Long cranks, thick gaskets, squish, port timings……….

[sime66]

When I was rebuilding my engine pxguru threw one of his cheeky hints in about a longer crank, which I dismissed when the subject quickly went on to squish, thick gaskets, port timings etc., and was forgotten (nearly), so I could get my engine running again without the aggro of it all. This morning there’s another little reminder:



Every now and then there’s still a little cheeky hint that reawakens my curiosity, so I wondered if we might have a little chat. To me at the moment it’s all snippets of info, which feel like they might all come together and make sense if I put my mind, and your collective experience to it – I know I’m supposed to be book-worming ‘tuning’ this winter; I’ve had a break doing my rust, but head, squish etc., is sort of where I’m heading next.

This is what I think I know, without making assumptions that might be wrong:

A longer crank is going to increase my capacity, needs a bigger base gasket (or head, or combination of both?), will affect squish (which needs to be measured and adjusted accordingly), and will also affect port timings (which is my ‘Devil’s Work’, H – ‘cos taking a dremel to my barrel scares me).

Here’s what I don’t know (or rather, the current extent of what I know I don’t know):

· What squish would be ideal now, and would be ideal after?
· Which way are which ports affected? Longer/shorter, earlier/later? Is that good, or does it need correcting?
· Does this apply to increasing base gasket without longer crank, or is it different then or just nonsense?
· Are any simple improvements doable to squish or base gasket that don’t require barrel butchery, or is that a waste of time?
· There’s some relationship between capacity and squish that must relate to the compression: is that just maths? Is it important to know?

I don’t need talk to go all technical or all workshop, nor do I need to be reminded that my DR kit requires no further adjustment than aiming at the bin; just a better understanding of the basic principles. I’ll be going through the books too, and scanning the Internet, but thought a kick-start from you chaps might help me along to understanding those cheeky little messages better.

I’m more looking to understand it better, than planning any actual jobs. If there’s dumb or naive questions here; it’s a subject I’m just starting on.

(That’s my extra hour this morning gone, writing that little lot).

[henri]

basically a "long crank" gives a bigger "swept volume" , the head profile an size determines the cc of engine ,a long crank just stuffs more mix into the space ,upping the compression an fuel/mix load=bigger bang ,
your squish measuring is to make sure theres a ring of turbulence formed at outside edge of piston where it almost meets the head at tdc ,getting a good burn/flame front using up all available fuel/air in the charge ,a stock motor can have a squish anywhere from 1mm out to 3mm ,anymore an it will be a really "flat" state of tune ,measuring an adjusting the gap is to get it as close to 1-1.5mm ,the optimum, less than 1mm an at high revs n temps an the piston/conrod can actually stretch enuff for contact to be made, this is a BAD thing, an gives more bang from ya bucks than you want ,literally,
its not really possible to answer ya porting questions here ,theres loads of other questions ya need to ask first , what you want from lump,which exhaust gives the power at which revs you want to port to , which induction system are you planning ,rotary valve/reedvalve/cut or full circle crank , once youve made these decisions an measured what timings ya kits got ,then you can decide where you want to alter timings an by which method ,which is a massive can of worms where no 2 tuners agree on anything ,
you ask for simple things with no butchery ,well apart from checking an setting ya squish the rest is all complications with no "magic fairy dust" 1 solution fits all type of stuff .1 question begats 10 more choices ,have ya read the tuning thread on the smallframe forum, it is clearer than anything my 1 fat typing finger n brain can manage ,
an ya dr kit isnt bin-fodder ,its the best plug n play kit on the market ,on matched cases it gives a certain bhp hike with usable torque with lots of different exhausts an carbs ,with reliability , it does this by not being too radical in its design , the mallosi is in the mid ground an the polini gives more but needs more work an only gives its best with exhausts designed for it, out of the box the dr gives its all an theres not much more ya can squeeze out of it ,the others have more in reserve that tuning can bring out ,if ya see what i mean , H

[vespasco]

Ill have a go although porting is not my strongest subject.
A longer crank doesnt necessarily mean you need a bigger (or indeed smaller) base gasket.
On my pinasco 225 for example, i use a long stroke with stock gasket and everything lines up (almost) perfectly. But yes i had to make a head to suit! If youre talking of changing to a long stroke on a good set up with short crank then yes,chances are you would ideally need to pack base and the head to end up with the same squish as before.
With longer stroke you also run the risk of the bottom of the piston hitting the crankcase at BDC...(like wise to the head at TDC)!

Ideal squish - in the tuning books the rule of thumb seems to be the smaller cc the smaller squish you can use:
100cc - 125cc = 0.7 - 0.9mm
175cc - 250cc = 1.0 - 1.4mm
But if youve ever checked the squish gap on a stock px then youll see just how much you can get away with!!!! (P200 for eg - 3mm)!?!?! Theres various reasons for such a large squish.... Lowercomprssion which helps overcome cheap fuel issues, allows for more retarded ignition, lower emissions, better fuel economy?!
But do remember the 'squish' figures gven above seem to be for getting the most power out of your tuned engine, without taking into account reliability and longevity etc.
A longer crank would extend port timings...the (exhaust and transfers) ports will be open for longer, which could be a good thing or it could be a bad thing, depending on the actual port timings.. Its not qute as simple as it seems.... Altering exhaust timings for eg affects the transfer timings and the lead time (the time inbetween when the inlet closes and the exhaust opens).

Another way to alter the ports but not the timing is to make the ports bigger/wider, on the horizontal axis. Again there are 'rules' that say just how much you can do this by. 65% of the surface area of the cylinder/piston?

The way i start,if i dont plan on doing any port modifications (i like to think the companies who make these kits has some idea although i seriously doubt that where the pinasco 215 is concerned)!!! is to line up the top of the upper most piston ring/top of piston to the bottom of the exhaust port...... From there i would easily work out my ideal base thickness.... This then leaves the deck height to check (top of piston to top of cylinder)... Hopefully the upper piston ring will be no less than approx 0.5mm from the top of cylinder (you dont want the pistn coming out of the top ofthe cylinder)!!...and hopefully wont be so large that you would need a negative squish head (a head that sits insde the cyliner,not on top) .this is where you would likely adjust the squish gap...on the head....as opposd to skimming the top of the cylinder) which by doing so will also alter the compression in the head!!
The width and height and angle and surface area of the squish band on the head all contribute to what we know as 'the squish'...(it's not just the gap between head and piston)!!
If you adjust only the squish band (by machining for example), you would also alter the cc of the combustion chamber!! Working out the ideal cc size of the head, (and thus compression ratios) can be done with maths but then there are correct and uncorrected values to consider....(piston at top of stroke or just above exhaust ports).... Compression ratios around 9:0 - 11:0 seem to be safe for most road going vespas over 125cc...
Once the ports are at their optimum height/width then flowing the ports is probably the next best thing to do without going OTT.

I Havent had my breakfast yet...head hurts...need more coffee.... I know someone else will explain/understand this much better than me!!

[vespasco]

Oh crumbs!!! There were no replies when i started typing my reply!!!!!

[sime66]

Wow! Very much appreciate the replies from the usual suspects.
Can’t go into great depth now, but realise now that I need to read more before I’m able to ask the right questions.
Will read both replies thoroughly later – must now put apron on and attend to Sunday Roast.
I can see, other than back to the books, a squish measure ‘as-is’ might be all I want to do for now, but I’ll get back to it all later on today….

(been waiting to see how you got on with your weekend jobs, vespasco)

[vespasco]

Oct 26, 2014 10:31:25 GMT sime66 said:

.

(been waiting to see how you got on with your weekend jobs, vespasco)

Me too.. Im not convinced ill have time today!!!! It took me all week just to get the motor out of the frame @10 minutes a day!!!

[pxguru]

Oh look what I started
Just to get one thing straight, all I will suggest on here are things that will do no damage and can be reversed. And changes that will generally make your scooter faster will usually be less economical. Standard looking fast scooters are currently my thing.

One thing about kits is they are made to fit any engine. As every engine is different in terms of tolerances a kit will rarely ever be perfectly set straight out the box. The one easy thing anyone can do with DR kits is set the barrel height, this is really important if your aim is to get the most power out of it. DR tend to be always under, or way under. For a standard DR kit the squish clearance is not as important as the barrel height, anywhere between 1 and 3 mm squish on a standard DR is fine.

[sime66]

They’re little jewels of contributions, pxguru; I always think there’s more meant than said, like little clues, which is good ‘cos it gets me thinking.
So going back to what you meant in the other thread, rather than me charging off and bogging myself down in the technical mire; you’re suggesting that there is probably a safe improvement to be had from setting the barrel height correctly to suit the engine tolerances (raising it, in your experience, because they always seem to be under), and as long as there is sufficiently less than a maximum squish already, the barrel height adjustment won’t matter to the not-so-critical squish on a standard DR, as long as it remains below the maximum and within the 1-3mm range, after the increase for the barrel height setting (raising)?
If that’s correct, then the next question would be how to determine by how much the barrel should be raised, or is that the trial and (safe) error bit?
I’m thinking if you know your squish, you then know how much you have to play with.
I’m aware there’s a difference of opinion on how safe it is; I’m just making sure I understand what you meant. I have taken note, H, what you said about needing “to measure n check your engine before randomly switching gaskets hunting a quick fix” on the other thread. Also noted, H, is that your optimum of 1.5mm, leaves far less room for trial and error than 3mm; and your in-depth squish stuff above, and the other subjects, which I’ve only brushed over so far, vespasco. The difference of opinion inclines me towards the ‘don’t fix what ain’t broken’ approach, just for now, other than to measure my squish sometime out of curiosity.
I still haven’t yet read all the above stuff properly, so that’s all for now – back to me bird……………

[vespasco]

The starting point for me with setting the barrel height correctly, is as above, top of piston/rings with bottom of exhaust port... Anyless than that, the pistons rings wont stay as cool, exhaust port is open for shorter period too, less time for hot gases to escape, heat build up, blah! Blah,blah!
The further the piston travels down past the bottom of the exhaust port, the more cooling the rings will recieve. It will also giving you longer exhaust port timing, which can be used to your advantage too! But for me id prefer to see that piston get back up to TDC again as fast as possible, not hang around below the exhaust port...
Thats a good base point to start at anyway, for any 2stroke cylinder. From there you can experiment!

Remember when you raise the cylinder and use the same head you will loose a little compression, as the volume of the combustion chamber increases.
For example, (from memory) on my pinasco 225cc, for every 0.38mm i raise my cylinder, i would gain 1cc volume in the head, and obviously also increase my squish gap, which would also lower my compression.
I think we're all singing from the same songsheet, just maybe different verses!

[vespasco]

Thats what i use too, along with other classic by Graham A Bell,
2 Stroke Performance Tuning...

www.google.co.uk/url?sa=t&source=web&rct=j&ei=q15NVPrvEs3B7Abu0YDgCA&url=http://iheartstella.com/resources/manuals/tuning/Graham-Bell-Two-Stroke-Performance-Tuning.pdf&ved=0CB8QFjAA&usg=AFQjCNEe1SK8icPjJqxJxt8uZVbQQPTpaQ&sig2=k-oWfrtrRCJzpJlTPA_1cw


And of course, along with sound advice from here (and there)!

[pxguru]

Oct 26, 2014 18:15:27 GMT vespasco said:

The starting point for me with setting the barrel height correctly, is as above, top of piston/rings with bottom of exhaust port... Anyless than that, the pistons rings wont stay as cool, exhaust port is open for shorter period too, less time for hot gases to escape, heat build up, blah! Blah,blah!
The further the piston travels down past the bottom of the exhaust port, the more cooling the rings will recieve. It will also giving you longer exhaust port timing, which can be used to your advantage too! But for me id prefer to see that piston get back up to TDC again as fast as possible, not hang around below the exhaust port...
Thats a good base point to start at anyway, for any 2stroke cylinder. From there you can experiment!

Remember when you raise the cylinder and use the same head you will loose a little compression, as the volume of the combustion chamber increases.
For example, (from memory) on my pinasco 225cc, for every 0.38mm i raise my cylinder, i would gain 1cc volume in the head, and obviously also increase my squish gap, which would also lower my compression.
I think we're all singing from the same songsheet, just maybe different verses!

For me setting of the barrel height is not directly about the exhaust port. Keeping the rings covered when the exhaust is fully open is totally true and necessary but not the focus of the set up. When setting up the height of a barrel it is the height of the top of the main transfer ports which is the highest priority. The exact amount of degrees set here for the decided purpose of the engine is fundamental for the entire engine set up. The closer the degrees are set to optimum without going over the more power there will be.

[sime66]

Righto chaps,
I’ve had a good read this morning; I understand what pxguru was getting at, which was the reason I asked in the first place.
I also understand that if I want to do anything at all I need to be able to translate port positions, sizes etc into timing (degrees), which may seem obvious to you, but still needs me to do some reading elsewhere. I did read the smallframe tuning stuff, yes H; I have it copied on my desktop. I’ve also made a start on both of those books about a month ago, I printed the first few chapters of each off before, so I’m going back to them. Stuff is not going into my brain and staying there like it used to, which is frustrating; age and alcohol taking their toll I suppose.
I can see you disagree about a basic start point, but I don’t even understand either of the arguments for the difference of opinion over priorities
I’ll leave you to sort out the boy, his cartoons, and Tellytubby software; I’m sure it has some relevance, but I’m not going to try to read any more on screen with that animated GIF all over every thread.
(Once would have sufficed - and that was done a month ago).

[vespasco]

Ah yeh! Gordon Jennings thats the one!
I absorbed as much as i could from these two books a while back, but what little went in is slowly falling out.
I dont think theres a difference of opinion, what i said still holds true, just not necessarily using my method as the ultimate base point! That was the limit of my knowledge on that particular subject until pxguru ji enlightened us all
I did think about lining the ring up with other ports but i never really knew which was best or why etc. I settled for the exhaust port method safe in the knowledge that its good practice and quite important (and it was far easier to check)!
Now, with this additional (brilliant) info i will inspect that area (top of transfers) more closely on dis/assembly. Next, how to find optimum transfer timings?!

*Google.... 'Lambretta port timing calculator' or similar.

C'mon .... share the biscuits

[sime66]

Lets say that ‘pxguru ji’ has polished your already precious gems.
A bit of quiet reading tonight, and it’s falling into place a bit more; no hurry.
I’ve looked at the timing calculators now, and I’ll find the relevant pages in the good book(s) to understand how they’re giving the results they do (a bit of trigonometry and Pythagoras by the looks).
There’s several calculators, but scooterhelp is familiar, so I’ll start with that:
www.scooterhelp.com/tips/timing/timing.calc.html
Roughly this:

[pxguru]

Not sure the books will help you so much. They are a little more theoretical than practical.
That timing calculator on scooterhelp works perfectly. 57mm stroke and 105mm conrod on a standard PX125/150.
Transfer port timing is normally stated in total duration time. The time the ports are open within the 360 degree period. The ports are obviously open the same amount of degrees either side of BDC.

[sime66]

I only want enough theory just to follow the topic when it comes up in threads. The books do get heavy very quickly, and lose me, so the maths itself has still got me beat, but I have got the same result as scooterhelp by accurate technical drawing (AutoCAD), so that’s good enough, just to understand it:






I might still work the maths out; just out of curiosity, but I do now understand. (Would like to do my own calculator, like the gear ratio one)
I think what I’m going to do later today is use vespasco’s previous theoretical port timings to do a timing diagram, and then convert degrees to mm between ports opening and closing.
Then I reckon I’ll have enough in the noggin to satisfy myself.
Need a brain leak test to make sure it stays there

[sime66]

It seems less important now I’ve done it, but doing it once to understand it is worthwhile for me anyway, so here it is:







Now I can go back to what you were saying about setting the top of the main transfer port in degrees, and following what that means in mm, and get a bit more practical with it; you were one step too many ahead of me before…. – Happy Days!

[sime66]

LITTLE CALCULATOR – READY TO TEST:
(I’ll email it to anyone who wants a play)



Only problem is I just realised I need to do mm BTDC back to degrees as well.....

[sime66]

LITTLE CALCULATOR (Now From Degrees or mm BTDC) – READY TO TEST:



(working):

[henri]

crikey , 1 day away with a killer head cold an you beggars are half way to re-inventing the shuttle , an who's animated 2 stroke cycle is that , it makes me dizzy ,
sime dont get too bogged down in the maths n theory off bell n jennings , there talkin about engine building/designing a lot of the time , your engines built n the designs set ,your aim is to get most out of what piaggio/dr have given ya , first read that stuff will make ya head spin ,but if ya drop into the less involved bits an use loads of page markers to jump back n forth before long it will start to make sense , bit of 5 pages a night bedtime reading an a "crib sheet" for any particularly relevant stuff will start to build ya own individual tuning manual ,
got to have a bath now,but will re-read the above n see if my befuddled mind can add anything useful after tea, H

[pxguru]

Wow, you have been busy
Just need to add a box in your little calculator to offset the static deck height ((how much the piston is below from TDC) there is always some thing!)

It won't be long now until you realise you havent asked the actual real question yet

[sime66]

I’m not doing any more thinking tonight, but a few questions did come to mind whilst I was doing the calculator, because I have a length, but don't know where on the barrel it is; roughly these (not really thought through yet, but since you asked):
1) At some time all this has to be turned into actually measuring on piston and barrel, and I can’t quite see how you would do that unless you mark/measure TDC and then remove it to measure the ports.
2) The other thing that’s been nagging me is that there are two rings and the overall distance between outer-side of top and bottom rings must come into it somewhere. (Where are we measuring from?)
3) If we then add in the subject of your ‘Static Deck Height’, which is something I’m not even going to think about tonight, then there is something (Some little magic key, that you hint at but won’t reveal, you tease) that converts the theory to the practical – not tonight though, or I’ll be at it all night……………
If either of you’d like to give any more clues, or comments, I do tend to have an hour pondering over the morning coffee; it’s just this morning it turned into a bigger job because I got a bit carried away with it.
I know it’s more than is necessary, H; but going right back to the start and building from scratch helps me to understand and remember it better. – Haven’t touched the books today, just maths and comparison with scooterhelp calculator results. Anyway, I confess, I actually enjoyed testing my memory of the maths, so it wasn’t just about timing. I’m in no hurry; this is all my winter, in the warm, project.

THE REAL QUESTION IS:……….answers?

This really is it for me tonight, but I reckon this ties in the questions I have coming together, your hint, and the static head height:
D, d, and H (and where on H we're talking about)
Deck height is where it all started - to make it as Manufacturer intended - so is it that I need to know the intended deck height, D; or the intended deck clearance, d; to know the amount out in mm and then degrees?



(4) I did also have a question about the acelleration/deceleration of the piston, and whether timing was ever thought of as actual fractions of a second, but I decided that they were such miniscule times that it would be a silly question, so I didn't ask it - except I sort of have done now by mentioning it, even though I decided not to)

[henri]

wow agin , an very enigmatic pxguru, but spose a guru has to be that way to earn the name ,
timing is always done in degrees rotation of crank sime , port heights an ignition timing can be done in mm of piston travel but is less accurate an confusing ,an never mind fractions of seconds degree's can be split down in to minutes an seconds of arc too , but that sort of accuracy isnt necessary.
an as a aside to ya accel/deccel question an the figures quoted earlier (by vespaco i tink) thats what squish minimum values are about , the co-efficent of expansion of material conrod is made from + the stretch of it due to rapid decceleration , he quoted .7-.9 for upto 125cc an 1-1.4 for 175-250cc ,which if memory serves is bells figures an relevant to japanese short stroke multi cylinder engines, i seem to remember it from my wasted youth n rd-lc's , all recent thought ive seen on vespa/bretta singles tuning puts a def min squish of 1mm ,with most aiming for 1.2 , if your going long crank i'd def stick to the larger figure as long conrod expands more coz there is more,an stretches more as it faces greater forces/stress on decceleration at the top of stroke ,
also i saw vespaco said port width has a limit of 65% of piston area , i always thought/worked to a 60% of bore diameter limit , with anything wider needing bridging or rings will pop into it n risk shattering , on the mallossi barrel i was given as its damaged on lower edge of exhaust port i measured it,as its the second mallossi ive seen this on ,an found it to be right on the 60% limit , scoot was still runnin fine an rings were whole but something had def chipped the bore ,maybe bad castings or porting work by mallossi had left a weakness.
an above sime your askin questions bout ring depth from crown of piston an where to time from ,ive always done it from piston crown as even if rings are below port edge the piston is still close enuff to the bore to give the same kind of seal as the sealing pad on crank, in the fractions of seconds between rings n crown uncovering a port theres only so much gas can physically fit through an to try n measure it an allow for it in computations is a unnecessary complication ,
an as for measuring ,the barrel comes off n a roll of mm square graph paper is stuck in an port outlines are drawn on , then port area can be measured ,an if deck height an crown at bdc was noted rough timings worked out in mm piston travel , then barrel back on an torqued down measured again in degrees ,theres ya base line , an i guess guru's hinted at question comes nxt , which i'm guessing is ,what ya want to do=which rev band you are tuning the ports for , thats when gas flow speeds can be added to the mix ,
will try n look out my old tuning black books tommorrow ,but theres a reason i hid em in the back of the most rarely emptied cuboard ive got, this stuff can become all consuming ,well for me anyway , an every bit ya learn just shows how much ya dont know ,am trying not to be sucked back in as tuneittis comes with a expensive cure , H

[vespasco]

And a double Wow! Wow! Now then, now then.
Impressive work!
And impressive contributions too. I like it when i learn something new and highly usable!
You re right about the port width thing henri (and just about everything else)!!! i was srruggling when i tried to type it (from memory) and yes i was quoting from said book.
I measure from the top of upper ring on my Pinasco but only because its a Dykes ring (fnarr! fnarr!) I couldn't resist! Otherwise, generally, it would be the crown as mentioned.
Im not sure if you've seen this sime.... Port timing calculator...
www.google.co.uk/url?sa=t&source=web&rct=j&ei=qgVQVLW0LsfdPd2RgeAE&url=http://lambretta-images.com/archive/porttiming.php&ved=0CB8QFjAA&usg=AFQjCNHBl7yk-3hCx62RuwHatav5YAiPjQ&sig2=JsUg7yYzxT12xEus3Mvu0A

Im going to have another read through later
Very intevezting !

[sime66]

I slept late, and have to rush out, so I'll go back over each reply in more detail later, but it occurred to me overnight that the static deck height (deck clearance, d above) must be, or at least partly be, the squish band. So I need it in my calculator to give me the reference point on the barrel. It must also be the start point for any adjustments to deck (for reaching optimum design, not for tuning).
For example, DR make the barrel and the piston, but not the cases, so the deck height is where the manufacturer's tolerances would be measurable.
somewhere there must be the deck height DR intended, from which the adjustments can be worked out, and also the deck height at the moment gives the current timing.

That's a bit rushed, and I might well edit or remove it a bit later, but it's what I was thinking in the early hours....

Edit

Still concentrating on what the keeper of esoteric knowledge was hinting at, and the facts that we’re talking about getting the barrel height as intended – not tuning, and the fact that I’m still trying to find my setting out point, I have done a little sketch showing the dimensions that DR are responsible for (DR), and the one’s that are from Piaggio’s case manufacturing tolerances (P). Some (*) might be affected by later changes of parts (e.g. crank, con-rod), and the deviation from ideal/design can be taken up either adding or skimming at the red blobs:



Basically DR make the barrel and piston, but they can’t control the position of it relative to TDC.

I’m thinking that the start point must be to measure the Static Deck Height, which must somewhere have a design reference, and might also tell us how much adjustment there is available because it either is the same as the squish band, or makes up part of it. (That is the thing I’m going to find out later today, but it’s going on to the head, which wasn’t my intention yet).

I’m still going to go back over last night’s posts, which I’ve read over quickly and will reply to, but I want to get this bit down first to keep the learning process in sequence.

Don’t want any wild horse chases after the stable doors bolted!

Being sensible again for a moment, can I check that we don't think about the dome or the position of the rings, or any piston ports; for doing this bit we're just thinking of the piston as a solid block whose face is the only point of reference? i think you've said that, but it seems odd when we're talking about fractions of a mm. (I'm also wondering how you can cut a barrel with sufficient accuracy too, but that's way off into horse goosing territory).

Another edit:

These are from the good books, taken out of context, but I’m just using them to show what I mean.

This one shows a step inside the inner profile of the head to make the squish band:



This one shows the squish band being the static head clearance, with the start of the inner profile of the head being flush with the face of the barrel:



Or am I barking up the wrong ball park?

Final edit:

Now I’ve got that lot above off my chest, here’s replies to what you lot wrote last night:

H:
‘The designs set’ – is that information somewhere then? what DR intended the dimensions to be? TDC relative to barrel, which isn’t fixed by Piaggio or DR, and I think is where I need to be concentrating.
‘Timing is always done in degrees’, and pxguru also said about timing being in terms of total port duration (I’ll find the exact words later), so we’re talking, with main transfer port for example about T.o. (degrees) then T.c. (degrees), and the degrees between as duration (degrees).
I think I don’t want to know about widths of ports just yet because that’s more tuning than setting the head (I think). (But it’s in the brain for later…..)
You’ve said that the piston is close enough to the bore (similar seal to crank pad) that the rings can be ignored. – You all seem to agree on that in fact.
I don’t think the question is about which rev band I’m tuning for because I’m setting up the head to design, not tuning, although pxguru did say early on about The ‘exact amount of degrees set at the main transfer port for the decided purpose of the engine being fundamental for the entire engine set up.’ – But the purpose of my engine (at the moment) is just to get the head height as designed though isn’t it?

Pxguru:
Does the ‘Static Deck Height’ come from actually measuring what’s there? Because by my reckoning there’s the actual (as it actually is on my scooter), and the intended/optimum/design.
If I still haven’t got the ‘Real Question’, I’m floundering a bit like a fish without a paddle out of deep water.

Vespasco:
I’ve looked up a Dykes ring (oh that’s rude), and understand what you mean, and think we’ve clarified the piston reference point now, so that’s good because it’s simpler than I expected. (About the only thing I’ve come across so far that is).
I hadn’t seen the Lambretta calculator, but I have now; those Lambretta boys do like to show off don’t they. Made my efforts yesterday seem a bit feeble – but not wasted effort because of the learning whilst doing my calculator. It’s gonna take me ages to replicate all those other calculators though, most of which I don’t even understand yet.



That’s me for now chaps; going to try to leave it alone and think about some other jobs for a bit.

[henri]

right , this is from very hazy memory but believe the "stock " deck height of vespas is 0 , as in not below or above ,but with piaggio manufacturing tolerances it can be 0 to -1mm , giving squishes from 1.5mm to 3mm ,with the tendency of the engines ive measured towards the later,
an sorry vespaco i'd forgot your running a pinasco an that they run a "dykes" ,with that design of ring i'm pretty sure theres a fancy equation you add on to the timing calculater to make allowances for the slightly diff gas flow .
i havent dug out my notes yet as there well buried an intended to stay that way ,but am dealin with a slight case of "tuneittis" with smally complications ,but from what ive seen recent what was cutting edge early to mid 80's is def well past its sell by date now, an is mostly jap/yamaha n motocross specific anyway ,so looks like this old dog is goin to re-learn a few new tricks along the way .
an to answer aquestion ya put to guru sime ,the static deck height is whats actually there on your set up ,piston on an barrel torqued down but no head , measured mostly with a Dial Test Indicater or verniers ,an its called static as theres no allowance taken for heat-expansion or decceleration stretch at this point , thats what ya squish minimum is for ,
an as px guru hinted an i'm sure off , tuning is like shootin birds with a shotgun , ya aim ahead so bird n shot are in same place at same time , for tuning ya ask where ya want to be an work out a route (aim) to get to ya target from where ya standing ,
unless he's hinting at what the sign above the mechanic in mad max 1 says " speeds just a matter of money,how fast ya wanna go" , maybe the question he reckons ya need to ask is "how rich am i ,n how poor but fast do i want to become", H

[pxguru]

Really very close to the point now. You have the idea though. Adjusting the base gasket to set up the transfer duration to where you need it to be for your purpose or set to just where it is supposed to be on the manufacturers design.
Measuring the static deck height is necessary for the calculations to be accurate. Head off, clamp the barrel down, hold the piston at TDC, then ram feeler gauges into the top of the piston until you feel a smooth transition with the top of the barrel. With practice this can be done with only two hands
To us degrees and time are the same thing. When it comes to transfer ports, plus or minus a few degrees really notices and this is no big distance. What Sime is nearly on to is not about spending money to achieve but just free power that was always available

[henri]

erm gaz , me n the missus had this discussion an came to a compromise ,she can start on top but i always finish there , or am i well off topic here , H

[sime66]

So, I need to know my static deck height and my squish to continue, both measured on my scooter as they are now, which isn’t going to happen any time soon, or without buying some more tools.
But getting back to pxguru saying that I’m close to the point, and thinking that the barrel and head are as DR intended, relative to each other, and that adjusting the barrel moves the piston the same relative to both if their mating surface is left alone; and from everything I read and watch referring to the power gain being from getting the squish right, should I be concentrating on squish, rather than the exact static deck height? And then using the static deck height as an adjustment in my timing calcs for the port timing? NO

That makes sense to me; I reckon that’s it:

You get the squish right, as priority, and then use the resulting static deck height as an adjustment in the timing calculator to then check and get the port timings right.

But I think as the head, barrel and piston are all DR, getting the piston right for head (squish) will get it right for barrel too.

^^IF ANYONE READS THIS IN THE FUTURE, IGNORE THAT BIT, IT IS INCORRECT.^^ READ ON (WE MIGHT GET IT RIGHT).....................

(Not sure if I made a logical step, or just ignored everything you just said 2 hours ago)

[vespasco]

I think i may have missed something. I'm not convinced getting the squish right first is the important thing. For a start ,what is the ideal squish? By our vespa px guide lines, we aim for 1.0mm - 1.5mm ?! Thats a variable you dont need in your calcs. But the static deck height would be needed for your calcs yes.
It is late and ive had a hard day and right now reading that, in my mixed up mind, i have more questions than answers..... Right now i think anything above TDC of the piston crown is out of the equation until the transfer timing is adjusted to optimum, which for the Dr. is achieved by raising cylinder 1mm. Is that true? (Faint alarm bells are ringing) What that would actually give in degrees of transfer duration i do not know but can only guess at around 124°?) Is that the optimum to aim for? I think ive gone round in a circle as im starting to ask myself what is optimum transfer opening/closing duration? and how is optimum determined ? And thats not the right question as i cant imagne how it would be answered (although i will still wait for an answer)! I thought i understood. I do need to read this all again and maybe even do a bit of revision myself!! It is getting late! I need sleeeeeeep...and if i cant I'll count the degrees of the crank going round and round and round...
Great thread tho'