sgian
Valued Contributor
Posts: 4438
Loc: MO
Reg: 12-25-03
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03-20-08 07:03 PM - Post#1392988
In response to Vaughn
It certainly is true that as the throttle opens vacuum drops, as can be seen when increasing rpm to 3000 rpm for example. However, when holding rpm at 3000 rpm throttle is decreased from when accelerating to 3000 rpm, so vacuum builds back up. So the vacuum advance is a factor when checking total timing, because if you hold the rpm at 3000 rpm for example, vacuum advance will kick back in.
My C20, for example, which has a mild Goodwrench/Targetmaster 350, holds around 15" Hg or more at 3000 rpm on the highway (I have vacuum gauge in the instrument cluster). Going uphill it will drop of course, but going downhill it will go as high as 25" Hg at 3000 rpm. If it can hold 15" Hg under load going around 60 mph, then it could easily get 15-20" Hg without a load at 3000 rpm (it idles at 800 rpm with just under 21" Hg). So that is enough for 10-20* vacuum advance on some canisters even at 3000 rpm.
| 04 Silverado Z71, 10 Cobalt |
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