That goes against what I've experienced. With a timing light on the engine, put in some throttle - say 1800 RPM or so - watch the vacuum advance pick up to about its maximum then give the throttle a quick blip and the vacuum advance backs off very quickly.
Well you can't see things bowing either!! ha ha. This is instantaneous, very quickly don't get it. I have experienced it. It's exaclty what 29NASH said. Are you out for show or go? If you have the advance turned up for max performance the over advance in economy mode will cause some pinging when you initially drop the vacuum and bring the revs up. Then you retard a little and it goes away. You also lose some power at WOT. It's seat of the pants power.
the last blower motor i had , we sat the distributor to max out at 30 degrees total advance and it was all in by 2200 rpm. Its all in how they are weighted and what springs , should be no need for a va***e advance.
I didn't see any mention of vacuum advance at all in that stuff, yet it said you can run "stock"ignition... No conclusions there... I would tend to think below the carb, above the blower would work as the distributor was designed since,basically, an engine "****s" and a blower just "****s" more to help the engine along. In either case, at WOT you aren't going to have any "manifold" vacuum available that might cause detonation. and how much total advance an engine can handle depends on the type of engine so generalities like "only use 34º max. or you'll grenade!" might work on a Hemi but be irrelevant on a SBC. That's all just theory though, as is most of what's above... including the anecdotes.
Exactly . there is no need for a vacuum advance . The stock distributor can and should be recurved to the specs above . No need for a vacuum advance . and yes , it does apply to a small block chevy . Been there , done that. and you can run a recurved stock Dizzy . Hense why they say you can run a stock igniton. It tells you everything you need to know .
But if what you just said is true, vacuum advances have never been necessary on anything... Lots of manufacturers wasting lots of money on vacuum advances? I can't believe that..
actually , alot of guys i know that run any cam at all unhook them . Myself included. you have to understand the amount of advance that a stock hei can produce , i cant quote the exact number but i know its above 32-34 . ANd the curve is very long . As stated by weiand above . You need to recurve the distributor to get the advance in quicker to prevent detonation . They dont come right out and say " recurve the distributor" but they shouldnt have to when they say you need a max 34 degrees before 2500 rpm . The only way to achieve that is to recurve.
What's the intended use? Strictly trailered to and used for racing only, or street engine? Strictly race, keep it simple to tune and lose the vacuum advance, street, you need more advance at idle in traffic to keep most engines from overheating and that is one thing the vacuum advance can do for you it probably won't really much matter above or below since most say the pressure doesn't even come on till past most car's cruising speed RPM. If you have a bib at the base of a carb already, use it, you probably don't have one in the manifold unless you drill one.
I wouldn't run vacuum adv on any perf. type application. If you INSIST on running it, it needs to be hooked to the carb's "ported" vacuum source (no vacuun at idle, but vacuum when you crack the throttle). Also, as your motor goes through it's RPM and vacuum signal starts coming back, the vac adv will prolly bring your "total" adv too high to be blower friendly(oodles of cylinder pressure, and too much timing). Don't do it. Limit, or have someone limit the mech adv in the dizzy, and I'll guess for starters, around 20 degrees initial, and 32 degrees total for a blown motor on the street(?). I ***ume it's a street deal, or why even discuss the vac adv, right? Now, all this is possibly for naught if your blower is loose, and way underdriven(no harmful cylinder pressure)
Whether vacuum or not on a blown engine is a separate argument from vacuum advance adding advance at low vacuum levels. Ported vacuum levels follow manifold vacuum levels fairly close once the engine is up from idle RPM. Ported vacuum isn't strictly a venturi device and doesn't follow air flow like it would in a carb.
Very interesting Mr. Squirrel. Educational and it oughta put a stop to some of the arguments about boost vs vacuum. Thanks for posting.
SQUIRREL - VERY informative...I guess that answers my question. As an aside, I would have rather gotten a Vacuum/Boost gauge for my dash, but SW doesn't make one in the style of my gauges...
Hide one back under the dash on the left side of the steering column. These two S-W's are maybe 6-8" behind and below the dash and visible to the driver only unless a viewer sticks his head into the ****pit.
Better luck running mechanical only. No Vacuum advance. You can ditch the V-advance and you'll have fewer problems. And above or below, that just depends what it's on man.
People who don't use vacuum advance on the distributor for a street driven engine are making a mistake. The only people who are making a bigger mistake are the ones that lock the timing at 34 degrees. Without vac advance working above the blower the engine only gets 10-12 degrees of advance at idle. This often results in poor idle quality, higher coolant temps and poor fuel economy.
...OR, I suppose I could cut another hole in the dash and add a vacuum gauge. The damn thing looks like an aircraft ****pit as it is. I'll probably just leave it alone. Since the Carters have no vacuum ports on them, it's simple enough to drill/tap the mounting plate for a port...
Seems gauge makers go out of their way to make sure not every gauge is available in every style, eh? So... I said screw it and put my non-matching stuff right up on the dash for all to see. At least I found a small tach and B/V gauge that match each other. Ridler judges and traditional freaks turn away. LOL. Gary
On mine I run no Vac adavance I also run a Boost/Vac guage in the intake I see the same results as the video (under light throttle cruising conditions about 15" of Vac, at full throttle with load 7 PSI) I wouldnt bother with the Vac advance but I would strongly reccomend A MSD BTM box it will save alot of headaches and possibly broken parts they like alot of initial advance anyway I run 22 initial and 34 total and use the BTM box to pull timing out under boost to prevent detonation
What style SW gauges do you have? Squirell's video proves the vacuum above the blower goes low enough to let properly sized power valves do their job with out boost referencing. Like C9 says - that should end a lot of arguments.
Try this: sit in rush hour traffic on a 90+ degree day until the coolant is at 220F, then shut down the engine and try to restart it, I bet that 22 initial will make your starter happy...
Keep in mind that there is a wide variety of blown engines out there...some, like mine, have low compression and not much boost, so detonation is not a worry at all (I run 87 octane almost all the time). Others are right on the edge of what the fuel can tolerate, and need boost retard, etc. There is no right answer for all blower motors, you really need to know what yours needs.
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