Hello, new to the forum here, I guess I could be called a "going to be" instead of a wannabe. Hope to get a 883 iron soon, if my dealer can ever get one in.

My question is regarding exhaust pipes, I mostly see the rear cylinder pipe appears much shorter than the front cylinder's pipe.
Is there a reason for this? or is it just a matter of fit/llooks. and If so, does it hurt performance per cylinder to have pipe lengths different front/back?

Great forum lot's of cool folks, tips, info, recommendations here, thanks to all for your posts!

Kind regards,

 

Actually, both pipes are "short". For optimum scavenging they should be something like about 68 inches long. The difference in length between the front and rear pipes on a Harley does not significantly affect "performance". However, the diameter and flow characteristics of various pipes drastically affect performance. To make it even more interesting, the same set of pipes on "identical" bikes can affect the two bikes differently.

If you go back in history you'll find a time when the board track racers used stub exhausts

The marketing of exhaust systems is an art. The design of exhaust systems is a science. Art (or image) usually wins over function and most guys hang inefficient big bore, low restriction pipes on their Harleys and then wonder why they get fouled plugs, really lousy mileage around town, gas contamination in the oil and worn out piston rings.

As food for thought: If a 1 and 3/4" exhaust pipe is about the right size for a 1340 engine (which is debatable), how can it also be about the right size for a 1200 or 883 engine?

 

Bentwrench
So, is it best to get pipes where both are as close to the same length as possible? I have seen some that appear to be that way. And if so, what pipes are a good recommendation for a 883 Iron that will provide the best performance / sound based on what you posted above?
Also, any other recommendations are appreciated....I have searched other posts regarding exhaust but haven't seen anything on both pipes being the same length

kind regards

 

i think i read on one of the threads that both pipes actually have to be the same length in order to maintain the same amount of backpressure in both cylinder heads

 

Back in the Day Drag Specialties sold M40 40 inch pipes for sportsters 2" dia if I remember right....

 

Go to Big City Thunder.com and click on exhaust dynamics, then scroll to the bottom of that and click on "Understanding exhaust: the how and why".

This article does the best job of explaining THE BASICS of what is really going on inside an exhaust pipe without getting into calculus. It's a violent place in there with tremendously strong shock waves moving both out and back in at really high velocities. Most people don't undertand where all that sound comes from. Making loud sounds takes power.

You might want to print out that article and then read it again and again, paying close attention to the fact that exhaust back pressure, in any and all circumstances, can only cost you power.

Back in the "old days" guys would rig a 3/4" washer on a screw inside their drag pipes so they could "adjust torque". All they knew was that with the washer turned sideways it made the bike run great around town and turning it edgewise made for good top end. It isn't back pressure that allowed the better low end torque, but they didn't know that.

Exhausts on Normally Aspirated engines operated over wide rpm range don't "make" power. A good (less harmful) exhaust allows the engine to make power. Modern exhausts for street bikes always have to compromise appearance, sound level, sound characteristics, general performance and "curb appeal" or desirability. Take a look at big team, big money flat track pipes. Pure pipe, and it only works at race speeds.

Read that article over and over until you really understand that you want an aftermarket exhaust that does the least harm and gives you the "sound" and appearance you want. Me? I want all the area under the torque curve I can get. Horsepower sells cars. Torque wins races. Torque makes a bike fun in traffic when you can crank on that throttle at any engine speed above idle and have it respond with a rip-roaring snort rather than carb farts, coughing and backfiring.

And when it seems you've got it figured, study some more until you can explain to someone how it is that on a carbureted engine, a certain exhaust pipe can make the rear cylinder run so rich that it will foul the spark plug at mid-range speed yet run clean, hard and strong in both cylinders at anything above 3,000 rpm, about 60 mph on a Sportster. No, there's nothing wrong with the carb. This is an every-day fact of operation and is the reason guys buy a half dozen carb jets chasing the problem but looking in the wrong place.

Hint: What is an AR cone, what does it REALLY do, and HOW does it do it?

 

I've recently installed the quiet baffles on my straighshot HS slipons and noticed my gas mileage has taken a dump. With the standard baffles in, I averaged 50 miles to the gallon. With the quiet baffles in, I am averaging about 39 miles to the gallon. I use the Fuel Pak. It has me "baffled" at the moment.

 

Bentwrench, excellent read, my assumption was correct that equal length pipes were the best bet, as they are with individual cylinder header pipe lengths before the collector on car engines.

Now I will start looking around at exhaust systems that have equal length pipes, or as close to equal as possible. any suggestions on brand/type right off the top of your head?
Thanks for the info!

kind regards

 

There are really too many variables to narrow it down to a brand or part number. Your expectations should guide your search. The reason absolute equal length isn't critical is the siamesed intake. Now if you were to run two separate individual carburetors you'd want equality for ease of tuning. The siamesed intake behaves differently at different RPMs due to resonance and inertia and mass air flow. What this means is that what is an "ideal" exhaust changes with load and engine speed. Ferrari is experimenting with variable exhaust for just this reason.

My recommendation would be to keep the exhaust velocity high by not going to a fat pipe. Don't ever surrender torque for horsepower for a street machine. Chasing horsepower gives you tunnel-vision and you end up concentrating on what's happening at 5,500 to 6,500 rpm or so. I almost never ride there.

Be careful of dyno charts unless you can get details of compression, cam timing, ignition module, valve sizes and so on. And what works with one specific set-up may not work with another slightly different set-up. If you can see dyno charts for a totally stock arrangement, then that's telling a story. Otherwise what you're seeing is "this is what works for us, on Tuesday mornings when its raining after Joe Slick spent 125 hrs to blueprint a 'stock' engine for us and running 20wt oil because we only wanted the big numbers". That's not the real world.

Read more and learn more so you can make really informed decisions. When you like one pipe over another because of appearance, be honest with yourself about it. We all have preferences.

Try to find an indy mechanic who rides a Sportster. They really know their stuff and they KNOW what works from experience, and like all the rest of us, they like sharing thoughts about Sportsters.

 

Got a web site for ya. HARLEY-PERFORMANCE.COM They got a bunch of good reads in there. Check it out. HOLLYWOOD