I've been working on getting Wayne's turbo setup working on my '66 Dart. One thing that's been nagging me is the issue of the vacuum advance. The Holley four barrel carb does not appear to have a vacuum advance port - or perhaps it does and I'm looking in the wrong place. I could run manifold vacuum to the distributor. This car is a street car so I do want the part throttle drivability improvements a vacuum advance system gives. Is there any problem with using manifold vacuum instead of ported vacuum?

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"Mad Scientist" Matt Cramer
'66 Dart - turbocharged 225
My blog - Mad Scientist Matt's Lair

Matt,

I am a wholehearted supporter of using vacuum advance, although I can't comment on using it with a turbo since I have no personal experience.

There should be a few vac ports on your Holley carb - one or two on the baseplate and possibly one on the primary metering block. One of the baseplate ones should be ported, but it's been awhile since I messed with one, so you'd have to test it. The block one should also be ported, but timed differently.

It is possible to use manifold vacuum for advance IF you have a fairly mild cam. The problem comes when you have a bumpy idle - (read: fluctuating vacuum signal). This causes the vac advance to fluctuate as well and you have a feedback situation where the idle fluctuations get worse in response to the vac advance. If you are using a close-to-stock cam on your turbo motor, then you can likely use manifold vacuum (I've done it on mild NA motors), but any long-duration/high-overlap cam will idle like %^*&.

Slant on,

Lou

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Home of Slant6-powered fun machines since 1988

Matt, I'm pretty sure that I saw a previous post about this saying that the port that you want to use has little or no vacum at idle and opens up as rpm increases.
Dan M.

Dan,

Thanks, you're right and I forgot to mention that. The standard "ported" vacuum source has no pull at idle, but gives a vac signal when the throttle blades are opened.

Lou

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Home of Slant6-powered fun machines since 1988

Ported vacuum port on the 4160 series Holley's is the one part way up the side of the metering block, not the ones on the base plate.

Another thing to keep in mind. It is likely a good idea to put one way check valves in lines that operate things like vacuum advance pots to protect them from boost pressure. My turbo II Daytona has check valves in many of the vacuum lines.

Also, to expand on the functionality of ported vacuum. You have none at idle because the port is above the closed throttle blade. As you open the trottle the porte is uncoveed and is exposed to vacuum as you approach WOT vacuum in the manifold again approachs zero. So, vacuum advance is only working at partial throttle cruising conditions.

FYI, depending on how the turbo is sized to the engine things will be different. My Daytona only sees manifold vacuum at idle and steady cursing on a flat road. Any movement in the throttle position it immediately drops to zero. Most driving the manifold pressure is between 5"Hg vacuum to 5 psi boost (max boost on my car is 14.5psi).

Cruise control, and the AC/Hest controls are vacuum operated. I have to take my foot off the gas to get the manifold vacuum over 15"Hg inorder for these items to operate.

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Dave Clement
68 Barracuda (Hot-Rod \6)
http://mysite.verizon.net/vze4pgxf/

Thanks to all who responded. I will have to contact Wayne for specifics about this particular carb, as he said it had already been prepped for boost, but I seem to remember him saying it didn't have a vacuum advance port. Luckily, this car is running a stock cam at the moment, and I don't plan to get too wild with the bumpstick - Desktop Dyno seems to think I can make 300+ hp with an RV cam and 10 psi of boost. Turbo motors don't really need big cams. Thanks to Dave for the warning about check valves!

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"Mad Scientist" Matt Cramer
'66 Dart - turbocharged 225
My blog - Mad Scientist Matt's Lair