Will a AMC 360 solve my gearing? - JeepForum.com
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post #1 of 47 Old 07-18-2018, 08:43 AM Thread Starter
scottyDive
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Will a AMC 360 solve my gearing?

CJ7 with 258, 33 inch tires with a 258 and TF999 auto tranny. I have the stock axles with 3.31 gears and because of the tires it's kind of a dog with the auto bogging down a bit. I can spend more than 2k on re gearing or put in a 360 for half of that. I am assuming the 360 will not kill the auto and the increase in HP will solve the dogginess I experience with the 258. Am I right?

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post #2 of 47 Old 07-18-2018, 11:16 AM
Matt1981CJ7
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Scott,

Torque to the wheels = Engine torque X transmission gear ratio X differential gear ratio. So let's look at some real numbers assuming you either change to 4.10 gears or swap to a 360 engine.

stock 258 torque = 210 ft-lbs
3rd (drive) gear ratio = 1:1
New differential ratio = 4.10:1

Total torque to wheels = 210 x 1 x 4.10 = 861 ft-lbs

stock 360 torque = 295 ft-lbs
3rd gear ratio = 1:1
Current diff ratio = 3.31:1

Total torque to wheels = 295 x 1 x 3.31 = 976.45 ft-lbs

So, a stock 360 with your current gears will provide approx. 115 ft-lbs more torque to your wheels in 3rd gear than your 258 would with 4.10 gears.

I hope that helps.

Matt


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post #3 of 47 Old 07-18-2018, 11:26 AM
John Strenk
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You would probably have to overlay the 258 and 360 torque curves and measure at each RPM but I think the 360 is going to win out in any way.

The only time that would be an issue is if you have a motor that only starts putting out more torque at high RPM's vs a motor that puts out more torque at low RPM.
Say a 4 cyd diesel vs. some trick multi carb, high HP V8.

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post #4 of 47 Old 07-18-2018, 11:42 AM
Fourtrail
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Scott, where is it a dog? Coming off of the line or accelerating while at speed at higher rpm? While Matt's calculations are correct that you will overall be able to put more torque to the wheels at max torque in high gear, the 360/3.31:1 will actually be about 50 ft-lbs of torque shy of the 258/4.10:1 at 1000 rpm in first gear. Through about 2500 rpm I don't think you will see much change, but with the way the torque curve falls off for the 258, the 360 will pull much harder at higher rpm.

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post #5 of 47 Old 07-18-2018, 11:57 AM Thread Starter
scottyDive
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Great info and discussion!

The jeep is always on the road and we have lots of hills around here. Off the line its just ok, but I am guessing it shifts around 1800 from first to 2nd at 15 mph and drops to 1400 then builds up again to 1800 to 2000 at 35 mph and then drops again to 1500-1600. Around 2300 rpm's in final gear I am doing about 65-70. Those shift points and hitting the next gear around 1400 to 1600 or lower is where its a dog. My hope is that a v8 will climb up in rpms with more gusto that the 258.

Scott
84 CJ7 258 Auto 2150 TFI nuttered 3" Rky Rd lift 33s
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post #6 of 47 Old 07-18-2018, 12:10 PM
Matt1981CJ7
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Fourtrail,

How did you arrive at the 50 ft-lbs less torque in 1st gear at 1000 RPM with the 360?

I ask because I haven't found torque curves for a stock 258 and 360. I do know the 360 max torque comes in at 2900 RPM, and at 2000 RPM for the 258. But I haven't found any info for lower RPMs.

Thanks,

Matt


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post #7 of 47 Old 07-18-2018, 12:13 PM
Shawn Watson
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Damn, Scott, those are some horrendous shift points. Only an industrial diesel would be happy down there. Looking at your highway speed rpm, I'd opt for a V8 vs making a 258 wind higher.


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post #8 of 47 Old 07-18-2018, 12:27 PM
Fourtrail
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Scott, sounds like you need to adjust your throttle valve to correct your shift points. Post 5 in the below link from JeepHammer covers doing the adjustments so it will shift at the correct rpm.


https://www.jeepforum.com/forum/f8/a...nkage-1318163/

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post #9 of 47 Old 07-18-2018, 12:32 PM
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Quote:
Originally Posted by Matt1981CJ7 View Post
Fourtrail,

How did you arrive at the 50 ft-lbs less torque in 1st gear at 1000 RPM with the 360?

I ask because I haven't found torque curves for a stock 258 and 360. I do know the 360 max torque comes in at 2900 RPM, and at 2000 RPM for the 258. But I haven't found any info for lower RPMs.

Thanks,

Matt

I used this chart from 74 and gave a bit more to the 258 since his is a 2 barrel. 190 at 1k for the 258 vs 230 for the 360 2 barrel. Found it on a Matador forum.
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post #10 of 47 Old 07-18-2018, 12:58 PM
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Thanks,

I think the minimal loss of torque at 1000 (roughly 2.5% less torque) in 1st gear with the 360 would be much less noticeable than the gain it would provide (roughly 13% more torque) in 3rd at higher RPMs.

Matt


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post #11 of 47 Old 07-18-2018, 01:24 PM
Fourtrail
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Quote:
Originally Posted by Fourtrail View Post
Through about 2500 rpm I don't think you will see much change, but with the way the torque curve falls off for the 258, the 360 will pull much harder at higher rpm.
Quote:
Originally Posted by Matt1981CJ7 View Post
Thanks,

I think the minimal loss of torque at 1000 (roughly 2.5% less torque) in 1st gear with the 360 would be much less noticeable than the gain it would provide (roughly 13% more torque) in 3rd at higher RPMs.

Matt

Correct, that is why I said the above and asked where he was unhappy with the power level.

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post #12 of 47 Old 07-18-2018, 01:36 PM
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As usual, we agree, Fourtrail.

One thing is certain, if he's cruising along at 55 and mashes the skinny pedal, the 360/3.31 combo will fly compared to the 258/4.10 combo.

Matt


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post #13 of 47 Old 07-18-2018, 02:32 PM
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The real comparison is a 360 with 3.31s vs a 258 with 4.10s as those are the two options, right?


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post #14 of 47 Old 07-18-2018, 02:43 PM
Fourtrail
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Quote:
Originally Posted by swatson454 View Post
The real comparison is a 360 with 3.31s vs a 258 with 4.10s as those are the two options, right?


Shawn

Yep, that is what we have been comparing.
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post #15 of 47 Old 07-18-2018, 02:48 PM Thread Starter
scottyDive
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Quote:
Originally Posted by Fourtrail View Post
Yep, that is what we have been comparing.
Quote:
Originally Posted by swatson454 View Post
The real comparison is a 360 with 3.31s vs a 258 with 4.10s as those are the two options, right?


Shawn

YUP - But with a TF999 Auto. Haveing a manual transmission allows a driver to wind out the engine to over come the gearing/automatic tranny

Scott
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70 Jeepster 225 HEI 3spd 2" Alcan lift
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