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28 Feb 2007, 20:14 (Ref:1854047) | #126 | ||
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28 Feb 2007, 22:37 (Ref:1854137) | #127 | ||
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Oops! Actually it looks like I transposed the 8 and the 9, I think it is 895 not 985 (based on 745.7 Watts to a HP) 1200 *0.746= 895. Phoenix, 883 would be PS, which is 735.5 Watts. (results are rounded)
AU N EGL, to do it with torque you use the torque to HP Conversion previously talked about. So, keeping it all imperial for you: HP = (ft.lb x rpm) / 5252 and BMEP = (HP * 895) / (Cap.Litre x RPM) so BMEP = (ft.lb x RPM x 895) / (Cap.litre x RPM x 5252) The RPM cancel each other, but sine 895 and 5252 don't go into each other easily we will leave them as whole numbers, so BMEP = (ft.lb x 895) / (Cap.litre x 5252) |
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1 Mar 2007, 07:52 (Ref:1854360) | #128 | ||
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Peak bhp.
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1 Mar 2007, 10:20 (Ref:1854448) | #129 | ||
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What is your peak torque and at what rpm? |
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1 Mar 2007, 12:58 (Ref:1854544) | #130 | |||
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1 Mar 2007, 17:44 (Ref:1854755) | #131 | ||
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what does b m e p stand for ?
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1 Mar 2007, 21:39 (Ref:1854937) | #132 | ||
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Brake Mean Effective Pressure.
It is the average cylinder pressure over the working stoke. The higher the pressure the more energy being put into the piston-rod-crank. The higher it is, the more the design has been "optimised". The key thing about BMEP is that it takes into account engine rpm, engine volume and engine power output. It’s the only equation to use when comparing engines from the perspective of saying which is more highly developed, BHP/Litre doesn't represent the work being don/rpm. So two 2 litre motors bother with 150 bhp, one does it at 5000 rpm the other at 7500 rpm, the one that can do it at lower revs is obviously more effecient (and this would be reflected in the BMEP... pulls out spread sheet and 13.44 vs 8.96) To help link it all in together: max torque, BMEP, and VE are usually at the same point |
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1 Mar 2007, 23:27 (Ref:1855012) | #133 | |||
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then there are some cars that have flat torque curves from oh say 2400 right up to 4800 rpms or almost flat. Nothing like torque to get out of corners and up to speed |
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2 Mar 2007, 01:08 (Ref:1855083) | #134 | ||
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Not true max torque most deffinatly can be above 5252, it is true that they cross at 5252
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Contrary to popular opinion, I do have mechanical sympathy, I always feel sorry for the cars I drive. |
2 Mar 2007, 08:02 (Ref:1855361) | #135 | |||
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2 Mar 2007, 08:32 (Ref:1855372) | #136 | ||
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so if the two cars had the same bemp and the same power to weight ratio they should be comparable on performance on the track yes?
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2 Mar 2007, 08:34 (Ref:1855373) | #137 | ||
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Ok, using todays version of the formulae -
BHP BMEP = 13.55 Torque BMEP = 13.52 Near enough given I am reading the figures off a graph. Peak torque is 127 ft lb at 7500. Just a bit higher than 5252! Out of interest it drops to about 120 ft lb at 9000 and only makes about 105 ft lb at 6750. |
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2 Mar 2007, 08:39 (Ref:1855374) | #138 | ||
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No, they would just be considered equally 'efficient', all other things being equal the large capacity engine would still be 'faster'.
Probably the figure is best though of as a benchmark on how well tuned your engine is. |
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2 Mar 2007, 11:04 (Ref:1855449) | #139 | ||
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Last edited by phoenix; 2 Mar 2007 at 11:06. |
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2 Mar 2007, 12:32 (Ref:1855519) | #140 | ||
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Unfortunately in a competitive series all it then takes is someone to put the effort into making the car lighter AND having the more powerful engine and you are stuffed.
And in a series with weight limits you CAN't make it lighter. |
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2 Mar 2007, 12:47 (Ref:1855525) | #141 | |||
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HP is caculated from TQ and Peak TQ can not be above 5252 rpms. As 5252 rpms is were HP = TQ. Normally Peak TQ is in the 4800-5000 rpm range http://vettenet.org/torquehp.html HP = ((TQ x rpms) / 5252 ) TQ in foot pounds |
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2 Mar 2007, 12:58 (Ref:1855534) | #142 | |||
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For instance at 5252 my engine is only making about 85 ft lb. A Formula Atlantic engine (or anything else on 325 degree cams!) isn't a 'normal' engine! |
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2 Mar 2007, 13:59 (Ref:1855581) | #143 | |||
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So at 5252 rpms the car is producing 85 hp too. In the States here most engine and chassie dynos are useing the SAE ( Society of Automotive Engineers) standards for determaining TQ and caculation of HP. This is now being considared the GOLD standard for all TQ and HP caculaitons. Using these standared, or to my understanding, TQ can not be greater the HP above 5252 rpms. Now the SAE does have different correction factors do to type of engine, altitude, and barrometric pressure. |
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2 Mar 2007, 14:26 (Ref:1855592) | #144 | ||
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As a number of people have stated above, peak torque can occur above 5252 rpm and does, often, in race engines. The formula does not preclude this from happening - look again at it or get a maths teacher to explain it to you. The only thing that happens at 5252 rpm is that both torque in ft/lbs and power in bhp are the same. The torque can still be rising, and if it is so will the bhp and not just because of the rising engine speed. Simple as that. Last edited by phoenix; 2 Mar 2007 at 14:28. |
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2 Mar 2007, 16:21 (Ref:1855651) | #145 | ||||
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I think there is a difference in how we define terms and SAE corected cacluations that may be causing the confussion. I can post of my dyno graphs latter. |
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2 Mar 2007, 16:25 (Ref:1855653) | #146 | |||
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These are entirely separate subjects. |
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2 Mar 2007, 16:26 (Ref:1855655) | #147 | |||
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2 Mar 2007, 16:49 (Ref:1855668) | #148 | ||
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Here is a link to the page detailing a 2 litre, V8 engine derived from a Yamaha motorcycle engine. You will see that peak torque is acheived at 8500 rpm and peak power at 10000 rpm. http://www.cyclonepowerltd.co.uk/products.htm From the information provided: You can compute, with the formula you know and love so well, that at 8500 rpm this engine will be making 270 bhp. You can also compute that at 10000 rpm the engine is making 158.6 ft/lb of torque. You can also compute that at 8500 rpm the engine has a b.m.e.p. of 14.065 bar You cannot compute what torque the engine will be making at 5252 rpm, but I would guess it will be very little. SAE correction has nothing to do with any of this. Simply, you seem not to understand the relationship between revs, torque and bhp and it would appear that we in this forum are unable to enlighten you. Last edited by phoenix; 2 Mar 2007 at 16:55. |
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2 Mar 2007, 17:00 (Ref:1855672) | #149 | ||||
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Source: http://www.cyclonepowerltd.co.uk/Image12.gif It is difficult to see where the exact hp and tq lines cross. It does seem to be slightly above 5252 at more like 5800. So we dont know the corrected values, but does seem to be a realtively flat torque curve from 5000 to 10,000 rpms Quote:
Any more dyno graphs, please post them not just as links so we all can be enlightened. |
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2 Mar 2007, 20:37 (Ref:1855795) | #150 | ||
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"
Dyno Correction Factor and Relative Horsepower So what's all this correction factor stuff anyway??The horsepower and torque available from a normally aspirated internal combustion engine are dependent upon the density of the air... higher density means more oxygen molecules and more power... lower density means less oxygen and less power. The relative horsepower, and the dyno correction factor, allow mathematical calculation of the affects of air density on the wide-open-throttle horsepower and torque. The dyno correction factor is simply the mathematical reciprocal of the relative horsepower value. What's it good for? One common use of the dyno correction factor is to standardize the horsepower and torque readings, so that the effects of the ambient temperature and pressure are removed from the readings. By using the dyno correction factor, power and torque readings can be directly compared to the readings taken on some other day, or even taken at some other altitude. How's it calculated? The Society of Automotive Engineers (SAE) has created a standard method for correcting horsepower and torque readings so that they will seem as if the readings had all been taken at the same "standard" test cell where the air pressure, humidity and air temperature are held constant. The equation for the dyno correction factor given in SAE J1349 JUN90, where: cf = the dyno correction factor Pd = the pressure of the dry air, mb Tc = ambient temperature, deg C The pressure of the dry air Pd, is found by subtracting the vapor pressure Pv from the actual air pressure. For more information about pressures and calculation of the vapor pressure, see Air Density and Density Altitude. Horsepower and Torque: Power is the rate at which work is done. When the engine torque is turning the crankshaft and power is being delivered, the resulting horsepower may be expressed as: which can be simplified as where: hp = horsepower, hp t = torque, ft-lbs rpm = engine speed, revolutions per minute This is a great formula. Basically it says that if you can keep the same amount of torque, then the more rpm you can turn, the more horsepower you get! " Source: http://wahiduddin.net/calc/cf.htm There may be some higher torque numbers above 5252, but they will not be by much. One quick observation is the RPM where the HP and TQ curvers cross. The two line cross at 5252 rpms. When you see a graph and the two lines do not cross at 5252 rpms, then the correction factor was not used, the dyno or tester is subject to question. Last edited by AU N EGL; 2 Mar 2007 at 20:39. |
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