1962 Vac vs Mech Only Distributor

[Jeff C.]

1962 340HP had dual point mech only advance distributor 1963 340HP had single point vac advance distributor. What are the Pro and Cons other than judging points ? My understanding is the vac adv distributor is more drivable and that both distributors have a maximum of 20 degrees of mech adv? My 1962 340HP engine is missing the dual point distributor. During a recent rebuild I had flat top pistons installed to reduce the CR to 10:1 but everything else is the same. The distributor I have is correct for the 250/300HP engine. Any help in understanding this subject will be greatly appeciated. Thanks
Jeff

[Duke W.]

Re: 1962 Vac vs Mech Only Distributor

Vacuum advance produces the necessary extra advance to produce maximum thermal efficiency with low density and exhaust gas diluted mixtures that occur at idle and cruise. Such mixtures reduce flame propagation speed, which is why vacuum advance is best for road engines. A properly matched vacuum advance control will reduce fuel consumption and heat rejection to the cooling system.

Both the '62 340 HP dual point distributor and all '63 distributors have a total of 24 degrees centrifugal advance. For a mechanical lifter cam you should use a VAC that is all in by 8" such as the NAPA VC1810. Search this number in the archives for further information.

Assuming your engine was assembled with a composition gasket the actual true CR is probably not much more than 9:1 and the engine will probably operate without detonation on regular grade fuel, or, at worst midgrade. I doubt if such a low compression with a late inlet closing SHP cam would need premium.

Claimed "compression ratio" of pistons is meaningless to any degree of accuracy due to variation in deck clearance, gasket thickness, and chamber volume. The only way to know the true CR is to take all these measurements, use a CR calculator, and manage the build to attain the target number. Most engines are half to one full point lower than what the owner thinks the CR is. This includes original Chevrolet-built engines which are typically half a point lower than advertised.

Most "engine builders" purposely assemble engines with low compression and high overlap cams. That way you don't get any detonation on premium, which most guys think their engines need. You also get poor idle quality, poor torque bandwidth, and poor fuel economy on more expensive fuel than necessary. Most "engine builders" don't "believe in vacuum advance" or think it should be "ported". The average customer doesn't know any better, so they go on their merry way. Ignorance is bliss!

This subject had also been thoroughly vetted in the archives.

Duke

[William C.]

Re: 1962 Vac vs Mech Only Distributor

Convert the curve in the 300hp distributor to match the 340 hp curve, use the vacuum can Duke recommended and you will be in business.

[Duke W.]

Re: 1962 Vac vs Mech Only Distributor

Actully with his low compression setup assuming it has a Duntov cam, he can put some light springs in and bring all the advance in by 2500 or less. This will improve low end torque and also add a little fuel economy improvement.

The optimum operating point at load is the ragged edge of detonation. Modern engines with knock sensors can maintain this point, but on a vintage engine you have to leave a little margin for worst case conditions, like a very hot day.

Duke