O2 sensor & A/F gauge in: help me tune it please

[Anonymous]

OK folks, I got my bung welded this morning ( ) and wired up my Stewart Warner A/F gauge. As expected, she's rich. From idle to cruise speed; at part throttle or wide open. Rich.

I've borrowed an image of the guage from SW so you can see where in the meter things are.

Right now (Holley 2300 500CFm w/stock 73 jets w/ the mixture screws ONLY OUT 1 full turn and a 5.5 powervalve):

Obviously, things need to be leaner accross the board. I should be targeting the lean end of STOICH right?

I have a set of 71 jets that I was planning to install along with an 8.5 PV (based on my vac readings being between 12-16). With your best guess, are the 71's going to be small enough to make that much drop?

What should my next step be? 71 Jets then retest? Other jets?

 

[MustangSix]

Most engines like to idle a little on the rich side. At stoich or leaner they usually stumble and have an occasional miss. At WOT you should be rich. The engine will make best power when it's in the 11.5 - 12.0 :1 range. At cruise you should be at stoich or a bit leaner. And at light throttle, you should be at stoich or a tad richer.

 

[Anonymous]

Thanks Jack. I think I'm going to drop to the 71's as I CAN'T get it lean/stoich.

 

[xctasy]

Carb discharges can be assessed like an EFI system.

Forget 73's, way too rich. Get some 68's, and then plan to go up to 70 jets when you've seen how lean of optimum you can get.

The peak jet size has governed by

a) jet sizes (variable from 40 to 100 call size)

b) PVCR (power valve channel restriction), a discharge diameter set by Holley for each part number carb. This doesn't change much on a stock Holley carb. It makes no difference if you have a 10.5 or 2.5 power valve, at wide open throttle, the channel restrictions add a bout 8 jet sizes to total fuel flow.

Here's a table of 500cfm Holley Jet power levels on our sixes. This assumes the stock #4412 power valve channel restrictions are used. It's based on an extensive search on existing jetting for known 500 cfm six cylinder engine combinations.

60 call size jet = 104 hp
61 call size jet = 108 hp
62 call size jet = 113 hp
63 call size jet = 118 hp
64 call size jet = 124 hp
65 call size jet = 130 hp
66 call size jet = 136 hp
67 call size jet = 143 hp
68 call size jet = 149 hp
69 call size jet = 156 hp
70 call size jet = 163 hp
71 call size jet = 171 hp
72 call size jet = 179 hp
73 call size jet = 188 hp
74 call size jet = 197 hp
75 call size jet = 206 hp
76 call size jet = 213 hp
77 call size jet = 224 hp
78 call size jet = 235 hp
79 call size jet = 246 hp
80 call size jet = 256 hp
81 call size jet = 266 hp
82 call size jet = 278 hp
83 call size jet = 289 hp
84 call size jet = 300 hp
85 call size jet = 312 hp
86 call size jet = 324 hp


Since you've got a good meter, the best option is to under jet, and raise jet sizes if your worried about leaness under wide open throttle.

Make sure you cross check the gauge, though. Any dynometer service station should have a calibrated fuel air meter to ensure there are no gross errors.

 

[Anonymous]

NICE info! Thanks. Here's where I'm at:

OK, put the 71's in and the 8.5 PV. With some carefull trimming of the mixture screws, I'm able to get the idle to here:

Is that better?

She still doesn't lean out too much more than that at cruise. Should it? At WOT, it jumps to about half way into the rich (where the first pic showed idle).

How does that look?

For best performance (not gas mileage), where on this meter do I want to be off idle (stompped on it at a stoplight)?

Since you obviously have extensive background knowledge (or at least figures), can you suggest a change to the acclerator pump, cam, or nozzles?