Optimum exhaust size?

**TygerTung** · 6th March 2005, 19:15

I have this badass 125cc honda engine, fully worked mate!

Anyway, I'm using a high compression piston with the 110cc head for extra high compression, a mild port job on the head, and a .020" oversize on the piston and bore.

I have a spare exahaust, and I thought I could make it into a straight through race pipe! what size should I make it? Just a straight through pipe that same size as where it comes out of the head?

Cheers

**TwoSeven** · 6th March 2005, 19:24

I think you'll also need a megaphone of some design on the end.

**Kickaha** · 6th March 2005, 19:37

Originally Posted by TwoSeven

I think you'll also need a megaphone of some design on the end.

Dead right ,I'll measure up the GN125 megaphone one and give you the dimensions,it should be close but we rev them to 11000-12000rpm and it will be obnoxiously loud for street use

**TygerTung** · 6th March 2005, 19:41

Does the megaphone make it go faster?

Should I just get the 110 exhaust, and then remove all baffles inside?

**Kickaha** · 6th March 2005, 19:50

Originally Posted by TygerTung

Does the megaphone make it go faster?

Yes

Originally Posted by TygerTung

Should I just get the 110 exhaust, and then remove all baffles inside?

No that wont work as good

Exhaust on the GN as follows
Header pipe 30mm ID, 660mm long
first cone 30mmID, 330mm long to 75mm ID
second cone 75mm ID, 70mm long to 60mm ID

With your high compression piston it would be a good idea to work out your comp ratio and also to check piston to valve clearance at full lift

**dangerous** · 6th March 2005, 19:51

Originally Posted by TygerTung

Does the megaphone make it go faster?

Should I just get the 110 exhaust, and then remove all baffles inside?

Well the CG pipe isent far off a megaphone, would just need shortning, you wont be able to get a WOF with a open pipe and you will attract too much unwanted attention.

My sugestion is to gut the stagerd baffeling in the pipe and replace it with a perfarated length of pipe with packing around it... make the end hole twice as bit too. Or stick the smallest turbo flo (resonator) you can find on it.

**FROSTY:Me** · 6th March 2005, 19:52

dude the art/science of exhaust tuning is just amazing. I'm guessing its a cb125 single or xl125
Anyhoo the ducks nuts as I recall it was to go for 1-1/4 pipe with a really smooth bend rather than the almost right angles the stock pipe has.
You then need a reverse cone megaphone "muffler"
The whole idea is to get the exhaust pulses working for you rather than against you. -They should actually help suck raw fuel into the cylinder as well as just dumping the exhaust gasses.
If ya realy want a 'badass" motor then you can muck around with different cam grinds and use the 6 speed gearbox from.... Opps dribbling from the ol bucket racing days

**FROSTY:Me** · 6th March 2005, 19:55

Originally Posted by Kickaha

Exhaust on the GN as follows
Header pipe 30mm ID, 660mm long
first cone 30mmID, 330mm long to 75mm ID
second cone 75mm ID, 70mm long to 60mm ID

Snap --funny innit -the years roll by but the rules dont really change :spudwave:

**TygerTung** · 6th March 2005, 20:08

Yeah, I don't think I'd be wanting to actually use it very much at all, as it would be ridiculously loud, and it would give me a bad headache, and damage my hearing I reckon.

Just if I'm going for a ride around the hills with my mate who's got a bigger bike, or I'm going bucket racing or whatever, I can put on the 'performance pipe', I'm not one of those fulla's in it for the wank factor, like some of those dudes on those american V-Twins

It's not somthing with heaps of street cred, and I ain't trying to make it so.

I got an XL125S piston, so the compression should be a bit higher.

I'm putting the whole bike back together at the moment, fully stripped down the whole frame, and got everything painted. The joys of being an aircraft engineer- I did it all for free

**TygerTung** · 6th March 2005, 20:22

Oh I have a 125 pipe on it now, but the 110 pipe is actually really smooth, so I can modify that.

Have already done a 5 speed gearbox conversion, it had a 4 speed before, so should be a bit better....

I modified the top of the airbox for better flow, didn't want to cut any holes in the side, cos it makes the induction noise too loud, I took it off once when it was a 110, and it was stupidly loud, gave me a headache quickly.

**Motu** · 6th March 2005, 20:24

How about a formula??

For the exhaust pipe length use...

L= V120/N

Where
L= length of pipe
N = engine speed in RPM
V = velocity of wave in ft/sec. Velocity of the wave is a bit tricky,but if you use 1700 ft/sec you should be pretty close.

With the pipe length we are dealing with SOUND waves,called resonance tuning.The diameter of the pipe is dealing with the gas slug itself,and we want it around 300 ft/sec.Use this formula...

V = piston speed/60 X D squared/d squared

where
V = gas velocity in ft/sec
D = piston dia
d = pipe inside dia.

To find piston speed in ft/sec.speed use this...

stroke X 2 X rpm divided by 12

Clear as mud eh.I used to fart around with this stuff when I was your age,hours and hours of enjoyable fun....well,if you're into bikes that is,and I was and still am.Almost got those formulas imprinted in my brain,but still had to go look for them....the books are almost falling apart.

Oops...just remembered,these figures will give you a pipe too short to use on a bike (seen the short headers on a fuel dragster?) just double it and catch the wave next time round.

**TygerTung** · 6th March 2005, 21:12

WOW!!! This is the most helpful forums I've ever found.

This will work on multi cylinder engines too eh?

For the RPM, perhaps I should tune it to where my max power is anyway? For example I think it's at about 9000 rpm, so I'll want all my power around that point for maximum fun.

**FROSTY:Me** · 6th March 2005, 21:15

Originally Posted by TygerTung

Oh I have a 125 pipe on it now, but the 110 pipe is actually really smooth, so I can modify that.

Have already done a 5 speed gearbox conversion, it had a 4 speed before, so should be a bit better....

I modified the top of the airbox for better flow, didn't want to cut any holes in the side, cos it makes the induction noise too loud, I took it off once when it was a 110, and it was stupidly loud, gave me a headache quickly.

Ohh mate Ive got a whole log book full of stuff I did to those old engines and bikes. Im a bit confused why ya staterd with the CG -except the piston is better than the cb.
If you aint after street cred -just go then Ive a couple of easy ideas for ya.
Front end wise the bees knees for you would be to use a gl145 forks and brakes.
If ya go to a pod filter then you ccan brace the flexiframe through where that big old air box went.
Throw out the plastic swing arm bearings and replace em with brass sleeves.
at the same time i'd weld strips of 25mm x6mm steel under the swing arm to brace it -you'll avoid most of the flexing.

**Motu** · 6th March 2005, 21:43

Originally Posted by TygerTung

WOW!!!
This will work on multi cylinder engines too eh?

For the RPM, perhaps I should tune it to where my max power is anyway? For example I think it's at about 9000 rpm, so I'll want all my power around that point for maximum fun.

Those exhaust formulas are for single cyls,all the old racers that developed these formulas had one pipe per cyl,check out the Guzzi V8,8 single pipes because they used resonance tuning.Multi pipes joining is another ball game,and they use tricks to kill the resonance wave.Tune to max rpm,trouble is as you improve the breathing your rpm at max power will go up,you just gotta keep working on it.