Diff Difference

[55 Fargo]

Hi all,now I know I have brought this topic to the forum before, but it's a topic I have not resolved quite yet, especially in my head.

Here's the issue, I have a 4.11 in my 48 Chrysler Coupe, as well I have a 230 Canuck big block, 3 spd standard trans, and a dry clutch, the car weighs approx 3500 lbs. I now have confirmed my 47 parts car has a 3.54 diff. If this diff is good, my plan is to swap the hog heads and call it a day.

Now how many on this forum run a 4.11 diff, with only a 3sp trans, with no OD. Is it worth the swap, will .57 make a huge difference, will it affect the performance of my present engine. The terrain around here is flat, and traffic is mostly light in my immediate locale.

The differences are as follows, 4.11 diff, with P 205 75R 15 tires, this equals to 4.11 x 744 revolutions of the tire per mile, which is 3057 RPM at 60 mph. A 3.54 diff would have with the same tires,P205 75R 15 tires, this would equal to 3.54 x 744 revolutions per mile, which is 2633 rpm at 60 mph. The difference at 60 mph, is 424 rpm. Being the hp max rating of these engines is in the neighbourhood of 3600 rpm, 424 rpm at 60 mph may be a big difference, or is it. Just tossing this around, what's the general consensus on this one with you guys..............Fred

[greg g]

Well the folks at the big time auto makers think a lockup torque converter is a big deal and they are only good for 2 or 3 hundred rpms. Take a three hour trip do the math and thats quite a few turns your engine doesn't have to make. It should be worthwhile if it is a simple change. I might also combine it with a 215 or 225 rear tire and that might make the change 6 or 8 hundred rpm at cruising speeds. Can't hurt. I have 4.11's in my plymouth business coupe. It is busy at highway speed.

[Normspeed]

Hi Fred, I'm currently running a 3 speed with 4.10 rear until my OD is rebuilt. If you don't plan to run more than 60 mph or so for long trips the 4.10 is adequate with the 3 speed. It will give you easier takeoffs from a dead stop. For long range highway driving with the 3 speed I think you'll like the 3.54 better and you'll get better gas mileage as well. It will be a little more sluggish off the line.

My flathead sings quite a song at California freeway speeds of 65-75 mph with the 4.10. Can't wait to get the OD done and back in.

[claybill]

fred....dropping from a 4.11 to a 3.54 is a big jump!!!!! you will notice better top speeds with the same rpm.....and easier driving.

your 230 will pull it with no problem.

you may notice milder takeoffs, but actually it will be easier driviing, without the chug chug of a low first gera. go for it. you might change your mind about overdrive!!

claybill

[Normspeed]

What I like with the 4.10 and built flathead six, you can really pull the hole shots on the competition at traffic lights. If I catch one sleeping, I can flat hammer a Prius for almost 1/2 a block...

[55 Fargo]

Thanx everyone, just came back from a test cruise, had it up to 70 mph on the speedo which in my guess is about 57 to 58 mph, my speedo is out about 10 to 12 mph at 60, because of the rear gears. the speedo originally had the 3.54 gears. Engine was reading about 185 at the gauge, and the rad temp, at the top with a meat thermometer was reading only 160, what a difference from back to front. the gauge is a cheap aftermarket type so it's suspect. Actually in my Chrysler book it says it should be 183 when the therm is fully open. My test ride is on a gravel road, would that make the engine run a little harder/hotter, at high speeds, I would think it would as pavement is a smother easier ride for a car.........Fred

[david lazarus]

Hi There

On my '38 Dodge I had exactly the same as you do now but 16 inch rims. I have swapped in a 3,23 diff and it is FANTASTIC. At the same time as diff went in I dropped new radial tyers on that were 1/3 inch shorter when measuring the diameter. This meant that my much lower ratio (new 3.23 V's old 4.11) was slightly "undone". But the car is SO much better cruising at the lower revs. As for hill climbing I would say that in most cases I can hit the bottom of the hill with greater momentum and the car still just climbs them with ease.

Just do it...you will be pleased!!!

Rgds Dave