blake7even wrote:
Wow. My head hurts.
Check!
blake7even wrote:
Well from what I can gather I have a couple of things to think about.
The BGM crank is forged. Thats why I like it. It's also currently unavailiable so may as well be forged from unobtainium. It's also not perfect for reed induction. But it's forged.
Here's what I got from that thread, give or take.
105mm rod on 57mm stroke = Good.
105mm rod on 60mm stroke = Bad.
High case ratio = Low transfer port (short duration) = shit exhaust is ok.
Low case ratio = High transfer port (long duration) = Good exhaust is neccesary.
This what you got???
To be honest the high-low business didn't stick in my brain... and did they draw a corelation between case volume and duration?
blake7even wrote:
Now if I use the that Bell crank (110rod 60mm stroke) it will lower the case ratio (as there will be more volume) and the 110 rod will lengthen the duration... I think!?
I will also have to space the cylinder off by 6.5mm but the port timing should be a nats cock off what it would be with a 105 rod?
So thats all good as long as the BGM Big Box does the business. I think.....
I'm
fairly sure that they were saying that the duration is more or less unaltered by the longer rod? Maybe I need to re-read.
I know the 6.5mm is close, but to be sure, use this -
http://www.race-base.com/2t-tools/steuerzeit/
These are a couple of the points that I took from that thread. As well as Richard's big thumbs up for his setup towards the end. TA = time x area - the other factor in duration - sorry if you knew that.
allyourbasearebelongtonik wrote:
By increasing rod length you can get more transfer TA at lower duration. This would make more torque at mid-range and the motor would not be peaky.
During experimenting on a stock Stella cylinder, I found that increasing transfer duration made for a peaky motor with no guts at low rpm, and thus for a slower engine. By increasing rod length you gain more mid-range and less peaky motor, thus making it faster. In real world conditions, like Deals Gap, you really notice how slow a peaky motor is.
A peaky motor can be fun to ride but pretty soon you wish you had some extra gears.
allyourbasearebelongtonik wrote:
When a rod length is increased, all other things being equal, both transfer and exhaust Time Area increases, regardless of the fact that dwell at BDC is shortened with longer rod. ...but by increasing rod length you gain exhaust TA and this is something worth exploring.
Another benefit is that longer rods increase crankcase volume, which should be done in any case (on scooters) but especially when a larger cylinder is used (to restore favorable crankcase compression ratio).
And of course the bell crank also increases case volume (and are less prone to twisting due to matching lobes).
One thing that I had forgotten until just now was the gudgeon pin size. 15mm for the Malossi but 16mm for the crank. He got around that by using a different piston... you obviously don't want to do that as A. you just bought it and B. they're such a nice design. What about having a bush made for the rod? Ought to be fairly straightforward for someone with a lathe.