Making a race car go fast entails many compromises, as we all know. For instance, a huge carburetor that may produce prodigious amounts of power at 9,000 rpm won't pull your Street Stock off the corner for beans. This situation must reach a compromise for the best overall performance around the track. Throttle response, low-end power, and sufficient power at the end of the straight are factors that must be balanced.
One of these relationships that is sometimes overlooked is that of the cam to the gears. There are any number of places that grind cams. There are a few that can knowledgeably recommend a cam for your engine. Fewer still are able to practically put themselves at your track and see what you need. It is not unheard of for a racer to tell a cam grinder that he needs more cam than he really does. The racer will say he has big ports and a large carb (wishful thinking), when in fact he has somewhat less-all in an effort to select the larger cam he thinks he needs.
Sometimes the racer listens to another racer's advice about a cam. If both the racers' cars were identical with identical driving styles, this might be OK. Driving style makes a difference. Some driving styles keep rpm up, while others depend on torque out of the corner. Both can work quite well. Again the trick is to select a cam and the appropriate gear to get the desired results.
As with just about everything in racing, the cam-gear combination is important. A 9,800 rpm Daytona motor might not pull your Street Stock out of the pits without destroying the clutch.
I see a lot of Street Stock/Hobby Stock racers that don't know what gear they have. Oh yeah, they know they have a 3.73:1 in the rear end and run in First in the transmission. But what gear do they really have? The final drive is the key figure. But one also needs to know tire size. Not the tire size designated by the tire manufacturer, but the circumference, or roll-out, as it is sometimes called.
Here is the way tire size affects gearing and speed. Select a tire: say, a 90-inch circumference DOT Street Stock tire. Then, let's say you have a TH350 transmission with a 1.52:1 low gear and a 3.73:1 rear end gear. This gives you a 5.67:1 final gear. Now, if you have a 7,000 rpm engine, the 5.67:1 gives you a tire speed of 1,235 rpm. With the 90-inch tire, your top speed will be 105 miles per hour. If you change to an 81-inch diameter tire, keeping the same engine rpm and final drive ratio, then the tire rpm will still be 1,235. Yet your top speed will have dropped to 94.5 mph.
You likely won't have a situation where all things stay the same except tire size, but this exercise should explain the relationship of tire size to gear ratios. Look to the chart on page 82 if you like math. This is the formula I use for turning engine rpm, gearing, and tire size into speed numbers.
The camshaft in your engine can be considered part of your gearing. A cam will make its power in a range that the cam grinder designs. Cams are seldom ever ground incorrectly. By the same token, there are few bad (new) cams. The wrong cam is only wrong in being improperly selected by the user. A cam is designed to make power in a certain rpm range. This power range can be wide or narrow as well as high or low. This power range must be realistically matched to your car and its performance
Higher horsepower numbers and higher rpms are always the focus of a racer. In some forms of motorsports where engine speed remains fairly constant, that focus is OK. In a stock car application the operating range can be quite wide. Most racers will see the tach at the end of the straight. Few will glance at the rpm in the turn just before getting back on the throttle. The high and low tach readings will give you and your cam grinder the rpm range of your engine.
I spoke with several cam grinders. From these conversations I found that the ideal would be to rev the engine right to peak power at the end of the straight. In practice, in order to make the best use of power, revving to about 10 percent over peak is often where you want to be. Also, keep an eye on the bottom end. The cam's power range should come in close to the lower rpm found in mid-turn. You probably won't achieve this unless you go about 10 percent over peak power.
This lower speed range is where the engine must pull hard to accelerate the car off the turn. Ideals such as these will be difficult to achieve because we don't have infinitely variable gear ratios. Thus, you could be asking for such a wide power range that the amount of power will not be sufficient. A cam can only be asked to do so much.
A cam designed with a narrow power range, say 7,500 to 7,900 rpm, can make a lot of power right in that range. However, it may have trouble getting a car pulled up to that range. The obvious point is that maximum power is not the answer.
So if a wide power range is too wide and a narrow range too narrow, then we must reach a compromise. This compromise is going to have a lot to do with the car's performance. The cam's compromise should be reached along with the gear's compromise.
Final drive gearing (transmission gear ratio multiplied by rear-end ratio), must be tailored to the power range of the cam and vice versa. It works something like this: A cam with only a slightly narrow powerband can be combined with lower gearing. This allows the engine to be revving a bit higher, say 10 to 15 percent higher, at the top end and be at the bottom of its power range when coming out of the corner. At the end of the straight the engine will be revving above its peak at the shut-off point, but little time is spent at the very top end of the straight.
The other way is to gear the car higher and use a cam with a wider power range. Doing this can keep the engine in the range where it makes the best power. With a smooth driver this can work well. The problem comes when, in traffic, the preferred line is not available. This can move the engine out of its best power range. Remember also that a wide range cam is more forgiving of driver error. A wide power range allows a driver to drive out of situations such as being sideways, where a narrow power range might slow the car tremendously.
The bottom line of all this is the engine's cam needs to be matched to the car's gearing for the best overall performance. I prefer to call a cam grinder and ask him what I need-not tell him what I want. He should have information and experience with many tracks and classes. He gets feedback from racers every day about what works in a given situation.
Your responsibility is to have all the proper information at hand when you make the call. This means information about the car, the engine, and the track. One thing I like to have is the low rpm number from the middle of the turn. This is something few drivers look for, but it can be important in selecting cams.
Finding the correct gear to match your cam can be difficult unless you have some experience to draw from. Your's or a friend's best guess might be the best place to start
Now run the car at the track. If the rpm is, for example, 10 percent below the power peak of the cam, then you will need to change the final drive gearing by 10 percent. If your cam grinder told you to rev the engine 10 percent above peak, that indicates the need for a total final drive change of 20 percent.
An example would be when your final drive ratio is 5.67:1 with an rpm of 7,000. Changing to a 6.80:1 ratio would increase the rpm to 8,400, a 20 percent change. The wheel rpm (the speed of the car) would be the same in both cases, and you would have moved the engine's power range to where you need it to be.
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|TURBO 350 TRANSMISSION |
|OE Gears |
|Low 2.52:1 ||Second 1.52:1 |
|TCI Automotive Gears (available) |
|Low 2.75:1 ||Second 1.57:1 |
|POWER GLIDE |
|OE Gears |
|1.82:1 (6-cyl) ||1.76:1 (V-8) |
|TCI Automotive Gears (available) |
1.76:1, 1.82:1, 1.89:1, 1.96:1, 2.03:1, 2.11:1
Street Stock racers, know your gears. TCI verified these ratios as available in the Turbo 350 and Powerglide transmissions.
Finding New RPM
RPMFinal Drive Ratio = Wheel rpm
To increase rpm while keeping the same wheel rpm:
Final drive ratio x 1 (percent increase) = New Ratio
New Ratio x Wheel rpm = New rpm
|7,000 ||= ||1235 || |
|5.67 || |
|5.67 ||x ||1.20 ||= ||6.80 |
|6.80 ||x ||1235 ||= ||8,400 |
This is the formula for finding how rpm and gear ratio relate to wheel speed (car speed). Using this formula, you can tailor your gear ratio to the needs of your cam.