Time,timing,time area ?

I hope no one minds too much, but perhaps, the issues raised in " what are we doing " are interesting enough to deserve a home of it`s own ?

There is nothing weird or excessive about 179*, call it 180 for convenience, for an exhaust open period. Indeed, given that enough blowdown and transfer time area can be provided, then 180* could suit a Bantam just fine .

As we all know, when the ex port opens a pressure pulse heads off down the pipe to the rear cone, is reflected off this to return to the open port just before its closure. The piston face in turn reflects this, diminished, pulse again to the rear cone and it then reflects once more to the just opening ex port, cylinder blowdown pressure combines with residual pulse pressure and an augmented stronger pulse travels to the rear cone to be reflected , and this process continues for each engine cycle in perfect.... resonance !
For this phenomenon to occur, the ex port needs to be open for an equal time as it is closed, so the crank angles must be equal and that can only be available at 180* spacings !
The down side to this is that with 180* the engine may not rev high enough by having inadequate blowdown time area over the transfers, a move to 190* timing will correct that, but torque may drop. This assumes, in turn, that the transfer time area its self is adequate and so the merry go round spins on .
As long as revs rise more than the torque declines then, all things being equal , and they never are, then power will go up !
So Derek is perhaps onto something here ?

Time for lunch now , catch you all later ? Trevor

Nigel,
Google " convert crank rotation to port height ", nice calculator there will give you all you require !

Trevor

yeah, ok, ive got that, thanks. recently removed loads of stuff off the computer as the laptop was getting slow. didnt think ide need that again..

Expanding a bit on ex. port timing, the details of the bench mark Aprilia makes for interesting reading. Jan Theil revealed that the power valve in the RSA cylinder intrudes into the duct/port by 4.5mm and does not start to ascend until 10,000rpm has been reached, also revealed was the port timing, with the valve fully retracted, at 202*. Using known engine geometry of 54.5 stroke and 120 rod we can establish that 202* equates to a port top edge of 25.8 from tdc , now if we add to that the valve drop of 4.5mm we get 30.3mm from tdc. Returning to the geometry and doing the maths for that dimension we achieve a figure of 184*, now where have we seen that before ?

So looking at the published power graph for that engine , at 10,000rpm the output is 33hp and with the valve completely open at 12,000rpm the power goes up to 50.5hp and peaks at 54hp with the revs at 13,000, over rev to 14,000rpm and the power is still at the 50hp mark !

Where does that leave Bantam motors running 200* ex. port timing and achieving a modest 22hp ?

Time, for a nice snifter of Port !!?? Cheers, Trevor

Trevor you are simply giving too much away my friend!.

I understand this type of info is common knowledge at the midlands Bantam meetings, but please don't give too much away too soon, like those photo's its the apprehension of what you cannot see, that makes it exciting or more interesting !.

may I say myself, "brilliant bit of theory/maths makes a lot of sense, A question for you Trevor, from you graph your engine produces max Torque at 10,2 to 10,400 max hp and max HP at 11,000 what exhaust port timing are you running. that question to Trevor.,

Also I found that on my engine during development the Torque went up when I drop the exhaust timing back, so did power?? but its back to where it was know but by grinding top edge, so increasing blow down slightly, also we have much better area in the transfers.


I have been pestering the hell out of Jan who has now actually replied to my post, he will pop along and take a look at some point. ! lets hope he stays generally he like photo;s technical INFO on anything two stroke, Please don't frighten him away, if/when he does, Jan's been involved over many years with engines that have won something like 36 world championships- and many different makes of engine such as Jamathi /Bultaco /minarelli /Garelli /Aprillia and many more.

He has never been involved with a Bantam he did not know what they were, but he does now.

Derek

Derek ,
Looks like you are going in the right direction with the porting work , but don`t get hung up on ex. timings alone, time area is more critical, and the pipe must be in concert with all of this and to get the revs you need blowdown . You can get away with a smaller value if the ex. area is big enough, but use large corner radii at the lower half of the port, this will help minimise short circuiting of gas from the transfers to the ex. port . Then with the duct outlet , for a single port , at around 90% of the port area you can rock n roll . However each engine is different so the goal posts are always moving , but one thing is for sure , you can never have too much transfer port area in any Bantam engine, if the charge streams are organised intelligently !



( 192* duration )

Trevor

Trevor,with the piston port controlled barrel if you can achieve the 192* exhaust opening time and the required area and 30 * blowdown and 130*transfer opening but NOT THE AREA what can be done? if you can evacuate the gases but cant re-fuel the cylinder sufficiently is there much point in having such exhaust/transfer timing?would it still be a forward change or a waste of time? would the tuning of that barrel be totally dependant on transfer area and everything else should be changed to suit what can be done with that?

Hi Nigel

good point there is certainly no need to have a large exhaust port if your transfers are not so, i think Trevor has said don't get hung up on timing, its the time-area and direction that matter's.

be interesting to identify what the others have, and their theory.

Derek

Nigel,
Apologies, I completely missed you post and the query !
I agree it is a dilemma, but the one thing you must ensure is that the port/duct , and always include the two as a single entity for they flow as such, is as efficient as possible. Now that's a pretty obvious thing to say but does need to be considered at the design/construction stage for it is tricky to replace metal that has been removed . I have seen dreadful bridged ex. ports that would flow far less well than a good single job and botched triples that flow worse than either !
If I were doing your next iron barrel, and I assume it`s a 175 ( or bigger ) then the following is what I would aim for, but perhaps with one or two qualifications .
If you have a robust piston with a reliable ring then you could take the ex. port width out as a chord dimension to 70%of the bore size, for 64mm that is 64x.7=44.8mm, with a pukka race piston you could squeeze 72% but here you may be in the realm of diminishing returns, so 70% is the more dependable option . Whilst on the subject of pistons, I have measured a quite a number and they vary enormously as to ovality and taper, but none were designed to run in an air cooled iron barrel so adjustment here could eliminate those unexplained seizures .
As a rule of thumb I have used a port with the following geometry. Top corner radii of 10mm with the top edge having a radius of 100mm, bottom corner radii of 12mm and lower edge radius of 80mm, this profile will provide for an easy passage for the ring and should keep it in it`s groove and allow for minimum bulging . If I was doing this I would have the lower edge of the port 2mm higher than the piston edge at BDC this will have no effect on blowdown but will assist the returning exhaust plugging pulse to get residual charge back into the cylinder. Having the large radius at the lower port region will reduce the tendency for fresh charge to escape straight out of the ex. port and contribute nothing to power production. The duct dimensions then will be calculated on area of the reduced size port, and having the outlet at 90%ish of that and the port/pipe boundary diameter at the whole port area, again the step will have no adverse effect . This reduced duct volume will keep gas speeds high, useful at lower revs, and have a reduced surface area that leeches away precious heat from the gas and help keep the pipe in tune and perhaps respond more predictably to the dimensions you factor into your calculations !
If you feel that the timing of 192* is too little, you could try making the port at that height and incrementally raise it by introducing a top edge radius, ala rsa, gas always flows better over a radius than a sharp edge . However, the acoustic energy pulse will be reduced thus affecting pipe action , always there are compromises to be made! It is only ever possible to make the best from what you have, the very reason I made my w/cooled engine, I eliminated most of the bad bits .

Hope that helps a bit ? Regards Trevor

cheers trevor, i have managed to get the bike going again, have a wiseco piston (63mm).Having removed 5mm from the top of the barrel the exhaust opening 29.3mm from the top(84*) was easily achieved and funnily enough this leaves quite a lot of metal sitting above the piston at bdc. I had previously  read that if the blow down time area could be achieved before the transfer ports opened that the hieght of the exhaust  port floor could be raised, so i left it at approx 2mm above the piston edge:cheers: .To get the 90% duct ratio could be tricky, could this be done by an insert in the exhaust header as it meets the barrels exhaust outlet, or is this too far away. If this is ok then i believe you say the step between the two  wouldnt be a problem or should it have a taperd or radiused edge? or am i way off!!  Again im sure ive seen something elsewhere which showed a header pipe section that had a small reduction in size close to the engines exhaust stub and then increased in size back to the its original dimension, a bit like squeezing it in( shaped like a sand timer glass thingy) or like the reduction in size of the area between the rear cone and stinger pipe on the exhaust of the aprilia.
Well the bike fired up, doesnt like anything below about 4500rpm and it vibrates the bike  like mad.. think maybe the crank balance may need  looking at or maybe get some gum shields and extra padding on the seat:idea: it really knarks me when an engine wont rev out like a road bike, nice ,crisp, smooth and from 800rpm up.. thanks again.

Hi Nigel ,
Taking the last bit first, that vibration you are experiencing, could be causing frothing of fuel in the carb float bowl and certainly will upset smooth running and assuming no other mechanical problems then that vibration must be reduced, for vibes sap power, and crack frames etc. A 50% balance factor works well, but vibration is always there, it is just a matter of where you reposition it for comfort and reliability .

The best of the RS Hondas have a barrel ex. duct outlet 41mm wide and 32mm high in a neat oval with a spigot transitioning out to 41mm round blending to the header diameter, this shape gave the best power. ON my w/cooled barrel the duct is 50mm long with an oval outlet, the pipe spigot blends to the barrel and tapers out to the header at 37mm .
One suggestion is to get the duct inner profile with an insert going in as far as you can practically go and welded to a thin disc which sits between barrel and pipe ,the insert can be blended inside the duct with Devcon, or similar, it won`t last too long but it`s no sweat to replace .
Modding the insert is easy for size changes and the oval shape will be fine with your existing round header pipe . The important bit is to reduce the duct volume how you blend to the pipe is much less so .

Hope this helps , Trevor

trevor, thanks for the info. i wonder if alpha will knock me up just one new crank for £100... ive just come in from the garage and ive had a look for any cracks in the frame. I couldnt see anything obvious .The last engine vibrated a bit and with my fast bend front wheel wobble it got me thinking if the head stock was cracked. Also used to see lots of fizzing in the inline fuel filter, which still happens probably more than before, looks like the engines coming out again then or its just gonna rattle to death.. thanks again.

Derek,
If you factor the data variables into any pipe calculation formula/software you can indeed come up with, what at face value, seem contradictory numbers . The tuned length that would suit, say, an RS at 12,000rpm with 40 hp could equally suit a 9,000 rpm engine with less than half that power .
The numbers would reveal that higher power would need a longer pipe as it runs much hotter and wave speed is higher, but needs to be shorter as rpm increases. Then you can play around with ex. port timing duration, keep rpm and hp the same but alter timing and tuned length changes once more, longer duration means more time and so pressure pulses can travel a lot further .
All of this just goes to illustrate that engines need a different pipe length for just about every scenario that can be imagined, when the average lap on an average circuit, etc,etc, is undertaken .
So it is not hard to imagine that the 1954 pipe could seem be compatible with today. From what I have been able to glean, the gas used then was around 80 octane .

Trevor

1951 -- or thereabouts. Pump petrol at 78 to 83 octane number. We mixed, 50-50  with 100% Benzine we fiddled at high cost for scrambling.

Compression ratio increased to 8.5 and Wow! did we go....!!

A question for Derek when I drop my reed valve cylinder down by removing a spacer
I can get 188* exhaust but my transfer are 130* so with my barrel I would need to
lower the transfers further to get the blow down rather than raise the exhaust? On your motor you talk of raising the transfers? Do you mean exhaust? Or are you trying to get more transfer area?

Regards Mike

Mike ,
Whilst I realise the question asked in your post is for Derek, the sub text is intriguing, I`ve crunched the numbers for the data you provide and it is all a little surprising ? If after removing the under barrel spacer you have 130* transfer timing, the pre- removal must have been extreme , so my question , in turn, is what was the thinking behind these port timings, whilst retaining the same blowdown period, and did it fulfil the design brief ?

Trevor

Mike both -" more transfer area", less time though, whilst increasing the blow down by lifting the exh timing but not the transfers.

I too am interested about what Trevor has raised Mike!, is this an error, please explain in more detail.

regds Derek.

Trevor no worries post away/Derek, this barrel was a 175 but linered down to 125 in build etc.
The liner was done by Fahron engineering in 1994 and worked out at 196* Ex and 136* Tr
design brief was me saying to Ron "can you make me a fast liner for this?" which in fairness
he did.

By dropping it down it turns out at 188* (was 196) 130* (was 136) .
Just using 30* as the blowdown would you suggest raise Ex or lower transfers?

Question for you both what blowdown would you use with 188* exhaust port timming?

Regards Mike

I`m a bit confused here Mike , in your first post you mentioned your reed engine (125) with 188* ex and 130*trns, hence 29* blow down .
Subsequently we have a 175 (non reed) with 196* and 130* trns, hence 30* blowdown, which , are we concentrating on here?

I have to say there is no place in the current Bantam world that 136*trns should even be considered never mind even used ! The worlds most powerful 125 2t doesn`t go that extreme, so no Bantam should . To do so will lead to such extreme ex. timings that real torque will be almost at peak rpm and that means no power band to speak of , think first, of 3 speeds . Anyone thinking of these numbers shows no understanding of the real needs of Bantam racing .

Cheers Trevor

Sorry to confuse its never been a 175 always 125 but made from a d7 cylinder and
Linered not a d1 and not linered, hope that makes sence.

To recap a 125 reed valve with 196* exh and 136* transfers - 4 race wins at lydden.

Can lower barrel to make 188* exh and 130* tr ...
Question is should I raise exhaust timing ? Or lower transfers?

Regards Mike

These numbers are a relic of the 60s thinking , high transfer ports, sky high exhaust port and miniscule crankcase volume squirting trans gas at warp speed, straight out the cavernous hole in the barrel wall that masqueraded as a port, the powder puff torque level needing 15 gears to keep the engine running in the 1,000rpm power band !

Actually Mike , you`ve answered you own question, 130* trans. timing is right at the top of Bantam timing, but the whole port areas will ultimately determine the port height , at the projected revs......time area ! I assume you have 5 trans ports in the reed barrel , so the potential is there, but if you have a wide variation in respective main transfer port, duct area/volume, then assuming a constant driving pressure from the crankcase, then the smaller will flow so rapidly that , again, the charge will take the line of least resistance and short circuit out the accommodating ex. port !
Extreme transfer port heights will start to open when cylinder pressure is still well above case pressure, particularly with insufficient blowdown, add a temperature gradient of hot to cool, then a lot of spent combustion gas will reverse flow into the trans ducts, pre-heating the incoming fresh charge and diluting it`s quality with obvious consequences for subsequent combustion efficiency . So , get the transfer phase wrong then decent power with a decent power band width is difficult to achieve and get outside that range and the bike is a handful to ride .

The answer to your question is, do both , drop the barrel a touch more if you can say to 128*, or perhaps introduce a touch of port stagger, add 32*blowdown, and you get 192* ex timing, max out your export width/area , for just a bit more than the blowdown duration, and the cylinder will be more clear of spent gas and reverse flow will be reduced , add a much improved power stroke, coming from the lower ex. port , bringing better torque providing for sweeter engine characteristics !
There is one caveat I would add to this in that I would caution against too aggressive a diffuser in the ex pipe, big centre dias., hence angles, will over- scavenge the Bantam cylinder, with dire consequences and the large surface area will shed heat and ruin the acoustic pressure wave and damage pipe performance !
As everything in a bantam race engine is a compromise , there is one beneficial effect of all this short circuiting in that this flow of cool-ish gas will help cool the piston crown !


Hope that is a suitable answer ? Trevor

That's a great answer Trevor many thanks, I've been away for a few days and struggled to get an internet
connection. I'll re-read again tomorrow to make sure I really understand what you are saying. I'm tempted
to leave the 188* Ex but lower transfers with devcon to 124*, Yep its 5 transfers and i'm also tempted to open
the rear port first then the T2s or B ports which are narrower than the T1s or A ports. I'd like to do a temporary
job first rather than take metal away at this stage.

I'm a bit busy ref motors as this season I have used my old piston ported, the reed valve and my new piston ported
and none are easy to ride, it would be good to make the 'reed' a tractable power house and may show me the way
forward for the future.. thanks again

Regards Mike

what a great post series, interesting point about defuser angle and big diameters, ref to over evacuation of the cylinder.


One more for Trevor, did you notice the the shape of the Aprilia bell mouth into the carb. very similar to the shape of the inner transfer, do you think this is also a good shape for the inlet port.

adding to this Trevor, most race two strokes seem to show a merry dance of gas as you open the throttle / while blipping, is this charge coming in and out, I have always assumed this is the waves coming in and out of phase as the revs rise it seems to disappear, this is right at the front of the carb, so buy having a Coanda shape here does this improve air flow/volume movement, or simply maintaining air streams to the boundary layer, so avoiding further turbulence in the carb bell month, and is this purely for top end / or both/all ranges.

Im sure there must be some effect here as the shape is so similar to a flying saucer wing edge. odd thats its very flatfish i was expecting to see a bigger radii.

Derek

I once came across a quote to the effect that, asking the question take five minutes whereas the answer takes two hours ! Today feels like one of those occasions .
Never really thought about that comparison before, but yes, a good, smooth flowing duct will work anywhere, if the geometry can be sensibly accommodated, but if you have a reed valve you don`t even need a port, just vestigial piston support !

There is a physical difference in the function the bell mouth for air entering and being expelled, air going in is just that, air, not until the carb jets have been passed do we have mixture. The function of the bell mouth is to flow as much air as is possible with minimal loss, one profile has proven to outperform others and that is an ellipse . The drawing, that i hope Ed will post for us, shows a very efficient shape that can be adapted for many carbs. Another favourable consequence of this shape is that "spit back" is reduced, CFD investigation at QUB has confirmed this to be the case, with the observation that short and fat comes out best. Reverse flow exits the bell mouth as a single contained column approximately the same size as the choke dia, ingoing air resembles more of a mushroom shape, like trying to get a quart into a pint pot, so the profile is obviously important . Getting this right enhances flow throughout the effective rev range, win, win !

Some of the old tuning books would have you believe that there is a mathematical formula for calculating inlet tract dimensions, but there are just so many variables in Bantam engines that to think so is pointless . Harmonics in the inlet are quite strong and vary with revs and can seriously upset carburation , particularly when the wave sign coincides with the needle jet/needle position. Negative, will draw up excess fuel and positive could pressurise the float chamber causing flooding , either way fuel will eventually be dumped out to atmosphere . Juggling the values for intake length , timing or case volume or carb size will help but closing the port earlier will aid general running and provide for a clean running engine.
Opening earlier, and closing when atmospheric pressure equals case pressure is the best compromise , as in a disc valve set up, or use a reed valve to reduce blowback and have a varying inlet cycle, as revs, hence pressure, self adjust and negative case pressure on one side of petals and positive on the other rapidly open the reed cage and inlet charge rushes in . When thinking of charge mass, the shortest practical duct length will have the minimum column inertia so will get the crankcase inhaling sooner. Piston port engines are the worst offenders in all of these aspects , and the very late closing messes with clean running . Yamaha got their old piston port engines running well but eventually gave up and then produced better engines !

This is a very complex subject with masses of possible variables and the homemade nature of Bantams throws up complications that generalities don`t always cover, but keep trying!

All this fantastic info... confirms I was just the GoFer-test-driver -- never the two-stroke, petrol engine tuner....

Just a point of interest: We dozy diesel engineers used a very simple way of measuring induction airflow. It was by a  venturi-bellmouth for a quick check on*** airflow into our  engines***.

Cheers!

PS-- Only works with unimpeded (not throttled) (or wide-open-throttle) airflow....