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Old 02-03-2007, 06:03 PM
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running no vacuum advance

I have a 350sbc with an HEI that does have vacuum advance. However I had the distributor recurved to 36 total by 2600rpm. When this was done the vacuum advance was disconnected. Is anyone else running theirs this way?

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Old 02-03-2007, 10:47 PM
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if you drive your car on the street at all you want to retain the vacuum advance. You'll get better gas mileage, and your motor will last longer and run much better overall. You may want to modify the rate and amount of advance but you do not want or need to eliminate the vacuum advance. Finish the job and dial in the vacuum advance. You can buy a adjustable vacuum advance from Crane or Moroso. its adjustable in both the amount of advance limit and the rate of advance relative to vacuum.
Usually you want to get around 48 to 51deg total initial+mech+vacuum advance while crusing on the hiway at hi speed and highest manifold vacuum. Some time, a vacuum guage and trial and error will tell you what the motor needs. Too much vacuum advance and the car will buck. Too little heats up the exhaust ports and cylinder walls and reduces mileage. Too quick coming in or going out as you go from part throttle cruise into WOT and the motor may ping. (diaphram spring preload) Generally Stock vacuum advance has too much advance travel (20deg is common) once the mechanical curve is hopped up. A little too much with 36deg mechanical. So you want to reduce the vacuum advance limit to about 10-15deg range (vac rod travel limit) and play with the vacuum rate (diaphram spring preload) So it kicks in and goes out just right, relative to engine load and throttle position.
its just a matter of playing with the set up till its right.

What cam is in your motor? How much manifold vacuum do you have at idle? What is the idle rpm?

Last edited by F-BIRD'88; 02-03-2007 at 10:52 PM.
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Old 02-03-2007, 10:50 PM
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Quote:
Originally Posted by oregon chevelle
I have a 350sbc with an HEI that does have vacuum advance. However I had the distributor recurved to 36 total by 2600rpm. When this was done the vacuum advance was disconnected. Is anyone else running theirs this way?
Including how much initial???
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Old 02-03-2007, 11:17 PM
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MY cam is a summit .454 lift and .216 each at .050 single pattern cam. My initial advance since the recurve is about 17. Manifold vacuum is 19 at about 600-650 RPM. Is it possible to hook the vacuum line from the distributor to the carburetor back up and run it like that or will that mess up the recurve I just had done?
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Old 02-04-2007, 07:17 AM
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running no vacuum advance

I have not used vacuum advance on my distributor for going on 4 years and my car runs just fine.There is no difference in plug life and no noticeable difference in gas mileage.I tried the Crane vacuum set with the cam to limit total timing,the car had an Accell adjustable vacuum can already on when I bought it.No matter what I tried there was always an issue with pinging at part throttle tip in.My buddy convinced me to try it without any vacuum,he doesent use vacuum on his Hi Per small block either.No more pinging at part throttle tip in,car works better than ever ! I think that just because something works in some situations doesent necessarily mean that that is the answer for everybody.If,in your application,your car works better without vacuum advance hooked up,leave it unhooked and enjoy it.
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Old 02-04-2007, 08:55 AM
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hey oregon,
the "performance curve" they did is based on WOT(pedal to the floor) and basically 0" of vacuum and 12/1 A/F....adding vacuum advance doesn't change that at all.... it only adds to the timing under light throttle

mount a vacuum guage in the car, 70mph crusin your vac is 18-20", A/F is 15/1, motor can handle 10+* more timing btdc to have a more "burn time" using a vacuum advance =better mpg

agree with Guy above, it's not a must have, it does help many cars be more pedal responsive around town, can help a little to keep plugs cleaner, can help auto trannys shift correct, long list of possible small benefits for how the car behaves

first check if yours is adjustable, stick a 3/32" hex in the snout...see if it turns either way....post answer?

sidenote: goal of messin' with the rubber parts on the other thread is to make the car "cocked for launch", "instant power transfer" like a sprinter in the starting blocks...makes a big difference for 60' times in a heavier car

Last edited by red65mustang; 02-06-2007 at 05:03 AM.
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Old 02-04-2007, 10:22 AM
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That ignition curve is not setup with vac advance in mind, hence why I think he left it disconnected. If you connected the advance canister to manifold vacuum you would have way too much timing at idle, not sure if the motor will like all that.

Even hooking the vac advance to ported I think you may run into issues under light load throttle conditions with an over advanced situation given the very high base timing settings. Could be very prone to detonation.
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Old 02-04-2007, 10:42 AM
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Quote:
Originally Posted by Rick WI
That ignition curve is not setup with vac advance in mind, hence why I think he left it disconnected. If you connected the advance canister to manifold vacuum you would have way too much timing at idle, not sure if the motor will like all that.

Even hooking the vac advance to ported I think you may run into issues under light load throttle conditions with an over advanced situation given the very high base timing settings. Could be very prone to detonation.
So true cause I've been there, done that! Most rigs I've seen with a mechanical only distributor have a lot more cam...
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Old 02-04-2007, 12:29 PM
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oregon,
Rick's post just reminded me, you still have the 3.08 gears in it,

fair to good chance you can't add enough additional timing with the vacuum advance and 3.08's to make a noticable "peppier" ride before the valves will complain with 10/1CR-3600lbs

put "vacuum advance" on the back burner till you get the (3.55-3.73) gears, then it's time to test
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Old 02-04-2007, 01:13 PM
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Quote:
Originally Posted by red65mustang
oregon,
Rick's post just reminded me, you still have the 3.08 gears in it,

fair to good chance you can't add enough additional timing with the vacuum advance and 3.08's to make a noticable "peppier" ride before the valves will complain with 10/1CR-3600lbs

put "vacuum advance" on the back burner till you get the (3.55-3.73) gears, then it's time to test
Just to remind everyone. Rear end gearing, camshaft specs., compression ratios, trans type don't matter as far as vacuum advance goes. Chevrolet used vacuum advance on all engines except the very hi hp models both big and small block from 140 to 454 cu.in. with rear end gearing from 2.41 to 4.56 and compression ratios from 8-1 to 10-1 with both autos and manuals.
I have always been an advocate of using vacuum advance on a street driven vehicle. If vacuum advance doesn't seem to make a difference on your combination, then maybe the combination needs more tuning and refinement. In over 27 years of working on cars and building engines, I have yet to see one street driven vehicle that couldn't be made overall more drivable with the use of vacuum advance.
I know some of you might disagree with me and that's fine. All I can say is the vehicle manufacturers spend millions on R&D and if they find they can get by without a part in a given application, they will not use it. A vacuum advance was used for a reason. Maybe some of us should consider this before we take what we have heard or read as gospel. Ok, the soap box is put away. Bring on the rebuttals.
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Old 02-04-2007, 01:20 PM
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Quote:
Originally Posted by topfuel
Just to remind everyone. Rear end gearing, camshaft specs., compression ratios, trans type don't matter as far as vacuum advance goes. Chevrolet used vacuum advance on all engines except the very hi hp models both big and small block from 140 to 454 cu.in. with rear end gearing from 2.41 to 4.56 and compression ratios from 8-1 to 10-1 with both autos and manuals.
I have always been an advocate of using vacuum advance on a street driven vehicle. If vacuum advance doesn't seem to make a difference on your combination, then maybe the combination needs more tuning and refinement. In over 27 years of working on cars and building engines, I have yet to see one street driven vehicle that couldn't be made overall more drivable with the use of vacuum advance.
I know some of you might disagree with me and that's fine. All I can say is the vehicle manufacturers spend millions on R&D and if they find they can get by without a part in a given application, they will not use it. A vacuum advance was used for a reason. Maybe some of us should consider this before we take what we have heard or read as gospel. Ok, the soap box is put away. Bring on the rebuttals.
Well said, no rebuttal here!
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Old 02-04-2007, 01:35 PM
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Agreed TF, it's impossible though to get the vac advance dialed in correctly IF the base mechanical advance curve is foobared up. This particular mechanical advance curve in question I feel is foobared for vac advance.
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Old 02-04-2007, 02:28 PM
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Thanks for all the responses keep em coming. I read them all and try to take in as much as I can even with different opinions. Yes Red I still have the 3.08s in it. Got a line on a 78 malibu that I may pick up and try to build a more strip oriented car out of it. If I do that will allow me to go ahead and put the 3.73s in the 8.2 ten bolt I have in the Chevelle right now. Figure it will be pretty safe if I only strip the Chevelle a couple times a year and take the 78 to the track most of the time. We'll see though I have not decided on the 78 for sure yet. It would be a total build. Body is & frame look pretty good but all else would have to be upgraded. As for the 17 intial and 36 total by 2600 rpm, I figure I am most concerned with performance at the track for now so would'nt that be the best setup? or would I run as good or better by going back to the vacuum advance. OBTW the initail was at 17 before we curved the distributor and just happend to fall there after we recurved it as well.
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Old 02-04-2007, 02:30 PM
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One thing you have realise is that if you're going to run a aggressive timing curve you'll probabily have to use higher octane gas 92+ even with a 8.5:1 cr. Thats life. Nothing stopping you from taming down the mechanical curve a little (go up in spring tension one one spring) from what your emchanic set up if you find that all in by 2600 is a bit too aggressive. I usually find a good bit of initial (14-16)+ a smooth curve that peaks out (32-36deg) at 3000-3500 is more than enough for a motor like you discribe. I've found thru much drag testing that very agressive timing curves are usually not nessessary to get best acceleration.
This slightly tamer mechanical curve will allow more vacuum advance with less tendancy to ping for exessive advance. You can also go down 1 or 2 sparkplug heat ranges from stock to good effect if you are an aggressive driver. (High performance) This really adds some knock tolerance when running aggressive timing. The stock recomended spark plugs heat range is biased more for around town driving where you need the hotter plug to reduce combustion deposits. WOT driving creates more heat allowing and requireing a cooler spark plug heat range to keep the plug tip form overheating (preignition and detonation) Don't feel you have to run 36deg total either. Many many motors make maximun power with less timing. 32-34deg. Running more timing than what is required to make max power just invites pinging sometimes a little less, is better. Experiment a little. It's all about overall balance.
What cylinder heads are U using? What grade of gasoline? What is the motors compression ratio?
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Old 02-04-2007, 03:22 PM
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Heads are Aluminum TFS 2.02/1.60 with 180 intake. I run the 91 or 92 ocatane what ever is the highest when I fill up. Compression ratio is somewhere around 9.25-9.8 to 1. I don't know for sure because my build sheet from summit gives me two different pistons dishes and I do not know which they used. Also the heads are 62-64cc depending on which tech guy talks to me from Trick Flow Specialties. The gasket is somewhere from .41-.38 in thickness so any way you get the point. I am not sure. When I did a cold compression check last year or so they were all from 175-195 or so I think. But who knows I cannot remember for sure if I even did that right. It is a B*%#$ to try and do a normal one because I cannot get to all my plugs without undoing the headers.
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