Now to everyones favorite part of a build, the engine. At this point you should have made some decisions about the performance goals of the vehicle. Taking those goals into account the suspension, steering, and brake system needs should have been identified. Now it’s time to identify the engine, transmission, and differential needs.
The first thing you need to do is determine the benchmark for performance you will use. Ultimately, the most accurate benchmark is track use. At a racetrack, performance levels may be very accurately measured and evaluated. On-track performance is not only a measure of how powerful an engine is but also of how well thought out and executed the vehicle build is. I believe putting a vehicle on a race track is a much more accurate indicator of engine performance than a dyno is, especially a wheel dyno that calculates HP and TQ at the wheels.
Now I’m sure that a lot of people will disagree with this statement, but there is truth in it. Many people seem to think that dyno numbers are the ultimate reflection of what an engine can accomplish. Big dyno numbers are regularly tossed around and taken as gospel. The fact is, dyno numbers are easily manipulated, and that more than one dyno sheet is inaccurate because of this. Sometimes these are just reflections of poor calibration, in other cases this is due to poor operator operation. The worst is deliberate, where a dyno operator manipulates the dyno in order to inflate the output numbers. A dishonest operator may do this in order to support their personal claims.
Another issue with dyno numbers is that the average enthusiast is looking primarily at the peak numbers the engine is making. Do not pray to the false idol of peak HP and TQ. Engines with lower peak numbers, but higher average numbers will outperform an engine with the opposite dyno numbers in virtually all real world applications.
I have worked on many vehicles that ran numbers at the racetrack that were substantially slower than what the dyno sheet indicated they should run. Often, by re-tuning the vehicle I can make the vehicle perform significantly better at the track, even though we may be sacrificing some peak numbers.
I have also worked on many vehicles that the results on the dyno sheet could not be duplicated. In one situation two other wheel dynos showed about 2% deviation from each other, but almost 18% less than the original dyno sheet. That translated into about a 65 HP difference in peak numbers at the wheels.
Now a dyno is a tool, and when used effectively, it’s a good tool. But when building a car, I believe it’s better to build for a certain race track goal. For example, let’s assume we are working on a 60’s vintage musclecar. The owner has no intentions of racing it but wants it to perform very well. He decides that a vehicle that will run 13.0 seconds in the quarter mile is what he wants. The HP numbers required to make the vehicle achieve that level of performance are calculated, and engine components are chosen based on that number.
Ultimately, he may not be able to claim the high HP and TQ numbers some enthusiasts brag about, but chances are pretty fair that his car will outperform theirs. You can always tell the owner of a car like that. He’s the one grinning from ear to ear.
(In Part IV I will share some information on choosing a realistic performance goal based on drag strip performance.)
