Quote:
Originally Posted by Frank Lee
Why put in a bigger rear end? Bigger gears turn harder.
Why put in a bigger exhaust? Velocity is good and small dia is it for low rpm.
Don't even bother with decimal point claims with short fills. In fact don't bother with short fills at all. Run it down to at least 1/4 tank before filling to have any semblence of accuracy.
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The 7.25 rear end with 2.94 gears is whining currently. Ususally a sign of improper setup if recently rebuilt or wear if not recently rebuilt (most likely), so the rear end is showing its age. I do not expect it to explode, but this particular differential has shown to be weak, and has a joking reputation that is is known to fail behind grandma and her slant 6.
The 8.75 housing I have is direct bolt it, and stronger, and I have both 3.23 and 3.55 gears accessible, and the gears change in an hour, so I can see which works better once the 4 speed OD unit is in place.
Exhaust: I am going to have a 2.5 run from exhaust manifold, to trans, and step to 3" from there back. Muffler will be a straight through Magnaflow, and exit just under bumper.
This goes against basic ecomodding theory. The reason is, this car will get a turbo down the road, and the 3" is sized for the turbo. It might be overkill in the short term while naturally aspirated, but the stock 1 7/8 stuff coming off the manifold chokes these engines. The head has terrible flow characteristics stock, and drinking straw sized exhaust does not help. The most common sizing on n/a is 2 1/4 or 2 1/2 so I am not overdoing it too much while n/a but will be sized right when turbo comes, and I don't want to do it twice. The existing set up is ok, but bangs and clangs, and is pinched over axle, through restrictive glasspacks.... not the model of efficient flow.
Quote:
Originally Posted by ALS
A 4 speed and 2.73 gears in the rear end should give you the best highway mileage. Best all around gears would probably be a set of 3.23's.
That Chrysler Automatic transmission is the biggest drain on your fuel mileage.
One the HP loss is probably close to 20%, and the auto box weighs an extra 100 lbs over the manual box.
The other factor is it doesn't have a locking torque converter. Slippage in the torque converter is killing your highway fuel mileage. The transmission swap should be good for at least 3 mpg maybe 4 if you get lucky.
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Right now it has the 3 speed manual. It was supposed to have 4 speed OD as part of the Dart Lite package, but mine is a very early unit, and got out the door with non Dart Lite options.
I have another /6 that I am going to build. Basic rebuild, with some internal tricks for oil drainback, reduced oil windage, etc. I will get it built, and swap the engine, trans to the OD, and rear end in one big swap, sometime next spring?