Custom exhaust systems

[snyderman]

I had a thread about baffles specifically, and that seems to have died, so I would like to open it up to any and all custom exhaust applications. My exhaust is rotted, and I want to do a 4 into 1, but I also want a custom look, and if I can save a few bucks in the process, so much the better! I also want to keep the peace with my neighbors! I am looking at buying a Mac header, I can save about $80 by not getting the pipe and can. That would buy a lot of pipe and baffles! I have been doing some research on engine exhaust, and there seems to be a debate about the need for back pressure. Some people suggest that the best systems have high velocity (small diameter pipe), and low backpressure. I found this link interesting, even though it is for cars.
http://www.mikekemper.com/elantra/diyexhaust/index.html
To be honest, I am more interested in the look and sound than all out performance, but I would like to keep my low end torque. I wonder if one could scale down his recommended pipe diameters for a gs engine. I know some of you have cut pipes and installed baffles. I would love to hear from you! Specifically the pipe diameter and baffle type and length, how it sounded and how it affected the carb jetting.

Thanks!

 

[Pendulum]

He's recommending 2" pipes for 1500-2000cc engines, so for a 750-1000cc, 1" would work according to him. However, I work in the sport compact performance industry and most exhaust systems for small 4 cylinder engines are 2.5" ID for normally aspirated, or 3" for turbocharged cars. I'm talking about Honda Civics making over 600hp at the wheels... So I'm going to trust my experience over that website. Half of 2.5" is 1.25, which seems reasonable. You could run 1.5" pipes, and it would decrease EGTs. I'm no expert, but this is based on about 5 yrs of experience tuning Japanese 4 cylinder engines.

For my custom exhaust, I'll be using 1.5" pipes into EMGO Shorty mufflers. I have a GS550.

 

[cowboyup3371]

He's recommending 2" pipes for 1500-2000cc engines, so for a 750-1000cc, 1" would work according to him. However, I work in the sport compact performance industry and most exahust systems for small 4 cylinder engines are 2.5" ID for normally aspirated, or 3" for turbocharged cars. I'm talking about Honda Civics making over 600hp at the wheels... So I'm going to trust my experience over that website. Half of 2.5" is 1.75, which seems reasonable. You could run 1.5" pipes, but it would increase EGTs. I'm no expert, but this is based on about 5 yrs of experience tuning Japanese 4 cylinder engines.

For my custom exhaust, I'll be using 1.5" pipes into EMGO Shorty mufflers. I have a GS550.

Half of 2.5 is actually 1.25

 

[Pendulum]

Half of 2.5 is actually 1.25

d'oh. I should probably type a bit slower and actually think about my math next time.

 

[snyderman]

I did a quick check, and if I did the math right, In terms of area, half of 2.5 is 1.76 and change. Shouldn't we be talking in terms of the area of the pipe?

 

[Pendulum]

I did a quick check, and if I did the math right, In terms of area, half of 2.5 is 1.76 and change. Shouldn't we be talking in terms of the area of the pipe?

Yes and no. Overall surface area is very important from an engineering perspective, but you're going to have to take a few things into consideration. For example, will the stock clamps even accept a pipe that large? Can a pipe that large be routed how you want? Also, 750cc is on the small side of the scale, so you could probably run 1.5". I'm going to measure the diameter of the stock pipes tonight.

 

[verde]

Rough estimates (calipers are awol) taken off my '78:

Header OD: 1.5"(relating to the stock exhaust clamp capacity brought up)
Header ID: 1.0"(it's double walled for those who don't know)

Scrapped the other stuff so not sure the ID of the collectors/mufflers in the stock 4-2 set-up.

To be honest, I am more interested in the look and sound than all out performance, but I would like to keep my low end torque. I wonder if one could scale down his recommended pipe diameters for a gs engine. I know some of you have cut pipes and installed baffles. I would love to hear from you! Specifically the pipe diameter and baffle type and length, how it sounded and how it affected the carb jetting.

Thanks!

Running open carbs and just the factory headers on my '78 I jetted slightly bigger than people quote for pods and a proper 4-1 system. Behaves well but definitely still "missing something." My '79 I run with open pipes built off the factory 4-2, but they are funky(long, square) so it actually feels spot on in it's behavior on the bottom end of things. Jetted just like pods/4-1.

 

[TheCafeKid]

1.5-1.75 OD is as big as you're going to be able to fit without complicating things. And even then it's gonna be tight. I used a Yosh race spec pipe on my 1100ES and it was a bloody PITA getting it off and on to change the oil. It's really tight around the down tubes of the frame.

Another thing you need to consider is the diameter of the mid-pipe. This, from my experience in trial and error with many many makes of pipes I used on my 1100, seemed to make a bigger difference than the header size. Keep in mind that this is all "seat of the pants" analysis.
The bike was set up as follows:
83 GS1100ES
BS36SS (1150 carbs) with stage3 DJ kit
KN filters
Timing slightly advanced.

The Yosh race spec pipe had a mid-pipes that was roughly 2.75" OD. This was the largest of the bunch I tried. Low RPM suffered slightly, power came on around 3K vs 2.5K with a stock setup or one of the smaller pipes I used. However mid range power was comparable to stock, if not a tick better, which is where most time is spent (4-6.5k) on city riding commuting.
Upper midrange and top end power was increased substantially, at 6.5K and above the bike felt like a different animal. It was powerful everywhere, but at the higher end of the useable rev-range (keep in mind in most cases with an otherwise non modified motor, the power curve drops dramatically around 8.5 or 9K so revving past this is a waste of time) the bike seemed to have a retro-rocket under it that fired right around then. The torque would stretch the arms and things got blurry real quick. In the twisties I kept the bike wound between 5-8K rpm to really charge out of the corner. It was a ball of laughs.

Same set up with a Vance and Hines pro canister type system. The mid pipe was substantially smaller. 1.75-2.0" perhaps. Low rpm was acceptable, mid range boosted noticeably, even over the Yosh, but that top end rush was greatly hampered. Keep in mind these 1100s were torque monsters so we're, in real numbers, probably not talking the difference between 5-10 HP at very very most either way. But the size of the midpipe seemed to move and increase or decrease the torque curve by my estimation more that adding useable, effective HP. This pipe would be great for tight roads, a lot of point to point light to light riding, as the midrange felt like it came on earlier, and stayed a bit later into the revs.

I also had a Yosh street pipe, smaller headers and the midpipe was somewhere between the two others. This pipe was just nice all around. Bottom end didn't suffer, midrange power came on at a nice point and the top end rush was there but not that crazy blinding arm-stretching launch that the larger Yosh provided (which I will say is quite addicting if you've got the room to use it).

The original pipe the bike came with was a SuperTrapp underbelly mild steel pipe with aluminum canister. I played around with how many discs I put in it, but anything less than 8 really choked the bike off. I think it probably performed worse than a stock system with less than 8 plates.
At one point I had as many as 17 plates in it (used their extended bolt kit) and I really couldnt ever get the best out of the bike or tune it for that pipe. No matter what I did with the jetting, it popped on decel, it had multiple flat spots, that wouldn't go away, they'd simply move depending on what I did with the jetting.
It was also loud. And annoyingly so. On a long ride the drone of that pipe would get on your nerves. It didn't have a great sound either. The yoshes had that "hive of really angry hornets" sound and when you got on it, had that great "whoosh" of a nicely tuned pipe. The 'Trapp just sounded tin-y and soft. And just got louder with power.
The VH pipe had that nice liter bike growl they're famous for but kind of broke up and distorted unpleasantly under power.

So, my point is, depending on what you're looking for, play around with the mid-pipe diameter as well. It seemed to me to have a more noticeable effect over the size of the headers themselves.

 

[snyderman]

Cafe Kid,
Thanks for the info, it will all help. I think I need to get my header and see what kind of room I have. The collector on the header is 2.5". What I was thinking is running a straight pipe off of that with either a baffle, a glass pac, or both? I was not planning on a mid pipe, I want to keep the whole thing as short as possible. I can relate to the top end rush, but I would probably be better off without that temptation. So could I Reduce to 1.5" at the header collector, and run a short 1.5" pipe with a baffle or glasspac and have a decent system? I would like to get something that would work with the jetting advice I got from MisterCinders : 122.5 mains, needle on 4th clip, and 17.5 pilot jets. I would expect to swap a few baffles and see how things work, but if I set my pipe up right, with the right diameter and length, It should be easy to try different internal components. I guess I am just looking to make sure that this is not a bad idea for some reason that I have not considered.
Thanks!