Crankcase compression debate

Traditionally, bantams have always run on high crankcase compressions.
Myself and Robbie agree to disagree! as I run an exceptionally low crankase compression with no particular issues.
What does everyone else run?

Hello again ,
To begin this investigation , or debate , it might be useful to , numerically establish, just what high and low really mean ?
Please don`t bring the disc valved Aprilia into the equation , it is so specific and specialised to it`s own class ! I don`t
see too many disc valved , six speed Bantams on the average grid !


Ah , the highs and lows of Bantam racing . regards Trevor

Hi Trevor,
Always difficult this one... I run a set of 100mm diameter crank wheels in an open crankcase ie: no stuffers at all.
If there was any special advantage I would say that it was the space below the crank that can easily contain a wrecked big end without it getting stirred up into the top end to create further damage!
But I doubt if few people racing at the moment have any actual figures as it is a swine to measure!

Cheers,
Alan

Alan , and all ,
Am i correct then in thinking then , that your low ccr is a damage limiting exercise and is not a specific tuning philosophy ?
I assumed the question related to potential power production and to the merits , or otherwise , of high or low ccr`s and
to perhaps establish a realistic ," Bantam " , concensus for others to follow

Raw numbers like , 1.5-1 or 1.3-1 , remain just that , they make no reference to the actual amount of combustable gas
that is actually induced or transfered . However , i would offer this , a big volume needs a longer time to fill and empty than
a smaller one given that both are subject to a simmilar pressure ratio , and it is only differing pressure ratios that moves
gas in an engine , and this can be both a good thing and bad .

Bye for now , Trevor

Hi Trevor,
You are long way in your belief! I have always said that as "genuine" crankcase compression only takes place over a period of around 20 degrees (dependent of course on your transfer and inlet timings) at lets say a peak rorque of 9,500 rpm then the actual compression takes place in 0.0035 of a second.
So a change of compression from say 1.1 to 1.3:1 has a very small order effect indeed in any direction, so my argument is why do it anyway as it adds up the mechanical likely hood of a disaster!
Cheers
Alan

Alan ,
I am sorry , but i don`t understand you opening sentence ?
Inertia , it is a long distance from the nether reaches of your caverous crankcase to the to the combustion chamber . It takes time to move a mass of
gas and for it to achieve optimum velocity , and by your own calculation there is precious little available , there by , compromising potential transfer .
Better to have the charge available close to the transfer ducts , and then to transfer more in less time .

Catch you again later , regards Trevor

Trevor,
Your statement "Am i correct then in thinking then , that your low ccr is a damage limiting exercise and is not a specific tuning philosophy ?"
I state you are long way in your belief..
I do not use low ccr as just a damage limiting excercise, but as part and parcel of my tuning, be it different to just about everyone elses version of tuning!
Hopefully others will put 10 penny into the cup?
Cheers,
Alan

Hi all
yes have been playing with crankcase compression and I feel its the way to go. the more you pack out the cases its seems to go better ?
how I see it if you take a pint glass it take a pint to fill it then half fill it with packing and it will only take half a pint
So is this why my bike was so crap at the bottem end at mallory at the weekend ? just need to reset up the carb now need less gas ?

evening, i believe the pivitol point of either high or low crankcase compression ratio is the blow down period. the ability of the exhausts ports size and opening time period to expell the burnt gases and lowering the pressure in the cylinder, before the transfer ports open. if you could achieve fully removing the burnt gases and lowering the cylinder pressure well below that of the crankcase / transfers wouldnt it be better to be pulling a new charge into the cylinder from a larger pool of fresh charge .

sounds like I got it wrong then???

Not so sure of that Robbie .. that you are wrong, I mean --

-- the volume of air-charge drawn in as compared with the swept volme of the piston is Volumetric Efficiency which improves with resonant pipe and port tuning (á la Nobby1 comment). This volume of air charge gets more compressed in the crankcase according to how much the packing is increased...
In other words the packing does not reduce the air charge -- just squashes it more....

Dunnitt??

I´ve explained on another topic about why I went AWOL -- was virus bombarded and hoping no one else on here has suffered the same.
Cheers,
JayBee.

Hello Robbie , Nobby and all ,
It is difficult to argue against your experience Robbie , when , reducing the crankcase volume of your engine provides a performance
boost . Is yours a 175 , if so , with it`s long stroke and long rod every little helps . With the big 186 , the stroke was greater at 62mm
but the rod was 116mm and all the gas volume was above the crank discs unlike Allan who has a large volume under the discs ,
one feature of which comes under the influence of crank rotation , it being difficult to move gas upward against the spinning discs .

Spot on Nobby , but then you usually are ! Most high speed engines become blowdown t/a deficient at peak rpm , again it`s the old
enemy , time ! If at transfer release the cylinder pressure still exceeds that of the crankcase flow reversal will take place and untill
pressure equality is established transfer cannot take place . The consequences of this will be reduced power as a result of shorter
transfer timing , simply less time , less gas flow . Far more serious is the very hot exhaust gas pre heating the fresh charge and
diluting it with non combustable residue . A very real consequence of preheating is the rapidly elevating temps can bring on deto
and/or holed pistons ! There`s loads more stuff to factor in , like , what are the effects of ccr at the very base of the power band .
low rpm , dodgy pipe influence , and so it goes on !

For those with the inclination , you could equate delivery ratio against against the ccr to determine just how much gas actually
occupies the case , and then set that against the combustion chamber contents . You may be in for a supprise !

Stay cool , regards, Trevor

I seem to remember a conversation with Mr P Tibbets during which he suggested crankcase pressure had a very small effect on overall power and was something you should not be concentrating on as other areas would provide a bigger overall gain.

I may be doing him some injustice as it was some considerable time ago but if he looks into the forum would be good to have his input.

Mike

Just spent an hour or so trawling through some computer files and discovered the following info . all are from the two guys responsible
for the design and development of the last of the Aprilia engines of 2008 , the year Jan Thiel left , so if they say it is so then they should
know . The comments are of a general nature but some are specific .

Filtering out the nuggets , ccr in the context of what would be "normal " then the full range is 1.3 is low , and high is 1.4 . the last rsw
engine with a 120mm con rod was in the high 1.2s . The case volume at tdc was about 650cc , depending to a small degree on the type
of cylinder and pipe combination used . Jan Thiel also suggested that too low a case volume could be detrimental but doesn`t offer a number .

Now to an out of topic snippet , but fascinating all the same .
Jan removed the complete exhaust system from an engine in dyno room , which had just shown 52bhp at the gear box , re jigged the jetting
and ign , ran the engin up and got.......... 17.5bhp !

Hope that informs and entertains , Trevor

Well done Trevor -- numbers at last! I was just going to say, despite the high & low references that have been made no one had put a number on the Primary Compression Ratio as Roy Bacon calls it in his, "Two-Stroke Tuning" book -- and you have....

Roy states the primary compression ratio should not exceed 1.5:1 as above this point there is little or no power increase to be gained.

I have seen that some recent Bantam engines have nearly 2:1 ???

Well remembered is another of the Founding Fathers of the Bantam Racing Club -- á la our Roy Bacon-- said it was wrong to exceed 1.2:1 ccr and 9.1 above (that´s closure of exhaust port calculating...) because of pumping losses ...


Cheers!
BOF is back!!

we measured this with an old piston with a hole in the center at btm dead centre and top ! it was 454cc at btm the bike was i think piston ported at the time, "from memory" several attempts where made and it was diffficult to do as the oil overflows from the transfer windows into the cylinder at BDC, easy to forget this,
also into the inlet and out the barrel, ! if not enough "thick grease" is used to seal the piston, I think there is a formula for odd shapes - "resonance of a flask" ? im not sure.

the transfers could be measured by filling the transfers with plasticine and then removing the plasticine and using archimedies theory to calulate the transfer volume..also ive read recently that pumping of the engine is only required to start an engine and that the combustion/exhaust process of the engine takes over there after..

...second line of Page 2 of Roy Bacon´s "Two Stroke-Tuning":-

"...Any increase (Crankcase - CR)also raises the pumping losses by a cube ratio law, that is, if the ratio were doubled , the pumping losses would increase eight times..."

So, if the CCR was 1.25 the pumping resistance would increase by 1.25³ = 1.953 times ...
............ " " 1.5 " " " " " " " 1.5³ = 3.375 times...
...and ......... 2.0 ...........................................................2.0³ = 8.0 times....

Thus going from 1.25 to 2.0 would mean 4 times the piston pumping resistance to crankshaft rotation.

This also happens with the final compression ratio where increase of piston pumping losses (piston pushing against compressed air-fuel charge on compression stoke ) is much higher than the CCR ....

So, in simple terms (I hope) the low-compression-ratio tuning has a lot in its favour [which Slick in the Isle of Man prefers***] because the port shaping in conjunction with pipe tailoring can give a dynamic compression-ratio (because of improved gas flow) higher than the calculated geometric (or Static) compression-ratio. PLUS the engine will run ón to higher max revs with a lower CR....

But, and this is a big BUT -- if the engine is to run on on a high-octane methanol fuel with obviously high CR the lower peak pressure of combustion and improved thermal-efficiency (power output) negates what has been said above. Much as in the case of the Walsh Bantam ....

Apologies for being so long-winded about a simple thing you all know....

***Slick told me his crteria is 7.5-8.0 CR for two-strokes he has tuned.

Cheers!
BOF.

= Boring OLd Fart!

Hi Alan

From what you have described of your CCC (crank case compression) I would expect an engine with a pronounced power band at high rpm. Low ccr will produce an engine with low gas speed through the carburettor at relatively low engine speeds. But to too high a ccr intaduces increased pumping losses at high rpm. Many people chase high CCC at all costs.
Alan I believe the engine you have described will have very little mid range. Too high CCC will squash any overrun. As with everything in a 2 stoke it depends on your own particular engine but extremes of CCC will never give good results.

Mick.

Do you think there is an ideal ccr and should it be different dependening on what type of induction you use?
Be it :- piston ported, inlet tract reed, crankcase reed, 24/7 valve! and the disc valve?( N.B. remembering we
cannot use a disc). Must admit its not something i've measured recently. Mike

Hello to you all ,
I popped the no pipe Aprilia item in to see if any one would run with it as it does dovetail nicely with what Nobby was suggesting
that , after starting the cylinder comp and pipe take over . That engine had no pipe and modest combustion pressure , so what
driving force got enough gas into the chamber to make 17.5 horse ? In the days of our long stroke iron engines that power level
could have won you a championship , and i`ll wager that quite a few current motors haven`t got much more despite 21st century
pipe technology , carbs and ignitions .

Dependable John B has done the maths again , initially the numbers look quite frightening but it is worth noting that the maximum
values can only occur for a tiny fraction of a second . In a full cycle the effect would be negligible , also factor the fact that twice per
cycle the engine is stationary and you would be hard pressed to induce or subtract any energy from a stationary engine . Actually
heat dissipation carries on at rest so mass energy levels do indeed decline , just a bit .

Mick Ps conclutions seem pretty much spot on , authoritative and well reasoned and as his motors go very well indeed , theory and
practice combine succesfully together , so take note !
I must say i have no direct experience of ccc influencing overrev capability , but if Mick says it is thats good enough for me . I do
have some direct experince with Steve`s RS , disconnect the solenoid activated power jet and the motor stops dead at 12200-12400
rpm , reconnect and it goes to 13500 . The extra revs past peak power bring a lot of ign retard and eventually the engine
fires past tdc releasing a ton of heat into the pipe keeping gas speeds high and maintaining performance a touch longer !

It may be worth considdering the place that Bantam engines occupy in the grand scheme of racing 125 bikes . 25 BHP was readily
achieveable in the 70s , fourty years on it is rare . So perhaps we are still at the "Lawn Mower " stage where a nice bit of crankcase
pumping compensates for shortcomings else where !


Stay in touch , Trevor

Hi Trevor and all
I popped along to www.pitstop -last night, for the first time in a while, Aprilia/Debi Hondas pipe dimentions shown there and the tail pipe detail" ! jan and fritz actually discussed crankcase comp ratio and its affects with large volume in the cranks cases both with piston reed and 24/7 item Mike talks of, so between page 43 through to 52 Im sure its in depth, alot more questions about this from some very knowlegible people one so from snow mobile racing.

page 44 to 48 is amazing, anyone wishing to tune a two stroke needs to look at these pages,

Anyway I "feel" (not think) the way the fuel burns has a lot to do with this subject, I have been doing some home work on this, the results are alarming, well staggering is possibly a better word, I think the reason we are all still struggling around 17 to 18 BHP is we are not all geneiouses, not sure that's the correct spelling, but it would be interesting to see who is running what cc-comp ratio, I worked mine out to be 1.45 to 1. on both my engines and our 175cc currently under construction.

o' yes one more point "Ifeel" Micks current bike is producing below the 17.5 you talk about Trevor, I would guess at 14 to 15.0. but he makes up for it in the corners, im sure his low weight, helps in others too, but i bet he will have more next year "I will" have to get my finger out, im trying in all areas. !

kind regards Derek Betts

Long, lower pressure, lasting longer after TDC combustion is what you want --
-- is what you get when on Meth ...

Cheers!
JayBee -- the BOF!

Mike, and all ,
It is very tempting to cherry pick ideas from exciting , sophisticated engines and expect them to transfer to a Bantam engine !
Were this the case , and remembering that the whole thing kicked off in the 60s , our engines should now be punching out
an easy 35+bhp , but they don`t !

There are only two realistic inlet systems to choose from , piston port or cylinder reed , the former has the advantage of simlicity
of construction and operation . The reed , to install correctly , is far more of an engineering challenge , i know as i`ve done both .
The piston port suffers from case , charge blow back out of the power band but can produce very good power figures . The reed
almost eliminates blow back but it`s self impedes inlet flow . The choice is yours to make , but the question Mike asked , does the
ccr need to differ from one to the other ? My experience suggests no , if you are in the range of 1.35-1 then a bit either way is of
little consequence . What does make a big difference is if you have a set up of high ccr , rubbish transfers and a big pipe sucking
away , that will conspire to eject a whole load of charge out the ex port , never to be seen again , and performance will be poor .
A good example of this is the 350tz , these engines have poor transfer ducts and a very high ccr , as a consequence of short
stroke and rod length . Those clever types that thought the std spec pipes were too skinny and subsequently stuck on aggresive
fat ones saw performance drop off , stick the originals back on perf came back . Eventually the problem was eased by installing
a longer rod and carefull re- profiling of the trans ducts .A classic case of too much being a bad thing , ccr that is !
the Aprillia is around 1.28-1 and i do know of a recently developed 50 that was at 1.45 , and that engine was a very intensely
developed using extensive sim and dyno time but it did show 20bhp . Two extremes , both high performers we are somewhere in
the middle , but with modest out put !


See you all later , Trevor

Hi Trevor and all.

I feel its down to our lack of Dyno time /resources.

Enthusiasum can waver when wives /children or work takes over.

Also it could be said our paddock is a "secretive societly" were tuning is not spoken too much about, unlike in years gone by, you could walk round and listen to many conversations of different groups thoughts or ideas on tuning and devlopments trying that day, they would openly discuss this, I feel this was lead from the top down, George, the hunters and still Trevor to this day.

If it was easy to get 35 hp, it would have been documented, or done !

I rely on others, for the real techincal bits, so I hope my enthusiasum allows me to ask the correct questions, to move forwards usually two steps and one back, !.

for me its all down to "enthusiasum" and "effort", as one once said its all skill - "skill has nothing to do with it", - other than the "rider".

My theory is its down to the two most vital eliments.
enthusiasum = gives us enough drive to find out what you need to know or learn about, if you dont know it makes you unaffraid of asking what appears to be studid or the obviouse to others, it gives us the pereserverence to keep going, becuase the solution is always in your next experiment, thought or idea.

Effort = to allow you to drive forwards, I have never seen so much effort put in from our little grid of modest purses, compared to some of the tents ive been invited into its actually quite alarming.

So my take is its all down to Enthusiasum and effort which in my book will always = results
("Hmm" I missed out the time bit )

I was going to apply to the Eintein colledge to have this formula logged officially as a "Betts formula" but some one beat me too it".

we dont have 35 Hp on our grid because no one has the enthusiasum /resources to make it happen.

Derek