Maurice

This is my first time setting my timing while actually having vacuum advance and with a HEI dist. Below are the specs for my engine. My question is that if I remove the vacuum and set the max advance to 36deg, then, I hook the vacuum back up, isn't it going to be well over 36deg? I am supposed to remove the vacuum when setting timing, correct? Do you all recommend removing the vacuum advance while drag racing?


Displacement: 502 cubic inches
Horsepower: 502 @ 5200 RPM
Torque: 567 @ 4200 RPM
Max. Rec. RPM: 5800 rpm
Fuel: Premium Unleaded 92 Octane
Spark Plugs: #4 Rapid Rire Gap .035, R43XLS Gap .035
Ignition Timing: 8° BTDC @ 800 RPM, 36° Total @ 5,000 RPM.
Engine Block: Cast Iron, Four Bolt Main Caps (P/N 10237292)
Bore & Stroke: 4.47" x 4.00"
Comp. Ratio: 9.6:1
Crankshaft: Forged Steel (P/N 10183723)
Connecting Rods: Forged Steel, Shot peened, 7/16" bolts (P/N 10198922)
Piston: Forged Aluminum (P/N 12533507)
Piston Rings: (P/N 12524293)
Camshaft: Hydraulic roller (P/N 12366543) Lift: .527" I, .544" E. Duration @ .050" 224° I, 234° E.
Cylinder Heads: Cast Aluminum Oval Port 110cc Combustion Chambers (P/N 12363390)
Intake Valve: 2.25" Stainless Steel (P/N 12366987)
Exhaust Valve: 1.88" Stainless Steel (P/N 12366988)
Valve Springs: (P/N 12462970)
Valve Lash: Zero
Rocker Arm: 1.7:1 (P/N 12368082)
Intake Manifold: Aluminum Holley flange (P/N 12363407)
Carburetor: Holley 850 cfm (P/N 12366996)
Distributor: HEI (P/N 1104067)
Starter: Gear Reduction (P/N 9000852)
Water Pump: Aluminum (P/N 14058915)
Timing Chain: Single Roller (P/N 3891519)

The ZZ 502/502 is not intended for marine use, and should only be used in 1976 and earlier pre- emissions street vehicles or any year off road vehicles.


[Modified by Maurice, 7:16 PM 9/5/2002]

Rockn-Roll

I can tune it up for you. But, I have a hard time seeing what needs doing unless it's in my car...it will only take a minute or two...and you can use my engine while I "tune" yours. :jester

Anyway, yes...the 36 degrees is mechanical only. Your vacuum advance should carry it up another 16 I think...for a total of 52. For drag racing (and any other driving) you should adjust the ignition components to provide the best ignition possible...you have both mechanical and vacuum for an HEI system. For drag racing you will be WOT all the time, thus mechanical is all you really need, but you may have some problems getting it to idle with 20 degrees of advance, plus the engine may need a little more advance when you shift to the next gear and drop the RPM's. So, in my opinion you should use both, and keep testing different setups to determine what is best for your engine and driving style.

BeaterShark

When people talk about total timing, they are talking about that max timing that is reached when at WOT. The vacuum advance will not do anything from 1/2 throttle and on. When you are rev'ing your car in neutral, you pretty much always have vaccum, so you set your total with the vacuum advance plugged. Under light loads (ie cruise), the engine can handle (and likes) more advance than under heavy load, so the vacuum advance is used on top of the mechanical advance for up to 50 something degrees.

It is also believed best to have the 36* of total advance in by around 3000 RPM.

lars

Maurice -
The 36-degree timing spec is the maximum mechanical advance, and this is checked at elevated rpm - you rev the engine to 2500-3000 rpm with vac advance disconnected and set the maximum timing to 36. When the engine drops down to idle, the timing should retard to around 12-16 degrees.

The vacuum advance control unit should be capable of pulling in about 16 degrees when it's fully "sucked in." If you're hooked up to manifold vacuum, this will give you your initial timing of 12-16 degrees, plus another 16 from the vacuum advance, thus producing about 30 degrees of advance at idle. If your engine does not like this much advance at idle, you can connect the vacuum advance control unit to a "ported vacuum" source on your carb. This will prevent any vacuum from being applied to the control unit at idle, but you will still obtain normal vacuum at cruise rpm.

At cruise rpm, 2500 rpm and above, your actual timing will be your full mechanical (36) plus all of your vacuum advance (16) for a max total at cruise of about 52 degrees. When you put your foot into it, the vacuum drops away, and you're left with just the 36 degree total mechanical timing.

You can pick off my vacuum advance tech paper from the Forum Tech Tips section - this talks a bit more about the vacuum advance and how to set it up.

Maurice

Thanks alot!! Now I know.