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

[farna@xxxxxxx]

One important note: I use the speed/tire size/rpm calculators at http://www.4lo.com/4LoCalc.htm to fine tune my ratio and tire size. Since I already knew from my previous Jeeps that I didn't want to cruise much over 2400 rpm, I set the car up for a cruise speed of 75 mph. I fine tuned by letting my gear ratio determine the tire size. I wanted to run 16" wheels, but couldn't get a 7" wide 16" wheel with a 5.5" backset -- it was hard to find a 15" wheel that size! Using the calulators on the above page I determined that I needed a tire ~26" in diameter. A 205/70R15 fit the bill at ~26.29" (actual tire diameters can vary by manufacturer up to 1/4", but +/- 0.30" is close enough). 205/65R16 would have been perfect to! If I wanted to up my most economical cruising speed to 80, I could change the rear axle or the tire. I'd be in the right range by going to a 205/75R15 -- a 27" tire. 78 mph is right at 2400 rpm (2412.106 rpm, but since tires vary a little so will rpm) with the 75 s!
 eries tire, 2473 rpm @ 80 mph. 78 is close enough, but I could go the extra 50 mph without hitting the "enrich" point of the computer (which I believe is 2500 rpm -- I always say under 2400 because it's easy to pick up 50-100 rpm going down hill without noticing). A 3.31 gear would also put me right at 2400 rpm (2395.42) with the 205/70R15 tires. Tires are cheaper to change than gears unless there's a really big change needed though! The car will take 215 tires in the back, so that's what I'll get next time. The extra 0.5" of tire diameter will decrease rpm by 45.5 @ 75, which will still put me close to 2400 rpm @ 75 (2363.09 vs 205/75R15@ 2408.54 rpm). 

A lot of thought needs to go into gearing and tire sizes. But you need to know your engine/drivetrain and what works best first. The best advice I can give is to find the torque peak first. This will be the rpm the engine is rated at. Consider that the middle of your targeted cruise speed, then go + or - 100 rpm. My engine is a bit tricky. I don't know the aftermarket cam's torque peak. Stock 258s peak at 1800-2000 rpm. I think it's safe to assume my slightly longer duration and lift cam would peak a little higher. From experience with Jeep 4.0Ls and my 4.6L, I know 2400 rpm is my target. So then I just have to figure how fast I really want to cruise on average. Since the speed limit in most places is 70, 75 sounds good to me. 80 would be nice, but I can always bump up a few rpm to get there when/if needed. Set up like it is, the car REALLY likes the 70-80 mph range, and delivers good mileage at that speed. I'm sure it will deliver better mileage at a slower speed, but then !
 there would be more shifting in and out of OD for hills. I'm positive that air drag and the amount of power needed to maintain 70-75 mph is the main factor in why I get better mileage atthat speed with 3.55 gears than with 3.07 gears.

Stock 4.0Ls are rated as peaking at 3200-3500 rpm, and one site lists 4000 rpm as the peak. I don't know what happened here -- I've never driven a 4.0L vehicle that felt like it peaked torque at that high an rpm. Sounds like a marketing ploy or a change in the way torque is rated. I thought for a few moments that it might be the CAM causing the higher fuel consumption over 2500 rpm, not the computer system -- but then my 91 and 93 Jeeps did the same thing with stock 4.0L cams. This leaves everyone in a bit of a quandry. 

Prior to 1990 the peak torque is right at the rated capacity, at least for carbureted engines. The last 4.2L made in 1990 for the Jeep Wrangler was still rated at 210 lb/ft @ 2000 rpm. The carbureted version of the 2.5L four is rated max torque at 2800 rpm, which makes sense. The MPI version max torque rating at 3250 rpm sounds about right for a four, I don't know where the max at 4000 rpm for the 4.0L comes from. Again, I think it's just marketing knowing consumers compare numbers, not types of engines, so they move the rating up in rpm to compare with V-6 engines. The rpm at such a comparatively low number like 2000 rpm doesn't look good when new model V-6 engines have their ratings so much higher. I'm thinking marketing just publishes a nice number at a comparable rpm, but then I'm just guessing. I'm totally stumped at why the high numbers when I know the engine pulls so well at much lower rpm!! 

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