BSA Bantam Race Preparation Manual

That is a very good suggestion Pete, and very worthwhile being included and having a chapter to it`s self. However, someone has to volunteer to compile the article for it to be included?

Sad to say that currently there seems little enthusiasm, beyond that of mine and Derek`s, to see the new manual reach publication! Given that production details are there in place that is puzzling, and disappointing. One wonders therefore, at what may be going on "behind the scenes"?

Trevor

Earlier on -- on here -- I thought I saw a list of names of those who had offered to contribute?

Wouldn´t it be a good idea to make a list of subjects(objects)  that need updating to start with?.

I am regular a Dumb-Dumb as far as all this goes but I guess -- mostly by the pics of Racing Bantams I see on here -- there are a lot  of changes without asking...

I mean,  that  without the latest in cycle parts there´s no point to having a fantastically better engine -- is there?  

Frame, brakes, forks and rear suspension and other hang-on bits -- surely have all changed since 40 - 60 years back.

Frame for example:  -- Icarus-1 & Icarus-2 had Cup-&-Cone bearings in the steering head --  aren´t they superceded by tapered bearing? these days? Drum brakes -- on the rear still but what are the advances with discs? Suspenion has become a science instead of an art. When a dealer sold  me a pair of Girlings and a pair of  AMC units he  said try these and see which are best --     there are  experts now  with a list of questions for the rider to answer before the seller spiecifies  what´s really needed ...

Lots  of things with the engine of course -- seals have improved, they seal better and drag less -- oil in the gearbox -- what´s in use these days? Clutch actuation has improved and so has the cluch itself ...

I am just gabbing on here -- not really knowing what I am talking about -- but trying to boost the promised contributors into contributing....

Race Bike Preparation  has a lot of essentials that cannot be left out and way back in 1967 they weren´t as they are today but they were just as essential ...  

 
Of course, the engine can be updatede at the same time -- loads of lovely stuff  -- that so many know of...

Cheers!

JB.....

Perhaps I should not interfere ....

But don´t give up on the manual -- perhaps the ones who said they´d contribute are working on their bit and will show when ready....

I have reminded myself that I only wanted to race and let Derek Neil do the donkey work whilst I prepared cycle parts -- again to his advice which he had gained from expereience of building a Villiers 250 Special and what his brother Colin Neil, excellent senior Bantam racer -- Told Him -- who also worked for Dr Joe Ehrlich at Hatfield.

There is a lot specialist -- topskilled -- work involved and the Bantam Racer has developed from many many hours of frequently secret work -- which in many cases was proudly shown to the rest of us ....

I agree with a Manual -- for the Wannabees -- it is handing the goodies to them on a plate but the fact is BSA Bantam Racing does need more new blood ...

Perhaps thde latter word is not appropriate....

Don´t despair -- Go for it!

Cheers!

John-Boy -- as the US Hill-Billy Series -- a yokel from the woody-wooly hills.

Thought for the day!

Trying to optimize performance parameters within a two-stroke engine is like trying to release a 10 digit combination lock, you won`t know if you have the solution until the last digit is entered; but if someone gives you 5 of the correct digits you will be half way there!
So, where can you find the 5 digits and perhaps even more?

Trevor

I cannot imagine that anyone noticed but I have taken a sabbatical recently. I felt that the complete lack of response to the idea of Terry Beckett Chat convinced me that I had only the past to contribute. I don`t really get on with the past very well being more for onward and upward but the concept of long experience with this simple two-stroke becomes irrelevant when the promise of technology will somehow overcome the need to think and understand. However, the question of the five digits set me thinking again. Only that there are no digits, only matters of importance in an understandable order.
My first thought is: Do not overcomplicate it. Try to reduce it to the simplest components. e.g. Take the D7 or whatever. What was it designed to do and at what rpm.
There is no doubt in my mind that the most important aspect of two-stroke operation lies in the transfer of gas into the cylinder and the things that influence that. So let us mould the transfers, measure carefully the taper factor they used and its capacity to pass the gas for the job. (don`t measure the window, it is the neck that is important) With that evidence to hand the obvious questions arise. What do we want it to do ? What % increase is needed ? Be modest. Settle for a figure from which you can improve. It is obvious that to pass more gas and faster the taper must be shallower and at the same time the capacity must be increased.
I`m not sure that I am on the right track. All I have promised is quite hard work, the need for tools and the need to think and grow a skill. Living in the past again. Why not go and buy the bits and a barrel/piston/head set off the shelf. I`m sure that is the way to look with an analysis of the bits and what they can do. Is there a need for another digit? Terry

I´d like to do one of them sabbaticals Terry? are they buchsheesh for OAPs? Learn a bit about the ONE I am due to meet -- soon....???

What one needs -- to go racing with a 3-speed Bantam is a nice list of gear-ratios according tyre OD and engine performance -- I had such a list with young Edward Pickering -- couple of yeras ago now -- that included tyre size (Dunlop Triangular -- I think!) and final drive ratios (rear & gearbox sprockets) but it would be out of date re modern tyre size and engine max revs but that could be easuly sorted.

Question about exhaust ports that always had me puzzled -- rectangular versus oval used by top racing marques virtually for the same job -- Any comments?

Cheers!

JayBee -- ex BRC Chairperson and almost completely ex.....................

I feel you don't need to re-invent the 2 stroke Bantam engine to go racing
and agree very much with Terry s sentiments. My old Bantam is the one that
reflects the Bantam Tuning Manual 54x54 Mick Scutt spec circa late 70s.
Its improvements have come with 1- Retard ignition 2- Exhaust to suit.

Its disappointing Terry, like you say people don't respond to posts but they
do read them and gleen info from that. Maybe that is the best we can hope for?

JB excellent piece in Motor Cycle News - Interview with Slick Bass where he
gets in the fact that he spent time in an oil drip (for safe keeping) at race
meetings while his father was "RACING A BSA BANTAM"

Thanks! Which week´s issue was the MCN, Mike -- don´t get it anymore and Slick´s too busy to tell me he´s being intrerviewed -- can order it here, dýer see...

At Cadwell when Slick was about 4-and-a-bit he somehow got playing with my burnt out clutch bits and virtually covrered himself in burnt-black oil.

Seems it didn´t change much when  Jimmy Wells had Slick as his apprentice and told me Slick was always knee deep in oil so he called him Slick as in Atlantic Sea, Oil-Slick... The lad when with Carl Fogarty told reporters it was because he was fast and accurate.... Particularly fast when away from the track ....

You are right  about re-inventing the Bantam Racer but a list of things like  gear-ratios and %s of oil-petrol and tyre types and places to buy fuel etc... etc ...would come in handy for the beginner -- or even the Old Hand ...

Power to Powell Elbow....***

Cheers!

***most unnecessary but it seemed time I said it!!!




John Boy....

JB its this weeks MCN 4th November.

Yep gear ratio's tyres etc etc should be in it, and a lot more besides, I'm meeting up with Derek at the NEC Show to chat about this and many other issues. Think Trevors done a load already...

Thank you, Mike.

Good -- Good-- on both counts, its time to get things sorted.

Thought of another idea: Daughter Kate ought to have copy she can send me. She´s also very busy with horses and singing but finds tíme her old Papa....

I hope we hear about the meeting ....

Take care.

John-Boy....

John, you did wonder about what an exhaust port profile should be, have a look at this; provision is made for maximum blow-down angle area and incorporates piston ring safety with the compound radii easing the ring into it`s groove. One more digit  towards determining the full combination?

Cheers, Trevor

Thanks Trevor!


Bit late -- I am learning a bit about tuning two-strokes. I rememeber:   back in the ' 60-`70s there were fully oval and fully rectangular exhausts ports (small radii in the corners) and it seemed then that the fully oval  must have had an effect on several aspects of the gas exchange and Blow-Down, I mean as the fully-oval  port opens and closes there must be a sudden 'squirting' effect which might involve Bounilli-Effect or something??

Did I miss something -- somewhere?

It all happens so quick though -- 200 times a second in some cases!

Cheers!

JayBee....

John,
      I`m not sure if the Bernoulli equations would come out right in the case of engine ducts, the combination of boundary layers, skin friction and such like interfere with pressures and might skew the results? But I sure won`t be getting the slide rule and log tables out just out of curiosity!

Cheers, Trevor

Sorry Trevor!

You are right -- it must be a hell of an equation -- or lots of equations ...

Thinking more about port flows  than I did when racing -- in those days I´d cock my leg over anything with a suggestion of power....

Its sort of a lot of squeezing -- ennit?

Cheers!

JayBee....

Just to prove a point and to show that even the factories don`t always get it right , have a look at this sliced through, scrap TZ125 barrel exhaust duct. The sudden area enlargement costs power; answer; bore it out, freeze shrink fit a sleeve in, hand dress to a correct outlet profile that reduces the duct volume and surface area and gain a couple of hp, at least that`s what the dyno shows, so you don`t have to just take my word for it!

Just the sort of reasoning and practical advice that might be found and explained in a suitable form of modern Bantam reading material?

Trevor

Thought for the day-2

Is the position of peak torque, within an engine`s escalating rev-range, found at the rpm at which optimum scavenging occurs; and, are there other variable engine performance parameters that have an impact, and is one variable more dominant than another?

Now, where could I possibly go to find the answer to this and maybe other questions?

Trevor

I can´t answer for the two stroke but with the 4-stroke the max torque would be made to occur where the engine was compelled to breath its best: hence we had variable valve timing, "tuned" intake ports and tuned-length intakes and exhausts.

The exhaust turbocharger was a different matter -- adds the advantage of bumping the torque by its "Torque Back-Up" characterstic of increasing torque with load-decreased speed.

There was a racing Jagaur ( cannot rember which XJ...) that had two peaks in its torque curve. Its torque increased with speed to a maximum and then drooped for a few more hundred revs and then increased again to a scond max, greater than the first...
Achieved -- I think by the variable valve timing and tuned intake and exhaust pipe lengths ....

Not sure this is what you wanted to hear --
-- rather, see!

Cheers!

JayBee.

On the contrary John, that accords very well with my take on the two-stroke engine. Where mixture strength, ignition timing and of course the exhaust pipe, playing its part in surcharging the cylinder. I guess the best scenario is when a combination of all of the variable factors are at their optimum efficiency. In the case of the Jaguar, the cylinder filling goes through two cycles where one is perhaps not as good as the following, by virtue of a variable valve timing that is then optimising the various static duct lengths and picks up a tad more torque.
Big advantage that the two-stroke engine has over it`s four-stroke counterpart is that the exhaust poppet valve opens comparatively slowly when compared to a two-stroke exhaust port. The 2t exhaust port cracks open a just about maximum speed, around 1-1/2 times the average piston speed, the implications for tuning the exhaust pipe lengths are heavily in the 2T favour!

Great fun trying to get the old brain cells to analyse it all though?

Cheers, Trevor

Right Trevor!

All happening too quick -- it is like looking  at the rivets around the door of a Jumbo Jet when boarding  and wondering how the hell this great chunk of metal and other material can ever get off the ground ....

Its like what happens in a 2-stroke cycle the air is persuaded to do nigh on the impossible  and we take it for granted that we never expected anything else...

I wondered how it sounds there in the crankcase when the ports open  and are trying to Gulp, Gush or Gobble at frenetic speed  ....

Oh heck! I am out of my depth, as the Bishop said to the Actress in the cellar....

Cheers!

John-Boy.

Thought for the day.....3

Triple cone exhaust diffuser effects from header end to start of centre section.

Partial Overview.

The dilemma we are continually confronted with is that the exhaust pipe is constructed of pre-determined fixed lengths, angles and diameters, but wave actions inside the pipe are transient and dynamic and temperature sensitive. As a consequence, the diffuser can only produce a working depression over a certain period of time, but engine revs are continually varying as the engine accelerates up and down through the rev band. Therefore the point of maximum depression created in the cylinder can move in relation to BDC as the engine speed varies. The boundaries set by the proportions of the pipe`s tuned length, that the header and diffuser combined make up, are rigidly set; but the resonance and possible exhaust port acoustic superposition are determined by the exhaust port timing and the actual tuned length dimension. Pressure fluctuations created have to be in sync with port timings, failure to arrange this leads to reduced torque and restricted effective power range!
By adjusting the cones it is possible to bias the efficiency of diffuser action to act where it is of most benefit within the engines design parameters; for instance, at the base of the power range, at peak torque or in the over-rev. It is then possible to change the diffuser suction curve by design changes, but it will always only ever be early or later in the cycle as the engine speed rises.
It might therefore be argued that transfer flow is dependent, in large part, upon what is “allowed” by exhaust flow?

Trevor

This short piece is part of the broader article I prepared some time ago in anticipation of the forthcoming Tuning Manual rewrite. The topic sub-title was intended to be: Deflagration to Detonation..... but it might just as well have been.... phfffffft to Ka-Booom, a more visual analogy of what might happen in the average Bantam cylinder for the unwary!

A combustion process should propagate in a highly turbulent, yet controlled exothermic reaction until all of the available fuel/air mixture is consumed, and be completed in the shortest time frame possible.

The temperature of the un-burnt end gases contained within the squish band is affected by a number of factors but the main ones are, ignition advance, effective compression and cooling efficiency. Fuel octane rating and lead content play a significant role but as everyone seems to use Avgas 100LL that then by definition is not a variable!
Effective compression is the sum of the static compression, as set by the volume of the combustion chamber at tdc, and the dynamic compression. The former remains a numerical constant but the latter is extremely variable, and reflects the actual quantity of mixture being trapped after exhaust port closure.
Dynamic compression increases with charging efficiency; more fuel/air mixture in the mixture per engine cycle – better trapping efficiency; more mixture retained in the cylinder per cycle, and finally, scavenging efficiency – less exhaust residuals remaining behind in the cylinder per cycle. A lot of different engines may have an identical static compression ratio but the dynamic will all vary, it is in the nature of the individual state of tuning! It is also in the nature of things, that in attempting to improve on one aspect you may impair the remainder. The synergistic sum of the parts will determine the final outcome, and it will almost certainly involve compromises.
As these three efficiencies increase so then does the effective compression within the cylinder, as the piston approaches tdc, become higher, more fuel is burnt creating more heat and pressure and thus more power.
The engine then reaches a stage where the end gasses trapped in the extremities of the squish band spontaneously detonate ahead of the advancing flame front. This is due to the chemistry of combustion producing free radicals (1) because of excess heat, and/or, uncontrolled pressure rise. Once radicals begin to form they become self-sustaining and cause a runaway reaction process, if left unchecked the engine self-destructs in very short order! Hot running, marginally air cooled engines will be far less tolerant of the sequence of events that can lead to and promote detonation than their water cooled counterparts.
There are options to control detonation; static comp can be reduced, and or, pegging the ignition timing advance back, and a jet up of the carburation to internally cool the engine with extra fuel, but this action will definitely cost power. As the individual engine spec approaches its theoretical limits of charging, trapping and scavenging efficiencies, it becomes important to achieve a balance between pressures, temperature and ignition advance in order to maintain the piston in survival mode!
There are some benefits that can be included in the initial engine design stage that can be used to cool the piston crown. For instance, appropriate axial and radial angling of the transfer ports can encourage the beneficial Coanda effect that keeps cool transfer mixture flow attaching to the piston crown, thereby taking heat away from the thermally stressed piston crown.
The mass of mixture contained within the combustion chamber at the point of ignition can only produce a `finite` amount of energy, this is commensurate with compression and ignition advance numbers. A good deal of this energy in the form of heat is transferred to the piston, cylinder and head and eventually into the cooling medium, be it air or water. So in one sense, this energy is lost and can play no part in power production, one down side to the whole thermal cycle.
It is by pushing and juggling the limits of all of these factors in some sort of organised synergy that is part of the `black art` of Bantam engine tuning that maximises power production, but that also demands reliability as well!

Trevor

The numbers in parentheses ( ) indicate a topic expansion in the 2016 handbook.

Thought for the day....4

The greater the burn duration, in both time and crank angle, to complete the combustion process, the longer the un-burned part of the charge pre-heats and pressurises its way towards detonation. So it follows then that the maximum dynamic compression ratio that the engine can tolerate must be low enough to prevent melt-down! Low compression, big bores using slow burning fuel, that by definition have long flame travel distance/times, are the worst offenders in this situation
The shorter the time period that complete combustion can be made in, the higher the compression ratio can safely be raised.
Traditional thinking would suggest that lowering the comp ratio would incur a drop in performance, but there are other possibilities to exploit that might mitigate this!
A larger volume above the rapidly closing exhaust port will allow for more useful charge, lurking in the port duct, to be stuffed back into the cylinder. As the expansion ratio will therefore be lower, the pressure drop after TDC will be lower, but the average pressure between TDC and Exhaust Open will be higher. That being the case then there will be more exhaust gas energy available for the exhaust system to work its magic on?

Trevor

Are you saying that given the right variables of exhaust timing etc that low-ish compression ratio isn't a bad thing

Yep, that is correct Dan, BUT, and there always seems to be question mark over everything Bantam!
The only way to liberate more energy from the available fuel is to increase the compression ratio, and that certainly is the case with Avgas 100LL.
With an increase in comp comes a higher cycle efficiency, and in particular at lower rpm and therefore lower cylinder filling; the 175 Bantam engine with its enforced single point ignition timing lacks enough advance to produce sufficient power low down. So if values for delivery and trapping efficiency remain substantially constant then the only re-course is to raise the static comp ratio. Conversely, if you can get the engine breathing more efficiently so the static component of the dynamic compression can be reduced with some benefit!

Trevor

The Closed Cycle

Is defined as the point from where the exhaust port closes until it re-opens.

I would like to start this article with a partial review of what I feel to be relevant examples of the dilemma facing designers and engineers in their struggle to achieve efficient combustion and which have resulted in reliable power increases, and can be taken as useful guides for application to Bantam thinking.

Way back in the 1950s Jack Williams of AJS was straining to extract more power from the 350 7R engine. By introducing an off-set inlet port with more downdraft the extra turbulence created in the combustion chamber made for a more complete burn. Next was the introduction of squish to energise the mixture prior to ignition, with piston to head clearance of just .025-.030in. In its final 1960 spec BMEP rose almost 14bar at 8,000 rpm.
Following this is the ground breaking 3 Ltr Ford Cosworth engine. Duckworth found a common thread running through the engines he had developed prior to landing the Ford contract namely that of getting a half decent combustion burn at high rpm without the excessive ignition advance that allows enough time for burn completion.
Combustion efficiency is the fraction of the potential heat energy of the fuel used, to that which is actually produced and released to the engine. Taking this a stage further Duckworth also included in his assessment, heat energy loss to the engine parts, the piston crown in particular, through an extended combustion duration. Also, he was anxious to limit the loss of efficiency with poor combustion after TDC with compression dropping from the descending piston.
Third and last of the examples is the old 3,000cc F1 engine revving that was capable of 20,000rpm. If we take the bore as 100mm (it makes the sums a bit easier) and an in-cylinder temperature of 400*c together with the fuel`s laminar burning speed of 1500mm/s, then with a 270* of crank rotation to fully enflame the 100mm bore, the max speed the engine will achieve is just 1,350 rpm! Such is the fundamental importance of turbulence during the combustion process, even with the historical time spans involved, all of the three examples have the common thread running through them that……..Turbulence is critical for efficient combustion!

Now it can`t have escaped your notice that all three of these engines highlighted are four-strokes; once the cylinder is in the closed cycle that makes no difference to the physics of combustion principles.
However, going in the opposite direction, we have the awful mistake that Yamaha made with the TZ 250G cylinder head. That cylinder head had an off-set chamber design with huge a huge 60mtrs/sec squish velocity area on one side and zero on the other. The end gasses trapped in the Mach-velocity area morphed to a supersonic compression wave at the slightest provocation, jet down or adjust the timing and melt-down became instantaneous.
It didn`t work then and it won`t work today, thankfully Yamaha realised their mistake and used the head for just a year! So even the factories can get it wrong, kind of encourages me a bit to know I`m in good company?
The sources of turbulence prior to trapping can be identified as piston motion, continuing all the way to TDC, the incoming transfer mixture streams and perhaps not altogether appreciated, the plugging effect of the returning exhaust wave pulse. At exhaust port closure, what is in the cylinder is all you get to play with, then as the piston approaches TDC the squish begins to have major influence.

A Bantam race engine having its combustion chamber geometry optimised for running correctly on Avgas, won`t survive very long if unleaded 100 pump gas was used in its place. So John A S was correct in suggesting that the type of fuel to be used should be the first consideration when designing the chamber profile. When racing the RS125 it was a simple matter to swap from Avgas to Optimax, drop the comp with a different head insert, richen the carb up and tweek the timing. Miss out one element however and a seizure was almost certain. A good RS 125 will overheat seize or detto with anything less than an NGK 10.5/11 grade plug, so no race Bantam should run with less than a 10 grade, ever!

The two fuels react in entirely opposite ways, so demand a different approach to get the best from them and for the engine to remain reliable.
In general, unleaded `hates` compression, but loves timing, and needs to be rich. Avgas, or any leaded race fuel, needs comp, `hates` advance and responds better when lean!
In a typical race scenario where we have average port/duct geometry, average pipe design, average ignition curve but using Avgas then we need comp up to 14:1 to at least to be able to get the stuff to begin to burn well. With unleaded, drop that ratio a couple of units. Whilst unleaded undoubtedly burns faster than avgas, which is desirable, the extra comp from avgas tends to negate that advantage, swings and roundabouts. But what is certain that faster combustion=more power, HCCI has no flame front and combustion is over the whole piston crown and so heat absorption is greatly reduced.
Notwithstanding all of that, if your engine cooling is poor, then fix it, fresh charge entering an engine should not pick up too much heat as that reduces its density, depletes its oxygen content and contributes to less combustible mixture being transferred, so makes less power.
It might also be worth stating that a compression ratio of 20:1 will be no problem to an engine making no power?

Whilst the chamber volume for different fuels will vary, the one constant is the squish function.
As a percentage of the bore, the band with that work every time for Bantam engines is in the 45-50% range. If you have an `Uber revver` up at around 14,000 then you will need to think again.
In any race engine the squish height needs to be zero! As that is not sensible for mechanical reasons then at maximum revs the clearance needs to be just shy of the piston clipping the head, with a touch of extra safety if you have a less than rigid crankshaft, and/or a heavy rod and piston.
This set up with high squish velocity up at around 30/35mtrs/sec increases turbulence, first and most importantly, in the end gases and helps flame speed. As detonation is visible at the piston crown edge, then if there are no end gasses, there is no detto!
Most of the time the thinking around squish is concentrated on its effects at high revs, but low to mid-range power also benefits from gas movement out of the squish band, a very handy benefit of helping acceleration, despite the rigid enforcement of a single ignition point on the 175 engine. To this end, on my w/c engine the squish height is set .5 mm and the bore and .7 at the junction with the chamber bowl, with no radius on the squish band edge, just a rub over with wet-n-dry to deburr. Micro eddies spin off from this sharp edge and contribute to the vigorous movement of mixture into the central combustion area. For any one the least bit interested you can google `Kolmogorov micro scale` and get the info on this?
In one part of the intro mention is made of F1 engine having a non-turbulent, quiescent, combustion chamber and achieving no revs. Likewise, how is it possible to have a 125cc race Bantam running at 11,000 rpm and achieve complete combustion in around 40 degrees of crank movement? As you have probably guessed by now the answer is turbulence. The rapidly ejected squish mixture is blown into the chamber bowl and strips of burning mixture are flung around creating individual fires in every little corner. The more complete the burn, the higher the flame speed, the better it all is for power. I now use a flat top chamber with a large corner radius blending to a small parallel section that abuts to the sharp squish edge, a bath-tub type of configuration. This configuration drops the plug closer to the piston for a similar comp ratio as the old hemisphere. The flame path is shorter and so complete combustion can achieved is a reduced time scale and ignition timing can then be adjusted to initiate firing just before TDC. Consequently there is almost no negative work for the crank/piston to do in fighting rising compression as TDC is approached.
A toroidal head can get the plug even closer to the piston crown and so shorten still further the flame travel from electrode to cylinder wall. They seem to be most effective when used in conjunction with a flat top piston and a tapered squish band diverging at a small angle. Lot of RS125 engines use them and do very well but the comparatively sharp edge of the flat crown to the skirt can be prone to detonate when the tuning envelope is pushed to the limit. Another down side is that the lack of a crown radius precludes the attachment of transfer mixture to the radius by means of the Coanda effect, and a lot of piston cooling is provided by cool transfer mixture flowing over the hot crown. For a comparable volume a true toroid will have a greater surface area than the bath tub type.

At TDC, the lower part of the combustion chamber is formed by the piston crown. If we calculate the surface area of say a 54mm and a 64mm pistons there is a considerable difference. The 54mm piston that I use has a crown height of 2.5mm and the 64mm has a 3mm height. The respective crown areas are 2310 sq/mm and 3240 sq/mm, so it is easy to see there is a large percentage difference between the two. Factor in the surface area of the remainder of the chamber the difference becomes significant.
The amount of energy available to the combustion process is finite, and a lot of heat energy is transferred to the surrounding metal (head-piston-cylinder wall). The larger that surface area the greater the heat soak into the cooling medium becomes. It is at this point that crank geometry comes into play. A 58mm stroke with a 125 long rod of 2.155 ratio, revving at 9250rpm is going to have a long lazy arc through TDC, there is plenty of time for that big surface area to absorb heat. The 54 stroke engine with a 110 rod of 2.03 ratio revving to 11,000 is going to swing through TDC in pretty short order compared to the larger engine. Then as the piston descends the much larger surface area of the 64 bore is leaching away heat at a greater rate than the 54mm bore, a 1mm strip of a 64mm bore is 201 sq/mm and the 54 is 170 sq/mm. By the time the exhaust port opens there is a considerable difference in wave speed energy for the pipe to do good things!

It is always difficult to separate out one single function in an engine for comment because all the events are interconnected, mutually dependent, variable and transient; from the atmosphere at the carb bell-mouth to the exit at the silencer to atmosphere, even the air its self is a variable.

Trevor


Most of the salient points highlighted in the above really deserve an article exclusively devoted to their speciality, so apologies for the brevity in some areas.