Effects of Timing

[Derek]

After reading about many people adjusting the timing to something other than what was specified, I was hoping that one of you gurus or physicists would be willing to explain the effects of timing. What happens to engine performance, reliability, wear, gas mileage, etc., as timing is advanced or retarded in relation to specification? Why?

[Maineah]

OK I’ll try this one. Timing is the relationship between the spark the piston position. It takes time to set the fuel on fire so one advances the timing to account for this, so by the time the piston is where to belongs it can take full advantage of the explosion of the fuel. Now if it is too advanced it starts way too early and tries to send the piston back down when it is coming up! This is spark knock not a good thing. Late timing is just the opposite it is all ready past the point of greatest power on its way down. No knock but lack of power and poor mileage. Motor homes work hard all the time with all the smoothness of a brick they are hard to push through the air so advanced timing ( as in too advanced ) becomes a greater problem and has a tendency to do bad things to engines. So the good people at Toyota (and others) try and come up with just the right amount of advance to get the best overall performance. Fuel plays a part in the scenario also as does temperature, humidity, and altitude. Generally advancing the timing helps fuel economy and power but there is a fine line and many variables exist so it’s best to do what the manufactures recommend. I hope this rudimentary explanation helps.

[keitholivier]

Derek

If you have a modern engine, there is nothing to adjust anymore. But with older engines (like my 22RE) one has a distributor. Setting timing with one of the EFI vehicles can be tricky, since the manual requires disabling of some vacuum circuits to get a stable timing (there is a vacuum advance and at idle one has a good vacuum, hence an unreliable point to adjust timing.

I decided to forgo the method specified in the manual and retarded the timing to get a good idle, from which point I progressively advanced the ignition while road testing until I could just detect the onset of pre-ignition. The pre-ignition tends to occur when under heavy load (which as we all know is a frequent occurrence in these beasts..). I then back off carefully again until no pre-ignition could be detected.

In my case I had just rebuilt the engine and I found that with the suddenly restored compression I had to run premium gas to get any reasonable amount of timing advance. Prior to the rebuild there was no such issue, but once the ignition was optimally adjusted I got quite good gas mileage running on premium fuel. The best being 22mpg in relatively flat terrain in Iowa when the west winds died down. On average, the engine used 14-18mpg running almost full throttle for 5000 miles.