[ttjsk812@aim.com]

Well I have a 355sbc with a pretty mild cam (536 lift). Would I benefit from running manifold vacuum or should i stay with ported vacuum. I heard vehicle will run rough with manifold vacuum but Idk. If anybody knows let me know. Thank you.

[Sw1tchfoot]

You just have to try both and see which you like. I run full manifold vacuum for better idle quality and it seems to help with drivability, especially when cold.

Just sitting at the factory 10 degrees BTDC (Ford 302) then bumping the timing up and up, it's amazing how much smooother my engine runs with more timing.

[ttjsk812@aim.com]

So do I need to retard my timing before I hook up manifold advance so it doesnt ping. Also where is that port located

[cdminter59]

With manifold vacuum you have vacuum from idle until almost wot and you don't have any vacuum at idle with ported vacuum. It will depend on your setup whether you need vacuum at idle. Do you have vacuum advance on your distributor? What is the duration of the camshaft? If it's a big duration cam with alot overlap where you have low engine vacuum and have power disc brakes you would probably use ported vacuum. Where is your initial timing set now? Set it around 16* and you should have a fitting that screws into the intake behind the left side of the carburetor. Another place would be on the throttle plate of the carburetor.

[oldbogie]

Originally Posted by ttjsk812@aim.com

So do I need to retard my timing before I hook up manifold advance so it doesnt ping. Also where is that port located

Maybe or just turn the idle down.

The difference between ported and manifold is timing when there is or not vacuum advance. The ported sources from just above the throttle plates when they are nearly closed at idle, so the port doesn't see manifold vacuum until the throttle is opened enough to expose the port to the manifold vacuum.

The manifold vacuum port goes directly to a locating under the throttle blades so it sees manifold vacuum all of the time.

Choosing which port to use is a function of the cam timing, idle speed control, and idle emissions requirements. For a mild cam that doesn't need to pass emissions using full time vacuum advance will let you have sufficient idle speed with less throttle opening and idle fuel flow, but this tends to run idle NOx emissions up.

If this engine uses the timed port, the idle speed will naturally want to be lower and will have to be adjusted up to proper speed with more fuel and air delivered by the carb. This often results in run-on when the engine is shut off, tends to increase unburnt HC emissions but reduces NOx, often develops a stumble or jerk at the transition point.

Once the timed port is exposed to the manifold vacuum by the throttle blade opening, either it or the full time source will work the same at the vacuum diaphragm.

For a wild cam which port is moot as the throttle blade is typically open at idle beyond the position of the timed port so it and the full time port will behave the same unless the throttle blade is drilled to supply idle air in which case the sources go back to operating as they would with a mild cam.

In large measure which to use will be that which makes you and the engine happiest.

Bogie

[75gmck25]

For the original poster - based on the question in your 2nd post, I don't think you understand how ported vs. manifold vacuum works. Switching from one to the other should not require a change in static timing, and you should not need to adjust it to avoid pinging.

At idle
-- Ported - you will only have static timing (e.g. 10 degrees BTDC)
-- Manifold - you will have static timing, plug whatever you distributor vacuum advance adds (e.g. 12 degrees more, for a total of 22). In most cases you just have to adjust the idle speed to compensate for the additional advance at idle.

Light acceleration (medium/high vacuum) - both cases will usually have equal advance, since the port is uncovered and now it is providing vacuum to the vacuum advance. At this point your mechanical advance also kicks in since it is based on RPM. If you get pinging its probably because your vacuum advance (or mechanical advance) is adding too much.

Cruise (high vacuum) - both equal; you will have total advance that equals static, plus vacuum, plus mechanical based on RPM. The additional vacuum advance is what helps gas mileage at cruise, as long as it does not go so far that it causes pinging.

Full throttle (low vacuum) - both equal, static plus mechanical only, since vacuum is so low that there is no vacuum advance.

Bruce

[454C10]

Ported timing advance is an emissions device.

If you are not emissions testing your engine then use manifold vacuum.