eyefi Posted March 31, 2002 Share Posted March 31, 2002 i have been looking at my cat back exhaust today and noticed, for the first time, 2 x 20cm long bulges in the pipework. what r they for?? ta Quote Link to comment Share on other sites More sharing options...
Guest Mycroft Posted March 31, 2002 Share Posted March 31, 2002 Tuned length (harmonic) expansion points. Kakimoto design these as vortex inducers and the entry and exit of the bulges (which are entirely separated by an inner wall) are pulse tuned to specific lengths, the formula for these is well known. One of the great falacies out there is that a simple pipe going from the headers to an exit at the rear is the best flowing therefore the least restrictive exhaust you can get....WRONG. Noisy?, YES. more powerful than a well tuned silenced exhaust?, NO. Exhaust passes the Turbos in pulses, the turbines break this up a little and the gas which is expanding all the time is also confined by pressures within the pipe that confines it, this creates congestion at various harmonics along its length, this is a restriction, if you make the exhaust a specific length (very short, just about 1 metre) then the problem is vented to the atmosp(here and there is no problem with restriction, any pipe that just goes from front to back even if it expands slightly along its course is not very good at all, do the same thing with expansion points at the 1st and 3rd and maybe the 6th harmonic the car will really jump in performance. I spent a couple of hours designing my system this way and gained just over 25hp, and 35lbs of torque, but more importantly I gained 12hp/20lbs at just 2000rpm, ask anyone and that is a HUGE jump, the calculations are just one level beyond Calculus so quite easy to perform. Quote Link to comment Share on other sites More sharing options...
Guest Martin F Posted March 31, 2002 Share Posted March 31, 2002 Quote: from Mycroft on 4:16 pm on Mar. 31, 2002[br] Exhaust passes the Turbos in pulses, the turbines break this up a little and the gas which is expanding all the time is also confined by pressures within the pipe that confines it, I agree with your logic here, but what are these 'pressures within the pipe that confines it' ? Quote Link to comment Share on other sites More sharing options...
Guest Mycroft Posted March 31, 2002 Share Posted March 31, 2002 As the engine revs up the pulses gain on each other. Quote Link to comment Share on other sites More sharing options...
Phil Wall Posted March 31, 2002 Share Posted March 31, 2002 So I guess that means later and higher velocity exhaust gases pulse out into the slow ones in front causing pressure within the tube. Quote Link to comment Share on other sites More sharing options...
Guest Mycroft Posted March 31, 2002 Share Posted March 31, 2002 Yep, making the acceleration thru the rev range a little tardy, you need to 'assemble' these pulses and set them in motion from a harmonic node and pass them along to another if you do this perfectly (not really possible as the length is a 'movable feast' so to speak, a bit like Easter falls on different days each year) you can actually make the earlier pulses 'drag' the others along. I measured my expansion points in cc and accepted only ±10cc. Quote Link to comment Share on other sites More sharing options...
eyefi Posted March 31, 2002 Author Share Posted March 31, 2002 very interesting, thankyou what does Kakimoto have to do with this exhaust, do they make them for hks? so i would gather that the 1st harmonic is covered by the 1st bulge, the 3rd harmonic by the 2nd bulge, could the 6th be the back box? its bloody huge, its alot larger than other sports exhausts ive seen. or is this just for silencing? this exhaust is quite quiet in general (ive had people ask me to start it when its running!), but not too quiet at WOT . engine braking sounds awesome. im getting a full decat downpipe soon, is there anything i should be looking for in that (except no cats). Quote Link to comment Share on other sites More sharing options...
Guest Mycroft Posted April 1, 2002 Share Posted April 1, 2002 I don't know the HKS design but the Kakimoto have the interior wall, it was a sort of wrapped up question 'is it a simple bulge or is it a by-pass' sorry it was a bit cryptic. If you have a Flute or Recorder to hand (don't we all?) you can play different notes by forcing the sine wave in the 'exhaust' further down its length (that is a very simplified explanation) the harmonics correspond to the holes that form those notes. (again simplified) Yamaha, used to have a public website which gave some the formulae for designing inlet and exhaust tracts, it is no longer there. Shame. Quote Link to comment Share on other sites More sharing options...
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