Cummins 4BT & Diesel Conversions Forums banner

1 - 14 of 14 Posts

·
Registered
Joined
·
164 Posts
Discussion Starter #1
sorry if this is a double post, but i was new to the forum and wondering if a rear diff upgrade is neccesary for a 453t repower. it is going in a 1984 chevy c20. the engine is stock and i am using a eaton fs5005 5speed transmission. if it is required why is that? is it due to the torque of the engine being almost off idle or because of the multiplication that takes place when you have a ten or thirteen speed trans? thanks for the insight
 

·
Registered
Joined
·
1,394 Posts
Really it's hard work to find a replacement diff that is ideal. What a lot of the guys do is put an aux overdrive box in line and leave the diff alone.
 

·
Detroit Section Caretaker
Joined
·
3,761 Posts
Depends on what you have and what you want the truck to do as to what you need to change.

Usually rear axles are changed to get a more favorable axle gear ratio so you can travel the speed you want on the highway.

Use a gear calculator to find out that answer. For the 4-53T in a light truck and for good economy plan on about 2,000 rpm plus or minus a hundred or two at your desired cruising speed then figure what ratio and tire size you need to get there.
http://www.onlineconversion.com/bigger_tires.htm
 

·
Registered
Joined
·
164 Posts
Discussion Starter #5
ok, thanks guys for the info and experience. by the way junkmandan this is phil from morgantown lol. i saw alot of rear end changing around and was concerned. mine has a 3.08 to 1 ratio and the eaton fs trans is a 1:1 fifth gear. i was doing the math looks like redline will get me about 80mph assuming 2800 is top engine speed. love the forum too, so much information here i spent three hours here last night just drinking it all in.
 

·
Registered
Joined
·
1,338 Posts
Phil------It's been a loooooooooooooooooooong time, but if I remember right we set no load with a digital tach at 2940 to give it the 2800 full load speed, then adjusted the electric tach [pulsed from the alternator], close on the top end . Wasn't concerned what the electric tach said at idle .
 

·
Registered
Joined
·
164 Posts
Discussion Starter #7
Hey junkmandan, at work the other day I was putting a clutch in a Ford 4000 tractor. The tach on it was a mechanical drive that ran off the generator with a cable. Looks like a adaptable setup for our engines.
 

·
Registered
Joined
·
1,338 Posts
Hey junkmandan, at work the other day I was putting a clutch in a Ford 4000 tractor. The tach on it was a mechanical drive that ran off the generator with a cable. Looks like a adaptable setup for our engines.
Cable maintenance is another problem, generators were big and bulky.........If your flywheel is off, drill a hole in the housing in line with the ring gear teeth, then tap for SAE 3/4 fine thread,[or was it 13/16ths?] That could be slightly more accurate than electrically pulsed off the alternator.........But you don't needed NASA accuracy.
 

·
Registered
Joined
·
1,602 Posts
3.08 ratio should be just about right for a 53 series, with a direct trans. If you do need to change a rear end for some other reason, 14 bolt, ff axles are plentiful and cheap.
 

·
Detroit Section Caretaker
Joined
·
3,761 Posts
3/4", it is a pain to do in place, but it can be done, ask Grigg.

That's a picture of the 3/4-16 tapped hole for a tachometer pickup.
It can be done in the truck even with the flywheel on the engine. Best to remove the transmission and slip a piece of sheet metal in there to protect the ring gear from your drill point. You'll need to grind the point off the tap drill to get full depth (or remove the flywheel).
You should double check but I think the hole is 4" back from the surface of a #2 flywheel housing, likely same for other SAE sizes. Still worth double checking.

Grigg
 

·
Registered
Joined
·
1,165 Posts
yea i have that set up in both my trucks. Does anyone have a part number for the sensor? It looks like this:



It matches with a Motorola tach (i have identical ones in both trucks), not sure what else it'll work with. I need another sensor if anyone has a part number i can't find one on that.
 

·
Detroit Section Caretaker
Joined
·
3,761 Posts
1 - 14 of 14 Posts
Top