Vacuum Advance and why to use it on the street

Plowman · #1 02-24-2012, 04:05 AM

I didn't want to post things on a for sale thread, and got some help from my son just to get the ball rolling. I appologize for giving a negative spin on the NOS vac advance.

Surprisingly, there were several on that thread who did not seem to understand the relationship the VA has on an engine with today's pump gas.

There is actually a fellow who writes a decent explnation at this link:
http://www.kitcarmag.com/techarticle...on_timing.html

Quoted from that link "Any original or high-performance aftermarket replacement distributor must have the ignition spark advance curve(s) tuned for your engine and the gasoline you will be using. Most of the high-performance replacement ignition systems, such as an MSD distributor, come with a very conservative ignition advance curve installed in the distributor. A new MSD distributor comes with a selection of advance springs and bushings so you can set the advance curve you want. However, most vacuum-advance-equipped distributors have too much advance for the reformulated gasoline of today."

Locked out distributors and too much VA at idle contribute to overheating (glowing red) pistons rings and premature blow-by. Yes you can drive it, but is not ideal.

Hopefully we get some good posts and comments here, and with some patience I'll try to share my understanding and techniques to get ideal street tunes on todays fuels. We could all learns from a decent thread- myself included.

**Salvatore** · #2 02-24-2012, 04:31 AM

have a stock distributor in my Z/28. Has been curved and VA locked out. Never had pre-ignition, pinging or glowing headers. Run 12 degrees at idle and around 36 at 2,800 rpm's. I run 112 Sunoco mixed with 93 BUT really don't need it. Ran 93 octane for about 10 years with a lead additive but that is not the reason I run the 112 mix now. Kinda run my carb on the rich side if that matters. Its more of a winter setup which IMO is a little safer but that could be old style thinking. Thanks for posting Paul.

Keith Tedford · #3 02-24-2012, 04:35 AM

The original factory advance setup in 1969 wasn't even for performance. It was to pass emissions tests. Our 427 came with #68 primary jets and had lean surge at sixty mph. The advance curve was long and slow, not coming in full until about 5K rpm. I forget the exact number any more but I bumped up the initial timing and played with various springs and weights. The difference in performance was impressive. That was when we could get 104 octane gas. With the 91 octane today you can't run nearly as much total advance. I had our L78 running reasonably well but it sure wasn't running original factory specs in the distributor. It will run but not all that well.

Hemicolt · #4 02-24-2012, 05:32 AM

There is so much that is being left out here.
Compression ratio and camshaft specs. play a huge part in how much pressure and heat are in a cylinder. Anyone who has looked into camshaft design knows timing events and overlap in a cylinder cycle will determine pressure bleed off.
My engine has thousands and thousands of miles on it. No melted plugs, ever.
Every year I pull the rocker arms to check wear. While doing this I always do a leak down test and have never found any problem in the ring and cylinder package. Engine has no blow by and uses no oil.
This year as well as every year in the past, since the car has been built, I will drive this car in heavy traffic and enjoy not having to pull over due to over heating.
To say that every motor needs a V.A. and can not run a locked out dist., due to todays fuels, shows you may need to play catch up. Contact Patrick James at Pro Systems, if you would like to check my information.

VintageMusclecar · #5 02-24-2012, 02:18 PM

My 496 uses a OE distributor with a 22° mechanical advance curve which is all in by <4000 rpm. I run 12°-14° initial and 34°-36° total advance, and with ~11.2-1 compression it runs fine on 93 octane gas. (I also run a 160° thermostat to help ward off detonation)

When I had the distributor curve set, we installed a vacuum advance can which provided very little advance--if memory serves, only around 8°-10° degrees since I knew I would have to keep the total advance limited to be able to run pump fuel.

I have tried hooking up the vacuum advance several times since I got the car running, and every time, simply hooking the hose up literally renders the car un-driveable. Running mechanical advance only, even with a 256°-264° @ .050" cam and 108° LSA, the engine will pull down just fine to <1500 rpm in 4th gear with only an occasional minor hiccup--by 1600 rpm it is totally smooth. Hook up the vacuum advance and it instantly becomes a bucking, snorting Bronco Bull ride from hell. No doubts here, this combination does not want, or need vacuum advance.

As far as super quick advance curves and/or locked out advance curves;

Years ago I had a 75 Monza with a nice running 400 small block in it....very mild build, never saw the high side of 5500 rpm. It had an HEI distributor with an aftermarket curve kit installed, and I originally had it set up with the medium weight springs which provided a similar curve to what I now run in my Chevelle.

I tried experimenting with the advance springs one day, and simply swapping the medium weight springs for the light springs resulted in a full .4 loss in e.t. At first I thought it was a fluke--there was no tire spin and my butt-dyno was telling me the engine was FAR more responsive at lower rpm. I made another run--again, it was .4 off. I stuck the medium weights back in and the .4 immediately came back.

Again, that combination simply did not like a fast advance curve--despite what my butt-dyno was telling me.

In regards to locking out the curve entirely, one of the more well-known and respected members here tried that recently with less than stellar results. I'll leave it up to them should they wish to divulge the details. I personally have used a locked out distributor one time with successful results, and that was on my old ~800 HP 572 in my last Chevelle. (Of course, the 4K+ converter probably hid whatever low speed driveability issues that may have been present)

I'm not saying there is never a time or place for either a locked out distributor or an aggressive curve. What I am saying is that it is entirely dependent on the combination.

My $.02 worth of real world experience. [img]<<GRAEMLIN_URL>>/smile.gif[/img]

Hemicolt · #6 02-24-2012, 02:56 PM

"I'm not saying there is never a time or place for either a locked out distributor or an aggressive curve. What I am saying is that it is entirely dependent on the combination.

My $.02 worth of real world experience."

Very well put. A little knowledge can be a dangerous thing. These blanket statements saying that "one rule applies" is the wrong thing to be telling people.

It's like the carb. trick he told me about using on the tunnel ram setup. I can't argue if adding the wire to the metering plate will or will not work when it comes to cutting down the fuel at idle.
We will however try this when we run the engine on the dyno. Before and after results will be recorded and I will report the findings for everyone to see. The reason I will try it both ways is because of what was said above. Every combination will require different things.
I'm still thinking the amount of of vac. at idle is going to determine how much fuel is pulled through the idle circut. I may be wrong in the end, but I'm not going to tell everyone you can or can't before trying it first.

To go in and blindly make the modification without testing it both ways is foolish.

**WILMASBOYL78** · #7 02-24-2012, 03:13 PM

I will certainly bow [img]<<GRAEMLIN_URL>>/worship.gif[/img] to the experts and can only offer my "real life" experience. We have a number of L78 Novas which are street driven and one which is occasionally run in the 1/4. They all have factory points distributors...we run vacuum advance on all of them. The engines all have the factory L78 cam, 11 to 1 pistons, big heads, etc..basically stock L78 motors...nothing tricky. For fuel we run a mix of AV Gas or race fuel mixed with 93 pump gas...no issues. Only one of the L78 cars has a "curved" distibutor...our black 1970 TH400. I have never tried running the cars without the vacuum advance...never saw the need to. Most of the factory BB solid lifter motors (68-70) had the #355 unit which had 15* advance..this is the vacuum unit that is in all of our stock distributors.

Again...my info is directly related to driving these cars for many years and knowing how they run. I can't comment on how other people's cars run or perform...only my own. I maintain that "fuel alone" does not determine the results related to vacuum advance or the lack of it...there are many factors that impact performance and tuning.. and each situation is a little different.

wilma [img]<<GRAEMLIN_URL>>/

[/img]

**Salvatore** · #8 02-24-2012, 04:20 PM

Tom, did you write that on your own or was someone there coaching you?

**PxTx** · #9 02-24-2012, 04:31 PM

There really is a lot to seting upa a VA tune one an old engine with todays gas. Unfortunately, implying hooking it up and saying it doesn't work because isn't the real world things ran poorly isn't a very fair evaluation.

Sam, It seems like you tune is pretty safe. I would not be afraid of running that in the street. What is the reason you choose not to run a VA?

Keith, you are right about the factory tune and configs. They aren not ideal for performance and certainly were not intended for life today. Part of this thred could evolve in to how to tune and old engine for great performance and street manners with todays fuels.

One of the challenges can be that todays unleaded reformulated gasoline is that it is designed for use in a modern computer controlled electronic fuel injected engine with the goal of lowest possible exhaust emissions. The computer that operates with a modern fuel injected engine automatically adjusts the air/fuel mixture and the ignition spark timing in order to obtain the lowest possible exhaust emissions along with the best possible power and drivability.

The leaded gasoline that was sold at your corner gas station until the mid 1970’s was easier to tune for because a tuner could “read” the spark plugs and look at the tailpipe “color” to determine the air/fuel mixture. Today’s reformulated gasoline does not leave any “color” on the spark plug unless the air/mixture is very rich and the tail pipe color is almost meaningless. Most people think their engine is running too rich because the exhaust fumes burn their eyes; this is not true. An engine with a rich air/fuel mixture will have excessive carbon monoxide (CO) in the exhaust; carbon monoxide is an odorless gas that is quite deadly if you breathe too much of it. A engine that has a misfire from things such as: a rich air/fuel mixture, a lean air/fuel mixture or incorrect ignition spark timing will have a lot of unburned hydrocarbons (HC) in the exhaust. Exhaust gas with a high unburned hydrocarbon (HC) content will burn your eyes and make it difficult to breathe

Tracy, I say a lot is being left out, so let's fill in the spaces. If you can, help us oout and get your guy Pat to post here, or maybe try your best to post things he has written in context? Also, since you enjoy the car in heavy traffic, why did you choose to lock your distributor? One last question, what are your leakdown results and what is your threshold of "good" for a street driven motor wiht thousands of miles on it?

Eric, the way you wrote your, it almost sounded like you were talking about hooking the intake to ported vacuum, not manifold vacuum. I hope you don't take this the wrong way- more saying this for the sake of those reading. Ported Vac= emissions cars (with EGR) and Manifold Vac= muscle cars. There are some people who do not follow this but they appear to be trying to use a "custom tune" for help with stock type tire drag racing and probably shouldn't be mixed in here as that could be a great seperate thread.

Tom, I know everyone hates blanket statements, but 15 degrees VA is too much for todays fuels. We've done some great, non-stock tunes wich would pass for stock drivability that work great on todays fuels.

I'm not expert here either, and I've copied and pasted some things above from post my Dad has made and form the Henry Olsen. I have been setting up cars for about 20 years and there are still things I continue to learn from my Dad. Maybe someone else can shed some light on the how and why things are done in their tune for us all to benefit from. Looking forward to the technical discussion.

Keith Tedford · #10 02-24-2012, 04:59 PM

When we bought our L78 car in '98, we could still get Ultra 94 and similar. Now, all I seem to find is 91 and the engine would ping. I could back off the timing until the engine stopped pinging but then there wasn't enough timing on the bottom end which can lead to over heating. That's where shortening up the advance curve came into play. You have to figure out where your engine works best on the bottom end. Then figure out where it works best in the upper end rpm for advance. Then tailor the distributor to suit. In my own clumsy dumb ass way I have managed to get close to 200K miles out of the 427 and have run the L78 on unleaded for about 37K miles with zero problems. Running a little rich on the jetting also seems to help with the pinging. If I can have my cars running fine, it can't be all that difficult, yet I'm always reading stories of red hot headers, melted pistons, over heating and engines detonated to death. There are vacuum canisters out there with different amounts of advance. Pick a suitable one. If not get an adjustable unit.