60' F134 - Would switching Tranny to SM 420 Improve Top Speed?

60' F134 - Would switching Tranny to SM 420 Improve Top Speed?
CiviJeep

CiviJeep

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Location
Lincoln, NE
Vehicle(s)
1960' Jeep CJ5
I recently found a SM 420 Transmission with granny low for sale locally.
They wanted $250 for it, which i thought might be a little high but I haven't looked at the condition yet. To see if it would even be worth my time I had a few preliminary questions for the experts at jeep-cj.com.
Currently Running:

Original F134 Hurricane with 3 Speed Transmission ( Runs great, doesn't need to be replaced )
Right now my top speed is 45-55 without hills
Questions:

Would switching to this transmissioin ( 4 speed ) significantly increase my top speed without changing the engine? ( Going 65 with about 300 hundred extra pounds of gear and a winch would be ideal ) Eventually I plan to replace the motor with a chevy for extra power since I will be carrying hundreds of pounds of camping gear, kayaks, bike racks, a winch, etc.

In addition, would adding an Overdrive also increase my top speed?
I heard this Transmission is good for mounting with chevy engines, is that true?
 
Its not the tranny that holding back your top speed, its the engine. Since both of the transmissions have a 1:1 final drive ratio, there will be no difference in top gear.
And while an Overdrive would change the final drive ratio, I doubt that the little 4 banger will be able to pull the CJ at speed with anything less than a large downhill drop to help it along.
Whats wrong with going 45-55 in a classic anyways? :)
BTW, My first CJ5 I bought had a GM 151 /3speed/5.38 gears/29" all tires. I drove it from Central WA state to southern Idaho to the base I was stationed at, loaded with extra axles, and everything else I got from the PO. Did 45-55 all the way there and enjoyed every minute. It was the first time nobody honked at me for holding up traffic on the hiway! Got lots of thumbs up, tho. :)
 
the SM420 won't help your top speed but instead it will benefit your rock crawling. It has an granny low first with a ratio of 7:1. Thats about as low as you'll find. It has been used by GM and for that reason would be a great choice for a chevy swap. Adapting to your Transfer Case might not be as easy but a company such as Novac or Advanced Adapters should have an adapter that should help. Then you'll need new drive shafts.
I would not bother putting the SM420 into your jeep unless you also put in a GM small block. And if you do you'll have the power to go as fast as you want (as long as your axle gears aren't too low)
 
Transmission won't help your top speed at all. You have a 75HP 4 cyl. motor trying to push a not so aerodynamic and realatively heavy Jeep.
My 3B with same motor would be lucky to go 50mph and it would be white knuckle at that considering it's steering and brakes.:eek:
 
CiviJeep said:
I recently found a SM 420 Transmission with granny low for sale locally.
They wanted $250 for it, which i thought might be a little high but I haven't looked at the condition yet. To see if it would even be worth my time I had a few preliminary questions for the experts at jeep-cj.com.
Currently Running:

Original F134 Hurricane with 3 Speed Transmission ( Runs great, doesn't need to be replaced )
Right now my top speed is 45-55 without hills
Questions:

Would switching to this transmissioin ( 4 speed ) significantly increase my top speed without changing the engine? ( Going 65 with about 300 hundred extra pounds of gear and a winch would be ideal ) Eventually I plan to replace the motor with a chevy for extra power since I will be carrying hundreds of pounds of camping gear, kayaks, bike racks, a winch, etc.

In addition, would adding an Overdrive also increase my top speed?
I heard this Transmission is good for mounting with chevy engines, is that true?
Click to expand...

T-90 top gear is one to one.
SM-420 top gear is one to one.
A stronger Transmission & seven to one granny are the benefits.:D
 
I had the 134 /T-90 Transmission with a Overdrive hit top speed of 70 the ole 3b shook like hell though now its got a 4.3 /sm 465 cant wait till its finished:D
 
54cj3b said:
I had the 134 /T-90 Transmission with a Overdrive hit top speed of 70 the ole 3b shook like hell though now its got a 4.3 /sm 465 cant wait till its finished:D
Click to expand...

70 mph in a 3b? the only way mine would see 70mph would be out the back door of a c130 at 10,000 feet or more. :D
 
Sasquach said:
70 mph in a 3b? the only way mine would see 70mph would be out the back door of a c130 at 10,000 feet or more. :D
Click to expand...

I've seen a GM 151 go off the tailgate of a C-130, but it was no where near 10,000 ft.
Hell, I've seem a Sheridan tank go off the tailgate of a C-130 & it wasn't more than 15 ft. off the deck. LAPCE Low Altitude Parachute Cargo Extraction.;)
 
Cheepjeep85 said:
wanna talk about something that is itimidating as hell, watch 100 paratroopers jump outta planes right in front of you. my wifes brother in law was a jump master at fort campbell, we got to go watch a jump one day. that is some crazy :dung:.
Click to expand...

I didn't know they jumped out of perfectly good aircraft at Ft. Campbell, home of the 101st air assault division.
Although I have seen a thousand man drop at Ft. Bragg N.C. Where I was stationed.:patriot:
 
Cheepjeep85 said:
he spent 4 yrs at fort bragg, but that was before i met my wife, they jump at fort campbell i watched it with my own eyes
Click to expand...

I did basic & airborne school at Ft. Benning GA. in 79'.
Then on to Ft. Bragg.
 
cool, man watching that made me want to join but not bad enough to actually do it. thats probably wrong but i don't respond well to people yelling at me at the <-BAD WORD-> crack of dawn. i thoroughly appreciate everyone that does serve our country past or present though. i couldn't tell you why they jump there, i can remeber my wifes bro in-law saying something about them having to have so many jumps in order to keep their airbourne designation, which from my understanding they still have
 
Cheepjeep85 said:
cool, man watching that made me want to join but not bad enough to actually do it. thats probably wrong but i don't respond well to people yelling at me at the <-BAD WORD-> crack of dawn. i thoroughly appreciate everyone that does serve our country past or present though. i couldn't tell you why they jump there, i can remeber my wifes bro in-law saying something about them having to have so many jumps in order to keep their airbourne designation, which from my understanding they still have
Click to expand...


It's not for everybody.;)
As far as "Jump Status" goes, it gives you as small increase in pay.
Even as far back as I was the 101st. were "Air Assault".
I suppose like you say they keep jumping to maintain their status.
After all the the shoulder patch still says 101st. Airborne.;)
 
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