What Simulation and Dyno Numbers Mean

Many of you get the simulation curves and data I email to you, and while we most often discuss Torque and Horsepower, there’s a lot of additional data that you have been asking me about.  This document is an aid to help you understand what the numbers mean and how they relate.

Horsepower and Torque:
We all talk horsepower, but did you know that dynos ONLY measure torque!  Torque is the “twist”, but horsepower is a calculated number based upon the produced torque at a given RPM: (horsepower = rpm x torque / 5252).  Since HP is derived mathematically, based on the torque produced at a given RPM, HP and TQ curves always cross at 5252 RPM because of this relation to time.  If you see HP and TQ curves that don’t cross at 5252, you know you’re looking at a bogus set of curves.  So why does a huge diesel engine, that produces 800+ ft lbs of torque, have only about 300 HP?  It’s because it produced that torque at a low 1900 RPM.  So a drag race engine that also produces 800 ft lbs of torque, but does it at 6000 RPM, is producing more than 900 horsepower!  If YOU had to produce the twist with a lever, wouldn’t it be harder to maintain that twist as the rate of spin increased?

Air/Fuel Ratio:
For a gasoline motor the A/F ratios should range between 10:1 and 15:1.  Usually the most horsepower is created at about 12.5:1.  You can run an engine lean during a light load cruise to get better mileage.  I do this often with EFI motors where I have the ability to control the EFI system.  If the motor has enough torque to keep the car at a steady cruise, I have run as high as 16:1.  Idle is typically in the mid 13 to 14:1 range.

BSFC (Brake Specific Fuel Consumption):
This is simply a measure of how much fuel per hour an engine uses.  A typical, normally aspirated V8 will consume around .5 lbs/HP/HR.  The lower the number the better.  NASCAR motors run about .43, and Pro Stock drag motors run about .4.  But this is NOT an indication of rich/lean conditions.  It’s merely a measure of how efficiently an engine is consuming fuel for a given power level.

VE (Volumetric Efficiency):
This is a ratio, expressed as a percentage, of the actual volume or air drawn into a cylinder (and therefore available for combustion), vs. the maximum theoretical amount possible.  You would think that this could never approach 100%, but in reality many well-tuned normally aspirated engines come very close to 100%, and supercharged engines commonly produce numbers above 125%.  CAM timing, the amount of cam overlap, as well as intake and exhaust tuning are critical factors.

Spark Timing:
The curves I send out include the predicted spark advance at various RPMs; but remember, this is at full throttle.  An asterisk (*) indicates that the spark was retarded to this value to avoid detonation.  These numbers help you understand what the theoretical timing curve should be for a certain engine.  Some engines require a “reverse curve”; such as a supercharged engine with a mild cam and little overlap.  This is a case where combustion pressures will be quite high and spark timing may have to be retarded rather than advanced.

Atmospheric Conditions:
All simulations are done assuming a STP - Standard Temperature and Pressure.  But the dyno is real world under real atmospheric conditions that never match the STP standard.  The dyno applies a correction factor to adjust for changes in atmospheric conditions but it’s not perfect.  An engine will produce different power levels on different days and even back to back runs may be off by several percent due to heating effects. The message is: “Don’t get hung up on a certain HP number”.  The best you can do with a dyno is look for deltas, trends, and the effects of changes, but try to give the total HP and TQ values lower priority.  The BSFC, A/F Ratio, and VE are actually better indicators of how well tuned or efficient an engine is.