Dan Timberlake

Hi John,
I've enjoyed your posts and articles for many years. I'm especially glad you are feeling well enough to participate on these forums again.

Your excellent article TIMING AND VACUUM ADVANCE 101 has been circulated far and wide over the years. I'm hoping you have a little time to help me understand it a little better.

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"lean mixtures, such as at idle and steady highway cruise." I understand that cruising mixtures are lean for fuel economy, but at least back in the carburetor days it was reasonable or necessary to shoot for a mixture of around 3.0 % CO or about 13.4:1 or even a little richer to achieve decent idle quality. At idle or cruise the manifold vacuum is pretty high, so the the condition of the gasses in the cylinder when the intake closes is rarified, with not many molecules of nitrogen, oxygen and fuel and etc residing together. So I'm thinking idle is rarified, but not necessarily all that lean.

I understand how rarified need more ignition advance, for sure.

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History of vacuum source for advance. The article is generally reported as saying "After 30-40 years of controlling vacuum advance with full manifold vacuum...",

Attached (I hope) is an image showing a few pages from an Audel's New Automobile Guide from the 1940s, predating the emission era by a few decades .

It describes the function of one ported vacuum scheme ("carburetor side," ) , and 2 "engine side" (unported) . For those of us used to downdraft carbs the engine/carburetor side reference looks backward since an updraft carb (also popular once-upon-a-time) is shown.

The text on Audel page 1242 says "carburetor side" (ported") is the most popular. Manufacturers back then were making their choice for reasons other than emissions I'm pretty sure. Like maybe steadier idle, less sensitive to small rpm changes due to the crude idle conditions of the day.



I can imagine that ported vacuum may have become virtually standard to cause higher idle exhaust temps for cat converters etc, but I'm pretty sure ported manifold vacuum was used a fair bit before emission control became king.

thanks for any clarification you can offer,

very best regards,

Dan T

SWCDuke

Idle mixtures must be richer than cruise due to low mixture density and exhaust gas dilution. Cruise mixtures can be as lean as possible to avoid misfire due to mixture distribution issues that can cause one or more cylinders to be lean enough to misfire. Typically most carbureted engines can run close to stoichiometric (average of all cylinders) under most cruise conditions.

Best fuel economy is usually about 10 percent lean of stoich., but it requires very even fuel distribution, which the Rochester FI system can provide. That's why FI engines like the 360/375s will deliver about 10 percent better fuel economy with the same gearing as their 340/365 SHP brothers with carburators.

Stoic. results in the lowest aggregate engine out emissions and maximum oxidation/reduction efficiency by the catalyst. That's why modern emission control systems maintain cruise/idle mixture at stoich. with O2 sensors and port or direct injection that maintain very even fuel distribution.

Search for threads started by me and download the tuning seminar pdf. It goes into more detail on the optimum mixture and spark advance requirement for all operating conditions.

Duke

tbarb

Your question about when ported vacuum was first used is interesting. I am a fan of full manifold vacuum advance but I have read where others swear by ported vacuum advance.

FWIW, I do think ported vacuum advance can work just fine but only with a completely different advance curve than the emission era. What I am talking about is something like 18-20* initial timing and 18* centrifugal timing along with 10-15* ported vacuum advance. In some reading I have done people swear the engine runs better with the ported advance.

MikeM

Despite what you often read here, your assumption is absolutely correct!

I believe the main reasons different vacuum sources were used for engine tuning was because of the low octane of the gasoline available then.