exhaust changes Trevor Amos

Trevor what have you done to the front cone or difuser, to lift the power/torque at higher peak, just guessing

1) increased the rear of the difuser angle, this would shorten it -
2) requiring the centre section to be longer but not by a lot as
3) to avoid the shorter reducer norrowing the PB you lave lessened the taper on the revers cone, so making it longer.

as I have no Idea about theory on the front down pipe, I do not know how the length or taper affects the Power curve.
But if you have done this it would also have increased the pipe volume by quite a bit so again aiding torque? I',m not sure on this either.

could you confirm/point us to what affects the power like this, ?

have you increased the comp ratio by reducing the head volume,?

tell me if im in the right "ballpark" ? or wrong as it all helps

regards Derek

Hello Derek and all,
The only changes made during this investigation were to the pipe specification and to the ignition timing ,
which was a consequence of the increase in revs , the initial timing was left unaltered , so we saw a retard of
a few degrees fron 11-11500 rpm . The comp ratio was left unchanged , making multiple changes complicates
and distorts the resulting information !

This is what i changed , all lengths expressed as a percentage of the tuned length from piston face to tailpipe entry

Header was reduced in length by 8% but angle increased to maintain original outlet diameter .
Diffuser lengthened at the front to compensate but its outlet position reduced by 1% but diameter remained the same .
Rear cone included angle was increased by 3* because its length was shorter by a few mm .
Volume changes come as a consequence of length and angle changes and not as a design, prerequisite number.

It might be that the top end improvement came simply from the rear cone giving a shorter duration , but stronger
return pulse picking up the extra revs ? There is a lot of information to assimilate and i have yet to plough through
it all , as ihave said before the devil is in the detail .
Take comfort Derek from the fact that no one has all the answers , least of all me , and you are certainly in the ball park !

Hell of an exhausting topic this , i need a rest ! Trevor .

`Scuse me taking over Trevor...

What I know of it for max revs after the peak of power -- is somewhat like Trevor said -- the sharper the angle of the nozzle -- (or final cone) -- the less the overrun of revs. The first cone, the Diffuser with a sharper angle influences the amount of peak power and it is of course the Nozzle distance from the Diffuser and the Nozzle distance from the exhaust port that determine the max Full Load revs...

As Trevor said, one change at a time is the proper way....

From what I have read recently the sharp angled Diffuser & Nozzle with "Belly"² in between is ancient Kaadan history. It seems the "Belly"² has mostly vanished and that shallower angled diffuser and nozzle come together are now favoured over Kaadan style where these modern style pipes give smoother lead-in to Full-Load power (coming in by the pipe) and a longer overrun of revs after peak power. In all these cases the diffuser is always a lesser angle than the nozzle -- a typical modern data would be 8° to 10° for diffuser and 15° for Nozzle but there are many variations to this theme that work very well for racing snowmobiles, Gocarts and the like....

The tapered header pip -- about 20" from piston face to start of Nozzle -- boost performance mid rpm range to low rpm but, most times, do so AT SOME EXPENSE of peak power -- so it is possible that this and shallower angled expansion chambers make for smoother power take up but knock a little bit off the top end... A parallel header pipe seems best bet....

The Tailpipe dia and length are usually well recommended data but if the development time is available -- and some progress has been made with the above -- a longer tailpipe can be tried to see if it helps the biottom end and mid speed-range power WITHOUT, of course, knocking off anything at the top end.

It is a hell of a lot of Suck-it-&-See that doesn´t always show up in simulation exercises. I did it all on the track and a lot of that was subjective so I could have done much better -- however, I knew exactly when my Pipe Power came in and slipping the clutch (which I di a great amout of...) smoothed it out anyway.



² ... "Belly" was Walter Kaadan´s name for the parallel portion between Diffuser & Nozzle and its max diameter -- if I remember correctly should -- be, that its cross-sectional-area is 4 times that of the area of the exhaust port.

My Memory might not have got that right -- so could someone please confirm or refute -- please!

Good Luck & Cheers!

Hi all

Thank you for that Trevor, ! I have to say I was surprised to see you had increased the taper of the revers cone! you got me there, "always thought this would reduce the run-on ability of the engine, or narrow the power band, but increase peek pressure over shorter period.!

the bit you lost me here with is "reducing the length of the front pipe" and incresing its taper I understand to acieve the same diameter- what affects and where does this affect power/waves - anybody! or is it all part of the piston the revese cone length calculation ?/ formula. ?

the only other observation I have to make is its amazing how such small length changes 1% and 8% have such an effect, you could lose one of these with poor weld !

and thanks for adding to this John - its all helpful.

Kind regards Derek