Damn OBDII
I've got some 4.0L build issues. The vehicle in question is my '98 Jeep TJ, 4.0L, automatic with 4.56s and 35s. The specs for the 4.0L build-up are as follows:
'01 low-mileage complete long blockHesco Aluminum Head with spec for 9.5:1 compression ratioHesco H264 cam advanced 4-degreesHesco roller rocker kitHesco double-roller timing chain kitLate-model '99-up intake Rebuilt and flow-balanced stock injectorsAEM Brute Force intake with throttle body spacerDoug Thorley Header2.5-inch mandrel-bent exhaust with Spintech Sportsman muffler
Before, the Jeep baselined at 122hp and 154 lb-ft torque. After the build it put down 139hp and 152 lb-ft. A compression check led me to believe that the motor actually lost compression as compared to stock. Based on reading about various builds in Peterson publications, I got in touch with Mark MacNeil at the Dynoshop and we tuned the Jeep with a Unichip. That got me another 10hp and 36 lb-ft of torque for 149hp and 188lb-ft.
Eventually after some internet exchanges, Hesco ponied up for another head to get me to 9.5:1 compression which is where I was supposed to be initially since I hadn't seen the claimed 30hp gain. The company also sent along a cam timing tool and a set of shorter pushrods to accommodate the milled head. I then tore down the motor and swapped in the new head. I verified again where the pistons were sitting in the head (within factory spec) and also that I did indeed have the cam properly advanced to 4-degrees BTDC.
1. The Jeep has dyno'd at exactly the same numbers with the higher compression head so I am looking for input as to why this might be. It drives OK, but for the equipment I have on the Jeep I am of the opinion that there is potential that has not been realized.
2. I also have an ongoing problem with the Unichip installation which has been there since day one. The Unichip will constantly flash the check engine light for misfires as well as other phantom codes that are not listed in my FSM. Use of a Mopar scanner reveals that there are multiple cylinder misfires occurring with the Unichip enabled.
MacNeil at the Dynoshop says that Unichip is aware of this, as it commonly occurs in Jeeps with automatic transmissions (it's supposedly a CPS issue) and that they are working on a software patch.
In the meantime, I have a tune that cost me $1,700 that is not satisfactory. Mark gave me a Scangauge II to clear the codes, but frankly, the CEL and misfire issue needs to go away.
Any advice would be welcome. The Jeep runs OK, but gas mileage is not that great (10-11mpg) and while I have a nice flat torque curve, I am thinking that I should have more. I'm wondering if there is another tuning option out there that might help me get this motor dialed in.Robert J. Yates, Riverside, California
I can't come up with too much more than what you've been told. The same thing happened to me with the installation of a 4.6L stroker in Jp's '99 XJ. The original 4.0L build (Performance Distributor's ignition, Gibson header and exhaust, 62mm throttle body, Hyertech programmer) dyno'd at 152hp and 200 lb-ft. The 4.6L stroker brought it up to 172hp and 224 lb-ft. The Dyno Shop dyno-tune with a Unichip brought it up to 184hp and 235 lb-ft.
While I didn't see a loss, the numbers just weren't what I was expecting either. I think ultimately the problem lies in the '96-up Jeep OBDII computer system. It's just not user-friendly when it comes to aftermarket modifications. They seem to support superchargers and other add-on mods OK, but as soon as you crack into the valvetrain and start messing with cam and timing events, the computer gets confused.
The Unichip will piggyback onto the stock ECU to control the timing and fuel tables to maintain the correct air/fuel ratio. It modifies these tables based off readings recognized by the factory computer taken through the factory sensors. I don't recognize the scale Mark used on the Air/Fuel chart you attached, but I'd be curious to know how closely the system is coming to leaning out during the dyno run. All those engine mods with the stock 19lb/hr injectors has me thinking you may be better off stepping up to some 21- or 23-lb/hr units. There's a possibility that the Jeep is experiencing some detonation under normal driving conditions that aren't simulated on the dyno. Remember, closed under hood temperatures, increased loads from acceleration and G-forces, and so on can all stretch the parameters of what is tolerable. Driving the vehicle with a wide-band oxygen sensor would tell you what's going on in the exhaust stream as you drive.
I wouldn't expect to see an incredible amount of difference just based on the compression. As a general rule of thumb, you're only going to see an additional 10-15hp increase per point of compression on a healthy V-8. And that rule of thumb is usually associated with compression ratios of 10:1 or higher. Going from 8.8:1 to 9.5:1 should get you a little power increase in a small 4.0L six, but the real upgrade is in throttle response and drivability. I'd also pop the valve cover and make sure the roller rockers aren't contacting anything as they saw. If there's any binding you'll get less lift with the new cam than you did with the stock one. To properly check you'll need to pull at least one of the intake and one of the exhaust rockers off and inspect it. Also, double check that you've got the proper lash - your hydraulic cam should require zero lash, but I'm sure you know all about that.
I'm really not an electronics guy, but I'd suspect the engine misfire code could stem from one of a few things. First, it could be possible that the engine is running out of fuel at certain rpms under certain conditions and is trying to fire the injector twice to compensate. I'm just speculating here - I've got no basis in fact. Second, the Unichip could be misinterpreting signals from the factory ECU as it scales timing back to prevent detonation under load conditions. Also, the fact that your CPS isn't exactly reading what the valvetrain is doing since the cam is advanced 4 degrees may have something to do with the phantom code issues, but that doesn't seem as likely to me.
I really do stand by MacNeil's work at The Dyno Shop. It's been my experience with six or seven different vehicles that I've brought them to work on that he and his crew really do know their stuff. I'd start by bring up the possibility of fuel delivery and lean/detonation condition issues that may occur in the real world out on the road versus what can be simulated on the dyno.
I'd also think about maybe stepping up the injector size. Your 19lb/hr stock injector may be able to deliver enough fuel through your engine's powerband under simulated conditions, but to do so it's firing for a longer period of time. To deliver X-amount of fuel, your stock injector needs to hang open X-amount of time. A larger injector is able to deliver the same amount of fuel, but in a much shorter firing time. It doesn't need to "hang open" as long to get the job done. It's possible with your advanced camshaft timing and the ignition timing tables being manipulated that the fuel and ignition systems just can't sync up properly to keep the codes from being thrown.
Sorry if all I've done is confuse you. Again, it's mostly speculation on my part, but hopefully something I've spewed out here may help you with your situation.
Got a tech question you're just itching to get answered? Send it on in to Jp magazine, Your Jeep, 6420 Wilshire Blvd., Los Angeles, CA 90048, or e-mail christian.hazel@jpmagazine.com.