Straight Pipe It? As in straight pipe from the Catalytic Converter Back -OR- straight pipe from the exhaust manifold back?
Keep a few things in mind. Engines are coming pretty well designed and optimized off the factory floor. I would NOT assume that its like the 70's were the designs had huge compromises to keep costs and prices down.
You can't flow out more from the engine, than you can flow in and vice-a-versa. So concentrating freeing up the exhaust while the choke point in air-flow through the motor is the intake system, its NOT going to get you anything at all, mileage or power.
Keep in mind as well, the engine is a balanced system, freeing up the exhaust more than will help, usually doesn't "hurt", but disrupt the balance to much, you'll hurt it instead of improve it, or just make no difference at all.
The old carbuerated engines, needed a strong vacuum signal through the intake and carb, reducing the back pressure too much would reduce that vacuum signal going through the intake. EFI, that is NOT usually a problem, but two things can happen, free up that exhaust too much, it might go outside the EFI's calibration far enough it can't adjust to the changes and loose power and economy. OR, since the EFI has to maintain emissions standards, the exhaust changes are screwing up emissions and thus the EFI is adjusting in response to that.
Regardless, there is enough testimonials here that its pretty clear straight pipes on a Commander, like a lot of modern vehicles, hurt more than it helps.
All Jeeps have bad fuel efficiency? Ummm, Renegade, Patriot, Compass, the new Cherokee? That all have decent fuel efficiency they are also heavily criticized for being "soft" Jeeps that based on on-road car platforms will some improvements to suspension and drivetrain to provide better off-road ability that falls short of traditional Jeep ability.
A couple of things to keep in mind;
The WK and XK are based on the same platform, in fact they are both WK platforms and XK is more a body code/designation. They were built on the same assembly line. So similar mileage between the very similar vehicles should NOT be surprising.
Today's car manufacturers are massively regulated, there are thousands of regs and standards that the vehicles must meet. Crash standards and Mandated Safety Equipment that increase vehicle weight significantly and mandatory emissions standards that significantly reduces engine fuel efficiency.
To build a body with sufficient off-road strength and a 4WD/AWD drivetrain only adds to the weight and drags down the fuel efficiency even more. The more fuel efficient Jeeps suffering in off-road ability is good evidence of that concept.
V6's making the same mileage as the V8's, lets keep in mind a lot of work has gone into making engines more efficient, only to be dragged down by emission standards. What you're really seeing is as efficient as possible engines, and it takes that much fuel to move a vehicle of that weight, while staying under pollution standards, regardless of the number of the cylinders.
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Last edited by Mongo; 10-11-2015 at 12:29 PM.