Re: AMC 4-cylinder in a Spirit (Revisited)

[farna@xxxxxxx]

Tom, it's the way your post was worded for one thing. How about instead of just slapping someone down you explain your reasoning in the post next time? That would stop (or at least slow down) any misunderstandings right away. I don't think you meant to "slap anyone down", But the "THEY ARE WRONG" kind of hit things the wrong way. Yes, I understand that we (you and I) are approaching this from two different angles. Given all the facts, neither of us are WRONG -- just two different ways to look at the problem. I don't think you're considering all the facts when you assume the Jeep engine/trans combo should perform the same as a similar size, but vastly different, engine does in a similar body. I made the same assumption when first setting up my car, as AMC did use 3.07 gearing with some OD combos in the American. Granted, my Classic is more car, but then I have a lot more engine that produces lots of low speed torque. It DID pull a 3.07 easily, but it didn't deliver the best p!
 erformance or fuel mileage.

The 63 Classic came with much higher gears than 3.55, but try operating a 4.0L with much higher in a car about the same weight as the Cherokee and mileage goes DOWN. With the power and torque of a 4V 360 (with full torque coming in at 1800-2000 rpm) my engine should have performed very well with a 3.07 axle, but it lost fuel mileage with the AW-4 OD trans. The OD ratio is the same as the old Borg Warner ratio (0.70:1). I think the car will do a bit better with 3.31 gearing (which was stock with the old 195.....6 w/OD), but that's about as high as it could go without losing mileage again. I always felt my 93 2WD Jeep Cherokee could use just a step higher in gearing to, but then I wouldn't have thought half a gear (actually .48) would have made that big a difference behind a torquey motor either. 

I'm pretty sure Andrew hit the nail on the head -- or at least in the general area. Cam and tuning (computer programming in this case) make a big difference. The Jeep engines are set up for the lower gears, therefore shouldn't be geared to high for best performance and mileage. The car engines were setup differently. I'd be cautious about dropping the rear axle ratio to the same as similar engined cars. I'm certain it can be dropped below 4.10, but my experience with Jeep tuned engines tells me you don't want to drop it that much. 3.55 should work well in the much lighter car (as you pointed out), even a 3.31 might be acceptable, but dropping to a 3.07 would likely be a mistake. 

The 4.0L used a 3.07 with a five speed, and I can tell you from experience that it's to high a gear even for a 4.0L (had a 91 4.0L Comanche, 4x4 five speed, 215/70 tires). It started a bit slow even in first gear (except on a level) and there was way to much shifting between 4th and 5th when driving in moderately hilly terrain -- not big hills. It felt like it needed more gear. The saving grace was the 4WD -- it had a low range in the transfer case. I had to use that to take off up an incline with a heavy load -- first gear wouldn't do it! That was with a 4.0L -- I can imagine what a four cylinder would be like! I'm confident I'd have got better mileage with 3.23 or 3.31 gears instead of the 3.07s. Some of the gearing is marketing. Another problem is the EPA check course isn't very realistic. Under ideal conditions I'm sure the 3.07 gears are acceptable (they were), but that is maybe 20-25% of driving -- and that's being generous! If you want to give up anything resembling p!
 reformance, go with the 3.07 gears. It will be sluggish and won't deliver optimum mileage for most drivers. 

A lower gear (3.31-3.55) will keep you from flooring it just to get out in the road. Highway mileage will not likely suffer either, but losing 1-2 mpg on the highway should be well worth the 2-3 mpg gain in overall average mileage. When I dropped from 3.07 to 3.55 gears I went from an average mileage of 17 mpg to 20 mpg. Highway mileage went up about 2 mpg (now ~22 mpg @ 70-75 mph) and city mileage went up 3-4 mpg (now 17-18 mpg). 




On September 27, 2005 Tom Bunsey wrote:

> Frank & Company:
> 
> Please pay attention to my entire post, AND to the original question concerning gearing.
> 
> (1) I don't argue that the 4-cylinder in a TRUCK would have 4.10 gearing.
> 
> (2) But he doesn't want to put the engine in a truck; he wants to put the engine in a CAR. FOR GAS MILEAGE!!!!
> 
> We aren't talking TRUCK gearing; he wants to know what PASSENGER CAR gearing to use. We're not talking about load hauling a ton of wood or whatever in a pickup TRUCK, we're talking about a passenger car rated at about max 700 pound payload (see owner's manual for details).
> 
> What I am saying is that when AMC used a similar sized engine in a Spirit, these were the rear end ratios they used.
> 
> Historically, trucks use deeper rear end gearing than the same engine in a passenger car. You know this, I know this.
> 
> The guy raising the issue wants to put this engine in a passenger car. He isn't going to haul firewood, or plow his driveway, or go off road with it. He is going to drive the car on the highway!
> 
> As I recall, the 1982 Spirit with the IRON PUKE and the 5-speed was rated at 37 mpg, highway. With a 3.08 gear. And a .85 5th gear (overdrive).
> 
> THIS IS WHAT IS WANTED. Mileage!
> 
> NOT pulling Jeeps out of ditches!
> 
> Tom Bunsey
> 
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