Drag car A/F ratio and jetting question

[JamesonRacing]

Warmed up the drag car today and focused on my Innovate A/F ratio meter.  I have it set to read lambda, so 1.0 is 14.7 gas A/F ratio.  At idle and warm up, the gauge reads 1.1 -1.2, occasionally blipping into the read "Lean" region on the display.  Running up to about 2k with no load, the A/F ratio is nominally 1.0.  Then I foot-braked the car and held the engine fully loaded as much or more than the brakes could hold.  The A/F ratio was fairly steady at 0.70 (10.3).  Video doesn't appear that I'm trailing black smoke, but I think there may be more power to be had.

I know that you're not supposed to tune to a number, but it would seem that I may be fairly fat on the main jets.  Seems like around 0.85 (12.5) is generally where engines make the best power, so I've read.  Would a four-number decrease be a good start?  The carb is a R-9375 first generation 1050 dominator, current jetting is 93 (f) 94 (r).

Thanks for your advice!

[My427stang]

Can you record it?

Any way to run it WOT and shut it down and read the plugs?

I'd imagine with airflow at speed it could be doing something different than in your foot braking test, not to mention it'd probably take a bit to clean it up if it was loading up a bit at lower rpm

Out of the box jetting for the carb is 92/92 so although it's richer, it doesn't seem like it'd be that fat on the booster

[JamesonRacing]

No record function on the gauge, but I may be able to borrow a gopro camera and perch it on the roll bar to record the A/F gauge and tach the next time I'm at the track.

I know the A/F gauge can be fooled if the engine is misfiring or if the headers are sucking fresh air, but this seems to stay very steady, and there's no hint of misfire.  Right now I don't have any reason to not believe what the gauge is displaying.

[My427stang]

We run an early Dominator like that on a 67 Chevelle with a 440 inch iron headed roller motor and had to lean it out a bit, but I don't know what's in it now, owner keeps the books in the trailer

I personally would sneak up on it and just start with the factory 92/92 and see what it does, especially watching trap speed and read the plugs to see if it matches what your meter says.  Lean is fast but too lean can break things, so go easy

Just to clarify though, I didn't think the meter was necessarily wrong, but at speed, airflow to the carb may be doing different things compared to a restrained motor with no forward motion.

[jayb]

On a race car with open headers, no way you can rely on the A/F meter unless you are at wide open throttle.  David, I'd go for that GoPro and then review the data after a pass down the track.  If you are still seeing 10s on the air/fuel, I'd start to lean it out.  You are correct that normally you will see 12.5:1 to 13:1 when the engine is making peak power, but tune for the best MPH regardless of the number.

[JamesonRacing]

Thanks for the direction Ross. 

I didn't think you were doubting the meter...that was me speculating about using a different measurement tool without having confidence in the accuracy of the measurements.  It's that cognitive dissidence when information conflicts with other things your seeing, particularly that the engine runs strong, not trailing black smoke, and not jetted all that large; but the A/F meter showing way rich.

Where did you get the info on factory jetting?  The Holley spec sheet I checked showed the carb having 86 jets.  But regardless the engine is going to want what it wants.  I think I'll risk the neighbor's scorn and see if a minor jet change moves the A/F ratio in the right direction.

On a race car with open headers, no way you can rely on the A/F meter unless you are at wide open throttle.  David, I'd go for that GoPro and then review the data after a pass down the track.  If you are still seeing 10s on the air/fuel, I'd start to lean it out.  You are correct that normally you will see 12.5:1 to 13:1 when the engine is making peak power, but tune for the best MPH regardless of the number.

I'm running 3-1/2" Dynomax bullet mufflers, so I have about three feet of pipe after the O2 sensor, so I probably get a little more stable number than with just open headers, but I see your point.  Hopefully will be able to get back to Byron in a couple weeks and see if I can find a 1/4 second and come back with a nine-second time slip.

[Nightmist66]

I think I'll risk the neighbor's scorn and see if a minor jet change moves the A/F ratio in the right direction.

I'm sure they"ll love it. 

PS: I'd love to catch the car at the track next time, good luck.

[thatdarncat]

Both my copy of the Holley Illustrated Parts & Specs book and my 1995 Holley catalog agree with Ross, 92/92 stock jetting on a List 9375. Good luck.

[Barry_R]

Sneaking a visit to the forum(s) before everybody wakes up - - been out for most of the week with family stuff....

If that carb is what I think it is - - old school 3 circuit - - it's going to be swimming rich with the original calibration.  A lot of guys from "back in the day" would end up completely plugging the intermediate just to get them under control.  The original intent of the Holley intermediate was for post-launch enrichment on a tunnel ram or "return to WOT" enrichment on a NASCAR application.

Definitely need to record/log the data on a pass, and compare what the meter says to how the plugs look.  Would not surprise me at all to see you pulling a ton of fuel out of that package and getting quicker each time.

[fastback 427]

I have the same 9375 series 1050 on my mustang. It runs pig rich at anything less than wot. I've tried leaning it down, air bleeds, power valves, no power valves, still too rich. After more research I was told,like Barry said, it was a tunnel ram carb, and it's tough to make a three circuit carb run right on the street. I talked to Jesse at Bigs performance carbs, he swears he can calibrate the circuit's so it will lean it down. I'll be sending it to him over the winter to try it out.

[My427stang]

To answer your question about jetting, the 9375 is 92/92, the -1 version uses much less jet.   That should be the 3 circuit version Barry talked about, unless of course it was modified, which Barry also mentioned.

[JamesonRacing]

Thanks for the info, insight, and great advice!  The carb's history is unknown; it was a carb off a friend's 496 BBC that had been sitting on the shelf and I needed something set up for gas instead of methanol.  Seems to get down the track pretty well, and may do even better with some cool Autumn air and a bit of tuning.

[CaptCobrajet]

I don't think you will be able to tell from a footbrake test.  It is likely in the transition there.  FWIW, an engine will generally 60' better if it is a little too fat on the starting line, before you go WOT.  I think I would start reading A/F after it goes into 2nd gear, and then all the way down track.  It will see the same rpm at the 2-3 shift that it sees on launch, but it will be in "slow motion" in high gear.

[KMcCullah]

Hey David-My bet here is since you had your intake ported, your pulling harder on that 1050 now. So a slight bump down in jets makes sense to me. Maybe 3 steps....

The 1050 I've been messing with is a 3 circuit deal also. It's fat as hell in the middle circuit too. I keep having to remind myself. A Dominator was NOT designed to cruise down the road at part throttle. WFO is where all the circuits work the best. With tons of air being crammed down the venturis.