Exhaust size Insight

[Dan Hunter]

Recently I'd adjusted the last segment of my 4" straight pipe and didn't get the clamp tight enough as it slipped out. I noticed it walking around the car while fueling. It was running at the time and I crawled underneath and was able to slide it back in place.

What surprised me was how 3 or 4 feet of straight pipe brought the idle speed down so noticeably. I'd read opinions on exhaust size and that 4" isn't necessary but at this point I'd beg to differ.

 

[6.5L]

I am running full 4inch straight pipe with a 3inch down pipe and it never made a difference with my idle speed. I think the recommendation is for performance wise. Straight pipes really don't matter on diesel's because any back pressure is supplied via the turbocharger, so anything after that is fairly open minded. The larger the exhaust, generally the lower EGT's since hot exhaust gases can escape away from the turbocharger more efficiently. I would argue that anything above 4inch isn't needed on these, but I also would not advise a full 3 inch system. 4inch seems about perfect for these, especially if you are running any form of aftermarket turbo.

 

[Project57idi]

No exhaust is the best exhaust [for power] on a turbocharged engine...

Not sure how or why exhaust length or diameter would make any difference... Especially since idle speed is monitored and controlled by the ECM. Or am I wrong? It's happened before

 

[radtech07]

That's the same thing I thought. That is a computer controlled engine and should adjust on the fly I would think. Maybe it's one of those coincidences that aren't necessarily related.

 

[WarWagon]

Plugged soot trap or other exhaust restriction? Someone shove a banana in your exhaust? This makes no sense.