MarvBarrish

Slightly off topic here but I'm currently putting a new carb on my hot rod. It's an Edelbrock Thunder series, 650 cfm carb. It has 2 outlets for the vacum distributor connection. One timed and one constant. The instructions don't tell me how to differentiate which one I would use. So I'm sitting here wondering. I'm guessing the timed outlet but that's just a guess. The distributor that I'm using is a MSD HEI sitting in a SBC 383 motor. Any thoughts on how to tell which outlet to use would be appreciated. Regards, Marv

JohnZ

You might want to read this:

http://www.camaros.org/pdf/timing101.pdf

MikeM

Pretty simple really. Just start the engine and put your finger over first one port and then the other. The one that sucks with the throttle closed is the full time. The one that doesn't suck is the part time.

MarvBarrish

John, that is a great article. Thank you very much. After reading it my guess of using the timed port looks correct. For the record, I know which port is which just unsure of which one to use.

Mike Ward

Really?

EDinPA

claysmoker

Why?

mechron

Why X2???? These old engines run/idle much better with full vacuum. That is the way they came. The timed port system was just another stupid way to beat the smog tests out here in Cal in the early 70s. In Cal about the same time all cars were required to have another stupid device taped to the top rad hose that stopped the vac until the engine was up to temp.

My customers HATED the loss of performance on their cars, but sorry, You had to pass smog. After passing smog I would just switch the vac lines back for free.

DansYellow66

The next post will probably be why does my motor run hot, why is my gas mileage so bad and how do I get it to idle decently.

wmf62

Bill

Plasticman

Marv,

Please re-read the 2nd from the last paragraph. You want full time (manifold) vacuum advance, not ported (timed) advance.

Plasticman

Avispa

Actually the linked article has a couple of mistakes. First, even pre-AIR cars came from the factory with the vacuum advance connected to "timed" vacuum (it's really ported vacuum; an orifice in the throttle plate is located just above the throttle blade at idle and becomes exposed to manifold vacuum when the throttle blade is opened). I guess they just didn't want to call it a "ported vacuum port." The idle mixture is somewhat richer than the part throttle cruise mixture.

At least in the 1966 CSM, the vacuum connection is not described, but the 1966 assembly manual clearly shows connection to the timed vacuum port on a Holley 4150 in RPO section L72, sheet C2. Sheet A2, RPO L-36 is not clear. UPC 6, sheet C2 shows the vacuum advance can connected to the timed vacuum port as well on a Holley 4160.

So, your car came from the factory with the advance canister connected to timed vacuum. Lots and lots of space has been used up on this forum debating the merits of timed vacuum advance vs full vacuum advance. What is really entertaining is a lot of the same people saying breaker points and oilite bronze clutch pilot bushings are just fine, thank you, because the factory did it right the first time are the same ones putting full vacuum to the advance canister and telling people they're stupid not to do the same.

And the gadget mechron is referring to was screwed into a tapped hole in the water outlet. Called a thermostatic vacuum switch and it switched the vacuum advance from ported to full when the water temp got to 230 degrees.

MasterDave

Full manifold vacuum is correct.

stratplus

That is an awesome article, thanks. It is great for us that tinker but don't know what we don't know.

So that I can understand the difference.

What would the difference in the article be for timing and vacuum (except for points and condenser) for an electronic distributor, like a Pertronix?

MikeM

I think there's a likely possibility that Chevrolet hooked the vacuum on some of the 1960's SHP engines to ported spark as an attempt to eliminate customer complaints of pinging during throttle "tip in" operation. I'm pretty confident the engineers knew the full time vacuum was best for performance.

I never saw the "gadget" Mechron is talking about clamped to a hose but a similar "gadget" was the TCS (transmission controlled spark) system introduced in '70 or '71 on Chevrolets. During cold engine operation, it blocked off spark advance in the reduction gears via a vacuum switch controlled by a temp sensor that screwed in the cylinder head to measure engine temperature.

He is right about that feature killing engine power.

Mike Ward

Even more amazing and entertaining is the folks that make the owners of the Jegs and Summit rich by buying everything they peddle as being an 'upgrade'. Some of us have tried the aftermarket stuff and the OEM and figured out for ourselves which is actually better.

Others will have difficulty understanding the concept and usually need a dog to keep them from wandering off while out in a pasture.

JohnZ

The only pre-'68 non-A.I.R. exceptions to full manifold vacuum were the '66 L-72 and the '67 400hp and 435hp 3x2's; everything else had full manifold vacuum to the distributor.

MikeM

................and the '63 327/360.

JohnZ

Yup, and Engineering fixed that for '64.

MarvBarrish

It's times like this that it's shows why I didn't finish high school. What was throwing me in my thinking was that none of this is vintage but all new. Of course no smog as it is in a 38 Chevy coupe. Thanks for the help, I knew you guys would know. Regards, Marv