Triumph Rat Motorcycle Forums banner
  • Hey everyone! Enter your ride HERE to be a part of this month's Bike of the Month Challenge!

1 - 14 of 14 Posts

·
Registered
Joined
·
245 Posts
Discussion Starter · #1 ·
Ive been doing a fair bit of reading lately on exhaust theory, especially from here:
http://www.rbracing-rsr.com/exhausttech.htm
I understand that the motor is an airpump & that intake & scavenging can be aided by tuning & matching the exhaust to your cam.
What I am still trying to wrap my head around is that if back pressure should be reduced to the minimum, why dont more guys run drags? What makes a baffled Your Brand Here pipe run better than a straight through pipe?
Is it because the straight pipes arent designed properly to utilize the pressure waves?
If brand X pipes are better than tors due to less restriction, then why arent drags better than brand X, due to NO restriction?
S.
 

·
Registered
Joined
·
8,877 Posts
The 4 valve head on these motors makes them very touchy as far as what pipes work on them,You can about forget anything you read based on other motors on these bikes.These motors dont seam to like stright pipes but they seam to like all most stright pipes and a little change can mean alot.Maybe me 10 hp or more.You can allso move the power around in the rpm range with little changes in the pipes.
 

·
Registered
Joined
·
245 Posts
Discussion Starter · #3 ·
These motors dont seem to like straight pipes but they seem to like all most straight pipes and a little change can mean alot...
That gets down to it right there. Do you think they don't like the straights because they are the improper length?
For almost straights, ie mildly baffled, the length of the pipe doesnt seem to matter at all! Short or long, just get your jetting right & go.
Im going to build some drag pipes & am trying now to figure out the mathematically correct dimensions to make them run properly. I might start a little long & trim them down to fine tune.
If they don't work well Ill throw in some baffles & tune from there. I guess the length wont matter then
S.
 

·
Registered
Joined
·
245 Posts
Discussion Starter · #5 ·
I realize that both are critical, but how does a guy calculate where to start?
Obviously there will be some trial & error, with small changes then testing, but how are the optimum specs calculated? :confused:
Ive been searching high & low for some formulas but am not having much luck.
On my other post about drag pipes somebody said that the optimal length for a straight pipe was 68", but I want to see where that comes from!
S.
 

·
Registered
Joined
·
1,629 Posts
Sharp...exhaust theory is a science unto itself and books can be written on the subject. Exhausts are a balance between low to mid range torque, maximum flow for horsepower and acceptable sound on the street. A large tradeoff. The reason why unrestricted exhausts aside from sound are not ideal on the street is about torque. Think about pulses or waves of air as finite entities with size, mass and momentum. If an exhaust is too free flowing or opened up, each pulse of air can be slowed because each pulse has lower velocity due to increased cross section and hence kill momentum or inertia of air flowing through the motor. This robs torque from the motor. The same fluid dynamic btw also applies to airbox design.
Engineers anguish over the balance between small enough pipe cross section for adequate exhaust velocities preserving torque pitted against maximum flow for horsepower and sound...accoustics at all RPMs including under load without objectionable harmonics.
Very light skim and others will supplement....
George
 

·
Registered
Joined
·
8,877 Posts
both phil and george are telling you right.The only real way to find out what works best on your motor is with much dyno testing.THAT CAN COST YOU MUCH TIME AND MONEY .You are better off finding what works for others and useing that.Some pipes such as the 2into1 D&D I run have proven to make power on about any motor you put them on.That came from much testing .even motor to motor one might like a pipe one way and the other another.
 

·
Premium Member
Joined
·
8,211 Posts
You also have to decide where you want the power. Low back pressure devistates your low and mid range power and increases your wot power. This is why exaust valves are now in place on sport bikes. At idle the ecu shuts that valve almost closed and it reallydoesnt open right up until high rpm.

Next time you see a gsxr kneel down on the throttle side where the belly pan ends and you will see two cables attached to a pulley which goes into the collector. The cables go up to a little frame mounted motor that the ecu controls.
 

·
Registered
Joined
·
245 Posts
Discussion Starter · #11 ·
Rodburner - Thanks for that link! Posts 47 & 48 are right on! I cant believe I missed that thread... I swear for every post I write Ill search 10 more.
Mike, Im definitely not looking to throw money at dyno time for a modest gain when I can buy a set of pipes with a known value, but with some forethought & planning I think it'll make a good learning project. Hell im already learning a lot!
I find it fascinating that you can tune your power to a specific rpm by varying pipe length.
S.
 

·
Registered
Joined
·
258 Posts
..
I find it fascinating that you can tune your power to a specific rpm by varying pipe length.
S.
Oh man, that's just the start of it.

As you're quickly becoming aware, the difficulty with a project like this is that there is no good place to start. When approaching it from scratch, there are a lot of interdependent variables that have to be solved for, so you have to just make some numbers up in order to get the others. If you don't have experience building pipes, you have no basis from which numbers to make up and which variables to fill with them.

My recommendation would be to keep this all theoretical for the time being. Sure, you can build a pipe, put it on the bike, and see how it goes, but until you're willing to put the bike on a dyno (repeatedly, making modifications to the pipe between runs), you're going to have a hard time gaining any real understanding of how pipe design affects actual performance. Also, the first try probably won't get it right, and will likely produce some really rough spots in the torque curve that could be tuned-out very easily on the second go.

I say that based on my experience with chassis design, which I've been casually studying for about seven years. Exhaust seems very similar, in that it involves balancing a lot of interdependent variables, and while the basic theory is extremely simple and accessible, turning it into a functional machine is anything but. I'm finally gearing up to build my first actual frame sometime in the coming year, after working out countless others (probably well over a hundred, if I still had them all to count) on paper. Had I built my first attempt, or my twentieth, it probably would have been rideable, but it certainly wouldn't have been any good.

If you just want to build a pipe, read a bit, design something, build it, put it on the bike, and enjoy. It will certainly not be the worst pipe ever put on a motorcycle engine, and if you do your homework, it might even make decent power. If you really want to understand how to build a pipe though, and build something that will make a lot of power, you're in for a much longer process.
 
1 - 14 of 14 Posts
Top