Basic modifications for newbies
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The problem with this approach is that you're starting at the wrong end of the vehicle. Unless you're starting with a scratch build, you're probably modifying an OEM vehicle which was engineered at the factory to provide good gas mileage (numerically low ring and pinion, (probably something in the 2.70 to 3.00 range) and a tight torque converter (maybe somewhere around 1,200-1,400 rpm stall). | The problem with this approach is that you're starting at the wrong end of the vehicle. Unless you're starting with a scratch build, you're probably modifying an OEM vehicle which was engineered at the factory to provide good gas mileage (numerically low ring and pinion, (probably something in the 2.70 to 3.00 range) and a tight torque converter (maybe somewhere around 1,200-1,400 rpm stall). | ||
| − | This vehicle was designed for all sorts of people, the vast majority of whom expect good fuel mileage and quiet operation. The cam, intake manifold and other parts that came stock in the vehicle were matched to the ring | + | This vehicle was designed for all sorts of people, the vast majority of whom expect good fuel mileage and quiet operation. The cam, intake manifold and other parts that came stock in the vehicle were matched to the ring and pinion gears in the rearend and the stall characteristics of the torque converter to accomplish this goal. When you begin changing parts on/in the motor, you are upsetting this balance of parts and operational characteristics that were built into the vehicle at the factory. |
The first things that a newbie usually changes are the cam, intake manifold and carburetor (if working on a pre-efi motor) in an effort to make the ol' hoss a world beater. | The first things that a newbie usually changes are the cam, intake manifold and carburetor (if working on a pre-efi motor) in an effort to make the ol' hoss a world beater. | ||
| − | First things first. The cam must be matched to the static compression ratio of the motor. The OEM's have super computers that tell them exactly the timing points to be ground into the cam to match the c.r. and make power at the rpm's the public expects. This is usually idle to around 4,000 rpm's or a little higher. Any cam that you bolt into the motor will have an operating range of roughly 3,500 rpm's. In other words, it will be efficient from idle to 4,000 or 1,000 to 4,500 or 2,000 to 5,500 or 3,500 to 7,000 or whatever, depending on the valve opening and closing points ground into the cam when it is manufactured. It will also have a wide Lobe Separation Angle (max lift intake point after top dead center added to max lift exhaust point before top dead center and divided by 2) for good manifold vacuum to properly operate power brakes and other vacuum operated accessories and contribute to a smooth idle (Grandma doesn't want the motor going RUMPETY-RUMP when she's on the way to bingo). An OEM cam might be measured at 114 to 118 degrees LSA for instance. You can figure the Lobe Separation Angle yourself. Let's say that the intake centerline is 106 degrees after top dead center and the exhaust centerline is 114 degrees before top dead center. Add 106 to 114 and divide by 2 to find a Lobe Separation Angle of 110 degrees. | + | First things first. The cam must be matched to the static compression ratio of the motor. The most important timing event on a camshaft is considered by most knowledgable people to be the intake valve closing point. The intake valve begins to open at some point before the piston gets to top dead center on the exhaust stroke. The piston decends in the bore with the intake valve open and the cylinder is filled with air/fuel mixture. After the piston gets to bottom dead center and starts back up on the compression stroke, the intake valve closes and compression begins. If we are beginning with a stock motor on which the static compression ratio is somewhere around 8.5:1 and put a cam in that has more duration and closes the intake valve later than was designed for at the factory, the piston will push some of the air/fuel mixture back through the still-open intake valve and cylinder pressure will be lower than it was with the stock cam that closed the intake valve at exactly the right point to work with the stock static compression ratio. This is the #1 mistake made by rookies, believing that changing the camshaft alone will make more horsepower. If you're going with more cam, then raise the static compression ratio also, so that the cam and static compression ratio are matched. The OEM's have super computers that tell them exactly the timing points to be ground into the cam to match the s.c.r. and make power at the rpm's the general public expects. This is usually idle to around 4,000 rpm's or a little higher. Any cam that you bolt into the motor will have an operating range of roughly 3,500 rpm's. In other words, it will be efficient from idle to 4,000 or 1,000 to 4,500 or 2,000 to 5,500 or 3,500 to 7,000 or whatever, depending on the valve opening and closing points ground into the cam when it is manufactured. It will also have a wide Lobe Separation Angle (max lift intake point after top dead center added to max lift exhaust point before top dead center and divided by 2) for good manifold vacuum to properly operate power brakes and other vacuum operated accessories and contribute to a smooth idle (Grandma doesn't want the motor going RUMPETY-RUMP when she's on the way to bingo and there are a lot more little old ladies buying cars than there are hot rodders). An OEM cam might be measured at 114 to 118 degrees lobe separtation angle (LSA) for instance. You can figure the Lobe Separation Angle yourself. Let's say that the intake centerline is 106 degrees after top dead center and the exhaust centerline is 114 degrees before top dead center. Add 106 to 114 and divide by 2 to find a Lobe Separation Angle of 110 degrees. You might also see LSA expressed as lobe displacement angle. This is not to be confused with lobe center. Lobe center is the point of maximum lift of the lobe, after top dead center for the intake lobe and before top dead center for the exhaust lobe. |
Now, the newbie comes along and decides that the motor needs more cam. In most cases, he has no idea what the static compression ratio of the motor is or the piston deck height or the squish clearance or anything else about the interior of the motor. All he knows is that he wants the RUMPETY-RUMP that he heard coming from the Super Comp motor he heard at the drag strip or the hot rod down at the Sonic Drive In. What he may not know is that the motor in that Super Comp car has upwards of fifteen to twenty thousand dollars invested in it and is maximized for racing. It idles like that because the cam has to be very aggressive to work with the 12.0:1 to 16.0:1 static compression ratio that is built into the motor. It may have been designed to make power from 4,500 to 8,000 rpm's for instance and will be coupled to a very loose torque converter that stalls at around 5,000 rpm's for instance. As far as the hot rod down at the Sonic, that owner may have used a cam such as a Thumpr to get that sound and the car may not be as fast as it sounds. | Now, the newbie comes along and decides that the motor needs more cam. In most cases, he has no idea what the static compression ratio of the motor is or the piston deck height or the squish clearance or anything else about the interior of the motor. All he knows is that he wants the RUMPETY-RUMP that he heard coming from the Super Comp motor he heard at the drag strip or the hot rod down at the Sonic Drive In. What he may not know is that the motor in that Super Comp car has upwards of fifteen to twenty thousand dollars invested in it and is maximized for racing. It idles like that because the cam has to be very aggressive to work with the 12.0:1 to 16.0:1 static compression ratio that is built into the motor. It may have been designed to make power from 4,500 to 8,000 rpm's for instance and will be coupled to a very loose torque converter that stalls at around 5,000 rpm's for instance. As far as the hot rod down at the Sonic, that owner may have used a cam such as a Thumpr to get that sound and the car may not be as fast as it sounds. | ||
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