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Dyno Question - HP Corrections
In recent weeks Club Cobra threads have discussed HP loss corrections on chasis dynos (18-20%).
What are the additional corrections for high ambient teperature and high pressure altitude? For example, 354 HP and 412 ft-lbs of torque on a chasis dyno in Reno during Hot August Nights at 100 degrees. |
Jeff, this is from Andy's site. Various HP calculators. Seems like elevation is in there , but don't know about ambient temp. I have no comment about the engine in question :D:D
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Bill,
You left out the link! Are we supposed to guess? TURK Drunk typing is illegal! |
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Ok Jeff,
I am trying to figure it as precisely as I can. If the outside temperature is 102, and let's say the altimeter setting was 29.95 what do you think the dew point was on that given day. Or should we call the cessna Pilot Training school? I am trying to find out what exactly the density altitude for that day to figure out exactly how much HP was left there in the parking lot. I hope you paid attention while you were in ground school. It is going to be more than 40 HP loss I figure. TURK |
I calculated, if there is ZERO allowance for density altitude and we used the 4400 for elevation the HP loss is almost 54 HP. Thus making the sea level numbers around 408 HP at the rear wheels, and adjusted for the 20% for the rear wheels we are right at 490 HP at the fly wheel.
Not bad for a engine that gets 14.536 MPG at increased speeds. Now for some reason the density altitude was at 5000, the numbers come up to 500 at the fly wheel. TURK I hope this helps. |
TURK,
For an incredibly long and painful discussion on density altitude and how to calculate it go to: http://rshelq.home.sprynet.com/density_altitude.htm Density altitude makes a huge difference! I found an engine tuner's calculator at the following website: http://rshelq.home.sprynet.com/calc_hp_dp.htm Using the following variables: Air Temperature 100 degrees F Altimeter Setting 30.10 inches Hg Dew Point 42 degrees F Altitude 4412 feet I came up with the following calculations: Relative Horsepower 82.6% Air Density .9678 kg/m3 Relative Air Density 79% Density Altitude 7847 feet Virtual Temperature 102.2 deg F Actual Air Pressure 25.61 inches Hg Actual Vapor Pressure .268 inches Hg Relative Humidity 13.9 % Dyno Correction Factor 1.21 Note the density altitude of 7847 feet and Dyno correction factor of 1.21. Plugging the Density altitude of 7847 into the calculator Bill provided I get 83.3 HP loss to the rear wheels on the chasis dyno due to the high elevation and temperature in Reno. 354 + 83.3 = 437.3 to the rear wheels 437.3 X 1.20 = 524.8 at the flywheel Not bad my friend, not bad at all! Jeff |
Higher density=higher combustion pressure=more power. A po' mans supercharger. Same basic theory behind intercoolers or packing your intake with ice at the drag strip.
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I like your figures better. There will be a 11% surcharge.
Tomorrow, I will either be going by or calling the engine builder in Sacramento. TURK |
The brand and style of chassis dyno will also have an effect on the power readings, with the Mustang brand of dyno (load type)always showing a lower power reading than the more widely used Dynojet dyno (inertia type).
20% correction to get flywheel hp seems a little high with a manual trans though... http://tech.rennlist.com/performance...einerFerch.PDF |
isn't the typical parasitic loss as they call it for such things as drive line, exhausts, air cleaners etc for a manual trans car more often used at a 15, 17 or 18% ratio vs 20? ie, to convert from rear wheel hp rating to an ESTimated flywheel hp, divide the rwp number by the reciprocal of the estimated loss % : ex: 410 rwhp at 15% loss equates to est flywheel hp of 482 ( 410 divided by .85)...410 rwhp at 17% loss equals 494 engine hp and at 18% equals 500 engine hp. if a 5 spd the chassis dyno pull should have been in 4th gear for 1:1 ratio. some variabilities exist between chassis dynos but think a 20% loss figure is too high to use even with a less accurate chassis dyno brand. many engines seem to engine stand dyno at numbers much higher than an est calculation from a chassis dyno of the same engine in a car vs engine stand. however, i suspect the enigine dyno numbers are under more ideal heat/humidity/ density conditions than the real world of being IN the car and in many cases the parasitic loss is much greater ? bill
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I was told 18% loss with a manual transmission and as high as 25% with a fluid drive. Is that more like it?
TURK |
Turk,
I've found the chart at the bottom of THIS PAGE to be about on par with what I've regularly heard. Oh, and don't bother emailing the guy for further information - his own 1200hp street driven car savagely transported him to the great dyno room in the sky a while back..... |
U-2:
Always keep in mind that peak HP values are always at very high revs since HP is a calculated value based on Torque and RPM! HP = (torque X RPM) / 5252. Torque values in the 2000 to 4500 range should not be sacrificed for high RPM HP since that where street engines spend almost all of their time. The HP at lower RPM can be far more useful than the HP at 6,500. Bob |
I have always been less than keen on "correction factors" that are given as a percentage. I think thqt the parasitic loss through the gearbox, driveshaft, and rearend to be constant for a paricular combination. The amount of force required to turn those components a given disance / speed is irrespective of the power output of the motor. A particular car with a 250 hp motor may lose 20% at the rear wheels, but the same car with a 500 hp motor would only use 10% of it's power to do that work. So if the owner claimed a 20% correction factor, he would be padding his ego by 25 hp in this fictitious example.
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Well, I guess it's my turn to throw
a monkey wrench into the whole thing. Chassis dynos, like flow benches are an excellent tuning aid for an individual, but not so great for real world comparisons to other peoples cars or motors. I have done chassis dyno work and am here to tell you there are way too many variables, assumptions, and calculations to make to get accurate data. For example, I still have some connections at Ford testing division (actually most of it is done by Roush Racing Ent.) I have a '01 F-150 Lightning and was told, by the testers, that the drivetrain and parasitic loss is 11.2%. Yes, 11.2%. We tested a friends totally stock Lightning and it made 342 hp at the rear wheels. 11.2% of 342 is 38.3 342+38.3=380.3 hp at the flywheel. Guess what for rates the engine at: 380 hp. We raised the tire pressure from 33 psi to 45 psi and the power jumped to 351 rear wheel horsepower. 11.2 % of 351 is 39.3. 351+39.3= 390.1 flywheel hp. 10 hp just by adding air to the tire. The cheapest 10 hp on the planet! My Lightning made 390.2 at the rear wheels. At 11.2 % that = 433.9 at the flywheel. I guess what I'm trying to say is there is so many variables involved that even with todays hi tech equipment we are still only making educated guesses. IMHO, --Mike |
Hey guys, I might be able to put to bed the debate on flywheel horsepower to rear wheel horsepower loss. I have just completed the installation of my Dyno-Jet chassis dyno. We will be testing the same engines on our DTS engine dyno and the chassis dyno in most of our Superformance installations. Now we will know what the power loss of the drive train and some of the different header and side pipe combinations. Hope I can be of help. Eric of Performance Engineering.
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Eric...now, some very interesting comparisions. I look forward to the results. Bill.
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