Page 1 of 3 123 LastLast
Results 1 to 10 of 21

Thread: A/F ratio

  1. #1
    cook1
    What A/F ratio should I be looking for at cruise (3300 rpm) with about 9" of vacuum. Anyone got any thoughts.

  2. #2
    Senior Member
    Join Date
    Feb 2009
    Posts
    678
    What A/F ratio should I be looking for at cruise (3300 rpm) with about 9" of vacuum. Anyone got any thoughts.
    i would think that if u had around 14-1 A/F ratio at idle...everything from there up is fine (cruise etc)...just set ur carb with a vac gage to get the highest vac reading possible
    fastrat

  3. #3
    Taylorman
    How are you measuring your a/f ratio?

  4. #4
    Unchained
    What A/F ratio should I be looking for at cruise (3300 rpm) with about 9" of vacuum. Anyone got any thoughts.
    I run an EFI system at 14:1 while cruising at around 10" vaccuum.
    It varies a little as the engne runs between 13.8:1 and 14.2:1
    At a 3000 rpm cruise speed the injectors are running at 1.8 milliseconds per revolution on a batch fire EFI system.
    At WOT I'm at 12:1 / 7000 rpm / 6.9 milliseconds per revolution.

  5. #5
    cruser
    Unchained,
    6.8 mS open per revolution, 8.6mS per revolution = 80% duty cycle on the injectors. Not much left but, from what I have gathered your fast enough already.

  6. #6
    cook1
    I am using a lm-1 wide band a/f meter. Mine runs around 13.5 at 3300 if it goes any leaner than this it will start to missfire and surge. Does compresion effect this. It is only 8 to 1.

  7. #7
    SmokinLowriderSS
    Compression shouldn't affect that, no (unless someone has experience making me mistaken). You should ideally be somewhere in the vicinity of 14:1 all across the RPM spectrum. The fuel doesn't care how fast your engine spins, it only cares that it and the air are in the right proportion to support combustion completely and reliably. 14.7:1 is perfectly stociometric (in balance). too lean or rich and you will loose the fire.

  8. #8
    Nubbs
    If you're just cruising, part throttle, light load, I would set it as lean as it will run. No need to dump all the extra fuel in if you don't need it.

  9. #9
    cook1
    Jet boats are under load at 3300 rpm so I know that 14.7:1 is to lean. In a car 14.7:1 is ok at a cruise speed becuse it under light load 14-17" of vacuum. This motor is at 8" so it it just above where the powervalve would open on a car. My guess is that it probably does need to be 13:1/ 13.5:1 for better power and at wot at 12.5:1/12.7:1. My question is at 14:1 under load will most motors start to lay down because of a lean mixture like mine does?

  10. #10
    Unchained
    Unchained,
    6.8 mS open per revolution, 8.6mS per revolution = 80% duty cycle on the injectors. Not much left but, from what I have gathered your fast enough already.
    I'll have to check a datalog to see what % duty cycle I'm running at 6.9 ms. WOT.
    I've got 160# injectors so don't think the duty cycle is a problem.
    As I recall 100# duty cycle was about 10.5 ms.
    I cut the fuel pressure back to 35# from 42# and remapped the fuel curves this spring and I think it has helped my low speed cruising and idleing.

Page 1 of 3 123 LastLast

Similar Threads

  1. A/F Ratio
    By Taylorman in forum Gear Heads
    Replies: 8
    Last Post: 01-11-2006, 01:27 PM
  2. Compression Ratio?
    By TexasJet in forum Gear Heads
    Replies: 3
    Last Post: 09-11-2004, 08:42 AM
  3. Rod/Stroke Ratio
    By Taylorman in forum Jet Boats
    Replies: 24
    Last Post: 09-06-2004, 06:27 PM
  4. Air/Fuel Ratio
    By Taylorman in forum Jet Boats
    Replies: 7
    Last Post: 01-17-2004, 06:58 AM
  5. Oil Ratio
    By jonnyd in forum Outboards
    Replies: 19
    Last Post: 03-13-2002, 11:05 AM

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •