DIY under-body exhaust - CBR400RR - Custom Fighters - Custom Streetfighter Motorcycle Forum

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Old Streetfighter Motorcycle Forum Post 02-05-2012, 05:52 AM   #1
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Default DIY under-body exhaust - CBR400RR

Hello!

I've finally taken the plunge and decided to de-clutter my little CBR400RR. I removing the shitty DIY SS free-flow exhaust and planning to fit an under-body shorty in it's place.

Currently the bike has a 4:2:1. The 4:2 section is great, the 2:1 is seriously fucked up with rust. It's so bad that I'm going to censor it in the pictures.

Anyway, there is the before pic:



Here is a crappy paint design I'm thinking of using (not to scale):



My question is should I link the two pairs together to balance them? One side will be shorter if I use this design. Maybe I should Rebulid the 2:1 section in the muffler and split again for the twin hole design.

I've got a nice little DC inverter MMAW setup. I'm planing on using mild steel; I don't care if it rusts away after 2/3 years; I'll replace it before then.

Any suggestions or criticisms are welcome.

Cheers,

Chris
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Old Streetfighter Motorcycle Forum Post 02-05-2012, 06:58 AM   #2
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Default Re: DIY under-body exhaust - CBR400RR

ok. for starters, the valves in these engines are TINY. losing backpressure could result in them burning out very rapidly. secondly, without baffling this will be very loud and more than likely shoot flames all over your foot.

you'll need to keep the pipes the same length, otherwise one end will scavenge faster than the other and result in misfires.

otherwise, this is pretty cool; I used to have the 250rr and loved the way they sounded.

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Old Streetfighter Motorcycle Forum Post 02-05-2012, 08:55 AM   #3
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Default Re: DIY under-body exhaust - CBR400RR

Quote:
Originally Posted by n0regret5 View Post
ok. for starters, the valves in these engines are TINY. losing backpressure could result in them burning out very rapidly. secondly, without baffling this will be very loud and more than likely shoot flames all over your foot.

you'll need to keep the pipes the same length, otherwise one end will scavenge faster than the other and result in misfires.

otherwise, this is pretty cool; I used to have the 250rr and loved the way they sounded.
Good points. In that case I'll go with something like this:



The blue stuff is a probably going to be SS wool. The orange thing is a baffle. I use half moon slits. I'm guessing that I should try to keep the flow rate of the muffler chamber similar to the headers to stop the exhaust gas from slowing down.
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Old Streetfighter Motorcycle Forum Post 02-05-2012, 09:05 AM   #4
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Default Re: DIY under-body exhaust - CBR400RR

Actually like this:

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Old Streetfighter Motorcycle Forum Post 02-05-2012, 09:42 AM   #5
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Default Re: DIY under-body exhaust - CBR400RR

I'd still wanna run it with two outlets after the baffle....


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Old Streetfighter Motorcycle Forum Post 02-05-2012, 10:03 AM   #6
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Default Re: DIY under-body exhaust - CBR400RR

I agree. Like So?

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Old Streetfighter Motorcycle Forum Post 02-05-2012, 10:51 AM   #7
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Default Re: DIY under-body exhaust - CBR400RR

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Originally Posted by ChrisE99 View Post
I agree. Like So?

That the kiddie....


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Old Streetfighter Motorcycle Forum Post 02-05-2012, 11:02 AM   #8
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Default Re: DIY under-body exhaust - CBR400RR

When I was trying to decide on mine I was looking at various options...

One was very similar to that. except, I was Gunna do the baffled section like a shortened sidepipe .. with the perforated outer sleeve (err...sorta,like an AC cobra but with dual tips)


SAD, I know... But I liked it

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Old Streetfighter Motorcycle Forum Post 02-05-2012, 11:11 AM   #9
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Default Re: DIY under-body exhaust - CBR400RR

Very shiny.

Here is a cost post of my build thread. It's still in very early stages. - http://www.customfighters.com/forums...ad.php?t=54482
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Old Streetfighter Motorcycle Forum Post 02-05-2012, 12:15 PM   #10
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Default Re: DIY under-body exhaust - CBR400RR

Just get you system repaired, fitting an under engine silencer like that will not make the bike run better. You'll lose midrange. Exhaust systems are the length and design they are for a very good reason. Cutting out 18 inches or so of the midpipe will screw up the resonance needed for the scavenging pulses. A short open pipe is fine if you are on the strip and just going flat out wide open throttle all the time, and you just need the spent gases out as fast as possible, but in the real world on a roadbike, you want to retain the low and midrange performance. The little 400cc bikes lack torque to start with, and with what you're intending you'll lose the little available torque you have.
I've seen all manner of exhaust systems fitted to the bikes that I've had in for setting up on my dyno, and the best performance is from a stockish style of pipe, underseat systems mess up the midrange just as the under motor systems do, same thing, by adding another 18 inches or so into the exhuast system again plys havoc with the resonance and timing of the scavengine pulses, bikes that have underseat systems or under engine systems as stock are designed from the outset to run with them, and will have different cam timing, ignition, etc to take use the increase or decrease in pipe length.
Look at the various aftermarket exhaust systems available for your particular model, they all look pretty similar in construction and design, that isn't a coincidence, it is because the design layout works best through the entire rev range.
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