Modified BMW 2002 Ignition Timing

Category: Engine | Related: BMW 2002, Carburetor, Ignition timing, MW 2002

Nearly everyone knows by now that in order to make an engine run efficiently, you must get the fire started before the piston reaches top dead center (TDC). And the faster the engine is turning, the sooner the fire needs to be started, because the time it takes the mixture to burn is independent of engine speed. This is known as ignition timing advance. However, more complete combustion of hydrocarbons requires less advance, maybe even retard firing after TDC.

The best horsepower figures for a given engine will probably be achieved by re-curving the distributor in a test session with the engine on a dynamometer. However, a few generalizations can be made from experience.

Pre-emission control distributors are generally better.
Timing should be advanced to the point just before "ping" occurs.
A little "ping" just coming off light throttle is probably O.K.
If you have to pass an idle emission test, a 1968 to 1971 distributor is probably the best compromise.

There are no simple recommendations here other than to try more advance, but don't exceed more than 40 degrees total; 30 to 35 degrees is probably preferable. Generally, use more advance with a 1 or 2 barrel carb; less with multiple carbs. Less still with turbo-charging.

Carburetion/Jetting/Metering (Richness) and Throttle Controls are all by-products of carbon monoxide and unburned hydrocarbon emission standards.

First, Throttle Controls.
On many models, vacuum devices keep the throttle open on deceleration to reduce over-richness. Sometime this function is built into a by-pass circuit in the carburetor body itself. The function doesn't really hurt power, but it eliminates engine braking and can be very startling to someone not used to it. I see this as one more part to break and an annoyance as well. We should all try to avoid breakage and annoyance.

Second, Carburation and jetting or fuel metering.
Whether your car is carburetted or fuel-injected, the problem is undoubtedly the same: lean mixtures Gas is power, as they say, so more is better. In fact, adding more fuel may even increase gas mileage.

Since best horsepower occurs at about a 13:1 air/fuel ratio, best mileage occurs at about a 14:5 air/fuel ratio, but lowest emissions occur at about a 17:1 air/fuel ratio, and so most non-catalytic converter equipped cars will improve performance and mileage when the intake mixture is richened from the stock factory setting.

Another negative side effect of this government imposed leanness is extremely elevated operating temperatures. Hence EGR. This shortens the life of everything that gets hot: head, block, valves, valve guides, pistons, rings, engine compartment wiring, hoses and seals, exhaust pipes and mufflers, etc., etc.

For a street car, the preferred compromise is to jet the engine to run about a 14 or 15:1 air/fuel ratio for highway cruising and a 12 to 13:1 air/fuel ratio for one-half to full throttle. This preserves good gas mileage while maximizing reserve power and engine life.

As for the brand of carburetor, almost any brand can be made to work if someone is willing to investigate the compromises built into it by the factory. Which is to say that on a BMW 2002, for instance, there is no need to replace that 2-barrel Solex with an identical Weber just change the jets!

An article of this type would be incomplete without a discussion of oil, spark plugs, gasoline octane and CD ignition. So let me say:

Use oil in your engine. Use a brand name. I like Oilzum. Thinner in winter, heavier in Summer. Change it often.
Use spark plugs in your engine. Use a brand name. I like Champion and Bosch. Hotter heat ranges in Winter; cooler ranges in Summer.
Use gasoline in your tank. For racing, I like Sunoco 260 or 280, or Union 76 Racing Fuel. Otherwise, use only as high an octane as the manufacturer of your car recommends. No more. Buy the cheapest gas available. Use a fuel filter always and dry-gas occasionally.
The benefits of a CD ignition system are mainly in the pocket of the manufacturer of the system, after you have bought it. Better you should buy a good AM-FM radio. At least you can tell when it's working.
CD ignitions do not improve mileage significantly or performance at all; what they do do is reduce the number of times you have to replace points, adjust the mothers, etc. And they allow plugs to be gapped wider, so they foul less often. And it's easy to tell when they are not working: the car stops! ed.

Let me also give those of you who may be about to spend money on street performance equipment a jump on article four:

Lately, automobile performance has suffered significantly because of noise emission standards and ever longer and more tortuous intake systems. The German cars BMW and Porsche especially have suffered greatly. In both these makes, replacing the last muffler in the exhaust systems with a straight-through or Corvair turbo muffler will uncork a quick seat-full of horsepower.

In the next article I will discuss improving vehicle handling by revising springs, shocks, brakes, anti-sway bars, suspension geometry, wheels and tires, in a systematic approach to better vehicle dynamics.

Robert Bosch Automotive Products Master Catalog #2211 02

If you want to know what's what, go to your nearest import parts distributor who handles Bosch and ask for a copy of the above. It's about a half inch thick and will explain such mysteries as the new Bosch numbering system, both old and new part numbers for tune-up/electrical/wiper parts, starters and repair parts, generators, alternators and repair parts, fuel injection system parts and a complete cross-reference to after-market brands as well as old to new part numbers.

This catalog will help you find out what other automobiles use the same part you happen to be looking for . . . very helpful when you're just about to resort to having a part flown in air freight because you can't find it locally.

Author: Stan Simm