diesel exhaust

[thomas89]

alright i own a 1990 suburban 2500, 6.2 litre engine... it gets on average 16.5 mpg in the city and 14 mpg highway. i was wondering if i got straight open exhaust on it if it should get better mileage... i've heard that diesels don't require any back pressure from the exhaust to run right but i had exhaust on it a while back and it seemed to get 10 mpg anywhere!?!

[Red]

Welcome to the site thomas

In high school I had a friend with a diesel F250, upping to a full 4" brought his MPG from 10 to 12-14ish or so. Might help, not too sure. What kind of tires and when was the last time you had your diff fluids changed?

[2TonJellyBean]

What kind of highway speeds?

[theclencher]

Oh yes, I and everyone else you bless with that contraption can hardly wait to listen to that thing rattle past. I definitely recommend no mufflers for everyone wanting to increase fe, or wanting to harrass their neighbors, or make a redneck statement.

[skewbe]

LOL, those loud a$$ harleys suck major a$$ too.

And it is especially dumb because the bozos riding them probably think they are "all american".
http://i141.photobucket.com/albums/r46/Motorbiker_photos/NewsPics/oldNewsPics2/japcrap.jpg

There are tons of faster/quieter/lighter/far less expensive/better looking/reliable bikes out there (thanks Mr Suzuki!).

[SVOboy]

There are always rumors floating around about bigger exhausts being better for mileage, but I think it really depends on the vehicle.

Only thing I would caution is not to eliminate your cat.

Welcome to the site.

[Sludgy]

I've thought about a bigger exhaust on my 6.0 Powerstroke, but I always come back to the same thing:

Bigger exhaust pipes help most at high rpms. But for fuel economy, I want to keep engine RPMs as low as I can. At low RPM and gas flow, backpressure should be negligible in the stock exhaust.

I just can't see doing a $500 experiment with so little chance of success.

Yanno, I just thought of something. Maybe I sould actually test the backpressure in the stock exhaust......

[thomas89]

the truck is a 3 spd th400 i usualy go about 70mph on the freeway...i also have highway tread 265/75 r16 tires nothing real aggressive...the truck doesnt have a turbo or a cat all it has are the mufflers

[Red]

Swap a 4 speed auto or a manual gear box?

[rh77]

BTW, welcome to the site...

Diesels need to breathe to be efficient. Unfortunately without the turbo, any exhaust treatment is going to be loud. "Dealer's Choice" on that one...

Biggest thing -- try to slow down. You have a big brick cutting through the air, which takes a lot of fuel. No matter what kind of mod you have, this could get you the biggest savings.

Consider an engine block heater to get things moving from a cold start. At 130 hp and 240 lb-ft of torque, you may be on the low-end of the power-weight-FE ratio. Does it seem to really need some throttle to get it going? If/when it's time for a replacement, an Isuzu turbo-diesel may be a good option. Otherwise, keep it tuned-up and look for signs of wear.

On a side note: Clench -- we all know you dislike loud exhausts, but do you have a suggestion to help this man's FE?

Instead of throwing our arms up in the air because, OMG, someone with a large vehicle is asking for help -- let's take the time to address the challenge.

As always, I throw-in the caveat: for the next purchase, think "what's the smallest vehicle that can fit my needs". Until then, we're here to help. Even after then too, I guess :o

RH77

[theclencher]

I should have given thomas some more background the first time...

Anyhow, I'm confused by this: "i was wondering if i got straight open exhaust on it if it should get better mileage... i've heard that diesels don't require any back pressure from the exhaust to run right but i had exhaust on it a while back and it seemed to get 10 mpg anywhere!?!" If you "had exhaust" on it before and got 10, but are getting 14-16 now- presumably with exhaust ("the truck doesnt have a turbo or a cat all it has are the mufflers") what's different?

I am suspicious of Red's (or his friend's) claim of a four inch system yielding a 20-40% mpg increase. It flies in the face of everything I *think I* know about exhausts. Plus, if it was true, you can bet it would have come from the factory that way.

On a different note, it seems to me if you are getting better mpg in the city than on the highway, you are likely burning up too much fuel by pushing massive amounts of air aside with that big square sled. Also, high speed is the enemy of that old-skool tranny w/o lockup converter and OD. Slowing down, being steady or using cc, and airing up those nice high psi, highway tread tires should help.

There are many vehicle mods and driver techniques that remain consistant whether you are driving a ZX2 or an Excursion. Instead of repeating them all again I would urge you to do a search here for all the wonderful knowledge that's already posted.

[Red]

I am suspicious of Red's (or his friend's) claim of a four inch system yielding a 20-40% mpg increase. It flies in the face of everything I *think I* know about exhausts. Plus, if it was true, you can bet it would have come from the factory that way.

The guy does drive like an @$$ and the thing is lifted, sitting on 38" boggers so it might be off some....

[omgwtfbyobbq]

Yeah... Given that a driver can double or half the EPA rating, I would guess that it's kinda hard to attribute any mileage change at all to the exhaust.