Weber tuning with Lambda sensor - Page 2 - Alfa Romeo Bulletin Board & Forums
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post #16 of 188 (permalink) Old 05-05-2014, 04:33 PM
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#2 Remember, low speed jetting ADDS to high speed through transition into the main circuit. A rich low end fills in a bit of rich mixture to the high speed circuit, BUT not the other way around! If you are running as lean as you dare on the high speed, and then LEAN your idle circuit, you are in for trouble. Conversely, if you are close to too lean on the main, and RICHEN the idle, the main will richen VERY slightly. This does not work the other way around. The main circuit will not change idle mixture as there is inadequate vacuum. To many this is a confusing concept. Clearly Richard understands this from racing experience, and I ALWAYS run my idle mix a bit rich with Webers.
I'm pleased you technical investigations have confirmed what OLD Weber users have always known. Ed has confirmed "what to do" with emission Webers that function is a similar way but with a "cleaner" and leaner idle circuit.
I know that this thread is not about emissions Webers but if you have them and you are reading this you should be aware that the interplay between the idle and main circuits is the opposite of classic Webers. With emissions webers, for reasons that I can explain but not here, the idle circuit does not have an additive effect on the main circuit and changes to the main circuit do affect the idle circuit. The biggest surprise that I got during my recent dyno testing was seeing the idle circuit lean out a bit when I put in bigger air correctors. Who would have thunk that?

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post #17 of 188 (permalink) Old 05-05-2014, 04:37 PM
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For reference here's the relevant chart from the Bosch manual. Max power is produced at about 12.6:1, a bit far from stoich. That's one advantage to keeping the carbs slightly rich, the other two are that:

- The power curve is a lot flatter in the rich region, so variations in mixture have less of an effect on engine output, and

- It gives you a bigger safety margin in conditions where the mixture might otherwise go lean causing detonation or burned valves

The disadvantages are obviously fuel economy and emissions, but those weren't much of a concern when these cars were shipping with carbs.

EFI cars run at 14.7 for emissions reasons most of the time, but at full throttle are designed to go rich for the same three reasons above.
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post #18 of 188 (permalink) Old 05-05-2014, 05:47 PM
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You might find this thread from a couple of years ago useful:


http://www.alfabb.com/bb/forums/carb...o2-sensor.html

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post #19 of 188 (permalink) Old 05-05-2014, 06:01 PM
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Here is my recent dyno curve for my Spider. You can see that the torque curve held up well until the AFR dropped below 11. This supports Tom's message that there are several reasons to stay on the rich side of stoichometric.

We were adjusting the Webers to maximize the area under the torque and power curves rather than maximizing the peak numbers. We made less total power when we ran a bit leaner.
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post #20 of 188 (permalink) Old 05-05-2014, 08:11 PM Thread Starter
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Really interesting and information, it's mind blowing how tuneable these analog devices are capable of! I've been doing a lot of tuning on air cooled Porsche motors lately via AEM stand alone managment where afr is altered via a couple key strokes yet it only makes me appreciate webers more.

Can someone chime in on how spark plugs factor into afr? Like I mentioned I did this tuning with NGK BP6ES plugs, for the sake of trial and error I popped in a brand new set of B8ES (my previous plug choice) and didn't notice much of a difference as far a driveability during my test drive....though a plug check showed the b8es seemed to stay whiter than the bp6es.

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post #21 of 188 (permalink) Old 08-09-2014, 01:41 PM
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A discussion on tuning Webers with wideband AFR started in this thread http://www.alfabb.com/bb/forums/carb...dations-2.html
but we think that it is more appropriate to continue it here.
I posted this data that I got using a Zeitronix ZT-2 on my 2L Spider when I did WOT runs in 3rd and 4th gear on an uphill stretch of road.
This agreed well with the data from my dyno runs.
I then attempted to lean out the mixture below 4000 rpm by swapping the 55F21 idle jets for 50F21's. It had NO effect. My conclusion is that this confirms my idea that with emissions Webers the idle jet has no effect upon WOT operation.

So I have given up on the AFR in that range and I will next have a go at leaning the top end above 6400 rpm where the drop in AFR seems to coincide with the roll off of the power curve. The first step will be to repeat the WOT runs of a few days ago but going to 7000 rpm. Then I plan to switch the AC's from 210 to 230 and see if it makes a noticeable difference.
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post #22 of 188 (permalink) Old 08-09-2014, 05:38 PM
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You may not have run the 8's long enough since with NGK the higher numbers are "colder"
From another site.....
NGK:

NGK indicates the heat range in the middle of the plug number. For example, BCPR6ES-11 has a heat range of 6. (The number after the - is the Gap.) NGK plugs are colder the higher the number, hotter the lower the number.

Starting with part # BKR6E-11 (Heat Range 6), a colder plug would # BKR7E-11 (Heat Range 7), a hotter plug would be # BKR5E-11 (Heat Range 5).



EXCEPTION: NGK Racing Plugs: (Any NGK plug that begins with the letter R)

For NGK Racing Plugs, the Heat Range is located AFTER the hyphen.

Example: R5671A-10 has a Heat Range of 10. A colder plug would be # R5671A-11 (Heat Range 11), a hotter plug would be # R5671A-9 (Heat Range 9).

Some NGK Racing Plugs are also available in half heat ranges. These are displayed as a 2 or 3 digit number after the hyphen. For example, R6120-85 has a 8.5 Heat Range and R6120-105 has a 10.5 Heat Range.

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post #23 of 188 (permalink) Old 08-11-2014, 10:22 AM
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Air corrector jets

I did 4 more runs today, first 2 with 210AC's (the ones that were in when we dyno'd it) and then 2 with 230's I did the runs in 3rd gear and took it a little over 7000 rpm each time. The data on run 1 was close to run2 and run3 was close to run 4 so I just plotted one of each. It is clear that the bigger AC's leaned out the AFR everywhere when I was at WOT. AC's are supposed to affect the top of the range more than the middle but I don't see that. Any comments?
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post #24 of 188 (permalink) Old 08-11-2014, 11:49 AM
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I think your AC's are too large or you could go up in main size. Typically when tuning a Weber carb the AC's and main jets should follow a 50 point spread. i.e. using 150 main jets then you should be using 200 AC's.

It also looks like the graph start at 3500 RPM. If this is true than the mixture has not been effected across the entire range. 3500-4000 RPM is typically the transition from idle to high circuits.

I would be curious too see what happens if you went up to a 180 main jet with the 230 AC. If you start losing power on the top end you know you have went too far.
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post #25 of 188 (permalink) Old 08-11-2014, 01:06 PM
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We arrived at 150 main, 210 AC on the dyno. I did not have anything bigger than 210 and this combination made the best power - 169 hp at the flywheel. My torque/AFR curves show that power falls off when the AFR goes below 12. Bigger main jets with 210 AC made less total power - area under the curve.

My experiments with idle jets showed that transition is below 3000 rpm at WOT on this motor and with these Webers. I posted that data somewhere. I started those runs at 3000 but stopped at 6500 as I was focusing on the 3000 - 4000 where I lose torque due to the mixture being too rich. Today I was focused on the high end so I started the runs at higher rpm.


Quote:
I would be curious too see what happens if you went up to a 180 main jet with the 230 AC. If you start losing power on the top end you know you have went too far.
That may be something to try when I next have it on the dyno.

Ed Prytherch
79 Spider
76 Suzuki GT500
2011 Jaguar XKR

A little government and a little luck are necessary in life, but only a fool trusts either of them. - P.J. O'Rourke

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post #26 of 188 (permalink) Old 08-12-2014, 07:52 PM
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I've been intrigued and would like to log some data for accuracy. I have the Innovate gauge and Bosch lambda sensor but the data logging doesn't get great reviews.
In researching I found this...."In general, however, optimum horsepower is achieved between 0.85 and 0.90 lambda (12.45 AFR to 13.2 AFR using regular gasoline)." I doubt any of us are running regular gas and I'm wondering what if any differences 89 or 93 would make.

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1974 GTV
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post #27 of 188 (permalink) Old 08-12-2014, 08:19 PM
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I doubt that octane number has any effect. There may be differences between oxygenated and oxygen free fuels. Ours with 10% ethanol has some oxygen. I think that racing gas has no oxygen. I ran racing motorcycles on 100% methanol in my youth and the AFR for them is very low as the methanol supplies much of the oxygen that it needs to burn.

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post #28 of 188 (permalink) Old 08-13-2014, 03:37 AM
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"In general, however, optimum horsepower is achieved between 0.85 and 0.90 lambda (12.45 AFR to 13.2 AFR using regular gasoline)." I doubt any of us are running regular gas and I'm wondering what if any differences 89 or 93 would make.
Octane will have no effect. Octane is simply a rating of a fuels ability to resist pre-ignition; nothing more.
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post #29 of 188 (permalink) Old 08-13-2014, 08:02 AM
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I agree. Why mention it at all? It could have something to do with temperature? I'm not a chemist but methanol is just an alcohol and behaves like any normal fuel with CO and water being the remnants. Race gas, ie nitro methane is a blend of methanol and nitro, oxygen and nitrogen, which is what supplies the oxygen. I did a cruise run this morning and was seeing a steady 13.2 @ 4, 000 in 5th, 83 mph which was about 5/8ths throttle as best I could tell.
EDIT! I spoke a little to soon, ehanol does add Oxygen to boost combustion. Nitromethane is the extreme example.

The passenger seat is 15 miles an hour faster than the drivers seat.

currently
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71 Berlina in 2L restored driver
the ones that got away:
1959 750 series Giulietta Spider Veloce
1962 Giulietta Spider normale
1965 Giulia Sprint normale
1972 GTV
1974 GTV
1974 GTV
1977 Spider
1974 Spider

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post #30 of 188 (permalink) Old 08-13-2014, 09:44 AM
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To return to the main subject: I decided that my main jets and air corrector jets were all too big. So today I did a run with 140 main and 160 air and then one with 145 main and 160 air. The 140/160 is still a bit rich at the top but I can see that I have leaned the low/middle range relative to the top - exactly as the text books say. I think a 140/170 will be a good combination based on the changes that I have observed when I swapped air correctors with the 150 main jets. I will post the plots when I think that I have nailed it.
It occurs to me that 135 main/160 AC might be even better but I am hesitant to use a relatively small main jet with these cams and 34mm venturis.

Ed Prytherch
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76 Suzuki GT500
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A little government and a little luck are necessary in life, but only a fool trusts either of them. - P.J. O'Rourke
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