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Sat Aug 13, 2011 5:58 am

With today's electronic distributors, you can easily modify your timing curve. It can reach full advance (30 degrees) at any given RPM. Some can be modified electronically, others mechanically.

My question is simple, however I'm not so sure about the answer. Can the rate at which a distributor advances, or how quickly it gets to full advance, affect your air/fuel reading?

In other words, if a distributor reaches full advance at 2000rpm, could it case a lean condition. And on the other end of the spectrum, could it cause a rich running condition if it reaches full advance past the 3000rpm mark?

I'm just trying to understand if there is a corrolation between the two, and if there is, how significant is it? Or at which point would you notice a difference in the way the engine performs air/fuel wise.

Sat Aug 13, 2011 8:17 am

you aren't changing the A/F by changing the timing. I've talked about this before, but combustion happens in the chamber, and in the exhaust system.

For example, if your timing is way retarded a lot of the combustion will complete in the exhaust. If you have a catalytic converter a lot will take place in the Cat too.

I would not try to use the Wideband to dial in ignition timing. Timing is relatively easy to do. But timing will not behave properly to adjustments if the A/F isn't "close". The same way jet changes will not behave properly if the timing and jetting isn't "close". You have to find "close", before you can find "perfect".

Timing and Jetting have to be done together. Most people are too advanced because they are too rich, under WOT. Then under light throttle they are too retarded.

Sat Aug 13, 2011 9:37 am

Can you elaborate on how to dial in your engine timing correctly?

From the information I see that getting the jetting close is the first step. Then how do you dial in the curve and the total advance?

From what I understand the leaner the engine is the less total advance it needs. What happens if it is a low compression engine then will it require more total advance since it is less efficient?

My understanding of the timing curve is that an engine will detonate at a certain advance throught the RPM range so the correct timing curve would be a few degrees before this detonation occurred.

Sat Aug 13, 2011 10:04 am

74 Thing wrote: the leaner the engine is the less total advance it needs.

A leaner fuel air ratio has less gas in it and is harder to light, so leaner needs more advance, hence why a lean light load cruise you can end up needing 40+ advance.

Leaner is a slower burn taking more time for the flame front to push the piston and therefore needs to be lit earlier to push the piston at the right time and make more tq.

Sat Aug 13, 2011 4:53 pm

Krusher,

I am talking about total advance while you are talking about light load with vacuum advance kicking in.

I am wondering how to properly dial in advance throught the advance curve.

I have read numerous posts from John at Aircooled.net advising that a leaner mixture needs less total advance like for instance 28 BTDC where a 32 BTDC total advance may be an indication of too rich of a mixture.

Here are a few quotes from him from this thread related to AFR and timing:

http://www.thesamba.com/vw/forum/viewtopic.php?t=377085&highlight=lean+advance

"engines that "like" 32-34 degrees of advance, like it because they are WAY TOO RICH. The rich (cool) mixture takes forever to get the burn started, so they need the extra advance to get out of their own way. When you get the A/F correct it will run better on 28 total. Timing affects jetting, and jetting affects timing. Hacks don't know this, tuners do.

IOW, one mistake is causing another. Too Rich A/F is RAMPANT in the VW Hobby, FWIW."

"Let me rephrase. After retarding to 28, YOU MUST LEAN OUT THE JETTING. This will behave as if you added timing, since leaning it out speeds up combustion (adds timing). If you are too rich you need more timing than 28 to get the car to move out of it's own way.

If you go way rich, you'll need 40 degrees of timing, etc. The richer it is the more timing the engine needs. I have discussed this in detail in the past. Adding timing to correct an over-rich mixture is a band-aid."

Sat Aug 13, 2011 5:09 pm

it depends at what load, and what is "lean".

At WOT lean burns quickly, and by "lean" I mean 13:1 instead of 12.* or 11.* or even 10.*:1. Which is what a lot of guys have.

But at light throttle, "lean" is 16-18:1, and this burns slow and needs MORE timing due to the slow burn

Sat Aug 13, 2011 7:31 pm

Thanks for the clarification!