Lambretta Boost Port & Cylinder Scavenging

[drunkmunkey6969]

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.

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 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 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.................................

[RICSPEED]

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

i agree

[Wayne Miller]

Most of the bike cylinders that Ive looked at have the boost port directed towards the spark plug, If the boost port is desgned to help purge the exhaust and the good stuff into the right area then what was Mr Shephard thinking?

[Adam_Winstone]

I'm getting slightly confused by this... surely a steeper angle to line/direction of bore/piston travel means that the boost is aimed more at the opposite side of the bore, in this case the exhaust port?

^ okay, I've just looked at the photos and now see that you are measuring at 90 degrees to bore, rather than in line with bore. Phew, this makes more sense to me now. I was considering it in terms of direction of bore/piston travel, which would have translated to wanting a shallower angle, rather than a steeper one.

Yep, I understand the logic and think that it is probably sound but know that the main and secondary transfers open slightly earlier than the boost and at an upwards angle, resulting in the boost never having a clear line of sight (as the screw drivers/pointer shows) but rather is coming in slightly later and having to oppose the charge that is already present and pushing upwards. As such, the 2 streams must meet and work against one another as they go up the bore, with the boost helping to give direction to the existing charge but never simply pushing it out in a straight line. This must be a resultant direction angle of 2 opposing forces, rather than considering them in isolation.

If I could remember much of studying structural forces for design then I could probably put a better discussion argument across... but I can't.... so I can't.

Hopefully some of that will make sense?!

Adam

[drunkmunkey6969]

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.

[drunkmunkey6969]

Also...I'd say its commonly accepted and explained in most two-stroke tuning literature that the standard angle for a boost port is 55-65 degrees, and usually on a centre plug head.....whereas on a Lambretta the angle is less at circa 45 degrees and the bowl in the cylinder head is offset in the opposite direction......the two items seem to have gone in opposite directions to each other?

[Avantone]

Interesting simulation of a cylinder without a boost port.

Looks like the main and aux transfer do a good job of pushing the burnt charge round and out, but it appears to leave a pocket of unburnt, unexpelled gas low down in the cylinder.

Is there an argument that a late closing, modestly angled boost port would help fill this void and push the remaining burnt gas out?

I'm pretty sure TS (RIP) knew what he was doing. MB has an extensive article on the TS1 and at no point mentions altering the boost port angle (he does talk about widening). He does however say there's particular benefit in getting the expansion chamber choice right, so perhaps it the charge needs a kick up the arse to get it scavenging really effectively.

Maybe directing the port higher improves peak power but little else - After all the cylinder was built specifically for a low revving motor with a 4-speed box - Maybe these constraints do require a different answer to 2-stroke convention?

Lots of questions, but with the number of people tuning these, I'd assume the answer is well understood............

[drunkmunkey6969]

I suppose it depends how the main and aux trans are angled in both entry and roof angles which dictates how efficiently they scavenge without a boost port?

Often rear (aux) trans are angled upwards to assist the roll up the back wall and into the head/bowl. Also on some race cylinders the angle of entry on aux ports can often scoop/flick back towards the exhaust port where port area/stud spacing permits.

But that's another problem we have to contend with......typical trans on a production cylinder might be of a 5:3 ratio in favour of main:aux. The TS1 doesnt follow that ratio in standard trim. Also a known calc for trans area is 1.2 - 1.35 times bore.....where again the TS1 falls short. So in this instance, due to its restrictive nature, its maybe difficult to compare the main and aux ports of a TS1 to a 'fuller' designed motor where stud spacing and entry angles are not a restrictive issue?

[tony]

Interesting one. I used to reverse the port opening on all my race motors. The reason came about from a lot of testing,dyno work and lots of barrels! In 1990 there werent any fancy pipes available for road or racing for vespas . I was lucky to have Graham at Kegra make me a high volume , strong angled pipe. So.. I worked to get the best out of it I could. The reverse sequence idea worked for this, certainly at low rpm it helped but lost a little at high rpm.
There are reasons for this of course. Losing flow velocity through the trans is one.. the pipe over scavenging the boost straight out the ex port (if opened too early) is another.
I would say Terry Shepard looked at what pipes were available at that time and how a lambretta engine worked when he designed that boost angle. There are some nicer designs around today but the high end tuners are designing pipes for the really big gains...
Looking at modern jap stuff gives the clues and so I would say pipe design is the way to go really.

[drunkmunkey6969]

When you say pipe design is the way to go.....in what respect? Of course a bespoke pipe for a specific tune will always win over an off the shelf 'generic' pipe, but that's a given. Setting that aside Tony, what are your thoughts on the thread topic of boost port angle......?