vac advance

[HotRodPC]

Lots of info to absorb and think about here. Thanks for all of those who having input, especially Rich's detailed explanations. I still haven't changed my mind for a daily driver, but still great info to consider nonetheless. I never thought about it, but it does seem to reason that Emission controlled and non emission controlled vehicles can be different. Seems intial settings could take care of that too though. Also emission controlled vehicles tend to have TVS (Temp Vac Switch) to keep timing from advancing by way of vacuum until the engine is hot enough to allow vacuum to the distributor.

 

[firebane]

My 1806 instruction sheet doesn't read like your picture. It says that you need to determine if your distributor is timed or runs off full vacuum.

So are my distributor advance weights different if it is timed rather than full manifold ?

Is the distributor the deciding factor or is it referring to keeping things the way they were ?

Maybe we need to include full knowledge of distributor vacuum advance in this discussion ?

To be honest timing is a whole black art science in its own. Timing without vacuum, timing with vacuum, timing with different weights etc etc.

Its no wonder so many people are confused on how to properly time a sbc engine.

 

[rich weyand]

To be honest timing is a whole black art science in its own. Timing without vacuum, timing with vacuum, timing with different weights etc etc.

Its no wonder so many people are confused on how to properly time a sbc engine.

Just to put a marker here for anyone searching and finding this thread, here's what I have for timing and induction.

Edelbrock 1406 with properly adjusted electric choke. Mixtures set with NGK AFX Powerdex A/FR meter with seven-wire sensor. Idle: 13.5:1: Cruise: 14.5:1; Power: 13.5:1; Secondaries: 12.5:1. Step-up springs modified to engage below 9" of vacuum (8" springs stretched 1/4").

Thermostatic air cleaner with hot-air induction off the headers and cold-air induction from in front of the radiator via induction hose off the snorkel.

Stock GM HEI distributor with stock vacuum advance (15 crankshaft degrees) and stock centrifugal advance (20 degrees). Vacuum advance connected to manifold vacuum. Base timing at 17* BTDC, which with the stock distributor gives 36* BTDC all in (base + centrifugal), 51* BTDC at cruise (base + centrifugal + vacuum), 32* BTDC at idle (base + vacuum). Idle at 700 rpm in neutral, 500 rpm in gear. Running 87 octane pump gas.

Performance is excellent in all modes. No transition sags, off-idle hesitation, or throttle response lag. Starts on the first revolution of the crank after a couple of full throttle pushes to prime the intake manifold. No difference in performance from at least 100 degrees F down to 0 degrees F ambient temperature once the headers come up to temp and start delivering warm intake air.

 

[Driver4r]

I just typed a whole page then my phone died.......

 

[Driver4r]

Here I go. I'm going to try to remember everything I typed.

You guys are making this alot more complicated than it needs to be. It's not rocket science. It's pretty simple in my mind.

Rick, I can plainly see that you are "book Smart". Here you are quoting page after page what someone else has said or printed somewhere else. I on the other hand and I'm sure alot of others here are "street smart". Meaning that with physical labor, we went out and with trial and error found out what works better. Not by reading it out of a book. But by actually taking the time to teach ourselves.

And you keep quoting your crate engine. And basically try and wrap the whole theory you preach around it. Truth is. It's pretty stock. It's not a hi-po engine like most would see one. So you can't preach manifold vacume around it. It's not 1 way fit all AT ALL. It's every engine for itself.

My 355 isn't a monster by any means. But it runs HARD. And it runs strong. It's probly quicker tthen alot of "street cars" nowadays. If we go back to the 70's, I would have no chance. But compared to today's vehicles. It's pretty quick. But guess what. It like vacume advance locked out to be snappy on the throttle. The only reason anyone should need vacume advance is for constant rpm cruising. Cause you recollect vacume and it advances. Cause about the time you achieve cruise rpm. Your cyntriphical weights pull off cause the engine is at a steady pace.

If it wasn't for cruise efficiency and people not knowing how to tune. You could easily tune with distributor clocking and weights.

Now if you want. We can start talking hi-po engines that wouldn't even think of hooking up vacume advance. Back when hot rodding was at its high in the late 60's to early 70's. Why was it that the most high performance distributors didn't have provisions for vacume advance??? Because it's not needed.

 

[rich weyand]

I post links to other people's stuff. What I write is my own: I'm not quoting anyone. I actually know all that stuff.

Street smart? One thing you forget in all this: I'm 61 years old. I WAS THERE. You know, back in the day, when as you say hot rodding was at its peak. All those cars you say you wouldn't have a chance against. Yeah. That's what we ran. And you're right, you wouldn't. And we didn't run 70s vehicles unless we ripped all that crap off and retuned them. Including returning them to manifold vacuum. We ran used 60s vehicles. Or 50s vehicles we put monster engines in -- and then more often than not replaced every driveline piece all the way back one at a time as we dynamited them.

And we did read. We read hot rod magazines, and learned the theory behind what we were doing.

I also know that my engine is pretty stock. It's what I wanted. Living with a hot rod as a daily driver is a PITA.

Why do performance distributors have no vacuum advance? Because when you're racing, you don't need them. You only care about the timing when you have your foot in it, and all that street stuff is just more stuff to break. Dirt track racer I know has a small motorcycle clutch for first gear to get rolling; high gear is splined-shaft direct drive, from the crank right through to the rear end. Not very practical for a street car, but for racing, simple is better. Street engine, that's different.

Oh, and centrifugal weights don't pull off at steady speed.

Hey, do what you want. It's your truck. I'm just trying to put some real knowledge and experience out there for those who are looking for it. And I've accomplished that.

 

[SkinnyG]

I'm a child of the 70's, and started my apprenticeship back in the 80's. All I knew was ported vacuum, because that was what I saw coming into the shop; that's the way it was done. I've rebuilt distributors, I've run distributor machines, I've re-curved them and whatnot.

When I built the 350 in my C10 (heavily ported heads, 230°@050 108°LSA, 10.9:1), I studied up on vacuum advance, and chose manifold vacuum for my engine. I read all the opinions either way (and I read a LOT), and I tried both. Based on all the information I read as well as my own experience, ~I~ chose manifold vacuum.

Rich's explanation above, is actually the most clearly explained and most logical reasoning behind manifold vacuum. It is clear that he truly knows his stuff.

But, like he says, it's your truck - do what you want. Take all the information given, and make your own choice based on that information.

 

[Driver4r]

And I was referring to the 355 I built for my monte. Not the one in my truck. I can see how you got those 2 switched.

 

[MadOgre]

Hey Rich oh I mean SkinnyG oh no I mean Rich.....Oh im just confused