Thread: Why I tune to 12.0:1 A/F ratio (0.82 lambda) and why you should too. (all welcome)

I thought higher compression motors made more power because the mixture was compressed more, which resulted in a stronger explosion, not a faster one. Perhaps we are thinking the same thing with regards to this, but saying it differently. If the explosion is 'stronger' it would push the piston down 'faster', but not necessarily burn any faster. Unless I have this totally misunderstood.

Yes, do detonation avoidance example.

I'll do a detonation avoidance example. Give me a second.

As for your comment on the stronger explosion, yes we're talking about the same thing. When it comes to combustion energy, you have two main things. Mass, and speed (pressure and temps are factored into the speed). All else being equal, raising the compression ratio doesn't increase the mass of the mixture any, so no additional energy can come from that. The additional energy must come from the burn rate, which it does. Denser air and fuel will burn faster than otherwise.

With forced induction, you get additional mass, and additional speed. However, again, the speed is the only thing important to consider when dealing with the ignition timing. You can't (and don't want to) compensate for the added mass.

Detonation avoidance example for @gomba:
For these examples we have to imagine what happens with a slow burn rate, and a fast burn rate, and how starting them early or on-time or late will affect things.

Fast burn rate:
Very early - With a fast burn rate, if you ignite the mix much too early you have the piston still rising while the pressure front rushes down the cylinder. This compresses the remaining unburnt mixture like an evil sandwich with the pressure front on top crushing the mixture against the rising piston on the bottom. This is a recipe for detonation disaster. The stakes are high. Even if you don't detonate, you're still working against the rising piston which causes a huge power loss.

Early - With a fast burn rate, if you ignite the mix slightly too early you have the piston still rising while the pressure front rushes down the cylinder again. Bad news. Again, even if you don't detonate, you're still working against the rising piston.

On time - Still with the fast burn rate, if you ignite the mix perfectly on time, the pressure front still works to squeeze the remaining unburnt mixture against the rising piston but the piston reaches the top and starts falling again hopefully without causing any detonation, and then the pressure front hits the piston at 20 degress ATDC and you get the most power possible from the system. This is assuming you can avoid detonation, which is often not possible on common engines with a good bit of boost. So you have to compromise. Read on.

Late - So we still are working with a fast burn rate, but we have to ignite it later than ideal to avoid detonation. This is 75-95% of examples you'll ever run into. The flame propagation might not even finish reaching to the edges of the cylinder by the time the piston reaches TDC and starts moving downward again. You have basically no chance of detonating based on added pressure, and the flame front still catches the piston on its way down maybe around 23-28 degrees after top-dead-center. You've hit the piston late, but not too late, and you have a good burn rate so you get some good power out of the system.

Very late- Then of course you can ignite things so late that you're in the weeds...




Slow burn rate:
Very early - With a slowburn rate, if you ignite the mix much too early you have the piston still rising while the pressure front moves down the cylinder. This compresses the remaining unburnt mixture like an evil sandwich similar to before. This is a recipe for detonation disaster not any better than before. Even if you don't detonate, you're still working against the rising piston which causes a huge power loss.

Early - With a slow burn rate, if you ignite the mix slightly too early you have the piston still rising while the pressure front moves down the cylinder again. Bad news. Again, even if you don't detonate, you're still working against the rising piston.

On time - Still with the slow burn rate, if you ignite the mix perfectly on time (which will obviously be earlier than the perfect timing in the previous example), the pressure front still puts a good squeeze on the remaining unburnt mixture against the rising piston simply because of how much earlier you need to ignite it. The piston starts moving down and the flame front catches the piston on its way down exactly at 20 degrees after top-dead-center and you get a good bit of power but not as much as in the earlier example of perfect timing. This is assuming you can avoid detonation, which is often not possible on common engines with a good bit of boost. So you have to compromise again.

Late - So we still are working with a slow burn rate, but we have to ignite it later than ideal to avoid detonation. This is 75-95% of examples you'll ever run into. The flame propagation isn't even finished reaching to the edges of the cylinder by the time the piston reaches TDC and starts moving downward again. You have no chance of detonating based on added pressure (heat), but the flame front doesn't catch the piston on its way down until maybe around 30-45 degrees after top-dead-center (that piston really starts accelerating away from TDC now). You've hit the piston way late, and you have a slow burn rate so you get very low power out of the system.

Very late - Who wants roasted marshmallows?


Because of where most cars must operate, it is better to go with the faster burn if you're looking for power. At no point is the slower burn better, except for a little bit of detonation avoidance in a small window of ignition timing.
The faster burn is much less forgiving if you ignite it early, obviously, but the power potential is higher and if you're late it is going to always produce more power. Hopefully that's enough info to at least help others make up their own mind about how they'd like to do things.

And don't forget there are plenty of other things to consider other than pure power output. This is just one side of the coin.

Awesome read! I learned a lot

So how do you determine if your timing is close to or at 20° after TDC ?

^^ I have a feeling it will heavily depend on where your torque peak is........But I could be wrong on this

The byproduct of correct timing is maximum torque/horsepower for a given RPM and air mass. So, a dynamometer is the best tool for determining how things are going. If you know EGTs for beast torque, you can also use those relatively reliably since EGTs are highly dependent on intake timing.

Keep in mind, you need some serious octane rating to get the best timing without detonation on most setups.

So Ben are you saying I am right??

Sure, why not?

I've been setting a NA VE with N1s and this thread has been really helpful. I have a question though, the owner had the distributor advanced almost all the way but there was not any knock, is there anyway to know if we are starting combustion too early besides getting it to a dyno?