Vacuum Advance and why to use it on the street

[PxTx]

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]

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.

[WILMASBOYL78]

<span style="font-style: italic">&quot;Tom, I know everyone hates blanket statements, but 15 degrees VA is too much for todays fuels.&quot;</span>

Totally agree...but, as I stated, we are running a mix..typically 50% Av gas or 112 with some 93...I also throw in some lead additive..and a little Marvel oil for good measure. I've tried the octane boosters, but I think the mix is the best solution...

Now..to my Italian friend from Santorum Land...

<span style="font-style: italic">&quot;Tom, did you write that on your own or was someone there coaching you?&quot;</span>

Yes...all on my own Sammy...but, when I'm really stuck I check with Cumby..at his age he has to know everything [img]<<GRAEMLIN_URL>>/rolleyes.gif[/img]

I'm glad it's Friday...wilma

ps..we got snow last nite

[VintageMusclecar]

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: pxtx</div><div class="ubbcode-body">...
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 &quot;custom tune&quot; for help with stock type tire drag racing and probably shouldn't be mixed in here as that could be a great seperate thread....</div></div>

Tried it both ways, ported and manifold, no difference either way...still un-driveable.

I have my own theories why this (and several similar builds I've been involved on) exhibited similar &quot;symptoms&quot;. Maybe I'm right, maybe I'm out to lunch, all I know is I've found methods that consistently work for me. Your mileage may vary (pun somewhat intended [img]<<GRAEMLIN_URL>>/wink.gif[/img] ).

Engines with limited low rpm vacuum (read: big cams) seem to be extremely sensitive to the (often drastic) timing changes that can result from the vacuum advance system. Just changing the base timing from 12° to 14° on my current 496 results in ~100 rpm gain in idle speed and requires re-setting the idle trim screws on the carb. If 2° has that pronounced of an effect, imagine what a wildly-swinging timing curve does as the vacuum advance adds and removes timing as the engine begins surging. Mechanical advance isn't affected by vacuum, only rpm, and as such eliminates that variable.

Besides, the car can blow the tires off pretty much at will in the first 3 gears as it sits now...how much more low end does it &quot;need&quot;? lol. [img]<<GRAEMLIN_URL>>/grin.gif[/img]

Going slightly off-topic but along the subject of variables on differing combinations, here's a little something to ponder over...

The 3418 Holley on my 496 was pig fat rich across the board in stock configuration on this engine and required significant recalibration. I pulled 3 jet sizes out of it on the dyno and picked up significant power (and it was still rich, I just ran out of dyno time to do further tuning). Once the engine was in the car I found out that it also needed a ton of fuel pulled out of the idle/transition circuits to be happy.

That said, I built another 3418 for a car that belongs to a member here with a stock L78, and aside from dropping the jetting down several sizes, it runs tip top for them.

Again, to reiterate... every combination is different, there is no &quot;right&quot; or &quot;wrong&quot; way to approach the tune under <span style="font-weight: bold"><span style="text-decoration: underline">all</span></span> circumstances. You have to experiment to see what does (and doesn't!) work. [img]<<GRAEMLIN_URL>>/smile.gif[/img]

[Tommy]

My last smallblock acted the same as Erics when I attempted to use the VA. Locked out it ran wonderful. For the last several years I have not used any VA on my engines and most of my Dizzy's (dual points) don't even have a Va canister option. The MSD unit I have is the only one with a canister on it and it absolutely did not want to co-operate with me. Totally undrivable. If I knew it was beneficial and might prohibit a melt down and could make it work I would try it but so far it hasn't resulted in any issues not using it. I hammered it up and down the interstates from Alabama to Kentucky and the Smokey Mtns for hours at a time and never worried about a thing. Hook up the va and it won't pull a greasy string....

Tommy [img]<<GRAEMLIN_URL>>/[/img]