The makings of exhausts

Originally Posted by dizzyblonde

Would you really like to hear a V8 engine under you, at full chat, for long periods?
Cause thats what you'd get if you were sat on my VRap.....and it ain't feckin quiet at idle with no cans!

Didnt know your Raptor was a V8?
Must be a new model!

[yorkie_chris]

Originally Posted by AndyBrad

i didnt realise it was a myth?

Well find me someone who can explain how making an engine use power pumping exhaust gas out adds power then.
Loads of people blather on about it but none of them can give an explanation that holds water.

On engines with a lot of cam overlap then adding some can increase midrange, EXUP valves etc. But there's a lot lot more to it than a simple case of flow and pressure. I think it all comes back to cylinder scavenging and wave movements.

[dizzyblonde]

Originally Posted by startrek.steve

Didnt know your Raptor was a V8?
Must be a new model!

obviously not, but you reallly don't wanna hear it with just the headers*, I was purely giving as an example why you wouldn't want to be riding around with no cans.
I could put the vid back up, but I can't be ar$ed.


*actually you would!

Originally Posted by yorkie_chris

I think it all comes back to cylinder scavenging and wave movements.

i agree, i can see how it can work based on the old CD nozzel principal but am unsure as to how this would translate .

When gasses flowing it does some very very strange stuff. its all about expansion ratios really so i can see how you could get different sorts of pressure recovery within the system to make it more efficient.


real world swapping exhausts depends a lot on the engine and the systems feeding it.

We use hydro-valves on manhole installations, the idea is it builds up a head (back pressure) the increased pressure then causes a greater force to be applied to the orafice impellor. Once the water passing through has built up enough force to get the impellor to start turning it literally propells the water out of the manhole as a vortex, not only increasing the flow rate but also maintaining a constant flow rate once it reaches its upper limit. Thats up until a time when the volume of water within the drainage system no longer surcharges the unit, at that point the impellor will slow and the flow rate drop back down to an eventual trickle.

^^^That makes sense, because of the hydrovalve unit, so with no valve unit within an exhaust you get no such vortex and no increased flow - actually makes sense. Doesnt answer my question about making my arrow can a stubby though, am I likely to get a mid range bog by knocking off a few inch from the end can?

thats different because its got the mechanical advantage of the impellor. what your talking about with backpressure in an exhaust isnt actually packpressure (although people think it is) its about restricting the pipe size so that the exhaust gasses are so fast they basically pull the next chambers gasses out alowing a bette charge into the cylinder (how i understand it) thus you can get better power. Increasing the exhaust dia slows the gas but allows higher flowrates so basically its a balancing act between the two. bigger bores allow more gas out (good up top) but smaller pipes help midrange but can restrict flow uptop).

Thats how i see it anyways. Im sure yc will set me straight.

the thing is you then get into stuff like reverse megaphone type exhausts that try and do both.

Originally Posted by AndyBrad

thats different because its got the mechanical advantage of the impellor. what your talking about with backpressure in an exhaust isnt actually packpressure (although people think it is) its about restricting the pipe size so that the exhaust gasses are so fast they basically pull the next chambers gasses out alowing a bette charge into the cylinder (how i understand it) thus you can get better power. Increasing the exhaust dia slows the gas but allows higher flowrates so basically its a balancing act between the two. bigger bores allow more gas out (good up top) but smaller pipes help midrange but can restrict flow uptop).

Thats how i see it anyways. Im sure yc will set me straight.

the thing is you then get into stuff like reverse megaphone type exhausts that try and do both.

IMPELLOR! thats the word I wanted, cheers Andy post edit'd now.

"what your talking about with backpressure in an exhaust isnt actually packpressure (although people think it is) its about restricting the pipe size so that the exhaust gasses are so fast they basically pull the next chambers gasses out "

Isnt that just the principle of jetting though? Designed to increase the velocity at which the exhaust is ejected, not to increase the volume of what is ejected. I thought the key principal in automotive exahsuts was achiving the highest volume of exhausted gas, whilst also trying to balance it with a consistant flow rate.

thats how i see it volume is controlled by the bore size?

[yorkie_chris]

Originally Posted by AndyBrad

thats different because its got the mechanical advantage of the impellor. what your talking about with backpressure in an exhaust isnt actually packpressure (although people think it is) its about restricting the pipe size so that the exhaust gasses are so fast they basically pull the next chambers gasses out alowing a bette charge into the cylinder (how i understand it) thus you can get better power. Increasing the exhaust dia slows the gas but allows higher flowrates so basically its a balancing act between the two. bigger bores allow more gas out (good up top) but smaller pipes help midrange but can restrict flow uptop).

Thats how i see it anyways. Im sure yc will set me straight.

the thing is you then get into stuff like reverse megaphone type exhausts that try and do both.

Pretty much.
Also stuff like 4-2-1 systems vs 4-1s and crossover pipes to help get that scavenging effect at whatever rpm.
Reverse megas are trying to create or change the pressure waves.

But yeah this is why I say backpressure is BS because there's so much going on to describe it by such simple and misleading term.

I'm not sure singles have the same things going on to the same effect. Not sure you'd notice much difference with a couple of inch off the can.

Originally Posted by yorkie_chris

I'm not sure singles have the same things going on to the same effect. Not sure you'd notice much difference with a couple of inch off the can.

In that case, does anyone have a riviting tool I can borrow