johnc Posted February 9, 2001 Share Posted February 9, 2001 Intertia dynos like the Dyno Jet are almost always optimistic in the absolute horsepower numbers they report (a lightened flywheel, underdrive pulleys, etc. will show a horsepower increase on a Dyno Jet dyno). Load dynos are more accurate at reporting the actual horsepower an engine is producting. http://www.fordmuscle.com/archives/2000/03/project1199/index.shtml http://www.supras.co.uk/resources/dynos.htm http://www.shotimes.com/dyno.html http://www.foxvalleykart.com/foxvalle/dyno1.htm Saying that an engine produces "n" horsepower based on a run on an intertia dyno is probably an inaccurate statement. Saying the engine "a" produces 10% more horsepower than engine "b" as tested on the same inertia dyno can be a very accurate statement. ------------------ John Coffey johnc@betamotorsports.com [This message has been edited by johnc (edited February 09, 2001).] [This message has been edited by johnc (edited February 09, 2001).] Quote Link to comment Share on other sites More sharing options...
Guest Anonymous Posted February 9, 2001 Share Posted February 9, 2001 You commented on the lack of precision(by citing a wide spread of numbers from the same car tested multiple times back-to-back), not a lack of accuracy. Obviously with the high acceleration rates of a chassis dyno you'll get exxagerated results from lightweight spinning parts. But this will decrease accuracy, not precision like you stated above. ------------------ Morgan http://z31.com/~morgan/s30 http://carfiche.com Quote Link to comment Share on other sites More sharing options...
johnc Posted February 9, 2001 Share Posted February 9, 2001 Morgan, Your dyno chart example indicates where our disagreement is. You are talking about Power from the perspective of accelerating the vehicle. My original post and discussion is about Work (torque) produced by the engine. Horsepower is torque over time and is generally measured by engine rpm, not spinning drums. I agree the Dyno Jet can be accurate based on what it is measuring, but they are completely inaccurate when referring to engine torque and horsepower. The engine in your chart examples is not producing three different levels of work. The original premise of this thread was that engine of "x" car wasn't making enough horsepower and some absolute horsepower numbers generated from an intertia dyno were posted as an example. My contention is that the absolute numbers posted are inaccurate as a reflection of the horsepower producted by the engine. My whole point is to get people to think about the numbers they are quoting. And its not a "cop out" to say that lightweight flywheels and underdrive pulleys do no add horsepower to an engine. They don't! A vehicle may be able to accelerate faster, but that power increase is reflection of mass reduction, not a torque increase in the engine. > You just contradicted yourself - you say a > lighter flywheel doesn't add power. Then > you say power is work per unit time(which > it is). Correct, and the "time" measurement is engine RPM (as I stated), not 1/4 mile time. > Obviously a lighter flywheel lets the car > accelerate faster, which reduces the time > period over which the same amount of work > is done. Viola, your flywheel makes more > power. I agree, but not in the engine. The car can accelerate faster because the same amount of work (remember, no torque increase) has less mass to accelerate. The resulting measurement device (in this case, an inertia dyno) sees that as more power (work over time measure - in this cas turning drums), but there is no increase in work (energy). > You are picking nits, espicially for > someone who doesn't the difference between > accuracy and precision OK, I give, what's the difference? >And I still say dynojets are more than > precise enough for tuning and > informational purposes. We agree completely and what you've said above is exactly what I said earlier: "They are good for comparing one vehicle vs. another or one mod vs. another and coming up with relative numbers (most often a percentage improvement)." "Saying the engine "a" produces 10% more horsepower than engine "b" as tested on the same inertia dyno can be a very accurate statement." ------------------ John Coffey johnc@betamotorsports.com Quote Link to comment Share on other sites More sharing options...
pparaska Posted February 10, 2001 Share Posted February 10, 2001 Morgan, are we so feeble that you just can't stand us? Quote Link to comment Share on other sites More sharing options...
TimZ Posted February 10, 2001 Share Posted February 10, 2001 Well, so much for mature, open discussion... Somebody needs to either cut down on the caffeine, start smoking again, or maybe just go to bed earlier... Anyway, it's true that in alot of cases, the turbo can make more boost in the higher (lower numerical ratio) gears. However, the magnitude of this effect is greatly dependent on the turbo/engine combination, as well. In general, to get the most repeatable and not coincidentally, highest power readings on an inertia dyno, you really need to be in as high a gear as possible. This is due to several things. Somewhat higher boost for a given RPM is one, as Morgan already mentioned. This effect can be negated to some extent by increased heat in the intake charge, since the intercooler usually doesn't work as effectively without a 60+mph airflow source for cooling. As a result, the longer the run takes, the higher the intake charge temps get. Also, frictional losses through the drivetrain are known to be considerably lower in the higher gears. Probably the most significant reason to use a higher gear has to to with the measurement method itself. The acceleration of the drum is measured by calculating the rate of change (or derivative, if you've had calculus) of the rotational speed of the drum. If you know anything about control systems theory, then you know that differentiating a signal results in a noisy output, and the effect gets worse as the rate of change of the input gets higher. So, if you use a lower gear for your dyno pull, the rate of change of the drum speed is considerably higher, and as a result, your power calculation becomes much, much noisier (i.e., less accurate, and less repeatable). This effect could very easily explain John's account of the Viper seeing a wide range of power outputs, or Morgans pictures of different power outputs in different gears. Also, John's points were well taken re: the interial dyno's sensitivity to flywheel mass, etc. Actually, John, you got me thinking a bit about the underdrive pulley effect. I had always considered the power increases from these to be due to the accessories having less parasitic drag at lower RPM. I'm pretty sure that this is true for the water pump, at least in some cases. I hadn't considered their contribution to the rotational inertia of the engine, but in retrospect they most certainly do contribute to this. Also, John is absolutely correct in stating that the horsepower readings that come from decreased rotational inertia of the drivetrain are not real power increases. You must remember that with an inertial dyno, the relationship between the acceleration of the drum, and the real power output of the engine is actually dependant on the rotational inertia of the drivetrain PLUS that of the drum. Unfortunately, the drum is the only part that you know for sure. For most powertrains, though, the weight of the drum dominates, and the effect of the powertrain rotational inertia is kept to a minimum. This will cause a definite amount of inaccuracy in the power readings, as John contends. Lightened flywheels allow the vehicle to accelerate faster, but it's for the same reason that lightening the body makes the car accelerate faster - less inertia. I don't think that we'll see too many arguments that making a car ligher increases the engine's power. Anyway, the point is, as I believe has already been stated, that inertial dynos can be very repeatable run to run, if the dyno operator understands the limitations of the device. For comparing car "A" to car "B", they are less accurate, but not completely useless. Because, except for extreme cases, the weight of the drum dominates the reading, I would guesstimate that comparisons between cars are still fairly valid, with a window of uncertainty of maybe 5%. In other words, if my car dynos with more than 5% more power than the next guy's it probably actually does have more power. Quote Link to comment Share on other sites More sharing options...
johnc Posted February 10, 2001 Share Posted February 10, 2001 I assume by precision you mean repeatability, correct? If that's true, I feel that a lack of repeatability in testing indicates a lack of accuracy - either as a result of equipment or operation. Let's take my Colt Gold Cup as an example: if I can't repeatedly hit the 10 ring at 25 yards can you say the gun is accurate just based on that sample? Regarding the lightweight flywheel and the underdrive pulleys - neither of those items add to the horsepower of an engine because they have zero effect on the volume of air moving through the engine or the amount of fuel being used. Yet an intertia dyno will report the addition of either of those parts as an increase in horsepower. Engine horsepower is torque over time (engine RPM). ALl the above being said, I think I am guilty of "picking nits." But its important that people be aware of the limitations of dynos. ------------------ John Coffey johnc@betamotorsports.com Quote Link to comment Share on other sites More sharing options...
Guest Anonymous Posted February 10, 2001 Share Posted February 10, 2001 > Regarding the lightweight flywheel and the underdrive pulleys - neither of those items add to the horsepower of an engine That's a cop out. If more power gets to the wheels then you have more power. It doesn't matter if the engine is making more power, or less is being used elsewhere, it's still more power moving the car, which is what matters and which is what a chassis dyno measures. > Yet an intertia dyno will report the addition of either of those parts as an increase in horsepower. Engine horsepower is torque over time (engine RPM). You just contradicted yourself - you say a lighter flywheel doesn't add power. Then you say power is work per unit time(which it is). Obviously a lighter flywheel lets the car accelerate faster, which reduces the time period over which the same amount of work is done. Viola, your flywheel makes more power. You are picking nits, espicially for someone who doesn't the difference between accuracy and precision And I still say dynojets are more than precise enough for tuning and informational purposes. Variances of 10-20% like you cite above are the result of operator error or something broken or wheelspin or who knows what - they are simply more repeatable than that. Just look at anyone who uses them - make a coupla runs that are practically identical, put a new exhaust on, make a few more runs, and you can see the difference. The end result is that they work properly, just like anything else, only when used properly. Here is an example of power measurements at the wheels: Know what the difference is between those? First one is in 1st gear, second is in 2nd gear, and the third is in 3rd gear. Are they wrong? No, they are perfectly accurate. Does that car make more power in one gear than it does in another gear? Yes it absolutely does, as do all turbocharged cars. Point: make sure you're not only getting the data correctly, but that you're reading the data correctly as well. ------------------ Morgan http://z31.com/~morgan/s30 http://carfiche.com Quote Link to comment Share on other sites More sharing options...
Guest Tom Scala Posted February 10, 2001 Share Posted February 10, 2001 Chassis dynos will give lower HP readings for automatic cars if all else is held equal. That's why it's very hard if not impossible to compare one vehicle directly with another using only dyno numbers,especially when the auto trans is equipped with a high stall convertor. Take you basic 4000-7000RPM SBC in a stick equipped vehicle and take a reading. Put an automatic with a 4000 stall convertor in the same car and you'll lose HP on the dyno through the covertors' inefficincy & slippage but the auto car will et quicker 10 times out of 10. Quote Link to comment Share on other sites More sharing options...
Guest norm[T12SDSUD] Posted February 10, 2001 Share Posted February 10, 2001 The only real world dyno slip that matters is a 1/4 mile time slip!! Because in the end, bragging about HP means nothing if your total package can't vanquish the opposition!! Later,norm ------------------ "Ya Gotta Drive IT like ya Hate IT!" Quote Link to comment Share on other sites More sharing options...
pparaska Posted February 10, 2001 Share Posted February 10, 2001 Norm, good point Tom, there's an interesting comparo in the latest Chevy High Performance wher they raced the same car first with a drag built light-weight rotating component TH 350 automatic, and also a racing 4 spd Jerico. Very interesting. The stall converter took about 30 hp away at top end (7400 rpm) due to converter slip. The guy also had to rev to 7400 with the auto versus 7200 with the manual to get teh same trap speed. Quote Link to comment Share on other sites More sharing options...
Guest Tom Scala Posted February 10, 2001 Share Posted February 10, 2001 A loose convertor will lose 1/4 trap speed but usually,if it's matched to the torque curve of the motor,produce better et's. My GTO lost 1.5 MPH going from a 2200 flash stall to a 3200 but gained 2.5 tenths. You won't have to rev as high with the loose convertor since rpm's don't drop as much between shifts. I shift at 5400 and the rpm only drops to 4100 after the shifts. With the tight convertor it dropped to under 3500. A stick would be worse. My engine is tuned to run in that 4000-5400 range so it works for me. After all it's who gets there first not how fast youre going when you get there. Quote Link to comment Share on other sites More sharing options...
pparaska Posted February 11, 2001 Share Posted February 11, 2001 Wow, this is getting away from the original topic a bit, but no biggie. Tom, the car in the CHP mag went quicker and faster through the quarter but was less consistent with the manual (0.032 sec spread versus 0.007 with the auto). I was kind of expecting the auto to be quicker. I don't have the numbers off the top of my head, but it was around a mph faster, on average with the manual, and around a tenth? quicker? Of course, if you're racing brackets, it'd seem the auto is the definite winner since you'd have much less chance of breaking out. The crowd at the test enjoyed the manual car more than the autos. I guess the rev limiter at the tree bouncing at 6000 rpm, then dropping the cluthc, and the 7200 rpm between power shifts helped make it more interesting to them . Quote Link to comment Share on other sites More sharing options...
Dan Baldwin Posted February 15, 2001 Share Posted February 15, 2001 I'd say these dyno results don't look *too* terribly bad. A broad power band and flat torque curve that does drop off way too early. Interesting that someone mentioned he'd be better off with less carburetion since his power peak is at such a low rpm. Actually, he needs MORE carburetion to MAKE more high-rpm power. He needs 45mm webers with 36mm chokes in order to make power above 6000 rpm. Assuming that the handy-dandy equations in "How to Modify Your Nissan/Datsun OHC Engine" are correct: throttle bore = sqrt(disp/cyl * max rpm/1000)*.82 main venturi = sqrt(disp/cyl * max rpm/1000)*.65 136 rwhp isn't too bad for a mild 2.8 liter L-series engine, maybe 170 at the flywheel? Definitely more to be had, though. Also, regarding big numbers that are regularly bandied about for NA L28s, any dyno numbers that are from the engine builder are to be taken with a HUGE grain of salt. Not meant as any kind of dis to engine builders, but it is definitely in their interests to "correct" their numbers a bit more than an independent dyno will. Dan Baldwin '71 240Z 3.1 10.1:1 CR, 2" SUs, 290/.503" cam *maybe* 155 or so rwhp (will dyno one o' these days) COMSCC #7 SPB Quote Link to comment Share on other sites More sharing options...
Guest MegaShaft_2000 Posted March 23, 2001 Share Posted March 23, 2001 quote: Originally posted by Drax240z: I just stumbled across this page. http://zccw.org/Club/MembProfile/DerekS/details.htm You can see his dyno results from a link on that page as well... Someone please tell me if I'm overlooking something, but how come the torque and HP curves don't intersect at 5252 rpm like they are supposed to? Quote Link to comment Share on other sites More sharing options...
johnc Posted March 23, 2001 Share Posted March 23, 2001 Huh? Where is it stated that the hp/torque intersection point is fixed? What if you're engine doesn't run higher than 5,000 rpm? What if your engine is redlined at 17,000 rpms? ------------------ John Coffey johnc@betamotorsports.com Quote Link to comment Share on other sites More sharing options...
Guest Fast Frog Posted March 23, 2001 Share Posted March 23, 2001 John: HP is a calculated expression of work formulated several centuries ago. If anyone has taken high school physics, there's usually some discussion regarding how many HP does it take to lift a certain wt one foot off the ground. I've long since forgotten the exact formulas and such, but in automotive terms the "wt" becomes ft lbs of torque and HP is the expressed "work" that is created by torque moving up the rpm scale. To put it simply, 5252 is a constant in that torque/HP equation; and when HP and Tq are shown on a graph, the their lines cross at 5252. (HP=Tq X rpm / 5252) At 5252 on a graph, Tq and HP are equal. Quote Link to comment Share on other sites More sharing options...
Guest Tom Scala Posted March 23, 2001 Share Posted March 23, 2001 Horsepower is kind of a fictional number that isn't directly measured but derived from the actual measured torque output.It expresses the amount of work done in a given amount of time. HP=TQxRPM/5250. At 5250rpm torque and horsepower will always be equal. Quote Link to comment Share on other sites More sharing options...
Guest MegaShaft_2000 Posted March 24, 2001 Share Posted March 24, 2001 quote: Originally posted by Tom Scala: Horsepower is kind of a fictional number that isn't directly measured but derived from the actual measured torque output.It expresses the amount of work done in a given amount of time. HP=TQxRPM/5250. At 5250rpm torque and horsepower will always be equal. Exactly.. so why are the lines intersecting way before 5250? I think the results are a little off on that dyno. PS- Now I see that the page is down...what's up with that? Quote Link to comment Share on other sites More sharing options...
Kevin Shasteen Posted March 25, 2001 Share Posted March 25, 2001 Okay; its raining, windy & I have no garage/nor a budget to begin my project....so I'm gonna attempt to explain the Infamous "5252" w/a story. For starters the "5252" is not a representation of a tangit of "Torque" or "HP" so the assumption that they should cross on a dyno is incorrect. I think we should obtain this info straight from the horspower's mouth....Mr. Watt himself. Scottie: prepare the Timemachine. Adjust the gauges for a trip back in time! (Cue the dry ice effect); set the timers for only a few minutes w/Mr. Watt & ENERGIZE! Ooooh-Aaaah! Where is that weird sound coming from? Why is my visual blurred? Where is that weird "Dreamy" music coming from? Uh-oh, my world....its disappearing/turning like into a vortex/wormhole of some sort...its..pulling...me...in/cant///resist it. I'm spinning out of control; Heeelllppp mmmeeee I've fallen & cant get up! Oh-hey; we're here now. I'm looking around & seem to be at a minor's camp. A lot of strange burly minor guys are looking at me (scary); guess I should've changed from my PJ's prior to making the trip! There he is...Mr. Watt; Yea-you....Mr.Watt could I speak to you, please; thanks. I know Mr.Watt that you are busy-so am I as I only have a few minutes as I've crossed the space-time coninuem to ask you about your Steam Engine & how you came to the HP/Torque formulas. Dont look at me like I'm crazy....these PJ's are in "style" back in my time. Would it help if I told you that your formula for HP/Torque have been labeled by the "Systeme International des Units" (S.I.) as a standard for HP! Yea, thought that would get your attention. Now; please explain how you came about your formulas & dont tell me it came to you after an apple fell from a tree-that one's been done already. Mr. Watt says he needed something to rpl the draft horses at the mining camps; it was their job to pump water out & their procedure was tedeous & inefficient. So he developed his steam engine to do the job. He noticed that the, now repl.horses, were plodding in a circular path, pullin at a right angle on the end of a 12ft. lever attached to a capstan; the capstan was geared to operate a pump. Watt says he estimated the horse pulled w/a force of 180 pounds. The circle had a circumference of 2 x 'pi' x 'radius of 12ft' = 75.398224 feet. The horse could make 144 trips around the circle in an hour or 2.4 trips a minute, for a speed of 180.95573 or about 181 feet per minute. Mr. Watt further explains that he converted the horse's ability into measurable leverage (what we call torque), Watt then says he multiplied 180 pounds time 181ft. obtaining 32,580 lbs.ft. per minute. He rounded that to 33,000 lbs.ft per minute, or 550 lbs.ft per second, which became the norm for 1.0 hp. Ahhh, thanks for the explaination Mr. Watt. However, I have just one question. WHY DIDNT YOU MULTIPLY 'FT x LBS' INSTEAD OF 'LBS x FT' as it would've been much easier in my time to say "Ft.Lbs" instead of having to say the laborious "Lbs-Ft"? Mr. Watt says....oh-no; the visual is getting blurred again; there's that funky music. Here comes the vortex/wormhole..sucking me back. Scottie-I need more power! What do you mean more power will fly her apart? Fly her apart then-I NEED MORE POWER! Oh; too late. We're back. Now that we know where 1.0 hp came from-we need to apply it to modern day engines. The horse walking in that 12' circle applied force at the output of the lever , that force was applied to the capstan at the center of the capstan; a modern day engine does just the opposite. It delivers force at the ouput end of the crankshaft. Envision a 1.0ft lever at a right angle to the crank; as that crank rotates, the free end of the lever will follow a circle w/a radius fo 1.0ft. Watt's definition of hp involved a force in pounds, applied over a distance in feet, for a time of 1.0 minute; which is the circumference of a circle w/a 1.0ft radius multiplied by crank rpm's. The circumference is 'pi' x 2 x 1ft; or simply 2 x 'pi' = 6.2831853 feet. So the total distance the free end of the lever will go in 1.0 minute is 6.2831853 x rpm. The product of that calculation can be multiplied by the known torque of the engine to find the total pounds-ft of torque per minute. The result can then be divided by Watt's pounds-ft per minute for 1.0 hp (33,000) to find the engines hp. That works out to be the following formula: Horsepower = 6.2831853 x RPM x Torque/33,000 Of course as we mathmaticians will recall; always get the lowest common denominator; so we now have to divide both the upper/lower by 2 x 'pi' and the formula now comes to: Horsepower = RPM x Torque/5252 Tada! Told you we would discover the infamous "5252". It is nothing more than a lowered common denominator of the 33,000 lbs.ft per minute Mr. Watt's horses worked. BTW: it you know HP but not torque you can also use the formula: Torque = 5252x HP/RPM Sorry if I rambled; slow day here in N.Tx. Kevin, (Yes,Still an Inliner) Quote Link to comment Share on other sites More sharing options...
Z-Gad Posted March 25, 2001 Share Posted March 25, 2001 Thanks Kevin for the colorful explanation!! Quote Link to comment Share on other sites More sharing options...
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