gear ratios / speed / power / revs

[sime66]

This is a bit long-winded, but I’m trying to get an over all understanding from some of my own threads, and some other recent threads, which are related. Several threads recently have touched on gear ratios / speed / power / revs:

vespasco: vespa.proboards.com/thread/4359/lacking-power
fredperrybruce: vespa.proboards.com/thread/4333/new-clutch-confusion
bigd: vespa.proboards.com/thread/4347/spark-plug-question (henri 15-9-14)

There’s other threads, but these are the recent ones that got me trying to better understand the whole subject. Some of you lot probably already know about the scooterhelp gear ratio calculator, which I found here (or your own knowledge/reference on the subject):

www.scooterhelp.com/tuning/vespa.gear.calc.html

..and I got this result for my current clutch and gears, which ties in very well with the speeds I’m getting in each gear before changing up*:



By feel and ear, my scooter seems to like 6000 revs, according to that table, and I don’t feel the need to overdo it, except sometimes just to see what it will do. When I’m referring to my revs, I’m going by my speedo, which I reckon is about 10kph over, just roughly, *so accelerating on the flat I’m changing up at approx 40, 60, 80, and comfortably passing 100, then pushing on sometimes (kph on my speedo). I like my scooter being nippy in lower speeds and gears, and I’m not particularly bothered about going much faster for longer at the moment, given the little amount of dual carriageway riding I expect to do next year, but I’d like to understand this better, so I have a few questions:

1) I asked about a year ago, and fredperrybruce asked recently about changing the clutch gearing. I can see that if I put a 22T clutch into the calculator I get a better top speed, but there is then the question of power and revs, which I don’t really fully understand. Is it just trial and error/experience that dictates what engine can pull what gears?

2) There’s the question of which clutch gears mesh with which cush/primary – eg. A 68 is OK with 20,21,22, but not a 23?

3) Am I right in thinking that reference to the T5 4th gear modification only applies to p200s because my PX150 already has the same 4th gear as a T5? So that’s a red herring when talking about a 125/150 engine?

4) Those three above are about gearing, but then vespasco asked about lack of power, low revs, and ways of improving it. I imagine this is the entire fuel, air, capacity, flow/porting, timing, combustion, expansion, exhaust etc… - too many variables with each individual scooter to be specific, but is there an overall explanation, or a simple example; ie taking my scooter or vespasco’s current project as an example and giving ways of how to improve power. – Is it just anything that increases the amount or speed of air/fuel that can be sucked in, exploded, and exhausted?

5) Is the power of the engine the main/only thing which dictates the maximum revs achievable in each gear? This leads on to questions about limited rev CDIs, advancing/retarding timing, what is a good target max revs, and whether an overpowered, undergeared scooter will just keep on revving until it goes bang – don’t want to turn this thread into a giggle; would really like to try to understand it all a bit better if anyone has the time to guide or explain any bits of it in any more detail.

6) Is a longer crank, just an increase in capacity? Or is there more to it than that?

7) With increased revs, you’ve then got the subjects of cooling, wear, and reliability, so there must be some knowledge there that I’m missing, and it also then leads back into fuel, 2T oil, air, carb and jetting.

Hope some of that makes sense; didn’t want to write loads and lose the questions in amongst the explanation, but needed some to give a start point – even to show where the gaps are in what I’ve picked up so far, to try to learn a bit more on the subject.

[henri]

the max revs is mostly limited by the "breathing" of the engine an the port timings ,a expansion chamber exhaust ups the revs a bit as does a free-er filter n carb ,but most gains in revs are from "flowing" the inlets an tuning the exhaust n ports,during port timings you set the squish an "deck" height (where piston is at top of stroke relative to barrel end)an play with comp rates ,
a longer crank not only gives higher capacitty but diff compression ratio's aswell as with various barrels slightly diff port timings.
a badly geared or good geared scoot will go bang if rode at "wot" max revs all the time ,just wrong gearing makes this more likely/tempting as scoot is thrashed more often,
if switching clutch primary it is poss to just switch clutch ,but best to swap both primarys to get the ratio you want ,an saves the old 1 wearing new gear faster ,always best to switch gears as sets/pairs .
if you want a quick/short/better intro to tuning look in the stickies on smallframe forum , thats 2 stroke specific an goes into it a lot deeper/clearer, theres also more complex gearing calculaters there n links to more .
its not just about gettin big bang from large charge of fuel an then gone down a free exhaust , its findin right balance of fuel in ,burnt efficiently an economically ,usin harmonics in exhaust to scavenge well n havin a good "balance" so engine dont implode ,then a clutch that will handle it ,then its gears to get power to move scoot as you want .
theres no magic 1 thing that will make scoot "tuned" fast ,but loads of little expensive mods that must work together , just about to post in the scooter related my latest experience of a " tuners" work , read that it wont teach tuning but will show how not too ,ok , H

[sime66]

Excellent reply, H; thanks. I'll have thorough read through that lot above first - just scanned through it quickly and there's bits that answer immediately, and bits that I can see leading to more questions.
I did join the Small Frame Forum to have a general nose around; I'm working through the old posts when this forum is quiet.
Intro to tuning sounds like the sort of thing I'd like to read, so I will go over and have a look at their stickies.

Thanks again.

[vespasco]

I love this stuff.great questions.
My memory's so bad I need to kickstart my brain and recall what little ive learnt...
Firstly,

Believe nothing, no matter where you read it, or who said it, no matter if I have said it, unless it agrees with your own reason and your own common sense.
(Buddha)


"If youre bike doesn't have the horsepower etc......".. Thats whats happening to my motor. Hence my earlier post.
Unfortunately the gear ratio calculator does not match to my particular set up.
76mph at 7000rpm seems a little off.
Im comparing that to my SIP speedo / rpm readings and speeds.
This will probably take me an hour on my phone!

1) its not all trial/error/experience but thats by far the best way!
I trawled forums, studied the 2 clasdic 2 stroke books available online for free..Jennings is one author, Bell is another. Its all on my latop...I cant remember exact name.
But take caution as these 2 books do tend to concentrate on max bhp, race running motors..
If you haven'got the power you wont produce the revs.
In my recent experience what dictates how to make max bhp (from a known good working motor) is all in blue printing, optimising tolerances on assembly, fine tuning the timing, carb, jetting, plug type, so many things. (then.I play it safe and dumb it all down a tad for max reliability and durability.). These are things you can learn and do yourself... Re designing cylinders, heads, pipes is more art,experience, trial n error, with biilding blocks in science formulas.
The faster you go, the more power is needed, to make it go even faster you need that motor performing extremely efficiently to to be able to do this.
(of course different cylinder port timings, stroke length etc will heavily effect this).. A small leak could show up at higher cranking and combustion pressures and ypu would loose power.

2) I dont know the details but I know its all out there somewhere, online but in various different places!?!

3)yes thats correct!!

Late type 150 1st gear is the same as late px200

2nd gear is pretty much universal on all px 125,150,200 on early and late type gearboxes

3rd.. Help..I have stuff crossed out in my little Hong Kong Book of Kung Fu Vespas..
One of my notes says T5 3rd gear is the same...yet elsewhere on the same page its crossed out?!

4th gear yes T5 is the same as px125 and will fit straight in a 200 gearbox

....my.notes also reveal, when I was riding my pinasco together for the first time, I wrote I could actually down gear to a 34 tooth 4th gear, as opposed to the T5 4th eith 36 teeth। (stock 200 is 35 teeth), as I had so.much power and torque in 4th... Now I feel like I need to fit a T5 4th gear to.help it rev more with less power

4) see above^^^ and... Er... Yeh.... Its pretty.much about getting your mix (and enough of your mix to supply the added demand) through the motor as fast as possible, firing in the combustion chamber at the right time and then out again...all at a perfectly balanced sweet spot, generally controlled by your choice of cylinder kit port and induction timings.
How well the kit would run would be mainly determined by the choice of carb (and crank inlet type etc) and exhaust/ expansion pipe.

For example I know (Maybe) that if I fitted a proper expansion pipe and a 28mm carb to my motor it would go faster/pull more in 4th gear...But only if that whole motor was blue printed or better. No leaks. Tight toletances on crankcase etc..(which im.beginning to believe mine aint quite 'optimised'!

5) other smaller components such as bearings, lubrication??!
Id say yes. The motor produces the power, keeping it an.optimum and making more at the top.of the scale proves difficult, especially for a vespa.. The gases do.not have as much time to escape when its revving faster ू this is when it starts to tet difficult, and keeping things cool.enough to disperse the likely excessive heat it eill produce.

6) straight sawpping a long stroker with a short/stock on the same cylinder would give you longer port timings. This can give you more power with the right carb / pipe combination (and of course the head etc).
Also, my head is spinning with thoughts that a long stroke travels further up and down the cylinder, making it more heat efficient but is in effect slower than a short stroke. If I wad to.build a sprinter I would go short stroke as opposed to long. For a tourer, long stroke.
But you woyld only see the benefits if those cylinder port timings were well suited.
You could also take it a step further and using a longer stroke with a more effective well ported cylinder to suit but thats not so vespa.

7) my take is to make it capable of max revs bhp but tweak it rich/resyrict it slightly, as opposed to the ragged edge, which is where max power is.
Cooling... These motors are air cooled. Normally anyway! The faster the.motor goes/more revs/the faster that piston travels up and down, the hotter it(the piston crown and combustion chambet) gets.
Ali cylindere are obviously the better choice.
How long the exhaust gases are in the combustion chamber also effect the heat build up.
But more importantly, its really the fuel that keeps your 'motor' cool (piston/cylinder /head) and stopping it from seizing. If you have enough fuel supply and demand to keep it cool at high revs then id say there's more power to be had.
Reliability...stock p200 for sure. Any stock, well maintained px really. I used this as a bench.mark for my tourer. Trying to keep a happy medium between piaggios specs and tuning guides. Taking note of compression ratios, timings, tolerances..
Carbs are such finely made things. And they.need and the cylinder.bore needs and the bearings need a suitable fuel,oil to work at the extra revs,extra heat produced. which all produce extra wear!!

Fuel keeps it cool!! B)

[sime66]

Wow! You two have excelled yourselves. I’ve had a quick read through that lot vespasco (after beers & on your phone - splendid work!); there’s a bit of info overload going on, and if I make a start on it now, I’ll be up all night, so I’m knocking it on the head in a minute.
Had a read through your ‘how not to tune n resto’ a couple of times too Henri, but that’s going to give me nightmares as well.
That’s without going near the SmallFrame Forum tonight.

I found those two books online – that’ll be like going back to college! If anyone wants them they’re here:

www.vintagesleds.com/library/manuals/misc/Two-stroke%20Tuner%27s%20Handbook.pdf
&
3cyl.com/mraxl/manuals/bell/performance-tuning-graham-bell.pdf

Cheers chaps; too much to go into now, but plenty to get my brain round, and loads of questions coming your way……….

[pxguru]

Impressive post but can't resist going over a few points.

6) A lot more to it than that! A long stroke crank fitted in a barrel ported (or made for) for a short stroke crank will make the scooter more sporty and rev higher. Trying not to be scientific at all; a piston does not travel at a constant speed, as it needs to turn around and come back up, while its down the bottom going further the ports remain open a little longer letting in more mixture. More mixture being compressed, more bang, more revs.
You can vary the amount of max revs with base gasket thicknesses but even the thinnest will rev higher than the short stroke crank would have in the same barrel.
A long stroke crank in a barrel made for a long stroke crank will make an excellent tourer.

7) The PX has excellent cooling and actually doesnt get hot enough in the UK heat. There are never temperature issues if the carb is set up right at all throttle positions.

[sime66]

Thanks for that pxguru. I’m glad you got involved; always value your sage words. I’m happy with the numbers and science, it’s the 'old-hand' terms that lose me more, which is why I deliberately ask my questions in layman’s terms if I can. I’m going to read all these replies, question by question to see if I understand, or need to read-up elsewhere if info is missing, or need to clarify with any of you. There’s a lot to understand, so it won’t be a quick job, but none of your efforts are wasted and I appreciate all the replies.

I’m going to add a question, which I know hasn’t got a simple answer, but is a big point missing from my questions in my first post:

8) An exhaust has to be big/open enough to allow exhaust gasses out quickly enough for whatever’s going on in the cylinder, but also needs to hold gasses long enough to give the pressure to keep the stroke going? I think that’s basically it. At my stage I’m just using off-the-shelf exhausts with general knowledge of what is currently regarded as best for my scooter; for example I’ve gone from the Sito+ to the SIP Road2. I actually have no idea how or why the SR2 is better than the Sito+ other than that it’s because I’ve been told it is, so if it’s possible and worthwhile to answer; how have the physical differences between the two made a difference to my power?

And whilst going through all this (on the forum, not in technical literature) is it safe to take it that when talking about revs we’re talking about power? An increase in revs is an increase in power? Power is achieved by increasing the revs, by whatever means?

If so, taking my PX150 as an example. I’ve done three things (so far) to improve the power – kit, exhaust, carb; does each of these have an increase in power that can be estimated and translated into revs or vice versa? Taking the 6000 as a baseline for a standard PX150, what might each of those three changes mean in power/revs? I know that is an oversimplification, and doesn’t bypass the knowledge given above, but it might help me understand a bit better at this stage of learning before I dive in.

I am going to stop the questions now, and try to learn a bit more before posting again…………

Thanks chaps!

[henri]

yep agree with pxguru on the piston accel/deccelaration on long strokers n its effect on port timings , my first answer was tryin to keep it simple for sime rather than overload an blow his brains ,but thats tuning ,its never simple A+b never=c ,unless D,E,F,G an the rest are taken into account ,
its why i try not to think/do it , lost too many hours/years of teens an cash too tryin to make yam rd lc's trip light speed , an just kept fallin off em anyway , H

[sime66]

See below..............(right to bottom)
v
v
v

[fredperrybruce]

lol,,,, just read all the posts on this thread and as informative as it all is ,it has totally baffled me and I'm still none of the wiser of what clutch set up I need???

[sime66]

.....the point being that it isn't just a question of which clutches will physically fit, or which gearing changes will work; there is far more to it than that, as I think (haven't checked back but will in a minute) was about the last thing I posted on Your clutch confusion thread. I started this thread, not for clutch selection, or any ongoing problem I'm trying to sort, but to actually try to begin to understand it all. I don't understand it either, yet, which is why I'm asking the experienced chaps, here.

[sime66]

Right, I've had a quick scan back through your thread, and it was left that you were going to do some investigation.....
I've livened that thread up again, so we can keep this one separate:

vespa.proboards.com/thread/4333/new-clutch-confusion?page=1&scrollTo=30694

[fredperrybruce]

I totally understand where your coming from Sime66 I wasn't having a pop!!! I'm still very a novice on all things vespa related hence me asking questions every bloody week,, It's very much a real head F**k all this gearing lark for me!!! but very greatfull for all the good advice you have given me without you and the other tec guy's on here I would be proper in the do-da

[sime66]

Simple is good, H. I understand that the piston has to accelerate, decelerate and stop before it goes back. One of the bits I can see I really don’t understand though is what you mean is ‘port timing’; anyway I’ve had a bash at pulling it all together…

Initially I started this from the point of view of gearing, but you can’t really look at gearing without a bit of understanding of power first. Working from pxg’s little phrase:

MORE MIX = MORE BANG = MORE REVS (= MORE POWER?).

I’m looking at the ‘BANG’, and how the Size, Rate, Quality can be improved, but must also keep in mind Safety, so that its best bang isn’t its last.

From your three replies, the things that can be done to improve this BANG are improvements to:

a. Compression Ratio
b. Port Timing (Need to understand better)
c. Volume
d. Volume Flow
e. Volume Quality (Fuel/Air)
f. Bang Quality (Timing/Good HT)
g. Bang Safety (Cooling/Lubrication)
h. Others I’ve missed……………..?

Bang size:
Compression Ratio: (How Squashed the fuel/air mixed is before spark). Affected by Kit (volume), long crank, squish. Compression itself is dependent on rings and leaks.

Bang size:
Port Timing: (Size, amount of time, and ease of flow for fuel/air mix to flow via ports through crank case to cylinder and out during cycle). Affected by Kit (porting), long crank, porting mods (dremelling ports). Still vague on this…….
(To do with which ports are open for which part of the cycle, and in relation to ignition timing, how big they are (how long open), and which other ones are open at the same time??)

Bang size:
Volume: (Amount of air/fuel mix per bang) Affected by Kit (volume), long crank, base gasket.

Bang rate:
Volume Flow: (How quickly the volume is refilled (thus revs)). Affected by carb, filter, kit porting and porting mods, exhaust.

Bang size/safety:
Volume Quality: (Fuel/air mix). Affected by carb, filter, jetting, 2T mix, tolerances/leaks, fuel quality.

Bang quality:
Bang Quality: (Efficiency). Affected by Timing, jetting, 2T mix, fuel quality, plug type (clean for spark), Stator, CDI, HT, bearings, lubrication, tolerances/leaks (‘blueprinting’),

Bang safety:
Bang Safety: (Reliability - keeping it banging – cooling/lubrication). Affected by jetting, 2T mix, bearings, seals, lube, barrel type (ali not cast iron), tolerances/leaks, keeping on safe side of max – off ragged edge.

This is only meant to be a start point, for additions and corrections; not a finished job. It’s OK if it’s a bit simplified as long as it isn’t wrong – I’m not going to learn and understand it all overnight.
My bleedin’ computer crashed whilst I was writing it; I lost some work and my train of thought, so there might be things missing that I think I’ve included but lost.
So far I’ve only got into what’s been written on this forum yesterday; I haven’t touched the PDFs or SmallFrame Forum for info yet; this lot is plenty to get my head round for now.

Once I’ve got these right in my head I’ll go back to gearing, but I’ve picked up:

1. Any mods to clutch should be matched with a change to the other half of the primary, for meshing and even wear.
2. Gearing mods are possible, if the power is there to pull them, the best way to see if changes work is trial and error, or asking you chaps, meshing of changed gears has to be considered.
3. Before any changes I need to know what gears each of the standard engines has, which should be pretty easy to find online.

I’m not doing this to get my hands dirty yet; it’s just to understand it better for now.

[vespasco]

Be careful sime...
Heres what I got up to after reading too much .....

combustion... [/URL
Expansion....
And...
Er..? Consumption!?

[sime66]

I've made a start on those pdf manuals this evening - printed off the intro, and each pdf's chapter on cylinder heads.
Going to go through it chapter by chapter, and try to keep you guys interested while I squeeze you for info!
That's this winter's project; something like this; one subject at a time -  lots of notes, lots of questions:

[vespasco]

Ill be looking forward to learning more too so will be paying attention.

[henri]

stil thinkin thru above post,but earlier ya asked "port timings" , a rotating crank moves the piston in mm straight so much per degree of rotation, so people will say raise/lower port heights in mm but talk of "openings/closings/overlap" in degrees , on a vesp with rotary induction youve a inlet port/valve formed by crank web an timings are increased by shaving off the edges , but if thinkin of tunin that far ya might aswell/better go full circle/open crank with reed valve ,lets you open up inlet tract n stick a better carb on ,
then youve got the transfer ports in barrel n exhaust ,timings here are altered by removing metal from edge of port opening ,or with some kits/engines you can alter em slightly by base gasket packers , am tryin to remember but think its a malossi smallframe kit thats ok from the box but with a .9mm base-packer hits a "sweet spot" is general opinion over on the "dremel it do death" forum,
theres a lot of "tricks",opinions,trends an bad info on tuning ,but most can be distilled down to simpler ideas n understood enuff to do what you want without takin a advanced degree in engineering ,but first ya got to do some hard thinkin of ya budget ,an either do it all yaself n hunt bargains or splash loads of cash quickly n retire to ya "lab" n experiment , 1st takes time ,lots off,2nd just cash ,lots n lots off,
my vote goes for ,cheap old px150e lump,an a steep lurning curve, KISS ,(keep it simple s#####) choose kit n exhaust n primarys ,match barrel to cases n leave rest alone , switch that with lump in scoot an see wether its what ya want , an plan/dream/save for nxt step, H

[sime66]

All noted, H.
Think you're right except if I make an engine, I might prefer to put it in a different frame - another can of worms! (forget I said that).
Anyway, whilst eyes and ears are open for a project, indoor book-worming is going ahead anyway, and attention is turned to sorting out a couple of scooter jobs.
Started a new thread to deal with those, which are my priority....

[sime66]

Just to tidy up this business about port timing, for now, and the extent of the level of my knowledge in general. I’ll use my manky DR (The Michael Barrymore of kits) to show you why I need to learn some stuff before I consider Dremmelling anything to death. Probably not a good example, but its all I have to hand.
Heres the cylinder and head, the piston etc, and a sketch of the interesting half of the piston:













I can see three main channels, so I imagine that’s the ‘3-ports’ – that’s all I know about it, and I’m not even sure about that.
All the stuff on the piston itself – haven’t got a clue (piston porting? transfer ports? – just words; not a clue); I need to understand it before I can have a clue about what to do and what differences it might make.
Likewise I can see on the head the area that is the combustion chamber and the area that is the squish band, but that’s all I know, and I only picked that up yesterday – from scan-reading the pages briefly as they were coming out of my printer. I basically know bugger all. Which is why I’m keen to learn a bit, over winter, before spending and grinding.

[vespasco]

This may help understand the ports www.vf750fd.com/blurbs/stroke.html
Its quite basic but so is the vespa!
Transfer ports transfer fuel from crank case to combustion chamber
Exhaust ports.. Yes alright, you know!
The timing of these could be, for example, as follows:
Starting at the top....

0* TDC - crank/carb inlet of fuel to the crankcase is open.
14* atdc max force is pushing the piston downwards
21* ATDC crank inlet begins to close
74* adtc crank inlet is closed
94* atdc the exhaust port begins to open
117 atds the transfer ports open
180* -BDC
(The weight of the flywheel helps keeps the motor rotating - inertia)
244* atdc - transfers closed
253* atdc the carb/crank inlet starts to open
268* atdc the exhaust is closed
341* atdc (or 19* BTDC) the spark occurs
360* - 0* start again
(I wouldn't say the above are ideal timings or 100% accurate )!
Feel free to add anything.

Piston port could mean cylinders that suck fuel straight into the cylinder first, not crankcase.
The 3 ports you see on the piaton are the transfer ports.
'3port' cylinders and cases refer to the 3 ports you see on the crankcase and bottom of barrel.
The T5 has 5 ports for example
The 2 little holes would be boost ports? (Im not familiar with the DR kit) they could also be to do with cooling /expansion rate of piston??

Im not so clued up on porting details , having never actually physically attempted doing any myself, theres others on here that can explain it much better. i just looked into doing a long stroker on a stock 200 cylinder. When measuring timings and strokes it made me 'ponder' to say the least. Without too much work involved too.

Cylinder head design is something i looked into, again using the guides comparing them with piaggio and from other people who had done similar. See previous pic ^^^^^^^ shows my fruits (and my attempt at a stock looking expansion pipe). If i had to redesign the head again i would aim for a slightly narrower squish band, slightly larger combustion chamber and slightly smaller squish height and slightly more compression. And putting all these things together to make sure it all burns fast at the right time and makes that bang and not over heat is the fun bit.
All motors produce max downward force at 14* ATDC. Or something like that !?

Here's Ed with the weather.....

[sime66]

That’s an impressive effort for 1:00AM on your phone – very much appreciated.
Good to see it broken down into more steps and with degrees alongside the stages too; very helpful – it’s all falling into place. I’m getting a bit ahead of myself now, but I think I can see four reasons for porting mods – there’s probably more, but this is first thing in the morning for me (05:30), and I want to order my paint and stuff for scooter jobs before I get distracted by the day, so I’ll have a much more thorough read this evening:
Porting:
1. To open up, to increase volume flow
2. To reshape, to improve flow
3. To raise or lower, to alter port timing
4. To match casings to barrel
I’m still going through the two intros of the books, but when I get to the chapters on cylinders and heads I’ll start afresh, with clean threads for them, and refer back to this.
Really good info, thanks.



There's a couple of things about Carol Kirkwood that appeal more! - (depends how tight her top is )

[henri]

just 1 quibble with above ,piston ported means instead of a rotary valve induction into crankcases the inlet is controlled by ports/skirt of piston .charge still flows into crankcases for "primary compression" where its pressured so when piston top comes below transfer ports upper edge it rushes into combustion space pushing exhaust gas out . rotary or disc induction doesnt have these inlet ports leaving more piston skirt to cut transfers n boosts into .
right ya piston has 1 main transfer port,big hole n 3 supplementary ports on larger rear (opp exhaust) side , as piston comes down these give a route for charge to travel up into combustion chamber,reason for splitting em is it increases surface area without making a port so large rings will pop into it ,dr kits i think have cast rings (smallie 1's do anyway)an rule of thumb is port width cant exceed 65% of barrel diameter without welded bridges across it , an thats width measured around barrel circumference , not straight across port opening ,
your 2 side transfer ports are controlled by piston crown an cut-outs at piston skirt ,the cut outs are there so descending skirt (always a good thing)doesnt mask the inlet to transfers .an the small holes above the piston pin are boost ports , these do 2 jobs ,helping cool the piston crown an creating swirl of charge in combustion chamber ,thats why there offset the centre line ,
as said earlyier port timings are measured in degrees of rotation , when varying them usual practice is to decide when ya want em to be ,bolt barrel to cases ,rotate to the degree you want measured on a degree wheel an scribe a line or pen mark where piston crown is , remove barrel n look whats needed , then the timingscan be altered by removing metal or by moving barrel up or down in relation to crank, most people when refering to "porting" are actually talking about changing port shape/size more than timing an getting better "gas flow" ,
from what i know the dr's come with pretty spot on timings (dont get the m barrymore jokes,sorry)an just benefit from gas flowing an careful setting up (right deck height/squish/comp etc) thats why there rep is a "bolt on" improvement . other kits mallosi/polini etc have the rep for benefiting from more intensive work ,
an from memory vespaco's comment bout max thrust at 14 degs atdc sounds bout right ,but its not about the bang above but down to inertia/angular leverage an the mechanics (turning moments) of turnin linear piston travel into circular crank rotation ,so i wouldnt worry bout that bit , unless ya going to design n build a new crank design as its critical to "balance factors" an counterweight/crankweb design then. but dont reckon we'll go that far today eh , H

[henri]

just 1 quibble with above ,piston ported means instead of a rotary valve induction into crankcases the inlet is controlled by ports/skirt of piston .charge still flows into crankcases for "primary compression" where its pressured so when piston top comes below transfer ports upper edge it rushes into combustion space pushing exhaust gas out . rotary or disc induction doesnt have these inlet ports leaving more piston skirt to cut transfers n boosts into .
right ya piston has 1 main transfer port,big hole n 3 supplementary ports on larger rear (opp exhaust) side , as piston comes down these give a route for charge to travel up into combustion chamber,reason for splitting em is it increases surface area without making a port so large rings will pop into it ,dr kits i think have cast rings (smallie 1's do anyway)an rule of thumb is port width cant exceed 65% of barrel diameter without welded bridges across it , an thats width measured around barrel circumference , not straight across port opening ,
your 2 side transfer ports are controlled by piston crown an cut-outs at piston skirt ,the cut outs are there so descending skirt (always a good thing)doesnt mask the inlet to transfers .an the small holes above the piston pin are boost ports , these do 2 jobs ,helping cool the piston crown an creating swirl of charge in combustion chamber ,thats why there offset the centre line ,
as said earlyier port timings are measured in degrees of rotation , when varying them usual practice is to decide when ya want em to be ,bolt barrel to cases ,rotate to the degree you want measured on a degree wheel an scribe a line or pen mark where piston crown is , remove barrel n look whats needed , then the timingscan be altered by removing metal or by moving barrel up or down in relation to crank, most people when refering to "porting" are actually talking about changing port shape/size more than timing an getting better "gas flow" ,
from what i know the dr's come with pretty spot on timings (dont get the m barrymore jokes,sorry)an just benefit from gas flowing an careful setting up (right deck height/squish/comp etc) thats why there rep is a "bolt on" improvement . other kits mallosi/polini etc have the rep for benefiting from more intensive work ,
an from memory vespaco's comment bout max thrust at 14 degs atdc sounds bout right ,but its not about the bang above but down to inertia/angular leverage an the mechanics (turning moments) of turnin linear piston travel into circular crank rotation ,so i wouldnt worry bout that bit , unless ya going to design n build a new crank design as its critical to "balance factors" an counterweight/crankweb design then. but dont reckon we'll go that far today eh , H

[sime66]

Now chaps, that’s a lot of info to take in;
I pretty much had the hang of your earlier post, vespasco; and then yours, henri fills in some gaps, but is going to take a little longer to slide into my brain. It’s helped understand the piston itself better, which was the bit I was completely blank on, although after getting my head round the cycle without the piston transfer ports, I’m having a little difficulty picturing it with, but I’ll get there. For now though, on this piston there’s one main and three supplementary transfer ports (how and when and why the piston transfer ports do whatever they do I’m still trying to picture – just need a bit more time with it – I can see them open to the main transfer ports, but not to the combustion chamber; are they filling that earlier?), and there’s the little boost ports for cooling piston crown, and the mixing too, which is why they’re offset. The skirt cut-outs make sense to me too.
Specific degrees etc aren’t as important at the moment as understanding the overall principle, which I am getting. – I don’t think I pick new info up as quickly as I used to though.
The actual basic cycle is quite clear to me now, although it’ll take a few more reads before I remember the exact sequence and numbers (they’re not exact, I know). I did a little table for myself, using your timings, vespasco, and the most suitable image in the sequence from the GIF. The only thing that doesn’t work is the 19 BTDC spark. (The GIF shows it at TDC, so I cheated). My brain works better with pictures than just words.



Here’s the GIF, for those that like that sort of thing (just me probably – I’ve slowed it down to make it easier to follow):



This looks a useful way of thinking about degrees and stroke-stage, maybe to put some numbers against, but times running out today; maybe when I get some quiet later on…



I did also begin to understand the expansion pipe from reading the link in your post, vespasco – header, divergent, chamber, convergent… – and the pressure waves; but I don’t want to do too many subjects at once or my brain will pop. I didn’t realise the mix went down the header and got shoved back at all.
I see the images have a reed valve; I believe the crank web does that job for me in my engine. I don’t need to know the answer now, but at some stage I’ll have to look into that one; pros and cons etc.
Also, maybe for now it’s sufficient to know the difference between porting for timing, and porting to match cases with kit, which I follow too.
Anyway, that’s a good bit of learning underway there. Probably more than enough to take in for a while, but really helpful stuff – cheers.

(The Barrymore is before you two came to join in the fun – nik called my scooter a poof):