU read better, fixing this idle issue and hopefully the mpg issue, or if that's just a lost cause.

Basically, I'm open to advice and would love to get some perspective.

Thanks!

[Retlaw01XJ]

I've read some people advance the 239 2* to 4* to get the ECU to respond correctly, but I've never advanced or retarded a cam so I don't know if that's something I can do... or even it it would work.

The 239 cam has more valve overlap than stock or mild performance cams.......That will produce a lower vacuum which I suspect the ecu has problems with. Advancing the cam is unlikely to change that. Besides, advancing the cam would close the intake valve sooner, increasing the dynamic compression ratio and make the engine more likely to ping.

I have no experience with ecm mods or tuning, but a good tune sounds like a good idea.

[acer44]

This is awesome advice guys - a huge relief off my mind.
Thanks!
I'll pm flynryan!

[Retlaw01XJ]

Let us know how it goes!

[gradon]

Advancing the cam will reduce the overlap, thus raise the dynamic compression. You would need an adjustable timing set in order to do it properly. +4* would be worth a try.

[Retlaw01XJ]

No, the valve overlap is ground into the cam, you can't change it by advancing or retarding the cam timing.
Valve overlap is the time when both intake and exhaust valves are open.....exhaust is closing and the intake is just beginning to open.

[Cheromaniac]

The 239 cam has more valve overlap than stock or mild performance cams.......That will produce a lower vacuum which I suspect the ecu has problems with. Advancing the cam is unlikely to change that. Besides, advancing the cam would close the intake valve sooner, increasing the dynamic compression ratio and make the engine more likely to ping.

I have no experience with ecm mods or tuning, but a good tune sounds like a good idea.

Exactly. Advancing the cam will only increase the DCR and the engine's tendency to ping without solving the low vacuum problem.
A good tune is indeed essential so that the MAP sensor doesn't respond to the low vacuum by sending a signal to the ECU to dump in more fuel. It seems FlyinRyan has that covered.

[acer44]

Thanks all for explaining the overlap and timing.

I was worried about advancing, which I don't have any experience doing so I don't trust myself. So that made me lean towards putting a 231-4 or 232-4 into the engine. But, from the sounds of things, that will defeat the purpose of stroking the engine since it won't allow the extra volume in the cylinder fill like it should.

From what I gather, I should ignore the fact that Comp Cams markets the 239-4 for the 1,400 to 5,700 rpm range, and pay attention to the fact that it lets more air/fuel into my stroker.

To me, this also highlights that the stroker mechanics are only half the equation. The other half is that the computer has to be setup to compute the larger volume and 239-4.

Is this all correct? Am I getting it right?

Man... I'd love to get better than 10mpg. I've been keeping a gas log... about ready for another fill up and only have traveled 160 miles on my tank so far.

[Russ Pottenger]

Any updates since the PCM tuneup?

[FlyinRyan]

Unfortunately, I have not heard from the OP since November. We exchanged several PM's and left off stating that he would not be available to do this until January. He hasn't logged into this site in nearly 2 months which is not encouraging. I hope I hear from him, as I would love to (and can) help him out. I've done several more strokers since then and have not only expanded my library of stroker calibrations, but I have had time to further refine them as well. Any questions about what I do, feel free to ask...

[Desertjr]

Flying yawn I've got a few questions can you pm me?

[acer44]

Sorry guys! It's January, and I'm here!

Ready to get the ball rolling with Flyin' Ryan, just got done searching through my email for the contact information.

[acer44]

Any updates since the PCM tuneup?

A bit ago I built up the MAP adjustor and tuned it a bit lean. I started keeping a gas log earlier so I could have a before and after comparison. I picked up a bit more than 1mpg, so I can't wait to see what I can do with the custom tune.