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Topic Review (Newest First)

  • 07-26-2008, 03:06 PM
    MACHINEHEAD1
    So once I get my 30k worth of info, what do I do? Spend another 30k on combustion chamber cnc programs/swirl-tumble annalisis/camshafts/pistons/liquid flow deally from GM. Should have just coated the piston tops and put in another 2 degrees of cam. This is why most of us dont look to much at BMEP. Especially when my customer is going to the river in 2 weeks, already has his rooms paid for, mother in law coming in from out of town and I dont even have the rail kit back from pollish yet. Build em for a living and you dont have much time to theorize about the " Id like to try this spacer and valve adjustment today". You might have to wait for the next build to push the envelope a little further.BTW my job on dyno day is to STARE at the A/F and knock meter. Just stare at the lights
  • 07-25-2008, 12:33 AM
    scott foxwell
    Quote Originally Posted by Warp Speed View Post
    Well...............

    BMEP numbers registered from dyno's tell nothing really except observed/corrected torque/CI in that cell, which can easily be done with a calculator. Comparing BMEP's from differant dyno's even gets worse depending on weather correction factor, room temp, fuel temp ect. Also, some dyno's use differant friction tables to adjust their correction factor. This will scew the results/data also as no 2 engines are the same.

    The only way to form a "Real" BMEP table/map is to base it off of actual observed combustion pressures in the cylinder in relation to crank position (combustion analysys). That way, the true work can be measured and pumping losses figured.
    These sensors are about $30,000.00 a set (proper ones anyway) and don't last that long, so for most shops it is not cost efective.

    It has been an interesting thread, but I think it is just like racing dyno numbers, WAAAAAY too many variables to be useful information.

    IMHO
    bottom line.
  • 07-24-2008, 03:11 PM
    Warp Speed
    Quote Originally Posted by Warp Speed View Post
    Well...............

    BMEP numbers registered from dyno's tell nothing really except observed/corrected torque/CI in that cell, which can easily be done with a calculator. Comparing BMEP's from differant dyno's even gets worse depending on weather correction factor, room temp, fuel temp ect. Also, some dyno's use differant friction tables to adjust their correction factor. This will scew the results/data also as no 2 engines are the same.

    The only way to form a "Real" BMEP (read IMEP?) table/map is to base it off of actual observed combustion pressures in the cylinder in relation to crank position (combustion analysys). That way, the true work can be measured and pumping losses figured.
    These sensors are about $30,000.00 a set (proper ones anyway) and don't last that long, so for most shops it is not cost efective.

    It has been an interesting thread, but I think it is just like racing dyno numbers, WAAAAAY too many variables to be useful information.

    IMHO
    To correct myself, as you can see in the post above, BMEP's are just that, a figure generated by observed torque/displacement. Because of all the variables between dyno's, cell conditions and correction factors used. The only useful data would be straight from the combustion chamber. Anything else is just a measurement that is guessing at the variables present.

    Ya, I think that's what I was trying to say!?!?!
  • 07-24-2008, 01:18 PM
    ptz
    camshaft.
    if it doesnt make cylinder pressure without detonation you won't have good bmep...

    VE...
    if your camshaft does not mach the flow numbers on the heads you won't have good bmep...

    below pertains to two strokes..
    they use port timing instead of a camshaft...
    but if you think outside of the box...it opens up the mind...


    this is reposted without permission...


    Brake Means Effective Pressure


    The following notes are in the SI, system international, or metric system for simplicity. This also reduces the need for unnecessary and confusing constants in the derived equations. The only concession will be to include the power in horsepower.

    When an engine is tested in the laboratory it is generally connected to some form of "brake", or dynamometer. As the slang name implies this device maintains a constant engine speed independent of the throttle position and the power produced by the engine. The outside casing of the dynamometer is allowed to rotate freely and is restrained by a load cell at the end of an arm of known length. This load cell behaves like an electronic spring balance, and after calibration outputs a force value (F) in Newtons. The distance between the centreline of the dyno. and the load cell is the length of the torque arm (L).

    The Brake Torque of the engine can then be written:

    Tb = F x L (1)

    The idea of torque can be illustrated by a see-saw with different masses at different distances from the pivot. The same torque is produced by a small mass m acting at a distance 2d from the pivot point as a larger mass 2m acting at a distance d. Thus the see-saw is in balance. In a vehicle the torque is the ability of the engine to turn the wheels , (similar to the ability of the engine to turn the dyno.), at a particular speed.

    The work done by any mechanism is defined as the force exerted multiplied by the distance over which it is applied. When pushing a weight along the floor a distance d using a force F, the work done is equal to F x d. The work done by the engine per revolution is the force F times the distance traveled by the force in one revolution of a circle radius L.

    Work per revolution = 2 x Pi x F x L = 2 x Pi x Tb ( From eqtn 1) (2)

    Power is the rate of doing work, i.e. how fast the weight was pushed across the floor. For the engine the Brake Power is given by:

    Pwrb= ( Work per Revolution) x (Revolutions per second) = 2 x Pi x Tb x rpm / 60 = Pi x Tb x rpm / 30 Watts = Pi x Tb x rpm / 30000 kW (3) = (Pi x Tb x rpm / 30000) / 0.7457 Hp

    The brake specific fuel consumption, bsfc, is then defined as:

    bsfc = (fuel consumption rate) / (Power output) = M.f / Pwrb kg/kWh (4)

    In order to understand bmep we must consider indicated mean effective pressure (imep) and make some comparisons with the theoretical ideal Otto cycle which the two-stroke cycle engine aspires to emulate.

    An ideal Otto cycle is shown in Fig. 1 and can be described as follows : From position 1 at exhaust port closure the piston ideally compresses the gas in the cylinder, which has the properties of air, with no heat loss to the walls, (isentropically), to top dead centre (TDC). The gas is compressed according to the equation:

    P x V ^ Gamma = Constant (5) where gamma is the ratio of the specific heats of the gas ( a number which is constant for a particular gas at a particular temperature). Gamma is 1.4 for air at atmospheric temperature, for mixtures of air and fuel or mixtures of air and exhaust gas or pure exhaust gas gamma is less than 1.4. For pure exhaust gas at: 1000 Deg K Gamma approx = 1.31, 500 Deg K Gamma approx = 1.35

    Heat is then added to the gas "instantaneously" at TDC and the cylinder pressure rises to a peak still at TDC. The gas is then allowed to expand, again with no heat transfer, according to equation (5) until the exhaust port opens. When the exhaust port opens the heat is rejected "instantaneously". There is no gas exchange during the cycle, heat is only added and extracted from the gas, so the cycle does not have the open cycle of a two-stroke cycle engine, (the time when the exhaust and transfer ports are open). The ideal Otto cycle is compared to a two-stroke cycle in Fig. 3 and the effects of non-ideal processes can be identified. The pressure history can be presented in several different ways which are used to look at specific areas of the engine cycle. As well as the overall shape of the cycle ,several important parameters can be noted from Figs. 3 and 4, such as the cylinder pressure at exhaust port opening and closing, peak cycle pressure and location and if several consecutive cycles are plotted on the same axes cycle to cycle variation can be observed.

    The coefficients of compression and expansion are the actual values of gamma measured from the real expansion and compression processes. These are difficult to determine from pressure - crankangle, or pressure - volume graphs, but my be determined from the gradient (steepness) of the straight lines if the data is plotted on as log P versus log V as shown in Fig. 5. Mathematically this can be shown:

    From eqtn 5 P x V ^ Gamma = constant
    Log ( P x V ^ Gamma) = Log (constant)
    Log P + Gamma x Log V = Konstant
    Log P = - Gamma x Log V + Konstant

    Compare with y = m . X + c

    Where m is the gradient and c is the intercept, (where the line would cross the x axis if extended).

    As stated above the real cycle does not follow the ideal and so the real coefficients of compression and expansion differ from 1.4, (ideal for pure air), due to temperature effects and gas composition. The inefficiency of the burning, or heat addition process, can also be seen in Figs. 3 to 5 as the real cycle does not achieve the same peak cylinder pressure as the ideal and the peak is not at TDC. The use of in-cylinder pressure traces and other data which can be calculated from it can help to give an understanding of the combustion processes happening within the cylinder and hopefully help develop an optimised combustion chamber, ignition timing and observe detonation.

    The ideal work done during the cycle is the work done on the top of the piston during one complete revolution, (two-stroke). In this case the force F is created by the gas pressure p times the area of the piston A, the distance is x moved by the piston. If the motion of the piston is separated into very small increments, dx, then the total work produced per cycle is the sum of all the work done during these small movements. (I'm avoiding the use of words like calculus and integration)

    Work produced per cycle = Sum of all [ F x dx ] = Sum of all [ p x A x dx] = Sum of all [ p x dV ]

    Where dV is the small change in cylinder volume caused when the piston moves a distance dx. The imep or indicated mean effective pressure is the average or mean pressure in the cylinder, which when multiplied by the cylinder swept volume would produce the same work out of the cycle as the real pressure. The ideal work from the cycle is the area enclosed by the loop as shown in Fig. 6 and is the same area as the shaded area calculated from the imep. The use of the word indicated is historical and comes from the fact that the devices used to measure cylinder pressure were called indicators, and the graphs produced were called indicator diagrams.

    The imep is the ideal average pressure which the engine can output. Friction of bearings etc. and the pumping losses of the crankcase are not taken into account, it is calculated from the cylinder pressure only. The bmep or brake mean effective pressure can be calculated from the measured torque and is related to the imep by the following equation.

    bmep = imep - fmep - pmep

    Where fmep is the friction mean effective pressure and pmep is the pumping mean effective pressure. All are calculated in the same way as imep ie. the pressure times swept volume which gives the required work per cycle used for friction or pumping. Bmep is a useful comparitor of engine performance and state of tune, although, care must be taken to consider the rpm at which the engine is operating to determine the power potential. Different engines can be compared directly as swept volume has been removed from the power or torque figures to calculate bmep. The relevant equations are shown below along with a comparison of several engines tuned by Factory Pipe.

    bmep = Power / ( Swept Volume x revolutions per second)
    bmep = 2 x Pi x Torque / (Swept Volume)
  • 07-24-2008, 01:11 PM
    cfm
    Quote Originally Posted by Sleeper CP View Post
    Anyone have anything new to add ?

    Sleeper CP
    To this thread...nah. People aren't really getting the BMEP thing. No biggie really.

    I read your other thread Sleeper - I wish you good luck and keep us posted.
  • 07-24-2008, 12:47 PM
    RiverRacer
    Quote Originally Posted by Warp Speed View Post
    Come on, inquiring minds want to know!
    I've shaken the Military/Industrial complex enough for this week!!! I've got to get to work here or I won't be making it out to the River until Labor Day.

    RR:
  • 07-24-2008, 12:29 PM
    Warp Speed
    Quote Originally Posted by RiverRacer View Post
    Always!!!!!!!!!!!!! Just Kidding!! PSYCH!!!!!!!!!!!!!!

    I sent you a p.m., Jon. My adding anything would start another meltdown, which may be somewhat entertaining for most, but may be harmful to others.

    I'd rather not go there on this thread.

    RR

    Come on, inquiring minds want to know!
  • 07-24-2008, 11:49 AM
    RiverRacer
    Quote Originally Posted by Sleeper CP View Post
    Anyone have anything new to add ?

    Sleeper CP
    Always!!!!!!!!!!!!! Just Kidding!! PSYCH!!!!!!!!!!!!!!

    I sent you a p.m., Jon. My adding anything would start another meltdown, which may be somewhat entertaining for most, but may be harmful to others.

    I'd rather not go there on this thread.

    RR
  • 07-24-2008, 10:41 AM
    Warp Speed
    Quote Originally Posted by Sleeper CP View Post
    Anyone have anything new to add ?

    Sleeper CP

    Well...............

    BMEP numbers registered from dyno's tell nothing really except observed/corrected torque/CI in that cell, which can easily be done with a calculator. Comparing BMEP's from differant dyno's even gets worse depending on weather correction factor, room temp, fuel temp ect. Also, some dyno's use differant friction tables to adjust their correction factor. This will scew the results/data also as no 2 engines are the same.

    The only way to form a "Real" BMEP table/map is to base it off of actual observed combustion pressures in the cylinder in relation to crank position (combustion analysys). That way, the true work can be measured and pumping losses figured.
    These sensors are about $30,000.00 a set (proper ones anyway) and don't last that long, so for most shops it is not cost efective.

    It has been an interesting thread, but I think it is just like racing dyno numbers, WAAAAAY too many variables to be useful information.

    IMHO
  • 07-24-2008, 09:36 AM
    Sleeper CP

    Bump

    Anyone have anything new to add ?

    Sleeper CP
  • 01-29-2008, 01:12 AM
    scott foxwell
    Quote Originally Posted by Sleeper CP View Post
    I gotta hit the sack, but do you think those flow number's will support 1,000 hp? I'm not sure that it will.

    Sleeper CP
    Dependes on the ci and rpm. Remember, I made 820 with <350cfm
    467 @ 7000 with 270*@.050 is only using 315cfm, at 260*@.050 it's using 327.
  • 01-29-2008, 01:09 AM
    Sleeper CP
    Quote Originally Posted by steelcomp View Post
    Fully ported, Not bad...still weak on exhaust at 73%.
    I gotta hit the sack, but do you think those flow number's will support 1,000 hp? I'm not sure that it will.

    Sleeper CP
  • 01-29-2008, 01:07 AM
    scott foxwell
    Quote Originally Posted by Sleeper CP View Post
    As you know it is one reason your bad ass "little" 467 made so much power. Crower and Mummert look at max flow number's for just a second they are more interested in mid lift number's and port velocity. Big flow number's by themselves are meaningless.Sleeper CP
    True, true, and true. Any number by itself is meaningless (in this context for you literalists) but in context max flow numbers have their value.
  • 01-29-2008, 01:04 AM
    scott foxwell
    Quote Originally Posted by Sleeper CP View Post
    I know you know, but Geoff and Danny loved the numbers on my heads for the 4.6 bore and 4.25 stroke pulling through a single carb. Lot's of variables you know.

    I'm giving some serious thought about a Steph's pan and the bigger carb and throwing it on the dyno this winter.

    Sleeper CP
    Big Inch Ford Lover
    That's why I was asking about your CSA. Max flow numbers are really not important since you're not lifting that far. What are the mid lift numbers? What's your rod length? (probably asked that)
    What's the port volume? What was the V @ the MCSA? Where is the MCSA (how far from the valve?) What's the throat dia?
    Stephs pan and a bigger carb and run it to 7500...
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