Bookshelf

When I'm going 70-75 empty on the highway, I'm making around 8-9 psi, when towing a 6K open trailer, slightly more. Empty, I get 18-20 mpg, loaded, 13-15. My truck(96 in sig) is currently stock w/ the exception of a BHAF and a 4" exhaust. Has anyone gotten better mileage w/ a housing change, because it should lower boost? First gens make less cruising boost, I attribute that to the larger housings that are stock. Thanks for any input.

97 D-Tec

Theres no many factors that go into fuel economy its not even funny.. down here at least one of the major factors was where i got my fuel from. For some reason our local Mobile station must be selling crappy fuel because my fuel economy droped from about 18 to around 13.. i switched gas stationsand went to sunco and it ended up going back to 18. this is all runing back and forth to college making it a total of two hours of drive time a day at speeds ranging from 50 to 90 mph (being stuck behind slow people, passing them, crusiing all on a two lane road) that 18 mpg is unloaded and my cruising boost at around 70 seems to be 4-6 psi im not sure if thats because of the housing change (14 instead of 12) or not

infidel

As D-Tech said there are many factors that go into mpgs. Probably the biggest is speed and the increased aerodynamic drag that comes with it.
More speed equals more fuel which needs more boost to completely burn it.
You could say that more boost equals worse mpgs but it's really more fuel that is the culprit. It's more the volume of air rather than the pressure of the air (boost) that influences mpgs. A bigger turbo supplies more volume at less pressure, higher pressure = higher egts as the more compressed air is the hotter it is.
Basic equation that never changes is it takes X amount of fuel to move Y amount of weight at Z speed.
Problem with a bigger turbo is slower spool up (boost slow to build) so it may be a draw depending on the type of driving you're doing, stop and go vs straight interstate driving. Stock 12cm turbo is a compromise, faster spool up for smoke free acceleration but with a wastegate to prevent over spooling once the turbo starts spinning.

You might be able to learn more here http://dodgeram.org/tech/dsl/FAQ/turbo_faq.htm

Ronco95

infidel made some good points. Let me add to it. Boost is a product of engine load. Ever notice how you cannot make boost in Neutral? No load, no boost.
Your 285's are pretty tall for one. 4X4 is taller than 2WD.
If you switch to a true 31 inch tall tire you can pick something up in mileage. If you can safely lower your truck 2-3 inches you can do better also.
My truck has gone 750 miles on a tank of fuel down the interstate at 70 M.P.H. It takes right at 33 gallons to fill it back up. You do the math.

Bookshelf

I'm aware of the many factors that go into mileage, and have routinely gotten 22-24 mpg w/ my first gen 4x4's as long as I'm not making short trips. I know I could pick up some w/ a tire change, taking the ladder rack that's on the truck off, slowing down cruising speed etc. I notice that on first gen trucks which typically have 18.5 cm housings stock, that their cruising boost is 3-4 psi, while mine in my 96 gets 10 w/ a 12 cm. I know there are other factors, but I'd bet that if I put a larger housing on my 96 that cruising boost would drop, which is why I was curious if anyone noticed anything mileage wise w/ just a housing change. The 20 I've gotten isn't too bad considering the extended cab, 4x4, tires, and rack, just trying to improve.

whereswaldo250

The best fuel mileage with my first gen is 21.6 with 4:10's at 60mph. I have to agree that with short trips my fuel mileage drops dramatically.

wcbcruzer

If I rev it in park I can make up to 5 psi boost at about 2100 rpm or so.

Ronco95

Making boost in park is very odd. There simply should not be enough load to do it. Do you have mechanical or digital gauges wcbcruzer?

JohnX

Yep, same thought here. Mine will not boost in park.

wcbcruzer

I had my camera with me so I took a short clip.

http://www.youtube.com/watch?v=O847tnHUQzw

I think your trucks do it too but you probably have a 0-60 psi boost gauge, so you don't notice it. Mine is 0-35, mechanical.

Sorry, don't mean to hijack thread.

Ronco95

I have the Autometer mechanical 35 PSI gauge also. I will stand by my statement. You have a demon under your hood. Treat it well.

I don't have an explanation here.

wcbcruzer

Well that's just weird. I'm going to have to start a new thread on this:

https://www.dieseltruckresource.com/...90#post2278090

I never thought anything about it until now.

66gaza

Me too, and no boost in neutral.

Gaza

13FOX

I cruise @ 5psi boost between 60-70mph and 6-8psi between 75-80. Get decent mileage on highway. Drops is town.