2005 Dodge Magnum SXT - Auto>Manual Conversion- 10.3% decrease in fuel consumption

[mcrews]

Originally Posted by t vago

Fair question.

Top gear on the dead 42RLE had a ratio of 0.69. Top gear on the replacement NSG370 has a ratio of 0.84. This represents about a 21.7% increase in propshaft rotation speed, for a given vehicle speed. If I were to have left the original 3.636 rear end in place, that should have resulted in a definite increase in fuel consumption.

The FDR was 3.636, while it is now 2.87. This represents about a 21.1% decrease in required pinion input speed, for a given vehicle speed.

These two changes almost cancel each other out, and the net decrease in engine speed (about 3.9%) does not account for the 10.1% decrease in fuel consumption.

Put it another way - Engine speed at 60 MPH was about 1850. Now, it's about 1780. There is no way that this change in engine speed (~3.9%), by itself, would have resulted in the measured decrease in fuel consumption.



Because I forgot? I only regarded the rear end swap as a necessary adjustment to counter the larger top gear ratio of the installed NSG370, nothing more. I figured that the 3.9% drop in engine speed that I calculated back in April of last year, would have given me a very modest 2% decrease in fuel consumption. This is based on the rule of thumb that a percent change in fuel consumption will be about half the percent change in the rear end ratio. The 2% change would have been rather hard to measure as it is just above noise.

You do bring up an interesting idea - What if I had left the original 3.636 rear end in place? Engine speed would have been about 2250 RPM at 60 MPH, which would have resulted in a net increase in engine speed of... 21.7% from when the 4-speed was still installed. I would have expected a 10.9% increase in fuel consumption just from swapping in the NSG370, if I were to have treated the engine speed increase as a result of swapping in a (nonexistent) 4.43 rear end.

I like a man who can run the numbers!!

[ksa8907]

If i remember correctly, the intrepid (42le) with 3.66 gears was 2000 rpm @ 60 and with 3.89 and 215's instead of 225's i was running about 2200@60. I still brushed 30mpg average with the 3.89's.

[ksa8907]

Originally Posted by t vago

The Jeep Wrangler used this transmission on the 3.8L V6 engine (which was what made this swap possible - the 3.5L and the 3.8L V6 bellhousing bolt patterns are nearly identical). The Chrysler Crossfire also used this transmission, but with a radically different bellhousing bolt pattern.

The first gen intrepid came stock with a 3.3, same as 3.8

[cRiPpLe_rOoStEr]

Originally Posted by t vago

Not sure if transmission weight itself would have played much of a part. I lost maybe 50 lbs or so from the swap.

Sure it's not the most important reason for that mileage improvement, but is not to be neglected at all.

[RustyLugNut]

Originally Posted by cRiPpLe_rOoStEr

Sure it's not the most important reason for that mileage improvement, but is not to be neglected at all.

I can't remember what the rule of thumb was for performance gained from weight reduction.

I should quit driving around with my wife. She's 100 pounds.

[Fat Charlie]

A navigator/commo/spotter that only costs 100 pounds is golden.

[t vago]

Well, made a few discoveries since last posting -

I had flashed the Magnum with a DiabloSport canned performance 93 octane tune a few years ago, and left it in there for up until a few days ago. I found out that this canned tune had caused my cold idle speed to be about 3000 (three thousand) RPM, and warm idle to consume about .55 gallons per hour.

I reverted the Magnum's tune back to stock, then played around with the stock tune with the limited tweaking capability built into the Predator. Basically, I advanced ignition timing by 8 degrees from 0-4000 RPM, and experimented with lowering the idle engine speeds (idle in P/N w/ no AC, idle in R/D/3/L w/ no AC, idle in P/N w/ AC on, idle in R/D/3/L w/ AC on).

Turns out that advancing stock ignition timing does wonders with fuel economy, above and beyond with what the performance 93 canned tune did. I got 28 MPG on a mostly-highway 29 mile drive last night, then got 25 MPG on a 6.5 mile city drive about an hour later.

Also learned that messing with the stock idle speeds is counterproductive, even with lowering the speeds by 50 RPM. When warm, the Magnum would attempt to idle at the lowered idle speed, then somehow the engine computer would experience some sort of underflow condition and cause idle speed to jump up to about 2000 RPM. I returned the idle speeds back to stock settings, and that solved the idle speed problem. I am also consuming about 0.35 gallons per hour in idle, now.

Also, the performance 93 tune messed with DFCO. The Magnum is now more willing to enter DFCO when I let off the gas.

[Fat Charlie]

Nice work!

[oldtamiyaphile]

I feel like it's been left out than an Auto on it's last legs may have had issues that caused poor economy. I had a 1.3 Suzuki with dead trans that struggled to get 25mpg.

A manual swap+ EOC would have sent that into the 60's.

[t vago]

Originally Posted by oldtamiyaphile

I feel like it's been left out than an Auto on it's last legs may have had issues that caused poor economy. I had a 1.3 Suzuki with dead trans that struggled to get 25mpg.

It's certainly within the realm of possibility. I did not consider it as a factor, however, because

• Out of 58 model year 2005 Dodge Magnums being tracked on Fuelly for gas mileage, mine made the top 5, with an average of 22.7 MPG, at the time the transmission died;
• The torque converter locked up normally and showed no signs of slippage;
• The transmission clutches all had normal measurements at the time the transmission failed (OD=125,UD=37,L/R=43,2/4=46);
• The transmission fluid was not burnt when I drained the transmission upon initial diagnosis; and
• The rear carrier assembly of the transmission had literally shredded itself into two separate pieces, once I took apart the old transmission to find out what had failed.

Here's the part that failed. It's part of the rear carrier assembly, the part that transmits engine power through 2nd, 4th, and reverse. Part of this rear carrier assembly is still inside the transmission. In the background, you can see the rear annulus, front carrier assembly, and a couple of large snap rings.



Here's a picture of a complete and unbroken rear carrier assembly. The part I'm holding in my hand is the bottom of the complete assembly.