How does boost affect fuel economy?

[jeffreydmet]

I have a 99 K3500 dually that I would like to maximize fuel economy on. On a previous thread where I asked about fuel economy people with stock trucks like mine pretty much got the same mileage. I noticed some were getting significantly better though. They were apparently running with more boost than stock along with a few other mods.

Does anyone have an idea on how boost affects fuel economy and how this can be changed on my truck without affecting reliability?

[Scottpearce]

I have a 93 C3500 dually, 4.10 gears, 3.5" exhaust. The only difference from stock was a k&n filter and the exhaust. I have routinely gotten 12 to 13.5 mpg with this. Then I got the Heath Turbo-master wastegate controller. I have not gotten less than 16 mpg since (unloaded, that is) With my camper in the back (approximately 2750 pounds), and towing our four place jet ski trailer (approximately 4300 pounds), I weigh in at around 13,800 pounds. Also has the aerodynamics of a rather LARGE barn door. Before the turbo-master, I would get 9 to 9.5 mpg, now it is getting 10.5 to 11 mpg. The ONLY difference between these two numbers is the turbo-master, so I would have to say that increased boost definately helps with economy. My max boost was around 7psi before the turbo master, and it is now 13psi.

-Scott Pearce

[jeffreydmet]

Scott
What is your boost level while cruising unloaded at interstate speeds?

[damork]

My 95 has a stock intake and muffler, tailpipe but I run the BD Race chip. I seem to get better economy running 65 than 75, and usually I see boost levels of 5-6 psi at 75, 4-5 psi at 65 and 3-4 psi at 55. 55 - 60 is where I've had my best economy, and it was when I was running on a very hot 100 plus degree day on level highway with little wind.

I've checked the race chip against the factory chip and find the race chip does a little better (every gallon of fuel it has burned has been logged). I've been planning to run a plot of fuel economy over the miles and see what impact thermostats, timing gears, chips, synthetics all have on the economy. Seems the next logical step is to try a free flowing exhaust.

[tom.mcinerney]

I think that having timing advanced more than dealer specs improves power/efficiency at expense of emissions. Heads with NA precups are said to increase mileage, and decrease max power a few percent.

[Scottpearce]

jefferydmet,

Running unloaded on flat roads, I am barely showing any movement on the boost guage (about 1/2-1 psi). Start pulling a moderate hill, and the boost comes up to about 4-6 psi (depending on severity of hill).

I am thinking of switching the ring and pinion to a 3.73:1 gearset. I would like to go to 3.42:1, but am not sure the transmission would take the strain when loaded. I only run the 13,000# + maybe 10 trips a year, the rest is mostly unloaded, just people. I figure even with the dual wheels, I could pull 20+ mpg with the 3.42's. I am hoping for 18 with the 3.73's (unloaded, of course)
My dad pulls about 11,000 pounds with his 318 ram, and he has a 3.55 gearset. It handles it fine, but I don't think the chevy transmission is as strong as the mopar. Not intending to start flaming, just my 2 cents.....

[rjschoolcraft]

Most folks would say that the 4L80E (assuming this is what your truck has, you don't say) is better and stronger than anything Mopar has ever put on the road. It is a very tough transmission.

[G. Gearloose]

Scott I don't think you will realize those number with just a gear change. Milage is not directly proportional to rpm with these engines.

[john8662]

Actually, RPM has alot to do with fuel mileage on these engines, I have seen it said many times here in this forum that the fuel milege is best at around 1900 to 2000 rpm. After that it suffers. Also 4x4 has alot to do with poor mileage.

[moondoggie]

Good Day!

For a really anal (& not directly related to your question) record of my mpg, look at What's your fuel mileage with a load? (http://forum.thedieselpage.com/cgi-bin/ultimatebb.cgi?ubb=get_topic;f=1;t=004439#000014)

Blessings!

Brian Johnson, #5044

'82 6.2 1500 4X4 1/2T pickup, 4spd man w/ OD, bone stock, 335K+, "In Rust We Trust"
'89 6.2 4X4 1/2T pickup, bone stock, 146K+
'95 6.5TD 2500 4X4 heavy 3/4T pickup, Gear Vendors Aux. OD, 184K +
'95 6.5TD 1500 4X4 3/4T Suburban, Kennedy exhaust, 200K +

[G. Gearloose]

Well that not my experince.. Look at the numbers being thrown around. 20mpg dually from just gears? not realistic. No doubt 1900 rpm is optimal, but not THAT optimal.
One the most fuel efficient 6.2's I've had sang at 2600rpm at 65. , and got 22 mpg. K20 4.10 and mc465. Switching from an auto to manual trans will typically net better results than gears.

[jeffreydmet]

ScottPearce

I changed my K3500 dually from 4.1 to the 3.73 and put taller tires to effectively go to 3.43. I only saw maybe 1 mpg better. However I like the truck much better now because you can actually pull at a decent speed in 3rd when needed and it runs quieter at interstate speeds.

I often pull with a 4000 lb trailer and 2-3000 lb in the truck without much difficulty. My trailer doesn't catch much air though.

[Scottpearce]

3.42 gears in a 93 c3500 crew cab dually will get slightly better than 20 mpg. There is one in my hometown (Bellwood, PA) that routinly gets about 21 - 21.5 with the same size tires as mine (235 85 r16). He originally had 4.10's also, and went to 3.42's He never pulls anykind of load though. He normally has about 1500 pounds in the bed, and four people in the cab. Internals of the engine are stock (118k miles), and the only changes are a free flowing exhaust and intake filter.

I don't see why I won't improve 1 to 1.5 mpg (I am now getting 16-17.5 mpg) with a switch to 3.73's from 4.10's. Remember, both the truck I am refering to, and my truck are 2wd, not 4wd's (well, I guess they are 4wd's, but all four of the drive wheels are in the back! ;) )

JeffreyDmet, I am pretty sure about the 3.73's being ok towing the 14K, just not the 3.42's

On the strength of the mopar vs chevy transmissions, I am just going by what I have been reading on this forum, that is all. No real world proof either way. I do know, however, that a lot of the cummins rams came with 3.55's, while you rarely see less than 3.73's in the fords and chevys. Keep in mind guys, I HAVE a chevy! Not trying to put them down. I HAVE historically been a mopar man though.

[Phil Garrity]

I don't know if my truck is typical, but it is a 94 extended cab, 195K miles, 265/75/16 tires, 5 spd, 3.42 gears, BD race chip, -1.88 timing with gear drive timing set. Exhaust is 3" downpipe, to a 3.5" straight pipe with no muffler or soot box. I get 18.5-19 mpg empty at 75mph. RPM is 2050-2100. The best milage is at 65mph, but I don't drive at that speed enough to make a conclusion as to how much of an increase I could get. I pull a 28' camper a couple of times a year and milage goes down to 10-11. When towing I drive by the EGT gage (no more than 950) rather than by speedometer. I do not recommend 3.42s if you are going to tow quite a bit. Overall for me the 3.42s are best. One of these days I want to install a gearvendors OD and that will make towing a lot better. Changing gears for milage is a losing proposition, I don't think you will ever re-coup your expenses.

[DogDiesel]

I normally don't use my 1T SRW 4x4 5sp L-C 6.5TDA for daily driving, but I recently parked my little economy pickup and pulled insurance and tags. Ouch!
Filled up twice this past month, one $98 and one $94 fillup. Mileage was 19.97 and 17.1 empty, one 30 mile towing stent on 2d tank.
I have given up on categorizing economy, with this math, my little economy truck by just fuel mileage alone (40-45 MPG); insurance is paid in two months driving, not even counting the reduced maintenance costs. I intend to drive my 1T for 5-8 more years at least, and when I get back from winter vacation, I will put insurance and tags back on the turd truck and save my tow vehicle.
I wish you guys luck, but I found an extra insurance policy using a ECONOMY vehicle as daily beater is the key to fuel economy. I concentrate on towing power, and towing economy on my larger truck.

Realizing not what you want to hear, but high teens on fuel economy is peak on these trucks, as well even the new 1 tons, and with as far as I live from work, and my driving, the Arabs and Venezulans will get rich, and me poor. Maybe consider a sweet motorcycle? Kool?

Wayne

[ 10-02-2003, 04:59 PM: Message edited by: DogDiesel ]