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| Don't Believe Everything You Hear
"I Have a 10,000 CFM Carburetor For Sale -- I'll throw the Golden Gate Bridge in for Free" By Don Terrill I'm not going to point fingers, but the companies that advertise bogus carburetor CFM numbers should be ashamed. OK, bogus may be too harsh, but they ARE deliberately trying to mislead consumers. Remember what they say "If it seems too good to be true, it probably is." A CFM number by itself means NOTHING without the test pressure it was measured at. For example the same carb measured at 25" will show more CFM if measured at 28". So, what these companies have done is just quote their numbers at a higher pressure drop. But it get worse, not only don't they use Holley's(R) pressure drop number, they also don't subtract for the fuel. Holley's(R) measurement method subtracts a percentage from the CFM for the flow loss caused by the area taken up by the presence of fuel. Since 99% of these carburetors are decedents of the 4150 Holley(R), I think everyone should stick to Holley's(R) method. Since these companies aren't going to change anytime soon, here's what you can do: (1) Find out what pressure drop the CFM is at, and did they subtract for fuel. OR (2) Get the measurement of the venturi and throttle bores, then match to your combination. Don't be fooled by these Tonic Salesman. |
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| Answers To Your Camshaft Questions
What is meant by basic RPM? The camshaft’s basic RPM is the RPM range within which the engine will produce its best power. The width of this power band is approximately 3000 to 3500 RPM with standard lifter cams, and 3500 to 4000 RPM with roller lifter cams. It is important that you select the camshaft with the “Basic RPM Range” best suited to your application, vehicle gearing and tire diameter. Why is cruise RPM at 60 MPH important? When selecting a new camshaft, you can raise or lower the engine’s basic RPM range. It is important to be sure the vehicle’s drive train is capable of matching your selection. The cruise RPM at 60 MPH is a way of rating your rear end gearing and tire diameter to determine if these components match the RPM potential you are desiring. What is camshaft duration and why is it important? Duration is the period of time, measured in degrees of crankshaft rotation, that a valve is open. Duration (at .050” lifter rise) is the deciding factor to what the engine’s basic RPM range will be. Lower duration cams produce the power in the lower RPM range. Larger duration cams operate at higher RPM, but you will lose bottom end power to gain top end power as the duration is increased. (For each ten degree change in the duration at .050”, the power band moves up or down in RPM range by approximately 500 RPM’s.) What is the difference in advertised duration and duration at .050” lifter rise (Tappet Lift)? In order for duration to have any merit as a measurement for comparing camshaft size, the method for determining the duration must be the same. There are two key components for measuring duration— the degrees of crankshaft rotation and at what point of lifter rise the measurements were taken. Advertised durations are not taken at any consistent point of lifter rise, so these numbers can vary greatly. For this reason, advertised duration figures are not good for comparing cams. Duration values expressed at .050” lifter rise state the exact point the measurement was taken. These are the only duration figures that are consistent and can accurately be used to compare camshafts. How does valve lift affect the operation of an engine? Lift is the distance the valve actually travels. It is created by the cam lobe lift, which is then increased by the rocker arm ratio. The amount of lift you have and the speed at which the valve moves is a key factor in determining the torque the engine will produce. What is camshaft lobe separation and how does it affect the engine? Lobe separation is the distance (in camshaft degrees) that the intake and exhaust lobe centerlines (for a given cylinder) are spread apart. Lobe separation is a physical characteristic of the camshaft and cannot be changed without regrinding the lobes. This separation determines where peak torque will occur within the engine’s power range. Tight lobe separations (such as 106°) cause the peak torque to build early in basic RPM range of the cam. The torque will be concentrated, build quickly and peak out. Broader lobe separations (such as 112°) allows the torque to be spread over a broader portion of the basic RPM range and shows better power through the upper RPM. What are intake and exhaust centerlines? The centerline of either the intake or exhaust lobe is the theoretical maximum lift point of the lobe in relationship to Top Dead Center in degrees of crankshaft rotation. (They are shown at the bottom of the camshaft specification card as “MAX LIFT.”) The centerlines of the intake and exhaust lobes can be moved by installing the camshaft in the engine to an advanced or a retarded position. Generally speaking, the average of the intake and exhaust lobe centerline figures is the camshaft lobe separation in camshaft degrees. How does advancing or retarding the camshaft’s position in the engine affect performance? Advancing the cam will shift the basic RPM range downward. Four degrees of advance (from the original position) will cause the power range to start approximately 200 RPM sooner. Retarding it this same amount will move the power upward approximately 200 RPM. This can be helpful for tuning the power range to match your situation. If the correct cam has been selected for a particular application, installing it in the normal “straight up” position (per the opening and closing events at .050” lifter rise on the spec card) is the best starting point. See here for spec card information. Why is it necessary to know the compression ratio of an engine in order to choose the correct cam? The compression ratio of the engine is one of three key factors in determining the engine’s cylinder pressure. The other two are the duration of the camshaft (at .050” lifter rise) and the position of the cam in the engine (advanced or retarded). The result of how these three factors interact with one another is the amount of cylinder pressure the engine will generate. (This is usually expressed as the “cranking pressure” that can be measured with a gauge installed in the spark plug hole.) It is important to be sure that the engine’s compression ratio matches the recommended ratio for the cam you are selecting. Too little compression ratio (or too much duration) will cause the cylinder pressure to drop. This will lower the power output of the engine. With too much compression ratio (or too little duration) the cylinder pressure will be too high, causing pre-ignition and detonation. This condition could severely damage engine components. How does cylinder pressure relate to the octane rating of today’s unleaded fuel? In very basic terms, the more cylinder pressure we make the more power the engine will produce. But look out for the fuel! Today’s pump gas is too volatile and cannot tolerate high compression ratio (above 10.5:1) and high cylinder pressure (above approximately 165 PSI) without risking detonation. Fuel octane boosters or expensive racing gasoline will be necessary if too much cylinder pressure is generated. How does an increase in rocker arm ratio improve the engine’s performance? The lobe lift of the cam is increased by the ratio of the rocker arm to produce the final amount of valve lift. A cam with a .320” lobe lift using a 1.50:1 ratio rocker arm will have a .480” valve lift (.320” x 1.50 = .480”). If you install rocker arms with an increased ratio of 1.60:1, with the same cam, the lift would increase to .512” (.320” x 1.60 = .512”). The engine reacts to the movement of the valve. It doesn’t know how the increased lift was generated. It responds the same way it would as if a slightly larger lift cam had been installed. In fact, since the speed of the valve is increased with the higher rocker arm ratio, the engine thinks it has also gained 2° to 4° of camshaft duration. The end result is an easy and quick way to improve the performance of the existing cam without having to install a new one. Remember, whenever you increase the valve lift, with either a bigger cam or larger rocker arm ratio, you must check for valve spring coil bind and for other mechanical interference. Please review the previous sections concerning these matters Must new (Standard Design) lifters always be installed on a new camshaft? YES! All new standard hydraulic and mechanical camshafts must have new lifters installed. The face of these lifters have a slight crown, and the mating lobe surface they ride on has been ground with a slight taper. The purpose of this is to create a “spinning” of the lifter as it rides on the lobe. This is necessary to prevent premature wear of the lifter and lobe. Therefore, these parts will be mated to one another during the initial break-in period. Used lifters will not mate properly, causing the lobe to fail. If you are rebuilding an engine and plan to re-use the existing cam and lifters (in the same block) it can be done, as long as the lifter goes back on the same lobe it is mated to. If the lifters get mixed up, they cannot be used, and a new set will be required. The new lifters would also have to go through the break-in procedure to mate to the old cam. Can used roller lifters be installed on a new camshaft? YES. “Roller” lifters are the only ones that can be re-used. This design lifter has a wheel (supported by needle bearings) attached to the bottom of it. The lobe the roller lifter rides on does not have any taper. This is a very low friction design and does not require the lifter to mate to the cam. As long as the wheel shows no wear, and the needle bearings are in good condition, the “hydraulic roller” or “mechanical roller” lifter can be re-used. What engine oil and lubricants should I use? Do not use synthetic oils during the initial break-in period for a new camshaft. Use a good quality grade of naturally formulated motor oil during this period. If you choose to use synthetic oil after the engine has been broken in, change the oil filter and follow the oil manufacturer’s instructions. When using either regular oil or synthetic it is important to pick the weight oil that best matches your engine bearing clearances, the engine’s operating temperature, and the climate the vehicle will be operating in. Use the oil manufacturer’s recommendation to satisfy these conditions. Should I use “Oil Restrictors” in my engine? No, the use of oil restrictors is not reccomended. The oil is the life blood of the engine, not only lubricating but cooling the engine components as well. For example, a valve spring builds in temperature as it compresses and relaxes. This increase of temperature affects the characteristics of the spring’s material, and if excessive, will shorten the life of the spring. Oil is the only means the spring has for cooling. How do I prime the engine’s oiling system? It is critical that the engine’s oiling system be primed before starting the newly rebuilt engine for the first time. This must be done by turning the oil pump with a drill motor to supply oil throughout the engine. If this is done with the valve covers off, you will be able to see that the oil is being delivered to the top of the engine and to all the valve train components. |
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| Where Do You Place The Ballast?
Ballast Placement Tips Do you want your racecar to be faster? Do you want your car to be more consistent on those long runs? Getting the ballast located correctly is a key component for a faster racecar. Proper placement of the ballast is actually a free speed secret. These simple tips will allow you to go faster around the turns and provide more grip in your car. First and foremost you should be very weight conscious when constructing your car. You would be amazed at how all of those little things add up to extra weight. Be sure to look for any weight savings. It is very difficult to find weight savings in five-pound blocks. Look for quarter pounds! Those little things will add up if you pay close attention. Strive to build a car that is as lightweight as possible. Never compromise safety for weight savings. There are plenty of places that you can save weight but safety should always be the paramount concern. We race for fun, make sure that you build a safe racecar. Now that you have a good lightweight car you will need to add ballast to get the car up to the minimum standards set by your sanctioning body. Check your rulebook for the maximum left side weight allowed. You will want to be as close to the maximum left side weight allowed as possible while maintaining the minimum total weight. Never run your car heavy for the sake of more left side weight. Next, check with your car builder for his recommendation on front to rear weight percentages. Verify that your car is "race ready" excluding the ballast operation. Now that you know what you want for rear weight you can begin finding a home for the ballast. You need to locate the ballast as close to the Center of Gravity of the car and as low as possible while maintaining the minimum total weight rule, maximum left side weight rule and car builder recommended rear weight. In other words, you want the ballast to be located in the smallest area possible. Insure that you properly attach any ballast to the car. Do it right to insure safety. For example, lets assume that your car weighs 2500 pounds race ready but without any ballast. Lets also assume that you have a 200 lb driver, your minimum weight allowed is 2900 pounds, your maximum left side is 56% and your recommended rear weight is 50%. With these assumptions you will need to add 200 pounds to get up to the minimum weight. Lets also assume that your car has 50% rear weight without any ballast installed. We now need to mount the ballast to our hypothetical car. Since we are starting with a rear weight percentage that matches our car builder's recommendation we need to add ballast and reach our goal of 50% rear weight at the rules mandated 2900 pounds. When mounting the ballast we want to concentrate the ballast in the smallest possible area. To illustrate the point, we would want the 200 pounds of ballast to be mounted in a concentrated area within the car to meet our target. We would want to avoid placing 100 pounds near the front of the car and the other 100 pounds near the rear of the car. By concentrating the ballast into a small area versus spreading it out your car will go faster. The same idea holds true for the left to right weight distribution as well. You want to mount the ballast in a small area rather than taking the easy route and placing some ballast in the left side frame rail and some ballast way out on the right side frame rail. Take the extra time to build proper ballast brackets between the frame rails to attain the desired left side weight. Avoid placing ballast (or anything heavy) to the right of the Center of Gravity. You will see that your static weight numbers can be the same whether you mount the ballast in a concentrated area or if you spread out a 100 pound block on the left rail and the other 100 pound block on the right rail. Statically this will look fine on the scales but dynamically the spread out scenario will slow your car down and wear out your tires faster. The same idea applies to front to rear weight. Avoid placing some ballast in the front of the frame rail and then another amount of ballast at the rear of the rail with an air space in between. Slide the two chunks of ballast together. Focus on concentrating the ballast into the smallest possible area and spend the time building brackets to meet the goal. Why will the spread out ballast placement slow you down? Lets picture a simple example. Picture a playground teeter-totter. The teeter-totter pivots in the middle. The pivot is compared to the Center of Gravity in your racecar. Now picture 500 pounds of weight on both seats of the teeter-totter. The seats compare to the left and right frame rails. You can see that in the static position that the 500 pound weights would balance out. However, when you put the teeter-totter in motion that much weight would require much effort to get started and even more effort to stop once it got moving. If the teeter-totter were moving fast you would be crushed trying to stop the movement with 500 pounds out on each end. Your springs and shocks would have to control all this dynamically moving weight that is rocking back and forth. Front to rear movements would have to be controlled as well. Now picture the same example with one revision. Instead of having 500 pounds on each seat with a balanced teeter-totter, lets move the 500 pounds in from each seat until we end up with 1000 pounds directly over the pivot point (this would be the same as our CG in our racecar). You would notice that the teeter-totter is still balanced. The weight would be carried directly at the CG or pivot point. Once the teeter-totter were put into motion it would be much easier to control compared to the spread out version that had the 500 pound weights clear out on the seats. Just think of how much easier this situation is on your springs and shocks! By concentrating the ballast into the smallest possible area you reduce the amount of weight that has to be controlled once the car is in motion. You reduce the amount of back and forth motion in the turns and front to rear weight transitions under braking. Weight transfers occur in more controllable amounts, which will result in a more efficient, and stable handling racecar. Another way to think of it is using your own body as an example. When you carry heavy items you hold them as close to the centerline of your body as possible. Typically you hold heavy items against your chest. With the weight against your chest you can carry the weight with less effort and you have more control once you begin moving. Most people do not carry their groceries into the house with a bag in each hand and their arms fully extended. Obviously with the weight extended way out at the end of your extended arms the groceries would be difficult to control and you could be thrown off balance very easily. Keeping the weight closer to your body or CG is much more efficient. Now that you understand this principle lets take it another step forward. When building your car you should strive to keep all support items as close to the CG as possible. Avoid mounting the battery out on the right frame rail. Batteries are heavy and need to be located just like ballast. Try to mount all of your tanks, electrical items, fuel filters, hoses, drink bottles, radio boxes, or any support items to the left of the CG. Avoid mounting anything to the right of the CG whenever possible. Using this strategy will allow you to place more ballast in a concentrated area. Simply by planning your mounting locations you can make your car faster by properly placing the ballast and support items. It may require more initial effort but the cost is effectively zero and the benefit keeps giving throughout the life of you racecar. If you are conscientious mounting all of your racing components you will be able to place your ballast closer to the CG and low to the ground while still maintaining your maximum left side weight and desired rear weight. The result of placing the ballast in a concentrated area is a racecar that is more nimble. The car will change directions much quicker. The racecar will be more responsive. Tire temperatures will be reduced, tire wear improved, lap times will go down, your car will have more grip, be more consistent and your chance for victories will rise. |
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