Rapid Tune-Up Tricks For Your Favorite Holley Carb

John Reedy ran the SuperFlow 902 dyno for us and helped with the carb thrashing. The K&N dyno facility is first rate and usually filled with the thunder of big-inch Rat motors.

John Reedy ran the SuperFlow 902 dyno for us and helped with the carb thrashing. The K&N d

Dyno Test
Here's where our testing became interesting. Ultimately, we discovered that this engine wanted less fuel to make more horsepower with the air/fuel meters peaking at around 13.3:1. As the power kept increasing, the spark plugs all looked good. Most of the change in power during this test can be attributed to the jet change (two leaner in the front and four in the rear), since that has the effect of lowering the entire curve but really doesn't affect the shape of the curve.

In an attempt to alter the shape of the curve, we first eliminated the lower of the two emulsion holes in the cartridge, leaving only one 0.028-inch hole in each cartridge. Then we calculated the area of that 0.028-inch hole and converted it into two 0.020-inch holes (which calculates out at 3 percent larger) and found that this combination reshaped the curve, adding a little more fuel at the top. By using two holes, we think this tended to stabilize the fuel curve over the wider rpm band. This is a difficult thing to accomplish since we're looking at an overall rpm band of almost 3,000.

The best way to evaluate the carburetor modifications is to pay attention to the changes in fuel flow. The biggest changes occurred when we changed jets, but we noted that the fuel flow change was not consistent all the way through the rpm band. The most drastic alterations in fuel flow occurred at the bottom with the jet change. When we modified the emulsion hole size and position in the cartridge, we were able to effectively richen the top end of the curve. At 3,200 rpm, we reduced the fuel flow by 27 lb/hr from 146 to 119, which is an 18 percent change. But at 6,100 rpm, we reduced the fuel flow only 17 lb/hr, which is only a 7 percent change.

As you can see by the power numbers, this half-day's worth of playing on the dyno was equivalent to about 7 lb-ft and an almost 6hp average. More importantly, these improvements show what can be done with a slightly more sophisticated approach to carburetor tuning.

Click here for the Holley carburetor tuning dyno test results

Just adding a K&N X-stream lid to a 14x3-inch filter was worth a 4hp average over a paper filter.

Just adding a K&N X-stream lid to a 14x3-inch filter was worth a 4hp average over a paper

K&N Air Filter Test
While we had the use of K&N's dyno, we also thought we'd do a quick air filter test, something we haven't done for quite a while. We used the same carb we had just tuned with the RapidJet, starting first with no air cleaner and without the SuperFlow air turbine on the carburetor. It's worth noting that our baseline numbers were not the same as the final number used in the previous RapidJet test, since we noticed a slight loss of power when the air turbine was removed.

Most everybody knows the story about the K&N filter, which uses oil-impregnated cotton gauze to filter air efficiently. As a matter of fact, the dirtier the filter becomes, the more efficiently it removes dirt from the air. First, we tested a bargain-basement 14x3-inch paper filter, then a K&N filter of the same size, and then the addition of a K&N X-Stream filter lid. The best combination was the K&N filter with the X-Stream lid. We also tested a taller, 4.5-inch filter element, which surprisingly lost a little bit of power. Of course, had we spent as much time tuning the engine for that filter as we did with the air turbine, we think that slight amount of power would have returned. Basically what we found was that our engine preferred a flat air filter base with an X-Stream lid and the shorter 14x3-inch filter. If you compare the X-Stream lid Test 4 against the paper filter Test 2, you can see the K&N filter and lid combo was worth an average of 5 lb-ft and 4 hp. That's pretty good for a simple air filter swap.

Click here for the air filter test results

By Jeff Smith

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