General Category > Quadrajet Carb Talk and Tips

Lowering BSFC numbers on dyno

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Cliff Ruggles:
Hire someone to machine a full set of metering rods for you from the same cores.  Some racers are more "anal" than others.  I have customers that want ever .002-.003" from .030" to at least .060" and a couple bigger sets for higher altitude tracks.

Can't argue with them as they are at the top of the pile for what they are doing.

Once you have a fully machined/accurate set of metering rods the tuning begins.  I like the tubes to be shortened apprx 3/16" and .036" hole size.  Some racers claim going bigger helps, others go smaller.

LOTS of opinions out there and you'll find that the "old timers" that sit around at the big races and run WAY under index without exhausting much effort have things pretty nailed down.  They have made so many runs they can get up in the morning, take a deep breath, look around and go pick out a hanger and rod that will be good to go for that day, and change to exactly the right combo when the sun comes out or a front blows in.

I like to be more "scientific" than that and prefer to use a very well made set of rods and log all the runs and learn from the results, using DA calculations, track conditions, etc. 

Dyno runs help some, but not dynamic so things change considerably when you get to the track and engine between the fenders with a hood on it.

Other variables are fuel pressure, N/S size and float settings.  All sorts of opinions there as well.  Some racers run tiny seats and sky high fuel pressure, others low pressure and huge seats.  Some run the small floats, others the large ones, a few run brass (NOT recommended).

I don't do a lot of that sort of work these days, absolutely no money in it for the time spent, and the customer base more times than not knows WAY more about it than I do, or at least that's how they come across on the phone.  Most are also cheap and don't want to spend any money, so countless hours on the phone to make a $10 sale doesn't work well for me as I'm too busy with other things.

I still don't mind helping out, and whether they will tell you or not, (most do not) I have carburetors prepared here whipping up on just about everyone at many of those events especially in the newer Pure Stock and FAST Classes.

Not sure where you fall in the learning curve, just getting started or decades of experience, but one piece of advice I will give here is to figure out how to do two things BEFORE you do much tuning.

Keep the bowl FULL on hard runs all the way down the track and make sure you have taken steps to keep ALL the fuel in the bowl on a hard launch.  Until that happens it's difficult, if not near impossible to nail everything else down as far as tuning is concerned........Cliff

tenxal:
     Thanks, Cliff.  The secondary air bleeds measure .039 and are 1.050 in length, (measured from the underside of the carb top).



     I keep the fuel pressure at 6 psi (at the carb) and it's rock steady during a run. Fuel regulator is a bypass style, fuel bowl stays full, float level is 1/4" and I never go bigger than the .135 needle/seat. Generally, the 'G' hangar has given the best performance on a given day, whether foot braking or on the two-step. I've been involved with QJets in NHRA class racing since the late '70's...they are by far my favorite carbs to work with. My thinking is that if we could get the BSFC down to the low-mid .400's like we can with the 4GC's and Holley stuff, we'd see some significant gains. Maybe not possible with the non-booster secondaries?  With a private dyno facility available to me now, we've had the chance to dig into this a bit more.

    Again, thanks for the assistance. I've got another virgin 1901 Edelbrock to do and I'll be ordering from you fairly soon.  :)

Cliff Ruggles:
If you are leaving on the floor (2-step) you might want to take the POE out of the equation.  Some do and some don't.  Depending on the size of the holes leading into the POE voids it changes the fuel curve and can continue to add fuel all the way down the track.

Easiest way is to simply remove the tubes and see how it works.  If it doesn't stumble all over itself coming up on the brake you should be good to go.

The hanger becomes a much smaller player once you go to a full set of machined metering rods with long tips on them.  They are a little finicky to machine (brass) and I like to zinc plate them when finished....very time consuming so expensive.  My eyes don't serve me as well as they used to either, so it's a full day in the lathe to make a custom set these days, and with the zombie apocalypse going on I can't keep up with parts orders let alone get to all the emails and phone calls!

Hopefully things will settle down some, but right now I can't get near a carb or any shop work, just doing parts and some "tech" for folks who spent a little money with me........

77cruiser:
You ever try this with the bleeds?

77cruiser:
You ever try this with the bleeds?

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