Dyno Terminology—Fresh and Refresh

 

The new SuperFlow 902 software has some subtle differences in channel headings in our test data compared to our original 25 year old  SuperFlow 901 software. For long term DTR members I need to clarify the difference, and for new DTR members I need to explain the channel information in our dyno printouts, and what each channel means to us.

 

EngSpd RPM self-explanatory—this is RPM, and on engines with digital tachs, the readings have proven to be identical from dyno to sled. But analog sled tachometers are another story—vast differences can be seen from digital dyno RPM to analog sled tachometer readings. So it pays to have analog sled tachometers calibrated electronically (Aaen does this for a small fee) or with a digital tach like those sold by Stihl (less than $100) for tuning chainsaws. Those are perfectly accurate. But keep in mind that the peak HP RPM on our dyno tests is with pipe(s) extremely hot. And since the speed of sound increases with temperature, the HP peak in the field may be at lower RPM (trail riders on-off-on throttle) or at higher RPM (lake runners or mountain climbers who might spend minutes at WOT). Sled dragracers used to think that they achieved best acceleration by clutching to the “peak torque RPM” which often occurs 2-400 rpm lower than dyno test HP peak. But now we know that we clutch to a shifting HP peak—cool pipes on takeoff will make peak HP several hundred RPM lower than hot dyno test pipes, and finding that constantly changing peak HP RPM is critical in creating best acceleration.

 

STPPwr CHp horsepower, corrected to 60 degrees sea level baro 29.92 in hg.
This is derived by taking RPM x torque lb/ft divided by 5252. Complicated initially but it takes into consideration the twisting force and how quickly it can be done (RPM). HP is what we need to create acceleration and maintain top speed. Read on:

 

STPTrq Clb-ft this is the twisting force, or torque that the crankshaft is exerting at any given RPM, corrected to 60 degrees sea level barometer 29.92 in hg. But by itself, torque does no work—it takes speed—lots of speed—to do meaningful “work” (HP). To emphasize that, I sometimes talk about me (200 lb Jim) riding my Schwinn bicycle with 12” long pedal cranks. With my 300 lb girlfriend riding on my shoulders (facing forward of course), by grunting and lifting my butt off the bike seat I can make 500 lb/ft of torque at the pedal crank! That kind of torque should create wicked acceleration, shouldn’t it? But my spindly legs can only muster 3 RPM! So based upon the math RPM x torque lb/ft/ 5252 = HP that means that I can make about ¼ HP. Not bad for a 62 year old. But if I were instead twisting the throttle on a Honda 90cc scooter with 8 lb/ft at 5000 rpm I would be making 7.5HP—accelerating much harder—even with my 300 lb honey riding with me! No contest. It takes HP to create acceleration. We need a combo of torque and lots revs to do work—to make HP. 

 

FuelAB- fuel flow into the engine in pounds per hour. This is a combination of the two dyno fuel flowmeters (flowmeter A and flowmeter B), measuring fuel flow from the sleds’ EFI pump to the rail, or from the mechanical pump to the carbs. Flowmeters can be used individually to measure N2O flow separately from carbs/ EFI. But combining the two flow readings is necessary for computing BSFC.

 

FulA_B- fuel flow from EFI pump to rail (A) minus fuel flow from bypass regulator back to the tank (B). Can’t use a – so we must use a _ instead. Polaris uses this type of system.

 

BSFCAB and BSFA_B- pounds of fuel per HP per hour. 25 years ago when I bought this dyno system there was no manual to explain the significance of this number. Early adventuresome guinea pigs (including all of the current eastern US aftermarket sled modifiers) and I proceeded to just tune engines blindly to max HP and beyond until engines seized. But after our 100^th piston or so, we realized that there was a pattern to this destruction if we went too far beyond max HP.  Fuel flow is meaningful information and we finally understood that our race engines would make best power at @.55 lb/hphr and pump gas engines needed @.70 lb/hphr to be reliable. Don’t go lower than that! But today, with modern race engine configuration two stroke race engines can make best power closer to .50 lb/hphr and new stock engines can be completely reliable at .60 lb/hphr and even lower! SkiDoo ETEC EFI systems allow way less short-circuiting, and a greater percentage of fuel flow contributes to HP, and not blowing out the exhaust thus reducing BSFC to bizarrely low numbers.  New cooling system designs like those used by Arctic Cat employ modern “reverse” cooling systems like all modern automotive race engines, deliver the lowest temp coolant to the combustion chambers where it’s needed most allowing way more deto-free power! High volume “bathtub” coolant passages are passé, and smaller volume but higher velocity “shrinkwrapped” head coolant passages scour heat from combustion chambers way more efficiently! They use high velocity, turbulent coolant flow to maximize heat transfer from hot engine parts to cold heat exchangers. “Low and slow” is for smoking barbecue meat, and for smoking pistons!

 

Air_1s scfm- This is the airflow through the engine in Standard Cubic Ft per Minute. The dyno flowmeter is affixed to the airbox, or inserted in the insulated duct from the DTR refrigerated air system to the sleds’ air intake.

 

AFRAB and AFRA_B ratio- this is the mathematical ratio of air pounds per hour/ fuel based upon the weight of the air (converted from CFM based upon air density) to pounds of fuel per hour. 10/1 is rich (richer than 10/1 can result in misfire), and 17/1 is lean (leaner than 17/1 can result in misfire).

 

LamAF1- the SF902 uses an Innovate wide band A/F ratio meter to measure exhaust gas, and give us A/F ratio readings. The O2 sensor can be plumbed into the exhaust via a bung welded into the pipe, or with a dyno probe—with a long ¼ ID steel tube inserted colonoscopy-like into the stinger/ muffler. This reading can show leaner than the mechanical reading if oxygenated fuel is used.

 

LM1air- if there is no airbox and no possibility of accurate airflow readings, then approximate airflow SCFM can be determined by comparing mechanical fuel flow and the wideband A/F ratio. The math is done by the 902 computer and SCFM is shown.

 

ExhPrs psig- this is the average gauge pressure inside the tuned pipe(s) measured with a combination pressure transducer/ open element temperature probe in the fat part of a pipe center section. To generalize, 4.0 psi seems to be optimal in creating max HP in two stroke engines. If backpressure is lower, then power might be sacrificed. But then there is less possibility of detonation-producing active radicals being packed back into the combustion chamber by the pipe(s) return sound wave(s).

 

Exh_1 deg F- this measurement tells us that a pipe is at optimal temperature for best HP. Also it’s critical in telling us where peak HP occurs. The most savvy clutch tuners/ dataloggers can use this particular dyno test data to ensure that their engines run at max HP RPM from clutch engagement to the end of the track, or to the first turn.

 

DenAlt- “Density Altitude” in feet. This is a computed combination of barometric pressure, temperature and humidity averaged out into “feet”. This is particularly useful for tuners of carbureted sleds. With dyno power related to a particular jet size and density altitude, a Mikuni sled rule can be used with a $200 Kestrel 4500 hand held race computer to optimize jetting regardless of altitude or temp! And yes it works for racers and trail riders that run from well below sea level DA to over 7000 ft.