Wide-open throttle seems to get all the attention when it comes to carbs. Whether on the dyno or on the strip, carb tuning will usually come down to the wide-open mixture-a jet change here or there, the big squirters and pumps, and once it's pulling as hard as it can, it's time to put away the wrenches. While it would be wrong to neglect a carb's calibration while running all four butterflies on end, it is just as wrong to believe the job is done at that point. Consider the big Holley 750 HP fitted to our solid-roller 408 small-block in Project Talladega. There was no denying the sharp tune found when the linkage yanked the carb to full noise-our dyno and dragstrip calibration sessions ensured that fact. No, the problems here were much more subtle, the kinds of things you could easily neglect with a shrug.
To really dial in the fine points on Project Talladega's Holley 750 HP, we brought the car
Getting to the specifics in the case of our Laguna, it was only when the car was not straight ahead and wide open that the carb's performance inadequacies made themselves known. Starting with its behavior right at idle, the stroker small-block was not quite happy. The idle seemed unstable, with excessive rpm pull down in gear, and the feeling that a two-foot driving style was required to keep the engine lit at a stop. Sure, this might be put down to the big .600-inch lift, 242/242-at-.050 COMP roller cam, but we had to think there was room for improvement in idle quality, even with the big cam.
From the jagged idle the engine would snap to attention if the throttle was stomped, boiling the hides at will. But at more meager inputs the results were far from satisfying, with a rough tip-in and light throttle transition to pedal inputs. On the jammed Southern California freeways, the poor light-throttle response was tormenting. Finally, when pressed hard around corners, the seamless straight-line performance just seemed to fall apart, with the engine coughing and sputtering against the g-force. Under hard extreme braking, the engine would just want to stall. When you are serious enough about cornering performance to run R-compound tires on the street, this is nothing but bitterly disappointing. On the autocross course, the let down was especially apparent. While such a set of circumstances would be enough to make an enthusiast seriously consider a change to fuel injection, we had to think that some massaging to the carburetor combination could yield very satisfactory results without breaking the bank.
We needed better fuel control during hard cornering, and the solution was custom angle-cut
The Fix Is In
Rather than just accept the shortcomings of our carburetor setup as par for the course when running a radical carbureted small-block on the street, we figured with some expert tweaks, the all-around performance could be dramatically improved. To prove that point, we headed to one of the country's premier carburetor specialists, The Carb Shop in Ontario, California, and took notes as they worked their magic. On the job was technician OJ Bretzing, who disassembled our fuel mixer and then went through the unit with a fine-toothed comb, making minor modifications to achieve specific results. Nothing done to the carb could be considered a major reworking. Rather, the modifications centered upon intelligent changes that specifically addressed some of the problems with solutions that come from knowing the intricacies of how the carb functions. To begin, Bretzing disassembled the carb, carefully cataloging all of the parts, and then treated the mixer to a bath in a sonic cleaning machine to start with impeccably cleaned parts. The carb was then reassembled, making the required changes as the build progressed.
Side by side next to a standard Nitrophyl float, the difference in profile is readily appa
The first modification was to the carburetor floats. Center-hung Holley floats were initially designed with acceleration at the forefront, and when g-forces are active during cornering, the fuel in the bowls responds by sloshing. The modification here is to take the standard Holley Nitrophyl floats and shave the material at the corners, converting the float from a rectangular profile to a trapezoid, resembling a triangle. What this does is concentrate the center of buoyancy away from the outer extremities and moves it to the center of the bowl. Thus, fuel sloshing left or right will have a lesser effect on the float position. To seal the pores of the Nitrophyl float material, the cutaway surface was coated with 5-minute epoxy. The epoxy not only seals, but provides a small amount of additional mass that dampens the float's response.
Our primary throttle plates were equipped with air bypass holes. With an objective of incr
Next on the hit list were the needle and seat assemblies. Here the internal restriction was enlarged from the stock .097-inch orifice to .120 inch. The actual dimensional change might seem small with an increase of only .023 inch in diameter, but this represents a change of 53 percent in cross-sectional area open to fuel flow. The higher flowing needle and seat assembly makes the carb much more sensitive to float inputs, allowing the bowls to fill faster once the needle separates from the seat.
The fuel in the bowl is only useful to the engine if it reaches the jets, and under very hard acceleration or braking, this can become difficult. As a nod to fuel control, both the front and rear jets were fitted with short jet extensions, which place the fuel pickup point virtually dead center in the bowls. This strategy provides the best opportunity for the jets to find fuel whether accelerating or braking.
When replacing throttle butterflies it is important to center them precisely in the bores
Longer bowl vents were also added to the main body, making it less likely for fuel to escape the carb while the vehicle is pressed into extreme maneuvers. Keep in mind when contemplating longer vents that there needs to be adequate clearance to the air cleaner lid, otherwise the vents will not vent, defeating their purpose.
Addressing the light throttle and transition irregularities, the first item Bretzing keyed on was the drilled primary throttle plates. Drilling the plates has long been a popular modification to allow additional air into the engine at idle, particularly with long duration cams. While this can work well, it is a primitive approach, since this allows no metering activity to take place at the throttle plate holes. The holes effectively bypass the normal idle circuit, acting as a measured vacuum leak. While the holes allow less idle throttle angle, if the air bypassed is in excess to the engine's requirements, the result can be compromised low-speed metering, particularly in the transition from the idle circuit to the transfer slot. On our carb it was determined that the low-speed, low-throttle driveability could be improved by increasing the throttle angle at idle slightly, making the transfer partially active at idle. The theory here is that transition to the transfer slot portion of the idle circuit will be more active, covering the lean hole at low throttle inputs. To that end, the throttle plates were replaced with standard butterflies without holes.
Our needle and seat assemblies were modified by enlarging the internal restriction from .0
Also addressing the driveability and carburetor's low-speed transitions, the primary to secondary linkage was modified, replacing the long link positioned at the lower linkage hole to a shorter piece engaged in the upper hole. This will delay the rate of the secondary actuation. The idea here is to delay the point of secondary engagement and slow the action for improved signal to the secondary booster prior to the secondary coming into play. Bear in mind that this is a transitional change, and at wide-open throttle the secondary still comes to wide open the moment the throttle is at the floor. Similarly, we found that the pump shot was weak at the initial portion of throttle actuation, causing a lean stumble with slow throttle engagement at part throttle. The cure here was a swap in pump cams, replacing the pink cam with the more aggressive green cam. Either cam will give the full pump shot with the throttle stomped wide open, but the green cam will give more fuel earlier in the throttle.
Short jet extensions help keep the main metering circuit in the fuel under both accelerati
So what did this moderate level of carb modification do for our Laguna? Putting theory to the test on the open road, we found the changes delivered as intended. We had the carb making good on all counts, from cleaning up the idle, with improved transitions and low-speed, part-throttle manners, to effectively metering even when giving the chassis a real workout. A carb can be a very efficient and cost-effective way of delivering fuel, and with the right level of know-how, it can be calibrated to cope with varied driving conditions. Based on the way the car behaves on the street, The Carb Shop has the expertise to get it dialed in.
Where The Rubber Meets The Road
We've hit a number of autocross and road race events with the Laguna this year, and one of the biggest hurdles we've experienced is keeping the fire lit during hard braking and cornering. Enigmatically, faster road courses pose less of a problem than slower autocrossing, which is why the NASCAR guys never see this issue pop up. With our Carb Shop mods in place, we noticed improved street manners right away-this is due to the more aggressive accelerator pump cam, the plain throttle blades, the slower linkage, and a change from a 10.5 to a 7.5 power valve. These mods had the added benefit of improving street driving, particularly part-throttle response, which was an unexpected bonus. The following week we took the Laguna to the Costa Mesa Goodguys autocross, and noticed the improvement right away. Some stumble is still evident-you can't eliminate it 100 percent-but it is much more manageable. Braking is where we saw the most improvement, with the Laguna experiencing not a single stall in the stop box all weekend. Along with the improvement in street manners, we're now rethinking whether we need to step up to fuel injection, which we always had in the back of our mind. We like the carb's simplicity and cost-effectiveness, and now with a near perfect tweak, we can see years of trouble-free operation ahead. -Johnny Hunkins
Although an increase in diameter of .023 inch does not sound like much, the cross-sectiona
Where The Money Went
The Carb Shop stripped, cleaned, rebuilt, and modified our carb for a price of $320 including parts and labor. Some of the small parts used in modifying our carb, and their individual mail-order prices (via Summit Racing) are listed below.
|Holley pump cam assortment kit:
|Throttle plates (111/16 inches):
|Nytrophyl float kit (each):
Slowing the actuation of the secondary improves the part-throttle response by allowing the
This change in secondary timing will bring the secondary on line when the booster signal i
Because our Laguna is set up for corner burning as well as strip performance, taller bowl
To aid in light throttle enrichment, the pink accelerator pump cam was replaced with a gre
While the performance mods promised to cure our carb's idiosyncrasies, it is important not
With The Carb Shop's reworked carb, the change in character in Project Talladega was notic