Gain with a larger exhaust?

[Stlheadake]

I currently have an 06 LLY STOCK exept for the 4inch exhaust and guages. I am averaging 18MPGs mixed driving 80%hwy 20%city every day. Which right now is the bulk of my driving. I DO average 22 on the hwy only. I have been presented with an opportunity to swap some equipment I am not using for a new 5inch stainless Magnaflow turbo back.

My question is, does anyone think there are gains to be be made going from the 4inch to the 5inch? It looks GOOOOOOD!!!, but that doesn't necessarily translate into gains. I am rationalizing that it may increase my mileage a little more, I will probably have to change my intake to see any more MPG gains.

Just asking for some more info or thoughts. Always looking for those 'extra' MPGs

[More Power]

Scott,

The big exhaust systems do look great. And yes, you should see some modest performance gains with one - though it would likely take a chassis dyno or time slip to see the difference. You'll not likely see the difference from the driver's seat. As far as fuel economy, you should see a slight bump, but everyday variables could mask what you calculate on a tank to tank basis. However, if you'd developed a spreadsheet for fuel useage over a couple years and tens of thousands of miles, you should see a slight increase in MPG with the big pipe.

Before going big, talk to Kennedy about EGR response to a larger exhaust system. EGR flow is, in part, determined by exhaust backpressure. If the EGR flow isn't correct, it could set a code or otherwise affect performance.

Jim

[Stlheadake]

Thanks for your thoughts Jim. I made the trade. I didn't think anyone was going to respond, and I let my heart override my brain. Actually I didn't get hurt. I swapped a generator that I haven't used in a year for the BRAND NEW PIPE! Plus I can use the thing ANYTIME I want/need it.

I have seen a slight bump in my around town driving (I have worked VERY hard at keeping the variables as close as possible). I have a question about the EGR for you guys that have been doing this for a while....

I have switched back to the stock ECM on my truck. I have had EGR codes a couple of times, but they have been cleared. It seems they turn up when I a pulling heavy and either coasting down a slight hill or something. I don't get them ROUTINELY.

OK the real question, I THINK I understand what the EGR is doing in a very basic way. It is poking exhaust back into the cylinder to be reburned. IF I were to block off the EGR (I know I'd have to have the finger sticks to fool the stock ecm) theoretically there would be more volume of fuel in the cylinder (less exhaust displacing usable fuel), but would that translate into better 'efficiency' of the engine?

In short, does blocking the EGR translate to a little better in mileage? I hve read some pretty bad things about the EGR on a diesel, and how it just clogs up anyway.

[DmaxMaverick]

The EGR is "off" anytime you are on the power. It only operates at low RPM with no load on the engine. It shouldn't effect mileage, one way or the other, in/of itself. If you do extended idling, it may have some effect, but I don't know any specifics.

[Stlheadake]

Hmmm, I thought that basically under load the valve opened and recirculated the exhaust. I was backwards, so my theory was too! Hmmm.... now what? I was sort of hoping in the back of my tiny brain that blocking the EGR off would have some positive impact on my mileage however tiny it may be!

Back to square one!

[DmaxMaverick]

If it operated while under power, it would dump your boost (from the intake side) right out the exhaust. The EGR would be operating backwards.

[Stlheadake]

That's why I ask you guys, you know your stuff!! As for the EGR, it makes sense, I just thought it worked differently.