How free flowing should the XJ exhaust be?

[Quills]

I have an inline 6, 4 liter and I am trying to squeeze the greatest gas milage I can out of it that is why I need your peoples opinion on my exhaust. I have stock headers with the stock head pipe next I removed the cat converter and put strait pipe in its place followed by my flowmaster 40 series delta and then a mandrel bent tailpipe. Is this too free flowing for good gas milage? Is there some general rule in relation to gas milage and exhaust flow resistance?

 

[HOUSEDIVIDED]

Your exhaust is only as good as your air intake. Do you have the stock air intake? A free flowing exhaust can hurt your gas mileage and power if you have a restrictive air intake.

 

[WB9YZU]

OPTIMAL FUEL ECONOMY? Re: How free flowing should the XJ exhaust be?

If everything else is stock...
If you put your stock exhaust on your XJ, including the catalytic converter, you will likely get the optimal fuel economy your XJ is capable of, assuming that it is in good tune.

Tuning an exhaust for "less restriction" improves fuel economy on vehicles that have fixed fixed delivery systems like carburetors as it causes them to run lean. Your XJ should compensate for any increase in flow by increasing fuel delivery and negate your modifications.

So in effect, you make you plan on spending big bucks on an aftermarket muffler and also making it (technically) illegal to drive on the highway for no reason.

You will never polish that turd and make it get 30 mpg. In all honesty 23mpg is the most highway mileage I have heard of from a 4.0L , and that was a manual, stock 4.0L with stock tires, stock exhaust. City is around 15mpg. I think they stickered at 21/15 when new.

You haven't told us of your mileage, what you have for gears, tires and the lot. If you have a lifted rig with larger tires, you mileage will reflect that.

Ron

 

[mountainpoo]

im running with stock headers, stock cat, and a thrush glass packs muffler with no tail pipe and it hasnt changed my mpg. honestly though, id rather pay for gas more often then a $2000 fine and ticket for having no cat.

 

[KSXJ]

yes, but some states don't have cat laws. But to answer your question I haven't seen a full exaust system or just a glass pack change my mpg or how my engine acts.

 

[mk153smaw]

optimal is not super free flowing. you want restriction built in. Start with a nice header and progressively get smaller as the exaust approaches the tail pipe. For example if the outlet of the header is 2.5 your end pipe should be 2" or slightly less. You are doing this since the exaust is cooling as it travels becoming more dense and exerting less pressure. By making the tube smaller you are accomodating the reduced expansion while maintain the all important back pressure for your motor.
Many people do not realize this and go too big on thier exuast then wonder why the performance takes a hit, not even considering that they just killed the back pressure from thier system.

 

[asp]

Re: OPTIMAL FUEL ECONOMY? Re: How free flowing should the XJ exhaust be?

You will never polish that turd and make it get 30 mpg. In all honesty 23mpg is the most highway mileage I have heard of from a 4.0L , and that was a manual, stock 4.0L with stock tires, stock exhaust.
Ron

i got 24 on a tank of 100% highway. at the time i had stock everything except for k&n intake on a 4.0L and i do in fact have an ax15 with 235/75/15's with mediocre to bad tread left. i got over 360 miles on the tank. arty: but yeah, if you polish a turd it's still a turd. just remember that you're pushing a brick through air... and you'll accept the bad gas mileage, or thing of all the guys in wrangler style jeeps that have it even worse for gas mileage.

 

[broqenjeep]

Just filled-up today got 16 mpg JALENA 88 4dr 4L aw30-40 4.10 ford 8.8 30 in. rubber stock exhuast to cat 22' glasspack and 2.25 to bumper 310,000 miles

 

[gradon]

MKs explanation above is why Borla designed their header and cat back system to go 2.25" from header to muffler in and 2" muffler out and tailpipe.