Ok, It's been a long time since I played with a vacuum advance distributor and I need a refresher from you guys. I'm working on a friends 67 camaro with a 350. It's got an later HEI dist, edelbrock performer intake and carb. Do you hook up the dist. vac line to constant vacuum or port vacuum? I'm confused because I'm getting pros and cons on both hook ups. I'm told to connect to constant vacuum (vacuum at idle) so it will advance much faster but there is a chance of detonation. Then I'm told to connect to port which (will only send vacuum at higher rpms) so the advance will happen only on higher rpms but less chance of detonation. Any inputs are greatly appreciated. Thanks

 

I Have Always Used Constant Vacuum. For Me It's Easier To Get The Timing Dialed In. Sick

 

How is it easier to get the timing dialed in? No matter where it is connected to you still need to disconnect the vacuum advance.

 

I Adjust My Timing Using A Vacuum Gauge ... And No I'm Not The Only One ... Timing Lights Are For Rookies ... LOL ... Sick

 

Hey Sick, How do you adjust timing with the vac. gauge? Do you tune it to get the highest vacuum? Do you disconnect vac. source while adjusting? Should I still try to keep my maximum advance at around 36 deg. total? Thanks

Mike

 

My Cam Has Such A High Duration 307 @ .050 That It Doesn't Have Near Enough Vacuum At Idle To Advance The Distributor. If Your Running A Stock Motor On Constant Vacuum, You Will Need To Disconnect Vacuum Advance To Time It With A Vacuum Gauge. I adjust For Maximum Vacuum Then Back Off Two Degrees. I Usually Can Make The Two Degrees Back By Adjusting Idle Mixture. I'm Not Sure I Know What Your Asking About Disconnecting The Vacuum Source? Sick

 

The purpose of vacuum advance is to get additional spark advance on top of that provided by the mechanical advance in order to maximize fuel economy. At part throttle, when the manifold vacuum is high the spark is advanced, while at WOT when the manifold vacuum is low there is no vacuum advance provided. At WOT all advance is provided the mechanical advance mechanism. If you have a hot cam, the vacuum is usually too low to cause any additional spark advance and things work better if it is disabled. The vacuum advance should always be disabled when setting the timing with a timing light.

 

So Since My Cam Duration Is Over 300 ... You Think I Would Be Better Off Without Vacuum Advance? Sick

 

I don't understand, you adjust the carb idle mixture and that changes your timing back 2 degrees? Please explane.

 

Sorry For The Confusion Sometimes I Don't Use The Correct Terms Assuming Everyone Knows What I'm Talking About. First Of All We All Know That Timing Advance Matches The Ignition Timing To The Burning Rate Of The Fuel And The Speed That The Engine Is Turning. Many Factors Effect The Rate Of Fuel Burn. Cylinder Pressure, Vacuum , Amperage, Fuel Octane, Air/Fuel Mixture, Temperature, Cubic Inches, Spark Plug Positioning, Air Flow Turbulence, Load, And What Not. ALL Engines Will React Differently To This Conditions. Even More So With Engines Using Aftermarket Parts Such As Cams. If You Relyed On A Timing Light Without Factoring In All The Other Conditions, You Would Be Doing Yourself and Your Engine A Diservice. Vacuum Guages Can Be Used As Well To Adjust Timing As Long As You Remember All The Conditions That Effect Ideal Engine Timing. Vacuum Gauges Are Set Up In Units Of Measure Called "Inches Of Mercury". I Have Found When Using A Vacuum Guage To Adjust Timing, That The Point Of Most Vacuum Is To Far Advanced For Your Engine To Run Properly. Why? Because Of All The Other Conditions Which Effect Rate Of Fuel Burn. It's Not Completely Accurate. Nevertheless I Have Found That If ( For Instance ) Your Point Of Most Vaccum Was 15 Inches Of Mercury And You Retard Your Distributor ( Normally Just A Degree Or Two ) Just Enough To Bring The Inches Of Mercury Down To 13, The Two Inches Of Mercury Can Be Regained By Adjusting Idle Mixture. Why? Air/Fuel Mixture Effects Rate Of Fuel Burn Which In Turn Effects Amount Of Vacuum. I Have Found Using This Method Can Get You Pretty Close To Ideal Ignition. Why This Much Concern For Two Degrees Of Mercury? Vacuum Effects Rate Of Fuel Burn. This Is My Own Expeirence. I Only Use A Vacuum Guage When I'm Trying To Remove A Dead Spot In Acceleration And What Not. The Best Gauge I Have Found For Timing Is My Hand On Top Of The Breather Cover. It's Very Easy To Feel Premature Or Post Detonation If You've Done It A Thousand Tmes. Sick