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Go Back   Dodge Cummins Diesel Forum > 2nd Gen. Dodge Cummins 94-98 12V Forums > 94-98 Powertrain
94-98 Powertrain Discussion of components that are directly involved in the power production and all that is needed to get and keep the truck moving . Engine , Transmission Ect...NO ADVERTISING

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Old 10-25-2011, 09:33 AM   #1 (permalink)
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Mileage Tweaks / Tuning

I want to tune for best mileage, I really don't pull anything and normally am running empty, but I do a lot of 75mph highway and 60mph highway driving. My truck is basically stock right now with 285-75-16 tires (yes I know smaller tires are better for mileage but look like crap), stock exhaust with catalytic converter, stock air intake system, no mods to fuel plate, or waste gate etc.

I am mechanically inclined, (was a car mechanic about 2 lifetimes ago) and do most of my own work ie KDP, leveling, tomorrow putting in a borgeson box etc.

What are some things I can do (without spending an arm and leg) that will have the biggest impact in increased mileage?

Seems to me BHAF is a given as is 4inch exhaust and losing the cat....what else?
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Old 10-25-2011, 11:08 AM   #2 (permalink)
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You also wanna up your timing to around 16*. I would also get a slightly larger turbo like a jkid hybrid hx35 this will allow lower boost levels and more air flow while on the highway. Also if your truck has high miles maybe get your injectors sent out to be cleaned and have the pop off reset. That or new injectors. And besides that you can up the power and tune out the smoke.
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Old 10-25-2011, 11:54 AM   #3 (permalink)
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Personally I think the BHAF is a little over rated and gives little performance increases. I say this because it is hard to not suck hot air out of the engine compartment. I think modding the stock air box so it draws directly from the cowl makes just as good an improvement if not better.

I believe the cat delete and free flowing muffler will give you bigger rewards. I purchased a SS free flow 3" muffler from NAPA for less than $80, a section of 3" pipe and some clamps and did that. I did all this for around $100.

The timing is a big thing. With 3.55 gears I personally believe 15.5* is good for a daily driver since this will keep your best timing in the RPM range. If timing has not been touched, I would imagine it has slipped a hair anyway.

I second the hybrid turbo. It's a great little turbo for a lightly modded engine.

For a little added kick in the seat of the pants when getting on the highway, passing and such you would benefit from a 3k GSK. This will keep you fueling past 2,000 RPM when you lay into it a little. I personally like a #10 fuel plate. It fuels harder, but if properly tuned won't smoke everybody out. Remember, as long as you don't smoke heavily, you're not wasting fuel.

Also, keep in mind as you start messing with the fuel, the trans may not like it. A good billet converter and someway of raising line pressure will set you back some money. Leave the fuel plate alone and most likely the stock trans will be fine. Playing with the timing, exhaust and air won't bother the trans.
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Old 10-26-2011, 08:23 AM   #4 (permalink)
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ok thanks guys!
Truck has an appointment at Bridgeway Cummins tomorrow to get the timing bumped. New air and exhaust should be on it by Saturday.
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Old 10-29-2011, 02:04 PM   #5 (permalink)
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I don't think you will see ANY gains with a BHAF. You stock air filter & box are not a choking point with a stock engine, turbo & exhaust. I'm willing to bet the filter minder thing on top of the air filter box is still all the way at the top. Until you see your turbo sucking hard enough to start pulling that down (with a clean air filter inside), save your money to spend elsewhere.

A bigger exhaust and loosing the Cat will let your turbo spool a little quicker and let the EGTs drop faster after pulling a hill which will help.

I agree with the idea of the governor spring kit making it more driveable (or at least more fun to drive) especially when accelerating up to speed on freeway on ramps.

Moving the stock fuel plate forward will also give you some more power if/when you want it. this will not change mpg either way unless you start putting your foot into it far enough to use the extra fuel that would now become available.

I think a slightly bigger set of injectors would help too. With these diesels, makeing them more efficient makes for better economy (and more power if you put your foot into it). I don't have any first hand experiance with them, but I like the idea of having 7 holes in the injector nozzel instead of 5(which is stock), this idea keeps all the holes small for good atomization and spreads out the spray pattern better for a more complete burn. AND also allows more fuel to get into the cylinder when you stand on it!

The spendy item that I've heard local guys praise, is an aftermarket torque converter with a lower stall speed. which basically lowers your RPM's across the board. If you currently cruise 70mph @ 2100RPMs, that same 70mph will then be down around 1800RPMs, which is more in the peak economy band of the engine. Torque converters start around $700 and can go to over $1000 and that is just the part. If you are not going to put it in yourself add labor to that too. then since you have all the fluid out of your transmission you may as well change the filter in there and add new fluid.

Beyond that you have to start messing with the air-o-dynamics. Your miles per gallon will be higher at 55mph than the 75mph you mentioned due to the amount of air pressure working against the front of your truck.

If you search you will find other threads on here that talk about improving mileage.

a tapered top over the bed, as high as the cab in front and tapering down to the tailgate.

smoothing out the air turbulance in the front at the grille helps.

I've read that the lack of the factory, little rubber strip under the front bumper can cost you 1 mile per gallon. lowering the front skirt and adding side skirts reduce turbulance under the truck. Lowering the whole truck will have a similar effect.

smooth style wheels, like a pizza pan let the air slip by easier.

highway tread radials instead of off-road mud & snow tread cause less friction on the road (as does a good front end alignment) and disturbs the passing air less.

You have a nearly 7,000 pound truck that you are propelling around and it has a certain size that are conteding with. It is NOT a Prius!

have fun with your mods. please report back what your miles per gallon is now and what it becomes after each mod so others can see what mod had the most/best effect.

My trk gets me about 18mpg to and from work (non highway) and in the 14-15mpg towing a 28 foot travel trailer that weighs about 7,000lbs.
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Old 10-29-2011, 03:16 PM   #6 (permalink)
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Even with a lower stall t/c, your highway cruise RPM will NOT change... unless you arent locking up... It will potentially reduce slippage when cruising at speeds below lock-up...

Chase
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Old 10-30-2011, 03:08 AM   #7 (permalink)
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Tune for minimal smoke with starwheel and quick turbo spool with preboost screw (less fuel wasted then)

Get your 2" leveling kit off.

Better torque converter and a shift kit.

Drive with no more than 5-10psi boost. Basically drive by the boost and try to stay low in boost psi just same as running a vacuum gauge on a n/a gasser to montior and maintain high vacuum as possible to minimal fuel usage. More vacuum = less fuel, same as less boost = less fuel for diesels.
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Old 10-30-2011, 10:46 PM   #8 (permalink)
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Quote:
Originally Posted by chasespeed View Post
Even with a lower stall t/c, your highway cruise RPM will NOT change... unless you arent locking up... It will potentially reduce slippage when cruising at speeds below lock-up...

Chase
X2. A converter wont have any affect at all on your rpms once it is locked up. If you are @ 2100 rpms at 70 with the stock converter it'll still be exactly the same with any aftermarket converter.

Sent from my phone, probably while driving and eating a sammich
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Old 10-30-2011, 11:39 PM   #9 (permalink)
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With only enough refills to say almost nothing I got better m.p.g. unloaded at 54 mph and 1500 rpm than I did at 1600 rpm and a lot better fuel economy than driving at 70 doing around 2k all on the interstate getting the truck home.

I will not say anything below is correct. These are all things I found on here and saved for my own use. I can not give credit to the original posters but if you did a search for each line you could find them.

------------------------------------------------
3.54's optimim fuel mileage is attained at 1700 rpm

I notice my mileage really starts dropping after about 1900 rpm also.

Our engines like 1600 to 1800 rpms for economy

The #1 factor in getting better fuel mileage is to REDUCE HORSEPOWER DEMAND.

This is a combination of vehicle configuration, condition, load and DRIVER OPERATION.

Keep your pyrometer under 600F and keep your boost pressure under 5 PSI when solo.

I actually gained a few mpg when I stepped up in tire size. Watch your pyro when youre driving and stick it as close to 550 - 600* as you can. I run 62 mph and get 22 - 25 mpg

I usually stay between 60-70 and almost never over I try and let my egt's settle at 600*

t makes peak torque at 1600 rpm and peak hp at 2200 ish. so any thing over 2200 is a waist of fuel. when accelerating keep your rpms down as low as possible

As a matter of fact I was playing with gears and speed to see a few differences. I'm using my truck (in sig). Cruise set at 55 MPH.

5th Gear = 1,600 RPM = 2-3 PSI Boost (Better for MPG)
4th Gear = 2,100 RPM = 5-7 PSI Boost

And knowning that high boost pressures means more fuel... Better to keep the pressure lower for highier MPG numbers.

Anytime your cruising down the Highway and pushing 0psi of boost, you are going to be at optimal gaines for MPG. I start pushing 2psi at 68mph. So every set-up is different.

I have a 04 dually 4x4 LB and if i drive sensable @ 65 i will get 21 MPG
@60 i get 23 MPG
@55 i get 25MPG
-----------------------------------

Nothing above was about tuning. It was all about increasing mpg by driver control. Perhaps it can be helpful. Do not forget, in stop and go driving a 3 1/2 to 4 ton piece of metal is not going to give good fuel economy regardless of what you do.
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Old 10-31-2011, 12:26 AM   #10 (permalink)
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Quote:
Originally Posted by chasespeed View Post
Even with a lower stall t/c, your highway cruise RPM will NOT change... unless you arent locking up... It will potentially reduce slippage when cruising at speeds below lock-up...

Chase
I was gonna say, if youre locked up in O/D before 55 how will a lower stall speed help? Also, if you're doing alot of highway driving at speed slightly taller tires will help somewhat by lowering your final drive ratio and in turn RPM
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Old 10-31-2011, 12:35 AM   #11 (permalink)
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Quote:
Originally Posted by TrailTamer View Post
Basically drive by the boost and try to stay low in boost psi just same as running a vacuum gauge on a n/a gasser to montior and maintain high vacuum as possible to minimal fuel usage. More vacuum = less fuel, same as less boost = less fuel for diesels.
More vacuum = MORE fuel in a N/a gasser.
Same idea though, watch boost gauge and keep as low as possible.
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