Terry Beckett Chat.

I`m looking forward to Mallory next week-end. Haven`t a clue what to expect. There was an expression "The pretty ones don`t go". At the beginning of the season, of course there will be a lot of pretty ones. I`m not sure what the Bantam Racing Club are fielding. Are they all 175s now? I used to prepare a 175 called the Golden One for the IoM TT week. I seem to remember waiting in Ramsey for the `fast`men, chasing them up to the hairpin and then racing them to the Waterworks. Great fun. Oh, by the way, when are the Bantams actually racing during the week-end? I will, of course, be looking for information about the specs in use to build up a general picture of what we are dealing with, that is if they will let me into their hard thought secrets. TB

Terry we the second race of the day on both days, so be there for 11am on saturday or 10.15 sunday? There are 2 races each day second race early afternoon probably....?

Thanks MJ. I`ll look you up if I can find you. John Colter and I will be there on Saturday. Thanks for the information. I looked up your many posts, lots of questions and real interest there.

Good Morning Terry,

It is good to see you are keen for the Mallory visit!

I am sure it will be interesting for both you and John!

The facilities have changed somewhat with more modern toilet block and race

control, but the lakes and wildlife are still around!.................

Best wishes for an exciting event!

Rex

My mobile Number is........... 07903 684530

Good Morning Terry,

It was amazing to see you and John at Mallory!

https://2img.net/h/s6.postimg.cc/uvdmxh9sh/Terry_and_the_lads_at_Mallory_Park.jpg

What a privilege for us all to have you both!......................

It will leave lasting memories for us all!

Thank you both for sharing those valuable gifts of priceless knowledge!

Have a Wonderful Day!

Rex

Looks as if it were a bit chilly -- except for the chap in leathers...


Whom was who -- or is it, which which is which?

Only time I went to Mallory it was a grass track. Sort of makes me wonder how I could possibly be writing this crud on here. I fell asleep on the way home on my Enfield Flea and awoke in a deep ditch. About this time -- 67 years ago -- I was driving a recovery truck in the dark of night in the wilds of Palestine with odd people ready to shoot at us just because we Brits were there.

How´s that for the Bantam Indiana Jones Conteast?

Cheers!

Go well and stay well!

John-Boy.

That'll be me in the leathers and my bike and spare motor, I took the heads off and we all had a good look with lots of ideas hints and so on banded about,

It was nice to meet new folks and chat about all things bantam etc, shame I'll not out again till lydden

It was a great and most educational day at Mallory, my first visit for many years. I enjoyed spending some time with the riders and mechanics and generally taking in a plethora of information. I must thank Rex and JC who were most helpful. My original idea was to produce a spec for a 175 Bantam that would provide a competitive base for novices and their standard machines. What I found was sophistication in a host of devices designed to keep the machinery safe and make it go. But 2strokes are only 2strokes after all and I was ably to look into cylinders, talk to people freely, examine contours and pipes etc. Tremendously interesting to me. Everyone was kind, helpful and open for which I was grateful. I was particularly interested in the cylinder heads and all that I saw were what I hoped I would see. This configuration plus the ignition systems in use I believe were responsible for MJP pointing out that the `pretty ones` do go even at the first meeting. It will be interesting to learn what c.rs are used. I forgot to ask.
Although there is a difference between looking and measuring there were things I noticed that are worth serious consideration and these, the business of exhaust ports, systems and pipes, I can talk about, perhaps discuss and chew over next time, soon. KISS

In the early days a young flying rider, Jack Machin, whilst doing inter race repairs, was kind enough to let me have a sight of his cylinder. I could have fallen down the exhaust port. So much for contaminating the transfer gas I thought. The value of that experience was lost on me until years later. The assumptions I had to make are reflected in the `thinking outside the box` that is the main stock in trade of the tuner.
Tuning: I am not an engineer. I had a small band with mixed skills, much like yourselves. My role was to bring assumptions/visions to fruition and sculpt them into existence. My advantage was experience with hundreds of two strokes for every use and the skill of a dozen or so riders over the years who made the assumptions acceptable truisms. Plus a strong propensity to question everything, to identify the doughnut and not get bogged down in the hole.
Any ideas I commit to paper are not tablets of stone. They are there for you to fit into your regime if you can use them. To me, the rider is key. It is what he wants that sets my objectives. (Sometime not what he thinks he wants) Some can win on anything but the vast majority need a wider margin of power and flexibility. As they progress the spec can be changed. I have had a host of experiences on this subject. I`ll chat about them sometime. I didn`t realise how much space this was taking so I`ll come onto the Exhaust proper next time.
Measuring: % of stroke and degrees. Fix your degree card to the crank and the pointer. Make sure the piston is at tdc. I used a micrometer for this located in the centre of the crown. Bend an old 10thou feeler gauge to the contour of the crown, trap it at the opening point and measure the length of the power stroke by butting up on the feeler gauge. Record the degrees after tdc. We then have power stroke expressed as a % of the stroke and and degrees so we`re talking the same language. Do the transfers while you`re at it. We`ll apply this to the exhaust system next time.

tb, to answer a question in an earlier  post regarding compression ratios ,

mine are in the 8.5:1 region. approximately 23cc in the head including the squish etc, as you saw on my engine that we took apart ,
I don't go to the 50% squish, mine is around 8.5mm wide when measured in a straight line , not sure what percentage that makes it, but I do seem to recall that if it were 50% then it should me something like 9.8mm,

I made it like this(50%) to start with but found it hard to get the cc's in the bowl part without having to go so deep that I'd need a spacer under the plug so I just came out in bowl width until I got the cc's I needed to make my cr 8.5:1 .

I note that rex and nick use a lot higher compression ratio than I do, something in the 14:1 type figures.

I, like a lot of us use neat avgas 100ll .

Dan,
Why not try a bathtub profile, simple to calculate, a rectangle and .25 of a torus (donut) the only difference, piece of cake on CNC lathe. Shorter flame travel, smaller surface area and the plug is down in the centre of the turbulent action provided by energetic squish keeping things nicely stirred up. Run a higher comp with safety and beef up the mid-range. Win, win!

Trevor

I bit late now I think, the metals gone from the head,

I tend to think(maybe over caution on my part) that high compression is the easiest route to a bit more power that has the highest risks to other parts

You`re an engineer Dan, make an insert and shrink it into the head, or have the option of changeable inserts, although heat transfer can be suspect with inserts.
A word of caution with higher comps, too much ignition advance can overheat things and also create too much negative work prior to TDC! Trouble is, too little advance and low comps help to drop the bottom out of the power band. Can be a tricky compromise?

What you need is about 28* of advance at the rpm point just under the base of your power band, about 12* at peak torque, retarding then quickly into the over-rev area, that puts more heat into the pipe to maintain it`s state of tune.
Oh, sorry, I forgot, you`re class is not allowed into the world of 21st century ignition? My mistake, apologies all round!

Trevor

Assumptions: We have, necessarily, to make assumptions about the way the exhaust system works and this how I see and have treated it. The port opens, the hot gas forms a bolus that, dependent on the way the exhaust opens, stays near the centre of the port mouth to exit. At 9000 rpm, say, this is a continuous stream but organised and still centred around the opening point. If the port does not have an effective opening time I believe the exhaust gas will not be so confined. Needless to say, the port takes the peripheral gas as well but it doesn`t have the power to attract the transfer flow. There are now two things going on here: The opening bang with have created a positive pulse that will be reflected off the rear cone. The effective evacuation of the gas is conditioned by the pipe design and its own outlet.
The Exhaust Port. The keeper of the power stroke, the bit that does the work, that I fight and compromise to preserve as you will see. Its timing, together with the transfer height determine the rpm and kind of power that other features can complement. So when and how it opens is of key importance e.g. Trials would need a simple oval port where the opening time is the effective opening time.(Although I understand they are higher revving these days) A road racer would have an effective opening time of at least 30% of its width meaning 30% of the with would open simultaneously. A careful chamfer would convince the rings that the port was oval. The gas would only recognise the opening time. The width of the port is whatever the physical features will allow, ring gaps etc. but as wide and a square as possible within the limits of piston reliability. A more pronounced chamfer at the port base is also helpful. So: It will open with a bang, not a cough, help to get the gas get organised quicker, send a positive message to the pipe and offer less resistance to the incoming transfer gasses. With this simple shape the piston rings are not overworked compressed etc. several times per rev. The fact that the port exit doesn`t equate to the port size should not surprise although it is expanding. The assumption is that it can`t be using it all. Just a word of caution. Have a care about the effective opening time. I just may bring your pipe on song for the first time.
We are not looking for rpm here but power and as much flexibility as the pipe design can offer.
Here is a test and some thoughts: Coat the piston top with engineers blue, scribe the port width and 30% of that width. Run the piston up and when it lines up with the 30% mark, stick your Vernier down and measure how much of the power stroke you have wasted.
Points to consider: 1. The gas is mindless hot or cold and needs every kind of direction. 2. Pipes are tremendously forgiving and terribly misleading. Get it right and the machine may be unridable. Think compromise with the rider in mind and race wins are possible. Next time, continuing the exhaust system; the pipe.

Sorry I`m a little slow in responding to some of the comments. The business of cylinder heads and the cult of the dreaded squish has raised it`s ugly head again. I`ll be pleased to chat about that so when I`ve done the pipe bit perhaps we can go onto that and cover Dan`s problem. I`m glad to see Trevor involved. I`ve read a lot of what he says and am really interested in the analyses that I could never undertake. KISS.

It is a puzzle to me why the 175 cannot match the 125s. As JC said, they have been developed over the years and the rules; water cooling etc. favour them. At present I have no access to a motor as raced with alloy cylinder, head etc. or the gear to measure it. But it has to be there that my interest lies with its obvious challenge. But, a two stroke is only a two stroke after all. They appear different but the principles are the same complicated by rules and valves of different kinds.
The exhaust pipe (expansion chamber). I repeat, the pipe is very forgiving and very misleading. This is how I see it. In the end, getting somewhere near with flexibility, will do the job. But it will be the port design and other factors which will decide the day meaning a machine that the rider can enjoy and preferably win with. My assumptions were backed up be the Production racers whose heavy pipes were a mass of convoluted silencing gear that had to remain standard and yet the motors would respond to port changes in a very positive way. But back to Bantams and reality. I expect the gas to expand and to control it I need a reasonably shallow primary cone to start the process. But there is something else going on. The pulse is travelling toward the terminal cone which will generate a higher speed return so to separate these differences I need a parallel section. The diameter is important and decides the choice of angles for the terminal cone. This parallel section can be adjusted by inserting an appropriate cone making the primary cone trumpet shaped if you like. I cannot emphasise enough that the reflective angle of the terminal cone should be calculated in the light of the rpm and what power band the rider can tolerate. The sharper the angle the narrower the spread potential. It will be worth a chat about `the power band` sometime. We still have to control the gas which is now pretty much a spent force. It is the tail pipe, surprisingly critical. The length is relatively unimportant but the diameter, if too small, can cause heat problems. e.g. I would use 22mm dia. for a 125. There are many drawings to go at and the 125 group are well catered for. For the 175 (186 ?) I would need quite a lot of information and the means of getting to it. I have no assumptions about the pulse. I only know they work in both inlet and exhaust and can improve the power output considerably. I also know they can work over a broader range of port time that generally expected and this places the emphasis heavily on the port timing and its application. I`m not sure what the silencing was meant to achieve but having quite a lot to do with silencing I would imagine the actual exit would be 22/3mm,say, and the perforated stuff about 24/5mm. There is a lot more to say about pipes and experiences with them. I`m sure it will come up again. Next time: Cylinder heads and compression ratios.

Here you go Terry, a short, visual explanation of current rear cone/tail pipe junction design. The inset image is a "de Laval" nozzle that exploits the interplay of velocity and pressure. It`s use renders tail pipe length and diameter non-critical, and nullifies any potential return waves from atmosphere as irrelevant. However, getting the best from this requires that all other design criteria are fully optimised!

Trevor

Love it Trevor. That is just what I envisaged. Terry.

Interesting: I did some serious editing of the bit on pipes when I thought it became overly long. I was talking of the tail pipe dia. "I would use 22mm dia. as a guide for, say, a 125". I then edited out the following sentence "How this pipe is attached to the rear cone can have a negative effect. Much better the tail pipe is shaped to go over the rear cone and be reinforced than it protrude into the cone or the weld produce an unpredictable lumen." Your drawing looks like a flexible tube possibly of heat resistant plastic. I`m sure it would be helpful to know. Many thanks Trevor. Terry.

Looks rather like something I made last year lol, of course it assumes that the gas is supersonic before it gets to the smallest point

Dan,
Particle velocity in a Bantam pipe will never achieve supersonic level. That might be achievable in GP level engines for a very brief period immediately after exhaust port opening. Supersonic flow takes all of this out of the realm of plain acoustics into that of gas-dynamics, we don`t want to go there!

The energy levels from the cylinder, post combustion, determine the level of driving force that propels the gas, it won`t travel faster than that force, in a 22hp Bantam that is quite low, relatively!
To be able to move gas in an engine you must have a pressure differential, if the difference is 1 then no gas can move. If you have a difference from cylinder to duct/pipe of 2 then the flow will achieve Mach1, the speed of sound. Achieving a higher pressure may improve bulk flow but particle velocity will not go beyond Mach1 and to try to do so will deplete energy that could be beneficially used elsewhere, flow is then said to be “choked”
Pipe dimensions, particularly lengths, are dependent on the individual engine energy level produced, you can`t capitalize on what you don`t have! Expansion in the main flow direction toward atmosphere will increase velocity, and perpendicular to this fills the diffuser body. Juggling the ratio of these two expansions determines the overall profile of the diffuser.


Temperature is one determining variable to particle velocity, the coolest part of the pipe will be from the largest diameter towards the tailpipe. It is easily possible that pipe temps vary 150*c from front to back. By definition then, flow there will be sub-sonic. The nozzle at the tail pipe entry will increase velocity of the gas to sonic level and the larger diameter just after will increase pressure, it is this that renders tail pipe length non-critical. We all moaned at legislators imposing silencers on us, but in fact it prompted the free thinkers to act.

What actual diameter the nozzle may be is determined by the amount of gas the engine produces, not directly on its cubic capacity. A tailpipe restrictor will aid the overall efficiency of the pipe but you need to have the engine optimized to get the full benefit. This will account for the similarity of sizes between engines despite the fact that there is a difference of 50% in their swept volumes.
The 186 boys, it would seem, have a long way to go yet?

Trevor

that's what mean Trevor ,does the bantam get to sub sonic before it gets to the restriction? otherwise there's no benefit I guess, as it leaves the restriction it speeds up, make you think about other inlets in the cylinder for example , do the same ideas move to transfers? and does higher gas speed mean more flow and mass? or just better mixing and extracting in the exhaust case.

Yep, you got it Dan!
If we look back at the days before silencing there was just a plain parallel tail pipe. Pressure waves would traverse the pipe hit atmosphere and return a pulse back along the tail pipe to mess things up. People were even calculating out the impedance of those tail pipes and equating that to a specific length. First, a silencer helped to dampen out some these acoustic waves and later, the insert increased pressure to oppose any remaining returning waves. One observer described these events as a bird flying against a storm force wind, it did a lot of wing flapping but got nowhere, a rather nice analogy!

Many years back whilst still racing, I tried a large diameter tail pipe into which I slotted various thin wall tubes, some into each other, in an attempt to find an optimum diameter. It did little good, I ended up at around 22mm. With engines then offering little power, the tail pipe fiddling was largely a waste of time, my efforts would have been better spent carving away at the ports! Pipe technology was in it`s infancy then and we had little idea of what we were doing. Doctor Blair, as he was back then, developed a convenient formula and off we went, or so we thought!

Transfers are different in that at port opening the mixture in the duct is inert. Again a pressure differential is necessary to initiate flow, and the case must be higher than the cylinder or there will be reverse flow or no flow.
A large volume, long duct will contain a large mass of inert mixture, a short, smaller area duct will contain less inert mixture. Subject both to the same pressure difference and the small duct will get flow underway sooner and with a higher velocity than will the large duct. However, the large duct has the potential to flow a higher mass at reduced velocity, but in a reduced time scale if the ports remain the same height for both.
It is also worth pointing out that pressure differential imparts only acceleration to the mixture, when the pressures equal out it is then that maximum flow is achieved, how long that flow lasts and it`s magnitude is down to the diffuser you have designed into your pipe!

Getting the balance right is the tricky bit and multiple ports and ducts help a lot, lots of port area with reasonable duct volume.

Trevor

Steady on now. I didn`t get to the transfers yet. It is a complex subject and I think it will have to be done in, at least two parts, as simple as I like to keep it. I was interested to hear mention of Dr Blair. From time to time I was threatened by graduates who were going to set up tuning businesses. I realised later that I needn`t have concerned myself. By the time they hit the ground they already had a trained incapacity with loads of theory but no practical experience. In this Forum I am dealing not only with engineers but people who have done the riding and lived through the experiences. It means a new learning curve for me because I have never committed any ideas to paper before and I am enjoying the experience. Terry.

Cylinder Heads and Compression Ratios. This is how I see and treat it.
The real effort is confined to exerting a downward pressure on the piston. With the exception of some of the 50cc racers(quite a class at one time) where space is prohibitive. I have never seen these odd shaped designs. I took considerable significance from the TZ 250, a pretty quick device by any measure. It had a clearance of about 1/8"
where we thought the squish should be and an apology for the chamber itself. Outside that I think the head should obey simple rules i.e. to compress the gas into a space that that produces maximum pressure on the piston. In our case I`m thinking of ,say, 9000 rpm at around 10:1. Just a word about the squish: To me it is a cult, everything centres around it and it is very dangerous, prolific in Bantams and 50cc specials where damage is common. The explanations for its use are so plausible. It keeps the area cool, it creates turbulence etc. and on and on. So we wind up with a head that has a top hat chamber that exerts pressure on a small central part of the piston. It loses sight of the objective almost completely for a whisker more power that may not be rideable. Happily, the squish bands I saw at Mallory were on the modest side as though the riders knew instinctively that wider spelled disaster. Then, what have I seen used and found to be effective and safe and where does the squish come into it ? The chamber should be triangular with gently curved, convex sides leading down to a modest radius and into a squish area that is only determined by how many cc you want the head to hold. The clearance is not critical but we want as much gas as possible squeezed into the chamber. Compression ratios are presented, from the mists of time past, as a No1 power producer. Fit a Todd head on a standard Bantam and the difference is considerable. In my book, the c.r. is not something to cling to as a power producer so much as how to keep it at bay while we get everything else, that really does produce the power, together. And how can that be measured? Ask the rider. The dynamometer can be very helpful in all sorts of ways, and I speak from experience, but it cannot simulate something far superior; the rider`s opinion and his sense of confidence on race day. I must admit I found the dyno intriguing and very moreish but of doubtful use out there except in the paddock café. Next time: Transfer ports.
Additional notes: I think the best approach to Dan`s problem is to find someone skilled who can fill the sides of the chamber and start again with something simple; as above.
Sometime I must chat about riders.
I like the expression `resonator` The Petter Harmonic as an example, is a static two stroke diesel which I found so interesting.
Also the story of the `Stainless Steel Exhaust Pipe. Fabulous. Terry