FRAM NVRAM Board

[Jayme]

yeah you will find the dizzy vac and cent advance curves arent actually best for the engine anyways... at idle 25 degrees is fine for most engines, if you have a big cam, you can go even higher. easiest way is to adjust the spark in realtime with the engine running, you will see the MAP go down as it becomes more efficient and the IAC motor closes up. you should be able to see and hear the sweet spot.

[Dylan]

30kpa is a nice vac for idle

[BennVenn]

I had it at 35deg at idle for highest vac which is miles away from what the service manual suggests.

My logic is, at WOT you want a relatively low amount of timing as the cylinder has a full charge, high desnity and will burn pretty quick. As the RPM increases, you increase the advance to make up for the burn time. As manifold vacuum increases, increase timing to make up for a lower charge density. Does that sound about right?

[vlad01]

I had it at 35deg at idle for highest vac which is miles away from what the service manual suggests.

My logic is, at WOT you want a relatively low amount of timing as the cylinder has a full charge, high desnity and will burn pretty quick. As the RPM increases, you increase the advance to make up for the burn time. As manifold vacuum increases, increase timing to make up for a lower charge density. Does that sound about right?

forget what the factory manual says. VW were very hit an miss with everything they did and those figures are based on carby dizy combo which are miles from optimal.

Even their engine combos are so miss-configured its not even funny.


My advice, shake it over a bin and do what your doing.

[BennVenn]

Timing Calc.xls

(20.5 KiB) Downloaded 283 times

Nowhere near as good as Jayme's but I think its a start.

[BennVenn]

I had it at 35deg at idle for highest vac which is miles away from what the service manual suggests.

My logic is, at WOT you want a relatively low amount of timing as the cylinder has a full charge, high desnity and will burn pretty quick. As the RPM increases, you increase the advance to make up for the burn time. As manifold vacuum increases, increase timing to make up for a lower charge density. Does that sound about right?

forget what the factory manual says. VW were very hit an miss with everything they did and those figures are based on carby dizy combo which are miles from optimal.

Even their engine combos are so miss-configured its not even funny.


My advice, shake it over a bin and do what your doing.

I'll do just that. I've got some numbers which I know are good for my motor at the extremes, RPM+MAP, and what idles good. I'll have to massage the peak torque region a little but without a dyno it'll be a bit tricky to get right. I'll stick with what's safe then get dyno time when the supercharger goes on.

[Jayme]

I had it at 35deg at idle for highest vac which is miles away from what the service manual suggests.

My logic is, at WOT you want a relatively low amount of timing as the cylinder has a full charge, high desnity and will burn pretty quick. As the RPM increases, you increase the advance to make up for the burn time. As manifold vacuum increases, increase timing to make up for a lower charge density. Does that sound about right?

yes thats generally correct. you want more advance as the vacuum increases. you should have the least advance at WOT.
regarding WOT, its a bit tricky.. yes you generally need more as the RPM rises, but you then need Less as the charge density increases. you can use your torque curve to see the charge density, or look at your VE table, which should follow the same curve as your torque curve. so depending on your cam and torque curve, the ideal timing at WOT generally increases with RPM, then as you approach your peak torque, it decreases again, then as the torque falls off, it increases again.

[BennVenn]

That sure got the little brain ticking trying to visualise that, kind of like a volcano sliced in half, with the hole in the top the peak torque. Maybe with more rounded edges. Maybe I'll keep the curve flat , tune the VE, use that as a guide, then repeat...

Edit: Found a stack of torque curves for my motor configuration. What kind of timing should I pull around peak torque, 2-3 deg?

[Jayme]

yeah tis tricky as you have the need for timing to increase with RPM constantly fighting the need for timing to decrease with Torque, so depending on your cam profile - if its fairly tame, sometimes you just need to flatten out a bit before peak torque. one of those things thats really hard without a dyno.
a few years back me and a mate were playing with perfecting the timing map on his V8 with 286 cam by the seat of our pants... we would increase Spark in each cell until we could hear knock and then back it off a few degrees to see where the spark curve landed. knock isnt a good indicator whether you are near the ideal MBT spark (DYNO!) ... but this was just all we could do on the road. attached is where the curve landed.
others here have played with Dynos a lot, so maybe someone else can chime in with a few examples of properly done MBT spark curves?

VE vs Advance.PNG (26.51 KiB) Viewed 6212 times

[BennVenn]

Here is what I've put together, based from a linear timing advance from idle to 4800rpm and 20 to 100kpa. I've taken the torque curve which peaks from 1800 to 2200, and reduced the timing a few degrees at WOT and smoothed it down back to 20kpa. I've rolled the edges off down to 0deg from about 4800rpm as I don't want it to go past about 4500 max.

It is a lot lower across the board than the pulsar tune so I expect it to be not quite as powerful, but also no more knock. I'll do repeated WOT pulls in 2nd and keep bumping the timing up till I don't notice a difference or knock, whichever comes first.

Does this resemble a spark table at all?

curve.PNG (34.46 KiB) Viewed 6207 times

I've just had a look at the pulsar VE and Spark, they seem to boost the timing at roughly the same time as VE.