Air/fuel ratio?

Rev · 02-13-2005, 07:12 PM

I recently had my car dynoed. This shop had all the bells and whistles for their DynoJet including an exhaust gas analyzer. The car dynoed well for me (280@5700/293@4000).

The printout included the A/F ratio for the entire run. In the usable rpm range the A/F stayed between 12.7-13. The dyno operator thought that the 12.7 area was a little fat and recommended maybe slightly smaller jets and/or power valve number.

I thought the 12.7-13 was about ideal for a carbureted car. Any opinion, recommendation, or other thoughts are appreciated.

Rev

Jeff Chambers · 02-13-2005, 08:01 PM

I'll usually shoot for 13.0 on the low end of things. Stochiometric for gasolines is 14.7:1 (hottest point). Too bad you couldn't have re-jetted while you were on the dyno to see the effects of one jet change on both A/F and HP/TQ. A single jet size is 7% change so IMO it probably would have leaned you into the 13.0-13.4 range and I'd bet would have been worth a little HP.

Rev · 02-13-2005, 08:13 PM

Actually, I did go from #70 to #68 on the primary jets a week after the dyno pulls. I tested it on the Gtech, but really couldn't see a difference. Might have even dropped a couple hp. The plugs are a very light tan now which is about how I like them to look.

Seems like atmosphereic conditions make a lot of difference on the Gtech hp/torque runs though, so repeatability is always a question. They actually would have let me tune on the dyno for a price.

Rev

02-13-2005, 07:12 PM	#1
Rev Registered Member Join Date: Nov 1998 Location: Houston, Tx. Posts: 3,887	Air/fuel ratio? I recently had my car dynoed. This shop had all the bells and whistles for their DynoJet including an exhaust gas analyzer. The car dynoed well for me (280@5700/293@4000). The printout included the A/F ratio for the entire run. In the usable rpm range the A/F stayed between 12.7-13. The dyno operator thought that the 12.7 area was a little fat and recommended maybe slightly smaller jets and/or power valve number. I thought the 12.7-13 was about ideal for a carbureted car. Any opinion, recommendation, or other thoughts are appreciated. Rev __________________ '66 Coupe, 306, 350-375 HP, C-4, 13.07 e.t., 104.8 mph, 1/4 mi. O.B.C. #2 '66 coupe

02-13-2005, 08:01 PM	#2
Jeff Chambers Moderator Join Date: Aug 2000 Location: Milan, OH Posts: 2,699	Re: Air/fuel ratio? I'll usually shoot for 13.0 on the low end of things. Stochiometric for gasolines is 14.7:1 (hottest point). Too bad you couldn't have re-jetted while you were on the dyno to see the effects of one jet change on both A/F and HP/TQ. A single jet size is 7% change so IMO it probably would have leaned you into the 13.0-13.4 range and I'd bet would have been worth a little HP. __________________ Jeff Chambers 1990 Mustang GT 10.032 Seconds / 137.5 MPH 14-time Street Warrior World Record Setter CRT Performance 2001 Tropic Green Mustang GT - 12.181 / 113.2 MPH 2002 Ford F-250 Crew Cab 7.3l Power Stroke - 17.41@77.2 "There's nothing boring about a small block automatic shifting gears at 9400 rpm!"

02-13-2005, 08:13 PM	#3
Rev Registered Member Join Date: Nov 1998 Location: Houston, Tx. Posts: 3,887	Re: Air/fuel ratio? Actually, I did go from #70 to #68 on the primary jets a week after the dyno pulls. I tested it on the Gtech, but really couldn't see a difference. Might have even dropped a couple hp. The plugs are a very light tan now which is about how I like them to look. Seems like atmosphereic conditions make a lot of difference on the Gtech hp/torque runs though, so repeatability is always a question. They actually would have let me tune on the dyno for a price. Rev __________________ '66 Coupe, 306, 350-375 HP, C-4, 13.07 e.t., 104.8 mph, 1/4 mi. O.B.C. #2 '66 coupe

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