Re: Vespa intake timing: let's talk numbers

Jason ( ͡° ͜ʖ ͡°) /

> Simon Nyi Wrote:

> -------------------------------------------------------

> The Bell book says that the intake port should close no sooner than ten

> degrees before the transfers open and vice versa. So take a port map,

> figure out when the transfers open and close, convert mm to degrees of

> rotation, give ten degrees on either side, and there's your safe

> maximum.

> just realized that might not be clear. the intake port should close at least ten

> degrees before the transfers open, no less, and the intake should open no sooner

> than ten degrees after the transfers close.

I tried to resist the urge to bump this ridiculously old thread but I couldn't. I figure I'm probably not the only one that ends up in threads like this to learn and figure stuff out . . . and I just wanted to correct some misinformation that threw me for a loop and most likely did for others returning to this thread for vespa knowledge.

Bell says in his book the exact opposite actually, regarding when the inlet port should open. And he never mentions anything about precisely when the port should close in relationship to the transfers because there is generally no amount of usable and reasonable transfer/intake timing that would ever bring the port close even within that range to the transfer open point. You'd have to be running ridiculously long intake period and even more ridiculously long transfer timing. I think? Either way I suppose on that half of the statement Simon was somewhat correct because it's good for the inlet to close a bit before the transfers open to build pressure while the mixture is in there.

Here is what Bell actually says:

"It is usual, when good low speed power is required, to open the inlet port about 5 to 10 degrees before the transfer port closes (ie 120 to 130 degrees before TDC) and to close the inlet port at about 55 to 60 degrees after TDC. This results in an inlet duration of around 180 to 190 degrees."

Cartsen does a good job of showing this math in practice in this video that I'm sure many of us have watched (he gets into this stuff right before the 27 minute mark).

This is also confirmed on Ciao NL. The translation is kind of crude but same exact idea:

"Graham Bell says in his 2-stroke tuning book that the coil ports can be opened up to 10 degrees flush with the inlet, I have adhered to that very strictly until I encountered many cases of more than 10 degrees where it just goes well. So it may be slightly more than 10 degrees. Suppose you close flush ports 124 degrees before TDC, then you may open inlet 134 degrees before TDC." (edited)

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