n00b: what makes a slip-on a slip on?

[roy-nexus-6]

The time has come for me and my stock s4 cans to part ways. I am thinking maybe some shorty megaphones would be nice.

My question is this: how is slip-onishness measured?  I mean, obviously the stock pipe generates back pressure etc but how is that back pressure measured? What is the unit?

By way of example,we'll call this unit BPs.

1. Stock pipes = 20 BPs.
2. Shorty megaphones = 10 BPs
3. Termi slipons = 20 BPs.

Obviously, the shorty megaphones are not entirely compatible. At what point are cans completely incompatible with the bike?

I've hunted around the board, but never found the issue framed in terms that would allow me to call up a dealer, and say: "Hey, I like those XYZ cans on your website - what is their BP rating? ....22? Ok, they sound like they might just work!"

[Buckethead]

My question is this: how is slip-onishness measured?

The difference between a "slip-on" and a "full system" is just a matter of what comes in the box. A slip-on system will have mufflers that mount up to your stock headers. A "full system" will, as the name implies, include everything from new headers and mufflers, as well as everything in between. If the manufacturer makes them, you may get some "db killers" that mount in the end of your slip ons OR full system that can make it quieter. They will add back pressure, but that may or may not be a bad thing.

I mean, obviously the stock pipe generates back pressure etc but how is that back pressure measured? What is the unit?

Back pressure in any given system is a measure of how great a volume of air it can flow. If you really wanted to quantify it, it would probably come down to something like "PSI @ X RPM @ X distance from the exhaust valve."

The back pressure of the stock pipe is not "measured" in any meaningful way, as far as I know. What manufacturers use when selecting a stock muffler is "(DBM at X RPM)/Cost."

1. Stock pipes = 20 BPs.
2. Shorty megaphones = 10 BPs
3. Termi slipons = 20 BPs.

On my bike (2006 Sport 1000) the stock system was ridiculously quiet. To the point where I heard more chain noise at 70 mph (4th gear, ~4500 rpm) than I did exhaust. Now I have BoomTubes and it's obnoxiously loud at that same speed. But since I got the "mildly-loud" baffles in the pipes, I'd wager my exhaust actually has MORE back pressure than my stock system. For a more detailed example, PM ducvet and ask about Monstermash's shorty exhaust tips. He had internally-baffled exhaust tips mounted to the udder on his S2R and it was creating enough back pressure to actually cost top-end horsepower. Sounded amazing, tho. 

At what point are cans completely incompatible with the bike?

When you can't figure out a way to mount them.

Hope that helps! 

[roy-nexus-6]

Thanks for the reply, Buckethead. I'm very keen to get rid of the stock pipes, and this helps enormously.