Got Time? (Part 4 - Fini)

The vacuum gauge will show zero to 2" or so on the same engine at idle which indicates ported vacuum.
Raise the rpm levels and you will see the vacuum levels raise in coordination with the rpm.
This because the ported vacuum is tapped into the venturi and as air flow increases through the venturi there is a pressure drop.

Pulling the vacuum line to the vacuum can on an idling engine will retard ignition timing as shown by the timing light if - the vacuum line is connected to full time vacuum.
There will be no change if the vacuum line is connected to ported vacuum.

Very much the same thing happens when a tachometer is utilized.
Pull the vacuum line from a full time vacuum source and rpm drops.
No rpm drop is realized when pulling the line from ported vacuum.

In lieu of either the timing light or tachometer, simply listening the engine and noting whether the rpm drops will tell you what you want to know.

You can easily see that connecting the distributor to ported vacuum will increase the timing considerably past what is optimum for an engine under load once the rpm comes up.
With detonation resulting in most cases.
Keep in mind that the vacuum advance system is also a vacuum retard system depending on engine load.

To strike off on a short tangent here, pinging, which some call detonation - and it is - is usually caused by two things.
Assuming a reasonable compression ratio for the street and an adequate gasoline octane figure for the engines requirements.
Pre-ignition is one and it’s caused most times by a piece of carbon glowing red hot in the combustion chamber or a sharp edge somewhere in the combustion chamber.
Many times these sharp edges can become hot enough to ignite the fuel/air mix early - as does the hot carbon piece - and pre-ignition results.
Detonation is initiated by firing the fuel/air mix too early.
In both cases - pre-ignition and detonation - with the fuel/air mix ignited early, the piston still rising and compressing the fuel/air mix in the combustion chamber creates very high pressures which will ignite the remaining fuel mix in the chamber very much like a diesel engine does.
The sharp metallic ping you hear is from the collision of the two flame fronts and is the result of a very high and potentially damaging pressure spike.
Nuff said there.

Distributor curve or curves is much bandied about.
What it boils down to is simply the amount of advance vs. a particular engine rpm.
Once you have it laid down on graph paper with say, engine rpm at 200 rpm steps and the distributor advance having one degree steps you can see the resulting curve on the paper and that’s where the name comes from.

One last little note and that on the style of timing light you use.
The newer dial-back timing lights have their advantages, but if you’re running an MSD - or similar - ignition amplifier box you’ll end up with the timing considerably retarded.
With a multiple spark ignition box use a standard timing light.

I ran across this little bit last summer when I had a little extra coin and figured a dial-back timing light would have some real advantages.
Especially so on engines that didn’t have the maximum all-in timing figure marked on the dampener.
I shot the timing on the roadsters MSD equipped Buick engine and found the timing showed about 12-15 degrees or so too far advanced.
This was with the light set at zero.
Shooting it again with the old faithful standard timing light showed the timing right on the money.
Easy to see there would have been a lot of lost performance if I would have reset the timing using information supplied with the dial-back light.

Ya know, dad was right when he said that running a stock engine I needed to stick to what the manual said.
He said too, I couldn’t go far wrong in using the manual figures with a mildly modified street engine as well.
My experience bears out what he told me and I find with mildly cammed and reasonably carbureted not too high a compression ratio street engine that the factory figures work quite well.

Once you get into the serious engines - and that doesn’t mean a low comp SBC 350 with killer cam - that you need to do some experimenting with ignition as well as fuel curves to get where you want to go.

In fact, most of us need to listen to what the cam manufacturers are telling us when they make cam recommendations.
You can’t go far wrong in either following their recommendations to the letter or at the very least deciding on what you need for a cam and selecting a cam one step milder than what you thought you needed.

I find that most guys select a cam and stick with it come hell or high water.
It’s interesting to realize how few have run more than one cam in a particular street engine.
Racing engines, another story, lots of experimentation needed there, but trying two or more cams in a street engine can tell you a whole lot.

Part of it has to do with vehicle weight and gearing, no doubt about that, but for me the aim is to have a strong engine with good mid-range torque that runs crisply.

About four cams, three intake systems and a couple of ignition set ups later I think I’ve succeeded.
The Buick engine in the roadster is a bored out 455 now measuring 462".
It’s an easy starting, strong running, quick responding dependable cool running engine that runs on 87 octane and will idle all day in hot summer traffic with nary a whimper.
It’s civilized, easy to drive and when called upon it’s just hell on the on-ramps.

Sort of a Jekyll and Hyde kinda deal.

As it should be I think.

Hot rods are all about performance in my book and looks are secondary.

If not, why bother?

C9

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~