Posted by T.O.O. on May 19, 1998 at 20:52:19:
The World According To T.O.O. # 1 What Is Horsepower, And Do I Need It?
Not having read any of the posted material today, I’ve decided
to select a starting place that’s always been a favorite ….Horsepower.
What is horsepower other than the formula HP= Torque x RPM / 5252?
Other than something to brag about having, it’s a totally useless term as far as I’m concerned (unless you’re talking steady state, like an airplane, steady cruise, etc.). I can’t count the number of programs I’ve been involved in that went backwards as we produced more horsepower. Torque is another issue, I suspect you’re thinking about now, and it is, however, perhaps not to the extent you may be thinking.
Any street car or race car is continuously accelerating or decelerating, and, assuming the brakes work and the handling is good, typically, the rate of acceleration is the way we judge the car’s performance and "power". As we’re talking about accelerating a given mass, torque is the most important component, assuming that we’re simply talking HP vs. Torque. If you can’t produce torque at the lower RPM ranges, it’ll take you all day to reach a "higher" RPM where you have torque (or HP.), and then you shift gears, and you’re back where you started.
If someone is discussing high HP normally aspirated engines of small displacement, I automatically think "RPM" when I hear the numbers, and for those of you unaccustomed to my rambling messages on this board, I do not like RPM. My definition of RPM is Ruins Peoples Motors, and I’ll bet you that you’ll not find any experienced engineer or engine builder who will dispute this notion. I don’t care if you’re discussing imports or domestics, the higher the operating RPM, the higher the stresses, and the sooner you’ll experience parts failures. Before you jump my ass, I think that there are many automobile manufacturers who have done really great jobs in design, material selection, and assembly quality to provide the public with cars with "sufficient" bottom end torque, and reliable upper RPM torque that’s outstanding, and the fact that many modify these engines to produce 2-300% more than the factory anticipated is further testament to the designed and built in quality. However, my point remains relative to RPM. Take two identical engines and run them each "loaded", with one at 4000 rpm and the other at 6000 rpm. Which one would you bet will fail first?
Every program I’ve participated in since 1969 has been designed to build engines that run on torque, and lower rpm than the competition’s engines. If you have torque, you simply gear the car accordingly, and you’ll match the competitions speed, and more importantly, you’ll be more likely to be running at the finish. We used to "recalibrate" the tachometers in our NASCAR, (old) INDY, and ProStock programs to read 1000 rpm high, so when other teams would "casually" look at the tattle tales after running (ProStock was a hoot, as the competition could watch the tach. on the on-board TV broadcasts), and they’d all go deep gear there cars and blow their engines. I damned near shut down some manufacturer’s teams in several racing series because the foundries couldn’t provide enough blocks to replace all the blown ones.
Let’s go to the mystery area that’s really what makes the car run….or accelerate. I call it Transient Response or Recovery Time. How long does it take for an engine to recover from being yanked down 2000rpm on a shift and accelerate back to the redline again….that’s the mysterious quality that all killer engines have, and it’s not something that will show up in "conventional" dyno testing. Quite the contrary, I’ve NEVER seen an engine that possessed this quality make big HP. We rarely even look at HP#’s when testing (I stopped that in ’77). The only "number" I’m concerned in is: How much time does it take this engine to accelerate a given "load" from point A to point B. If the rpm range you anticipate operating in is for instance from 5000 to 8000rpm, the combination that will pull the "known load" from the bottom to the top the quickest will ALWAYS be the engine to run regardless of HP. Several years back, one of our NASCAR programs was so dominant that everyone said we had at least 695 HP. to run the way we did, and I will admit that we did have some engines that approached that number, however, all the "big HP." engines required more than 1.5 sec. more time to accelerate the load from 6000 to 7600rpm than another "special" engine we’d built. The "special" engine ran almost 10 mph higher lap speeds and it never made more than 590 HP. on the dyno when we decided to "check".
Transient Response is going to become a major topic of discussion in my future writings, as will torque and HP(for what it’s worth). We’ll travel through cylinder heads, manifolds, cams, engine geometry , etc. and we will continuously look at how each affect the "big picture" which is "what happens when you nail it".
Next topic will be more generic: What ingredients are necessary to make "useful" power, and what is "useful" power. …………………………………T.O.O. …………………………………………………