Mark,
Great explanation!
Who knew we'd actually get to use some of that physics, we had to take in
college ;-)
- Bryan
>X-Unix-From: mhaynes@ball.com Wed Aug 4 09:18:09 1999
>To: "'chuckc@ibm.net'" <chuckc@ibm.net>
>Cc: "'spridgets@autox.team.net'" <spridgets@autox.team.net>
>Subject: RE: Timing the 1275 ? (vac. advance with a sidedraft)
>
>No bother, You've just made me have to think, that's all.
>
> It's not the volume of vacuum (i.e.how many cc's of air pulled
>through per unit time) but the differential in pressure relative to ambient.
>by placing the vacuum take-off ahead of the throttle plate and perpendicular
>to it, Bernoulli's law applies the vacuum (a volume of gas/fluid moving at a
>specific rate across an opening will pull gas/fluid through said opening at
>a rate proportional to the relative diameters).Thus while the manifold
>vacuum is near 0 at full throttle, the vacuum APPLIED to a perpendicular
>hole at the throttle plate (the place of greatest restriction/highest air
>velocity)is as high as possible relative to the engines' air demands. Since
>the vacuum system isn't open ended, it doesn't change the airflow to the
>engine, it just applies a differential signal to the membrane in the vacuum
>advance mechanism, which is hooked (mechanically) to the advance plate in
>the dizzy. If this is too technical, I can try again, but I think it's
>pretty understandable
> Does anyone understand differently? Am I missing something?
>
>Mark Haynes
>
>> -----Original Message-----
>> From: Richard Shipman [SMTP:mbelect@mindspring.com]
>> Sent: Wednesday, August 04, 1999 8:42 AM
>> To: Haynes, Mark
>> Subject: RE: Timing the 1275 ? (vac. advance with a sidedraft)
>>
>> Hi Mark,
>>
>> I'm really trying to get a good handle on this...
>>
>> Isn't the vacuum level exactly the same inside the manifold and just in
>> front of the throttle plate at full throttle, since there's no
>> restriction?
>>
>> If so, then how can placement of the vacuum pick-up effect full-throttle
>> advance?
>>
>> Also, is the (manifold) vacuum pick-up placement on my '74 Midget just
>> plain
>> bad engineering? Does it work backwards?
>>
>> Sorry to be a bother...
>>
>> Richard
>>
>>
>> At 07:47 AM 8/4/99 -0600, you wrote:
>> >Manifold vacuum decreases as your throttle plate opens. The dizzy wants
>> full
>> >advance at full throttle. if you use carb vacuum,the dizzy gets an
>> >appropriate indication of the engines running demands otherwise, you get
>> >less advance at full throttle, and more at idle . just the reverse of
>> what
>> >you relly want.
>> >Mark Haynes
>> >'62 Sprite XSP motor
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