The Bantam Forum.

I would question the true octane number of AVgas you are able to get JohnSBantam -- from what I have heard it can vary according to where you are and which standard is being used by the authorities to measure the Avgas octane number. Two standards I know of are by "Motor" and "Research" test method: MOZ and ROZ where ROZ states 98 octane number for Super Plus petrol and MOZ says it ir 88. I have heard that AVgas is said to be in excess of 101 so that everyone is sure of being above 100 octane number for aircraft fuel.

Theoretically the difference of thermal efficiency against CR is that from 6:1 CR to 8:1 is 5.5% (51.5 to 57%) increase -- and 8:1 to 10:1 is only 3% (57 to 60) and similarly 10:1 to 12.1 is slightly less than 3%....
.... So going from 8:1 CR say, to 12:1 is about 6% improvement in thermal efficiciency which does not seem much on here but out there, racing is considerable.

I imagine that the latter with AVgas -- changine 8:1 to 12:1 CR -- becomes critical on correct Air-Fuel ratio at all conditions with the ignition timing being varied accordingly... Otherwise its bang.... ---- !! silence.

Now, way back in the dark days, just after WW2 we could only get Pool petrol which at its best was around 80 octane and CRs were around 6.5:1 with tuning to 8:1 CR being a big challenge. A source of pure Benzine became available and many of us bravely upped our CRs in excess of 9:1.
I ran on 9.5 :1 compression ratio with Pool mixed with pure Benzine(50-50) having >160 octane number even without TEL (tetra ethyl lead) which we guessed gave us about 120 octane number.... Several competitors (scrambles and grass) went higher on CR -- some with disatrous results which may have been attributable to weak mixture or too advanced timing....

I would have thought in your Domain methanol was permitted and then its 16 or 18:1 CR with no great difficulty. Going above the latter is where theThermal-efficiency V CR curve flattens out and its hardly worth going any higher....

Cheers!

JayBee....

Did a little googling.  Life is not simple! Following is lifted from the Shell site.

Avgas 100. The standard high octane fuel for aviation piston engines. It has a high lead content and is dyed green. There are two major specifications for Avgas 100. The ASTM D 910 and UK DEF STAN 91-90. These two specifications are essentially the same but differ over antioxidant content, oxidation stability requirements and max lead content.

Avgas 100LL This grade is the low lead version of Avgas 100. Low lead is a relative term. There is still up to 0.56 g/litre of lead in Avgas 100LL. This grade is listed in the same specifications as Avgas 100, namely ASTM D 910 and UK DEF STAN 91-90.

Avgas 82 UL. This is a relatively new grade aimed at the low compression ratio engines which don't need the high octane of Avgas 100 and could be designed to run on unleaded fuel. It is dyed purple.

Which would be more suitable for a two stroke - 'standard' or low lead?

Jimmie,
Interesting research you have done, however, the club rules state that the only permissible fuels are petrol and Avgas 100LL, no others are mentioned so are not permissible. By petrol I assume that garage forecourt or supermarket products are intended and not the rocket fuel available only in sealed drums from specialist suppliers.
None of that will prevent certain people from clandestinely using specialised fuels in order to gain an advantage over their rivals. Same goes for extra curricular gearbox parts that are reported to be appearing, again the rules are explicit, where only Snell type and exact copies of original BSA parts are allowed.
Getting back to fuel, perhaps the ethical and moral path to go down is to permit only unleaded petrol to be used and that which is available at garage or supermarket pumps. Oh, and unleaded is a damn sight faster burning fuel than Avgas and vaporises at much lower cylinder temperature and pressure, so what`s not to like!

Cheers, Trevor

'So what`s not to like.'  Old fibreglass tanks.

To answer JohnSBantam relative the contributions so far. If the Avgas 100LL is positively over the 100 octane number then I´ll say 9.5:1 Compressio Ratio is safe...

I say that in case you have an almighty big  blow-up and want to sue me John...

Fact is 10.5:1 would be safe if ignition and carburration variations were adjusted within the limits of correct combustion. But what about the Bantam >175 formula ? -- and its cruder ignition alowance  than the 125??

There is a microprocessor control of A/F ratio and ignition timing  correction relative combution & exhaust temperature that ensures reliability -- but is such allowed within the BRC Formulae for both 125 & >175 ?

The nice thing about it is that it can be programmed to momentarilly weaken the mixture to gain a small increase in power and then richen to ensure piston seizure does not occur....


OK -- so I am out of touch and over-age by a mile but I am still interested in what you Sprogs*** say!


Cheers!
JayBee....
***I am NOT going to use American Lingo and call you Rookies and I would never use GUYs ....

Trevor Amos wrote:

None of that will prevent certain people from clandestinely using specialised fuels in order to gain an advantage over their rivals. Same goes for extra curricular gearbox parts that are reported to be appearing, again the rules are explicit, where only Snell type and exact copies of original BSA parts are allowed.

But if people modify original BSA parts to make the gearbox more reliable they are not copies.

rules.... rules... rules...who decides?.... bsa bantam built with a cast iron 2 transfer ports one exhaust one inlet barrel. 3/4 speed box.how many race like this?  this is bsa bantam racing... anything else is a special... different class.... oh dear what happend? or is it just me?

and also im not happy about Tim Rice being dj on bbc2 radio on saturday mornings, sounds of the sixties just aint the same...

nigel breeze wrote:

 this is bsa bantam racing... anything else is a special... different class.... oh dear what happend? or is it just me?

You need to back a long way for the answer Nigel. The rules change to make the formula more attractive and for availability of parts. Anyone that thinks what some of the guys are doing with their gears is against the rules has the right to protest but they will end up with egg on their face.

thanks Trevor.
I am specifically interested in Compression Ratio for 186 which is a 64 x 58 cast iron barrel with Todd head. Tho could use a NED head.
My successful Jawa mate tells me to ignore CR and look at a cranking pressure up to 170psi

Have got 4 speed close ratio gears sorted, can easily use in D14 die cast cases, but thinking of modifying D7 sand cast so we can have bigger transfers with alloy barrel.

JayBee:
Very interesting; I am running 12.3:1 geometric CR with 104/110 leaded AvGas in Modified Class.
My experienced tuner likes this for consistency and readily available high octane.
Whats more. from local airfield it is a lot cheaper than FIM 100.
In Clubmans class I ran 98 pump gas in 125 with 12.4:1, later 14.3:1 with "pumped" Race Gas (outdated Av, no water content guarentee) off forecourt.

Could use Methanol, but watched learning curves of very fast fellow Bultaco / Ariel Arrow owners and many holed pistons, whilst I came second two years in Champs with slower bike finishing every race with no rebuilds !
health issues concern me a bit too.

I work under New Zealand Classic Motorcycle Racing rules, so not constrained by UK rules.
I would like to remain friends with most English Bantam racers, so I wont stir, fuel / gear issues !!!
My 25/22T 3speed top gears are direct copies of Snells.

(e)  Only BSA close ratio and BSA normal ratios gears or exact copies (e.g. AA Snell) to be used. Maximum number of gears: three if close ratio, and four if normal road ratios

VERSION 01-12-04 (MODIFIED)
Rules checked by: A.F. Moulden 01-12-04

The above is the current Bantam club ruling concerning the type and number of gears permitted, this was taken from the published rules governing racing specification.

I guess the elephant in the room here is the “exact copies” part of the ruling. I interpret that as meaning conforming to the original BSA design and production specification, and there are such drawings also on the site that can be used as a comparison and therefore as confirmation of any deviation from the exact, original specification!
How much modification is allowed within the context of “exact copies”?

Some years ago George Harris had a batch of main-shafts made up but this was done publically, was discussed beforehand, and they were available to all, for the benefit of all.
As for being covered in egg, well I guess no one will worry too much about that if what is going on, if anything, is revealed. If all is legal then fine, no harm done, if not, the miscreants will be exposed. Either way, following a successful protest and subsequent engine strip, the other competitors will get to know what is going on, and that can only be of benefit to them all.

Alternatively, and to coin the modern parlance, all of this may just turn out to be `Fake News`, time will tell!

Trevor

What is the correct method of measuring cylinder head volume, with sparking plug or one off solid threaded plug. I know it would make little difference either way. Just interested.

Hi JohnSBantam -- I'd say your tuner has got things right -- carb's  A/F and ignition timing. The walsh Bantam -- if I remember aright was on 16:1 CR using Methanol ....

What Ned says about cheats ought to have them feeling rough but I guess cheats never have any of that in their grey matter .....

My first ever road race was on Icarus-1 Bantam at Snetterton -- when Icarus-1 had a 150cc engine --  and several people said enter the Bantam Novice race with it 'cos no one will know and you won´t win anyway.....

I said, "But I will always know..."

During the 250 race in last position I wished I had done just that and then on the 5th lap I passed a 250 and finished 2nd from last.

It is stuck in my memory that I beat a 250cc machine  to the line in my very first road race on a  MC with 100cc less capacity.......

OK -- so that´s nothing to crow about -- but I like it because I didn´t cheat when I could have done with ease....

Like I said a dozen times on here already, Les White our team´s machinist found our 125 Bantam engine in a field. He saw kids playing on a heap of rubbish and having seen the engine at first passing went back when the kids had gone off to bed. Derek Neil tuned the motor and I bought CR gears at various MC oulets between Barking and Leyland (during several months of Liason with Laeyland) for shillings a time and we had a forty quid (plus a little bit) racing Bantam with an extra 1.5 sets of CR gears and cadmium dipped frame. The latter by appreciative Leyland Motors mechanics.
Leathers and boots at 25 quid ---- it WAS REALLY " Racing On The Cheap" as the Club Founders stated.

Cheers!

JayBee....

john bass wrote:

What Ned says about cheats ought to have them feeling rough but I guess cheats never have any of that in their grey matter .....
.

On the contrary John I never said anyone was cheating. I'm pretty sure I know who Trevor got this information from. The guy is obsessed with the thought of people cheating and once again he is wrong.

Never mind the cheating!
What about thoughts of my compression ratio ?
Would I benefit from using Elf 2T or VP C12 oxygenated fuels ?

John S,
           Your Jawa friend offers good advice in suggesting to you not to concentrate solely on C.R but to do so on cranking pressure. Or as I like think of it as the Dynamic C.R, which is delivery ratio, trapping efficiency and static C.R combined. As the two former elements increase with engine and pipe tuning the less static comp the engine will tolerate, nor is even desirable. Mind you, an engine could run at 20:1 if it produces little power! Racking up high static numbers just leads to an area of diminishing returns, however at lower rpm where there is less cylinder filling a high C.R undoubtedly helps. As indeed will increasing ignition advance, but a retarding ignition system is obligatory to keep the engine safe at higher rpm, so as always with Bantams, compromise is necessary.
Smaller bores of the 125s will usually tolerate higher compression when compared to their bigger bore counterparts, part of the reason being one of scale. For instance, doubling a cylinder`s linear dimensions of bore and stroke gives four times the surface area (which is where the ports are of course) but, eight times the volume to be filled, so in relative terms the port dimensions are halved in size!
What an engine can survive heat wise is determined in large part by the effectiveness of its cooling, iron 186 engines only enjoy what is ostensibly a 125 cooling regime and too high a comp ratio brings the risk of thermal melt down so much closer. Cooling the large surfaces inside that engine, including the cylinder head surfaces, with a con-rod length that dictates a lazy, slow rotation around TDC, allowing plenty of time for all of those surfaces to absorb combustion heat, which together conspire to frustrate thermal efficiency.
Fresh fuel entering the cylinder must not be allowed to heat up too much and the large surface areas of the 186 will do just that. In doing so gas expansion takes place therefore density is depleted and potential power is lost. Add in the likely hood of that over-heated charge bringing the combustion event closer to the critical detonation point then remedial action is needed.
From what I see of the ratios you are currently running on your various engines you seem pretty well on the mark. As you wisely point out, reliability always takes precedence over a fast but fragile and unpredictable engine. Tweeking is of course always possible but you seem to be doing just fine as you are with your considered and intelligent approach.
One thing I always encourage is to look at getting as much of the air flow that enters the open front of the fairing to actually impinge upon the cylinder and head fins as you can. Air is funny stuff and it always, always takes the path of least resistance and that is invariably not through the engine fin gaps.
Air also has mass and follows the Newton laws that it won`t necessarily go where you want it to unless acted upon by another influence. Worth thinking about!

Should you be interested I have some first rate information outlining the advantages and pitfalls of racing with methanol as a fuel, all garnered from hands-on racing and dyno work, you did mention that seems to be an option available to you. I could get it into a coherent whole and post it up, may take a few days though.
The dangers to health are very real from the adverse effects of methanol, so yes, full safety gear required.

Given the choice of fuels, VP C14 or C12 every time.

Cheers, Trevor

I wanted to post this article published by, The Bantam Racing Committee 1983, written by Mick Scutt, as a piece of first rate information when regarding compression ratios on Bantam engines. At the time I had my wife laminate the article along with the heading title: `Scuttie`s Dictum` and hung it in my shed as a precaution against over zealous elevation of compression.
Sadly I forgot but here it is now!

Trevor

What a treat, you don't get this sort of intesting info on facebook.

All the best,

Les2012.

After 38 or 39 years I met up again  with Mick Scutt at Cadwell`s  last meeting  of 2012 (or was it 2011´...?). He was in his leathers again and having another  go at bantam racing...

What the devil happened after that ...?

I heard he had fallen off and hurt himself at some poky little circuit but since nobody bothers on this site with things appertaining to racing and riders we --I mean me -- never get to know.

My most fascinating memory of Mick (how this sounds -- is awful --  I hope he is alright?) is when Robby Winston and Mick came to my house in Barking at --
--- like! Wait for it... -- 21.00 hours (with our kids in bed, hopefully asleep!!)  and asked if I´d do a bit of welding for the TT: -- reposition footrests and install a reserve battery carrier ... etc ... etc...

Mick had sat in the racing posture for the new rests position  and was then squatting behind the bike studying  it for
reasons known only to him whilst Robby and I were at the bench. A good ten minutes went by.  Mick was still in the Squat -- sort of like he would be when racing on the Island -- and later on still.  he was still there. I said to Robby I thought he was doing a lap of the TT in his mind and Robby told me, No, he´s been working late on the bike for days now,  and he is asleep.
  After the bits were done  Robby & Mick drove off to Liverpool at about 02,00 (a.m.)....r

I heard that Mick had not completed that TT because of a technical fault and wondered about my welding....????

If you read this  Mick, and I hope you do:  what's been happening???


Cheers!

JayBee.

Tuning.

As Mick said,  a bit at a time.. Too many I knew of,  tried  to do too many changes at once. Doing that has a complete new animal to tame.

Make one change at a time and test under the same conditions otherwise it is a test result worth nothing. I wonder how many amateur tuners never correct their results to a standard. Its oK if everything is done in one day  within a few hours of each test but as we all know the weather conditions change day-to day snd humidity differences-- particularly -- can make a minor  increase look so  significantly large a change is made that  should not have been done..... I know of an instance -- Lusaka, in Zambia,  and - where an engine run on the dyno had 125bhp at 8 '0' clock in the evening and  at noon next day it had 119bhp  -- 5 less....Much like Leyland engineers scratching their heads when they metricised thie 0400 engine they lost a similar  5 bhp -- to going metric -- and never found it again.

An interesting result with a race bike on the dyno was the loss of revs and power until the float-chamber was lifted above its normal road-running level. Seems their were vibrations on the dyno that did not exist on the road and the petrol in the float chamber was foaming and when the chanber volume was increased the A/F ratio came back to what it had been and the lost power was restored. At least  -- that was the "Educated Guess that was acceptable....




Cheers!

JayBee...
Boring Old F...

Jimmie,
No one else seems to have picked up on your query so you are stuck with me I`m afraid.
What I have always done with my race engines when checking CRs, is to lightly smear Vaseline around the ring area to create a leak proof seal. With the head screwed in place, rotate the engine to TDC, carefully inject very thin oil, or whatever option you fancy, from a 20cc syringe until the liquid reaches the top of the plug hole. Read off what has been expelled, or is retained depending on you starting datum, and subtract 2.3cc plug volume from that figure. If you are using a roadster plug then the inside of the plug will have a tiny fraction more volume than the NGK 10 grade plug I used. Another point to bear in mind is that my method assumes the plug lower face protrudes 1 mm into the chamber in its working mode, which is the correct position for a race plug, at least according to NGK it is! Also, be careful to allow air to escape as you inject the fluid, obviously trapped air bubbles will provide for a false calculation. Hope this works for you ok?

Cheers, Trevor

https://i.servimg.com/u/f58/19/61/73/03/th/20170216.jpg


Interesting how the discussion has moved from the relevance of the Forum to cylinder head compression and fuels! I think that shows how this forum gathers moss as it rolls along. Only my view, but Facebook and Twitter don't add value in the same conversational manner. I thought I might add a few words about fuels and the above photo came to hand. This was my first race (on the Bantam pictured) in 1970 at Cadwell Park. ( Please note new leathers ... that shine didn't last long!) I didn't need to think about fuels then. Most of the circuits had pumps, and you just trusted them to have petrol suitable for racing! I think most people expected the petrol to be 100, maybe 101 octane, and you filled your can, measured out your Castrol R as per the above photo, shook it up, and in it went!

In 2014, after a long break from motorcycle racing, ( but with the diversion of a jump jockey's licence in between ), I said hello to the classic racing world, bought a Greeves RDS Silverstone, which was the bike I last raced in 1973, and got back on track via a few parades. However, the simple world depicted above had changed, and I had some new things to think about when it came to fuels, such as unleaded petrol, synthetic oils, ethanol, Avgas variants, race fuels and race regulations, to name but a few!

My 1966 Greeves works manual says " petrol/gasoline should preferably be 100/101 octane and not below 98/99 unless the compression is lowered. The total lead content is important and must not be above 3.2 grams/imperial gallon or 2.6 grams/U.S. gallon." Even in those days, Greeves understood the protective, but cloying, properties of lead! In addition, I intended to carry on using the original fibreglass tank, and with all that in mind, it seemed that the use of ordinary unleaded pump petrol, at 95/97RON, and with 5% ethanol was not what Mr.Greeves had in mind!

How I made my fuel choice is possibly worth another post, but I've just had a fasciectomy performed on my right hand and my finger tips are saying enough. ( Please don't post any photos of that little op please!)

Tim Marlow

Trevor, many thanks for information. Somehow it reminded me of my innocence (stupidly?) as a teenager in the early 60s when I rode an Enfield Bullet. A friend picked up a seized two stroke and asked me to free it up. Engine on bench, plug out, diesel in. Why are my trousers wet?

Right again Trevor...

Something to do with fertile minds -- or what? Not that the Facebook founders were mindless, they never stopped laughing to the bank....

In searching for some of my past I once got on a web site that involved people in South Wales wishing to be objectionable and I was glad of being able to shut them out. The Welsh I knew, when in the army school, were lovely people. The ones referred to above, probably were not Welsh.

Poddy Phillips sent me an e-message on this site and all I got was his name. Now Maurice, if you read this please
tell the people on here what it was like to belong to the BRC with 660 members -- some of them never paying their membership or race entry fees and how we had a fantastic committee meeting in The Hoop & Grapes Holborn ...
every hic! -- hic! --- and hic! --- hic!
.
Cheers!

Jay-Bee ... snore!

[quote="les2012"] What a treat, you don't get this sort of interesting info on facebook.

Ain`t that the truth, Les

Cheers Trevor