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TDC and Degrees

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What is meant my 16 degrees off TDC or where does this angle measurement come from?

A circle has 360 degrees, TDC is 0 (zero) degrees. 16 degrees off (is this before or after) TDC is either 16 degrees after 0 on the degree wheel or crank pulley or 344 degrees on the degree wheel or crank pulley which is 16 degrees before 0.

Ok I thought so, does octane determination stem from the inability to maintain the current octane with the ~16 degree system. The lower octane would detonate well before 16 degrees.

Thats kind of like asking, "whats the best AFR". It all depends.

Octane is a construct (which means an ideal instance) that merely indicates a fuels ability to withstand detonation. The higher the octane the more resistant to detonation the fuel is when compared to other fuels used in identical situations. There are many variables that impact on an engines detonation threshold and only one of them is the octane rating of the fuel.

The reference to 16 degrees is simply to show at what point ignition produces maximum force on the crankshaft which gives maximum acceleration of the piston down the bore. It is all about leverage and if you take a look at a circle and mark out TDC (0), BDC (180), and various degrees (such as 16, 45, 90, 270) you will see that 16 degrees is still fairly high up and provides good leverage to the crank.

Higher octane means higher required temperature to ignite the fuel:air mix. More aggressive tuning means higher pressures in the cylinder.... which means... higher heat. You want the "steady" outward-progression of the flame-front to ignite the air:fuel mix... not the high temperature in the cylinder... which would make it all ignite at once (explode) ie. detonation - this is what breaks engines.

This is why direct injection is so good; required octane is lower for a given engine.

If I'm understanding your question correctly, the point we want to achieve maximum cylinder pressure (generally around 16 degrees after TDC) doesn't depend on the octane of the fuel. This is simply the point where we can achieve maximum mechanical advantage from the cylinder pressure and hence maximum torque.

What we are trying to achieve when tuning the ignition timing is to start the combustion event at the right time so that maximum cylinder pressure occurs at approximately 16 degrees ATDC - This is called MBT timing which stands for Maximum Brake Torque timing, or Minimum timing for Best Torque. If the octane of the fuel is low or the compression or boost is very high, it may be impossible to advance the timing enough to achieve MBT timing though.

All the guys above got it right. But if you mean base timing, for example on a honda is 16 BTDC (before top dead center) meaning its 16 Degrees advanced. Anything past TDC would be considered ATDC (After Top Dead Center) "Retarted". Base timing is different from ignition timing in that it is used to sync the ecu with the physical ignition timing of the engine. For example if you have a honda motor with a distributor and its set at 10 BTDC and ignition map calls for 15* the actual ignition is firing 5* retarted which will make the car feel slugish but if its advanced it might cause knock. All the base timing does is sync the ecu, it doesnt matter if you use 0* or 15* or 5* but if you lock the timing in the ecu then use a timing gun to confirm cyl is firing at that exact value.

You're confusing him with "retard" and "advance". These are both *relative* terms... not absolute. And the example you gave is 5 degrees ADVANCED... not retarded.

To add a little bit to my previous reply Octane is also the fuels ability to be compressed. If a fuel is compressed beyond its ability to handle it will detonate. Traditionally fuel is drawn or injected into teh air stream before the intake valve so it is compressed with the air in the cylinder. Direct Injection works so well because the fuel itself is not compressed as much as it would be because the much of the fuel is injected after the air in the cylinder is compressed.

Cetane, on the other hand, is a rating that deals with a fuels ability to combust in pre compressed air (like a diesel engine). The higher the cetane the better the fuels ability to limit what many call diesel knock. Many of the advances in diesel performance have simply come about because of Common Rail (which petrol engines have had for years) and Direct Injection technology (which petrol engines are now using as well).

The ability to tune the combustion event (petrol or diesel) to exert maximum leverage on the crank at any given rpm is what we try to do within the confines of a particular engines design. (Just thought I'd add diesel to the conversation as I'd love to tune a Duramax V8 or one of Fords V8s)

Don't forget that heat is a by-product of compression. Im not sure compression-pressure itself has any bearing on detonation. In other words... its solely the heat... not the pressure, that starts the combustion (reaction) process. By the way... the fuel is not compressed... the air is. The air molecules are compressed and thereby heated... that heat is transferred to the fuel molecules surrounding the air molecules.

I should have said the Air-Fuel mix is compressed in traditional fuel systems, my bad. Here's a brief article.

http://www.bellperformance.com/blog/bid/101182/What-Does-Octane-Do-In-Gasoline-Octane-Ratings

Material like this was part of the auto apprentice course back in the 1980s (in Australia) I assumed it was common knowledge in the automotive field today.

If the ecu thinks you have it set to proper 15 and you really have it set at 10 it would be Retarted not advnced.

See, thats what makes your post so confusing. With your example, one imagines a distributor [statically] set at 10* BTDC. The software is commanding 15*. Therefore, the ignition is [electronically] ADVANCED from your base timing of 10*.

you are over thinking it. what i meant is ecu commands 15 and thinks it getting it but its actually getting 10, because the distributor was set incorrectly 5 degrees retarded. anyway maybe were trying to say the same thing in different ways.

You're talking about "synchronizing".