sm420

[currupt4130]

looking into swapping in an sm420 for the durability and 7.05 to 1 first gear, which gives me a 78:1 ratio with the standard 4lo, but i want to kno about how it drives on the street between 2nd and 3rd gears. i know what the ratios in both transmissions are and i dont think itll be too bad but if anyone knows someone who runs it or runs one themselves id love the input. right now i get good gas running 4th gear in my ax5 which is the same as the 4th gear in the sm420 so going with the 420 would also prevent me from changing out my R&P. thanks
-sam

 

[Junkpile]

A four cylinder and a three speed transmission aren't the best combination in the world. The relatively short rpm range in which that engine makes decent power is going to make it have problems trying to deal with the big rpm drop between gears. It can be done, and you'll love it offroad where the super compound low will let the engine spin up quick and feel like it's a built V8, but on the street I would imagine it to be a hard combination to live with.

 

[Junkpile]

I know it's a four speed, but you won't be taking off from stoplights in granny low.

 

[currupt4130]

RE: need space for tires, what

well id actually have to be taking off from granny to get me going, second gear is smaller than my first gear is and getting first gear going is a little tricky sometimes

 

[currupt4130]

RE: ICE AND SNOW FOR A TJ?

bmp

 

[Junkpile]

Selling the Jeep (ouch)

With a set of 35s, that probably measure closer to 34s, you'll be spinning it up to 4500rpms to reach about 15mph. You're going to shift as soon as you start moving. Things like that get aggravating in stop and go city driving. You can live with it though.
It would be awesome offroad.

 

[currupt4130]

RE: Re: Here are some progress pics

i found some gear calculators and its not gonna be as bad as i thought. just hop into second at around 5 mph, take second to 17 mph(2500 rpm) and then shifting into third at 17 mph will drop me to 1200 rpm and from then on it should be smooth sailing

 

[jps4jeep]

'99 Wrangler Sport, 6 cyl, 5mt. 48K, 32's, cage, more!

I have a sm 465 in my 77 chevy, and will be swapping it into my YJ when this transmission goes, that truck has been in the family since new, and no complaints except for the lack of over drive.

And it if funny when you let people drive that particular transmission and they have no clue so they throw it into Lo and let the clutch out and stall the vehicle! HAHAHA

 

[currupt4130]

yea driving my jeep will be off limits to most people after this, i let people run it to the store if were doin something you know, but its getting to the point you need classes to drive the damn thing

 

[Bounty__Hunter]

Where is Oregon in all this fun??

Don't forget that the SM420 is a very heavy truck transmission, so the parasitic drag of it's internals will be much more than what your AX5 produced. You will notice performance suffering even if the gear ratios were the same. I don't think you'll get the driveability you're looking for, I'd spend the money and time on regearing the axles and leave the light transmission behind the 2.5L engine.

 

[Junkpile]

Don't forget that the SM420 is a very heavy truck transmission

That's a good enough reason for me not to put one under one of my Jeeps, without getting into the other negatives. Even a 4 banger is going to rip that 8.8" apart if you get stupid in compound low. There's no way I'd put a NP231 behind a granny gear either. It's a total mismatch for the rest of your drivetrain in my opinion. Some people love them.....I'm not one of those people. I'll shut up about it now. If you do it, please post some in-depth results for us.