This statement about ignition timing

[Truckedup]

Does this make any sense?

To create peak power an engine/any engine needs to hit maximum rpm velocity at 14^ past top dead center. That is where the gasses have lost all there power.

The initial timing is set with ONLY this in mind. 28* BTCD as a firing point is not an arbitrary number. It is calculated to equal max velocity at the 14* point after TDC.

The only time it is to your advantage to change the total timing is if fuel or boost pressures will allow this. If they do allow meaning more power per cibic inch or cc then the extra power in the mix can be fired sooner, and still reach max velocity at 14* after.

On the other hand if you have crap fuel it will benefit you to retard the timing and fire later but the en goal is still 14* after.

This is the easiest way to explain this. Timing is NOT a modification that makes power. It is a control of when the power is delivered in the engines cycle.

In an otherwise healthy engine, advancing the timing will create more heat soak in the combustion chamber, and loose power due to the heat being absorbed rather than used as force.
Same applies to retarding the timing. The heat will be dumped out the pipe before it can be fully developed into force for the piston cycle.

[mag2555]

It makes sence in general, but that 14 degree rotation number is highly dependant on the bore to stroke ratio of the motor in question if you ask me!

[peejay]

Any time someone throws out a hard number like that, my BS meter rises.

MBT doesn't care if maximum pressure (er, "velocity" I guess?) happens at 14 degrees or 18 or 23 or whenever. Peak is peak.

[twl]

I think that there is a lot wrong in that statement, but it seems like he is trying to say that the MBT goal should be at 14*ATDC.

I'd say that he sort of knows what he wants to say, but doesn't fully understand the concept, and is recalling some things he heard others have said.

[Dan Timberlake]

Hi Truckedup,

Are there particular statements you question?

regards,

Dan T

[Walter R. Malik]

Hi Truckedup,

Are there particular statements you question?

regards,

Dan T

The hard number of 14 degrees ATDC needed for EVERY engine, is one...

[engineguyBill]

It is not correct to assume that all engines react to timing parameters in the same way. Total timing is dependent upon induction design, combustion chamber design/efficiency, fuel type and several other factors which may or may not be controllable by the engine builder/tuner.

Higher octane fuel actually "burns" slower than lower octane fuel does. Therefore high octane fuel might develop more torque/horsepower output with more timing advance. The fuel mixture actually burns and produces power through a fairly wide range during the combustion process. Maximum power output typically continues until 70 to 80 degrees ATDC. Maximum effort high RPM drag race engines begin to open the exhaust relatively early in the combustion cycle, which results in "blow down" into the headers. This blow down produces positive pressure in the primary tubes and more importantly results in a reverse depression which helps to fill the cylinder during intake stroke at very high RPM, albeit a very narrow RPM band.

[Truckedup]

Hi Truckedup,

Are there particular statements you question?

regards,

Dan T

Like others have mentioned I don't believe in absolutes.....It was a discussion about tuning vintage British bike engines. The success I've have with my bike is tuning the engine the way it goes best for the situation....

[automotive breath]

Heres a good description of what is needed.

"The trick is to maximize heat release from the A/F mixture, minimize the amount of
heat release before about 10 ATDC, maximize the heat release before about 20 ATDC
and finish up the burn as quick as possible after 20 ATDC"

[les327]

In general Do boosted engines open exhaust valve later to utilise the higher pressures,

[140Air]

[quote="Truckedup"]Does this make any sense? [quote]

The whole post is not completely clear to me. However, 14 degrees is in the range between 12 and 15 degrees which I have read is an optimal range of crank degrees for peak pressure to occur. I am not aware of any authority that sets a precise figure for all engines.

[turbobaldur]

In general Do boosted engines open exhaust valve later to utilise the higher pressures,

Exhaust valve opening time is a balance between harvesting the energy and losing it via pumping losses. A late EVO means the piston needs to push the exhaust gas out of the cylinder. An early EVO means the energy (pressure) of the exhaust gas can do the bulk of this pumping work while the piston approaches BDC. One can see where exhaust port flow comes into play here in making the engine as efficient as possible.

[MadBill]

It's a worthless statement. The terms are incorrect, the grammar is garbled and the premise flawed. ("peak cylinder pressure should be at 14° ATDC.") It's true that for most SI engines not limited by detonation that optimum spark timing results in cylinder pressure peaking at a crank angle somewhere in the mid to high teens, but so what? Not one in ten thousand builders have access to the required measuring equipment to determine LPP but virtually anyone can arrive at optimum advance via dyno, timing clocks, etc.

[user-23911]

Velocity is another of those fancy words used in the wrong way by people who don't know what it means.

Definition

https://en.wikipedia.org/wiki/Velocity

[peejay]

Actually, pressure sensor spark plugs aren't all that expensive, nor are multichannel scopes fast enough to catch cylinder pressure per crank degree.

No they aren't CHEAP but they aren't $100,000 lab equipment anymore either. A really really good automotive-grade scope is about $2k new and the plugs are only a couple hundred bucks (so i'm told by someone who bought some for S&G. He also had an engine dyno in his residential garage) so we're getting to the ballpark of equipment that a decent pro shop could have. Heck, a lot of general auto repair places already have the scope...

The trick, like any other information, is knowing how to interpret it.