Emulsion tubes actually make a difference - this week's Weber lesson

With a hard wired in LM-1 A/F meter at the end of the headers, got to compare the common Ferrari Weber emulsion tubes of F24 (older earlier V-8s) and the F-36 (later cars). Both times used the same idle jet of 55 and main jet of 140. The stock 308 GTB air box was used with a K&N filter. (in this configuration the car, a 3.2 with 40DCNFs with 36mm primary venturis has 223+ rwhp on a Dyno-jet, without the stock airbox 241+ with well over 205 rw torque). I use the stock airbox day to day, but do have some other options sitting on the shelf for more spirited application. I have also found that the airbox/k&N makes about a 1/2 to 1.0 point richer mixture depending on the operating regime than the open trumpets.

Emulsion tubes have an effect on the mid range richness and response and sit between the main jet and air corrector jet.The F-24 is 'richer' as it is thinner and allows more gas in the well and has a different hole pattern than the comparatively 'leaner' F36.

F24s: The car idles with an A/F of about 12 - 13 with some variation, usually a bit richer. I tried going to 50 idle jets, but transition to mains was impaired. Toodling around in the city in the 30-35 mph range, the car was a bit rich running in the 11 - 12 range. When opened up went into the low 12s to 13s and cruised at 70 at about 11.9. Throttle response was good (much better than K-Jetronic as expected) and backing off throttle did not produce any over lean or over rich situations.

F36s: idled about the same, surprisingly seemed a bit more in the mid 13s, a chqange which I did not expect but there it is. The low speed behavior was about the same, but the numbers did change as more throttle was used. Upon higher throttle applications the A/Fs were in fact leaner as expected, with A/Fs now in the low 13s to 14. Never really got above 14.2 or so. No off throttle lean or rich behavior. Throttle response remained good ( maybe a little better?. Oddly, cruise was still around 12
Due to traffic safety, I did not really have an opportunity to put the spurs in. I did bury the throttle a couple of times and after the accell pump wore off never went above 14.2.

Summary: the F36 produced about a point leaner difference in some power-on regimes while was not as noticible at lower throttle settings. The idle effect I do not readily understand so am going to say I'm not sure. With these accel pumps, the tendency to go too rich at tip in was alleviated a little bit with the F36, thus possible giving a small improvement in an already excellent throttle response. I am looking at some options to bring my cruise A/F into the 13s as a target. The engine cruises at such low throttle openings it's interesting the balance I am going to do to keep a nice clean transition which is now aided by the richness of the idle jet. Options - off the top of my head may try a 52 idle (maybe not) or go to a slightly smaller main with the F-24 or F-36

To review some background: Depending on who you read or talk to, folks say max power occurs with A/Fs in the mid 12s to mid 13s. It has been my experience with multiple jettings on multiple dyno runs that this engine actually tolerates a wide range of A/Fs with almost identical output - from low 12s to 14. The accell pump richness to cover tip in is crisper in a bit leaner situation.

Ferrari V-8 K-jetronic cars in tune run A/Fs that are locked in concrete right at 14.2 or so from the dynos I've seen. Ferrari 3.2 engines with FI make from 210 - 218 hp rwhp based on folks I've talked to. I have heard as high as 221 rwhp I think on our own Rob Lay's 328 with tubi and K&N, but again, the numbers are not directly comparable from different machines at different times and a host of other factors. Anyway, the point is that the 3.2 does just fine on leaner, near stoichiometric mixtures.

Anyway, a bit of a ramble, but pretty cool to actually see a difference in side-by-side testing of the deepest darkest region of Weberdom, the emulsion tube. Hope this helps anyone else that wants to tweak their carbs for whatever reasons.

best to all
rt

 

Thanks everyone - I appreciate it and hope the information over the past year is helpful..

While I don't know the lower limit, I forgot to say from previous experience here that when the A/F hits 15 or better under load, it feels like the engine hits an rpm wall with large loss of power and a bad miss.

best
rt

 

It's definitely a touch leaner at part throttle according to the LM-1. It's interesting that with the F-36s it does wander well above 14 and even 15 in some off and part throttle applications. I am even going to go out on a limb here and say actually, the car likes to run a tad rich after an extended drive without the LM-1 attached so I could kinda jerk the car around a bit.
I think the newness has worn off and I am going back to the F24s, and consider having Pierce dial in some 137 mains to try to back off just a hair on cruise richness. That and may go back in and measure my float heights again.
Transition seems ok unless you hold it part throttle in one place at slightly open where you can feel a spot where it does lean a bit. Otherwise it punches you in the back like my 911 did. So Perhaps a bit more fuel in the well would be good. The personality of the car is that it likes it better if you are decisive and direct, no *****footing.

So will go back to the F-24s, and be happy with crappy gas milage.

Would love to hear anyone else's numbers if you have them.