Change of subject for Phil

Hi Phil ,
Seems a long time ago that the three of us were scoffing cake and slurping coffee, down here at Twin Gables ,
a lot of water under the old bridge since that time ! As you asked , here it all is , long time ago though.
Porting mods .
As we had an established level of performance it was quite obvious that there was no need to be clever and try anything

radical .What was needed was to revert to the std 54 bore, and at the same time correct a couple of mistakes i made
originally.If, in addition, if i could squeeze in some improvements garnered from subsequent experience and knowledge
then so much the better, you never stop learning in this game !
The exhaust port.
After experienceing some smearing of metal in the ring area of the piston face and once actually trapping the ring
in it`s groove i decided to slope the top edge of the port down to the corner radius , by 1mm . The loss of port area was reclaimed
by adjusting the size of all the various port radii .This way, with the timing unaltered , specific time area remained unchanged
and with it, flow characteristics, and , the pistons contact with the port top edge is more progressive . So far all seems well, early days yet.
The inlet port .
Not much to do here ,a little improved streamlining here and there adjacent to the area between reedblock and port .
I did adjust the port area where , at maximum reed lift and at max power rpm , at that point , there was sufficient area to pass
the required charge mass . Or so i hope !
Transfer ports/ducts.
The ports nearest the exhaust had only the top edge efflux angle increased , by a small amount, all other outlet angles remained the same,
along with its timing.The other pair remained unchanged but for the rearmost outlet angle, this was squewed round slightly to direct
charge away from the rearmost single port outlet. A very tricky and intricate piece of milling was needed here together with a oneoff
cutter.! My old mate Big Mike pulled of a wonderful job inside the barrel on both sides and the angle was continued in the thickness
of the new liner. The rear ,single port ,was left unaltered .
Finally, the inner duct walls were profiled to compliment the outer profile to provide a correct transition from duct inlet, at the crankcase,
to its outlet in the cylinder. Again ,tricky to pull off to maintain symmetry of profile, volume and do in both left and right ducts.
I cannot over emphasize the crucial importance of symmetry of all dimentions ,angles and timing as being the basis for efficient scavenging
in the transfer phase ,bringing with it , good running and high,specific torque production !
That was the thinking , and the engine seems to go reasonably well , in the company of the other guys, so perhaps i`ve done no harm .

Hope thats what you wanted? Regards Trevor.

Good evening Trevor,
very nice post and well explained in simple terms ,i am still a virtual newbie in the bantam racing scene so am on an up hill learning curve .i have worked for many years within the motorcycle trade so have a good grasp on how to make a production engine fly but bantam tuning is a bit different.
I am also a great believer in symmetrical porting within the cylinder,not easy to achieve on a fabricated watercooled cylinder maybe you could expand slightly on your method of manufacture when building one of your cylinders.Are your tranfers built up of sheet steel and brazed on ? if your methods are secret i will understand that you won't want to disclose your design but if you dont ask you dont get.
I have only seen inside a few bantam cylinders and am amazed with the different designs.
My cylinder is an updated by Tom copy of what colin hall used with the exhaust over inlet and 3 transfers ,the limiting factor i feel of the motor is the inlet port but it always gets great comments from on lookers. dont think it will ever be a winner .
would be happy to have any feedback on my set up and wait in anticipation of some of your design methods.

Cheers Nick

Good Evening Gents,

Have to agree nick. Especially from a person who has no knowledge and experience like you guys. Very well put by trevor and easily understandable.


Eddie

fab post trevor what about some pics to back it up. !

regards Derek

fab post trevor what about some pics to compliment it. !

regards Derek

actually a sketch may be better to explain what you were doing and why

regds Derek

Hi , Nick B ,
I haven`t ignored or forgotten you , i will do a post on the transfer ducts , ports and some of the reasons what i do
and the thinking behind it all . Unfortunately everything takes time , well it seems that way for me !

regards Trevor

Hi Trevor and all,
While my motor is apart i thought you would be interested in these images, check out the excessive wear to the gear selector flag. A major gear shifting issue at cadwell , thats my excuse anyway.
look foward to any comments.
cheers Nick

Hi Nick,
Sorry for the delay , but here we go , and you`re right , don`t ask don`t get , you did ask and you got !
I`ll do this in a couple of installments , tend to get confused during a lengthy post , and the end result
can be even worse gibberish !
The gas flow into, through,and out of any high performance engine, in any direction, is conducted in
an unsteady manner ! The definition of which, is where pressure, temperature and gas particle velocity ,
in any duct is variable with time . Where flow is unsteady it is also said to be turbulent in nature, and,that
flow cannot take place without there being an adequate pressure differential to induce particle flow from
point A to point B . Without pressure differentials your carb wouldn`t flow fuel, aeroplanes couldn`t fly
and the wind would not blow. The greater the value of this pressure ratio, and also it`s duration, so will
be the greatest movement of gas and it`s velocity .
There are down sides to this , but we won`t get into choked flow at Mach 1 velocities just yet , and i doubt
it could ever apply in a Bantam engine anyway !
Whilst all the ducts in an engine are of great importance ,both from their flow potential and constructional
constraints, they are , simplistically ," in and out " propositions .The inlet can be a lot more complicated
if it incorporates a reed valve assembly , but i`m sure you get the picture .
The transfers , however , connect the , almost , non-dimentional world of the crankcase where it`s
volume and shape change with crank and and piston motion, to the highly dynamic conditions in the cylinder!
The temperature gradient , created from the cool conditions at the crankcase mouth entry and the high
temp at cylinder exit, can create wave reflections which can impare overall flow rates .

So , the challenge is to create a passage which takes the gas from the crankcase , tapers upwards, reduces
in cross sectional area, turns through, around ,90 degrees and directs flow rearward ,away from the exhaust port ,
minimising charge loss .If all of this was not enough, the transfer cycle must be conducted in a shorter time frame
than either inlet or exhaust duration and incur minimal resistance to gas flow whilst doing so . Quite a tall order ,and
very easy to get wrong , resulting in poor torque production.

Next time i`ll get into lengths, radii , angles, diameters and such like ,leaving the esoteric and on to the empirical !

Regards , Trevor

Good evening Trevor,
I look foward to your next thread on this posting and we will all be wiser drawing on information that you have gained through your own
experiences. I am a great in believer in proving a theory.
It will be very interesting hearing about your ideas on port and duct dimensions and the reasons for these, just out of interest do you read the piston crown patination to achieve your optimum transfer duct angle? or have you got a special method.
I hope you had i look at the pictures while my engine was apart, i suspect you spotted the yamaha crank webs, a great way to build a crankshaft
for those like myself who have limited engineering shop access.
Just to finish what are your feelings on the formula in general as the days of garden shed tuning seem to have long gone !

thanks for now Nick

All interesting stuff chaps ....

Nick what was your gearbox fault exactly at Cadwell ?
You might change your worn selector flag and it may still have the problem - thats all.

I think your engine is a real marvel - I've seen inside it in the past and I like the way the
transfer flow changes from Ex/In side(rear) to boost port side(front).

Whats your inlet port area ? is it the same as your carb diameter?

I'm trying again" second time!

Hi phill Love the photo's the crank it a beauty is it a TM.


The Barrel "WOW" I remember having a conversation with Colin many years ago he had actually been campaining the bike for a number of years then decided to go through a lengthy scientfic process of maths machines and making /measureing gas flow's on a bench etc etc, he was telling me in the pits about what he had been doing in the off season, he spent months using all sorts of sophisticated equipment, balancing the transfer, flows, directions, angles and openings, and got a very disaponiting minor improvement in torque, after all that hard work, mind you he won the championship on it a year ot two later or was it that year ? After doing that exercise not sure "Colin" ?

what a great little bike with fantastic history - if ever you decide to sell, you will have a que a mile long, they will be lining up to purchase this one of Bantam Racings most famouse little bikes with a championship win too its history.

Great news my dad's been released from hospital, he looks to be on the mend. he will I'm sure be reading this tomorrow, I heared they had to kick him out in the end, its said he liked the food so much, he even went back for a second time, hopefully he's out for good now.
I told dad he's not having the Brunel University Log to read untill he's out, I'm only up to 1967 and the bike is being shared in races between John Sawer and P.Rossiter, taking it in turns, its been measures at between 69 and 73MPh, they had a expansion chamber comisioned form some guy in summerset, not sure who yet, but roy Bacon has been helping John has mentioned him, seems he's giving advice, the expansion chamber has not turned up John has chased it but British rail have said they are looking for it? kniowing them they probably still are , -
Dad this mean I have'nt finish reading it yet ?, you get it "when I have". !!!!!!!!!!!!


best regards Derek

Evening chaps,
Mike , gear selection was becoming difficult, mainly the flag was not moving the sliding gear enough to engage top gear.
The gearbox has always been great but this problem was becoming worse.I must say that i have tried to put as much mileage on the motor as possible
to gauge the rate of wear as this is now the best that i have had the bike going.i now have a much better idea of service times and can
move fowards from this.
Inlet port area is very slightly larger than carb diameter this being 34mm.
Derek , hope your dad is getting better and thank you for your comments.
just to let those who dont know my motor was a simplified version of the Colin Hall engine, built by Tom in i think 2008 (havent got the log to hand)
It has now proven to be reliable and i am aiming for further consistant running for the rest of the season.
regards Nick