power loss or gain with intake and exhaust changes??

[OLY-XSer]

whats the deal with power loss, or gain with changing the intake and exhaust? I have seen people post both sides of the argument. If you go to pods, glass packs, and go up in jets to compensate, what does it really do on a otherwise stock xs11? In my case a 80sg.

[natemoen]

The oem exhaust was very well engineered and pretty much anything you change airbox or exhaust will make you lose something somewhere in the rpm range.

[MaximPhil]

Hi Oly,
I see you have pods and free flow exhaust. Your set up will have scraficed some low end torque for more horsepower at the top end.Just how muh you would need a Dyno run to determine. 4 into 1 would get you more horsepower still. Original factory set up give excellent "rideability" modifications can gain something at the expense of something else.
Some times mods go backwards as trial and error is not engineering
Phil

[T99Ford]

Kind of along the lines Nate said. But more specifically, it aftermarket pods and exhaust tend to move the power bands to different RPM ranges, often with negative effects in others. For instance, the stock exhaust was designed for maxiumum "rideability", or, "anyone can push the button and go". Aftermarket exhausts move the power bands to the specified and desired riding range of the tuner. IE, a 4-1 tends to move the power band to the upper end of the RPM range.
A proper 4-1 can increase power at the top end over stock by increasing the amount of intake air in the cylinder. This is accomplished by a tuned length that is such that at upper RPMs, the exhaust harmonics set up in the pipe are traveling away from the cylinder drawing in air, and serve to increase the vacuum in the cylinder, thus drawing in more air during a given stroke. The trade off, though is that at just below optimum or tuned RPM, the exhaust harmonics are actually traveling the OTHER direction, or toward the air cleaner. This can pass exhaust gas through the carb (fueling it) and then the engine pulls it back in, fueling it again. This twice fueled midrange mixture is often seen as a 3K stumble on an XS, and will appear as an extremely rich A/F mixture on a dyno chart. Midrange tuning can usually only account for some of this. Of course, this all applies only if the pipe builder actually tuned the lengths of the exhaust system components for a specific RPM range.
A 4-2-1 exhaust reduces the negative impact of the 4-1 at midrange, and while it does not produce optiumum power, the exhaust is not allowed back to the intake cylinder, and so the huge rich midrange does not happen. Still not a power gain, but at least not the stumble so often noted. In both cases, though, the upper end should be improved over the stock system, albeit at the cost of the midrange, and even some low range. Thus the 4-1 and 4-2-1 systems are generally better for aggressive street or race applications, since the engine must be at the upper end of the RPM range to benefit them.
The 2-1 systems will often be of comparable performance, since the stock system is just that. Very rideable, reasonably flat A/F curve, and hence no perceived flat spots. These will give up power at the top end, though, for lack of exhaust scavenging benefits. Thus they are preferred for most touring/ commuting applications.
In short, to maximize your exhaust system choice, you need to be making that choice based upon a specific purpose, and choose accordingly. There is not right or wrong, just will/ won't fit the application.

The argument over pods vs. airbox is very long and even ongoing. I think most here would agree that there is a negilible performance increase or decrease with proper tuned pods. Many have trouble tuning the bikes to run with pods for some reason. Others have very good luck.

My cafe bike is equipped with a 4-1 Yoshimura exhaust, which did in fact improve top end power, and I have nearly tuned the midrange rich portion to as near a flat line as I can. I have had no trouble with the pods, although I must admit that I installed pods becaust I don't like the look of the huge airbox, and mine was junk anyway. Tuning them was a snap.

If you are looking for average and very reliable, easy ridability, stick with the stock system. If you are looking for a specific performance end, you will have to mod accordingly. Sorry for the long explanation, but believe me, it is much more in depth than this!

[natemoen]

Very good explanation T99. I am kinda interested to see how things go as I start reassembled of my xs here since I now have a 4-1 exhaust for it. We will see how it tunes out.

[OLY-XSer]

wow. thanks for the info. you guys know alot more about this then me. this is my first xs11. when i got the bike it ran like ****. and the stock mufflers was snaped off right behind where each header comes together on each head. I think the bike fell over in a garage or something. I don't think it was downed while moving because besides the exhaust there wasn't any more damage. Thats why i put on the glasspacks, cheap easy fix. I cleaned the carbs a few times to get it running right, as far as I could tell. It was still running lean though. So I bought the pods (and fuel filters) and started changing the jets since it was running lean anyway (probably because of the exhaust). I have not done a sync yet or tuned it, but I will as soon as I can. But after going up with the jets one step at a time untill I got to 45 pilots and 130 mains on 2 & 3, and 125 on 1 & 4, no more running lean. the spark plugs are burning all the same now. Even though I haven't sync or tunned yet its running great. I dont have any flat spots that I can tell. It pulls hard especially over 3k. Because I made all these changes at once I don't have anything to compare it to. I may have just got lucky with my uneducated changes.

[crazy steve]

One thing should be pointed out is that not all aftermarket exhausts are created equal, and if they've been modified since they were new, who knows what you may have.

The 'premium' exhausts like the Yoshimura, Kerker, and Supertrapp were made to work as a complete 'system' with the supplied muffler(s), and if something else has been substituted since, you may or may not get the intended benefit. That's one reason these systems if complete and original are still in demand and pricey when you find them. If you get one that's been altered, you may get the tuning problems without the performance. The only new exhaust systems made for these bikes is by MAC, and their's has a rather spotty reputation (some say 'MAC' stands for 'make another choice' ).

As for the broken exhaust, that's not unusual. I've seen several 'Special' exhaust busted right where the muffler joins the headpipe. If the mufflers are still good other than that and the crossover is intact, you might give some thought to having them re-welded...