the vacuum advance diaphram on the side of the distrubutor of carbureted car engines served to advance timing as RPM increased, since vacuum increases along with rpm. however I have noticed that by advancing the distributor timing too far at idle, this will increase low end power but also cause the infamous spark knock at higher rpm as Bigdaddy mentions.

this has always made me wonder why we didn't just advance the timing at idle, do away with the vacuum advance, and let the timing maintain all the way thru. but instead, the best performing ignition timing at high rpm is brought into play by this vacuum advance device, which allows the timing to be retarded at lower rpms, even though the lower rpms benefit from the advanced timing too. I'm sure there is more to it here than I can recall or maybe even been aware of, there surely is a reason for this, I just can't think of it, its been a long time since I looked under the hood of a carbureted engine.

Sorry this is of no help to the DS ignition topic at hand, but interesting none the less...

 

Hightower, Vacuum advance is ported just above the throttle plates and only advances off idle, as the throttle plates open further the vacuum drops there and the vacuum advance quits advancing the timing. It is put there to help get the vehicle a litlle extra hp when first starting out.

 

Yea Jed, the -10 is a HECK of a lot of retard in the timing curve. I'd like to see something in the area of -1 to about -3 from stock at the top end.

Scatterbrain, you wouldn't actually pull harder (say like a nitrous hit) you just would continue to build more linear power all the way through. Think of your dyno curve for a minute.... what retarding has the potential to do is continue to make more torque longer in the curve. This is of course if you are pushing the limit now. If you are running 11:1 and using 110 octane, your probably not going cool things down much... however you could possibly have a better fill rate/ratio. Not teo mention crank/piston position (which is really what all this is about and how timing plays it's roll).

Randall once told me that if it's working, then leave it there and be done with it. I didn't know much about the mighty rotax at the time and the learning curve was still fairly steap. Now that I've evolved in the rotax arena, I'm much further in motor efficiency and producing much more power. Now I need to go back to the "set to kill" principles that I've dealt with in racing.

HT, the vacuum advance of an HEI does change timing as vacuum increases and decreases. To get more detailed and specific, advancing the timing too much changes the point to which the flame front comes in contact with the piston. If the flame front contacts the piston to early the position of the crank/piston is that to which an adverse effect on power is promoted. Advancing the spark timing in the lower RPM absolutely produces more tq, but the opposite WILL happen in the upper RPMs. I'd like to correct you if I can HT on the vacuum advance on a distributor. It actually works in the opposite direction. The vacuum is strongest at idle and holds more initial timing at idle. As the rpms increase, the vacuum drops and the timing begins to retard. This is done in order to let car motors do two things.... Be more efficient, and to inhibit pre-ignition.

There are a bunch of math equations to show how this works.... If you want them, I'll post them on this thread.

Have I lost anyone????

 

If the info i have been given is correct and im sure it is.Russell karting has the program to set the mapping for you and i would guess so dose the master distributor i forget who they are but do a search.....i just cai'nt remeber there name "BRAIN FART" I just hate it when that happens.[img]i/expressions/face-icon-small-blush.gif[/img]

 

Thanks AZ, I'll check it out.

 

Quick question, sort of on subject. I installed a dynatek cdi, which advanced the timing 5 degrees. Is it possible to advance it more yourself, and have it benefit your power without wrecking anything? How does someone advance the timing themselves for more power? I have a 2006 650V2

 

You can call ron wood and ask about their advanced woodruf key for the crank. I remembered we installed one of these years ago. The key was cut in half, and was kinda tricky to finess it in there.

 

Mario is right about the offset.... kind of sketchy.

As far as the CDI being re-curved... you can't re-curve yours, but I'm sure you can get a custom cdi made. That said, a vortex will probably be a better choice. If you need something more than what it comes with, I'm being told that a customized curve can be ordered. I'd suggest though, that you get with someone and talk about your needs. More over, what are you trying to do with the curve.