ddestruel

Like the title says why are our 24v's getting less mileage than our old 12v's

my 02 HO and all of my friends who have 01's and 02's in stock and bombed form are getting between 15-17mpg on the highway running 65-70 up highway 50 and 80 in california central valley to tahoe.

My bombed 12 valve used to consistantly do the same trips witht eh same guys and i would always get 2-3 mpg more and if i feathered it 4-5 mpg. I figured they were rodding on it but now that i own one setup with the same tires as were on the 12v, i am just wondering what might be some of the contributing factors.

1 on top of my mind is the full time front axle dragging me down some? maybe?

Bocifus

With my '02 4x4 I am seeing 24 mpg regularly on the highway and 18-20 mpg around town. But I am on my 3 lift (transfer) pump. The FASS in in my future, after warranty, will let DC put in another if needed.

Casey

BigBlue

I believe that's the highest MPG I've seen out of a 24v 4X4. I got 25 once, but I had stock 265 tires and a Comp. Ddestruel, I'm with you on the full time front axle engagement issue. In the future, I'm gonna eliminate that and go with manual front hubs. Just don't have the money, time, or want to do it right now.

phox_mulder

I think a lot of it is the EPA.
Compare the 12 valve, 24 valve 2nd gens, and the 3rd gens.
Mileage has gone down with each incarnation.

The front driveline always turning isn't helping things either.


phox

Bocifus

Yeah, my B-in-Law has a 1st gen ('93 or '94) had some work done to it by a performance shop in Simi Valley, claims he is getting 28 mpg. Now that is

cmckinney

Nah. Doubt the front drive axle is pulling you down that much. I can still get 19-21 mpg with my truck but it's pretty flat in Texas.

I say it's the luck of the draw on the trucks.

-CM

ramcummins

My 12v gets around 16 with manual hubs unlocked
My 24v gets around 21 with axle spinning

firestorm

My 01 2wd gets 18.5-19, unless I am running 75 mph, then I get 17.5-18 mph. The best I have seen is 21, and that was one time right after I bought the truck.

ddestruel

i might wager that you have 4.10's on the 12v

all our trucks run 3.54's and tire sizes range from 265/75, 285/75 & mine with the 315/75's. all are 4x4's it just strikes me as odd, the centered injector should be a more efficient system than the angled and offset 12valve i would think in theory????

Could be luck of the draw but these things are based on a world of physics/mathematics. Definately makes it fun to ponder.

Maybe the EPA compliant fueling curve on these motors isnt condusive to efficient power production at the right time in the process?????

DZLPWR

I have heard its mainly EPA emission standards that made the 24v a little less efficent. Good idea huh? A little less emissions for the price of spending more fuel......

rattle_rattle

I'm getting a consistant 17mpg.

BoldtsWagon

From stock Cummins engine fuel consumption figures (by Cummins) published in the TDR, the 12 valve is better from 1500 to 1700 rpm with a peak efficiency at 1600 rpm. The 24 valve fuel efficiency peaks at 2000 rpm and is better than the 12 valve at all rpm above 1750rpm. The peaks are 22.25 hp-hr/gal for the 12 valve at 1700 rpm and 21.9 hp-hr/gal for the 24 valve at 2000 rpm.

The fuel consumption lines cross at 1750 rpm. The unmodified 12 valve will have better mileage than the 24 valve at less than 1750 rpm, the 24 valve will have better mileage at greater than 1750 rpm. At over 2000 rpm the difference for stock timing gets much better for the 24 valve. (7.5% or lower fuel consumption for the 24 valve above 2200 rpm.)

The efficiency is probably largely affected by timing. The 12 valve has fixed timing and the 24 valve can advance timing. Larger injectors, bigger fueling plates, ar advanced timing in the 12 valve gets all the fuel into the cylinder faster and effectively increases the timing and increases fuel efficiency and mileage. Fueling and/or timing boxes does the same for the 24 valves.

The other side of this coin is that truckers observed that advanced timing might increase fuel mileage but also reduced peak torque for pulling grades.

Basically, with stock engines, the 12 valve gets better mileage if operated at 1800 rpm and lower, the 24 valve gets better mileage at over 1800 rpm.

The 12 valve does have the advantage(?) that it does not have the flat torque curve of the 24 valve. The 12 valve will have peak torque at about 1600 rpm where the 24 valve is just getting started. Unfortunately, this also results in much clutch trouble in both the automatic and manual transmissions with the 12 valve engines.

Haulin_in_Dixie

And the cam is retarded on the 24v to comply with EPA, hence no CAT. Also helps Dodge by reducing low rpm torque to save the drive line. I have downstairs a stock '99 Dodge build engine and a commercial build '99. The commercial has the same cam part number but is just about one tooth advanced on the cam timing. Commercial has less EPA regulation. My understanding is that the 12v used the same cam timing, but with the increased torque of the 24v the cam was retarded to comply with epa and save the driveline.

California vehicles, gas and diesel, have had retarded cams for many years which is the lower hp on them. Detroit diesels were 5 degrees retarded on the calif build. A 325 hp v6-92 was 305 hp in california.

dodgeman01

all I can say is my dads truck (98 24v) suxs the fuel and is the same truck as my 94 just newer with less power. Does not matter if the box is on or off it still gets 5-6 MPG less than my 94. and my 93 gets 2-3 MPG better than my 93 I don't drive the 98 much anymore I would take a guess because I have not computed it but I think my 98 is getting around 13-14 MPG. and I'm not kiddin. takes me 1/4 a tank to drive back and forth to school everyday in the 98. on my 94 and 93 it barely moves the needle
DM01

BigBlue

Something ain't right there. I can do a 100 mile trip in my truck (3.54 geared auto with 35" tires) and only burn an 1/8tank. Run around town all week, and drive a 100 miles back that week. Over 400 miles on a tank.