We've all watched Horsepower TV and observed the casual "big word" thrown in periodically in a lame attempt to make us Hot Rodders at home feel, shall we say less than enlightened? WELL here are a few things I picked up over the years. What is " flame front velocity"
the burn speed of your air fuel mixture, and with the right mixture can be maximized, to a point.
It depends on the fuel used. The limiting factor is the BTU of the fuel used. The correct mixture can greatly enhance Flame front velocity.
Many variables come into play here. That's why Top fuel runs two plugs, they have the ability to light the fuel in two places, thus maximize Flame Front velocity.
The spot you're looking for when you retard or advance the timing for a given RPM is called "peak firing angle", which produces, "peak firing pressure"
As the piston approaches TDC the plug should be fired to obtain the Peak firing pressure. The two things that effect Peak firing pressure are:
1. timing
2. Mixture
Some would argue RPM also but this isn't true, RPM may help a cylinder scavenge better, thus maximize Air Fuel ratio/ another words mixture.
You'd never want the timing so early that Peak firing angle happens before TDC, this can be Very hazardous to a set of Rods and Crank.
The reason we "fire" an engine Before Top dead center is the Flame front velocity.
the time it takes from the spark of the first "kernel" of fuel till the flame reaches the cylinder wall is Flame front velocity.
The piston doesn't see its full "bump" or force on it's dome until the flame front reaches the cylinder wall.
By then the piston has gone past TDC if only a few degrees, and the controlled explosion "hurries" the piston back down on it's merry way.
Generally, the closer you can get the Peak firing pressure to TDC without getting on the wrong side of it the better. For mechanical reason the Crank has to be slightly past TDC in order to receive the hit without damaging it.
If the cylinder becomes MORE efficient at scavenging as RPM increases, then we can LIGHT the Air Fuel mixture later in the burn cycle, just past TDC to maximize Peak firing pressure.
Why is this? Well mixture can CHANGE the peak firing angle. What is the peak firing angle? It is the number of degrees of crank rotation past TDC that creates the greatest Cylinder Pressure. Cylinder pressure = more power.
However, there is a point where the gain stops and a drastic loss occurs.
As the piston travels further down the hole, the later you light the mixture and Peak firing pressure occurs, the MORE room there is between the piston and the head for the controlled explosion. By doing this you not only create unecessary overlap, you lower your comp ratio.
maybe this diagram will help?
So from the graph can we conclude that the air/fuel ratio affects the burn rate and combustibility of the mixture? In richer air/fuel mixtures, the rate of burn is accelerated causing the burn to occur more quickly. ( up to a point ) The faster burn occurs closer to TDC and generally results in higher peak firing pressures.
Conversely, a lean air/fuel mixture slows the burn and results in later and lower peak firing pressures.
I?ve heard a gazillion folks say at the track that you should run your car as lean as possible to produce the best power. This is simply not true. You should run your car so that the peak firing angle occurs at the point where you gain the highest peak firing pressure. This would not be lean or rich but would be near the correct Air Fuel ration for the given combination. Typically, the more fuel you can pack into the cylinder and still supply the Engine with the correct Air to burn the given fuel, the more power is available to obtain Peak firing pressure.
So how do you find Peak firing angle? peak firing pressure? and Flame front velocity? Well some of this can be measured with some very expensive equipment, a company called BETA makes a Peak firing pressure meter and transducer, but it is not made for continuous run.
Time on a Dyno or down the track would be my suggestion.
Now if I only had more time and money to do both! 
Hope someone gets something from this long winded post! whew! 
------------------
JS
"never submit yourself to scientific tests no matter how bad you need the money, for all you know they could have erased your memory and YOU WERE REALLY SOMEBODY BEFORE"
69 Mustang Pro ET Drag
[This message has been edited by jsracingbbf (edited 10-18-2004).]
Mustangsandmore Forums
posted 10-18-2004 11:00 PM
