Help with A/F Measuring

I think the closer you get to the cylinder the better, and individual readings are much better for setting up the carbs as well as diagnosing problems like air leaks.

 

The bung on wide band sensors are I believe 18mm, that is kind of big to put on every header tube.



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+1 (and you could never put the bung in so perfectly as to not compromise the inside diameter shape -- but that is where you need to sample since the adjustments are individual).

 

I ran a wide band on my 308. I thought there were specs that determined how close you should put to the exhaust port? Sorry, it's been a few years but worth checking
Philip

 

1 wide band o2 sensor upstream of the cat and at or just aft of the collector

 

Here's how I do it, I use an old Innovate LM-1 BTW.

The 5-8 bank has a bung welded (well, I had it done) on the exhaust manifold itself a few inches prior to the muffler flange. Make sure the bung is within 60 degrees of being vertical, meaning it's ok to be a bit canted but certainly NOT horizontal.

The 1-4 bank does not have one. If cam timing is dead on between each bank I don't find it's needed.

Anyway, this is the very best way to fine tune for power, but at the end of the day don't go chasing every slightly lean condition here and there because with carbs it's simply not possible to get them perfect. Carburetors are a blunt instrument and you can only really tune them for best power and 'good' or 'decent' drive and transition circuits (transition between idle circuit and main circuit which takes place around 3500-4k rpm). Plug in the a/f meter and do a good number of wide open throttle runs from 3k rpm to 7500rpm preferably in 3rd or 4th gear (I know, it's difficult to do this safely) and DATALOG it with the meter, then read the data and tune from that. These are brilliant tools.

Best power on the dyno I found to be had at an AFR of 12.9:1. I went from 253bhp 216lbs torque at the wheels on the dyno with 13.9:1 AFR to 272bhp 225lbs torque at the wheels with 12.9:1. Yes they are that sensitive. Obviously my engine with more displacement and compression/goodies is going to yield a slightly bigger difference, but proper tuning of a stock 308 can still yield 10-15bhp at the wheels which is equivalent power-wise to kicking a 115lbs female out of your car, although why on Earth would you do that? Anyway...it's noticeable

I know you have lowered your a/c size a touch, you may find some wide open throttle power with further changes to the main jet. Also note that upping the main jet size brings in the main circuit a bit earlier so if you're having a lean transition, 1 size larger main jet is the answer. Also note that a/c jets only really start affecting the mix north of 6000rpm, so if everywhere else is perfect, but when you get north of 6k things lean out a bit, just tighten up the a/c jet and you're in business.

Keep us posted, it's as much science as it is art.

 

The Innovate measuring tools are real good. The ports in the photo might work with a pyrometer. I don't know how accurate it could be.



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I got some O2 bungs from Summit and welded them in at the collector. I suggest pay a couple bucks more for stainless steel bungs, as the plain steel rust just sitting on the bench.

Great data! Which engine was this? I like the IRI equipment to set up mixture, and I totally agree that with carbs, you go for a "general" setting (unless you want to drive yourself nuts). The innovate is awesome 'cause you can just leave it installed. I've found throttle response is best as I approach stoich - on the rich side like 13.9 ish : 1 . This is on a V6 Dino engine. Also, I fortunately have sampling tubes on all the headers which are a pain to do one-by-one, but this is the best I've ever had it tuned. When I go richer, it might be stronger, (not using a dyno) and it boggs just a bit as I increase throttle. I just went down one size in pump jets ( from .45 to .40) but haven't had a chance to drive it - fingers crossed.

I really wanted to Dyno the Dino after the rebuild, but plans changed. Now I kind of regret it as it was the perfect opp'y to get get tuning sorted.