64 GMC stepside - The 1947 - Present Chevrolet & GMC Truck Message Board Network

Harleymike · 08-12-2017, 05:18 PM

Hey guys. New too all this.
I just picked up a 64 from Alberta. No salt there.
Its in good shape.
I have searched this forum, but Im trying to figure something out.
Mine has the I6 with 4 speed.
These 4 speed trannies, I have heard good and bad.
Pretty much first gear is useless but very durable trannies.
But, how much HP can they take.
I plan on putting a V8 in there and would be around 400 HP.
Is this tranny capable of handling that, or should I look at a different tranny.

Thanks for any help
Mike

**Clyde65** · 08-12-2017, 06:06 PM

I would go with a 5 speed mated to the 400hp v8, for pleasure of driving and highway use.

**Captainfab** · 08-12-2017, 11:49 PM

With the SM420, SM435 and SM465 four speed transmissions, 1st gear is not intended to be used in everyday driving. You start out in 2nd gear, so you basically have a 3 speed trans. First gear is used when low speed crawling or pulling power is needed. I see no reason a SM420 won't hold up behind a 400hp V8

For a frequent driver I have to agree that a 5 speed manual or a 4 speed auto OD trans makes more sense.

Brad54 · 08-13-2017, 01:49 PM

Captain is right about the durability of the trans.
I went through all this decisioneering for my Suburban. Like your GMC, I had a Dana 44 in mine, which gives a lot more options.
I made a chart that showed the gear splits with the Granny 4spd (yours), and a 4spd OD manual trans (since I had one and it was a bolt-in).
I figured effective ratios with the trans gears and different rear end gears, with an eye to final drive ratio at highway speeds.

With the Granny 4-speed, you could get 2.90 or 3.08 or 3.23 gears in the rear and still leave the line, with the 4spd OD I have (.72:1 final drive), I went with 3.42 gears.

I like jack-rabbit starting and 3.42 gears are still enough to pull a light trailer, so that was why I went with my choice.

5-speeds are a great choice too, but I didn't have one, and I don't know what's required to put them in our trucks as far as bellhousings and new cross members. The 4spd OD was a bolt-in for my '62 torsion-bar/X-frame truck with the bellhousing engine mounts.

-Brad

08-12-2017, 05:18 PM	#1
Harleymike Registered User Join Date: Aug 2017 Location: Courtright, Ontario Posts: 84	64 GMC stepside Hey guys. New too all this. I just picked up a 64 from Alberta. No salt there. Its in good shape. I have searched this forum, but Im trying to figure something out. Mine has the I6 with 4 speed. These 4 speed trannies, I have heard good and bad. Pretty much first gear is useless but very durable trannies. But, how much HP can they take. I plan on putting a V8 in there and would be around 400 HP. Is this tranny capable of handling that, or should I look at a different tranny. Thanks for any help Mike

08-12-2017, 06:06 PM	#2
Clyde65 1965 Chevy C10, 2005 4.8L/4l60 Join Date: Aug 2004 Location: DFW Texas Posts: 8,539	Re: 64 GMC stepside I would go with a 5 speed mated to the 400hp v8, for pleasure of driving and highway use. __________________ Clyde65 Rebuild of Clyde http://67-72chevytrucks.com/vboard/s...84#post8338184 69 Aristocrat Lo Liner build http://67-72chevytrucks.com/vboard/s...84#post7561684 *support our troops!*

08-12-2017, 11:49 PM	#3
Captainfab 60-66 Nut Join Date: Mar 2007 Location: Coeur d'Alene, Idaho Posts: 23,246	Re: 64 GMC stepside With the SM420, SM435 and SM465 four speed transmissions, 1st gear is not intended to be used in everyday driving. You start out in 2nd gear, so you basically have a 3 speed trans. First gear is used when low speed crawling or pulling power is needed. I see no reason a SM420 won't hold up behind a 400hp V8 For a frequent driver I have to agree that a 5 speed manual or a 4 speed auto OD trans makes more sense. __________________ Power Steering Box Adapter Plates For Sale HERE Power Brake Booster Adapter Brackets For Sale '63-'66 HERE and '67-'72 HERE and '60-'62 HERE and "60-'62 with clutch HERE Rear Disc Brake Brackets For Sale. Impala SS calipers HERE Camaro Calipers HERE D52 Calipers HERE 6 Lug HERE Hydroboost Mounting Plates HERE

08-13-2017, 01:49 PM	#4
Brad54 Registered User Join Date: Aug 2006 Location: Athens, Georgia Posts: 1,456	Re: 64 GMC stepside Captain is right about the durability of the trans. I went through all this decisioneering for my Suburban. Like your GMC, I had a Dana 44 in mine, which gives a lot more options. I made a chart that showed the gear splits with the Granny 4spd (yours), and a 4spd OD manual trans (since I had one and it was a bolt-in). I figured effective ratios with the trans gears and different rear end gears, with an eye to final drive ratio at highway speeds. With the Granny 4-speed, you could get 2.90 or 3.08 or 3.23 gears in the rear and still leave the line, with the 4spd OD I have (.72:1 final drive), I went with 3.42 gears. I like jack-rabbit starting and 3.42 gears are still enough to pull a light trailer, so that was why I went with my choice. 5-speeds are a great choice too, but I didn't have one, and I don't know what's required to put them in our trucks as far as bellhousings and new cross members. The 4spd OD was a bolt-in for my '62 torsion-bar/X-frame truck with the bellhousing engine mounts. -Brad __________________ '61 Suburban daily driver: off the road due to 180-pound 8-pt buck! '62 K-10 long-step project '61 C30 Camper, aka "Valdez" There's no cool like Old School



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