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Discussion Starter · #1 ·
Ok. So this post directly relates to a post I made about my carburetor tuning. I believe I got that down. Now I’m just double checking my timing. Here’s what I’ve got. With the vacuum canister unplugged and plugged off, I get 15 degrees initial. I rev it up and it comes all in at 36 around 3400. I’m ok with that. I was previously running my vacuum canister to a sparked time port on the carb. I switched it over to a full manifold port so the vacuum advance would come in at idle and I could tune my carb better with the assisted advance. Now I plug everything in. I’ve got 23-25 initial and when I go wot it climbs to 45 degrees advance. Scares the pants off of me. Is this normal?
 

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Discussion Starter · #2 ·
And I should probably add that I have an advance block off plate in the distributor. I’ve got it set to limit the vacuum advance to 5-8 degrees because I’m terrified of this. It’s a street fire distributor btw
 

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Yes this is normal in the garage as you can’t really go to WOT with no load on the engine so the manifold vacuum really doesn’t drop away as it will on the street.

On the street with a load on the engine it takes a lot more throttle opening than without any load so manifold vacuum follows a very different profile than when blipping the throttle in the shop..

Going to and holding actual WOT on an unloaded engine is likely to encounter RPMs that will damage the valve train unless you’re using high RPM springs and associated parts as if the springs loose control of the valves this presents the real possibility of colliding pistons and valves.

Bogie
 

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Discussion Starter · #4 ·
Yes this is normal in the garage as you can’t really go to WOT with no load on the engine so the manifold vacuum really doesn’t drop away as it will on the street.

On the street with a load on the engine it takes a lot more throttle opening than without any load so manifold vacuum follows a very different profile than when blipping the throttle in the shop..

Going to and holding actual WOT on an unloaded engine is likely to encounter RPMs that will damage the valve train unless you’re using high RPM springs and associated parts as if the springs loose control of the valves this presents the real possibility of colliding pistons and valves.

Bogie
ok. Cool. Thank you. Am I right in thinking that Hooking up the vacuum canister to full manifold would provide advance at idle? My theory in this is that I would get an extra kick in Rpms so I wouldn’t have to touch my curb idle screw. Now my transfer slots are golden.
 

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Timing is perfect as bogie said. Revving the engine without a load is not the same as flooring the pedal with the car moving. No load means high vacuum, accelerating puts a load in the engine thus no vacuum. Distributors have worked like that since the beginning of time. Normal cruise timing can be 40-50 degrees depending on the engine, car etc. and vacuum at cruise speed.
 

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ok. Cool. Thank you. Am I right in thinking that Hooking up the vacuum canister to full manifold would provide advance at idle? My theory in this is that I would get an extra kick in Rpms so I wouldn’t have to touch my curb idle screw. Now my transfer slots are golden.
If you’re running a cam you need to find what works for your case and that will change by how big a cam which includes the combined effects of duration, lift, and LSA, how much compression, the design and installation of the head’s and pistons as to how they affect compression, swirl, squish/quench, spark plug placement, and breathing as related to RPM. Then the environmental factors of altitude and weather. Plus if a really big cam and or very high compression and it’s effects on starting and idle.

An example is playing with base and vacuum advance. An engine with a lot of cam and compression will want a lot of early advance but a lot of initial can make starting a difficult exercise and shorten starter life. There are several ways to mitigate this by having manual control of the spark so it doesn’t start because the engine is cranking. By independently switching the spark you can give the starter a chance to get the engine turning over without fighting early ignition firing. A adjunct or substitute for this is to run less initial advance and a manifold direct vacuum advance so the cranking load is reduced in terms of early advance but as soon as it fires the vacuum advance pulls the ti up for a clean idle. Or you can put all the idle advance into the static and connect the vacuum to the timed port so under lower RPM cruise the vacuum will come in so you don’t need to run so much centrifugal in this RPM zone.

Basically you’ve got four toys to play with here between the base, two types of vacuum and the centrifugal to see what combinations work best for your engine in the situation you’re in. I’m more an experimenter rather that ridged follower of the book says this so I do that only.
And I don’t necessarily do it the same way all the time, I’m always hunting for the best trim that it likes.

Bogie
 

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Discussion Starter · #7 ·
If you’re running a cam you need to find what works for your case and that will change by how big a cam which includes the combined effects of duration, lift, and LSA, how much compression, the design and installation of the head’s and pistons as to how they affect compression, swirl, squish/quench, spark plug placement, and breathing as related to RPM. Then the environmental factors of altitude and weather. Plus if a really big cam and or very high compression and it’s effects on starting and idle.

An example is playing with base and vacuum advance. An engine with a lot of cam and compression will want a lot of early advance but a lot of initial can make starting a difficult exercise and shorten starter life. There are several ways to mitigate this by having manual control of the spark so it doesn’t start because the engine is cranking. By independently switching the spark you can give the starter a chance to get the engine turning over without fighting early ignition firing. A adjunct or substitute for this is to run less initial advance and a manifold direct vacuum advance so the cranking load is reduced in terms of early advance but as soon as it fires the vacuum advance pulls the ti up for a clean idle. Or you can put all the idle advance into the static and connect the vacuum to the timed port so under lower RPM cruise the vacuum will come in so you don’t need to run so much centrifugal in this RPM zone.

Basically you’ve got four toys to play with here between the base, two types of vacuum and the centrifugal to see what combinations work best for your engine in the situation you’re in. I’m more an experimenter rather that ridged follower of the book says this so I do that only.
And I don’t necessarily do it the same way all the time, I’m always hunting for the best trim that it likes.

Bogie
ok. Makes sense. I had to open my secondaries some when I was running on a sparked timed port and I was cutting it close in terms of opening them too much to expose the transfer slots. When I switched over to the full manifold I was able to turn the primary down because of the added advance. For the first time I’ve felt good about my setup. I’ve got 14 inhg at 800rpm. A steady idle.I tuned my idle air mixture screws to obtain that and throttle response is crisp.
It’s a 385 stroker. My cam is kinda conservative in terms of what’s out there. It’s a 230/236 and roughly a 540/546 (I’d have to double check the lift) with 112 lsa. I have AFR 195’s eliminators with 64cc chambers. My engine has 11.1:1 compression and I run 93 octane. Quench is .041. And I run a stater capable of cranking 14.0:1 compression
 

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ok. Cool. Thank you. Am I right in thinking that Hooking up the vacuum canister to full manifold would provide advance at idle? My theory in this is that I would get an extra kick in Rpms so I wouldn’t have to touch my curb idle screw. Now my transfer slots are golden.
Would depend on vacuum at idle, you have a heathy cam, what does it make for vacuum at around 8-900 rpm, enough to fully engage the vacuum advance? When you put in the vacuum advance restrictor plate in this can sometimes shift the spring range start point of the canister above the idle vacuum or worse put the canister actuator into mid point, depends what side of the actuator rod the little plate sits (in a typical HEI). Best to make sure if you run manifold vacuum its enough to fully pin the canister actuator to full advance and keep it there. If ported then spring range/rate less of a factor at idle. I always bench check distributor vacuum advance start point and range with hand pump and then compare to idle vacuum. Really nice to have a vacuum gage in the car too as real world info, go pontiac! lol

And dido 45 all in at cruise is just fine.
 

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Discussion Starter · #9 ·
Would depend on vacuum at idle, you have a heathy cam, what does it make for vacuum at around 8-900 rpm, enough to fully engage the vacuum advance? When you put in the vacuum advance restrictor plate in this can sometimes shift the spring range start point of the canister above the idle vacuum or worse put the canister actuator into mid point, depends what side of the actuator rod the little plate sits (in a typical HEI). Best to make sure if you run manifold vacuum its enough to fully pin the canister actuator to full advance and keep it there. If ported then spring range/rate less of a factor at idle. I always bench check distributor vacuum advance start point and range with hand pump and then compare to idle vacuum. Really nice to have a vacuum gage in the car too as real world info, go pontiac! lol

And dido 45 all in at cruise is just fine.
min getting 14 inhg at 8-900 right now
 
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