Making the power?

Who could the owner of this engine possibly be?

Superimposed upon each other are some power and torque traces produced by testing a collection of pipes during a recent single dyno session. The green trace is a classic of its type in that the tuned length is too long to sustain rising power increase beyond 10500rpm. We see this time and again, high power initially only to falter later where the returning plugging pulse arriving when the exhaust port is already closed. Some of the other pipes perform in a relatively similar fashion to each other, but there is one outstanding trace that peaks at the engine output of 25hp!
The curious thing is that this pipe did not do well initially, being unsuited to the engine specification back at the time. Subsequent improvements to porting geometry, combustion chamber, reeds and ignition, along with trying the other pipes all showed incremental improvements. It was the marrying of the initially rejected pipe with the upgraded engine that made everything `click.`
The next and very real challenge is to try and replicate the green trace from 7,000rpm and fill the area from 9,000rpm up to the peak around 11,000 of the red trace and maybe nudge the peak up a touch. So it looks like a lot of interesting work over the winter months.
The lesson here is that the exhaust pipe can never make for what was never there in the first place! It is by looking at a more efficient mass movement of mixture into the cylinder, the delivery ratio, and keeping it there, the trapping efficiency, that combined provides the potential to make the power!

Trevor

well Trevor, i wonder whos been so lucky as you say with what has been found, could this engine be improved by exhaust mods alone?

Hi Nigel,
             The short answer must be yes, to a certain extent, as can be seen from the read out the general spread of power is quite good so any ill thought through tinkering might be counterproductive. Exhaust pipes can only work with the finite quantity of energy they are provided with, and from the moment the exhaust port opens the energy level is depleting. It is the designer`s job to decide where this energy is utilised for best overall performance. For whilst the engine is dynamic the pipe dimensions remain static and the pipe has to deal with acoustic energy, thermal energy and exhaust gas, all of which vary with engine rpm! The first hurdle is the exhaust duct, some ducts flow really well with minimal losses, some are not very good having a low coefficient of flow, so turbulence robs energy from the pressure wave, even the cooling of the duct be it air or water has an effect.
The principle working parts of the pipe are the diffuser and reverse cone/tailpipe combination. The diffuser is timed by the header length and the reverse cone by parallel section down to the tuned length at the tail pipe. Diameters are a far more contentious problem for the widening angles of the diffuser use up energy in their functions, but some has to remain to be reflected back to the cylinder just before exhaust port closure to stuff useable mixture, that is lurking in the duct, back into the combustion chamber.  
As we have enough basic data we can determine that the engine produces at the crank; 8.8bar bmep, pretty good for a Bantam, the little 50cc job we looked at 10.6bar, an RS125 at 12.5bar and the Aprilia at 16 bar, so within the pantheon of 125 race engines Bantam power can be seen as pretty modest, then by definition, there is a modest quantity of energy available.

One thing that might easily be tried and could achieve an improvement is the tail pipe/insert diameter. On this engine the exhaust has a 23mm tailpipe, very safe and does an ok job but could a reduction to 22mm be beneficial? The smaller you go with tailpipe diameter the better everything gets, especially at mid to peak rpm, but at some point heat in the system rises exponentially and piston seizure or worse follows soon after. Do you risk that or still play safe. Shortening the tuned length could keep power at a higher rpm but will be way out towards the base of the power range.
Going too big on the centre section which implies acute angles on the diffuser runs the very real risk over-scavenging, from the rubbish transfer ducts so typically found in Bantam engines and losing working charge out of the exhaust port to be lost forever.
By far the most complicated part of the pipe is the diffuser, particularly if it has multi sections. The problem is we have a fixed set of lengths for the pipe that can only develop a depression over a certain period of time, but the revs are constantly altering as we accelerate and slow down. The real dilemma is that the point of maximum depression relative to bdc varies with rpm. It is possible to change the curve of the depression but it will always move from early to late in the cycle as rpm steadily rises, overlaying all of this is the fundamental need to keep usable power within the scope of only three speeds, so which part of all of this do you select from this merry go round for the first alteration in pursuit of the next hp?

My own view is that with this particular engine it is the cylinder ducts and porting that can further be developed in order to move more mixture into the crankcase then the cylinder and to keeping more in there prior to combustion, and of revising the combustion chamber further to be more efficient. In the final analysis, if the engine doesn`t make it, the pipe can`t use it, who said Bantam engines are simple!

Trevor

Hi Trevor.. I take it that was you i had a chat with today (sat)...I did come to your pitch as i rode out of the circuit, but you were on track me thinks...Hopefully i'll see u again on Sunday...Bantam Rule

Hi, Roy2cycle?
I think you must have me confused with someone else, I certainly was not `on circuit` at the time and place you have in mind. None the less it is great that you are on the Bantam forum and hope that your post is just one of many.
Tell us more about that impressive machine in the picture, looks a challenging piece of motorcycle engineering and could I suggest, a very worthy candidate for the Wacky files!

Regards, Trevor

Hi Trevor. Haha, i saw the impressive graph and went off on a tangent somewhere.... I met Derek and his wife (plus race bike) at silverstone on sat/sun. Yes, on seeing a "bantam" in the program, we hunted him down to have a look at the bike. The sunday race was pushed back to "last race" due to 2 red flags on starting or trying to start it and failing...Derek did say he had a slight hand in one of the red flag stops...oops.... We were all in the Bdrc stand waiting for Derek to show us his balls (haha) but all we got to see were Red flags and no balls.... Never mind ...we had two good days over at Silverstone for the cmrc return to a circuit they used to do yearly...The cafe, roast pork dinner was welcomed to a few tummys during the day....As to my Bantams and i have three of them, one twin carb'd crankcase reed, Ariel Arrow "250" engine, 2 rg400/rg500 engined Bantams one using Ariel Arrow top ends measuring out to 475cc and one with Peugeot speedfight top ends being 413cc. If you have a moment search "bsa bantam RG500" "Bsa bantam Ariel Arrow" or wrongly named "bsa bantam v4"....as its a square 4... The full builds are in the hybrid section on the Bsa Bantam forum...http://www.bsabantamclub.com/forum/hybrid-bantams/how-much-bantam/
Well done and keep up the good work....
Roy
northampton

Morning Roy,
Small world; before re-locating to down here in Somerset I used to live a few miles along the A45 in Rushden, and there are still a handful of `old` Bantam racers spread around the area.
After seeing your post and having a look on Facebook I guessed it was Derek and not me you were referring to. Actually he had a good weekend, stood on the podium and won a shiny new pot, so well done to him!
I must say from what you describe that is an impressive array of hybrid bikes you have, the high standard of craftsmanship, and sheer imagination does you great credit. It would be nice if we could we have some pictures of your entire menagerie for us to enjoy and marvel at?

Cheers for now, Trevor

Hi Trevor. Here is a nice (test) pic of the rg engine getting fitted with the scooter cylinders..

https://2img.net/h/i628.photobucket.com/albums/uu5/roy2cycle/DSCF1538_zps0a27d9af.jpg

Trevor...You may find you can scroll through my album by clicking on the > and < button's...

Fascinating!!

Looks lovely workmanship ...

Congratulations & Cheers....

JayBee....

Hi. Cheers for the nice comment on my built for fun Bantam

https://2img.net/h/i628.photobucket.com/albums/uu5/roy2cycle/14568215_157849598006365_3248946217622880809_n_zpsltypjpav.jpg