Gear change 3.73's to 3.31's Observations

[Hedgehog]

So, I've got about 250 normal everyday well documented going to work miles on my new set of 3.31 gears and I have some thoughts and observations:

1 - For highway driving these are great gears. At 65mph the RPM's dropped from 2800 to 2400 a 14% drop in engine speed.

2 - The CJ is much more comfortable to drive at 65mph. The engine isn't stressed and seems far more relaxed.

3 - With the old 3.73 gears the CJ would accelerate with authority all the way up to 4000 rpm where I think of as red line for the big V8. With the 3.31's, surprisingly enough the top end or the area where the engine seems to work for small acceleration gains is LOWER than before.

4 - With a 14% reduction in RPM's at 65mph and an engine that seems far happier I expected a decent increase in mileage. A couple MPG anyways. So what did I get? ... As before all information was gathered from my automotive GPS ...... After entering all the data necessary into my calculator while sitting at the gas pump and completely pissing off the lady behind me (I admit that I knew she was in a big rush and darned twitchy to get me to move even when I was pumping gas so it was time to cool her heels.) the results are in ..... AND ..... there is absolutely no change in my mileage at all. WHAT! How can that be! I spent a LOT of money to change those gears.

Conclusion: Time will tell about the mileage, but I'm not expecting miracles. The difference might show up in an average, in the long run 250 miles isn't a long distance to find a definitive answer to a mileage question. Over all I like the changes in the way the CJ drives, maybe in the long run, that's enough for me to be happy. ..... Right now I'm damned disappointed though!

 

[Lumpy Grits]

The AMC, V-8 redlines at about 6500 rpm.
Make sure your tach is somewhat accurate.
With the gear change-How do you confirm true ground speed? GSP??
Forgot-What size tires your run'n.
Any better gas mileage?
LG

 

[Hedgehog]

Grumpy - All you want to know and more is written in the original post.

Early on I asked about what the redline on an AMC V8 would be and was told by a few different jeepers that ~4200 RPMs was about where it should be. I know the engine or cam really seems to level off at about that RPM range.

My tack is an older Auto Meter tach. I don't know if it is actually a racing tach, but it was used on a jeep used on a track of some sort. The tach. was for all practical purposes NOS.

Yes, I'm using an automobile GPS for mileage and speed, a VERY cool and accurate feature. As is written in another thread my stock gauges are not accurate, the speedometer is ~14mph off, the odometer is off enough to make it useless for figuring out mileage.

Interesting, my CJ didn't have the stock speedometer and the odometer was way off so I have no idea of the real mileage on the CJ. However the Motor Vehicle Department insists on having a mileage number. So, when I first registered the CJ I said 100,000 and set the odometer at 00,000.00. The CJ now has just under 5,000 miles on it since major work was done on about every component. When the new Speed Hut Speedometer was ordered I asked them to program in 105,000 miles on the odometer. Now I won't have to "hem and haw" when asked about mileage. I'm sure the CJ has more than 100,000 miles on it, but I won't feel like I'm lying (never was my intention from the start) either.

 

[Lumpy Grits]

Read post #7
Aaron knows of what he sez-- Your 360 is the same block, etc.
AMC 401 Question: Max RPM - JeepForum.com

Another source.
http://www.jeep-cj.com/forums/f2/amc-360-redline-21498/

The AMC 258 i6 / 4.2l /AMC 258 i6 / 4.2l 6 cyl engine has a 4500rpm r'line.
LG

 

[Hedgehog]

Okay I won't argue, 5,500+ is about it before pieces start coming off. At this point I don't see me workin' her that hard.

 

[tarry99]

So, I've got about 250 normal everyday well documented going to work miles on my new set of 3.31 gears and I have some thoughts and observations:



4 - With a 14% reduction in RPM's at 65mph and an engine that seems far happier I expected a decent increase in mileage. A couple MPG anyways. So what did I get? ... As before all information was gathered from my automotive GPS ...... After entering all the data necessary into my calculator while sitting at the gas pump and completely pissing off the lady behind me (I admit that I knew she was in a big rush and darned twitchy to get me to move even when I was pumping gas so it was time to cool her heels.) the results are in ..... AND ..... there is absolutely no change in my mileage at all. WHAT! How can that be! I spent a LOT of money to change those gears.

Right now I'm damned disappointed though!

You may see some improvement if you were to take a trip that allowed you to keep the engine at a lower RPM for a longer period of time.......gear ratio's are funny, the negative of a taller / higher (closer to one to one ratio ) is the motor will have to do more work early and spend more fuel to get the vehicle up to X cruising speed........gear multiplication or lower ratio's allow that event from A to X Speed to be a little less work on the motor, but as you already know the lower ratio's keep the motor at a higher rpm at X Speed , thus burning more fuel at cruising RPM , hence the need for a 4th gear at 25% OD........... problem solved!

 

[Lumpy Grits]

Okay I won't argue, 5,500+ is about it before pieces start coming off. At this point I don't see me workin' her that hard.

Don't see any need to ever run it over 5500
Those old AMC 8's build torque from the Stop sign on.
LG

 

[hondo]

my bet on the mileage would be because on the highway you are using less fuel. but getting up to speed and starting after stop signs and such is using way more fuel. i am assuming that you didn't spend the entire tank on the highway.

 

[Hedgehog]

My trip to work consists of 4 miles to I-10 @ 45 with 3 stops, 10 miles on I-10 @ 65, 0.25 miles to the parking garage, naturally home is a reverse of that. So, during the week the old girl gets a steady diet of interstate travel. But, as you guys have mentioned, I hope for better mileage on long +1 hour interstate trips. Those might prove the gearings value other than the over all feel of the CJ.

 

[Only in a Jeep CJ]

What are the chances that your old 3.73 gears are in good shape and you don't wont/need them anymore???

 

[Hedgehog]

part of my deal with Willys Works was that he got my old gears. They were a little scuffed anyways. like the rest of the CJ they were driven hard. By the looks of your profile you are familiar with 3.31's and 3.73's. What are your experiences between the two gear sets?

 

[Kane]

Now that you are at 2400 rpm. Check to make sure you are at full timing mech advanced. That way the vacuum advance is doing it job. This will ensure the best fuel mileage. Some engines are tuned to a higher rpm. If it isnt then get some lighter springs and try to get it to come in by 2100 to 2200 rpm.

 

[Hedgehog]

I have been playing with the initial advance some. I as running with 12*, last week I backed it off to the factory 8* and can feel the performance loss. I could go with lighter springs in my DUI /HEI ignition. Fooling with ignition timing makes me a little nervous, but the DUI is fully adjustable so HEY why not give her a try. It should be fully advanced before 2800 RPMS, but I really don't know.