I haven't, but I am interested if anyone has, and what the results were...

 

It doesn't matter if it's turned up or stock, as long as your not blowing smoke your not wasting fuel. Also it's your right foot that gives you MPG's.

Gearing and tire size will determine MPG's also.

 

One example I'll give...well two...

OTR trucks that are governed at lower speeds generally give worse mileage than one not governed (but driven reasonably). Reason? They struggle on hills, they can't stay in that sweet spot.

I had an 86 Ramcharger with a 318, it averaged around 13mpg. I swapped in a warm 440 (same trans and drivetrain) and could easily get 16-17mpg driving sensibly. Reason? 440 barely worked to drag that big SUV around where the 318 struggled everywhere, not to mention the 440 was a blast to drive!

The biggest influence on mileage is the driver...mods will free up a little (but typically don't ever see a cost savings versus their cost to install).

 

I think the biggest thing fighting me is gearing. Pretty sure I have 4.10's in it because at about 55-60mph (GPS verified speed, my speedo reads about 7-10% high on the highway) I sit at about 1900 RPM. Tires are stock size and of the low rolling resistance flavour. I use cruise control every waking moment I have the opportunity and as long as I'm not pushing the speed above the limit I get pretty good mileage actually. Right foot is only a factor when buddy in his 4th gen thinks it'd be funny to see me try keep up down the dump road at 90mph haha.

Think I might try it just so its on record for everyone else how the truck does on a detune like this. Like sdeeter19555 mentioned the reduction in power might cause the engine to labour more, causing me to put more pedal into it than normal and may actually be counterproductive. I'll be sure to report back with my findings.

 

My old 92 D250 had the IP turned up and a 16cm exhaust housing on it and I always got around 25 mpg's. I had holes drilled into the front side of the air box and a free flowing exhaust.

Don't know if you do this or not but you should put some lube into the fuel tank to help make the IP last longer. It also keeps the small slide pin from sticking.

 

I always run Stanadyne Performance Formula additive as my year round anti-gel and lubricity modifier. It surprisingly helps a lot. It supposedly boosts cetane and you can actually feel a difference in how the engine runs with it. Better MPG's overall too. I tried some TC-W3 two stroke oil once and she wasn't happy running with that in the tank even at like 200:1

 

Make sure everything in the fuel system is up to par in your 30+ year old truck. Is the pump pressurizing fuel and varying timing with rpm like it should?
Are your injectors popping at the right pressure and atomizing the fuel properly?
Is your lift pump supplying adequate volume and pressure?
Thoughts to ponder.....

 

As far as I can tell everything is up to snuff, governor and variance systems in the IP are working as I can feel it change the fueling as I change load conditions/RPM, also have the KSB wired up to a switch so I can verify that timing advance DOES work in general. As for lift pump I can damn guarantee its in good shape. As I recall the old Ford 6.9IDI's called for about 1/2 to 1 pint of fuel out of their lift pumps through their fuel filter Schrader valve after 1 minute worth of running. I had accidentally loosened my fuel heater while changing my fuel filter last time and the o-ring did not seat correctly. I lost about 1 to 2 gallons of fuel, spraying up out of the hood 6 feet when I went to check, in about 30 seconds of running so I'd call it good. What a mess that was though. Only thing I can't confirm is injectors, though I regularly run an injector cleaner and lubricity additive. I'm not going to pull them until they are obviously failing, and even then good luck with finding availability of replacements in Canada! It's either order upgraded stage 2 and up injectors or bust currently.

As for testing a turned down pump I've been putting it off as my place of employment has decided to pay for my fuel costs because reasons? I guess they like me. Between that and riding my motorcycle for small commutes I haven't really had much reason anymore to pinch every drop. Maybe if I get bored today I'll do it for the hell of it.

 

You must not have the stock LP on your truck the stock one only puts out 3-5 psi of pressure.

 

Looks identical to the spare one I keep on hand in case it fails, just with a lot more rust on the housing haha. Maybe somebody modified it or stuck in a high performance one 20 years ago? I'd be curious to install a pressure gauge on it now to see where it's at if it weren't a hassle..... or if I had a gauge. All I know is its definitely supplying enough fuel at the moment haha. Maybe if I ever decide to address the slow oil leak from the gasket I'll take the time to see what it's putting out to see if I should be replacing it at that time.

 

A picture will tell which one you have, does it look like this one?

Alliant Fuel Transfer Pump - 1989-1993 Dodge Cummins | Thoroughbred Diesel

Alliant Fuel Transfer Pump - 1989-1993 Dodge Cummins: The Alliant Fuel Transfer Pump is compatible with 1989-1993 Dodge Cummins engines, ensuring efficient fuel transfer for proper engine function.

www.thoroughbreddiesel.com

 

CONCLUSION UPDATE:

So I've been running around for a tank and a bit with the pump 1 full screw rotation down from stock and these are the things I've found.

Total average fuel consumption is 19.2MPG. This is with short highway stints, mixed town driving, lots of idling and drive through visits, with the odd longer haul about 50 miles in length and moderate speeds. Not much change overall. Still about 750-800km per tank before I fill up.

However it does change a few things about HOW that fuel is consumed.

Pros:

- Truck runs way quieter, engine clatter is notably muffled, exhaust seems a little bit quieter too, no change in roughness of running though, still smooth.

- EGT has dropped about 25°F while under cruise conditions, loaded is no change (my probe is post turbo and likely not that accurate take that with a grain of salt)

- If I am just putting around town under 20MPH fuel seems to last way longer. City stop and go seems to be up around 20-22mpg

- Smoke is way down, despite being straight piped and having no muffler to catch any soot, aside from the odd puff at startup, I can fully lug the engine foot to the floor in a high gear and there is not even the slightest haze. It's a good look if you don't like little old ladies giving you death glares on the highway.

CONS:

Power is down noticeably, I've always been told the 22cm turbine housing trucks felt like an old dog that couldn't get out of its own way but MAN do I feel it now more than ever. This thing feels like my old 6.9 IDI. Nearly foot to the floor to accelerate uphill at a decent rate. I suspect this plays into the next factor.

- High speed fuel consumption is worse. I feel like to maintain speed above 30MPH you definitely need more pedal. Crusing back from my buddies house cruise locked at 50mph I averaged about 18MPG when I usually got 20. Haven't done much higher speed highway runs but I imagine it's much worse. Engine is definitely using more pedal to go over hills and maintain high speed. I feel like because my commute is basically accelerating up a hill to highways speed, maintaining for 3 minutes then coasting down a hill before stopping both ways this has negatively impacted my overall fuel consumption.

- Smoke is way down, can't shoot BMW repellent on command anymore if a little old lady in her fancy German car thinks it's funny to tailgate the decrepit old Dodge.


IN CONCLUSION: Definitely not for everyone. If you haul any sort of heavy load, do above 30mph more often than seldomly and don't have any current problems with EGT's or smoke I can't really advise it. However I can see a niche area where if you are running a stock muffler and use the rig to take the ole lady out to dinner down the block or get groceries every so often and you like a quiet(er) ride and would like to sip fuel a little more it might be worth a shot for you?

As for me I might go back up to 1.5 Screws above stock. I noticed better overall mileage for what my daily driving profile is. Less pedal needed to get up to highway speed if a bit too much fuel for bombing around town is a fair tradeoff in my mind. Plus seeing that 22cm H1C scream up to 30PSI of boost is something I've been missing! As with anything, your mileage may vary!!