diesel exhaust
 

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!?!

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?

What kind of highway speeds?

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.

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".
https://www.fuelly.com/attachments/fo...cf1e614f3f.jpg

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

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.

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......

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

Swap a 4 speed auto or a manual gear box?

Diesels need to breathe
 

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

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.

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

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.

The American carbs that Harley tried to use for years were unreliable. Those same Japanese carbs are on our ATV's, Jet Ski's, & even lawn mowers. The only thing better is FI.

Dave

6.2 is an N/A diesel so bigger exhaust probably won't help too much. if you could get your hands on a turbo kit for that it would greatly improve the milage and power.

this article may help even though it is performance oriented the concepts work for milage too.

https://www.dieselpowermag.com/tech/c...dup/index.html

I don't think adding a turbo will increase FE at all. Changing to a smaller engine with a turbo probably would.

without adding fuel it will add a little power but make the engine more efficent

I don't see how. A turbo uses exhaust pressure (which comes from pistons) to cram in more air; the fuel injectors add more fuel to match, don't they? Or does it just run leaner because diesels run widely varying ratios? If it just runs leaner, then you're using the pistons to push exhaust to turn a turbine to push more air into the cylinders...sounds like a recipe for loss unless it was running too rich beforehand.

the 6.2 runs pig rich when pulling any load. A turbo would lean that out and make use of the fuel rather than wasting it as black smoke. the main reason a turbo would help milage is the 6.2 n/a has 145hp but with a turbo it makes 60 hp more. This will move the 6000+LB Suburban much better than the N/A could ever do. The banks power pack claims 10% increase in milage with an increase of 60hp and 115 lb/ft.

I just ordered a 3" downpipe for my powerstroke to go with the 4" exhause it had on it when I bought it. Will post the results when I get it installed

Re: diesel exhaust
 

Phillip, back to back testing on several diesels showed a properly tuned n/a diesel see's no improvement with only excess air added.(turbo or supercharger) With a poorly tuned diesel, a turbo can band-aid it a bit and as you said use some of the previously unused fuel through higher cyl pressure causing higher chamber temps. But again, it was only a band aid and a proper tune up/inj replacement would of done the same thing cheaper.
Only when extra fuel is added with a turbo or supercharger does hp/torque go up. As far as milage, when the hp goes up (through the consumption of more fuel) milage drops. The only way around it is to replace a n/a 200hp engine with a smaller 200hp turbo engine. And THAT only works if you only require the max hp/tq rarely.

If you require 200hp (driving a rock crusher for example) than 200hp is 200hp. a big n/a engine will be burning x amount of fuel and the smaller turbo engine will burn almost identical amount of fuel to produce the same 200hp.
BUT...if you have a 200hp n/a engine in a crusher that only requires 100hp for regular loads, then the smaller turbo engine will be more efficient and not burn the same amount of fuel for the same 100hp.

Re: diesel exhaust
 

since we're digging up old threads...

NA diesel exhaust requires just as much attention as NA gasser exhaust. and will benefit from good manifolds/headers and properly sized pipes. bigger=better is for turbos, gas or diesel.

Re: diesel exhaust
 

NA diesel exhaust requires just as much attention as NA gasser exhaust. and will benefit from good manifolds/headers and properly sized pipes.
This is true.

bigger=better is for turbos, gas or diesel
This is not.
Too small gives quick response but limits flow.
Too big pronounces lag but overall flow increases.
Same as a turbo must be matched to cu.in and rpm.

Sorry about all the picky correcting, my pet peeves are old wives tales.
Another is that an engine requires backpressure to run or run correctly. This is absolutely FALSE!!!

A correctly tuned engine with restrictive exh makes "x" hp. When a free flowing dual exh is installed power drops off. Reinstall the tiny single exh and power comes back.
"see, engines require backpressure"!
Not quite, if the engine would of been retuned so it wasn't so lean (due to better flowing exh), the engine would of made more power/easier on fuel etc.
And, to add to above, replacing a 2" single exh with dual 4" pipes on a small block for example, will flow better than the restrictive single 2", but exh velocity would be so far from the ideal 300ft per second that the gain would be minimal compared to what it could be. Less restriction is better as long as the pressure wave doesn't drop too far and as long as the return wave hits the back of the exh valve during overlap to create scavenging.
(yes I build drag engines too, and am a tech inspector for Stock and SuperStock dragracing)