Lambretta Boost Port & Cylinder Scavenging

[Darrell Taylor]

just stumbled on this one and theres a lot to get thru on here ive dropped a few replys in blue

Thought it might be an interesting topic (could be wrong! lol) to have a look at the problems associated with the boost port angle (amongst other things) on the well known TS1 cylinder. I'm not quite sure why it was designed this way, but in my humble opinion it is to the detriment of power and performance of the cylinder set up.

how do we know its of detriment until a collection of tests are carried out to confirm or deny,like so many tuners if confronted with the question "why do you do it like that" i doubt the answer will be based on actual results as theres often too many variables to test that youd never earn a living doing the day job ,if you buy a std ts1, fit it, base dyno test then change boost roof angles,analyse the results then publish them on here it would be really good of you but to make the test correct other factors would cloud the results ,in changing the angle could mean raising the boost changing its duration and also increases area and increase the duct volume so if a positive result was found then what gave the improvement?



The main purpose of the boost port at the back of a schnuerle-loop scavenged reed-valve cylinder is to scoop out the dead exhaust gasses in the cylinder bowl and push/mix the new charge into place. I examined several cylinders today, and they are fall into the traditional category of 50-65 degree angle for the boost port.

the boost ports primary benefit is adding extra transfer area but doing so in a manner that can be used effectively to purge spent charge from the rear cylinder wall to help cleanse the cylinder prior to the fresh charge arriving in an attempt to maintain its purity ,pushing the old spent charge up and over the new charge arriving from the mains and secondary transfers , we dont want to push and mix the charge into place

The pic below shows the cylinder circumference along with the angle of entry for transfers (T1 & T2). It also shows (the big dot in the middle) the sweet spot where the boost port angle directs the gasses in the cylinder head:

the black dot needs changing to a black line to the rear of the cylinder as reading thru the whole topic i cant see mention of how the port flows in a working engine (as in with a piston slid down the bore) the roof angle only has been discussed and no consideration to floor angle or in the case of a ts1 no floor angle exists appart from the piston face/crown edge which acts upon all transfers as a power valve changing opening and constantly changing flow direction which makes the boost into a multi angled port ,you also have to consider where the fuel is delivered from be it horizontally or vertically which on a std ts1 is a combination as is delivered horizontally from the reed and part vertically from the piston windows/skirt in a loop when operating in loop scavenge piston positions



The TS1 cylinder however is angled in at a very low 45-46 degrees. The other cylinders all work off centre plug/squish heads, which essentially means that the boost port will scoop straight up into the bowl and scoop it out nicely....where as the TS1 not only has a very shallow angle of boost port, but also uses an off-set squish/bowl....which means the main flow from the boost port misses the bowl completely. Thus....the main advantage of having a rear boost port is largely diminished.

In the image below i rested a metal shaft into the head resting at the angle of the boost ports direction:


Performance cylinders from big manufacturers come in at much steeper angles:


This suggests that the use of centre squish heads and the redirection of the boost port angle is of benefit when tuning TS1 cylinders.................................

as you say Dan an area of interest worth discussion and potential for improvement ,good topic

[Darrell Taylor]

Yes Adam, the gasses do of course diverge, and the combined angles/forces give the resulting loop and purge. And you are right, the boost port is not to be considered as a singular item, I'm merely highlighting that particular shortcoming. Another one of which in my opinion is the fact that it does, as you pointed out, open later/last......on my cylinders I change that so it opens first.

on the std ts1 the boost port is positioned/duration timed to open last but in its working enviroment will flow first as the primary and secondary are subjected to blowdown flow reversal.
at the time of transfer opening exhaust blow down is continuing and will back flow down the transfers especially more so on blowdown limited cylinder designs and port durations as are present in scooter cylinders this will drag the spent charge across the crown to the boost (now ex port as it will evacuate out of every hole it is offered)

[Darrell Taylor]

I really do not think you are going to meaningfull scratch the surface of this topic with out some serious investment. If I were to consider how best to go about that it would certainly be a CFD programe (not cheap, but much cheaper than building and buying test equipment). In the end, it will all boil down to hypothosis and what people have done previously; which isn't a bad thing but don't expect a clear winner, if there were, there'd only be one two stroke tuning book.

i agree, only thing to do for those with time/money and facilitys to do so is start testing and see what results emerge,this represents a large investment in many ways which carrys well onto a later post

[Darrell Taylor]

Experimenting @ the moment with holes in the piston skirt aligned with the exhaust port
Theory being the reflected sonic waves not only help the scavenging when the exhaust port is open
But also increase the crankcase pressure via the holes when not

Some two-stroke theory I read a while back ~ Giving it a whirl " Nothing better to do" :biggrin:

The engine is running fine and readings fine
Going to log some egt + cht readings and change back to an unmodified piston "Winter Strip"

Who knows :wtf:

can see this causing heavy contamination and pre heating of the fresh charge,
did a dyno back to back and saw around 5% power loss across the range as a result of a similar scenario,i used malossi pistons in an ar50 race motor and the skirt needed cutting by around 1.5mm to clear the webs a swap to a customer cut piston(nearly 3mm) saw the ex port stil left open to the crankcase at tdc

[Darrell Taylor]

Dan do you remember "Tom the tuner" who you met at Elvington.

He rubbishes the TS1 saying the boost port angle is wrong. He says that the Harry Barlow TS1 alike kit has the angle right and "works". He said this at Wicksteed last sunday whilst giving me one of his "lectures".

be interesting if you had continued questioning why the ts1 is wrong and why the new one is correct and if there was any test data to substantiate it
a lot of statements are merely speculation until tested,as well as my own i might add

[Darrell Taylor]

For sure Ric. And thats what I said. This takes us back to the pipe design. Thats the only thing has changed that will affect really... So... someone says "thats all wrong" well... they maybe right... today.... but when it was designed, maybe not at all.

Its not a case of who is 'right' and who is 'wrong'.....cylinder port scavenging techniques can be a very subjective item, which is what makes conversations like this so interesting. And its certainly not a case of saying TS didn't know what he was doing, not at all.... despite possible difficult instruction from his employers, or what he did/didnt have to work with at the time. The aim of the thread is to analyse where we are at today, because as we select pipes, pistons, porting techniques and so on, we are only concerned with what is available today and the power we are creating today.....and hopefully debates like this will tease out more and more collaborative opinions in order to help us to continue to achieve those goals. That is my goal, as its always been with Scooterotica....to share information, to analyse and discuss. Hopefully then, we all benefit.

Not like the days of yester-year..... cloak and dagger tuning techniques, grinding details of jets....the 'dark art' of tuning and jetting.......b@11@x to that bullshit i say! Which other site photographed dozens of tuned cylinders and supplied port maps, port widths, dyno charts and jetting examples? Complimented with a variety of information (ever growing) to help improve things even further. Most tuners wont even let you see inside a cylinder they have ported.....we photograph them and publish them! lol And if you request it, when i dyno scoots, i include the final jetting spec on the graph!

dont think ive seen anywhere on here photo,s/port maps/widths/dyno charts/jetting specs for any tuned engines other than edens 240

I think its a shame that most tuners who are actually good at what they do remain aloof and don't want to share. Its appears to be a terribly vain and self-serving way of trying to place themselves in some sort of higher-echelon (legends in their own minds) wanting to distance themselves from anyone who takes a practical interest in tuning as if they are beneath them. Happy to tell you what you DONT know, but not willing to discuss what they purport to know. But this backward view of not discussing anything publicly is usually more to do with fear of criticism......these people are often not willing to put their theory's up for debate, or share their working knowledge. Don't get me wrong, every year brings new development and new discoveries that keep the top people....at the top, and i'm certainly not suggestion anyone would discuss these finer details, not for one second. But there are generally speaking, very well known, understood and published tuning techniques....available to all and sundry in any tuning book, but most of the 'high end' tuners wont even chance a public discussion on these topics, or publish a picture of similar work they have done to explain the how/why.......shame really.

i agree Dan theres many who spend more time visiting sites placing adverts for there services and products but never contribute to discussions or share any info take a look down your members list click on some names and check there posts and it paints a picture of there intent ,im in the trade and often contribute but havent advertised my services


BUT WHY SHOULD THEY TELL US ANYTHING........ITS THEIR LIVING! I hear people hark at the back......yes, it maybe...but the majority of people who are interested in the tuning techniques to such a depth, already do their own tuning or have a relied-upon tuner that they use repeatedly to do it for them....so they lose no business, in fact the sharing of information generally results in MORE interest in tuning and MORE business for the tuner, not less. Just look at where we are now with Lambretta and Vespa tuning, we are in a golden age....new kits from Cam Lam/Charlie, Ron Moss, AF, GT, Falc, Quattrini, Polini.......websites like this publishing/sharing information. Home tuners modifying cylinders, tuning companies selling more 'tuning' goodies than ever, sprint meets, new people art BSSO race meets, faster laps times, more powerful machines each year......

Inspect, analyse, discuss, modify, analyse, share.....continue to improve.


like mentioned on a few previous posts sometimes a large investment in time and money is made to establish tuning eureka moments on a particular engine that either the tuner has invested or his customer has paid for so for example 4 weeks of testing 30 different combinations of ex and transfer duration combinations on 3 new cylinder kits on 1 specific pipe is not gonna be followed by "hey everyone look what i found out"


But i digress...... ......in my experience the boost port on a TS1 is better employed at a steeper angle, combined with the use of a centre plug head. Thoughts?

[Dono]

After reading through all this info, I need a lay down :fb: I find the best way to sort out my cylinder is just pick the phone up and send it to Darrell Taylor :anbrgin; Job Done :anbrgin;

[Darrell Taylor]

Wot benefit is gained by having the boost port open before or at the same time as the main and secondary transfer ports??

over the years many methods have been adopted,and during the honda rs125/250 years of production have moved above and below suggesting on them theres pro,s and cons to both but these cylinders have huge blow down area and long ex durations
all recent race cylinders run the boost low and is my preferred placement on a blow down limited cylinder too ,my thoughts are id rather back flow a little spent charge/ex gas into the nearest primary at the front cylinder wall and get it back out the ex port again without mixing it across the full length of bore contaminating fresh charge from all transfers whilst travelling across to blow down a high boost port

[Darrell Taylor]

After reading through all this info, I need a lay down :fb: I find the best way to sort out my cylinder is just pick the phone up and send it to Darrell Taylor :anbrgin; Job Done :anbrgin;

cheers Dono,please dont send me anymore work i need a holiday
i best get some work done too,lost a morning reading all this

[Chris CSPE]

Is that it!!!!!!!!! Lol