Saving Old Seagulls

Is the Silver Century exhaust port too big?

I have just done some calcs on one engine that I have in bits.

Would the exhaust port be beter smaller ?

H-A

Not sure if size is the most important thing, I was always told when playing with 2 stroke bikes that exhaust and inlet port height were very important. So that they "open" at the right time.

Keep on reading H-A & you'll answer your own question.

I know you've got 2 of the best tuning books, just make your own decisons, you won't always be right, but any gains you get are yours.

If anyone could double or triple the power output of a Century, I doubt very much that they would publish it on here, a public forum

H-A

Now you've gone & done it You've opened a great big can of worms

Porting is one of my favourite subjects. Port area is critical, basically there are 2x schools of thought on port area. Hi Velocity theory vs Low Velocity theory

Hi Velocity theory says more speed gives greatest mass transfer. Hi velocity works in with the impulsiveness of induction combined with scavenge characteristics. Yamaha did alot of research on this on GP bikes around the early 1980's. It lead to engines with small ports cut low in the cylinder & expansion chamber exhausts & extremely high rpm (15,000~20,000) with narrow power-bands & 12~18 speed gearboxes to cope. I imagine they would have been hell to operate for the poor bike rider.
conversely;
Low Velocity theory says more cross sectional area gives greatest mass transfer. Maximising time-area values dominates here. This theory prevails in engines with moving exhaust gates. This design lead to engines with large ports cut high in the cylinder & expansion chamber exhausts & lower rpm (10,000~12,000) with wide useable power-bands & 5~6 speed gearboxes. A much more useable bike engine.

Here's a link to some basic porting theory for side-port induction

http://2stroker.net/jennings/aspirin_takers.pdf

So to answer the question about port area, It really depends on how you've setup the rest of the engine

Port timing is another subject entirely
Wave dynamics also another subject entirely

Rex-NZ

yes.. it is too big.

Lightbourn wrote:yes.. it is too big.

One can never be too big...you just need to know what to do with it

Anything ever come of this?