Twitch78

Hi all,

As I have put a 2008 R1 rear on the VTR, I am looking at putting the undertail pipes to suit. My question is, Can I run the exhaust from the front into the rear pipe near the header and then back into 2 so it runs straight to the pipes? instead of the 2 pipes into 1 and back into 2 that is under the bike presently?

Will this create an issue for exhaust flow, bike running etc. and the rear exhaust header if the pipe is run straight into a muffler effectively or does it need the bends and length of the exhaust to give proper back pressure?

Currently the rear exhaust runs down to the bottom of the bike and is run into the front exhaust pipe, then exits and forms its own again and curves back up and around to the left pipe.

I have attached a picture of the under pipe setup presently.

Good sound knowledge would help alot before I shell out more bucks!

helimd

I recently finished fitting a 2005 1000rr tail to my '05 VTR. This was a surprisingly simple mod that turned into a VTROM candidate. I'll post pics later. I was able to run a single pipe from the front cylinder up through the swingarm, then fabricated a y to bring the rear exhaust into the same pipe, then up to the muffler. Worked like a champ. Later

Twitch78

Awesome!!!

if I did the same but ran them back to 2 would the pressure be ok being 1 is long and 1 is short? Or could I run the front pipe straight up the swing arm to the muffler and the rear direct to the second pipe?

helimd

I don't think the length is the issue, it's the ignition timing. I doubt if my 'hawk will ever see a dyno to compare numbers, but from my conversations with aircraft exhaust gurus, the opposing timing of the cylinders will have each cylinder being scavenged by the opposites exhaust stroke. Certainly the joy of a v-twin. One aside to this. I am nearing 60 and not nearly as aggressive as I was 40 years ago, but still feel pretty good with the bike. Seat of the pants tells me there is no difference in power, but the sound is worth the effort spent on the mod. And, as you will see when I post the pictures, this is what Honda should have done with the exhaust from the start. And I love the look due to the fact that all the bodywork is all titanium in color, with no paint involved.

Twitch78

Thanks mate,

What do you mean when you say "the opposing timing of the cylinders will have each cylinder being scavenged by the opposites exhaust stroke."?

Is that a concern as far as putting something out of timing? Please forgive my ignorance, the more I ask the more I learn!

Cheers for all of your advice.

Paul

helimd

Back in the early days of opposed cylinders on aircraft engines, each cylinder had its own exhaust pipe, with no common muffler. Times changed and engines got bigger and louder. Mufflers were introduced and, with that, problems with noise. Early mufflers couldn't withstand two cylinders firing at the same time. So, firing orders changed, engines smoothed out, and maintenance issues disappeared.

As exhaust gas from one cylinder leaves the engine, it creates something of a vacuum which helps scavenge the gases from the next cylinder in the firing order, and so on. This all comes from a former Lycoming engineer.

Twitch78

awesome thanks,

So 2 independant exhaust pipes that run 2 totally different lengths of pipe to muffler should be totally fine? My theory was easier the breathing the better without going too large on the pipe size itself.

gotta_gofast

It really depends on how **** you want to get with exhaust and performance. For the undertail, I thought about sweeping the rear header up and bring the front header through the "now vacant" swingarm opening where the rear cyl used to plumb down. I would run a parallel X-pipe right into two mufflers under the tail. Once I choose a camshaft profile, I will have the info needed to figure out what rpm the pressure waves will meet. I won't be real scientific, but as long as both pressure waves don't meet at the X-pipe at the same time (between 6500-9500rpm) I will be happy. If both pressure waves do meet at the X-pipe during said rpm range, it would be in my best interest to move the x-pipe toward or away from the mufflers. While the length will differ from front to rear cylinder, the factory header does anyway. And there will be about 360+ degrees less bend in said setup than the factory. So, there are benefits and there are drawbacks to said setup. But I get bored in the winter months so I'm willing to try. lol

But the engine is apart and I'm currently porting the cylinder heads.... so custom exhaust work is months out.

Twitch78

Ok thanks mate, so in simple terms for me, if I ran the rear header straight into the left rear muffler straight under tail and the front header went straight under the bike, up the swing arm and up into the right muffler under the tail would that be ok? or do the 2 headers have to join and then separate for some reason?

Cheers
Paul

gotta_gofast

If math works out, I'd use the X-pipe to scavenge the pipes.... an easy way to gain velocity and create better torque down low at low revs. As said earlier, when one exhaust pulse passes the x-pipe, behind it is a low pressure signal. Since the exhaust pulse has mass (and is moving, therefore momentum) it passes through the x-pipe in one direction. The low pressure created behind it will pull in all directions (including the original header pipe it passed through, the other header pipe, and even pull against the high pressure wave in front of it). At higher rpm there is enough exhaust speed (velocity) to scavenge on its own. But in order to scavenge the exhaust at a low speed, we either use a cross over pipe to take advantage of the low pressure behind the exhaust pulse or we size each exhaust pipe so that the velocity of exhaust gases is great enough to scavenge. The problem with the latter, is that the pipe would hinder high rpm flow due to its small size.

Having a better background with 2 stroke powersports than 4 strokes... I've been tempted to tune a 4 stroke exhaust using a small megaphone and reverse cone (though, obviously it would not need to be as large as a 2 stroke expansion chamber lol and my goal is not cramming gases back into a cylinder but doing the exact opposite by drawing exhaust out).

However, I doubt there will be much of a difference in "felt" power on a streed driven motorcycle. Had you taken this machine to a track and ran consistant times pushing the limits, you may feel a decrease in power (by winging an exhaust system with 2 separate odd length pipes). Try it! Worst case is you keep your stock components and save as a plan B.

Tweety

Twitch78, you are missing the point of what people are telling you...

You can run two completely independant pipes of whatever lenght you want... It will work just fine, but you loose 1-5 bhp or so doing it...

The reason the pipes join as stock and in whatever else configuration is that the pulse from one cylinder is followed by a equal pulse of vaccum... That means that if you have two cylinders working in sync, you get more bhp as they help each other breathe better... Exhaust scavenging... All that has been said before, I just repeated it...

The "new" tidbit I'll add is simple... Exhaust scavenging is something that good engineers spend large amounts of time and a bunch of equations to calculate out... Ie, it's not as simple as making the pipes equal lenght, or just joining them at whatever point... That can just as easily decrease the output about as much as it can help...

So in simple terms, yes, two separate pipes work just fine, and then you will loose a bit, but not much peak power... But it will work, where as an experiment with crossovers and such can easily be in the category of "not working" if you haven't got a clue to what you are doing...

helimd

Tweety, you hit a home run with your response. This exhaust discussion has been going on for, literally, decades. Engineers have, as you pointed out, spent time and great amounts of money coming up with the perfect solution for horsepower. I am posting pictures of my hawk and hope everybody likes it as much as I do. At some point in time I plan on fabricating a more free-flowing muffler but, for now, I am enjoying the ride. Later