December 8, 2011

It's so easy for enthusiasts to get excited about horsepower numbers, rpm levels, and sound, while skipping some very key factors of a well-designed engine. Some build requests are realistic while others are not even close. And it's actually just as important for the engine builder to clarify the needs of the customer, as a totally mismatched engine can easily be an embarrassment to the builder and to the driver/owner.

This "Power Principles" series will focus on how you, the engine buyer or home builder, can better assess your needs and compare them against your wants. Yes, we have needs. Yes, we have wants. Yes, there are sometimes ways to meet in the middle. As we cover some different scenarios and viewpoints, keep your mind focused on one statement: Build the engine for the application. Don't make the application fit the engine. This will help you get the engine that you need and it will also help your engine builder's (or your) reputation stay in the clouds. We all like win/win situations, right?

To get an idea of what you want in an engine, here are some key questions that the average engine builder will ask of a potential customer:

    1. What kind of vehicle do you have?
    2. Is this a street, street/strip, or race application?
    3. Do you have a horsepower number in mind?
    4. If you could pick where the horsepower would peak, where would it be?

As we discuss the different varieties of applications and try to focus on the associated implications, try to answer these questions as honestly and realistically as you can so that you can better clarify your own needs.

Vehicle Specifications

Regardless of what your horsepower desires may be, almost everything depends on what type of vehicle the engine is going into, as vehicle weight plays a huge role in how the engine will work within its confines. A very heavy car or truck requires far more torque to get it moving than a light one will. In most cases, it's best to pair a heavy vehicle up with a milder cam and an engine that has a horsepower/torque curve geared more toward getting (and keeping) a heavy vehicle moving. A very light car, by contrast, can get away with a lot more in terms of how "peaky" an engine is. The torque curve doesn't need to be so high or so broad, and the horsepower curve can be shifted to the right to gain more power at peak rpm.

Let's think about some extreme examples to demonstrate this.

You're looking to build an engine for your '77 Ford Thunderbird. (Don't laugh, it happens). This car has a curb weight of 4,808 pounds (Stop laughing). Now, as a means of demonstrating our point, would it be prudent to stick a 7,500-rpm 347 in this car?

On the flip side, you're looking to build an engine for your Cobra replica. At around 2,500 pounds, it's not necessary to design an engine with a flat line torque curve (although there really are no downsides to doing so) because it takes much less torque to move such a light car from a dead stop. You would be able to get away with a little more cam, a little larger cylinder head port volume, quicker ignition timing curves, and, ultimately, steeper horsepower/torque curves that maximize horsepower. It's just a lot easier to get 2,500 pounds moving than 4,800 pounds.

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The mode of operation (how you drive it) goes hand-in-hand with the vehicle type. A street-oriented vehicle, which the majority of readers probably have, needs to have more of a focus on driveability, lower maintenance, and just less drama in general. It's very easy to get carried away with large camshafts and large cylinder head volumes, but these items don't always lend themselves to trouble-free operation. Street cars spend a lot of time at cruising rpm. That larger cam with the cool, lopey idle can cause bucking while cruising, and if you've ever experienced this, it doesn't make for a fun cruising experience. You'll find yourself grabbing the next lower gear or speeding up to get the engine running at a higher rpm.

This is where horsepower and torque curves come into play. The horsepower curve and the torque curve define how an engine will operate in its environment, and camshaft design has a direct impact on the curves.

For street cams, lower duration numbers will help keep the horsepower curve shifted toward the left side of the graph, single- pattern profiles help cut down on overlap and the possibility of reversion, and wider LSAs will help with idle characteristics, engine vacuum, and broader torque curves. Does this mean that you will have a weak engine because the cam is smaller? Of course not. Building an engine with a focus on the intended purpose can often make a car feel and run quicker, big cam or not.

It's very hard to have your cake and eat it too when dealing with operating curves. If you prefer the rev-happy engines, then you have to be prepared to have a softer bottom end. If you prefer the low-rpm torque monsters, then you have to be prepared for the engine to run out of breath at a lower rpm ceiling.

For illustration purposes, look at these two horsepower/torque graphs:

As you can see, the higher that you move the horsepower peak, the less average horsepower and torque you get. The curves actually take more of a linear shape instead of producing a "fuller" curve shape. Without the low-rpm torque, a heavy car would feel slow and sluggish. You can also see on the graph that represents the "fuller" curves, the horsepower starts to drop off at a lower rpm. In the same manner, you can see how this situation wouldn't be beneficial for a race-oriented engine that needs to rev and make horsepower up higher. Both scenarios have their places. There's nothing wrong with either one of them as long as the engine fits the application.

Horsepower Goals

We all want big numbers. Big numbers sell engines. Big numbers are bragging rights. For those reasons, it's very hard to get people to focus on the application instead of the numbers. However, this is one of the criteria that many builders use to qualify the customers and set them up with the correct powerplant. Again it's easier to talk in extremes sometimes, so we'll offer one "extreme" example.

"I want an engine for my super-fly '77 Ford Thunderbird. It's a car that I'll be driving almost every day, back and forth to work. It's got the factory 302 and 3.00:1 rearend in it, and I'm wanting something around the 600hp range."

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OK, let's use some of our theory in practice. This is a very heavy car, with the intended purpose of driving in a daily commute. Not a race car, not a street/strip car, but a street car in every sense of the wordùair conditioning, fuzzy dice, the whole nine yards. Now, there's nothing inherently wrong with wanting 600 horsepower, but 600 streetable horsepower from a 302 is pretty unrealistic. Even getting 600 hp from a naturally-aspirated 347 would take some very keen engine building skills, high-compression ratios, big cylinder heads, and a nicely sized camshaft. Now, if the customer would be open to going with another engine platform, such as a stroked big-block, or even a moderately sized small-block with forced induction, reality looms a little closer. What's our motto? Build the engine for the application.

We haven't reiterated the "wants versus needs" argument lately, but this would be a great time to do so. We have wants. We have needs. Sometimes we need to compromise and it's very often that our needs are of more importance than our wants. Our Thunderbird would be one of those cases. Yes, it would be nice to have 500-600 hp, but the customer needs torque, throttle response, and a usable powerband. They don't always realize that or what it means until they drop a high-strung small-block between the framerails and wonder why it is sluggish.