Wow! What A Change

[Bill I.]

Reset the timing on my dizzy (mech, no vac can) from 8* at idle to 14*. No pinging, runs a bit hotter, maybe 190 now. BUT what a change in get up and go. A scalded dog has nothing on this car now. Could just that amount of advance cause that much better performance? Thanks, Bill. 67 327/350.

[Ronald L.]

Re: Wow! What A Change

yup

[Jack H.]

Re: Wow! What A Change

For those who aren't familiar with the non-US term 'dizzy', it's common slang in the UK, Australia, New Zealand for what we call a 'distributor'. Comes from the fact that IF you were watching the distributor function without a cap in place, you'd see a rotor going around and around causing you to get 'dizzie'...

We can also digress to 'boot', 'bonnet', 'box' and other non-US slang terms for other common auto components!

[Clem Z.]

Re: Wow! What A Change

Reset the timing on my dizzy (mech, no vac can) from 8* at idle to 14*. No pinging, runs a bit hotter, maybe 190 now. BUT what a change in get up and go. A scalded dog has nothing on this car now. Could just that amount of advance cause that much better performance? Thanks, Bill. 67 327/350.

power timing,setting the timing to full advance with the engine reved up till the advance stops moving gets the best performance. most all chevy engines run best at 36 degrees total advance without the vacuum advance connected.

[Duke W.]

Re: Wow! What A Change

Reset the timing on my dizzy (mech, no vac can) from 8* at idle to 14*. No pinging, runs a bit hotter, maybe 190 now. BUT what a change in get up and go. A scalded dog has nothing on this car now. Could just that amount of advance cause that much better performance? Thanks, Bill. 67 327/350.

Your engine is on crutches. The OE distributor has a lazy centrifugal curve with a maximum of 30 degs. @ 5000, so increasing initial timing will improve low end torque, but yields too much advance at high revs, which will cost power in addition to promoting detonation.

You would be better off using lighter centrifugal advance springs and backing the initial advance down to 8.

If you read the lastest Corvette Restorer, you saw that total WOT advance of 38 deg. improved power about one percent over 36.

The '67 OE 300 and 350 HP engines have the same centrifugal curve, but different VACs due to the different idle vacuum characterisitics.

The better than expected low end torque of the Special 300 HP engine configuration is partly due to the aggressive spark advance curve, despite the very late closing inlet valve.

Your engine would also benefit from the addition of a 12" VAC - cooler operating temperatures in normal traffic and better fuel economy, and a 12" VAC is best for the L-79 cam versus the 15" VAC for the 929 cam and two custom cams discussed in the article.

Most guys have little idea of how much better performance and economy can be achieved by optimizing the spark advance map. Most OE maps are very conservative in terms of rate of centrifugal advance increase, and some OE configurations don't have an optimum VAC.

Duke