Jeeps dont seem to like high RPM's

[Rjackson32]

I own a '98 cherokee and I have been in new rubicons and such and I have realized that Jeeps don't really like the high RPM range. Why is it that when they hit about 5k RPM's they start to shake violently and not really accelerate as well as they do from about 2-4k? Is it bad for them to be in the 5k range. I mean, i dont bring it there very often, but every once and a while.

 

[langer1]

I own a '98 cherokee and I have been in new rubicons and such and I have realized that Jeeps don't really like the high RPM range. Why is it that when they hit about 5k RPM's they start to shake violently and not really accelerate as well as they do from about 2-4k? Is it bad for them to be in the 5k range. I mean, i dont bring it there very often, but every once and a while.

Because the 4.0L inline 6 is more of a heavyduty truck motor than a race engine.

 

[RichP]

It also has a retarder, not sure if rpm's effect it but it kicks in big time over 120mph... big time as in 'who thru out the anchor'...

 

[PaulJ]

The 4.0 is an engine that's not really intended for use above 4K RPM. Get a rice racer with little pistons and short stroke if you want to rev. The 4.0 is built for torque at low RPM.

 

[Cherokee91Red]

Why is it that when they hit about 5k RPM's they start to shake violently and not really accelerate as well as they do from about 2-4k? Is it bad for them to be in the 5k range.

Did all the red lines after the 5,000 RPM mark not lend a clue?

 

[Rjackson32]

The 4.0 is an engine that's not really intended for use above 4K RPM. Get a rice racer with little pistons and short stroke if you want to rev. The 4.0 is built for torque at low RPM.

I've got one, VW R32, but I was just asking the question. I dont bring it up past 4.5k very often just because its a truck. But when i do, I wanted to know why it shudders like it does. Is it bad for it, or it just doesnt like it?

 

[5-90]

Granted, the 4.0 is made for low-end torque - that's the mainstay of the inline six.

However, it's possible to run the 4.0 at high RPM - there's a "harmonic" at 6000-6500rpm that should not be loitered in, and I'm sure that you'll have to put a few bucks into balancing the reciprocating assembly if you want to live that high up for any length of time - with the low-RPM intended runtime of the 4.0, I'd not think a lot of work was done to make it spin high.

I probably "touch" 5000rpm in my 88 about once a year - just to make sure it will still be OK there. "Torque makes you quick, horsepower makes you fast" - and I'd sooner be quick than fast. Being "quick" makes evasive maneuvers much more effective...

5-90

 

[summitlt]

I had a parts truck that I regulary took upto 6300 RPM in 20 degree weather while the engine was still cold. It took it, you can "hear" the vlave float, it gets a different tone to the revs and doesnt rev anymore. I definatly wouldnt do it ot me daily drive, or anything I wansted to keep, bu tit made me feel better about hitting 6K missing a shift on my DD.

So basically, the I6 can handle 6300 RPM, for a logn period? Who knows, I did for about 15 seconds, and thats a LONG time when the motor sounds like that.

 

[5-90]

I would say, essentiallly, no. Not only do you have issues with valve float (which can be cured with a spring/lifter change,) but there is the "harmonic" I'd mentioned earlier can be an issue. As I recall, you'll hit it somewhere between 6000 and 6500 rpm - under, you're fine. Over, you're fine.

It probably has to do with the length of the crankshaft and distribution of impulses along that length - after all, I've not heard of such a problem with V6 or H6 engines.

It's also possible that this is apocryphal - I've not taken the opportunity to verify it yet (I consider the redline on my tach to be serious, I've never hit the rev limiter, and I barely break even 4000RPM in my rig!)

5-90

 

[raypla]

Gas is just to expensive to go over 3k let alone 6-7K.

 

[87manche]

Gas is just to expensive to go over 3k let alone 6-7K.

I hear that. I hit 4500 RPM in the Mj for the first time today in the 2 months that I've owned it. I don't think I'll ever do it again. Not only was she noisy as hell, the power there was just plain scary. In a light pickup truck it was not problem to fishatail the rear, even with 31's and 3.07s.

 

[XplodJeepster]

the shaking is probably the engine trying to flip itself over on the mounts.

with just me in the jeep I can get about 3-4 inches of side to side rocking just from a flurry of revs.

(the shifting rpm on the stack tach in a 92 laredo is ~2500 , all the gears in the 5 spd trans are real short and torquey)

 

[Rjackson32]

So the consensus is it can take it every once-and-a-while but it is happier and will live longer under 4k? I thought that was true, just wanted to check. Merry Christmas Everyone.

 

[Colorado_XJ]

That's how I've always treated my XJ's. Everything in moderation, grasshopper. Happy Holidays.

-Rich

 

[Dave in Eugene]

Limiter kicks in at 120 huh? Had no idea it'd do 120 and I'm not sure I'd want to be along for the ride.

Yeah I noticed mine shakes pretty good at high revs too so I just don't rev it high. Torque peaks on mine at about 65 MPH in 5th gear, not sure what RPM that is but I know it's a long way from where the shaking starts. I'm pretty sure HP peaks well below that level too, so there's little need to ever spin up that high.

I'll know that RPM after tomorrow hopefully, because Santa (that's me :wave: ) got himself a nice second hand tach cluster for being good. Mrs. Santa got him tools, his brother got him tools, his girls got him tools, and not just duplications of things he already had, but things he needs and can use for projects underway. His 5 mo grandson got him a big smile (Santa must have been very good). Some of that will help fix his broken oil pressure gauge and add the tach he wanted.

Merry Cristmas everybody.

 

[whatevah]

Limiter kicks in at 120 huh? Had no idea it'd do 120 and I'm not sure I'd want to be along for the ride.

a not-so-bright guy I know said he had his 96XJ close to 125mph chasing a friend driving a Camaro... I don't think I've ever had mine over 90, except for a few seconds at 100 when I was chasing a Dodge Magnum who was chasing a Mustang. made for a nice diversion during a 200 mile trip (albiet, not a smart one)

 

[bjoehandley]

the shaking is probably the engine trying to flip itself over on the mounts.

with just me in the jeep I can get about 3-4 inches of side to side rocking just from a flurry of revs.

(the shifting rpm on the stack tach in a 92 laredo is ~2500 , all the gears in the 5 spd trans are real short and torquey)

I've noticed that in my '98 too, plus if you keep mine in the torque peak long enough the whole truck starts to lean to the right.

 

[Dave in Eugene]

Ever see a Dodge V-10 truck fire up? They have a very noticeable nod to the right - must have lots of low end and a heavy flywheel. Makes me wonder how it's possible to take off in one of 'em without scratching the tires.

 

[dizzymac]

What the. hell are ya doin' putting a 4.0 up in the 5k range. I'm not sure but I hear tell you can the chair for that....4.0 abuse, 4.0 abuse......lol

 

[Rjackson32]

What the. hell are ya doin' putting a 4.0 up in the 5k range. I'm not sure but I hear tell you can the chair for that....4.0 abuse, 4.0 abuse......lol

Haha, yeah, I was asking if it was abusive or not. I guess it is. Ill try to keep it under 4k from now on. Thats why I'm on the forum.