Tuning a carburetor doesn't require a mechanical engineering degree by any means, which isn't to say that it doesn't take a bit of knowledge to properly set one up. If you're tuning for the street, then you'll need to take heed of idle quality, including off-idle throttle response, which is addressed this month in "Just the Right Size" on page 40. On the other end of the spectrum, tuning for all-out maximum performance at the dragstrip is a whole other ball game.
This month, we spent the afternoon at California Speedway in Fontana, California, dealing with less than ideal track conditions and sweltering temperatures well over 100 degrees F. Braving the heat with us was Matt Sendejas and his small-block-powered '71 Nova. What's interesting about the Nova is that Sendejas had literally finished piecing the car together the day of our testing. It had yet to make a single pass down the quarter-mile and while it was a slight gamble-OK, a big gamble-it was one that paid off, making everyone involved look like heroes.
To facilitate our testing, we enlisted the QMP crew out of Chatsworth, California, who own and operate one of the premier engine-building facilities in the San Fernando Valley. Considering these guys compete in the NHRA Comp class with a naturally aspirated Monte Carlo that dips into the mid-7-second range, we were confident they would be able to tune Sendejas' street/strip juggernaut.
Now it's important to note that the results you see here are very specific to our '71 Nova, and we can attest to the fact that every car will be different, depending on individual combinations including engine, suspension, and choice of transmission and gearing. Let's not forget that grand variable, Mother Nature. In our case, it was hot and miserable, with the density altitude ranging anywhere from as little as 3,500 feet to well over 4,300 feet. Nevertheless, we've provided all the details and results with the carbs' fresh-out-of-the-box and the numbers after being tuned by the professionals. All said and done, nothing blew up and we made rather remarkable gains. The only drawback was the nasty sunburn, but hey, it's all in a good day's fun and we've already set aside a date for the next outing.
Q&A with QMP
CHP: What's the biggest difference when it comes to tuning for the dragstrip?
Mike Consolo: I think it's much easier to tune at the dragstrip. You're just trying to get the right jetting and fuel flow, whereas on the street you're more concerned about off-idle qualities and having to deal with things like the accelerator-pump spring adjustments, squirters, power valves, and the accelerator pump cam. On the strip you're only worried about the initial leave and the performance at wide-open throttle.
CHP: There's a general tendency to block off the power valves in a drag-race application. Why is that?
MC: The power valves are instrumental to controlling daily drivability concerns. When you're at full throttle, the power valve isn't doing anything. By blocking them off, it gives you complete control of the fuel through the jets.
CHP: How do you adjust the mixture screws?
MC: I start by turning each screw all the way in and then backing them out by one and a half turns. I'll then turn a screw until the rpm fluctuates; nothing drastic, mind you, just until it wants to stumble, and I'll turn them out until the motor has a nice idle. Once I know the number of turns I'll set the other three to a similar setting. Also, my personal feeling is you should never have any of the screws vary by more than a half-turn.
CHP: We know every tuner has his own way of adjusting carburetors. How about you? What do you generally like to do?
MC: I like to open the butterfly a little bit before the squirter, but to do that I also run a larger squirter to eliminate potential stumbles. You may not see the hesitation by simply revving the engine, but when you leave off the line, it helps the initial hit of the throttle by supplying it with additional fuel. Again, for all of this to work, there is a fine balance of infinite adjustments that need to be addressed.
CHP: Have any additional tips to eliminate carb stumble?
MC: If you adjust everything and still have problems, check your timing. It's a real basic tip, but it's not uncommon to overlook how the timing is being ramped in. If you're still using the stock springs supplied with an aftermarket distributor, you may need to swap over to a lighter spring to ramp in the timing sooner. When you first roll into the throttle, if you only have 15 degrees of timing coming in at 3,500 rpm, it's going to cause a problem. The lighter spring will bring in the timing sooner, say, at 1,500 rpm, where you really need it.
Like any new combination, the initial 13.68 run was more of a shakedown pass to make sure nothing leaked and the car went straight; hell, the paint was still tacky from being sprayed the night before. (Seriously, you could smell the paint as you walked by-from five feet away.)
Our second pass with the 830-cfm Holley produced a traction-limited 12.34 at 121.17 mph out of the box. That's big mph, but it didn't hook, making the e.t.'s less than impressive. Back in the pits, checking the plugs revealed a lean condition, so Consolo fattened it up by blocking off the power valves and went up 10 jet sizes from 78 to 88s on both the primaries and secondaries. The result: a gain of 2.4 mph! The added traction dropped us well into the 11s with an 11.38, but the 123.59 mph was the real indicator of what the added fuel did.
It was only 11 in the morning and the temperature was 98-and climbing. We splashed the launch pad with a little VHT traction compound and were rewarded with an impressive 11.32 at 123 mph. For the second pass, once again the power valves were blocked and this time, the jets were bumped up eight sizes to 88s, netting us with an 11.29 at 126.65 mph.
Stepping up to the 830-cfm HP series was impressive, to say the least. During the first pass, we experienced a bit of hesitation out of the gate; regardless, the HP seemed to be the ideal match for our 415ci bullet with an 11.02 at 126.88 mph. To eliminate the stumble, the 28/29 squirters were swapped for a set of 37s. We won't deny it; it was a drastic change and ideally, you should only step up a couple of sizes at a time. The truth of the matter was our track time was running short. On top of that, we also jumped eight sizes on the jetting, from 86s to 94s. It worked; we bettered our e.t. with a 11 flat, which we'll attribute to better traction; however, the mph was similar. Still, we got rid of the stumble, and the Nova motored strong all the way through the traps. Again, if we had the extra time, we would have leaned out the combination by dropping a couple of sizes in both the squirters and jets, but that's another story for another day.
If you're wondering what's different between the conventional 830 versus the HP series 830, the HP comes with down-leg-style boosters, whereas the standard 830 has annular-style boosters. Other differences for the HP are stainless steel throttle plates with button-head screws, power valve blow-out protection, and nonstick gaskets.
Originally, the 830-cfm HP was going to be the final carb of the day, until we learned QMP had a 950-cfm HP sitting in its tow rig. Even with the limited time, we decided to give it a shot-and saw nothing less than a half-dozen arms flinging like mad to finish the swap.
From Holley, the 950 HP is jetted with 78s on the primaries and the secondaries, 31/31 squirters, and 6.5 power valve on both sides. This particular carb already had the power valves blocked (notice the trend?), was jetted with 84s all around, and was sized with a 35 squirter. Needless to say, this was the money run; the great pumpkin hooked and flew through the big end with a 10.89 at 127.58 mph run. Not a bad way to end the day!
We swap four Holley carburetors and tune for maximum e.t. and mph at the dragstrip.
Stock-suspension '71 Nova with a 415ci small-block, 28x12.5-15 Mickey Thompson ET Streets.
Carburetors start at $410.
Engine: 415ci small-block
Intake: GMPP Bow Tie single-plane
Cylinder: heads Dart Iron Sportsman II
Camshaft: Custom solid-roller (around 275 duration)
Pistons: SRP flat-tops
Rods Eagle: H-beam
Crankshaft: Eagle 4340
Front Suspension: Stock w/Lakewood 90/10 struts
Rear Suspension: Stock w/Lakewood 50/50 shocks & traction bars
Transmission: Turbo 400 w/10-inch 4,000-stall converter
Rearend: 10-bolt w/4.10:1 gear