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Outlawmack 10-26-2012 09:54 AM

I need some opinions
I got my cj running ok with the new coil, some rewiring and a trans shift adjustment. It's a 4.2 auto. And still doesn't want to go up a hill I have 31x10.5 tires on it. And from what I found 2.73 gears (ill check tomorrow). It's an 80 renegade With winter coming fast , I'm thinking I need to do a gear swap. I'm going to lift it the 1st week of December 4 inch with a 1.25 body. I want to run 34 or 35s.
Would you do 4.55 or 4.88s. Also an oc locker in the rear, 1 piece shafts an a lock right locker in the front. I'm not going to replace the tires till after winter. And only drive 65 miles a week to work

What are your thoughts on a 4bbl swap. Something like a truck avenger 470cfm. Of is the motorcraft 2100 a better way to go. I hate the carter that's on there now

Last thing. The HEI swap. Is it worth it? My current set up seems to be cutting out at higher rpm. I know it's not the coil. I'm thinking the distributor is taking a dump

Thanks for the help

derf 10-26-2012 11:18 AM

Are you going to drive this Jeep on the higway? If so, even 4.10's will be very short gears and send your RPMs through the roof. There's no way I would put 4.55 or 4.88 gears with a non-overdrive transmission and tires that size. If I were driving this Jeep daily I'd consider 3.54s or 3.73s. If I was doing a short commute only I'd give some thought to 4.10's but that's as much as I'd go. If you're going to step up tire size, then you should make your plan based on the tires you're going to have.

As far as the carburetor, it's hard to beat the Motorcraft 2100/2150. Just get one off a 304 rather than one off a 360. The Truck Avenger, to me, would be solidly in 2nd place.

When is the last time you replaced the cap and rotor on the distributor? What about the plug wires? Simple maintenance like that can make worlds of difference. A TFI or HEI upgrade will do more but if you're on a budget, a simple tuneup is almost always worth the money.

Outlawmack 10-26-2012 11:59 AM

Cap and rotor look new I'm going to replace the wires with 8.5 mm ones to match the coil wire I made. Just don't want to spend 50 bucks if I go to hei. As far as tires. I'm going to get either 34x10.5 swamper ltb or a 35x12.5 in march. Just don't want to regear twice. I drive to work, it sits a week then drive home. I can take back roads to and from if I need to

Outlawmack 10-26-2012 12:02 PM

And the reason I was thinking 4.56 is because it seams as if nobody has an automatic cj I don't know what the final drive is between the 3speed auto and a 5 speed

derf 10-26-2012 12:44 PM


Originally Posted by Outlawmack (Post 2928719)
And the reason I was thinking 4.56 is because it seams as if nobody has an automatic cj I don't know what the final drive is between the 3speed auto and a 5 speed

If it's a 70's Quadratrac, it is the GM TH400. If it was an 80's Jeep (given the 2.73 gears I'd say that it is) it is a Chrysler TF999.

Regardless, both transmissions have a 1:1 3rd gear. No overdrive.

The 5 speed manual was the Borg Warner T-5. That has a 0.86:1 overdrive.

If you're going to go to 35's, I would think that 4.56 gears would be plenty. That still puts you at over 3,000 RPM at 70 MPH and that's a little high in my book. If it were my Jeep I'd still consider 4.10's if I was going to do much highway driving.

Outlawmack 10-26-2012 01:31 PM

Thank you for the info. It's an 80 It doesn't see much interstate. Usually 55-60mph at the most. 90% is 50 and lower. The hills around where I live are a pain

derf 10-26-2012 02:17 PM

Yeah, 31s with 2.73 gears would be.

Stock that CJ would have had H78-15 tires (roughly 225/75R15). That works out to about a 28.3" tall tire. With 31's you really would want 3.09 gears to get close to stock performance. With 35's, you would want 3.54s.

Since that's the "economy" ratio, you can conservatively step up to something like a 4.10 and get better performance in everything but fuel economy without killing your engine on the highway.

derf 10-26-2012 02:23 PM

Best bet is to find out for sure what ratios you're running now. Then you can do a simple formula:

(new tire size/28.3) x current axle ratio = new axle ratio.

Because bigger tires are heavier, you'll want to round up to the nearest actual ratio: 3.54, 3.73, 4.10, 4.56, 4.88

That will get you back close to stock performance levels. If you want more off-the-line performance and don't care about highway use so much, step up one or two, maybe three ratios from there.

gcaughron 10-26-2012 04:23 PM

I found on my '82 CJ-7 that 3.54's were the best overall ratio for a daily driver that sees weekend use. I changed to 4.11's and the engine rpm's went up. You should have a model 20 t-case which does not have that great of a low-range in it so if you don't mind losing highway top end the 4.11's would be a good choice. Also of note is that 4.11 is the lowest recommended gear range for the model 30 front end for maintaining maximum ring gear strength per the Dana/Spicer manual. I have broken ring gears on both 4.56's and 4.88's.

derf 10-26-2012 04:26 PM

The Dana 300 was introduced in the 1980 CJ so his low range will be 2.6:1

Cj7otr 10-26-2012 05:05 PM

You can always keep a moderate R&P ratio (like 3.54) for hwy speeds and add the Dana 300 4:1 conversion. That's what I did and I feel that, for me, it's the best of both worlds.

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