News

3.2 with carbs - too rich - part 6

Discussion in 'Technical Q&A' started by snj5, Dec 10, 2003.

  1. snj5

    snj5 F1 World Champ

    Feb 22, 2003
    10,213
    San Antonio
    Full Name:
    Russ Turner
    Short story: WAY WAY WAY RICH throughout the whole range.

    Took the car for its first dyno run today as I tweak the carb settings. I succeeded all too well in avoiding a lean situation (no wonder it starts so great on snowy mornings).

    During the entire run the A/F was usually well below 12, peaking at 12 at 6000, then falling off to 10 at 7000. Did not get a downloaded graph, but the curve was very smooth (4th gear used). The curve leveled off at 6000 rpm and held close to peak hp through 7, only dropping a little. I think I can find a few more ponies leaning out the mixture, as well as going to a 36mm venturi for more flow.

    Carb: Weber 40 DCNF; 34mm venturi; 150 mains; 190 air correction; .6 idle; stock FI cams
    Dynojet

    rpm-----rwhp(SAE)-----torque-----A/F
    4.0.........137................180.........10.0

    4.5.........158................184.........10.0

    5.0.........181................197.........10.7

    6.0.........220................193.........11.9

    6.5.........224................181.........11.5

    7.0.........214................161.........10.1

    7.5.........212................149.........10.0

    With the 17% fudge puts me at 260 hp SAE (270 DIN) with suboptimally rich mixture and before any tweaking. I'm told that peak hp occurs around 12.5 to 13.5 A/F, so have a lot of room to improve. Sure explains the sooty tailpipes.
    Current plan:
    More air, less gas
    upsizing the venturis to 36mm for more flow, keep 150 jets in for first run, but have a set of 145 mains ready. Probably should have some 200 A/C as well. This jetting stuff is pretty cheap.

    Once I get the new distributor on will roll in a bit of advance as well.
    Cams probably at the first part of the year or so.

    Considering these first settings were a WAG on my amateur part, I'm pretty proud it runs at all. If I can get the mixture better with more flow, will have much improved response and peak hp. The response and drivability is already way ahead of the FI.

    As a side story, the dyno tech at this Mustang hot rodding place says it was an extremely smooth pull and a very smooth and flexible engine - and these numbers are about what he sees from a stock 5.0 liter mustang.

    Will be ordering the new jetting tomorrow. Damn, this is fun.
    All advice welcome, and thanks to all the great help from Philip, Paul, Mark, Kermit and many others over the past months.
    best to all
    rt
     
  2. f355spider

    f355spider F1 World Champ
    Rossa Subscribed Owner

    May 29, 2001
    16,562
    USA
    Full Name:
    Hugh G. Rection
    Sounds like a great start...keep us posted!
     
  3. pma1010

    pma1010 F1 Rookie
    Silver Subscribed

    Jul 21, 2002
    2,556
    Chicago
    Full Name:
    Philip
    Russ
    Great data. Please keep us all posted.

    Mike at Pierce may be able to give you some pointers on the re-jetting but my guess is going to the 36s is going to mean starting all over again.

    I am running 155 mains, 185 ACs with my 36s and I also think it is too rich (better that than the opposite until there is data). My guess is 150s (145s in inventory). Need data for the AC (175s, 165s in the tool box).

    Charts usually show a fair amount of HP gain from leaning it to get to 12.8 or thereabouts. Anecdotal comments support this too. Mike thought he found another 50 BHP on a 7-litre GT40 from leaning out the (Le Mans) settings for Leguna Seca. I'd be surprised if you don't find another 15 - 20 horses from the mixture and the venturis.

    Philip
     
  4. mk e

    mk e F1 World Champ

    Oct 31, 2003
    11,384
    The twilight zone
    Full Name:
    The Butcher
    Cool. Around 13.2 is what you're looking for, but better rich than lean. The 2 point swing you have is the battle you always fight with carbs...I think you'll find that the 36's tend to make it get worse, I'll be interested to see...I've been wrong before. Looks like you're close.
     
  5. Mike328

    Mike328 F1 Rookie

    Oct 19, 2002
    2,655
    Boulder, CO
    Full Name:
    Mike
    What do you mean by "Two point swing"? Dropping HP after a certain peak RPM?
     
  6. snj5

    snj5 F1 World Champ

    Feb 22, 2003
    10,213
    San Antonio
    Full Name:
    Russ Turner
    Mike:
    Two point swing in A/F ratio

    Philip -
    Could you re-comment on your drivability on the 36mm venturis? Am especially curious about the 2500 - 5000 range and throttle response.

    thanks
    rt
     
  7. snj5

    snj5 F1 World Champ

    Feb 22, 2003
    10,213
    San Antonio
    Full Name:
    Russ Turner
    Mark -
    will be doing this again after the holidays with the new jets - he said he would cut us some deals if we got a group.

    Any DC area folks interested?
     
  8. mk e

    mk e F1 World Champ

    Oct 31, 2003
    11,384
    The twilight zone
    Full Name:
    The Butcher
    Russ,
    For me it's going to depend on weather really, I'm a couple hours north so we get snow. But if the roads are clear....I do want to get the thing on a dyno.
     
  9. snj5

    snj5 F1 World Champ

    Feb 22, 2003
    10,213
    San Antonio
    Full Name:
    Russ Turner
    Got a Weber lesson (for me at least) on proper venturi sizing from Mike at Pierce manifolds as I was ordering the new jets, so thought I would pass it on.

    You should use the smallest venturi that allows you to spin to the rpm you are looking for. In my case, moving up to a set of 36mm venturis would be no improvement over the 34s which already allow me to spin to 8K+. Smaller venturis keep the velocity up and crispin the response. A too large venturi will not add to the top end and only flatten the response. A venturi is too small if the engine goes flat at high rpm and will not pull to the desired rpm, it will just get stuck at a certain rpm. That all being said, I am curious about trying the original 32mm chokes that came with the carbs once I get the mixture better sorted to see If I can pull up to 8K with the 32s and improve the response.

    The jets I ordered were
    .5 idles
    140 and 145 mains
    200 and 220 air correction

    This gives me quite a few combinations to try, especially if I swap venturis. He said to shoot for 13.2 or so.

    Fun fact: the difference between paper airfilter/airbox and just air box was 8 horsepower !!!! Unlike the Forza article, when I took the airbox off I gained 14 horsepower over baseline!!!!! Of course, no ram effect was in play on the dyno, but interesting numbers, don't you think. So where do I get a 308 K&N? :)

    As usual, any and all advice/experience/corrections welcome.
    This is great fun.

    thanks to all
    rt
     

Share This Page