Fiat 2000 TC air/fuel ratio measurement

[Slawo2000]

Hi
I have recently taken a measurement of air/fuel ratio of my TC 2000 engine. The test was done at full load at 3rd gear and wideband lambda sensor was used. You can see the results on the attached chart. What I wanted to ask is what actually correct value of lambda should be in order to get max. power through range. Different sources give different data on this subject - from 0.8 to 0.95. Assuming, that the correct is something between 0.85 to 0.9 it is clear, that the mixture in my engine is generally too rich, especially at top end (air correctors too small probably) and in range 2700 ¢‚¬Å“ 4000 (idle jets too big? incorrect emulsion tubes? float level too high?).
Could you please look at the chart and say your comment ?
Engine specification is following:
2ltrs Fiat Argenta engine, 130TC cylinder head, Fast Road intake camshaft (2.8 mm at TDC), 4-2-1 exhaust
Weber IDF 40 carburetors:
35 chokes
55 idle jets
145 mains
170 air correctors
F16 emulsion tubes

[Guy Croft]

Slawomir, hi

Definitions:

1. A/F ratio is mass flow rate of air/fuel and stochiometric (chemically correct) is 14.6 parts air to fuel by mass. For power go richer - towards 12 and for economy go towards 14. I say that in the most general terms.

2. Lambda = (A/F actual) divided by (A/F stochiometric) and if A/F actual is 14.6, so then lambda = 1. If the mixture is richer than stochiometric lambda becomes a lower figure, eg 0.8 and if lean it gets higher, eg 1.1

Your graph has Lambda not A/F values but if these are multiplied by 14.6 you will get the A/F ratio. Optimum Lambda for a normally aspirated competition engine is typically between 3500 and peak rpm should be in the range 0.86 - 0.87 (12.5-12.7 A/F) and not really much leaner, though of course dyno tuning can prove out the best settings and they do vary from one engine to another.

So in fact your engine is a bit over-rich in the 3000-3800 range (low lambda values of 0.775-0.825, 11.3-12.5 A/F ratio) and getting a bit lean in the peak torque range 4750-5250 (higher lambda 0.87 - 0.7, A/F ratio 12.7-13.14).

So your engine is a bit over-rich at the bottom end under full throttle and then leaning out at higher rpm. This is unlikely to be a jetting issue, in the first instance anyway: you're off the idle jets and progression drillings on full throttle and right in the working range for the main jet - the air corrector has little influence until 6000 plus. If you put a weaker main jet jet to lean out the bottom end it will lean out the 5000+ region too which you don't want. A 145 main jet is about the right size for that motor. You already have quite a rich emulsion tube (170, being smaller, gives richer top end - over 6000 or so - than one with, say, abigger air hole, 175 or larger)

I think the most likely cause is the emulsion tubes. I wouldn't be using an F16, never have on IDFs so I don't really know what it will do. I would use an F11 or richer F9 and see if it works better. I assume you have set the ignition timing to 34-36 deg total advance at (usually) 5500rpm and the silencer system is low back pressure (ie: straight thru design).

I hope that helps,

GC

[Slawo2000]

Hi Guy

Thank you for your reply. I will try to use F11 emulsion tubes, as you advised. I will do also some tests with bigger (180 and 190) air correctors. At the end of June, when I'm back in Poland, I will inform about the results.
I'm a little surprised, that emulsion tubes F16, which are proper for DCOE 40 carbs, don't work good in IDF's. Does it mean, that DCOE's need generally different jetting than IDF's ?
My exhaust system is a straight thru design, however I have never checked back pressure. Surely I have to do this. The silencer I have instaled is smaller, than the one you recommend.

[Guy Croft]

Well, yes, they are rather different, I mean for a start the fuel is falling under gravity in the barrels being a downdraft setup. This may sound a rather obvious statement but it's really by way of saying that different engines and carbs can need different emulsion tubes.

Like I said I've never run F16 in 40 or 44 IDF so I cannot comment, whereas I know F11 were used on the 124 Abarth Spider 44IDF - I had one and the the richer F9 is certainly an option too.

Read James Bowen's thread on jetting his X19 carefully, I have made numerous important observations regarding what does what in a carb and how to jet up.

GC