Sharp Yanks and Steady Pulls
I was recently talking to Car & Driver's Kevin Smith about Honda's hot 1993 Prelude, the one with the 190-horsepower VTEC engine. The new engine displaces 2157cc, and uses Honda's variable valve-timing system to allow it to make power all the way to its 7400-rpm redline and beyond. It's roughly 30-horsepower stronger than the engine in last year's Prelude Si.
But, Kevin complained, it doesn't feel all that fast. After all, with a 20 percent increase in power, why doesn't it accelerate a lot harder than the old car? Car & Driver's numbers show only a 0.2 second improvement in quarter-mile time, and only a 2-mph improvement in quarter-mile terminal speed. What gives?
Well, the interesting thing about the VTEC Prelude engine is that it actually displaces 98cc -- or about 5 percent -- less than the engine it replaces. Assuming equal BMEPs in both engines -- a very kind assumption, given that the higher-revving VTEC powerplant certainly has more friction -- that means it should make about 5 percent less torque. It makes its greater power through greater revs, delivering more smaller torque impulses in a given time because the engine is spinning faster.
But unfortunately, it's only going to spin faster in the car if Honda shortened the Prelude VTEC's gearing, and that is something the company barely did. The final drive ratio remains the same, as does the first gear ratio. Second through fifth have been shortened slightly, but only by about 7 percent. So the gearing reduction barely compensates for the displacement decrease, especially considering the VTEC 'lude is 2.5 percent heavier than the Si.
If you plot out thrust curves for the two cars, the VTEC version will deliver more push to the rear wheels on average than the Si because it can stay in a lower gear longer, due to its higher redline -- but the effect is much less than it would have been if the gearing had been shortened more. So, interestingly, by going for power through rpm, Honda has barely enhanced the VTEC's acceleration capabilities. It, has however, made the VTEC faster; its top speed, according to C&D, is 135 mph, or about 6 mph faster than that of the Prelude Si. That's close to the 8 mph increase that simple physics would predict for two cars with the same aerodynamic drag and these power levels.
If Honda really wants to make the VTEC Prelude a real road weapon, it's going to have to install shorter gearing, and a sixth gearbox speed to allow peaceful highway cruising.