RE-GEARED>>>>WOW RPM's!!!!

J

JEEPTUBE

NAXJA Forum User
Location
citrus heights
Recently i have regeared my trusty 97 XJ with 4.56's.
I have noticed a BIG rpm jump. At 65 im 2500 and up maybe 2700. I have an AW4 tranny and 33's.
Is this normal?
Whats everyone elses?
The weird thing is my 1992 XJ has the same 33 MTR's and can do burnouts with its 3.55 gears.
My 97 XJ with the 4.56 cant. I wonder if the tranny is taking a shit?
I dont do burnouts but everyonce in a while when i need to go.
 
If you regeared from 3.55 to 4.56 I would expect a large rpm change at the same speed. You can look up an RPM calculator on the web and see what the difference should be (Nominal).

As an example, one I just hit said that with 33 inch tires, a tranny ratio of 1 to 1 (3rd gear), and a speed of 60 miles per hour, the difference between 3.55 and 4.56 is 2,170 and 2,788 RPMs respectively.

As for spinning the tires on the 97, is this the one you have the trail armor on? Is it heavier than the older one?
 
JEEPTUBE said:
Recently i have regeared my trusty 97 XJ with 4.56's.
I have noticed a BIG rpm jump. At 65 im 2500 and up maybe 2700.
Click to expand...


Yes, this is a good thing. Revel in the added RPM's, they will improve your mileage and possibly your sex life.
 
I thought deeper gears would give lower RPMs... I mean isn't the engine turning the wheels with less force with deeper gears, ie, its eaiser for the engine to turn the tires.... which would be less RPMS, but if the RPMs are higher how will the milage be better... won't an engine running at 2000 rpms on the highway get better milage than an engine running at 2700 RPMs

I justy always thought going to deeper grears would allow for less RPMS and better crawl ratios etc
 
CRASH said:
Yes, this is a good thing. Revel in the added RPM's, they will improve your mileage and possibly your sex life.
Click to expand...

OK, that's it.

I'm regearing!
:loveu:
 
twisted_ed said:
I thought deeper gears would give lower RPMs... I mean isn't the engine turning the wheels with less force with deeper gears, ie, its eaiser for the engine to turn the tires.... which would be less RPMS, but if the RPMs are higher how will the milage be better... won't an engine running at 2000 rpms on the highway get better milage than an engine running at 2700 RPMs

I justy always thought going to deeper grears would allow for less RPMS and better crawl ratios etc
Click to expand...

To go the same speed at lower rpm's, you have to use more throttle. Running 3.55's and 33's is like towing a trailor constantly - your doing more work for the same speed. Once you re-gear, the engine doesnt have to work as hard to maintain speed, so you use less fuel.

The 4.0 seems to LOVE rpm's in the 2500-3200 range, its ok to run it there.
 
lower RPMs does not equal better gas mileage. the best simple indicator of gas mileage for a given speed is throttle travel. the air/fuel ratio is going to be pretty similar no matter what, so the more air you're sucking in (throttle depressed), the more fuel you're using.

I got 20mpg with 4.88s and 33s at about 70mph and keeping the speed pretty constant, which was a few MPG improvement over 4.10s and lower RPMs.
 
CRASH said:
Yes, this is a good thing. Revel in the added RPM's, they will improve your mileage...
Click to expand...

4:89s and 33"s is even more wow fun.

CRASH said:
and possibly your sex life.
Click to expand...

Left handed AND right handed!

:loveu:
 
BrettM said:
lower RPMs does not equal better gas mileage. the best simple indicator of gas mileage for a given speed is throttle travel. the air/fuel ratio is going to be pretty similar no matter what, so the more air you're sucking in (throttle depressed), the more fuel you're using.
Click to expand...

A gage for intake manifold vaccuum is actually the best indication of fuel mileage that you can easily install. Throttle travel does give some indication, but at the same throttle position intake vaccum will vary with rpm, and air flows at different speeds through the throttle body, so fuel consumption also varies.

Lower vaccum (higher numbers, since 20 inHg is lower pressure meaning more vaccum than 10 inHg) generaly correlates to lower fuel consumption. Intake vaccum is directly related to engine load. When you open the throttle completely at low rpm, intake vaccum drops down close to zero (or positive pressure, called boost, if you have forced induction). The highest vaccum reading you can get will be when engine braking, with the throttle closed.

Hopefully this makes sense, and can be useful. My point here, for the original post, is that if you had tried a vaccum gage before and after the re-gear, and saw higher vaccum readings after the re-gear at a given speed, that would indicate that you are getting better gas mileage.
 
Damn i didnt think id get back this many replies. Thanku very much everyone. How much heavier are the newer 97 and up XJ's than the older ones around 1992? I still dont get why the 97 cant roast em. Prob the freakin tranny. AHHhHH!! Oh and DAFFYXJ the only armor i have on it is the stage 1 rockrails and a stage 3 bumper. I was thinking that the newer XJ's were heavier due to the thicker metal doors and metal rear hatch.
 
Oh yes i will drag both these. But i have a feeling the red 92 is gonna beat the 97. Both engines run great. I do have a battery drain prob in the 97 though. It eats batteries. I have done a alternator check and its fine. Its so lame, if i leave the door open for more than 10 minutes-i wont even be able to start it. The battery gage when running sits less than half way around 12. Will that mean decrease in spark? Is my ignition pooped? I definitely need to find the drain first.
 
by the way, higher RPMs at the same speed gives you better crawl ratios it means there is more time in first gear and therefor more ability to go slower, or go at the same speed with more torque. thats why the big guys use so deep of gears. imagin being able to crawl at 3-4 miles per hour or less at 2500 rpms thats why they have such good crawl
 
twisted_ed said:
I thought deeper gears would give lower RPMs... I mean isn't the engine turning the wheels with less force with deeper gears, ie, its eaiser for the engine to turn the tires.... which would be less RPMS, but if the RPMs are higher how will the milage be better... won't an engine running at 2000 rpms on the highway get better milage than an engine running at 2700 RPMs

I justy always thought going to deeper grears would allow for less RPMS and better crawl ratios etc
Click to expand...

i'm sorry, but that just blew my mind. wow.
 
zthang43 said:
A gage for intake manifold vaccuum is actually the best indication of fuel mileage that you can easily install. Throttle travel does give some indication, but at the same throttle position intake vaccum will vary with rpm, and air flows at different speeds through the throttle body, so fuel consumption also varies.

Lower vaccum (higher numbers, since 20 inHg is lower pressure meaning more vaccum than 10 inHg) generaly correlates to lower fuel consumption. Intake vaccum is directly related to engine load. When you open the throttle completely at low rpm, intake vaccum drops down close to zero (or positive pressure, called boost, if you have forced induction). The highest vaccum reading you can get will be when engine braking, with the throttle closed.

Hopefully this makes sense, and can be useful. My point here, for the original post, is that if you had tried a vaccum gage before and after the re-gear, and saw higher vaccum readings after the re-gear at a given speed, that would indicate that you are getting better gas mileage.
Click to expand...
that's why I said "best simple way", in other words, without having to install something new like a vacuum guage.
 
XgeekstarX said:
i'm sorry, but that just blew my mind. wow.
Click to expand...

Well I am new to the gearing world and don't quite understand what everything mean. I had no Idea that deeper gears would do... All I know is I run highway all the time and needed to regear with my 32sx11.50 BFG M/Ts and was told to go with 4.10s best bang for dual purpose on and off, plus finding a ford 8.8 with 4.10s when looking for an axle swap was even better
 
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