Re: [T3] '68 SQ w/ dual carbs questions

[Jim Adney <jadney@vwtype3.org>]

On 10 Dec 2002 at 22:10, Steven Ayres wrote:

> I'm running an FI distributor with working vacuum advance connected to the
> stock vac port on a pair of '66 Solexes tuned for my 1641, with larger jets and
> later venturis. With these carbs the vac advance kicks in when the throttles
> open and tapers off  as they close for cruising. 

I'm gonna stick to my claim that this is hard on your engine, although I agree 
that it certainly will "feel" good. 

Running this way will give you in the neighborhood (depends on which FI dist 
you have) of 40 deg of advance at full throttle, but you haven't mentioned 
where it is timed at idle, so I don't really know where you end up. Since I 
doubt that you're set for any amount of retard at idle, I'd be concerned about 
the end result you get. The FI vac advance can may not give you its full 
advance with the Solex vacuum, so this may alleviate things. Your larger 
venturis will also reduce the vacuum response.

I have a friend who races (different cars) and he once set his timing by ear 
following a late night rebuild session. He was fortunate that one of his 
friends actually put a timing light on the engine the next morning, before the 
race, because the max advance was around 70 degrees. While everyone agrees that 
it might have felt good, it is also likely that he would have burned a piston 
and not finished the race.

Along the same lines I believe Keith was doing the same thing as you at some 
point and came up with his observation that he could have detonation at high 
speed and med throttle. In his case also I believe that the cause was actually 
excessive advance. I eventually checked with my mechanical eng friend who 
confirmed that extreme advance could cause detonation even under situations 
that would normally be considered non-typical for knock, but it takes a LOT of 
advance to do this. Keith is lucky he suffered little or no damage.

What few people appreciate is that the max advance figures that are given out 
for air cooled VWs (30-34 degrees BTDC) are for full power output only, but 
that the engine can easily tolerate much more advance under light loads. Since 
this gives better fuel economy, it is often done in the later cars by using the 
vac advance as I described before. Of course this requires a mechanical advance 
to take over the work that the earlier dists did with just the (different) 
vacuum advance.

-- 
Jim Adney
jadney@vwtype3.org
Madison, WI 53711-3054
USA


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