hello, I just need your thoughts about my setting 327 double hump heads 191,mild cam,headers, 4 barrel, Hei distributor, I have 14 ° at idle, vacume advance, unhooked, 34 at 2800 all in and 42° at idle with manifold vacume hooked up and 58 at 2800 with vacume hooked up,it starts good,throttle responce is great and it runs 10 ° cooler than with ported vacume, I have never checked total advance with vacume hooked up, could 58° be correct, thanks,mike
Sounds like too much vacuum advance to me. Be nice to see 42 degrees total with, what I always thought was better, ported vacuum. The vacuum advance may be adjustable. Put Allen wrenches in the vacuum port until you find one that fits. If it has an adjustment on the diaphragm you can make it there. Mine is all centrifugal at 40 degrees at 3k rpm.
Seems like way too much advance......those old heads like a lot of advance, but initial should be around 14, total should be about 36, all in at about 3000 rpm for a mild engine.
If it's running as good as you say it is, and not showing any signs of detonation on the plugs or piston tops, I'd run the hell out of it. That's just me though...
From your numbers, you have 20 degrees of mechanical advance and 24 degrees of vacuum advance. 20 degrees of mechanical is acceptable. 24 vacuum advance is probably too much. You should be in the 48-52 range for total with vac advance. It is fairly easy to limit the vacuum advance. Cheap kits are available. Mechanical advance can also be limited easily on the HEI. A little research should find the method. I like to run my SBCs at: 16 initial advance + 18 mechanical advance = 34 total +14 vacuum for 48 degrees. All in at 3000. Every SBC I have ever owned would respond well to this setup. There may potentially be slightly more power available with more timing, but 34 leaves some cushion for a load of bad gas or some other problem. I would run full manifold vacuum.
As was said... If the engine is running well...who cares what the numbers are !? To me, a timing light is a "verifier" of what the engine is happiest with, for your records. If you have a $150,000 car, with a $75,000 engine, yeah, check every degree, but...most of us don't. I haven't used a timing light in...a LONG time. I just dial in what the engine wants, a little at a time, check for detonation, and...drive on. Mike
The adjustment in the vac can only adjusts when the vacuum advance tips in. Not how much. I adjusted mine using a vac gun. It comes starts advancing at about 7 in and is all in at about 12.
Well, we all have our own tuning methods. I have had those $75K racing engines. Believe me, you better run those by the numbers. I recently built a 355 SBC for my avatar. Starting with a stock block and crank, I had almost $6K in that. A mild 350 horse engine. Nothing special. With that kind of money I like to play it by the numbers. I worked for an old engineer whose favorite saying was "The numbers don't lie!"
The way I was explained it, the vacuum advance is a fuel economy kind of set up. I was under the impression that there is 24⁰ advance in the centrifugal weights in the HEI distributor. Mine is set, without the vacuum hooked up, at 39⁰ total on the light, and it's 15⁰ initial. If I hook up the vacuum advance and cruise down the highway at 3000rpm, the vacuum advance pushes me into detonation, so I'm not running it.
Yep, vacuum advance is an economy thing. It will also give you a cooler idle. The centrifugal advance can be easily limited in the HEI. These pics show where to put a machine screw to limit the swing of the weights. There is already a hole there. I ground one side of the head in an arc to be able to adjust the amount of advance. The pics also show (not very well) the device I use to limit the vacuum advance. If you tailor your mechanical advance to about 18, you can dial in about14-18 degrees of vacuum advance. Your engine will idle better and cooler and perform better. Using the engine for a distributor machine makes it easy to tune in the amounts.
I prefer manifold vacuum, more advance when light throttle and lower cylinder pressure to keep from detonation. But cuts back the vacuum advance when you give more throttle and corresponding higher cylinder pressure. That seems to be best for a street driver.
Thanks for your responses, I have never used manifold vacume before. As I stated before it runs cooler, throttle responce better, rich exhaust smell better. And it idles smoother without having to mess with carb. I normally run about 60 mph at 2300 rpm, get on it every now and then just for kicks .I don't know enough about timing so I worry about burning piston,I guess if that was a problem it would show up in performance, thanks again,mike
There is a lot of useful info in this thread. Thanks Jaw. I have noticed a significant improvement in off-idle throttle response from using a vacuum advance connected to manifold vacuum.
I had the same motor. Could go to 37 total--but, had a vac unit that put it all in around 2500 or so. This was a set unit from, i think, MSD or one of the suppliers. No adjust, it was preset. Maybe you can get one from a parts store, not sure but i thought some 'vettes had the same thing. Really could tell the difference.
With that kind of advance the only problem I see is that you might get a little spark knock at tip-in. That's when you are running a constant speed and just accelerate lightly. If you aren't experiencing that, I would leave it. People get up tight about over advancing with vacuum advance, but they forget that the max advance is at very light engine load. As soon as you add load (which is where you might experience engine damage), the manifold vacuum drops and most of the vacuum advance goes away.
Thanks for your accurate input, @jaracer . Most rodders don't seem to understand vacuum advance and the difference between ported and manifold hook-up.
I am going to lower vacumm as I am not comfortable with 58 total. I have tried to adjust can without any luck I think the best thing for me to do would be to replace vacuum advance. Do the vacuum advances come with predetermined settings? I don't know why my jegs hei shows 24. Napa shows a vacuum advance that will fit my distributor but doesn't say how much it advances , mike
My sbc is a 427 10.5 with a accell hei 12 btdc 35 with 2500 and I think 58 with the vac advance 91 pump gas.You can make your own stop if you want to change it but it runs fine 90 degree outside temp. The engine will tell you what it likes.As soon as you touch the gas the vacuum falls off and so does the timing.
I have found that the aftermarket adjustable vacuum units can provide too much advance. The older points style Chevy distributors vacuum cans give 16 degrees of vacuum advance so I welded up the slot in the accel vacuum can on one of my cars to limit to 16 degree total and mimic the operation of the old “B28” vacuum can. One option for you. lots of ideas on here but personally I totally subscribe to manifold vacuum to the advance can to provide more idle advance for a cooler idle. And if your engine is running well with that much vacuum advance and plugs look good with no part throttle ping- I would probably keep it. It could be your mechanical curve is a little slower coming in and your transient timing combination (mech plus vac) is working well for your engine setup. matt
You can limit the vacuum advance by shortening the slot where the advance rod travels. I believe GM used rubber "bushings" to do this.
You 'Shivel-lay-boyz' have got-it-made when it comes to tuning the advance curve on your distrib.!!! SBF distrib. require a complete tear-down to get to the weights & springs in the BOTTOM of the distrib.!! (Sorry for the whining!) "I've found"...vacuum advance is much to do about a lot of nuth'in! Plug it off! ...END of problem. I don't think it makes THAT much of a difference in MPG in your average souped-up engine anyhow. With a snotty camshaft ...a LOT of initial advance really sharpens-up the idle and off-idle performance. I have light springs in my distrib.........figgered out how to 'limit' the total advance to 36*....and then I just screwed the distrib. around until I had 36* at approx.2000 RPM. Who CARES what the "number" is as far as initial advance goes!!! 36* total advance at 2000RPM.....NO vac advance.....She screams and no rattle. (IF it does rattle a little...that means I need to down shift to the next lower gear) Since we are "plagued" with computer controlled everything on modern cars....a simple carb and distributor is like a breathe of pure oxygen sometimes! K.I.S.S. (I think) is the best answer for these old hot rods. 6sally6
I just swapped from a set of 416 heads to 461 camel humps on a 350sbc. went from 12 degrees to 14 initial and 42 total. Those heads definitely like more adavance as stated before. It also seems to be happier now with the vac advance on the ported and not manifold vacuum. If its running well call it a win. Its easy stuff to play around with and learn what your engine likes, no two run the same.
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