Mopar Muscle Association UK

Hi

I have a 4.1 diff in my 8 3/4 inch axle.

Where can I go to get a 3.9 or similar in good condition, what is involved in doing one of these. I have a guy who rebuilt my mk1 escort diff, he used siny shims to get the load and preload spot on.

Is the same normally required on these american cars?

The car is a Dodge Dart 68.

Thanks

Jon

You will not realistically notice any difference between 4.10:1 and 3.91:1.
Don't bother.

Easier putting taller tyres on for a better effect.

exactly what I was thinking, unless you're planning long road trips, I wouldn't bother or switch to tall tyres, as Pete says.

3.55's are the best compromise in my view, but if it was my Dart I'd leave the 4.10s in

The 4.1 runs out of puff at about 60 ish MPH

It would go faster but its revving at about 3.5k rpm at this point

MPH is indicated on the dash, it could be faster. Bigger tyres are an option... hmmm

All Yank transmissions of that era have 1:1 final drive.

You have to rev the motor. Simple as.

This is why people put tall tyres on to avoid their ears bleeding.
Or cruise at 55mph.
Then batter it at the track.

Try a 528 or 572ci motor in that scenario......bye bye fuel consumption.

If the 400 you have is stock with single valve springs, then you are done by about 5300 rpm.

I have had modified and stroked 400's that revved to 7000.......

I agree with what was said earlier lot of effort for no real gain from a 4.10's to 3.9's

Pete wrote: I have had modified and stroked 400's that revved to 7000.......

I know of few others that zinged even higher RPM

DB's 360 went event higher!!!!!!

Then it went "Bang!"




Sorry, Paul......

Queue the "Have you got a Rev limiter?" Discussion......

Pete wrote:DB's 360 went event higher!!!!!!

My B1 used to rev freely & regularly to 7,800 RPM

It can be done but you wonder for how long.

Even lightened Hemi pistons are still really heavy; when you calculate how many times they go up, stop, and go down per second at even 4000rpm it is really scary. No wonder they go off with a bang......

With 4.10 gearing what sort of tyre height would be needed to make it " motorway friendly " 28 to 30" ???

Use the excellent "Automath Spreadsheet" that Dave posted on here to balance off Diff ratio, rpm, and tyre height to achieve the desired affect...

mopar_mark wrote:

Pete wrote:DB's 360 went event higher!!!!!!

My B1 used to rev freely & regularly to 7,800 RPM

Aftermarket blocks but I used to shift at 7600rpm and go thru the traps at 8200rpm with the belvedere with 28inch tyres, and shifted at 7600rpm and went thru the traps at 8000rpm in the roadrunner with 32inch tyres, both 4.56 gears

Pete wrote:It can be done but you wonder for how long.

Even lightened Hemi pistons are still really heavy; when you calculate how many times they go up, stop, and go down per second at even 4000rpm it is really scary. No wonder they go off with a bang......

I didn't like that 9.96 run.......7300 trap rpm with 1100 gram 6pk pistons and rods that weighed a ton...balanced assembly and external oiling or not....only did it the once...4.88's with 32's, a race converter 600hp is not good for 6pk engines.....apparently that engine is still running in a pop.

Les

"One careful owner", Jerry ????

Just shows what you can do.....

I find 29" tall tyres and 4.10 gears are acceptable on the street. Much less than that and you tend to keep looking for that extra gear you haven't got...