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Old 04-29-2018, 12:58 AM
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Revving the 4.0

So I rarely ever take my 1993 4.0 past 2500 RPM, but tonight after work I was having a little fun with my friend who has a 5.0 Mustang. While we waited at a long red light, we had fun having a "rev-off" (my YJ has no muffler so it actually has a fairly loud growl). The highest I went was somewhere around 4500 RPM. Again, these revs were just bursts, I didn't hold the pedal down and keep it there. Not sure if my 93 has a rev limiter or not but I did not want to find out the hard way.

This clearly seems to be a ping-pong question, as nobody can decide if revving in neutral causes damage to your engine. Am I just being paranoid or is that really not good for your engine?

And before you add, yes I know it's pointless and I know I can avoid doing it. But don't act like you've never had some fun with the skinny pedal before

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Old 04-29-2018, 01:08 AM   #2
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I bought my 94 sahara w 170k miles and have been regularly bouncing off the rev limiter for 10 years and almost 100k additional miles....

Offroad slick wot low range 2nd or 3rd gear hill climbs are the norm....
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Old 04-29-2018, 08:15 AM   #3
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Originally Posted by Kbh7607 View Post
So I rarely ever take my 1993 4.0 past 2500 RPM, but tonight after work I was having a little fun with my friend who has a 5.0 Mustang. While we waited at a long red light, we had fun having a "rev-off" (my YJ has no muffler so it actually has a fairly loud growl). The highest I went was somewhere around 4500 RPM. Again, these revs were just bursts, I didn't hold the pedal down and keep it there. Not sure if my 93 has a rev limiter or not but I did not want to find out the hard way.

This clearly seems to be a ping-pong question, as nobody can decide if revving in neutral causes damage to your engine. Am I just being paranoid or is that really not good for your engine?

And before you add, yes I know it's pointless and I know I can avoid doing it. But don't act like you've never had some fun with the skinny pedal before


4.0 TJ 2002 with 100000kms - regularly sit 3000-4500 while sand duning occasionally tickling the rev limiter and it seems quite happy (regular oil changes help I think)


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Old 04-29-2018, 09:40 AM   #4
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There really isn't much that can happen. In more high performance applications you can get valve float.
With that said the more your engine vibrates, the more likely you can rattle something loose.
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Old 05-03-2018, 08:42 AM   #5
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You can get away with that most of the time... But as an engine guy, I'll tell you the two things that cause catastrophic failures.

1. Over heating.
2. Over revving.

There is a balance there, if you are running cool, over revving is fine. But if you combine the two, and you continue to over rev an engine running over about 230, you can really start to pop gaskets and in some cases shoot a rod out the oil pan.
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Old 05-03-2018, 10:17 AM   #6
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You can get away with that most of the time... But as an engine guy, I'll tell you the two things that cause catastrophic failures.

1. Over heating.
2. Over revving.

There is a balance there, if you are running cool, over revving is fine. But if you combine the two, and you continue to over rev an engine running over about 230, you can really start to pop gaskets and in some cases shoot a rod out the oil pan.
If your engine is hitting 230, your already in trouble.
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Old 05-03-2018, 11:01 AM   #7
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I can't agree w either of those comments....
OVER revving is hard on an engine no matter the temperature
230 isn't gonna hurt anything .... w a 15 psi rad cap the boiling point is over 250
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TJ dash speakers that actually rock! Kicker sub and amp in center console
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Old 05-03-2018, 11:07 AM   #8
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When I was a kid, we'd rev our Ford flathead engines to whatever RPM (none of us had a tach) and say "That's good for them. It blows the carbon out".
How true that might have been on an old flathead I don't know, but it sounded good.
I replaced a bunch of first/reverse gear clusters in those transmissions from dropping the clutch at whatever higher RPMs the flathead V8 could make.
It's called "Pay to learn".
Now, I take it easy and just putt along and don't break anything.

Your rev limiter probably is designed to prevent valve float. At higher RPMs without a rev limiter a piston could hit an open valve with potentially disastrous results.
I think WOT acceleration is harder on a motor than an occasional blip to 4500 RPM.

Good Luck, L.M.
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Old 05-03-2018, 12:42 PM   #9
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Redline on my tach (4.2) is 4500. I rarely hit 3000 RPM.
I've removed my computer and thus my rev limiter if I ever had one.
After 35 years of scaring my self three quarters to death on Harleys, hotrodding in a car, truck or Jeep just doesn't have any appeal. Having survived to this point, I'm committed to making my senior years trouble free.
Even busting my knuckles repairing something I've torn up doesn't have any appeal.

Our gauges are there for a reason. Keep them functional and keep an occasional eye on them and you'll have fewer problems.
Easy does it make for an easy life.

Good Luck, L.M.
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1987 YJ-4.2L-Standard Shift- Re-manufactured Carter Carb-2" body Lift-31X10.5X15 BFG KOs-190K Miles No back seat.
HEI distributor with computer and all related relays and wiring removed.
12K Badlands winch with dual batteries.
Warn front bumper. Matching imitation Warn bumper on the rear.
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Old 05-05-2018, 11:03 PM   #10
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It’s too high of piston speed that’ll tear up a motor and with our long stoke I6s the piston speed at 4000rpm is close to a short stroke Honda at 5-6rpm
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Old 05-06-2018, 11:08 AM   #11
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Revving the 4.0

It is piston acceleration that tears up the engine and the higher the rpm the greater the piston speed and acceleration

With my old TR3 with a 3.6 inch stroke at redline 5000 rpm the mid stroke piston speed is about 100 mph so at 5000 rpm that piston goes from stoped to 60-70MPH to stopped 10,000 time in one minute as it reaches max speed both up stroke and down stroke for each revolution
Force equals mass times acceleration

You as rpm rises you have significantly increased forces on rotating assembly

Anyone who thinks valve float is the only issue is uninformed

Anyone who thinks just because tranny is not in gear that revs cannot damage an engine lacks basic understanding of physics


The 4.0 likes to run at 2-3 K RPM and the efi engines generally have built in rev limiters

But excessive rpm can damage any reciprocating piston engine even at normal operating temperature


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Old 05-07-2018, 02:32 AM   #12
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Originally Posted by Digger84 View Post
It is piston acceleration that tears up the engine and the higher the rpm the greater the piston speed and acceleration

With my old TR3 with a 3.6 inch stroke at redline 5000 rpm the mid stroke piston speed is about 100 mph so at 5000 rpm that piston goes from stoped to 60-70MPH to stopped 10,000 time in one minute as it reaches max speed both up stroke and down stroke for each revolution
Force equals mass times acceleration

You as rpm rises you have significantly increased forces on rotating assembly

Anyone who thinks valve float is the only issue is uninformed

Anyone who thinks just because tranny is not in gear that revs cannot damage an engine lacks basic understanding of physics


The 4.0 likes to run at 2-3 K RPM and the efi engines generally have built in rev limiters

But excessive rpm can damage any reciprocating piston engine even at normal operating temperature


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Very nice write up - anyone done the numbers on the 4.0L? Just landed after operating a 12hr flight so my brain is about as useful as tits on a rocking horse...


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Old 05-07-2018, 02:34 AM   #13
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Itís too high of piston speed thatíll tear up a motor and with our long stoke I6s the piston speed at 4000rpm is close to a short stroke Honda at 5-6rpm


Explains why mine tends to hold its own at about 33-3600rpm - any higher it tends to make more noise and not much more horsepower lol


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