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Discussion Starter · #1 ·
I have a 350 engine in a 89 GMC SIERRA 1500 CAN ANYONE TELL ME WHAT THE VACUUM ADVANCE CONNECTS TO?PLEASE!!
 

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You need to post some more info. Is the 350 carbureted or does it have factory throttle body fuel injection? If it has a throttle body unit on it and all the factory stuff then the distributor does not have vacuum advance on it because the computer controls the timing of the engine. If you have a standard pre computer large cap HEI distributor then the vacuum advance hooks up to either timed port or full manifold vacuum on the carburetor depending on what sytle of carb your using and also your setup if it can handle the full manifold vs timed port. Some differences on each location on how it works.
 

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Discussion Starter · #3 ·
My 350 GMC pickup is electronic ignition. Ole school and has the large distributor cap 2 barrel carburetor THANK YOU FOR YOUR HELP!!
 

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What two barrel carburetor are you running? Also what is the specs of your engine? Is your motor pretty much a bone stock build? You can try your vacuum advance on timed port or on full manifold. The timed port location was first used back in the very early 70's for emissions purposes in order to allow an engine to run hotter to burn off more carbon emissions and was a band aid feature really as they has the old smogger engines run on very little timing in order to run hotter and to meet the EPA stuff way back when.

So in order to allow an engine to make more heat they took more timing out thus adding more heat to the engine and also making it run a lot hotter because of less timing at idle. On the timed port location the vacuum advance will not start until you start to open the throttle and then the vacuum advance will come on as you cruise around in high vacuum conditions. On full manifold it will add the extra amount of timing of the vacuum advance at idle and in doing so it allows the engine to run cooler but also because of more timing it also produces more emissions but I won't get into all the details about it. Most engines will run fine with the vacuum advance and do better on full manifold in most situations but sometimes it does not like it but it depends on the build.

When vacuum advance is on full manifold it will be pretty much fully in and once you open the throttle and start to cruise it will function the same as vacuum advance being on timed port at steady throttle. The only difference and at times where you can get into trouble is when you might run into certain conditions of it running on full manifold and when your vacuum advance crosses paths with the mechanical advance coming in and make a little to much timing and you can get into pinging. The best thing is to limit your vacuum advance to give no more then about 12 to 14 degrees of timing and to have the canister adjusted to come on as quickly as possible on the least amount of vacuum that your engine can allow it to and without it being to quickly.

Its best to have an adjustable canister so you can fine tune it like anything else. The biggest difference is when on timed port as you open up the throttle the vacuum advance will slowly add the vacuum advance timing to your overall timing until you start to make the vacuum signal go down as you open the throttle up even more upon hard acceleration. On the full manifold part it will start to decline on the timing as well during acceleration and the vacuum drops but once the throttle goes back down the full manifold vacuum advance will give all the vacuum advance back even if the throttle is hardly open such as very slow speeds across town.

When driving in slow speed conditions the vacuum advance on timed port might not give off very much extra timing until the throttle plate is open up enough to allow enough air signal to activate the vacuum advance more as the vacuum signal increases with more throttle opening. Is one of those things as a trial and test sort of deal. On just about everything I have had built over the years, the full manifold has worked excellent for me but I also have a cam way bigger then stock and an engine that runs much hotter then a bone stock 350 as well and I need all the timing I can get with my build.

I don't know if your HEI distributor has an adjustable canister or not. You have to put a 3/32 allen wrench in and see if it catches and you can adjust it and you would have to read your instructions to see what direction it takes to allow the vacuum advance to come in faster or slower. If you don't limit it then it can add up to 20 degrees of timing while cruising minus the mechanical advance and if you get into to much timing while cruising it can cause you to have pinging with to much vacuum advance and if you don't limit it then at times it takes away from your mechanical advance of how much it can be allowed and at that point it will cost your engine some power and also fuel mileage and performance regardless of it being stock or not.

The thing is if your running say 12 degrees initial timing and your mechanical advance adds 20 degrees of timing and add that to your initial timing of 12 degrees that makes 32 degrees of total timing and then add the vacuum advance while cruising and you have another 20 degrees added then depending on your engine it might not be able to run with that much total timing with it being at 52 degrees and most older small block chevy builds with not fast burn style heads run best with a total timing of 36 to 40 degrees on average and with the vacuum advance not limited it can add up from 56 to 60 degrees while cruising and can cause a lot of pinging.

That is why you want to limit the amount of your vacuum advance and MSD make a limiter plate that is very easy to install and will limit it.
 

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We would need the exact carburetor by number. Prior to SMOG requirements say up 1964 or 65 pretty much all vacuum advance would be full manifold vacuum from a carburetor port. From the mid sixties on a timed port was added and it what the OEMs used to delay the onset of advance till the engine speed moved above idle.

It really turns out from the hotrodders stand point that this gives us another toy to play with in relation to cam and compression selections that deviate from those of the factory. It’s really something that along with the base set timing you can play with to see if one offers any tuning advantages over the other. I wouldn’t attach a rule that says one is specifically better than another.

The big difference is with full time vacuum you get the maximum advance available from the system as soon as the engine fires, if your running a lot of compression but not a lot of cam duration this can help getting the engine started by not running so much base advance then allow the vacuum to fill in the best idle advance from the system. This eases the load on the starter without having to wire an ignition delay switch. If your running a lot of cam you cam where idle vacuum is low you could connect to the timed port while putting all the idle advance in the base. In this situation the manifold vacuum will peak upward as the throttle opens and the RPM starts to increase. This can allow a slight delay in the centrifugal so that it doesn't over advance the upper RPMs but provides good advance at part throttle cruise.

Basically either vacuum connection direct or ported once the throttle exposes the port will function the same.

For a mild engine with low compression and mild camming if idle SMOG is not a concern then a moderate amount of base and full time vacuum will provide a smoother idle. If you have to pass an idle emissions check and the vehicle does not use a 3 way converter, then you want to stay with less base and timed port vacuum advance to hold the NOx numbers down.

Otherwise this is an exercise to see what the engine likes best based on your vehicle, the terrain and weather where you live.

Bogie
 
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