Basic modifications for newbies
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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. | 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. | ||
| − | 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. What he may not know is that the motor in that 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 | + | 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. What he may not know is that the motor in that 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. |
I'm just throwing these numbers around to show you that the cam in the Super Comp motor will not work in your street-driven 350 Chevy. | I'm just throwing these numbers around to show you that the cam in the Super Comp motor will not work in your street-driven 350 Chevy. | ||
I've gotten a little off track with my explanation. We need to go back to what a newbie should do to his vehicle FIRST. | I've gotten a little off track with my explanation. We need to go back to what a newbie should do to his vehicle FIRST. | ||
| − | The very first modification done to an otherwise stock auto/truck should be a different ring and pinion gear. A good compromise between mileage and acceleration in a street car is somewhere around 3.70:1 ratio and should be complemented with new shocks and new bushings in the spring/suspension link mountings. If the owner intends to change this vehicle into a semi-serious street/drag car, then additional aftermarket traction devices such as anti-wheel hop products and tires with different rubber compound should be considered. Even more serious competitors should consider mini-tubs or full tubs in the car to | + | The very first modification done to an otherwise stock auto/truck should be a different ring and pinion gear. A good compromise between mileage and acceleration in a street car is somewhere around 3.70:1 ratio and should be complemented with new shocks and new bushings in the spring/suspension link mountings. If the owner intends to change this vehicle into a semi-serious street/drag car, then additional aftermarket traction devices such as anti-wheel hop products and tires with different rubber compound should be considered. Even more serious competitors should consider mini-tubs or full tubs in the car to accommodate wide racing slicks. A large number of racers who show up at my track mount slicks on separate wheels (wide steel wheels will work just fine) to be bolted to the car after they drive it in off the street. |
See this article I wrote to understand why and how a shorter rear gear will accelerate the car quicker with no changes to the motor, transmission or torque converter. | See this article I wrote to understand why and how a shorter rear gear will accelerate the car quicker with no changes to the motor, transmission or torque converter. | ||
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Of course, when it comes to traction, some type of slip-limiting device like the Chevrolet Posi-Traction or aftermarket Detroit Locker or Auburn units or similar units will do an excellent job of hooking up both tires. However, these are not mandatory to get both tires to pull and not spin the passenger side tire. | Of course, when it comes to traction, some type of slip-limiting device like the Chevrolet Posi-Traction or aftermarket Detroit Locker or Auburn units or similar units will do an excellent job of hooking up both tires. However, these are not mandatory to get both tires to pull and not spin the passenger side tire. | ||
| − | With a front-motor, rear-drive vehicle, the chassis twists diagonally upon application of power. The left front gets lighter and the right rear gets lighter. The right front and the left rear get heavier. This is why you will see the right rear tire spin while the left rear hooks up on a car with a "one-legger" or "open" type differential. The right rear is light and needs additional weight applied to it. This can be accomplished cheaply and easily by installing an air shock on the RIGHT REAR ONLY to replace the conventional shock absorber on that side. Experimenting with the air pressure in the shock will allow you to equalize the weight applied to both rear tires on acceleration and "hook up" both tires without going to the expense of installing a locking device in the differential. Just keep adding air pressure to the shock until you have two equal-length black tire stripes on the pavement when | + | With a front-motor, rear-drive vehicle, the chassis twists diagonally upon application of power. The left front gets lighter and the right rear gets lighter. The right front and the left rear get heavier. This is why you will see the right rear tire spin while the left rear hooks up on a car with a "one-legger" or "open" type differential. The right rear is light and needs additional weight applied to it. This can be accomplished cheaply and easily by installing an air shock on the RIGHT REAR ONLY to replace the conventional shock absorber on that side. Experimenting with the air pressure in the shock will allow you to equalize the weight applied to both rear tires on acceleration and "hook up" both tires without going to the expense of installing a locking device in the differential. Just keep adding air pressure to the shock until you have two equal-length black tire stripes on the pavement when accelerating from a stand-still. I've done this many times and have seen other racers do it with equal success. An additional benefit is that the car will be easier to drive at the strip with an open differential that has been "weight equalized". |
Now, with the rear end nailed down and operating, it's time to move on to the middle of the car and take a look at the transmission and torque converter, assuming an automatic transmission will be used. If the transmission is a manual shift, then overlook the following information and go stiffer on the rear gear (4.10, 4.44, etc.) | Now, with the rear end nailed down and operating, it's time to move on to the middle of the car and take a look at the transmission and torque converter, assuming an automatic transmission will be used. If the transmission is a manual shift, then overlook the following information and go stiffer on the rear gear (4.10, 4.44, etc.) | ||
| − | If the car is very light (under 2,800 lbs with driver aboard), then a 2-speed automatic will work fine on the | + | If the car is very light (under 2,800 lbs with driver aboard), then a 2-speed automatic will work fine on the drag strip and should work on the street as well. Heavier cars should use a 3 or 4-speed auto. If fuel mileage is a concern in your world-beater, then a 4-speed overdrive auto is probably the better choice. I'll leave it to someone else to go into detail about the choices here, but the GM 700R4 has shown to be a good choice for non-computer applications. They can be beefed up to take considerable abuse. |
The next component to be addressed is the torque converter. It is mandatory that the converter is matched to the camshaft you'll be using. As was stated earlier in this article, stock converters will stall at around 1,200 to 1,400 rpm's, depending on the amount of torque produced by the motor among other things and that's fine when you are using a stock-type cam that begins making power at idle. But when you change the cam out for a longer duration/higher lift model, you're no longer making power from idle and the car will be a dog until the rpm's increase to the point where the motor is making power. One way around this is to install a converter that stalls higher than stock so that the motor comes up on the cam quicker and the car accelerates faster. Any cam you install in the motor will have an operating range of about 3,500 rpm's. In other words, it might make power from idle to around 4,200 rpm's (stock cam) or from 1,500 to 5,000 rpm's or 2,000 to 5,500 or 2,500 to 6,000 or 3,000 to 6,500 or 3,500 to 7,000. I'm sure you get the idea. It works in a certain "window" of operation and is inefficient below and above those rpm's. So, if you install a cam that operates between 3,500 and 7,000 rpm's and use a converter that stalls at 1,200, you can see that the motor will not be producing enough torque from 1,200 to 3,500 to move the car efficiently. The car will be a D-O-G. On the other hand, if you use the 3,500-7,000 cam with a converter that stalls at around 3,500, then when you nail the loud pedal, the motor will rev up close to the stall speed of the converter and you'll be making power and applying torque to the rear tires sooner. (PLEASE NOTE THAT EACH APPLICATION IS DIFFERENT AND THAT A PROFESSIONAL TORQUE CONVERTER MANUFACTURER SHOULD BE CONSULTED FOR THE EXACT CONVERTER FOR YOUR APPLICATION). If I could get a little plug in here, I know Jim Hughes personally and have for many years and I can, without question, recommend Hughes Converters. Jim is a man of integrity, both personally and professionally. | The next component to be addressed is the torque converter. It is mandatory that the converter is matched to the camshaft you'll be using. As was stated earlier in this article, stock converters will stall at around 1,200 to 1,400 rpm's, depending on the amount of torque produced by the motor among other things and that's fine when you are using a stock-type cam that begins making power at idle. But when you change the cam out for a longer duration/higher lift model, you're no longer making power from idle and the car will be a dog until the rpm's increase to the point where the motor is making power. One way around this is to install a converter that stalls higher than stock so that the motor comes up on the cam quicker and the car accelerates faster. Any cam you install in the motor will have an operating range of about 3,500 rpm's. In other words, it might make power from idle to around 4,200 rpm's (stock cam) or from 1,500 to 5,000 rpm's or 2,000 to 5,500 or 2,500 to 6,000 or 3,000 to 6,500 or 3,500 to 7,000. I'm sure you get the idea. It works in a certain "window" of operation and is inefficient below and above those rpm's. So, if you install a cam that operates between 3,500 and 7,000 rpm's and use a converter that stalls at 1,200, you can see that the motor will not be producing enough torque from 1,200 to 3,500 to move the car efficiently. The car will be a D-O-G. On the other hand, if you use the 3,500-7,000 cam with a converter that stalls at around 3,500, then when you nail the loud pedal, the motor will rev up close to the stall speed of the converter and you'll be making power and applying torque to the rear tires sooner. (PLEASE NOTE THAT EACH APPLICATION IS DIFFERENT AND THAT A PROFESSIONAL TORQUE CONVERTER MANUFACTURER SHOULD BE CONSULTED FOR THE EXACT CONVERTER FOR YOUR APPLICATION). If I could get a little plug in here, I know Jim Hughes personally and have for many years and I can, without question, recommend Hughes Converters. Jim is a man of integrity, both personally and professionally. | ||
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