Since there isn't general forum, I'll go ahead and post in here. Moved on from my 14R to a Bandit 1250 and have an exhaust question. Thought it was worth asking with all the dyno experience you guys have.

From what I understand, the longer the exhaust tract the more low end power you can extract from the engine. (To a certain extent)

The stock pipe on my Bandit 1250 is a whopping 21" if I were to mount a slip on that measures "17 or "18, is that enough to make a difference in low RPM power all things being equal with fueling? I do plan on having a custom tune done once I decide on an exhaust.

It's also worth noting there's a cat in the header, not the pipe.

Good questions, but you'd need to hear it from an engineer in the field. Things like air speed, supersonic theory, shit like that. But for entertainment purposes, you're on your own in the measurements and lengths of choice. Back in the day, pre-nascar guys would run zero headers and have 10 more HP on the field. I think they were the Flock Bros. running that trick. Dragsters run short headers and you don't hear about warped valves happening each round.

Theory wise it's real simple. The less bends the less friction. The more the air is forced to slow down(cat), the slower the event occurs rather than more rapidly. Add more to the theory you can't remove; is the even occurs in the reverse. So for every action, there is your equal and opposite reaction = Grunt.

From point A [being the open hole at the exhaust tip], to B, the valve opening/closing of that valve face circumference. From that length to length, you have to measure volume and how much is pumped thru that header length... if that is a formula that applies. Who knows where the best grunt is and what length for the headers alone. Now add the muffler and how fast will the 'speed event' of the air have to travel out of that maze.

And remember, that pipe has no air speed movement when the other 3 cycles are in play. Add the theory, 'that which stays in motion' where the valve is now closed kind of theory... we begin to add more cylinders sharing/exploiting those pulses. And what 'speed event' made the crank speed move faster with all those bends, cats, inner tubes in the muffler to snake around and slow down that crank speed = Grunt.

Bottom line, it's black magic and the old cut&try method gets you there.

I think what Auron is referring to is how back pressure in the exhaust system can actually improve low end torque/power. It is my understanding that this is typically the case when it comes to high lift & especially long duration cams where with enough overlap between the exhaust and intake strokes that what can happen is as fresh air & fuel is drawn into the cylinder some of this air & fuel also escapes out the exhaust valves resulting in overall less power.

A solution would be to use lower lift and/or shorter duration cams and try to eliminate any valve overlap (the time both the intake and exhaust valves are open at the same time) however this limits top end power. Take the Concourse for example, same engine as the zx14 but its "tuned" for more low/mid range power, most likely this is done by tuning the cams, exhaust system and intake tracts.

The other solution would be to use back pressure that is always present in the exhaust system to help keep some of this fresh air/fuel in the cylinder at lower RPM. As Hub mentioned, there's a lot of science involved with this and its basically "black magic" to anyone who doesn't understand the science. But in general what I have read is basically whenever you have a wave (exhaust pulse) which hits an area of different pressure (outside air where the exhaust system ends) not only is this gas sent out the pipe but it creates a reflected wave that goes back to the engine.

Its this wave that causes the back pressure, engineers can and do calculate out the resonance frequency for this pulse and can tune it to improve performance in the lower RPM range for a very narrow range of RPMs (think 2,500-2,750RPM). Its my understanding that there is a similar phenomenon on the intake tract which is why the velocity stacks (the tubes from the airbox to the intake valves) are a specific shape and length. This could often be seen on a dyno graph as a sudden spike in power in the lower RPM range.

So... what does this really all mean at the end of the day? Like Hub already said, guess and check. Put your new exhaust system on, run it on a dyno (if you can), look at the torque curve and adjust the fuel as well as the timing maps (if possible) to get the best power possible.

As a side note, many cars now have variable cam/valves to remediate this issue, short duration/lift for low RPM power, long duration/lift for high end performance. Also there was a motorcycle (Honda? Yamaha?) that had variable length intake tracts in order to squeeze the most performance out of the engine.

I would guess the bike feels very under powered stepping down from
the 14 to a 560lb bike with under 100hp

Changing slip on length by 3 or 4 inches isn't going to change anything.

Your best bet to gain a couple of torques and HP is a good tuner
working with the timing and fueling.

I rather be tuning the WATTS on an E-Bike and fuc the gas engine with the pulsified physics. So ends the carb to fuel injection, and now ends the normally aspirated, I ride a Harl-E. Until that road is met:

Let's exploit what little we can do. Here's some theory, then apply it in the real world; when one chases 'Grunt.'
Intake valve ~ Say we look in the shop manual, blueprint the numbers of the cam's timing in the book, that, or see where they are to begin with. Say the numbers match. What's left is the valve lash. So think intake and how to speed up the intake's event. I grasp HP by a new "Speed Event," not More Air. I'll get back to the intake, but first...

... say I remove the stock muffler, where the air snakes thru internal piping you can't see inside the muffler. Either way, just think how the air has to gate from one pipe to the next before it exits. Now, change the muffler to a straight exhaust. This now is a straight out shot, thus eliminating the snake effect. You uncorked a speed event that escaped sooner out the end of the pipe. Say you run without the air cleaner. The air flow becomes restrictive with, but unrestrictive without. There is a somewhat sooner event of flow because of the restriction removed. That was 2 simple moves that unclogged a gated/restricted/snaked thru/sped thru type of timed event made = More HP.

More HP means more torque converted to 'grunt.' So intake wise, we narrow the gap to the blueprint's minimum numbered gap. Why? We caused a sooner 'speed event' to occur by way of opening the intake sooner so the event happens faster. On the exhaust side you blueprint the gaps seat of the pants wise. In other words, say you let the exhaust's power stroke bleed out to its diminishing return and use a loose gap so it stays closed more. Or, tighten the gap to bp, and now the grunt stops short and the intake is ready for the Grunt Move. Make the grunt happen sooner, i.e., kill its diminishing return sooner than let it bleed out.

Make sense?

Lets set the length question aside, I'm just trying try to figure out the best way to get it to run well.

Update from my tuner:

He doesn't think the time it will take to extract an accurate air/fuel from the stock exhaust will be worth the effort. I'm guessing he thinks I won't be happy and complain. So basically I have to put a pipe on it to have it flashed and custom tuned, and he's the only guy in this state (beside a harley dealer) that can do it.

He claims I won't lose any bottom end power but I'm not sure I buy it, he's used to tuning race bikes, not bikes like these that blow their entire torque load at 3750 rpm.

All I want is smooth power off the bottom, I could be wrong but it feels lean and unhappy.

Reversion is what kills high rpm power output with straight pipe, unrestricted exhaust systems.

Auron,

Why not simply invest in a power commander and an auto tune module for your bike? I'm pretty sure my zx14 with the stock exhaust could have benefited from just an adjustment of the Air to Fuel Ratio (AFR) as I remember there was a lull in power in the low RPM band. Even with a map and aftermarket exhaust it was there then with the auto tune I was able to fix it.

A side note worth mentioning, with the auto tune you set the target AFR (say 14.7) and limits as to how much the module can change the fuel trims (say up to adding 10% more fuel or removing 10% less fuel) and the module makes adjustments on the fly until it reaches the target AFR. What I noticed is that while cruising in a high gear at low speed (around town 50kph/35mph) I was sitting at a nice 14.7 AFR but ANY kind of acceleration through the same RPM band resulted in terrible performance. I noticed that this RPM range had trims that were always WAY off from the adjacent cells (ie, 2,000, 2,250, 2,500, 2,750 RPM was like +5, -20, -18, +7 fuel trim) so when I adjusted those cells to match the adjacent cells (ie +5, +5, +6, +7) suddenly I had great response, AFR was still good, etc.

Basically what I'm saying is that even though the auto tune module is supposed to be set it and forget it, there is some checking that is needed and a bit of a learning curve. Also the POD-300 LCD module is super handy for tuning the power commander but its pretty expensive...

as I remember there was a lull in power in the low RPM band. Even with a map and aftermarket exhaust it was there then with the auto tune I was able to fix it.

Same here. 3000 rpm or bit less. It suddenly come alive. One AutoTune adjustment made a vast improvement and fine tuning I did later made it almost imperceptible. I'm sure it was the stock fueling for flies causing the trouble.

If you use AutoTune, hold the throttle at one position and let it rev as high as you want. let go of the throttle. Do not close it slow. Let it snap closed. Pull the clutch in first if you want. Do not let Autotune make any trims except for the TP you are tuning. Read my tutorial and you will get a better idea.

To hold one TP, you need a POD-300 or use a laptop velcroed to the tank cover so you have a TP gauge. Othrewise, you can guess and mark your throttle grip with tape. It's very important you hold the throttle at one position though.
It's all in my tutorial.

On my old 1100 Kat I installed some 92 or 93 GSXR-750 cams and the sling shot head. Sling shot heads have a dot cast on the side of them. When coupled with the stock flat slide carbs this made a tremendous amount of low end power. I can't remember the exact exhaust but it wasn't some exotic job that cost a lot of money it might have been a Kerker LOL. Since the Bandit motor is basically the same I bet you can tune that beast to the limit with some cheap ebay oil cooled engine parts.