Full Version: Dyno numbers explained
It would be great to start this with a explanation of all the info a dyno run, printout produces, what bsfc is, whats trail safe, how to tell a bogus, make the guy feel good run, exc. maybe pin it at the top for reference.
Need to know water temps and keep constant between runs,how fast the dyno run is made(acceleration rate).Need to compare runs so not to be blinded by a peak hp #.An EFI sled will dyno differently than carbed due to throttle position and loads.
most pertinant data is surely CHP (HP corrected to 60 degree F sea level dry air) and the RPM the HP peak occurs at. It's good to know length of time for the dyno test so you can interpolate where the HP peak might be on a shorter or longer field run, since peak HP RPM varies with pipe internal gas temp (call it pipe center section temp).
CBT (torque corrected like CHP) is confusing, since torque really does nothing. Some people are confused by torque, thinking that clutches should shift at peak torque. Since torque peaks often occur several hundred revs lower than HP peak on hot dyno pipes, what we're really doing is shifting (correctly) at cool pipe HP peak a few 100 revs lower than the dyno sheet indicates.
Fuel flow is a critical component of good dyno data. When we divide the fuel flow lb/hr by the actual observed (not corrected) HP we get Brake (from brake HP) Specific Fuel Consumption in lb/hphr. If you look on my DTR archives you will se that over the years my own assessment of what is a race BSFC and what is 85 octane good for long runs has changed over 20 years. That change in assessment has been the result of great improvements in sled engine design, efi, deto protection and engine cooling (see my free blog on combustion chamber cooling http://www.dynotechresearch.com/blog/default.asp). .80 is pig fat and .50 is wickedly powerfully lean. Todays stockers are often .60 or even leaner at WOT, a full tenth leaner and hence more powerful per pound of fuel consumed than the hot sleds of a decade ago.
A/F ratio is another important issue--10/1 is rich, on the verge of rich misfire, and 17/1 is the extreme opposite on the verge of lean misfire. Most sleds today make best HP at 13/1 but are safer at 11/1, cruise A/F ratio is fine at 13/1 for low emissions and good fuel economy.
No way to really tell when you've been handed a happy sheet. Computers can tweek results as the dyno operator desires. I have several dyno detective Jim stories on my website which are good fun but sad in a way.
Brake Specific Air Consuption is one we dont spend enough time with. My pal Greg Santry has a Superflow dyno in his home garage for tuning his own sleds, and he likes BSAC numbers to see how engines are using their airflow (trapping air and making HP or blowing unburned charge out the pipes?).
All of this is understandable after doing it for 20 years, and I often suggest to first time dyno users that they go through the DTR archives to see where all of this fits in to the current dyno tuning we do.
CBT (torque corrected like CHP) is confusing, since torque really does nothing. Some people are confused by torque, thinking that clutches should shift at peak torque. Since torque peaks often occur several hundred revs lower than HP peak on hot dyno pipes, what we're really doing is shifting (correctly) at cool pipe HP peak a few 100 revs lower than the dyno sheet indicates.
Fuel flow is a critical component of good dyno data. When we divide the fuel flow lb/hr by the actual observed (not corrected) HP we get Brake (from brake HP) Specific Fuel Consumption in lb/hphr. If you look on my DTR archives you will se that over the years my own assessment of what is a race BSFC and what is 85 octane good for long runs has changed over 20 years. That change in assessment has been the result of great improvements in sled engine design, efi, deto protection and engine cooling (see my free blog on combustion chamber cooling http://www.dynotechresearch.com/blog/default.asp). .80 is pig fat and .50 is wickedly powerfully lean. Todays stockers are often .60 or even leaner at WOT, a full tenth leaner and hence more powerful per pound of fuel consumed than the hot sleds of a decade ago.
A/F ratio is another important issue--10/1 is rich, on the verge of rich misfire, and 17/1 is the extreme opposite on the verge of lean misfire. Most sleds today make best HP at 13/1 but are safer at 11/1, cruise A/F ratio is fine at 13/1 for low emissions and good fuel economy.
No way to really tell when you've been handed a happy sheet. Computers can tweek results as the dyno operator desires. I have several dyno detective Jim stories on my website which are good fun but sad in a way.
Brake Specific Air Consuption is one we dont spend enough time with. My pal Greg Santry has a Superflow dyno in his home garage for tuning his own sleds, and he likes BSAC numbers to see how engines are using their airflow (trapping air and making HP or blowing unburned charge out the pipes?).
All of this is understandable after doing it for 20 years, and I often suggest to first time dyno users that they go through the DTR archives to see where all of this fits in to the current dyno tuning we do.
It’s good to have you back Jim. 
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