Back to Race Carbs
The purpose of a carburetor is to supply the correct ratio of air and fuel to the engine under all conditions. This is not particularly hard to accomplish, but it does require a thorough understanding of carburetion, and plenty of dyno and track testing.
The first step is testing on an engine dyno with data acquisition to accurately monitor air fuel ratios. These carburetors were developed on the dyno with a DePac data acquisition computer.
The DePac measures airflow, fuel flow, oil temperature, water temperature, air inlet temperature, exhaust gas temperatures, (And just about anything else you wish to hook up to it.) and has several programmable channels which allow you to interpret the data anyway you wish. This allows for monitoring of air fuel ratios, brake specific fuel consumption, brake specific air consumption, volumetric efficiency, cfm/hp, and a host of other valuable information.
Testing with this type of equiptment is very revealing. In addition to measuring the air fuel ratios, it allows a determination of what air/fuel ratio is required for best power.
It may surprise you to know that the air/fuel ratio required for best power is dependant on many factors, and the best power ratio "BPR" varies with rpm. For instance, a stock 12A will make best top end power with air/fuel ratios in the mid to high 13's, but will make best midrange power with air/fuel ratios in the mid to high 12's. A ported motor, depending on the shape of the ports, and atomization quality, can require mixtures as rich as 11:1 at lower rpm.
This does not mean that you can only have one or the other. A properly calibrated carburetor will deliver a relatively rich mixture in the midrange, and gradually lean out as the rpm increases. The result is good low speed torque for corner exit, and good top end power for the straights.
While every effort is made to simulate real world conditions on the dyno, the final answers can only be found on the race track. Afterall, we are not racing dyno's!
The G loads encountered on the track affect the fuel in the bowls, and adjustments must be made to maintain crisp throttle response on corner exit.
This type of tuning can only be done at the race track.
As of July 2,000, over 200 modified Nikki carburetors have been sold, with nearly 100 of them built for race applications. I have spent considerable time working with drivers at the track, as well as having feedback from customers all over the country.
The result of this track tuning, and feedback, is many minor modifications that improve dirveability, throttle response, and fuel control during high G cornering.
Most racers know from experience that a broad powerband is the key to quick lap times, but few realize that it is very dependant on the carburetor itself.
The key is to have the appropriate fuel delivery curve. Most of you are likely familiar with changing fuel jets to alter the mixture. I'll bet many of you using a stock carburetor have made a jet change only to find that you gained power in one part of the powerband, and lost in another. This is the result of an improper fuel delivery curve.
A very typical situation is that the stock carburetor goes rich at high rpm. If the fuel jets are chosen to correct the top end mixture, the engine runs lean on corner exit. This hurts lap times, and adds to the problem of stumbling off left hand corners. As you can see, the real answer is to correct the fuel curve, rather than just changing fuel jets.
The air bleeds, in conjunction with the emulsion tube, needle and seat, and fuel pressure, allow for changing the shape of the curve. Here is a series of dyno runs showing what was involved in dialing in a Nikki with enlarged primary venturis. This is part of the initial dyno testing that resulted in the "Super Street" Nikki.
By carefully studying these dyno runs, you can get a feel for the importance of carburetor tuning, and its effect on the powerband.
I cannot overstress the importance of a broad powerband. The stock transmission has a very wide spread between gears, and for most classes this makes a wide powerband necessary. If you are building good power at the upper rpm range, but the motor bogs, or is sluggish in the midrange, the car will not accelerate well. A broad powerband is essential.
Listed below is the rpm in each gear, after upshifting at 8,000 rpm with a first generation RX-7 gearbox. I'll bet you didn't realize how much time is spent below 6,000 rpm!
2nd. gear 4,827 rpm.
If two engines build the same peak horsepower, but one has weak midrange power, the engine with the strong midrange will run away from the other. The difference in acceleration would be so great that you probably could not convince anyone that both engines made the same horsepower! Consequently, a strong midrange with a weak top end will not accelerate well either.
To make this easier to grasp, consider that a horsepower rating tells you only what the engine is doing at a single point in the powerband. This is great for impressing potential customers, but it is a poor indication of the acceleration potential of the vehicle. The average horsepower throughout the operating range is what matters. The average horsepower is directly proportional to acceleration. A high average horsepower rating is the result of a wide powerband, or "Area under the curve."
Good top end is required for straightaway speed, and good midrange is required for corner exit. An engine combination that does both well will be very fast. And that is what these carburetors are all about!
3rd. gear 5,167 rpm.
4th. gear 5,586 rpm.
5th. gear 6,600 rpm.
In addition to a broad powerband, crisp throttle response, and the ability to smoothly modulate the power is required. A good driver uses the throttle to control the attitude of the car from the apex, to the exit of the corner. . Some of the more experienced racers I have worked with made considerable improvements in their lap times after adjusting the accelerator pump for maximum response. If the motor does not cleanly, and predictably respond to throttle inputs, car control is greatly diminished!
A car that jerks or lurches on throttle opening can be quite scary to drive through the exit of a fast sweeper. Any of you who have driven in the rain have an appreciation for the importance of driveability.
All the above issues have been addressed during the development and testing of these carburetors. The result is a very drivable carburetor with smooth power delivery under all conditions.
Those of you running on the street will appreciate the driveability, especially if you have prior experience with aftermarket carburetors.
As hard as I try, it is just not possible to build a one size fits all carburetor. Every application is slightly different. Motors vary from one to the next, fuel is different from one part of the country to the next, no two exhaust systems are the same, etc. In addition, weather conditions have a drastic effect on the operation of a carburetor. A savvy racer will keep his car running its best at every event.
A performance carburetor should have provisions for fine tuning. All Yawpower carburetors are shipped with extra jets, and complete instructions for fine tuning. This allows for optimum performance regardless of weather conditions, or changes that may be made to the car.
If you've ever disassembled a carburetor, you have seen the myriad of jets that control fuel metering from idle to redline. A proper fuel delivery curve is simply the result of calibration.
Every portion of the fuel curve can be altered by changing the size of these jets. (Or orifices.) The particular combination of jet sizes, from fuel jets to air bleeds, in conjunction with the emulsion tubes, and needle/seat flow rate determine the fuel delivery curve.
As much as I like to brag about my carburetors, there is no magic. It is simply the result of testing, and making the appropriate modifications. All the hard work has been done for you. All that is left is to fine tune it for your particular application.
To make this process easy, complete instructions are included with all carburetors, and I am always available to answer your questions. For those of you going to the track for the first time with your new carburetor, I am at the shop on Saturay, and will gladly work with you. Many customers take their cell phone to the track with them, and call after each session so that I can help dial them in. I do this because I want everyone with my carburetor to be as fast as possible.