Does A Dented Exhaust Pipe Restrict Power?

Thai Long Ly
by Thai Long Ly

Organic tuning

So how does slamming a suicidal rock, thus spewing hot juicy oil all over your rear tire, sound to you as you’re flying down your favorite mountain road? Sorta frightening, right? Because it is. Ask me how I know. Then ask me which of my favorite four-letter words I chose to spit out of my helmet were once I realized that this could have been really bad. Reeeeeealllly reeeeeaaalllly bad. I was thrilled to be climbing off my bike by choice as opposed to a sudden unplanned eviction over the side of a mountain. Guardian angel, this round’s on me.

But, seeing as this is, I found two positive things came of this unexpected “oily bike upright” skills test.

  • I got to exercise my ever-expanding friendship with Troy Siahaan. (“Hey buddy, I know you’re home and I’m waaaaay over here stuck on a mountain, but can you haul that sparkly truck of yours through rush-hour traffic and pick me up? Bring towels, as I’ll be spewing dirty oil all over the back of the bed.”)
  • I got an idea for a new business venture!
Oil is equally effective at lubricating the ground as it is your engine.

Let me explain. Rounding a blind turn at speed while avoiding a large gathering of rocks was what I thought I was doing as I suavely flicked my Aprilia Tuono 1100RR side to side. Think Lindsey Vonn dancing vivaciously through moguls on her way to Gold (I know she was a downhiller, but stay with me). Wrong. I was actually denting the snot out of the underhung exhaust collector and severing an oil line as I clumsily plowed through the most invisible rocks on display. Think Gilbert Gottfried ramming face first into bowling pins. Never bueno.

Once back to my garage, I replaced the oil line and scrubbed the oil off the rear of the bike and tire. I then went about searching for a new collector pipe (the collector pipe is where all four of the V-4’s header pipes converge into a single outlet). Surely a giant air impeding dent can’t be good for power, right?

In speaking to as many experts (and so-called smart people) as I could, I was not able to come to any conclusion as to whether or not this rocky “love tap” would affect power in any way. Some said, “absolutely replace it” while others quipped “don’t sweat it.” The delusional millionaire in me said “immediately buy a new one,” while the realistic musician biker bum sighed “leave it be.” What to do? I know. To the dyno!

Custom fabbed collector courtesy of Mountain Rock Motorsports. Look closely and you’ll see the top of the pipe has been pinched as well.

A call to Jarred Fixler over at Motorsport Exotica (Los Angeles, CA) would put an end to this first-world dilemma once and for all. I had my bike (with about 6000 miles on it) dynoed on his freshly minted DynoJet, and the pulls were still on file. A couple of pulls with the dented exhaust (and about 10k miles on my bike) and we’d know if and how the power was affected.

So, who here thinks restricting roughly 45% of your exhaust’s flow would be detrimental to power? Care to wager? And the answer is….

Unmolested pipe (around 6k miles): 151.6 hp, 79.2 lb.-ft.
Molested pipe (around 10k miles): 151.5 hp, 79.4 lb.-ft.

And there she am. I lost an entire 0.1 horsepower and gained a juicy 0.2 morsel of torque! An infinitesimal 1/5th lb-ft… more like a hoof than an entire horse, but as in much of life, some mo’ is mo’ bettah. However, there is a slight loss of power and torque around the 5500-6500-rpm range before gaining a hair of pull around 9200 rpm. But before you all go running to the tool box for a mallet, keep in mind this motor might have loosened up over time, accounting for the slight increase in power. Not to mention any number of operator variances inherent in even back-to-back dyno pulls. Also, I can think of far more effective and attractive ways to modify a bike. Perhaps that’ll be another article.

So until then, based on these findings, I can’t dissuade you from plowing through as many rock gardens as you can stomach.

Hell, maybe I’ll start a service that specializes in “alternative organic tuning.” For $159.95 I’ll show up unannounced at your favorite riding spot with a bag of rocks! Towing not included (contact Troy Siahaan).

Dyno runs courtesy of Motorsport Exotica, Los Angeles.
I’ve heard of guys running “slicks” on the street, but this is ridiculous.
Thai Long Ly
Thai Long Ly

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3 of 11 comments
  • Oldironnow Oldironnow on Jul 04, 2017

    Nice addition to the science of motorcycling.
    What does it mean, then?
    Back pressure wasn't increased because the cross section area at the dent is still greater then the smallest restriction in the system?

    • TL2Bass TL2Bass on Sep 16, 2017

      Honestly, I don't really know. That is waaaaay beyond my pay grade, trying to decipher the what's and why's of exhaust tuning. I did watch a fairly exhaustive (snare, splash!) video recently done by some car guys in which they literally beat the crap out of a set of headers on a car's motor. They dyno'd it after every bashing and they concluded basically what I have here. That it doesn't matter. There were some instances where the numbers changed in both directions (positive and negative, compared to stock) but essentially the take away was that performance remained the same and the numbers were infinitesimal.

  • Leroy Tyrebyter Leroy Tyrebyter on Jan 15, 2018

    These guys are hilarious sometimes but watch this video to see they found the same conclusions.