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Old 09-20-2012, 09:39 PM   #1
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what rpm's can a 4.0 I6 handle on the hiway?

guys, I need to know what would be tolerable Highway Rpm''S in my 97 TJ inline six..
is 2500rpm..or 2700 too much for an hour or two? I would like to put the 456:1 new gears into my three speed ... but to hit even those rpm's I would need 35" tires.. and keep it to about 60mph.

mY other choice is the 410 gearing already in the same jeep = with the 31's already on it, we are turning 2700 rpms at 70mph..

In order to make a choice I need to know what level of rpm's would be acceptable ..I know the I6 gets good torque at low rpm's... and red lines toward 5000.
Please Help me with this.. I have stayed awake many hours searching for what the engine will tolerate in stock form and with some 100K miles on it.
thank you in advance for the help folks/
dk

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Old 09-20-2012, 09:49 PM   #2
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Yeah good question, mine is the 4cyl TJ, with 134k miles new cooling system (still stock) and 265/75r15s so no big tires and 70mph is 2500-2700 rpm as well, can it handle it for an hour or 2? I will be driving it to the beach soon, an I will have 2 hours of 65-70mph driving, before I have 3 other hours of 55mph driving

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Old 09-20-2012, 09:53 PM   #3
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If you have a 3 speed auto then you have 1 to 1 in top gear. With 4.10 gears and 31's you would be turning 3100 at 70. 2700 would be 60 mph. Did you recalibrate your speedo yet? The 4.0 is a short stroke, long rod motor and will handle a lot more RPM than we normally throw at it. I would not think 3000 for 2 hours is a big deal.
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Old 09-20-2012, 09:53 PM   #4
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To me 3k feels like its the top end of what it can handle for long distance but maybe it could handle more,it is a 4.0l .
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Old 09-20-2012, 09:58 PM   #5
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The 4.0l will run at 3k rpm all day with no issues.... Except the mpg will suck even more than it already does.
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Old 09-20-2012, 10:02 PM   #6
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You sure you have 4.10 in your Jeep? I have a 97 4.0 auto with 3.55 with 31" tires and turn 2700 at 70. I had it there for hours on the interstate in 100 degree weather and it got to about 230. Normal is 210. Got off and did 55-60 and it came back down. Runs fine at that RPM's
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Old 09-20-2012, 10:12 PM   #7
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Would the 4cyl also be ok at 2500-2700 for 70mph for 2 hours?
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Old 09-20-2012, 10:15 PM   #8
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And I have a manual
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Old 09-20-2012, 10:16 PM   #9
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My 2000 TJ 4.0L, 3spd automatic stays at 3000 RPM'S at 60mph. 35" tires and 4.88's. I wish I had a 4spd auto for the freeway. It climbs, but I tow or trailer it to wheel.
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Old 09-20-2012, 10:24 PM   #10
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I run 4.56's and 33's... at 2800 or so... I am at 65-70 I usually run around 80-85 on the freeway ( I know... don't start... Just being honest) and run over 3200 to 4000 a lot when I downshift to 4th. I can do this for hours on the freeway from Reno to Sac (Through Donner Pass- mountains )

even in town, I don't shift before 3000-4000 rpms in first or second gear.

IF you have never nailed the right pedal to the floor and NOT shift til 5000 or better, you have no real sense of power of the 4.0... it's no V8, but surprising to say the least.

Don't think I abuse my TJ... I don't. That's just the way I drive and that will never change ....not even when I'm driving my wife's Wolfburg Edition Jetta.
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Old 09-20-2012, 10:25 PM   #11
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So I'm good and so is he haha
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Old 09-20-2012, 10:41 PM   #12
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the 4 banger will turn 4k all day long, my last one had 5th gear out so i ran 4th at 4k no problems.
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Old 09-20-2012, 10:47 PM   #13
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Originally Posted by Sahara707 View Post
My 2000 TJ 4.0L, 3spd automatic stays at 3000 RPM'S at 60mph. 35" tires and 4.88's. I wish I had a 4spd auto for the freeway. It climbs, but I tow or trailer it to wheel.
This was the reason I went with the AW4 when I did my manual to auto swap.
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Old 09-21-2012, 12:21 AM   #14
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is it a 410? or 355? well let's see...speedo not corrected.. lol here is what I did:

Quote:
Originally Posted by Plowboy1970 View Post
You sure you have 4.10 in your Jeep? I have a 97 4.0 auto with 3.55 with 31" tires and turn 2700 at 70. I had it there for hours on the interstate in 100 degree weather and it got to about 230. Normal is 210. Got off and did 55-60 and it came back down. Runs fine at that RPM's
>>I looked up a formula that gave the gearing when you plug in the tire size, 366, miles per hour, and RPM..

Gear ratio =
rpm x tire diameter
_______________________( divided by )
Mph x 336
when I put in 2700rpm x 31 divided by 70 x 336... I got 401760..

I am probably wrong in that I did not use the other possible formulas for correcting the m.p.h. ?? lol.. story of my life...
thanks.. any more enlightenment.? seems I will be OK ...
dk
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Old 09-21-2012, 12:35 AM   #15
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the 456 is the other half of the dilemma... I have to choose

Quote:
Originally Posted by TnDz TJ View Post
I run 4.56's and 33's... at 2800 or so... I am at 65-70 I usually run around 80-85 on the freeway ( I know... don't start... Just being honest) and run over 3200 to 4000 a lot when I downshift to 4th. I can do this for hours on the freeway ...
>>>I have to choose between whatever the gear ratio really is that produces the 2700rpms with 31s... and a new 456 I just bought and installed a month ago, in a jeep I am not keeping... I would prefer the brand new R&P if the motor can take the rpm's.. it is correct that the three speed auto's ratio is 1:1 like the manual in 4th, the 3rd gear in the auto is 1:1.. It looked like I would need to put 35's on it to reduce the rpm's with the 456... I could run 33's at similar RPM's and kind of similiar speeds, with a 410..

If the 456 is a viable option I would like to just swap the new 456, brakes, lockers .. the whole assembly both front and rear; from one jeep to the other.
less expensive than removing the two lockers to install on the three speed... and just driving it with the gears that are in it.. ( unsure if 410 or not now )..
I could just stay with the 31 in. tires by swapping the 33's that are on the full double axle assembly back to the manual donor jeep.
if it is not going to hurt it, then I am going too harvest all my new parts and put back only the tires from the donor jeep. Then I will lift the 3 speed a bit and install MT's onto its rims.. and could choose 35's or 33's..
dk
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Old 09-21-2012, 02:51 AM   #16
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Quote:
Originally Posted by kennisondan View Post
guys, I need to know what would be tolerable Highway Rpm''S in my 97 TJ inline six..
is 2500rpm..or 2700 too much for an hour or two?

No.


Quote:
Originally Posted by kennisondan View Post
I would like to put the 456:1 new gears into my three speed ...












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Old 09-22-2012, 10:17 AM   #17
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Quote:
Originally Posted by kennisondan View Post
>>I looked up a formula that gave the gearing when you plug in the tire size, 366, miles per hour, and RPM..

Gear ratio =
rpm x tire diameter
_______________________( divided by )
Mph x 336
when I put in 2700rpm x 31 divided by 70 x 336... I got 401760..

I am probably wrong in that I did not use the other possible formulas for correcting the m.p.h. ?? lol.. story of my life...
thanks.. any more enlightenment.? seems I will be OK ...
dk

I used your formula here to figure gear ratio too. It is WAY off. I turn 2450 at 60 mph on 31's That figures out to 425,319 What gears are those? Use this chart which is for 65MPH. 97 4.0 Wranglers are available in 3.07 and optional 3.55 gears only. 98 came the 3.73 gears for the 4.0

Tire Gear Ratio Charts
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Old 09-22-2012, 10:25 AM   #18
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Jesus, you guys are overcomplicating a simple subject......

OP, 2500-3k is optimal at highway speeds. With your 32rh 3 speed, you'll want 4.10 gears for 33" tires, 4.56 gears for 35" tires, and so on.
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Old 09-22-2012, 10:26 AM   #19
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Quote:
Originally Posted by kennisondan View Post
>>I looked up a formula that gave the gearing when you plug in the tire size, 366, miles per hour, and RPM..

Gear ratio =
rpm x tire diameter
_______________________( divided by )
Mph x 336
when I put in 2700rpm x 31 divided by 70 x 336... I got 401760..

I am probably wrong in that I did not use the other possible formulas for correcting the m.p.h. ?? lol.. story of my life...
thanks.. any more enlightenment.? seems I will be OK ...
dk

I see what you did. You ran the formula together....you can't do that!!!

The formula is (rpm x tire diameter) divided by (mph x 336)

So lets put in your numbers.......2700 x 31 =83,700

AND the next line is 70 x 336 = 23,520

NOW take 83,700 divided by 23,520 and you get......3.558!!!!!! You have 3.55 gears!!!!
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Old 09-22-2012, 12:32 PM   #20
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Sorry but these engines were only meant to operate at above 2500k rpm for no longer than 1 hour and 59 min. At exactly 2 hours they will blow up.
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Old 09-22-2012, 12:39 PM   #21
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Quote:
Originally Posted by kennisondan View Post
guys, I need to know what would be tolerable Highway Rpm''S in my 97 TJ inline six..
is 2500rpm..or 2700 too much for an hour or two?
Of course your engine can operate at those rpms, it can do so all-day every day.

In fact, 2500-2700 is ideal for bigger tires. When I regeared my TJ with its 4.0L engine and 5-speed transmission with 35" tires to the optimal 4.88, that allowed my engine to run at about 2700 RPMs at 70 mph which was just right.

When I upgraded the transmission from the 5-speed to a 3-speed automatic several years ago, my rpms jumped to over 3K at 70 mph which the engine was still perfectly happy at. I slowed down a tad for fuel economy reasons but not because I was worried about the >3K rpms.
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Old 09-22-2012, 12:55 PM   #22
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Originally Posted by Plowboy1970 View Post
I see what you did. You ran the formula together....you can't do that!!!

The formula is (rpm x tire diameter) divided by (mph x 336)

So lets put in your numbers.......2700 x 31 =83,700

AND the next line is 70 x 336 = 23,520

NOW take 83,700 divided by 23,520 and you get......3.558!!!!!! You have 3.55 gears!!!!
So I took this formula, trying to figure out my gear ratio :
2000rpm * 30 = 60000
65mph * 336 = 21840
Comes up with 2.747

Did jeep make 2.75 gears ? I have the 05 'x' with 6spd manual trans
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Old 09-22-2012, 01:09 PM   #23
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So I took this formula, trying to figure out my gear ratio :
2000rpm * 30 = 60000
65mph * 336 = 21840
Comes up with 2.747

Did jeep make 2.75 gears ? I have the 05 'x' with 6spd manual trans
You need to check your rpms when your 6-speed transmission is in 5th gear which is a 1:1 ratio. RPM calculators are based on not being in an Overdrive gear ratio. Based on your 2000 RPMs at 65 mph, I'm guessing your ratio is 3.07 but I didn't do the math to verify it.
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Old 09-22-2012, 01:34 PM   #24
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Quote:
Originally Posted by Brit Nick View Post
So I took this formula, trying to figure out my gear ratio :
2000rpm * 30 = 60000
65mph * 336 = 21840
Comes up with 2.747

Did jeep make 2.75 gears ? I have the 05 'x' with 6spd manual trans

Like Jerry said, you need to be in the gear that is 1:1 which for you is 5th. 6th is .84:1 So with that you have to take what you got and divide by .84 to get your gear ratio. That gets you 3.27 The gears available for you are 3.07 and 3.73 I bet your gears are actually 3.07
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Old 09-22-2012, 02:09 PM   #25
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Originally Posted by Jerry Bransford View Post
You need to check your rpms when your 6-speed transmission is in 5th gear which is a 1:1 ratio. RPM calculators are based on not being in an Overdrive gear ratio. Based on your 2000 RPMs at 65 mph, I'm guessing your ratio is 3.07 but I didn't do the math to verify it.
Thanks. I didnt realise that 5th was the 1:1 gear.

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Like Jerry said, you need to be in the gear that is 1:1 which for you is 5th. 6th is .84:1 So with that you have to take what you got and divide by .84 to get your gear ratio. That gets you 3.27 The gears available for you are 3.07 and 3.73 I bet your gears are actually 3.07
Thanks Plowboy..

Will redo the math on the way home in 5th
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Old 09-22-2012, 03:07 PM   #26
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A Big Thanks to all .. I figured I was out of my depth when I used mathematical equations.. lol..
the practice of law is a haven for math-weak english majors.. lol.. they just taught division..
if you divide any number by 1/3 ... the answer is : MINE.
I don't even do that math..
thanks... that helped me a lot... feel better about the RPM's and have a handle on that without fear of unknowns... have a better handle on the 456 gearing I want to harvest; and the impact of that ratio to 31/33/35 inch tires..
now I still have a decision or two to make .. but have some facts and more knowlege to work with.
dk
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Old 09-22-2012, 04:39 PM   #27
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wear vs pressure

It is better to run the engines at a higher rpm than to lug them. Same concept as sanding. If you want to smooth something you use light pressure. If you want to use the same abrasive and take of more material faster you use more pressure.

When you keep the rpm's up you spread the torque pressure over more rpms and, thereby, lighter pressure on the bearings. Also, the oil stands up better to the lower pressure and higher rpm, also reducing wear.

I run a 94 Wrangler with the 2.5/5pd/29" stock tires. On the trip back from Sturgis, SD to Minneapolis, pulling my 29' self contained trailer I maintained about 4,000 rpm all the way and got 11 mpg. Running 4th gear at about 70-75 mph. Total trip took 10 hrs including fuel time (6 fills due to small tank). Never hurt the engine at all. It is still the same engine and still holds 60lb driving and 40lb idling.

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