Effect of Weber mainjet size on cruise and accel

Another A/F run on the streets of America with an LM-1 meter.
**Caution - could induce sleep in those not rabid about carburation!**

To review, the car is a 3.2 QV with stock FI cams and four Weber 40 DCNF-12 carbs with 36mm venturis and a stock 308 airbox with a K&N.[308 stock is 40 DCNF-XX with 32mm venturis running from 125 - 135 main jets with typically F-36 emulsions and 200 - 210 air correctors]

Initially, the car was jetted with 55 idles, 140 mains F-36 and 190 air correctors. The car idled at an A/F of about 11.7 when cold and when warm about 12.1, with admittedly a sl higher idle speed by 200 - 250 rpm. Cruise A/F was about 11.5 or so and dumping the throttle kept the a/f at about 12.5 after the pump ran dry or so all the way up the scale to 6000+ (remember we are on the street here!). During tip in, it did have a tendency to lean a bit as the pump dried up, up to the 14s before it re-stabiliized down to the 12s.
I had tried reducing the idle jets to 50s which brought the idle a/f to 13.5, but really went very flat in transition, which seems to occur strongly about 3000 - 3500 rpm depending. Ok, back to the 55s.

Swapped some jets with FChat Weber Jedi Paul Airey (pma 1010) and installed 135 mains. While I don't like to change more than one thing at a time, I was a bit wary on the going to a smaller jet with the tip in leaness, so I hedged a little and re-installed F-24s emulsions. Idled still at about 11.5 when cold, 12.2 when warm. At legal cruise, defined as 3000 - 3500 she still kept an A/F of about high 11s, and would lean into low 12s if throttle opened a bit to go up a hill. So I guess I just proved what most folks already know is that at cruise, the Webers are not open enough to significantly get out of the idle/transition circuit. Well, if you, let's call it 'advance the cruise' level up above 3500 rpm, the A/F would drop solidly into the mid - 12s to 13 or so where it used to stay in the high 11s with the 140s. On a straight tip in to full open throttle from 3200 rpm the A/F would hold about 12.5 up until about 5800 - 6000 where it would slide up to about 13.2 for the 'rest of the way'. I will add, like stories of older carb'd Ferraris, she was happier after a high rpm 'clean the plugs off Italian tune up'.

Summary of what I have kind of learned today:
At cruise, you're probably still well on the idle/transition circuit until over 3500 rpm where the main jet cuts in. Changing one main jet size looks to buy you about a point on the A/F, and this is consistant with some full throttle dyno runs I did last year. You really do get the feeling the car is a bit happier on the rich side.
Looks like you can start seeing the A/C effect at about 5800 - 6000 rpm as expected.

The things I am looking at doing are leaning out the idle screws a bit from the last synchronization perhaps up into the high 12s/low 13s top allow some richness for cold starting and see how that affects cruise A/F.
I may also shim down to 34 venturis just to sharpen lower end response since I actually do drive my car places other than to the Dyno...

Any comments, experience or advice welcome.
Hope this helps someone.
best
rt

 

Another thought
I took out the car with the LM-1 off and flogged it a bit, as I do not do that with the LM-1 power clips on the battery and stuff hanging about. More noticible intermittant very slight bog in the high 3000s from moderate throttle application. Does not happen when throttle is applied slowly. Probably would not notice if I wasn't looking so hard at this transition range.
I do think this is a leaning out do to the 36 mm venturi air velocities that I did not have when it was much richer with the 140 jets. I think the richness of the 140s disguised a lot of air velocity things around transition down low while tooling around in the A/F 11 range. I may just keep this jetting and re-install the 34mm venturis for higher air velocity and see.

On a side note, it's not as edgy sounding with the new Ansa resonators in place of the open tips, but the deep smoothness of the sound is pretty soothing and way better at cruising. Amazing for an 8 cylinder.

Time to run in for a post-test pizza!

 

Update - it's amazing when what they tell you in book theory actually works

I re-installed the 34mm venturis (down from 36) in an effort to clean up the very slight lower end flat spot as described below which I thought was due to sl. lower velocity effect being unmasked by sl. leaner jetting. By Pierce Manifold's book calculation, 300 hp should be supported by a 33.5mm venturi, so I'm still in my target range. Also went down a gnat's ass hair on the idles to 52s from 55s to help with the rich cruise hoping that the previous large transition flat spot with the 36mm venturis/50 idles would be overcome by the smaller venturi's higher air velocity and stronger downstream vacuum signal.

Wow! Don't know what it did to the top end 6500 + peak hp, but the midrange response is wildly better - very easy to break the rear loose now with instantaneous response throwing you back in the seat with that woom-woom on dumping the throttle. Should be a bit more accurate with my throttle steering now. I think for a street car this is a better arrangement than chasing top end dyno numbers over 6500 rpm (although it is great sport). Will take a drive with the A/F meter on this weekend I hope.

So, for the couple of folks out there following this carb 3.2 (stock FI cams, so far) saga, we are now at:
40DCNF-12 (w/ the huge accell pump cam), 34mm venturis, 135/F24/190/52 with the rest at -12 spec. This is really tweaked now closing in on perfect for the street application. For the track would definitely use the 36s as they flow a bit better top end high rpm - would guess 5 - 10 hp at the top looking at some old dynos. It's an easy price to pay for the low end response - the seat of the pants mid range feel is more dramatic than I expected. Amd a ton more of Italian personality when compared to the previous standard K-jet slow response. Woo - hoo.

best
rt


OBTW - Although I got a lot of compliments on the snarl from the 4 individual Pierce carb filter boxes, I went back to a stock 308 airbox w/ K&N as the 4 individual filters were amazingly loud. Good for the track, not so good day-to-day and on trips.

 

Thanks, guys, I'm actually still learning the 'dark arts'. Appreciate the comments as most everyone else is fuel injected these days.

Interesting fun fact to know and tell, probably true for 308s and many carbureted Ferraris as well. Again, it's cool when what they tell you in the book theory actually seems true.

On a drive to Austin from San Antonio today, got to closely look at certain engine speeds with slow transition, If you very slowly open the throttle plates you can feel the transition to the main circuit in 5th gear depending on the road slope, right around 3700 - 4000 rpm. Of course, this is where we were cruising so could feel it go in and out with just a very minor little sound change where it went a wee bit lean for just a fraction of a second. I'm probably been smelling too much unburnt gas, but I've been doing this so long now I can almost feel and hear the A/F change.

Fun fact: Pretty much up through 70 mph AT CRUISE with lower throttle openings, you're still primarily on the idle/transition circuit. Of course during accelleration with larger throttle openings you on the main circuit. So, it's important to get the idle tuning pretty right, as that's a large part of daily driving. Above 80 mph you're now on the main jet system, even at cruise. You can feel it.
As my Ferrari seems to like a bit richer mixtures, may consider going back to the 140 mains, a popular upgrade choice with many 308 guys (although most of them except the early models are running 32mm/F36s so difficult to directly compare). This may give just a bit more invisible and smoother transition at small throttle movements around transition at 'higher' cruise speeds. It's already creamy smooth with larger throttle movements with no hesitation at all and a real buck in the butt when you punch it.

So if you've noticed a little spot in your carb'd Ferrari sound around there, that's what it might be.