What A/F ratios should I tune for? Idle? Cruise? WOT?

[62Ranchero200]

Background:

1962 Falcon Ranchero, 2,400 pounds on title.

250 .020 over, early 300 rods, custom forged pistons, near zero deck height, balanced and blueprinted, Clay Smith 274/274/108 hydraulic, adjustable 1.6:1 stamped rockers, heavy-duty springs with dampers, CI aluminum head (55 cc), 9.45:1 static compression, CI intake, 500 CFM Holley, DUI with about 20 degrees of initial advance, platinum plugs, CI stainless three-piece headers, 2" true duals with X-pipe and Flowmaster Series 10, TCI 2500-3000 stall converter, C-4 with shift kit, light duty six cylinder open differential with 3.50 gears. Now idles at about 1,000 RPM in park or neutral and maybe 500 RPM in gear. About 12.5" of vacuum while idling in park (no vacuum-operated accessories). Only tuning I have done so far is to set the idle mixture screws and idle speed.

Just installed an Autometer Wideband A/F ratio meter. The ratio stays around 12.5 at idle, fluctuating a bit erratically up to 13.5. Going down to about 11 at when the engine is revved and spiking briefly up to 14, then back down to 12.5 on return to idle. Haven't taken it for a drive yet, so don't yet know about cruising and WOT.

In general and taking my engine configuration into account, what ratios should I tune for at idle, cruise, and WOT? I don't care about fuel economy, but I want the engine to run as hard as it can SAFELY run. I've put way too much money into this engine to push it to failure.

Thanks
Bob the Builder

 

[First Fox]

Tune the idle for best vaccum and let the AF ratio fall where it may. Mine doesn't idle well at stoich, and prefers about 13 on our local e10 pump gas. It doesn't matter what it is at idle, there is no load and it is more important to have a good clean idle.

Even though mileage is not a concern, you should keep the cruise ratio as lean as possible to keep the plugs nice and tidy and again, no real load there so just jet it until it runs close to stoich as long as it drives well and has no surging.

WOT for a naturally aspirated engine on should be safe at 12.5 or thereabouts. You are giving up power if you are any richer than that and if you are in that area, you should be pretty safe. My car had a noticable difference in power between 12 and 14, but I live life on the edge and if I break something, I learn something.

 

[THE FRENCHTOWN FLYER]

WOT fuel/air ratio is somewhat a function of how good the fuel distribution is in your engine. If you had a dyno that could measure individual cylinder A/F ratios you could then set the ratio so the leanest cylinder would not be in jeopardy. You mentioned your desire to keep things safe. I know that in some dyno cylinder-to-cylinder fuel distribution dyno test I have conducted we had to set the overall ratio down in the high 11s to keep things safe.

 

[xctasy]

WOT fuel/air ratio is somewhat a function of how good the fuel distribution is in your engine. If you had a dyno that could measure individual cylinder A/F ratios you could then set the ratio so the leanest cylinder would not be in jeopardy. You mentioned your desire to keep things safe. I know that in some dyno cylinder-to-cylinder fuel distribution dyno test I have conducted we had to set the overall ratio down in the high 11s to keep things safe.

A Morse test. I always think of Lee Morse, the Ford development engineer, but it is not named after him. Lee discovered matched and cross sectional data from engine dyno tests...when Ford in the toal performance era from 1969 to 1970 were busy making Trans Am tunnel Port and small block Boss race engines into scrap in record numbers, a strangled and unstrangled oil supply test was made of valve spring life under load, and they discovered locking off the oil supply to canted valve Cleveland and Windsor Boss engines to prevent main bearing failure also created valve seat spot welding and valve spring failure. Ford development engineers sure are a smart bunch. To this day the old FoMoCo engineers are head hunted when NASCAR or AVESCO engines on new development regimes run into durability matters.

http://www.thecartech.com/subjects/engi ... sting3.htm

2) The Morse Method

An engine connected to a absorption-type dynamometer with load weighing gear and tachometer will enable the indicated power (Pi) and mechanical efficiency (ηm) to be calculated within reasonable limits of accuracy, providing care is taken to maintain the exact rev/min and particular attention is paid to the torque arm setting and the reading of the load figures.

The test consists of measuring the total brake power (Pb) with all engine cylinders working normally under full throttle, and the cutting out each cylinder in turn. With spark ignition engines it is a simple matter to short each cylinder in turn, and various gadgets are produced for this purpose. With oil engines, using in-line fuel injection pumps, the raising of the fuel pump cam follower with a screwdriver or similar tool will cut off the fuel supply to that cylinder without having fuel oil leaking around the test area. This is the case if pipe unions are loosened to prevent injection. When a cylinder has been cut out, the remaining working cylinders have to overcome the frictional and pumping loses of the cut-out cylinder.



Consider a four cylinder four-stroke engine:

Let A = brake power Pb of 4 cylinders which equals 4 Pi - 4 Pf

(where Pi = indicated power and Pf = friction power)

and B = Pb of 3 cylinders which equals 3 Pi - 4 Pf

thus: case A - case B = 1 Pi, which is the indicated power of the cut-out cylinder.

When each cylinder’s indicated power is known and added together, the result gives the total indicated power for the engine under those speed and load conditions.





Worked example

A four-cylinder four-stroke engine was Morse tested at 2000 rev/min. The data is tabulated below.

Pi (of the n th. Cylinder) = engine Pb(all cylinders) - engine Pb (No. n cutout)



Pi (1st cylinder) = Pi (1) = 32.87 – 23.73 = 9.14 kW

Pi (2nd cylinder) = Pi (2) = 32.87 – 23.82 = 9.05 kW

Pi (3rd cylinder) = Pi (3) = 32.87 – 23.35 = 9.52 kW

Pi (4th cylinder) = Pi (4) = 32.87 – 23.94 = 8.93 kW



The engine indicated power = Pi (1) + Pi (2) + Pi (3) + Pi (4)

Engine Pi = 9.13 + 9.05 + 9.52 + 8.93 = 36.64 kW



The friction power (Pf) = Pi - Pb = 36.64 – 32.87 = 3.77 kW

The mechanical efficiency (hm) = Pb/Pi = 32.87 / 36.64 = 89.71%