Exhaust tuning

[tony.mon]

I'm thinking about building my own stubbies, and that set me considering how much, if any, improvement could be made to the standard exhausts.
I know that the pipe length is critical (to the open end of the pipe or silencer)-
see this link for a (fairly) clear explanation:
http://www.challengers101.com/IntakeTuning.html

But this seems to mean that the linked pipes would be a bit of a compromise, and after all, the front pipe is a different length to the rear one.

I can't believe that Honda just made it up on the back of a fag packet and bolted it on, there must have been a bit of thought and design going on. And so I'm considering trying a pair of single pipes, each starting off at around 20 inches but with an adjustable sleeve that clamps on to the end to make each pipe longer.

Trying it on a dyno would give a base setting for best pipe length, I would hope.

From there, I'd merely replace the end part of the pipe with a silencer- a straight-through would work exactly as the open pipe, but if it has a baffle of the same style as the standard one then there are a bunch of tubes and baffles which effectively lengthen the pipe length inside the silencer.

But although this doesn't seem to be too tricky, would it merely tune for best horsepower, and would this be at the expense of torque?

And maybe Honda have deliberately tuned one cylinder for horsepower and one for torque- after all the two inlet trumpets are a different length as standard, and the main jets are also different sizes......

Any thoughts?

[IgorVigor]

If I remember right the rear carb is a 178 main, where as the front carb is a 175...
As for the different intake trumpets, I could be wrong but a shorter velocity stack pulls more air in, but due to the design of the firestorms airbox (Inlet at the front) the rear carb would collect more air easily as its canted slightly towards the airbox intake...
The front carb is canted the other way, so relies on the pure venturi effect....

AFAIK the reason for this is to keep the rear cylinder slightly cooler, as it wont get as much airflow over it as the front cylinder...

Seperate pipes will deliver the most horsepower, but pipes that join are designed for torque...

Its due to the pressure waves that travel down the pipes, so if they join, as the do on the firestorm (2 into 1 then back into 2), it creates a scavenging effect for the other cylinder as well...

If you have a true 2 into 2 system each pipe has to scavenge for its own cylinder, but with the 2 into 1 into 2 system the system works as a whole so the scavenging effect is enhanced...
The disadvantage is you lose peak HP, but you gain a better torque curve.

Hope that helps a bit :S...

By all means try it if you want, would be interesting to see the results...
You will probably noticably lose torque, but, and its a big but... If you tune the length correctly, the bike will have a much more prominent powerband...

Good luck either way...

[DPR11]

Heres my 2p`s worth, I put a small block V8 into the back of a 1981 vw van a couple of years ago, although it had a few more cylinders the principle is the same, obviously with the V8 being at the back i had limited space for the exhaust, i ran it with seperate pipes for a few weeks but did not run as well as it did when the pipes were linked together, the overall effect was much smoother and also sounded much better, dont know if that is of any use.

Danny

[Phil-VTwin]

As Igor said, you can tune for maximum HP or Torque but not both, there are plenty of exhaust tuning calculators available but your biggest problem will be space.

[Zakalwe]

Different inlet lengths are a compromise (AFAIK). When the inlet valve on an I.C.E. closes a shock wave is generated which travels back up the inlet tube. When this shock wave hit the inlet edge, part of the wave is reflected back down the inlet. If this is timed correctly, this pressure wave will hit as the inlet valve is open, and is a "free" way of forcing more air into the engine. This increases the volumetric efficiency of the engine. Each pot has a total capacity of 500cc, but the engine cant really fill the chamber as there are restrictions in the way (poppet valves etc). The amount of air the engine can pump is know as the volumetric efficiency of the engine. Timing the inlet pressure wave effectively stuffs more air/fuel into the engine.

At low revs a longer inlet means that the pressure wave takes longer to travel up and back down the inlet, thus matching the slower speed of the engine. At high revs the valves are open for a shorter time, so a shorter inlet means the wave is reflected back down more quickly. To gain the best compromise, inlets of different lengths are used, as the engine is required to perform across a wide range of revs. On 4-cylinder engines, you generally have 2 banks of different length inlets.

The best solution would be to have a completly variable inlet. Imagine 2 tubes that slide inside one another, powered by a stepper motor. As the reves rise and fall the inlets can alter there overall length*. Yamaha have a 2-length version (YCC-I) which gives a long inlet funnel at low revs, and at higher revs a motor lifts the inlet tops away, exposing a shorter inlet.




*The latest Norton racer uses exactly this system, powered by the stepper motor from a CD drive (of all things).

[Zakalwe]

Exhausts are tuned in a similiar way....to make the pressure pulse arrive back at the exhaust port at the right time (just as the valves are closing). This partly pressurises the port and stops the fuel/air charge escaping out the exhaust port (again increasing the volumetric efficiency)

[sirch345]

Well written Steve (Zakalwe), very well put IMHO

There's been loads written about exhaust etc as I'm sure you are fully aware Tony, but for first hand experience you can only get that by trying it yourself. I admire you for intending to give it a go. Having a dyno within reach is going to be one of your best friends.

Good luck with your new plans and DO keep us posted,

Chris.

[Zakalwe]

Well written Steve (Zakalwe), very well put IMHO

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