Exhaust back pressure

steell wrote:

Number three, you will piss off some people, and one of them may be crazy enough to try and do something about it. I live in a rural area, and every time a Harley with straight pipes rattles my windows I want to grab my shotgun.

A little noise reduction would be nice, at least get some with baffles

You will also go deaf after a while. Take it from an old goat, it's no fun hearing the whine of a jet engine in your ears 24/7 and always saying:

"HUH?"

bountyhunter wrote:

That was what I was raised to believe until I did some research on it recently. If reducing back pressure was always better, straight pipes could always be made to run better. In reality, they often run a lot worse.

"Back pressure bad, Exhaust Tuning Good"

The mile high view that I learned was:

The intake and exhaust flow of a properly designed engine have to sing at the same key. Screwing with one will usually make it worse.

The key to getting more HP is squeezing more air/fuel into the cylinder when the valve opens, and exhausting it efficiently. To that end, the intake is tuned to play with the exhaust.

Can't argue with that

It may seem odd the intake valve opens before TDC and actually stays open after BDC (even as the piston is rising). The reason is, the exhaust flow creates a vacuum (scavenging effect) and the intake valve opening before TDC aids in getting the fuel/air mix going in. The inertai of the fuel/air mix flowing means you get some extra in leaving the valve opwn as the piston starts back up. Fuel/air will still flow in a bit after BDC.

Or that.

That is the main reason you want to get the flow velocity in the intake as fast as possible before other effects like turbulence start screwing up the flow. Velocity increases inertia as the square of the velocity (1/2 x m x Vsquared). There are side effects of increasing velocity, so you need to get the sweet spot.

The speed of sound is the limiting factor

Some of the oddities I learned searching the net:

A guy who set a record at Bonnevile used flow modeling to increase the flow rate through the intake and exhaust... and the equations said they needed a rather thin exhaust pipe. The car broke the record. A veteran looked at the skinny exhaust pipe and said something like: "It will go faster if you get a bigger pipe and reduce the backpressure." They tried it and it went slower.

Back pressure bad, flow modeling very good

s?

The guys who tune intake systems admit it's a lot of experience and black magic. In one case, the guy polishing an intake manifold intentionally leaves rough spots at locations to create turbulence to slow the flow where it has to turn to get a smoother overall flow result. You can bet there are no textbooks that tell how to do that.

It's not magic, it's just not well understood by most people. If you want to spend the time looking, there are research papers available that will explain it. NASA, back when it was NACA, did a whole lot of research on airflow, and most of the archives are available online.

To paraphrase:

A technology advanced enough is indistinguishable from magic, by the normal person.

The only thing I can tell from personal experience is this: I cut the mufflers off my pipes and put on different ones and totally screwed up my bike. I bought a designed header/muffler set and it went like a cruise missile.

Straight pipes of the proper diameter and length work well in a narrow rpm range on a motor that was designed to operate in that same range. They suck on a street bike

A note on the ear damage comment:

I am 57 years old, and my hearing is still much better than that of those in their twenties that spent time in cars with over powered stereos. I actually don't know any male between 20 and 30 years of age that can hear as well as I do right now. My hearing has degraded, and it was merely normal to begin with, it's sad the way kids have destroyed their hearing

And then you have the Harley riders with no helmets and straight pipes, Huh? seems to be the response to any question asked of them

I think this is a record length post for me

I have to agree that if you really want to do the math and understand the physics behind exhaust systems you can make your own exhaust systems that will flow better for your engine. I however only understand part of it, have taken minimal math and physics classes, and don't plan on starting an aftermarket exhaust business to pay for all the hours and money that it took to figure it out. So instead I am happy to pay Kerker who did figure it out, for a specially tuned muffler. Those guys spent the time to figure it out and now I'm going to pay them for that time and a product that gives me more power then stock across the RPM range. I have however notice something about people and exhaust systems. If you take the baffle out of someones bike and then have them ride it they will probably tell you it has more power. However most of the time I can't notice a difference other then how louad it is. It seems to me that some people may think that their bike is making more power because its louder or because it just makes sense to them. Has anyone else noticed this our am I just being a cynic.

Becker,
You are not just being a cynic. There really are folks who think decibels=hp. I guess it's because they hear a powerful racing engine that is loud so they believe if they can make their street engine louder it will have more power. If that was true, the old BSA I owned as a kid would beat today's supersport bikes.

Taking the baffle out of a proper header/exhaust (kerker, v&h) simply changes where the bike makes more power. With the baffle it will have more low-mid range and falter up top. On my 650 with the cams it really likes to make more power above 8k, so that in combination with removing the baffle yes, it really screamed from 8k all the way to 10,500 where i normally shift at WOT. And anyone who spends enough time tinkering/racing cars knows that just running open headers is actually slower than running a proper muffler/exhaust set up.