FrazierMedina

I have a 1990 Jeep Cherokee Renix 5 speed, ax-15 4.5" long arm and 33's. it’s the base model and everything in the engine is stock, some new parts but no upgrades. Ever since I got the Jeep stock a little over a year ago, it’s had super slow acceleration. Taking me about 10 seconds to reach 60MPH using .4 miles of road. I’ve always assumed it was because it’s a 26-year-old Jeep. After looking at videos of 88-90 Jeeps accelerating fast, I was confused on how they are able to accelerate and have high rpm's no issue with both auto and manual. My tires have little to nothing to do with it since I have the same acceleration with 29's and 33's at same RPM. I've been having this issue before ever modifying my Jeep. With 29x9.5" tires 30x10.5" tires 31x10.5" tires and 33x12.5" tires, I've had the same exact acceleration at the same RPM's

I recently upgraded my cluster to one with a tach and no dummy lights. Here I noticed that the good shifting point is 2k rpm and it’s pretty loud at that (super 10 muffler) I don’t even want to take it to 3k RPM because of how much of a hard time it has getting there. 1: its uncomfortably loud (not an issue, just scary) 2: The Jeep feels like it’s going to explode. I watch acceleration videos on YouTube and see people at 4k rpm and no issue. In the videos their 5k RPM sounds like my 2k RPM.

Could there be an issue with anything not allowing me to accelerate fast or get to higher rpm's smoother?
Why do I have slow acceleration?
Am I just being a baby about it and not pushing it hard enough to get high rpm and fast acceleration?

aroundincircles

With 33" tires, if you did not regear your axles, you'll never get up to speed. Its like starting in 3rd gear all the time.

FrazierMedina

Like i said at the end of the first paragraph. It has the same acceleration at same RPM's with 29" tires. This has been an issue before i even modified the jeep in any way.

aroundincircles

http://cruiser54.com/

Read that first.

and why did you modify a vehicle before fixing issues? seems kinda backwards to me.

cruiser54

Your shift points are WAY too low. Take it to 3500 or 4000 RPM.

caged

yep, you just gotta put your foot into it more.

with a 5 speed, your axle ratio is low at 3.07, so a regear, or even swap out your axles for some out of an automatic. they have 3.55. although not the best, but it would be better.
ideally, you want somewhere in the 4.10 to 4.56 range. but a regear is pricey.

remember that a renix only has 170 horsepower stock on a good day, compared to an H.O at 190. so there will be a noticeable difference.

cruiser54

Nope. 173 in 87, 177 in 88, 182 in 89 and 90.

And just as much torque as an HO.

HO myth buster


Renix in 90 made 182 HP. HO in 91 made 190 HP. That's 8 HP difference.

HO only made more HP than Renix at higher RPMs and not a bit more torque. HO had a 58 mm throttle body versus a 52 mm throttle body on a Renix. That’s 20% more air available through the HO throttle body. The HO also had a better design header. See where I'm going with this?

It’s only a 4% horsepower increase…..

The whole 8HP was not mostly from the head, but from the bigger TB and better exhaust manifold.

Put a 60mm TB from www.strokedjeep.com on your present manifold using the Renix head, eliminate the "crush" in your headpipe with proper re-routing, and go for it.

HO stands for Highly Overrated.
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'90Cherokee

Simply not possible. With the 33s you'll have less acceleration and you'll be going faster than what the speedo reads unless you recalibrate it.

Shifting at 2k; even if the Jeep is in perfect condition it won't accelerate well.

Sounding like it's gonna blow up; either you're being panicky, something's wrong with the Jeep, or your tach is wrong.

cruiser54

Absolutely correct.

The 4.0 doesn't like to be, and doesn't need to be, lugged like that.

aroundincircles

agreed. that didn't make sense to me. I went from a 28" tire to a 31" tire, and it is a HUGE difference without regearing. (auto, 3.55 gears). and no way I would want to go to a 33" tire without re gearing.