More efficient above 2500rpm Why?

[karl4x4]

Anybody knows what happens at 2500 rpm?

I have a 1998 PCM (for auto trans running a 6 speed manual) running a stroked 4.7, Hydro cam DURATION AT 050 INTAKE 208 DURATION AT 050 EXHAUST 212 GROSS VALVE LIFT INTAKE .427 GROSS VALVE LIFT EXHAUST .438 SEPARATION 114, 9.2CR, 1994 head, 24lbs injectors, 2000 intake, cold air, headers, straight pipe all the way.

I have a wide band O2 sensor (in stock location) and digital gauge feeding a programmable analogue signal (programmed to simulate stock O2 sensor) to my PCM and I noticed that the engine always runs slightly rich with or without load until it reaches 2500 rpm. When it reaches 2500, under med or little load it will run at 14.7 or slightly leaner air/fuel ratio (much better on gas). Under hard load, it reverts back to slightly rich and WOT at any RPM it goes even richer. It's not speed related because it works in every gear.

Anybody knows what changes for this to happen? I would prefer if it always ran at 14.7 or slightly leaner except at WOT but when I program my sensor, it affects all RPMs and becomes too lean after 2500 RPM.

[mountaineerjeff]

its how it works basically. when running more throttle the map switches and dumps more fuel to deal with the higher power demands.

[karl4x4]

its how it works basically. when running more throttle the map switches and dumps more fuel to deal with the higher power demands.

That is true. What I am trying to figure out is what happens between 2400rpm and 2500rpm for then engine to change the A/F ratio for no particular reason other than engine speed. It does it in any gear so load is not suspected to be the influencing factor.

[Cheromaniac]

I noticed that the engine always runs slightly rich with or without load until it reaches 2500 rpm. When it reaches 2500, under med or little load it will run at 14.7 or slightly leaner air/fuel ratio (much better on gas).

That's funny 'cause mine runs the other way round. At low/medium load my engine runs an AFR of 14.7 or leaner at lower rpm (good for highway fuel economy) and at 3000rpm it suddenly becomes richer. Under part throttle acceleration it feels like a small shot of nitrous kicking in at 3000rpm and the Jeep takes off.
I think the reason is the way the ECU is programmed.

[SilverXJ]

I remember reading a thread someplace that stated the Jeep PCM has a lean spot at a certain to cool that cat converters.

[karl4x4]

I noticed that the engine always runs slightly rich with or without load until it reaches 2500 rpm. When it reaches 2500, under med or little load it will run at 14.7 or slightly leaner air/fuel ratio (much better on gas).

That's funny 'cause mine runs the other way round. At low/medium load my engine runs an AFR of 14.7 or leaner at lower rpm (good for highway fuel economy) and at 3000rpm it suddenly becomes richer. Under part throttle acceleration it feels like a small shot of nitrous kicking in at 3000rpm and the Jeep takes off.
I think the reason is the way the ECU is programmed.

Seems to be the logical way it should work. Now Im suspecting MAP sensor that may have a "sweet" spot. Something to check I suppose.

[SilverXJ]

Under part throttle acceleration it feels like a small shot of nitrous kicking in at 3000rpm and the Jeep takes off.
I think the reason is the way the ECU is programmed.

I'd be more inclined to contribute that 3000 RPM jump under accel to intake manifold tuning.

Seems to be the logical way it should work. Now Im suspecting MAP sensor that may have a "sweet" spot. Something to check I suppose.

Under cruising the PCM is relying minimally on the MAP sensor and mostly on the O2 sensors.

[karl4x4]

Finely figured it out. I tested my MAP sensor and its withing specs then,
After driving around for 2 hours, entering several o2 inputs to the PCM and recording the results on my data logger I realized that the PCM ignores all O2 sensors input after 2500rpm and the computer reverts to other sensors.

So I Just programmed my sensor to run the engine at 15:1 (I want to tune it to 15.4 like the graph but I have to find a long flat stretch. To many hills and red lights here and it screws up my data) and when it passes 2500rpm it still goes closer to 14.7 ish anyway. At WOT it goes to about 13

When its programmed properly, Ill do a full tank gas mileage check to see the difference.

http://www.mummbrothers.com/SRF_Stuff/images/Stoich.gif

[Cheromaniac]

I'd be more inclined to contribute that 3000 RPM jump under accel to intake manifold tuning.

That's also possible but my AFR gauge does suddenly go from stoich to rich at 3000rpm under part throttle so I think the ECU tuning does have something to do with it, especially since under WOT that 3000rpm "jump" in acceleration doesn't happen and the AFR doesn't suddenly change either.

[karl4x4]

I'd be more inclined to contribute that 3000 RPM jump under accel to intake manifold tuning.

That's also possible but my AFR gauge does suddenly go from stoich to rich at 3000rpm under part throttle so I think the ECU tuning does have something to do with it, especially since under WOT that 3000rpm "jump" in acceleration doesn't happen and the AFR doesn't suddenly change either.

What sie injectors did you put in? What year CPU?

[Cheromaniac]

Stock '92 ECU and Ford 24lb injectors. Mind you, I still had that part throttle 3000rpm "surge" even when I had my old 4.0 engine and stock injectors.

[karl4x4]

Stock '92 ECU and Ford 24lb injectors. Mind you, I still had that part throttle 3000rpm "surge" even when I had my old 4.0 engine and stock injectors.

I had the same surge when I was running my stroker with 24lbs injectors with my 88 PCM but same engine put into a 98 PCM no surge. I always had a 94 air intake until recently I put the 2000 one on. Still not surge. Just a little more torque

[FlyinRyan]

The way JTEC works, it has a series of limiters or thresholds that need to be expired for there to be enrichment. SO, depending on the calibration, enrichment is achieved by exceeding a certain time, rpm, MAP threshold. Where that happens is totally based on TPS, MAP, RPM as it is not a set threshold across the board. I completely re-configure this in my cal's. Later.....

[karl4x4]

The way JTEC works, it has a series of limiters or thresholds that need to be expired for there to be enrichment. SO, depending on the calibration, enrichment is achieved by exceeding a certain time, rpm, MAP threshold. Where that happens is totally based on TPS, MAP, RPM as it is not a set threshold across the board. I completely re-configure this in my cal's. Later.....

How would you normally tune engine other than the O2 sensor input?