General Category > Quadrajet Carb Talk and Tips
Suggestions For Next Drag Strip Testing Day
Cliff Ruggles:
I also noticed above "launching at 2000rpms".
Are you "power braking" to 2000rpms on the starting line?
I'd try leaving off idle instead if that's the case instead of loading up the drivetrain and suspension and taking the "hit" out of the converter.
Also curious who's converter it is, diameter, etc? I can't get past 1700rpm's with my 10" converter without the rear tires starting to spin and it flashes to 3600rpm's on a full throttle start with traction.
Also, what RPM's are you seeing at the finish line at 104mph?
Something else I noticed was running 78 main jets in the carburetor. Not sure why or how you ended up with that much jet? Also a mention of it running slower with 76 jets.
We tune the primary side separate from the secondary side for drag racing/WOT performance. I've set up scores of 73-74 Super Duty carbs and you don't need or want that much jet in them. I'd back that down some and go to smaller primary metering rods as well. Also remove the triangular cover over the adjustable airbleed screw and gently seat it, but leave the cover off for fine tuning if/as needed.
You mentioned 36 degrees timing all in by 2600rpms, how much timing is the vacuum advance adding?........Cliff
Brian B.:
Cliff I have a 13" Continental converter. My stock brakes barely hold on the starting line but I can power brake it to 2000rpm. With the exception of Bilstein front "racing" shocks My suspension is very slow. I typically leave as soon as the 3rd amber lights up without red lighting. RPM at finish line is 4000 with my 3:08 rear end. Transmission shifts into 3rd gear approximately 100 feet or so before finish. Accel Adjustable Vacuum Advance can adds 12 degrees without pinging or black specks on insulators of NGK 2771's, which are one step colder than stock and are NGK's recommendation for my application. As far as 78 jets go, car always seemed to run best times with those with no drive ability issues or fouling.
Brian B.:
Sorry..I forgot one other suspension mod. I clamped the front half of my leaf springs. I am sure this sounds like a lot of backyard engineering but I do most of my testing at the track rather than a dynamometer. Amateur for sure.
Cliff Ruggles:
Those are excellent converters but I'd leave at or just above idle speed. When you "load" the converter that hard it softens up the hit to the suspension and tires and can hurt short times.
I'd drop back the jet size at least 3 numbers and same with the primary metering rods. Don't forget about the air bleed screw in the airhorn under the cover. Then you will be able to tune more effectively on the secondary side of the carb at the track.
Clamping the springs is fine, but you need sub-frame connectors on those cars to effectively transfer weight. An adjustable pinion snubber will also be required to control wind-up of the center section.
"Old School" mods but they work. My car is set up the same way, and don't forget to weld the axle tubes to the center section. 8.5 diffs are only "spot" welded and known for spinning axles in the housing when hit with big power and good traction.
It's been at least 20 years now but I helped a customer set-up his Super Duty T/A pretty much the same as yours other than running an old HO Racing (may have been a Nunzi cam) flat cam. It was 240 something @ .050", and topped with Rhoads lifters. Motor was pretty much stock otherwise.
He used a TH400 with 3.08 gears and we ended up getting him to 12.0's in the 110-112 mph range. In addition to a LOT of carb tuning it took a lot of suspension work and good tires but those engines are a factory "hot-rod", just waiting to be exploited........
Brian B.:
Cliff...thank you for your insight and thumbs up on my converter. I should have added it is what was called the "Jim Hand Converter". I will try the carb modifications soon. Just curious about the customer's SD that ran 110 to 112mph. Was it equipped with headers? I'm trying to do all this without tube headers. So far I've taken a stock and option loaded 14.0 second car to 12.91 and don't want to use headers or a gear swap. Thank you!!!
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