Questions and Answers

Engine Power Versus Fuel Efficiency Q. Vince, I am about to buy a: 2002 Subaru Legacy Wagon 2.5 liter F4 Manual 5 speed roughly 40,000 km Fuel Injection ABS Power everything, AC, Cruise... Loaded

I would like to improve the engine's power while at the same time improving fuel efficiency. Is this possible? I understand how factors such as tire pressure, driving style, use of air conditioner and fan, and open windows on the highway affect fuel efficiency, but I would like to make actual physical changes to the car so that it can be more efficient.

Is it necessarily true that for a given driving style, acceleration rate and cruising speed, an engine that has been modified for power will be more efficient?

To ask the same question a different way: Will power modifications always result in better fuel efficiency when a car is driven "normally"? I am also wondering what are the most common/easiest/least expensive ways to accomplish this goal and which modifications you would favor.

I have been searching for a source of information relating to efficiency modifications but I have yet to find anything that I can use to determine how effective a given modification will be.

This would be useful so that I could figure out, based on cost and benefit, a logical place to start. Also, how far can I go before my car is considered to be "modified" and subject to outrageous insurance rates? I live in Canada, but I assume American regulations in this area would be similar.

I hope that's not too many questions, but I don't really want to ask an aftermarket parts dealer or anyone else who may be trying to sell me something. One final request: Could you please email your response to me after you have posted it on the site?

Thank you,
Kelly

A. These are questions that will take a months worth of answering. But I'll do my best in the space allotted.

First off, if you are looking for something to add to the fuel system or air intake or ignition system that will give you more power and better fuel economy, you will have a long, hard search. Every such product I have tested has failed to live up to their promises, or was not worth the expense versus fuel saved.

In other words if you took every fuel saving, power enhancing product and installed them on you vehicle, you might gain one or two horsepower and one or two mpg.

What people don't understand that modern engines are designed to get the maximum power and fuel efficiency they can. If the air swirlers and fuel ionizers were such a great product, all the car makers would be installing them at the factory. I have done tests on some of these and you can see the results in the Product Reviews section of this web site.

Another thing to remember is that generally power has a cost, and that cost is fuel. So if power goes up, mpg goes down. As my Grandfather used to tell me, "There is no free lunch", everything has a price.

Driving a car "normally" is subjective. I would consider the way I drive normal but you may not. And vice versa. And the guy next door his habits normal and ours are not. In other words, if you ask ten people what normal driving is, you'll get ten different answers.

As for what defines "modified", that depends. To me any change to the engine or vehicle in general would be considered modified. Especially if some one wants me to fix it. As far as Insurance Companies are concerned, I've seen one that wanted more information that Year, make and model. The only exception I know of is in the case of conversion vans.

Now if the insurance company is looking for any reason to raise your rates, as Prudential Insurance tried to do with me, they will consider new floor mats as a modification. So it is up to your particular insurance to define "modifications".

In short, in order to get the most power and fuel economy from your vehicle, use common sense when you drive, keep the well tuned, the tires at the proper pressures and read Gas-Saving Products: Facts or Fuelishness? and Gas Saving Tips and Suggestions.