MASSIE

I have always heard and on the few dyno graphs ive seen the torque and horsepower always cross at 5250 is this true with all dynos.

foff667

iTrader: (2)

HP is calculated using tq & yes they do cross...its actually at 5252 though

MeentSS02

iTrader: (11)

*ding ding ding*

Thread over. foff wins

DynoDR

iTrader: (1)

Just in case some people still dont understand where the 5252 number comes from.......

1 hp is moving 550 pounds one foot in one second or 33,000 pounds one foot in one minute. 550 x 60 sec = 33,000. Now since the circumference of a circle with a one foot radius is 6.283186 feet, we divide 33,000 by 6.283186 to get our constant of 5252.112542.

I just thought it may be easier to understand or alot of people dont know WHY 5252 is used!

93ChevExt5.7

Really good info, but with the differences between dyno systems (dynojet vs mustang and such) how would 5252 be accurate still? Sorry, its probably obvious - just not to me (wow - like that's something new ). Now agreed, is this read from the vehicle's tach or the dyno's interpretation of the rpm - maybe that's where the answer might be?

MASSIE

thanks for the info

pianoprodigy

iTrader: (1)

However, you have to look at the dyno graph Y axis. Many times the HP and TQ will be on different scales (TQ will peak at 500 and HP will peak at 550). This will obviously shift the crossover point. This is not as common with LSX vehicles since the torque and horsepower are generally close, but in turbo import applications (which generally rev much higher) where the torque and horsepower may be 200 "points" different (or more), you see it much more often.

Forced370GTO

the graphs always cross at 5252... the only time this doesnt happen is when the torque and horsepower are on different intervals... meaning you have two different values for the y axis. graph the same curves with one y interval and the graphs will cross at 5252, always!

pianoprodigy

iTrader: (1)

You don't say? (see above)

GOaT Cheese

That is the fundamental PROBLEM with Chassis dynos, they use a reverse engineered algorythm to "interperate" the h.p. output of an engine. I use the word interperate because that is exactly what it is doing by measuring the work at the rear wheels. Since horse power is a created figure, it doesn't exist, only torque (twist) exists, horse power is simply an efficiency rating of the created torque. That is why ALL engine dyno graphs cross at 5252, because that is how the mathmatical equation is done to "calculate" h.p. from real work being done, which is torque. The problem with the chassis dyno is that the actual measured torque at the rear wheels is in the several thousands of foot pounds ( My GTO puts out 15,200 lbs. of torque at the wheels), but the chassis dyno doesn't report ACTUAL rear wheel torque output, it reverse engineers a torque number back to crankshaft output level. My contention is that there is a fundamental flaw in ALL chasis dynos that makes it impossible to achieve an accurate rear wheel horse power level. And for the non believers, My GTO makes 610 ft lbs at the crank, with a converter multiplication of 2.6 (Munnsinger 4000), a first gear ratio of 2.48 and a rear gear ratio of 3.90. That equates to 15,340 lbs of rear axle torque. And at 3750 lbs race weight it runs 11.70's at 116. But the chassis dyno says I only make 550 rear wheel torque, where'd I lose 14,800 lbs!!?

Sigforty

One flaw in your calculations goat cheese. you do not dyno in first gear. so that multiplication should be 1.00 for 3rd gear in an auto or 4th gear in a 6 speed.

dynocar

Quote, "That is why ALL engine dyno graphs cross at 5252, because that is how the mathmatical equation is done to "calculate" h.p. from real work being done, which is torque."


Yes, HP is the measurement of work being done, not TQ. we can have a lot of torque with no work being done, such as putting TQ to a shaft but it can't turn it and we can have less TQ with more work being done, such as putting your engine at its peak HP RPM even though it is putting out less TQ, to go faster. A chassis dyno is reasonably accurate in measuring HP at the drive wheels which can give us a reasonably accurate indication as to how much HP or TQ our engine has at given RPMs. With this information, we can get a HP/TQ line graph the can accurately mirror the same graph from an engine dyno, minus whatever, so called, "drive-train" losses we want to factor in. This is what most people want to see, but there is more.

You did hit on a very important aspect of performance that many chassis dynos are capable of but no one is interested in, not your rear axle TQ, because tire diameter enters the equation, but better yet, tire patch roller force at given MPHs. I've promoted this often with no takers, but if we gave out graphs of this data comparing pulls, more people would understand why changing gears, TQ converters, how the engines peak HP RPM, etc, can increase tire/roller force thru TQ multiplication. Another words, how hard is the car pulling on the dyno tie-down straps per MPH. This is what it all comes down to.

GOaT Cheese

I agree, I am no expert on the topic, but I strongly feal that the current techniques tend to lead to misrepresentations of power output.

GOaT Cheese

I'll give you that, then my rear axle torque is 6200 lbs. at a 1 to 1 final drive ratio. Still alot more than any chassis dyno would ever attest to.

dynocar

I'm sorry I keep beating this dead horse, but I don't understand why you're so hung up on just axle torque, unless you just like large numbers. Your 6200 lbs, number (also "force") is only usable if your tire had a 1 ft radius because that 6200 axle torque number is in lbs/ft, it would be 3100 lbs if your tire was 2 ft, etc, as we work or way to the ground, not to mention we have to divide by two for each drive axle. So why stop at the axle and let's go all the way to the ground where it really counts and use "force" that also compensates for tire diameter. Most people understand that if your engine TQ has increased in a given gear, everything, such as axle TQ, ground force and acceleration will follow, whatever the numbers are. Many chassis dynos will give you force, roller torque (if you like big numbers) and more. Just call the dyno shop and ask for such info, many can go back to previous pulls and get such info if you're interested, which I'll give you credit for, you're possibly the first guy I have come across that is. More importantly, I attest to the fact that you will gain a new respect for chassis dynos.

GOaT Cheese

The actual number to me is completely irrelevent. The point I am trying to make is that there is a discrepancy in the way a chassis dyno represents the horse power that is generated. I understand the theory of how they work, a given mass is accelerated to a given speed, in a given time. My issue is after that, how the computer calculates a horse power figure (an artificial number to begin with) from an INNACURATE wheel torque output? Lets take into account all of your arguments, lets subtract circumfrance of the tire, and divide the axle torque by two (for both rear wheels) the torque generated at the tire contact patch is still alot higher than the dyno says it is. We all know that horse power is an efficiency rating of torque, horse power itself is nonexistant, only torque (work, twist, wats) exists. In order to generate an accurate efficiency rating (h.p.) you MUST beggin with an accurate torque rating.

dynocar

Quote, "My issue is after that, how the computer calculates a horse power figure (an artificial number to begin with) from an INNACURATE wheel torque output?"


I know exactly what your issue is and you are correct, when we see stated, "RWTQ", that is a total mis-statement, it should state, "engine TQ based upon using these RWHP numbers", and yes that is much less then actual RWTQ. The nice thing about these mis-stated RWTQ numbers though is that we can easily compute what the engine's crankshaft HP and TQ are with whatever drive-train loss (another mis-statement) multiplier is. Thanks for an interesting chat, in the interest of helping to draw our off subject discussion to a close, I'll let you have the last word should you choose to. But this does not mean that I agree with you on everything.

critter

iTrader: (9)

HP = Torque * RPM / 5252

They must cross at 5252 or you have serious problems.

Keep in mind that dynos like the Mustang actually measure torque and from that they can compute horsepower using the above formula. Dynos like Dynojet infer torque by measuring how long it takes to spin up the heavy drum the rear wheels ride on. (More precisely, they measure the acceleraton rate of the drum, but the effect is the same). They do not measure axle torque. This means that the weight of your wheels, flywheel, driveshaft, axles, etc affect the horsepower "reading". Adding horsepower will spin the drum up faster, but that absorbs power so that not all of your increase shows up on the meter unless you add in fudge factors. This is partly why you show less horsepower if you dyno in a lower gear. Of course some of the difference is due to transmission loss, but you will not see as much difference on a Mustang as you will on a Dynojet. In a sense, a Dynojet represents what happens at a drag strip better than a Mustang (unless you run the Mustang in sweep mode), but if you want a real horsepower number, the Mustang is better than the Dynojet.

dynocar

Quote, "Dynojet represents what happens at a drag strip better than a Mustang (unless you run the Mustang in sweep mode), but if you want a real horsepower number, the Mustang is better than the Dynojet."

Carl, I agree with everything but this. Every Mustang chassis dyno that I know of also uses the sweep mode. The Mustang represents what happens at a drag strip better because it simulates actual vehicle acelleration and engine load because the weight and wind drag factors are keyed in for the rollers load. As a matter of fact, on a Mustang with such simulation, it had a drag race program so an actual drag race can be run on it from a start with amazingly close ETs and MPHs compared to actual track times on most cars.

critter

iTrader: (9)

I think we agree :-)

We both say you can run a Mustang in sweep mode (which is a choice, not the only way it works) and that the Mustang is better.

I think both sweep and stepped modes have their place. Clearly you need sweep mode to look at the effects of a lighter flywheel or crank. I would choose stepped mode to look at the effect of retarding the cam 2* though.
Mabye that is just old habit though ....

EDIT: If you are internet dyno racing, the Dynojet is better because it gives higher numbers and you can get free horsepower by putting light rear wheels on and sticking 2.73 gears in it :-) But then real racers don't care about that.