Cummins going in '71, anyone here do it before? - The 1947 - Present Chevrolet & GMC Truck Message Board Network

TP from Cntl PA · 02-18-2005, 03:24 PM

I got a small straight 6 Cummins on the way to replace the 350 in my '71 C-20. It is a 6AT3.4 and about the size of a 292. I would have put a 5.9 in it but I was concerned about the length. Anyone here put a diesel(Besides a 6.2) in one of these trucks? My main concern is on the gearing. I have a load of Saginaw 3spds laying around and kicked around the idea of mounting one behind the original tranny backwards for a poor mans OD. I haven't looked inside one yet to see if it is possible, but I thought that would be sorta different and practical as well. I thought about mounting it like a devorced transfer case and then adding an aftermarket 3spd shifter in the cab to control it. In 3rd it would be like it wasn't there and you had stock driveline, then once out on the highway, put the second tranny up in second and match the main tranny up to the road speed and engine rpm.

Anyone ever try something like this?

BET68 · 02-18-2005, 04:14 PM

SOUNDS GREAT ,WOULD LOVE TO DO THIS TO ONE OF MINE ! KEEP US POSTED!!!

Chevy67Tk · 02-18-2005, 05:09 PM

Cool idea, keep us informed. The only thing I would be worried about is you might be running helical cut gears backwards. I can’t remember what’s in a Saginaw it’s been a long time sence I had one apart.

'71c204x4454 · 02-18-2005, 06:07 PM

Someone is in the prossess right now. He just posted in the 4x4 forum.
http://67-72chevytrucks.com/vboard/s....php3?t=141525

hgs_notes · 02-18-2005, 06:51 PM

If you put both trannys in reverse, you would go forward!

HG

tom hand · 02-18-2005, 07:40 PM

I don't understand the tranny thing...I really don't believe mounting a tranny backwards will buy you anything....it will have the same ratios if mounted forward. If you have the first tranny in first and the second tranny in second it would be the same as if you had the first one in second...the only thing two trannys would buy you is let you run a really high gear in the rearend....like a 2.73...and then you could put one tranny in first and the second in second for a really low gear for takeoffs....correct me if I am wrong

dwaite72lnghrn · 02-18-2005, 07:44 PM

The main problem is keeping the 3 speeds together long enough to get it on the road. Any diesel puts so much torque out at startup that the trans just can't hold together.

TP from Cntl PA · 02-18-2005, 08:17 PM

Quote:

Originally Posted by tom hand

I don't understand the tranny thing...I really don't believe mounting a tranny backwards will buy you anything....it will have the same ratios if mounted forward. If you have the first tranny in first and the second tranny in second it would be the same as if you had the first one in second...the only thing two trannys would buy you is let you run a really high gear in the rearend....like a 2.73...and then you could put one tranny in first and the second in second for a really low gear for takeoffs....correct me if I am wrong

The second one will be mounted backwards for OD effect. I know lots of trucks with second trannys around here for low, low gears, but I want to run the second tranny backwards for an OD. For example, with the second tranny in 3rd, it will be like the second transmission didn't exist due to the 1:1 ratio in 3rd. Once on the highway, you could put the second transmission in second and it would be an OD since it is backwards(Ratio would be backwards).

tom hand · 02-18-2005, 08:30 PM

Ratio would be backwards...duh...you are correct

02-18-2005, 03:24 PM	#1
TP from Cntl PA Registered User Join Date: Apr 2003 Location: Central PA....In the Berg of Nisbet Posts: 790	Cummins going in '71, anyone here do it before? I got a small straight 6 Cummins on the way to replace the 350 in my '71 C-20. It is a 6AT3.4 and about the size of a 292. I would have put a 5.9 in it but I was concerned about the length. Anyone here put a diesel(Besides a 6.2) in one of these trucks? My main concern is on the gearing. I have a load of Saginaw 3spds laying around and kicked around the idea of mounting one behind the original tranny backwards for a poor mans OD. I haven't looked inside one yet to see if it is possible, but I thought that would be sorta different and practical as well. I thought about mounting it like a devorced transfer case and then adding an aftermarket 3spd shifter in the cab to control it. In 3rd it would be like it wasn't there and you had stock driveline, then once out on the highway, put the second tranny up in second and match the main tranny up to the road speed and engine rpm. Anyone ever try something like this? __________________ '72 K-20 EFI'ed 250 Inline/4spd stake body, Farm Truck '71 C-20 Cummins Diesel Powered, In storage thanks to $5.00 diesel! '69 3500 GMC 305V-6/4spd, Still under reconstruction.......... Inlines Rule! 6 soldiers standing is better than 8 laying down!!!!!!!!

02-18-2005, 04:14 PM	#2
BET68 Registered User Join Date: Mar 2004 Location: BREVARD NC Posts: 156	Cummins SOUNDS GREAT ,WOULD LOVE TO DO THIS TO ONE OF MINE ! KEEP US POSTED!!! __________________ NAMASTE B&E 1971 LONGHORN C-30 1969 LONGHORN C-20 1968 DUALLY 1970 STEP SIDE PROJECT 1972 CST C-20 1980 M-80 MALIBU FOR SALE 1974 NOVA FOR SALE:

02-18-2005, 05:09 PM	#3
Chevy67Tk Registered User Join Date: Mar 2003 Location: PA Posts: 349	Cool idea, keep us informed. The only thing I would be worried about is you might be running helical cut gears backwards. I can’t remember what’s in a Saginaw it’s been a long time sence I had one apart. __________________

02-18-2005, 06:07 PM	#4
'71c204x4454 '71k204x4454 Join Date: Jan 2004 Location: Hood River Or. Posts: 137	Someone is in the prossess right now. He just posted in the 4x4 forum. http://67-72chevytrucks.com/vboard/s....php3?t=141525 __________________ 1971 K20 4x4 1964 buick skylark convertible

02-18-2005, 06:51 PM	#5
hgs_notes GEARHEAD Join Date: Aug 2003 Location: MN Posts: 6,126	If you put both trannys in reverse, you would go forward! HG __________________ If no one knows what you're doing, they can't tell you you're doing it wrong HG's Dream Car Build - Shelby Cobra Kit Build your own adjustable track bar 71 Long Fleet C/10 72 Jimmy 4x4 (Junkyard Jimmy) HG's Plow Truck '78 K35 Dually Bigass Farm Truck HG's thread of miscellaneous stuff

02-18-2005, 07:40 PM	#6
tom hand CCRider Join Date: Feb 2000 Location: Olive Branch,MS,USA Posts: 2,232	I don't understand the tranny thing...I really don't believe mounting a tranny backwards will buy you anything....it will have the same ratios if mounted forward. If you have the first tranny in first and the second tranny in second it would be the same as if you had the first one in second...the only thing two trannys would buy you is let you run a really high gear in the rearend....like a 2.73...and then you could put one tranny in first and the second in second for a really low gear for takeoffs....correct me if I am wrong __________________ 72 GMC Sierra SWB almost finished---- 84 Softail Olive Branch MS

02-18-2005, 07:44 PM	#7
dwaite72lnghrn Registered User Join Date: Jun 2002 Location: central pa Posts: 1,525	The main problem is keeping the 3 speeds together long enough to get it on the road. Any diesel puts so much torque out at startup that the trans just can't hold together. __________________ 72 longhorn c30 502BB/th400/fact air/4.11rear/ custom camper 72 bug(the better half's) under reconstruction 2009 HHR aqua blue 71 GMC k20 350/sm465 2009 Chevy hd3500 6.6 Victory Red crew cab dually Dave & Jeanne

02-18-2005, 08:30 PM	#9
tom hand CCRider Join Date: Feb 2000 Location: Olive Branch,MS,USA Posts: 2,232	Ratio would be backwards...duh...you are correct __________________ 72 GMC Sierra SWB almost finished---- 84 Softail Olive Branch MS

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