Hello All

[Conk]

Im new here and I like this forum

I have a 90 F250 Lariat with a 7.3
I changed the stock exaust to a 3" system just after the y pipe.
The muffler was a staight through round type, like on a 3 or 5 ton van
The Power is good and it has a nice loud sound too

But it seemed to kill my milage by 25%
I was getting around 20 now im getting only 15
Could it be my exaust system lacking any backpressure?
Has anyone had or heard of any trouble like this before?

 

[tonkadoctor]

Im new here and I like this forum

I have a 90 F250 Lariat with a 7.3
I changed the stock exaust to a 3" system just after the y pipe.
The muffler was a staight through round type, like on a 3 or 5 ton van
The Power is good and it has a nice loud sound too

But it seemed to kill my milage by 25%
I was getting around 20 now im getting only 15
Could it be my exaust system lacking any backpressure?
Has anyone had or heard of any trouble like this before?

Welcome....Yep I saw this exact same problem on the my Superduty....It was caused by my right foot wanting to press the pedal harder all the time to hear the sweet sound of a turbo diesel.

 

[dsblack]

Diesel trucks do not need back pressure like a car... Keep the sound down, and the MPG will go up!

Welcome to the site.

 

[sle2115]

NO internal combustion engine benefits from backpressure. I would say Tonka hit the nail on the head! Enjoy the sound, it is sweet!

 

[TLBREWER]

It has been proven that sound quality is inversly proportionate to fuel mileage.

Tom

 

[swampdigger]

I hate the term "backpressure". It sounds restrictive, like having something try and push down on your piston as it's coming up to push out the exhaust gasses.

It's supposed to mean if you make a gas flow through a smaller diameter pipe, it speeds up, and the momentum it gains by speeding up helps pull any remaining exhaust out of the cylinder. The high velocity gasses also completely exit the pipe, instead of getting halfway through, cooling down, and lingering.

This plays a crucial role on 2 stroke engines, but has the same effect on 4 strokes as well, it's just not necessary for operation.

You get increased friction with a smaller diameter pipe though, so you want to choose carefully. Too big, and the exhaust gasses won't get the nice velocity gains, too small and you get a restriction.

 

[Agnem]

I'll throw an ugly acronym into the mix. ULSD! My milage has been consistently lower since that crap hit the streets. Can't be the exhaust. Welcome to the site!

 

[Conk]

True But

I live in the country so I always take the highway to work every day.
I work in the next town.
I still drive at the same speed, around 65mph for about 20 miles 1 way.
I was getting 20 now 15mph
only change was the pipe to 3 inch
Truck runs perfect before and after pipe, just milage sucks now

Im gonna plug/restrick the pipe and try it next week.

 

[Agnem]

Well make sure your taking an average over several tank fulls.

 

[Dsl_Dog_Treat]

I can say that when I upgraded the exhaust on the Beef that was factor to a 3" from the y-pipe back, it improved the mileage.

I must agree with Mel on the ULSD cutting mileage. I've noticed a good 2-3 mpg decrease since that stuff has been in my area.
Now going french fry burnin, it has come back up a little bit after I succumbed to the fact that it burns cleaner and less smoke output at an 80% mix.

Another thing to consider also is the air filter. What type and condition is it in?

Good flow going out but not enough air going in.