[Healeys] Healey 100-4 timing

[Bob Spidell]

re: "I'm wondering if I have been over thinking this process and the 
stated advance is meant to be "at idle" with the vacuum connected..."

I've always disconnected the vacuum advance, at least for my BJ8 and (I 
think) for my BN2.  I believe I read this in the shop manual, but don't 
have it handy to check.  I've checked with and without the VA connected, 
and didn't detect much, if any difference.  If you've set up HD8s 'per 
the book,' you would be running on the slow run (bypass) circuit, and 
the closed throttle would (I think, not sure) cover the VA port.

I'm not sure the vacuum vanishes at wide-open throttle* (WOT). There are 
two types of vacuum: manifold and port; the type on SU carbs is, I 
believe, the port type.  The depression (vacuum) is created, at least in 
part, by venturi effect.  Hence, I don't think it goes away entirely, 
even at WOT, but I'm not sure.

If you google 'manifold vs ported vacuum advance' you'll get a ton of 
links; here's just one:

http://chevellestuff.net/tech/articles/vacuum/port_or_manifold.htm

So, the $64,000 question is whether the port in an SU carb is behind, 
right on, or in front of a closed throttle plate (this article says the 
purpose of port vacuum is to not advance at all at idle)?  First correct 
answer wins the internet for today.

* I've always felt the term 'full throttle' to be a little, er, 
confusing.  Since the purpose of the throttle, by definition of 
'throttle,' is to restrain/limit airflow, wouldn't 'full throttle' be a 
fully closed throttle?  The term may be evolved from 'fully open 
throttle,' but got shortened (similar, perhaps, to how the aviation term 
'on the backside of the power curve' probably gave us 'behind the curve.').

Bob

On 11/2/2018 7:05 PM, Michael Salter wrote:
> Thanks for your input on ignition timing Michael.
> I have always found the factory specs to be a little confusing.
> They state 6 degrees BTDC as the timing (which I presume to be 
> "static") and then state that the distributor produces 16 - 18 degrees 
> at 2000 RPM (distributor speed) which I figure to be 32 -36 degrees at 
> 4000 engine RPM.
> I interpret that to produce 6 + 34 (2 x 17) degrees BTDC at 4000 
> engine RPM for a total of 40 degrees which sounds like an awful lot.
> My engine does not sound at all happy with 40 degrees even with 110 
> octane!!
> I'm wondering if I have been over thinking this process and the stated 
> advance is meant to be "at idle" with the vacuum connected (not normal 
> practice as far as I was taught) which would add 6 degrees of vacuum 
> advance which would vanish at full throttle producing a net of 34 
> degrees over 4000 at full throttle.
> I have been setting the timing to 32 degrees BTDC with the vacuum 
> disconnected at 4000 RPM and the engine runs fey well and very cool, 
> in fact too cool without a 180 degree thermostat.
> I am running 8.3:1 compressions with an M cam.
>
> M
>
>
> On Fri, Nov 2, 2018 at 9:16 PM Michael Oritt <michael.oritt at gmail.com 
> <mailto:michael.oritt at gmail.com>> wrote:
>
>     Wayne--
>
>     Exactly right--it is total advance that matters and I arrived at
>     that number by gradually increasing advance and seeing the effect
>     upon water temperature at speed and under full load.  Anything
>     more and the car would begin to overheat. A bit too much less
>     advance and performance fell off.
>
>     BTW I am running a DW "fast street" cam and tubular header and a
>     Mallory non-vacuum advance distributor fired with their Unilite
>     module.  I too have a Smitty's five-speed and the Toyota box, 180
>     XAS's and 3.54 diff set give me about 72-73 mph at 3K rpm's indicated
>
>     Best--Michael Oritt
>
>     On Fri, Nov 2, 2018 at 6:03 PM Wayne Schultz <waschu2 at gmail.com
>     <mailto:waschu2 at gmail.com>> wrote:
>
>         Thanks to Michael and others I am going to stick with 34 - 35
>         degrees total advance for now. What, I think this means, is
>         that the advance at idle is secondary and not as important as
>         total advance. Cruising with the the Toyota five speed or the
>         original overdrive on the highway is right in the 3K RPM range
>         so I think this the perfect RPM to limit total advance. I
>         might make a run to my local airport and buy some 100LL fuel.
>         I have worked in aviation since 1967 so I have a few friends😃
>
>

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