I've heard talk about timing an engine with a vacuum gauge. Has anyone done this as a standard timing method? What are the benefits as opposed to using a standard timing light?

When using a vacuum gauge, I do understand that you set the timing at the point of the highest vacuum. The questions I have about this process are:

Do you jumper the ALDL?

Do you being the vehicle up to operating temperature before timing?

Does this method create or eliminate spark knock?

How does it affect performance and MPG?

This technique does not apply to your Fiero. Jumper the ALDL, connect timing light, set timing to recommended specification.

It's not possible with any stock Fiero engine, I believe Vacuum advance was only on carbed engines.

I'm no expert on it but I like the theory behind it, because you're timing the engine based on actual behavior. A timing light tells you where the timing is. Vacuum attempts to tell you where it should be, so they're not really doing the same thing.
If you do this, I think it makes sense to warm up the engine first, and not to jumper the ALDL since you're trying to find what gives you max vacuum under real operation.

That said, I tried it once and it didn't give me a usable result. The max vacuum occurred at a point where timing was advanced enough I was getting knock. From what I read at the time this is typical. But this result does suggest it's reasonable to use the method of advancing as much as possible without knocking.
Most people say to use a timing light, and I can't really say that they're wrong, but I like the idea of calibrating against real behavior instead of a fixed spec.

Another method I've heard of is to chock the throttle partly open, then turn the distributor until you find the max RPM. That seems it would be better for performance tuning than the vacuum method.
A big caveat though. These methods are all complicated by the fact that the ECM could do all sorts of crazy things with the timing under different conditions, so what looks ideal in the driveway might not be the same while driving. And that's probably the biggest argument in favor of trusting the 25yr old EPA-approved spec.

[This message has been edited by armos (edited 04-29-2012).]

I had heard about it being used on a Fiero, but there wasn't any real specifics about it, other than you adjusted the distributor to the point of pulling maximum vacuum. It was just one of those things that lost most of the detail as it was passed along.

Matt, I grew up with the old vacuum type distributors and sometimes I think I'd rather use one of those than some of today's setups. The vacuum advance on those distributors was only for advancing timing, not for setting it. Ideally, they were set by a timing light way back then, too. Some would advance to maximum RPM then adjust the carburetor idle set screw to bring the idle back down. Spark knock didn't seem to be much of a concern in those days, but when it happened, it sounded like marbles in the cylinder and you knew it was time to retard the timing. For the country folks, it was done by ear, not a timing light or tachometer.

If it were used on the Fiero, it seems that any attempt to use this method without the ALDL jumpered would be difficult because the ECM would be attempting to over ride the movement of the distributor. And we know that setting the Fiero timing with the ALDL jumpered doesn't result in it being set at the highest RPM setting when you are at the 10* BTC position.

Armos, the theory did sound good. It was that when the engine is pulling it's maximum vacuum, it's operating at peak efficiency. Right now the 86 and 87 are set with a timing light and the 88 Duke is set by technology. I don't plan to do anything with them until I get more info on the process and results.

The vacuum idle timing is a SWAG at best and in my book optimizing the idle is of little significance compared to optimizing the cruise and WOT timing.

Now if you wanted to experiment with the timing to find the highest vacuum when cruising at a steady 2400 rpm in top gear (or wherever you do most of your interstate cruising), then I could see some benefit. However, you would need a timing light to confirm you are making small 1 degree changes and to be able to return to the setting that had the highest vacuum.

[This message has been edited by fieroguru (edited 04-29-2012).]

Optimizing vacuum and optimizing rpm probably wouldn't occur at the same points. What I'd heard was to optimize vacuum and that would create a more 'correct' timing for the engine than by using a timing light. It may upset emissions or other things, though. Good information and thoughts, everyone. Now, if we could just hear from someone who's tried it on a Fiero. When I heard about this method, it was that it was done on a Fiero V6.

The automotive engineers probably use settings that will give the best average of MPG, HP and emissions numbers. Probably not the best settings for performance. I once had a new Toyota Celica. I kept it serviced by the dealer for warranty purposes. Gave me a good 23 mpg on the road, but I had a young independent Portuguese mechanic retune the car after dealer service and I got a whopping 30+ mpg with better performance. Every time it went back to the dealer, they would reset it and I was back to 23 or so mpg.

IMO the best way to time a street driven engine (if you're not worried about emissions) is to advance the ignition timing bit by bit until you can hear the engine pinging while under load (foot to the floor while going uphill in high gear), and then back it off a little.

I run regular gas, and I've done this procedure to both the duke in my '84 and the 2.8 in my '86 GT (as well as to a variety of Chevy V8s over the last 40 years). The mileage is better, the performance is better, the emissions are worse.

But you've got to be aware of what "pinging" sounds like, and NOT continue to run an engine with timing advanced enough to produce it.

I also want to make it clear that I have absolutely no experience doing this with turboed or supercharged engines.

[This message has been edited by Patrick (edited 04-30-2012).]