Lambretta Boost Port & Cylinder Scavenging

[RICSPEED]

TS (RIP)

sorry to hear that must have missed that ..respect

[tony]

Well as i said. You can use the boost to clean the barrel out. But I feel this is a waste of charge and loss of flow. Isnt it better to look at an efficient pipe? What you have done does work... and you can widen power bands by staggering the porting as the smaller ports will flow under less pressure hence 'streatching' the pipes range. But.. looking at modern design you will see that they generally open the ports together with a boost that is around 60 or more and you have to ask why and how. Maybe the head design and ex port angle they use today which wasnt used in the ts1 is a factor? Or maybe its that in conjunction with the pipe..There is so much to it. Changing one thing (boost in this case) maybe a solution here... but with a different pipe,ex angle etc you will find that maybe thats not the solution after all. As I said, I would stagger it all back to front. My grp 3 2000 motor uses this config. And its not slow. But if I had the know how to develop pipes to remove the need for using ports to clean the cylinder I would of.

[Rich_T]

I'm with Adam on this, however there are a couple of other points that are of interest.

Probably the first to make how certain cylinders end up the way they are, this may well have something to do with how the production cores are made and assembled into the tooling.

For example, you will see on a lot of Gilera and Piaggio barrels (I think TS1 as well) that there is a small lip on the edge of the boost port or on the supplimentary transfers. This is because of production requirements and machining tolerances, the clasic example of this is a bore being off centre by 0.5mm would have a great effect on the port heights (left or right transfers and the boost). To stop this possibility, the port edge is drawn in at 90 degrees to the bore so that the height of the port (not the angle) is always consistant. The way to get around this lip issue is to put those features directly into the bore core, this makes production a bit harder but the accuracy frighteningly good.

I'm a big subscriber to rolling the charge up the wall of the cylinder and took my leads from tamed down kart configurations. There is a debate between boost opening first, last or at the same time, different kart engines adopt all three of those options and still produce similar power (the karts I refer to are fixed gear so not peaky performance like the geared engines). GT kits are based around 30 degrees against the cylinder wall (60 degree in Monkey's measurement) and like Adam says, there are at least two other divergent charges combining plus different pressures and divergent angles from the ports which are also pressure/time dependent, not to mention the elapse time from the transfer charge crossing the portion of the cylinder to reach the bost port charge.

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.

[mnpes]

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:

[Adam_Winstone]

This is a very interesting read

[Avantone]

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:

Keeps the port a bit cooler too

[Rich_T]

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:

I wouldn't like the idea of mixing hot exhaust gas with new mixture, plus as the portholes you describe will be open BTDC as well as after any gain might only be lost from the previous open position or directly out the inlet (which would be open in all positions. I've never seen (what you describe) it before and there might just be a reason for that. But hey, if you have time and pistons go for it.

[Adam_Winstone]

I can't remember which fairly common text this theory is mentioned in but I've read it too. It may even be something scooter related; 60s tuning manual, Scootering tech feature, Webster's, etc.

[Special X]

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".

[Avantone]

I can't remember which fairly common text this theory is mentioned in but I've read it too. It may even be something scooter related; 60s tuning manual, Scootering tech feature, Webster's, etc.

Done it years back in the racer. The holes were small; 2 x 2.5mm holes that were only uncovered at TDC.

Theory was improved filling of the crankcase, cooler exhaust port and potentially more charge/lubrication running past the small end.

Proving the benefit is something else though .......,