so I got this question (Chevy 4spd swap for the GMC 5spd)

Area for Discussion of non-original and non-Theater Modifications to the GM family of WWII trucks. This would include engine, transmission, and driveline swaps, etc.
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daveyboy
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so I got this question (Chevy 4spd swap for the GMC 5spd)

Post by daveyboy »

This is all about the Hupla or debate regarding the Chevy 4 speed transmision swap for a GMC 5 speed with O.D. in a chevy 1.5 ton 4x4 G506.

Some say it aint worth all the effort.. Others swear by it
(But of course off the board and private emails telling me this)

Probably for fear of public ridicule.. I dont know...

I guess what I am saying is there seems to be two opinions here:

One says its the best thing since sliced bread. The other say its a waste of time, money and effort.

I beleive the later to be hard core 100% restoreres, probably why the responses I got ON LINE on other boards.

I belevie I really nerver got the answers I thought I would get.

So, I found this online automotive calculator.

http://www.bgsoflex.com/auto.html

So, for SH*Ts and giggles, I plug in some numbers.
---------------------------------------
Differential Gear = 6.67 to 1
Transmission Gear = 1.00 to 1
Tire Diameter (Inches) = 36.90
(as listed on the Coker TIRE site for a 7:50-20 NDT
Engine RPM = 2750
Computation Results:
Computed Vehicle MPH is 45
---------------------------------------------------------

I think 2750 is a "managable, working RPM" for a 235/261 straight 6 dont you think? I mean, we are told that the ww2 Chevy trucks are good for 45 mph.. Not sure what RPM is a "good working range" for the "dippers" but a pressure bearing engine I think 2750 is reasonable.

reading engine performace specs they seems to be capable of 3100 PLUS RPM's... Maybe more.

Comments so far?

Now, I take the same calculator and plug in the .79/1 over drive reduction ratio for the GMC 5 speed. I get the following:

--------------------------------------------------------------

Differential Gear = 6.67 to 1
Transmission Gear = 0.79 to 1
Tire Diameter (Inches) = 36.90 (same size again here)
Engine RPM = 2750
Computation Results:
Computed Vehicle MPH is 57
--------------------------------------------------

WOW.... So "technically" the .79 ratio reduction can give me one nice "boost" in road speed. Or, as I would be using the truck, no more than 50-52, I would reduce the "working" RPM's even further.

Am I missing something?

I mean, I have read LOTS of negative responses to why one would not , or should not install this O.D. transmission. "Very Little gain for all the effort" is one statement written.

Heck, I see one helluva advantage just by these numbers.

again, am I missing something? Drag? vehicle weight? Lack of HP?

Is 2750 RPM an "unreasonable" working RPM for a full pressure engine or no?

Its my understanding its fully capable of that RPM... and more.

Tell me your thoughts!

Anyway, I go one step further...

I figure I would NOT want to push this truck past 52 as stated above.

So, here I get:

-----------------------------------------------------
Differential Gear = 6.67 to 1
Transmission Gear = 0.79 to 1
Tire Diameter (Inches) = 36.90
Engine RPM = 2500
Computation Results:
Computed Vehicle MPH is 52
--------------------------------------------------------

Hummmmm..... ONLY 2500 RPM to achieve 52 MPH.

Seems like a good deal to me... So, would anyone here comment that 2500 is EXCESSIVE RPM for this engine?

Thanks all..

(Just looking for truthful, non-biased answers to a swap I want to make)
dr deuce
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Post by dr deuce »

I am one of the complainers of the 5 speed and it is not for your reasons. The 5 sp has it all over the 4 speed untill you take it apart, but that is another note.

My/thee problem with the 5 sp in the Chev is that in the CCKW, the power train sits about 3 inches lower in the chassis. The difference is the height of the standoffs for the starter linkage mounted to the bellhousing. Put one of them in front of the gas pedal which is essentially what you are doing when you put the 5 sp in a Chevy. The 5 sp has mounts for a parking brake handle on both sides. one of those will be pretty much right in front of the gas pedal and oh by the way you still need to "build a house" over the top of it (floor). I have never seen pictures published showing the inside of the cab with someones foot on the gas pedal. I have heard people explain how they cock their foot to use the pedal...

Years ago, I took one of my Chevys and installed a 5 sp and determined even before I got to modifying the floor that it was unworkable/unsafe as far as the gas pedal was concerned.

The newer engines (235/261) will rev higher and allow speeds over 50 mph. I do this all the time in my M6 Bomb truck all the time with the original 4 speed. Do the calculation on 3250 rpms. That is a 118% increase over the 48 mph stock Chevy which translates to: 56 mph
Dr Deuce Over 50,000 driven miles in a CCKW
1942 CCKW closed cab shopvan
1943 CCKW closed cab cargo w/M32 MG mount
1944 CCKW open cab LeRoi Kompressor
1944 CCKW open cab F1 Aircraft fueler tanker
1945 CCKW open cab cargo w/artic cab
1942 Chev cargo
1942 Chev K51 Panel
1944 Chev M6 Bomb Truck
1942 GPW Jeep
http://home.comcast.net/~cckw/wsb/html/ ... 59870.html
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