Torque is conceived in the...
Torque is conceived in the engine shop, but it's born right here in the dyno cell. Finding torque is actually more challenging than finding horsepower. This is especially so with a normally aspirated engine, as there is only 14.7 psi of air pressure to work with under ideal conditions. This makes a 1 lb-ft gain more significant than a 1hp gain.
Just about everybody who has picked up a wrench knows what torque is. Also, just about every racer understands that the more horsepower the engine produces, the faster the car is likely to go. What is not so commonly understood is the relationship between torque and horsepower. While this article is about generating torque, we must first understand how it affects output.
If we leave off the word horse in horsepower, we are left with power. This refers to the engine's ability to do a certain amount of work in a certain time. But because we have left off the horse part, the engine's power output is without units. Doing this for a moment allows for an easy way to see how torque and power are related. If we talk in terms of power, then the power of an engine is simply torque multiplied by rpm. If the torque multiplied by the engine's test rpm comes to a bigger number, then regardless of units, the engine has more power.
Ultimately, it is rpm that breaks engines. This means the engine will make more power without having to turn more rpm if we can get more torque at any point in the rpm range. Also, an engine with more torque can be geared higher, which means that the mass of the engine's internals, plus clutch and flywheel, will have less negative effect on the rear wheel output.
This CP piston is intended...
This CP piston is intended to deliver a 12.5:1 compression ratio. It does so with a minimum of valve cutout depth, the lowest crown (for better flame front progression), the highest top-ring placement possible, and the closest quench clearance possible. In addition, it is thermal barrier coated. This all adds up to more power by virtue of more torque.
Let's assume that traction is not a problem. Then, for a given horsepower, a high-torque, lower-rpm engine will always outperform one with the same horsepower but less torque and more rpm. This is what makes torque important. But making more torque can often be more difficult than making horsepower. The reason is that unless a supercharger of some sort is used, we can only work with 14.7 psi of air pressure. Now, let's look at 10 critical factors that affect torque.
A compression ratio (CR) as high as either the fuel or the rules allow is a circle tracker's number-one friend. This is especially so if the engine is restricted by means of a small carb or restrictor plate. If 10:1 is the limit, make sure you have 10:1. If a flat-top piston and a cylinder head combustion chamber volume is called for, go for every bit of compression you can find. That includes making the valve cutouts as small as possible, having the piston come out of the top of the block (if allowed, net quench clearances of 0.030 inch are OK, but with super parts and a known combo, this can be cut to 0.024 inch-test your combo first). Move the top ring up as high as possible to cut the ring-land volume. You need to talk to your piston supplier about this, as the ring-land groove and the valve pockets can intersect if it all gets too close.
Most of the air entering a...
Most of the air entering a 23-degree Chevy intake port hugs the outside wall and exits through section A of the intake valve, shown in the graphic here. Only about 30 percent of the air enters the cylinder via section B. Because the greatest flow is via section A, there is a strong swirling action. In the case of this port, it's counterclockwise.
Rapid mixture motion just prior to ignition helps promote effective combustion. By being in rapid and random motion, the charge burns faster and more completely. We get mixture motion from two principle sources: intake-port-generated swirl, and squish caused by the close approach of the piston to the quench area of the head. These two sources of mixture motion enhance two important aspects of combustion. First, there is faster flame propagation. The unburned charge is exposed to the advancing flame front for a limited time, so it is heated less and therefore less prone to detonation.
Fortunately, almost every 23-degree Chevy head has intake ports that generate swirl, so that is not a factor in most cases. This leaves quench clearance as an issue. Most small-block Chevy engines have a static quench clearance of 0.055 to 0.065 inch. On a typical 400hp 350, cutting the quench clearance by 0.010 inch and holding the CR constant is worth about 3-4 lb-ft. This means decking the block (or using a skinny head gasket) to get the quench down to 0.030 inch is worth about 12 lb-ft. Be aware the engine needs less advance with a tight quench clearance.
Although this Dirt Super Stock...
Although this Dirt Super Stock engine was built to very restrictive rules, attention toward keeping the intake charge temperature down was worth 20-plus pound-feet.
How do we know when the combustion process is effective? A big clue here is that the engine makes its best power without the need for a lot of advance. Having stated that, it should be pointed out that leaded fuels typically have a longer ignition delay time than unleaded. A guideline here is that a 10:1 unleaded fuel engine with a moderate but not overly restricted carb should make its best power with about 32 degrees total. If the advance for best power drops under 30 degrees, then the combustion process is almost certainly really good.
The principle factors contributing to good combustion are as follows: a tight quench, good swirl, correct air/fuel mixture, optimally atomized fuel, good fuel distribution, a big energetic spark, and low combustion chamber exhaust residuals. There's more, but if these factors are attended to, you are well along the way to success.