articfriends

iTrader: (1)

I have a general dyno question for people out there that have actual dyno results.We recently chassis dynoed a drag car(i know this is a boat forum).I have always understood that when your dynoing something that your torque and horspower ALWAYS cross over and are equal at 5250 as a dyno only measures torque and calculates horsepower and somehow 5250 is a constant. On this dyno session we rented a portable chassis dyno service and everyone that had been running there mouth about big hp numbers came over and made pulls with their cars. Our results and just about everyone elses had torque and horsepower crossing over all over from 3800 to as high as 4800,when questioning the dyno operators about the 5250 constant they said it was only a myth. The next day i pulled out my dyno sheet from my boat motor 2 years ago and sheets from 2 different circle track cars i dynoed in the past,sure enough,the torque and hp numbers did cross over at 5250. Is there anyone out there that has dynoed something that can look at their info and tell me if their numbers crossed at 5250 or a different number,thanks,Smitty

rmbuilder

Artic
Its impossible mathematically for the torque curve and the HP curve not to intersect a the rpm value of 5252. Here is the formula to calculate horsepower:
hp = (torque * RPM) / 5252
If you have your torque value times your rpm value divided by 5252 (and the RPM value is also 5252) the rpm and divisor cancel each other out and the the remaining value is HP= Torque.
http://www.carcraft.com/techarticles/868/
Here is a tech article explaining dynamometer procedure and formula.
Bob

articfriends

iTrader: (1)

rmbuilder,ive read same article in past and have seen the math before,thats why i was skeptical of this dyno operator, business and results,thanks for replying though,Smitty

cobra marty

? is there different scales on the HP and TQ curves? I've seen this befor where the HP is on the left vertical and is say 0-500HP and the TQ is on the right vertical and is 0-700TQ. Otherwise there is no way they won't cross at anything but 5252.

Infomaniac

I was a dyno operator for a number of years and have had this debate many times. The age old debate about whether to "build" the engine for TQ or HP depending on the RPM range the engine will operate at.


Yes the 5252 is correct if using SAE measurements.

Where does TQ and HP intersect if using metric units to do the math? (Nm, kW, kph).

rmbuilder

Info
Kilowatts = (Torque * RPM) / 9549
Here is how the 9549 constant came to being for the metric one. You convert RPM to Rad/s, and the Power is Kilowatts not just Watts, so there is the difference of 1000 times as well. So 1 rpm / 60 = rps = 0.01666 * 2PRad = 0.1047 Now 1000/0.1047 = 9549.29

Kw to Hp multiply by 1.34
Kw to Metric HP (ps) multiply by 1.35962
When converted they still intersect @ 5252 rpm
Bob

Infomaniac

Thanks Bob for the headache.

I guess it would intersect the same if converted back to HP.

Are Hp and Torque the "units" used in the metric system? What if not converted back to SAE.

The reason I am asking is due to a debate about "power" in general. Torque or HP at specific RPM targets.

Raylar

Having done a lot of dyno work on our engines and parts, I would suspect that the chassis dyno mismatch here could be from two things. One would be what kind of calibation was the dyno set up with if any? Two would be the various driveline abnormalities in tire slipage, etc. Not being a chassis dyno expert, I am not sure this engine dyno cross point would show up the same on a chassis dyno.
Raylar

MESABALANCING

Its funny someone else should bring theis point up because there are a couple of well known shops here in Miami that have a chassis dyno and most of the street car guys tune there and they have the same inconsistency with their dynos also . I am not sure why this happens if its not calibration but I attribute the inconsistency problem to be because the majority of the dynos do not have a resistance other than the weight of the roller it sits on and an engine with more torque would have a tendency to spool up the drum at a different rate on the other hand I have another customer in ft Lauderdale that has a unit that has a brake system and that unit does calculate correctly every car we dyno the engine that we chassis dyno has been consistant with our engine dyno minus the % loss.Sincerely Laz Mesa

rmbuilder

Info,
SI units are:
Torque: Newton meter (Nm)
Power: Kilowatts (Kw)
1 Nm = 0.737 lb-ft
Kilowatt (kW) 1 kW = 1.341 hp
T = Kw x 9543 ÷ RPM
Kw = T x RPM ÷ 9543
RPM = Kw x 9543 ÷ T
Artic,
Heres an article on how Dynojet corrects for tracive loss etc
http://www.idavette.net/hib/dynojet/
Im sure there are many variables at play with other dynos but I would be cautious of an operator that dismisses 5252 as a myth.
Here is an actual Dynojet run sheet with all values crossing @ 5252
Bob