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Ask Anything - May 2014

Rat Tactics

John Raczka; Golden, CO: I'm looking for some direction on my 1969 Nova drag car. It has a 468ci big-block Chevy, a Comp 11-250-3 flat-tappet cam, AFR 265cc oval-port Magnum heads with roller rockers, Hooker Super Comp headers, full MSD ignition, a TH400 tranny with a 3,500-stall converter, CalTracs, a Dana 60 with 4.11:1 gears, and a spool. I have been running an Edelbrock Air Gap intake with a Holley 850 carb and 1-inch phenolic spacer. I live in the Denver area, and I run my car at Bandimere Speedway at a 5,800-foot altitude. My car is street-legal, and I had never driven a high-stall-equipped car before. I always thought the car was sluggish from a start, which I related to the stall converter. The Nova ran 12.20s at 108 mph.

I decided to try a 1050 Dominator carb and used an adapter plate approximately 21⁄2 inches tall and no other spacer. OMG! The car woke up! It is snappy and a ball to drive. I guess it wasn't the stall. I had to tweak on the CalTracs because of traction issues. I only had one chance at the track, but it ran an 11.92 at 112 mph. Now for the question: I developed an oil leak at the rear of the intake, so should I just fix the leak or is there a Dominator manifold that will work better than my Air Gap and adapter?

Jeff Smith: The question is finding an oval port, single-plane intake manifold that will also fit a Dominator flange carburetor. I think that going to a single-plane intake would certainly benefit your combination. A good single-plane intake will make more peak horsepower and probably raise the peak horsepower rpm point by roughly 200 to maybe 300 rpm. This would also complement your camshaft with its specs of 240/246 at 0.050 and 0.574/0.578 inch lift. I looked in the Edelbrock catalog, and the best manifold for your application would be the Victor Jr. 454-O, (PN 2909, $289.97, Summit Racing). This will allow you to bolt that Dominator carburetor directly to the manifold, and the oval-port configuration should be very close. Don't sweat the interface between the manifold and the heads if there is a slight mismatch. You can port-match it if you choose, but honestly there is very little to be gained by spending any time port-matching. The majority of flow occurs in the middle of the port, and if you mapped velocity, you would discover minimal flow activity near the walls. That's why nobody polishes intake ports anymore—there's no benefit. Finally, in almost all cases, a dedicated Dominator manifold will be better than a 4150 flange manifold with a Dominator adapter. One thing that might have also helped you with this is the fact that you had a 21⁄2-inch-tall spacer between the carb and the manifold. If you have underhood room with the Victor, Jr. intake, a spacer would probably also be beneficial.

Of perhaps more interest is why the larger carburetor really helps. I spoke with Westech Performance's Steve Brulé, who has a ton of experience with big-block Chevys. He said that big-blocks are very responsive to bigger carburetors, and this isn't necessarily related to the engine needing more airflow. As an example, way back in the late '80s, Kenny Duttweiler was testing a normally aspirated Buick V6 engine on Edelbrock's dyno and discovered significant power increases when he adapted a 1,050-cfm Dominator carburetor to the little V6. The engine didn't need the additional airflow, but it did respond positively to the larger carburetor, which reduced the air velocity exiting the carburetor. This allowed the heavier fuel traveling at a slower speed to make the transition from vertical flow out of the carburetor to horizontal flow into the intake ports.

Following that theory, your engine probably responded both because of reduced air speed out of the carburetor and perhaps also to a change in air/fuel ratio. I noticed you mentioned that you race at Bandimere Speedway, which is in the mountains just outside Denver at 5,800 feet of altitude. This means the air is a lot thinner where you race. This also means that it would be very easy to make an engine run a very rich air/fuel ratio because of the greater elevation. I'll assume that the Dominator was already tuned for use at your higher altitude, but it is something you may want to look into a little closer. Further tuning with the carburetor or possibly using a tapered carb spacer might just deliver even better performance.

More Info
Edelbrock; 310/781-2222; Edelbrock.com


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