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oregon chevelle 02-03-2007 07:03 PM

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?

71C10 02-03-2007 11:50 PM

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???

oregon chevelle 02-04-2007 12:17 AM

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?

Guy Hiltz 02-04-2007 08:17 AM

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.

red65mustang 02-04-2007 09:55 AM

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

Rick WI 02-04-2007 11:22 AM

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.

71C10 02-04-2007 11:42 AM

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...

red65mustang 02-04-2007 01:29 PM

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

topfuel 02-04-2007 02:13 PM

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. :)

71C10 02-04-2007 02:20 PM

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!

Rick WI 02-04-2007 02:35 PM

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.

oregon chevelle 02-04-2007 03:28 PM

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.

oregon chevelle 02-04-2007 04:22 PM

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.

red65mustang 02-04-2007 04:31 PM

also agree Top Fuel....
vacuum advance is a good thing.....

but a performance curved dist (17base/19cent/2600)is a different animal and is much more sensitive to C/R, gears and cam etc....
ex: less than half throttle on a steep grade, at 50mph which in this case is at full timing (2600rpm), and with his 216/216 cam would show 13-14" of vacuum on the guage....based on vacuum, the vacuum advance thinks it's cruisin,"add 10*=20crank degrees" ....the available tq at the rear wheels at 2600 with the 3.08 gears is too low for the 3600lbs loading the motor on the grade..rattle rattle rattle ...change the gears to 3.55, 50mph is now more rpm's and there is alot more tq at the rear wheels and the motor will tolerate more vacuum timing.....probably 7-10*=50-56 total

(oregon, that's why I'm saying wait till you get the new gears, at 2600 with the 3.08's....adding about 4-6* will be the limit...and that doesn't do much)

if vacuum advance was independent of CR, cam, gears etc as you mentioned Top fuel...there would be one can and one curve for all motors.....totally not true....every stock carb/cam/motor has it's own vacuum advance curve and limits

xntrik 02-04-2007 07:30 PM

Vacuum advance cans come in various "types".

Total degrees of advance, the amount of vacuum needed to make the advance begin, as well as the amount of vacuum that makes it advance fully.

Adjustable types usually only alter the degrees.

There was a good Corvette site with a lot of specific vac can stuff listed. You might Google and find it.

Info is on this site too, IF it can be found. I know, I posted it.


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