this will be a learning exercise for the newer guys,

[grumpyvette]

this will be a learning exercise for the newer guys, below are two dyno sheets from another site thanks to new2novas
it compares the SAME ENGINE with TWO DIFFERANT CARBS,A 600cfm and a 750 cfm, notice that the 600cfm makes more power so if you just look at the power the 600 cfm carb would seem to be the better choice .......RIGHT??? WRONG!!!!

heres the 600

heres the 750

yes I looked at the dyno sheets and the 600 carb has a slight advantage AS THEY ARE NOW TUNED,
after BOTH CARBS are correctly adjusted I think youll find the 600 carb makes for a better street combo, is more responsive and gets slightly better mileage, BUT the 750 carb will make MORE PEAK HORSEPOWER, and its easier to fine tune correctly, myself Id go with the 750 cfm

why?
look at the dyno sheet closely.........
the engine was running BOTH richer and cooler with the current 750 cfm carb adjusted that way and that, loss of heat and rich mix cost you power!!
the BSFC should stay close to about .45
your oil temp should have been 215F minimum
and if you had both carbs running the same a/f ratio the 750 SHOULD have a slight advantage
you were running about 10 degrees hotter and significantly closer to the ideal a/f ratio with the 600 than with the 750

BTW you should be running about 12.8:1 a/f ratio for max power and about 14.7:1 for low emissions and mileage
at 9:1- 10:1 like the 750 runs now,and the oil temp under 215f your giving away a bunch of potential hp/tq
BTW you should be running about 12.8:1 a/f ratio for max power and about 14.7:1 for low emissions and mileage
at 9:1- 10:1 like the 750 runs now your giving away a bunch of potential hp/tq

 

[Ken]

... you should be running about 12.8:1 a/f ratio for max power and about 14.7:1 for low emissions and mileage

I've been trying to get a handle on this for a couple of weeks now Grumpy, in an attempt to understand and tune my set-up.

Recently I came across an A/F Mixtures & Characteristics chart from Jeff Hartman's book "Fuel Injection: Installation, Performance Tuning, Modifications":

• 6.0 - rich burn limit (fully warm engine)
• 9.0 - black smoke, low power
• 11.5 - approximate rich best torque at wide-open throttle
• 12.2 - safe best power at wide-open throttle
• 13.3 - approximate lean best torque
• 14.6 - stoichiometric air-fuel ratio (chemically ideal)
• 15.5 - lean cruise
• 16.5 - usual best economy
• 18.0 - carbureted lean burn limit
• 22+ - EEC and EFI lean burn limit

Sometimes I think I'm getting closer to understanding, but until then, I'm still trudging uphill. :L

Thanks for the article!

 

[SG4206]

I agree with Grumpy's assessment ; but consider what engine vacuum read from the real data sheet , not listed on this standard corrected printout. I would be willing to bet the 650 carb showed 1.5 inches of vacuum because the engine pulled almost 600 cfm at 6000 rpm. The 750 carb would likley only pull 1.0 inches at 6000 rpm which would produce much more power. Unfortunately , the 750 was too rich and killed any gain from the less restriction of the bigger carb.

In my opinion ; this engine can use a 830 cfm, vacuum secondary carb and make more usable power . Vacuum might only be .8 inches.

The mistake seen the most , is sticking a huge 850 mehcanical secondary double pumper on a engine like this for street use. That kills the throttle response . A 750 or 830 Vacuum secondary will only give what the engine demands under any load condition.

Grumpy - I 've got lots of dyno sheets and always learning something new from these wonderful tools.