General Category > Quadrajet Carb Talk and Tips

Lowering BSFC numbers on dyno

(1/3) > >>

tenxal:
   Great forum...thanks.

   I've been doing dyno work and the BSFC numbers on both my good performing QJets are in the high .4's to low .5's. I'd like to get that number down to the mid .4's or lower.

   Any recommendations and/or experience is appreciated! Thanks!

Cliff Ruggles:
Nothing really to worry about.  Way back when I was doing the KRE head testing and articles for Popular Hot Rodding's "Engine Masters" and HPP magazine we back to back tested my 1977 Q-jet against a very well prepared Holley 4781-2 850 DP carb.  The Q-jet actually made more power but the A/F wasn't quite as "clean" and BSFC a little higher. 

Since then I've repeated that testing a number of times on the same dyno and NOTHING we've ran against the Q-jet will outrun it, but it's just not quite as good at metering having those huge secondaries fed by two fuel nozzles vs carbs with boosters in them.

Interesting but twice after the dyno testing we carried the same carbs tested to the track after the engine went back in place and the Q-jet outran them by about .02-.05 seconds and a little higher MPH, verifying that the dyno numbers were indeed accurate.......Cliff

tenxal:
    Hi Cliff. The power piston is blocked on both of this one (no primary metering rods) with 68 primary jets. On the secondary side, I normally run either .056 or .061 rods with a 'G' hangar. On the dyno, the AN (.070) rods gave more power and cleaned up the BSFC a bit. I'm hesitant to lean the secondary side much more, though. I can certainly go to 66's on the primary and see what happens.

   Is there any air bleed mods that may help the BSFC numbers? I know the lack of boosters is a biggie on the secondary side and contributes to the higher BSFC's when compared to a well done Holley that can get into the low .4's.

    I've got two really good carbs so I'm not adverse to using this one as a 'mule' carb while we're on the dyno.

    Your thoughts and time is appreciated!  :)

Cliff Ruggles:
You can shorten the tubes slightly and open up the holes in them to help clean things up a tad.

You will find much more precise tuning by using custom machined secondary metering rods than factory ones.

Few if any are machined to the same specifications for tip length, upper section diameter, taper and included angle leading to the tips.

ALL of the successfully racers I do work for use custom machined secondary rods for accurate metering changes on the dyno or at the track.

What carb number...7029207?  Sounds like a SA or Super Stock type tune in one of those.....

tenxal:
   Yes, Cliff...NHRA Stock Eliminator small block Chevy.  :) I have two excellent carbs...the 7029207 I'm working with and an Edelbrock 1901.

   I know what you mean about the metering rods...the AN's I have are everywhere from .686 to .700 in tip diameter! The 0.061's are lathe made, as are the .056's. The AN's are unmodified and are .686 diameter and the taper seem to be consistent. I like the idea of working with the air bleeds. I'm headed to the dyno this morning and will check the air bleed sizes and touch base later. Is there a percentage of orifice change you'd recommend? If they are .036's for example...would a 10% change be advisable (going to .040)?

   Thanks again!

Navigation

[0] Message Index

[#] Next page