engine timing question

[HFK]

Can someone explain the impact of timing on performance to me? And also where you have it set on your cars. I have an '81 fuel injected spider. My mechanic is tweaking the timing to get me better performance, and I wish I knew more about it so I could provide intelligent feedback to him. I want to ask, but also want to be respectful of his time and let's face it, I don't want to sound totally ignorant!

He says that her prefers it to be advanced from the factory recommendation. He had it advanced to 8 (?), but backed it down when I said I was getting the occasional "backfire" type sound from under the hood when I started her up. I don't think I am getting as much acceleration power as I used to, but then I am really spoiled in that I am now driving aggressively again like I did when she was new. I have been treating her gingerly for quite a few years - its great to have a mechanic who really knows these cars!

Thanks in advance for your help.

[majicwrench]

Ignition timing is critical to good performance. It is stated as "8deg BTDC" (Before Top Dead Center) It only takes "X" long for the fuel/air mix to burn in the cylinder, so you want to start the fire at the precise moment to use the most of that burn. And to do that, you need the spark to happen Before the piston reaches Top Dead Center. Most of those 80ish engines, they backed off (retarded, or moved it closer to TDC) the timing a bit to help meet emmision specs. So yes, a lot of engines run a bit sharper with the timing "advanced" which mean further Before Top Dead Center. If you advance too far, the engine will ping, which is the fuel air mix burning and colliding with the piston as it is moving up the bore. If you retard timing, will not get the full energy out of the fuel/air burn, thus a drop in power. Backfiring from under the hood would not be from advancing the timing. If you do not have the power you used too, he may have backed it off too far. There is a specifacation for the timing on your car, setting to to spec is a good place to start.
The only feedback he should need from you is input on the power and the way it runs, AND whether or not it pings. He should be able to handle the rest.
Hope some of this makes sense. Been a hot day. Enjoying a cold one.
keith

[So Cal Mark]

your FI motor factory spec was 10 btdc, so setting it at 8 will hurt both performance and mileage. I'd push it to 12btdc and make sure the vacuum advance is working

[HFK]

Thank you so much for your help! Please be kind if I get the following totally mangled.

I think I understand the concept of "advancing" the timing making the spark occur further in advance of the piston getting to the top dead center (yes?). I don't understand why that would make a difference to the power, but maybe I do - is the amount of gas ignited dependant on the volume of space available in the cylinder? i.e. more advanced = more space = bigger bang = more power?

What was the backfire under the hood?

[majicwrench]

The timing concept --yes--that is correct.
Second part, power, is not correct. Advancing timing does not have anything to do with the volume of space available.

Like I stated above, it takes "X" amount of time for the fuel mix to burn. If you start the spark too late,(retarded) fuel is still burning, creating power, as the exhaust valve opens to pump out what should be the exaust gases. You are losing that energy.
If you start too early (advanced) the piston is coming up as the fuel is already burning, and the two collide, creating that dreaded and damaging pinging noise.

If you have it just right, the piston is almost to the top as the fuel mix starts to burn ( in your case, around 10 BTDC at idle is just right). It takes a tiny amount of time for the fuel to start burning and building pressure, that is why you fire it off as piston is coming up. Anyway, if just right, just as the piston passes TDC the burn really gets going, forcing the piston down, the crank around, and you down the road. In a perfect world, as the piston reached BDC all the fuel has burned, the exhaust valve opens, and the piston head up, forcing out the exhaust gases.
Hope this makes sense.
Backfire under hood can be any number of things.

[maytag]

it takes some time for the flame-front to travel across all of the fuel/air mixture in the cylinder, so you want to begin the ignition process as early as possible once the valves are all closed. The more of the mixture you are able to burn, the higher your volumnetric efficiency, meaning higher cylinder pressures, ie: more force pushing down the piston. Obviously there is a point at which you see a rapid fall-off, as the rapidly-expanding gasses can begin to push on the piston before it reaches TDC. Detonation (or "pinging") can also be a product of too-much advance.
Your backfire up the carb can be caused by too much advance, if you are soo far advanced that ignition begins before the intake valve is closed. But this would be in the extreme, I would look elsewhere.

[HFK]

Thanks for taking the time to explain!

[131]

its great to have a mechanic who really knows these cars!

If he's "Advancing" it to 8 deg and then backing off I would suggest he doesn't really know these cars.

[baltobernie]

HFK, in this post, "advance" is unfortunately a homonym. When your mechanic has rotated the distributor relative to the engine block to move the timing earlier than its previous setting, you can confusingly name that action as "advancing" the timing.

But most people reserve that term for the internal action of the distributor which occurs while the engine is running, with no intervention of the mechanic. In previous posts, you've learned that the spark must arrive at the precise time in the engine cycle to produce the best power and smoothest operation. The moment in the cycle varies with engine speed. Without getting too complicated, distributor design features one or more methods to vary this moment. You'll see posts here referring to "vacuum advance" and "centrifugal advance", and you can Google these if you're curious.

What your mechanic is doing is referred to as "setting the static timing", which is part of the tuning and maintenance process. I agree with 131 that he should know the location and factory recommended setting of the timing marks on the Fiat engine. Most folks attempt to get the engine to run best at factory settings before departing from them.

[HFK]

Hey guys,

I have no doubt about the ability or experience of my mechanic. The question may be inaccuracies in what I am saying, because I don't speak the language. However, with your help, I will get better!

[124ADDHE]

You should set your timing for 36 or so degrees @ 5500 rpm - done deal, the base timing doesnt mean a lot, most of our dizzys are a bit worn out anyways, this way you are @ a safe peak timing and wont hurt your motor.