Hard Data

[Anonymous]

I have been reading lately about air fuel ratios, and what AF ratio is going to make the most horsepower. From what I have read, mostly on different internet sites, your engine will make the most power at a 12.5 to 1 ratio. A friend of mine says that an engine will make the most horsepower when it is most effiecient, at a 14.7 to 1 ratio. Does anyone know where I can find hard data to substantiate either claim?

Thanks

In God We Trust, All Others Bring Data

 

[rickwrench]

If you have an OEM computer controlled fuel system maybe you can run 14.7, but that is right on the very edge of too damned lean. You will be burning every last bit of fuel possible, with little or no waste. So, very efficient. On the other hand, your cylinder temperatures will be high, and you better hope you've got good factory knock sensors. Otherwise, whenever you stomp the go pedal your engine will lean out for a fraction of a second, and ping. The knock sensors will back off the timing immediatly upon detecting knock or ping, saving your piston ring lands but dropping some bhps to do it.
Running at 12.5, you've got some wiggle room, and a little extra fuel cools the charge down to boot. Cooler charge is denser, etc, etc.
On a side note, the C5 corvette almost didn't get a true manual transmission because of it's knock sensors, the transmission shifting noise was confusing the sensors. That's one of the reasons there are so many paddle shift "semi" manual sports cars out there. To heck with that, I like to clutch and row.
Rick

 

[xctasy]

If all else is equal, 12.5:1 is 5 to 10% more powerfull than 14.7:1.

Unless its a 346 cid GM LS-1 or Z0-6. On these engines, leaning the mixture in the closed loop and open loop conditions, raises the hp because the ignition tables then are more agressively timed. The Chevy guys have purposly calibrated the engine to run a rich setting close to about 12:1 in some conditions.


These days, oxygenated higher alcohol gasoline has a different stiochimetric ratio, so the ideal ratio will vary, but 12 to 13:1 is it!

 

[rbohm]

8) on gas we found 12.5-13:1 a/f ratio makes best power.

 

[BB6Mustang]

This is the standard AFR chart for gasoline:

12.6 - best power - occurs because the extra fuel helps to cool the intake charge

14.7 - lowest emissions - best compromise between the high HC & CO in a rich mixture and the high NOx in lean mixtures.

15.4 - best fuel efficiency - almost all fuel is completely burned at this AFR

 

[Anonymous]

You can't say one a/f ratio makes more power than another. That changes with spark timing, cam sizing, compression ratio, RPM, etc etc etc.


-=Whittey=-

 

[Anonymous]

What ever happened to the Lean Burn engines?

19 to 1 air/fuel ratios are far enough into the Lean zone that the temp would go down into a normal combustion range.

Dodge had one that "almost' worked. But it was not a computer controlled system and it could not be maintained.

I assume it was the increase of NO2 emisions that stopped it.

John

 

[rbohm]

8) honda also had one the almost worked, remember the cvcc? it was also a lean burn engine, but again no computer controls. i think a lean burn engine would do well today emissions and economy wise, not sure about power though.

 

[66Monterey]

Don't forget the Civic VX of the '90s...about 17 or 18:1 ratio, computer controlled EFI. Took a pretty powerful catalytic converter to pass emissions, though.

 

[Anonymous]

I worked for Honda when the CVCC engine came out: it was a complicated, but interesting, system. It had 2 intake valves, one in the combustion chamber and a very small one in the 'precombustion' chamber above the main combustion chamber. The 2-bbl carb was not 2-stage, but one rich barrel (11:1) that fed the precombustion chambers and one very lean (17:1) mix that fed the regular combustion chambers. There were also 2 sparkplugs per 'cylinder', one in the regular combustion chamber and one in the 'precombustion' chamber, 2 intake tracts, too.

The way it worked was: the precombustion chamber was timed 3-5 degrees ahead of the main chamber in cam and spark (oh, yeah, it had 2 coils, 2 sets of points, too - some had 2 distributors instead of 8 wires for the 4-cylinder). This little chamber had a port into the main chamber that entered at a very precisely set angle. When the compression stroke was occurring, the precombustion would fire and the rush of gases into the main chamber caused a swirl effect of the rich mixture around the outside edges, near the cylinder walls. Then the regular sparkplug would try to fire the lean mixture, which would get help because the flow of the rich mix was right across the face of the main sparkplug. This implies that the main plugs had to be indexed properly or else it messed up the important flow pattern.

The rich mix around the outside kept the hardware cooler while the larger, leaner charge was burning to push on the piston. The result was more HP for the same amount of gas, but boy, were they hard to tune...

 

[CobraSix]

my first car was a CVCC...ah...the memories...

Slade