Chasing performance issue

[dasbrick]

So. Trying to chase down this performance issue with the MJ.

 

87 4.0 BA-10/5

 

The First and Second gear run fine, speed up fine, third gear is fine, but the fourth gear and ESPECIALLY the fifth gear just dog. Is this a fuel issue, or is this a transmission issue? I am a volunteer firefighter, and I need my speed.

[mnkyboy]

Has it been like that or is this new?

 

My guess would be 3.07 gears is your problem

[dasbrick]

Been since I got it, approx 1.5 months ago.

 

Yes, 3.07 might be my issue but my cherokee had 3.55 and it was PSYCHO fast. granted, some was the AW4 but still, it was really fast.

[mnkyboy]

if your comparing it to the 92 in your sig the MJ is gonna feel really slow.

 

the 4.0L HO doesnt have much more HP then your 4.0L renix but it delivers it a whole lot better.

[glundblad]

I have the same year engine as yours along with the same transmission and gearing. I have figured out that the gearing is so high, I rarely use 5th. I really understood what was going on after I added a tach (and posted questions on the forum). I thought about changing the gears to 3.55 or 3.73 but I figured out I would have the same result keeping it in 4th gear with 3.07. Then 5th is available for the interstate if needed (any who wishes to argue this point please chime in).

 

I have 28.9" tires btw.

 

I see you are in Michigan. I am about 40 miles west of Ft Wayne, IN. I don't drive my MJ right now due to the weather but if you are curious, contact me some saturday if you wish (in the spring or summer after the salt is gone). Just give me as much notice as possible. You can drive mine and see if you are having the same trouble. That assumes it is a close enough drive for you.

[Eagle]

Been since I got it, approx 1.5 months ago.

 

Yes, 3.07 might be my issue but my cherokee had 3.55 and it was PSYCHO fast. granted, some was the AW4 but still, it was really fast.

You are comparing an HO against a non-HO. The Renix (non-HO) version of the 4.0L engine has a torque peak at around 2400 RPM. It isn't meant to be a street rocket, it was designed to produce torque at low RPM. The torque peak on the '92 HO version is at 4,000 RPM. It breathes much freer at high RPM and revs easier, but it doesn't have the low-end power/torque that the Renix version does.

 

That said, the Renix easily cruises at extra-legal speeds and volunteer firefighters are not authorized to exceed the speed limit under any circumstances, so I fail to see a problem. My MJs and my original XJ are all Renix models. I don't drive that fast but 80+ MPH is not a problem. But 5th gear is useless under 55 MPH -- if you're trying to use overdrive at lower speeds, you're lugging the engine and just beating up on the entire drive train.

[dasbrick]

That said, the Renix easily cruises at extra-legal speeds and volunteer firefighters are not authorized to exceed the speed limit under any circumstances, so I fail to see a problem.

 

Here in Michigan, we have Public Act 300. States that a volunteer firefighter en route with a POV to a emergency call is considered an authorized emergency vehicle, and is allowed to disregard posted limits within due regard for safety. So we can speed, as long as it is safely. We also need to have a light visible from 360º at 500' and a siren that can reach 500'.

 

 

To everyone else: OHHHHHHHHHH. ok. sacrifice speed for torque. got it. Thanks all.