Piston Slap: When you need the opposite of timing advance?

Mark writes:

Looks to me like the turbo engine Gen 1 Corvair has a vacuum unit that works backwards! Timing calls for 26 degrees BTDC … can this be correct? Not my first air-cooled six, but it is my first turbo ’vair! Any help would be appreciated plus how to identify the turbo ignition distributor.

Sajeev answers:

After a little Googling (actually a lot of it), I kinda think the vacuum unit works backwards on turbocharged Corvairs too! Luckily, I need not think much further, as our resident Corvair expert once again comes to the rescue. So I’d better hand it off to Kyle Smith, without further delay.

Kyle Smith answers: 

The vacuum unit on your distributor is indeed backwards! The Corvair has a rudimentary turbocharger system by modern standards, and all of its safeguards are built-in, using hard parts. The main issue is detonation under heavy load and boost. Too-advanced ignition timing can cause this on the turbocharged Corvair engines, even at the modest factory boost levels. To counteract this, what was traditionally the timing advance (that’d increase the timing in a normal Corvair) is now a boost retard. If the canister sees manifold pressure, it retards the timing by up to 8–10 degrees (depending on the exact canister you have) as boost rises. There is still mechanical advance inside the distributor (springs and weights) but they do not come into play until above 3500 rpm.

The base timing can be set with the boost retard connect, unlike vacuum advance systems on naturally aspirated Corvairs since the boost retard does nothing under vacuum. (And that’s what it would see while setting base ignition timing). The factory shop manual is your best resource for confirming the factory timing specs and can also serve as a starting point to some finer tuning. Adding 1–3 degrees of additional advance is fine, so long as you do not hear any pinging under heavy load or acceleration.

The quality of your fuel, along with other factors, will determine where the engine will be happy. If the engine is happy, the driver will be too. Have fun with it!

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