Diverging cones in 4stroke exhaust

[MexicanGringo]

Im busy rebuilding an nc30 exhaust system to re-route as an undertail and was wondering if 2 expanding cone sections could improve performance as in a 2stroke.. The theory is to reflect sound waves back to the exhaust port and create a negative pressure at that point so as to rid more burnt exhaust gas.. Has anyone any knowledge on this??

[fastdruid]

IIRC from what I read years ago in theory yes it could be an advantage but the extra weight and space issues mean in
reality the very minor gain in power isn't worth it.

Druid

[CMSMJ1]

my guess is that no it would not be as benefical as a spanny is to a 2 stroke.

you need to think in lines of pulses bobbing up and down the pipes - they get to the end of the pipe and come back again. The shorter the pipe the faster they do it. The higher your bike revs the more it likes to have these pulses coming back quicker.

Just work out your lengths and get that spot on. Everything else, as long as you have good flow, will fal into place.

[V4Chris]

I could be wrong, but....

The 2 stroke engine has a combined power/exhaust stroke so any extra "suck" from the exhaust system is quite beneficial.

The divergent cone you are referring to in 2 stroke exhausts are used to lengthen the negative wave pulses so that when they return they have more chance of finding the exhaust port open. These cones are finely tuned based on distance from port, length and rate of divergence. Even then it can only be tuned for a small rpm range, hence a "power band".

4 Stroke engines have a dedicated exhaust stroke which is already pretty efficient at getting rid of all the burnt gases from the cylinder. The main concern of a 4 stroke exhaust is to just get rid of all these gasses as quickly as possible once it has left the valves. Tuning of these exhausts is mainly to make sure that the flow is not inhibited by bad bends and to get the diameter correct for a given flow rate. In these exhausts it is the velocity of the exhaust gas which is more important. There is less to be gained from tuning the pulses, and if you get it wrong things like valve overlap can mean you start sucking your exhaust gases back into the engine.

The two types of engines are completely different beasts, and exhaust tuning is a black art as there are so many variables.

Unless you have a way of measuring the effects of your changes you could do more harm (to performance) than good without a lot of experimenting.

[Neosophist]

Somone beat me too it, and they wrote a more comprehensive explanation!

I was just going to say, if it were of any benefit you'd have seen it on 4 strokers before now.

[MexicanGringo]

Cheers for the advice.. If the main factors then to improved performance are higher gas flow and no positive pulses pushing exhaust gas back into the chamber, i would think as v4chris said about bend geometry being the most important aspect.. Larger bore pipe should seem like a better option?but i know gas speeds are reduced in larger spaces as they expand and loose heat.. what i might try is the same bore size, but with a slight venturi just before the muffler to increase gas speed at that point, which would hopefully create a slight vacuum there and improve gas flow throughout the pipe.. Please keep coming with opinions!

[Marb]

It's not completely true that a the bigger the bore size the more power. At lower rpm's you definatly need some backpressure in the exhaust system, otherwise you have no power under 8000rpm. Try riding with no can, you'll have very little power left.

[zimm]

It's not completely true that a the bigger the bore size the more power. At lower rpm's you definatly need some backpressure in the exhaust system, otherwise you have no power under 8000rpm. Try riding with no can, you'll have very little power left.

not true if the carbs are re-jetted to suit.

[Marb]

True, but you also need to re-jet if your going to add diverging-cones/venturi's/exup valves, etc, etc.

[Morespeedvicar]

Hiya,
tapered headers anybody???? Yeah it will make a differance, thats why modern exhausts are tapered but its only very slight summat like a 15 to 20% increase in area. Maybe worth trying but a pig to make unless your very good at welding and have rollers or have a hydroformer at home!!!
From what i seen of pics of F1 car exhausts there tapered but in steps, i dont know if this is to keep the volumes the same (like in ducati pipes long and thin or short and fat so the pressure pulses keep in time with differant pipe lenghts) or to achieve the tapering effect i dont know, maybe both, as there not aloud exhaust valves....
Its something i've always wanted to try but never had the time to make.
Also if a pipes too big the gas will start to tumble rarther than flow especially around corners thats why micron made serpent headers to limit the speed differance in the gas as it goes around a corner( the gas has further to go around the outside of the pipe compered to the inside of a bend).
I would love to have a good look at a set of F1 pipes to see how big they actually are, because they dont look that big and flow 400 odd BHP of gas!
Maybe just go up a coulpe of MM in diameter allong the length (like go up a pipe size when you come to a weld point).

Cheers

Ian