Sanity Check - Exhaust

[bannind]

So I went in to have O2 bungs put in to make it easier to eventually go to EFI and if I run across a standalone wideband gauge at the salvage yard - use it to help tune the rig. While at the shop, the exhaust guy (cranky and old - which should be on his business cards as a selling point, but he is too cranky and old to have business cards) mentioned that the exhaust smell in the cab isn't being helped by 2 different facts.

1. I have Hooker headers with a cross over into a 3" in/out muffler with 3" back from the muffler. Too big of exhaust size.
2. It does exit behind the rear passenger tire, but is at a 45 degree angle rather than 90 degrees directly out the side.

His belief was that my 289 would never use 3" diameter tubing and thus the exhaust was slowing down as it left the pipe rather than getting propelled past the air bubble that is created behind the bronco. He said I would need a much larger engine to truly use the 3" exhaust. Rather than redo the entire thing, which he would do if I wanted to waste money, he suggested where the exhaust comes down over the rear axle - reduce it down to 2.5" tubing and the straight out the side rather than at the angle it is at now.

Thoughts?

 

[bmc69]

I don't buy that explanation for a sec. Yes, 3" is way too big for any engine but that's not causing the problem you are experiencing...which is very typical. It's simply the way the old bricks go through the atmosphere creating low static pressure areas behind them.

The location you show in your pic is exactly my preferred location for tail pipes.

 

[Broncobowsher]

The headers have nothing to do with it
The crossover has nothing to do with it
Changing the location or the length of the exhaust tip by a few inches can make a difference.

 

[pcf_mark]

Exhaust pipe diameter is meaningless after about 18" after the collector. So that argument is not valid. My pipes exit at 90 degrees and it was a massive improvement over straight out the back. Maybe 45 degrees the fumes can still be drawn in. It does look a little short - like it is exiting under the square pocket in the truck bed side and that is not good. Throw a tip on the end and extend it outward a bit and I bet the smell goes away.

 

[bannind]

I think I will go pick up an extension, just because it is a cheap test. Confirmed it is also time to hit the yard and find a wideband gauge and see if I can get the carb dialed in a little tighter.. Or hope that it all pisses off my wife enough I can spend the $$ on EFI. She doesn't even have to give me full on approval, as long as she doesn't say 'you're spending how much' - I could consider that an ok.

 

[ntsqd]

I had a 3" system on my '67 Ranchero with a 302. The FlowMaster rep at SEMA told me that 3" is too big for a 302, that I'd likely get "Constructive Interference" from it. That meaning that the exhaust pulses would slow down and run into each other in a way that would amplify them. Subsequent trials with that system finally resulted in a quiet system, but it took some doing.

Posts above to the contrary, I'm going with your guy's explanation though it may not be all of the problem. As noted there are other factors involved and all, some or none could be the reason why.

 

[nickgp]

I had a 2.5 inch system that exited just like yours. Fumes were an issue. After the full frame off build, I'm having it re done to exit at a 90 degree bend behind the pass tire. It may not look as cool, but sometimes you just have to do function over form!

 

[pcf_mark]

Exhaust rant starting...

Let's think about this statement:

"exhaust pulses would slow down and run into each other"

If one pulse slows down then they would all slow down right? How could they run into each other because they travel at the same speed?

The sound in the exhaust is leaving the head at approaching the speed of sound. The sound wave has very high amplitude (its loud) and very high frequency (moving very fast). These waves are flying around inside the tin box of the Flowmater muffler with no material to absorb them. That is why Flowmaster sounds tinny and loud. They put a V shaped metal piece to "reflect the waves". If you have a V in the exhaust path isn't going to reflect exhaust back up the pipe and cause reversion in the exhaust pipe? You know "constructive interference" - I would call that poor scavenging.

The Flowmaster guy was a moron. They still think it is 1988.

Sorry for the rant but exhaust design after the header collector plus 18-20" is meaningless and all about the sound.

 

[ntsqd]

If they all slow down what ensures that they all slow down at the same rate? Each pulse would have to have exactly the same energy and have traveled exactly the same distance. Not possible, not in a street engine. Maybe not even possible in an F1 or Cup Car engine.

Pontificate all you want, what he said about it and what he suggested to fix it was correct and worked. And has worked on other vehicles as well.

 

[Rox Crusher]

the other benefit of having it come out at 90 degree behind the rear tire is that is a bit more protective when wheeling.

My exhaust currently exits at 45 as shown likes to get between rocks and the quarter panel.

If it were 90, and tucked a little closer to the wheel, it would be less prone to damage.