DV: Other than having it all stay together, what can you share with us about bottom ends?
JM: Consider that every hp the engine will ever make happens above the top of the
piston. This means you don’t generally build any power with bottom ends, you just try and avoid losing more
than necessary. I don’t want to oversimplify things here because you can be bitten. There is a great deal
of pure science that goes into building the best bottom end. The difference between a good guy and the best
at the job, on something like a Pro Stock motor, could be approaching 100 hp. All the skill here is being
able to windage the crank for best results, cut piston and ring friction to a minimum, pull as much crank
case vacuum as possible and so on. In addition to this there are some important geometric considerations
such as rod–to–stroke ratio and its implications, pin offset, bore stroke ratios and so on. And here’s
another myth I want to blow into the weeds: For a given displacement, a long–stroke engine does not make
the torque more smoothly at low rpm, but that’s it. The argument that the longer stroke has a greater
leverage is offset by the fact that there is less push because the piston area is smaller. If anything,
the short–stroke motor is better at producing torque because there are less frictional losses due to
piston circumference than due to stroke length. I think too many car testers confuse low speed drivability
with torque.
DV: Your students get their fair share of dyno time, what is it you try to
impress on them to seek most?
JM: What it boils down to is this: We must get the highest average or greatest
area under the curve over the rpm band to be used. People like to debate torque versus horsepower where
they use torque to describe the bottom–end characteristics and hp the top end. If the engine has more
torque at 3000 rpm then it also has more hp at 3000 rpm.
I use torque as an indicator of cylinder pressure. More cylinder pressure directly
relates to more torque. On a street engine, if we use wide–open throttle from 3000 to 6000 rpm, we
must build to optimize in that range regardless of whether we quote it in horsepower or torque.
Torque per cube–being related directly to volumetric efficiency and compression–has
not increased nearly as much as power per cube in the last 20 years. This has come about because
today’s cylinder heads will breathe to far greater rpm. This means that though the torque produced
has only gone up marginally, the rpm at which that torque can be produced has gone up substantially.
In many forms of racing today the motors never go as low as the rpm at which peak torque is produced.
This being the case we need to be very concerned about the form of the power curve past peak power.
This is generally given little attention or even completely ignored. But consider this: an engine
that can hang on longer past peak power can also hang on a lower gear longer. This equates to greater
acceleration and that is what usually wins races.
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