BMEP and other technical terms
Discussion
Ok on Gazboys request on another post I'm going to cover my understanding of the term BMEP.
I'm actually going to shameless rip-off/quote some of my previous posts and copy and paste here.
BMEP stands for Break Mean Effective Pressure.
Think of BMEP it as specific torque- or torque per litre- that's all.
It allows you to compare how well an engine produces torque for the capacity, so you can compare submarine engines (say) with Fiat Cinqecentos!
Worked example:E36 3.2 litre M3 engine makes 350 Nm of torque at 3250 rpm .
The engine capacity is 3.201 litres.
So 350/3.201=109.34
109.34 X 0.1257= 13.744 bar BMEP.
And finally, to put things into perspective, I've seen the ignorant comparing pushrod Chevy V8 engines with twin cam 4 valve per cylinder VCT engines- thinking that GM haven't been trying- which is as pointless as comparing a Turbo charged engine with a naturally aspirated engine.
To be a bit fairer let's NOT compare boosted engines with naturally aspirated
The next thing should be to seperate home modifed cars from production cars by the manufacturers, the manufacturers are far more constrained (with emissions and durability etc etc), and there's alot of unscrupilous "pub talk"- with people claiming to get 100/Bhp / litre from naturally aspirated 2 valvers all the time!
This post will focus on the most difficult of all: Production naturally aspirated cars design to be sporty road units- NOT race engines.
The most difficult task is to obtain high BHP/litre from a Naturally aspirated engine (like BMW and Honda) and THEN achieve good BMEP (or specific torque at low speeds) Like BMW have with the E36 3.2 litre S50 M3 engine. Infact I would rate the E36 M3 3.2 litre way above in terms of achievement over the honda S2000, no other car can achieve close to 14 bar BMEP at 3250 rpm and over 100 Bhp/litre at 7400 rpm-such incredible range. (The new CSL is even more impressive as it achieves 14.3 bar BMEP and 360 Bhp from a 3.2 litre)
I’d say that in a production type of engine it is very very difficult to get 100 Bhp per litre from a 4 Valve. It’s also pretty impressive going getting 105 Nm/litre (77lb ft/litre)(13.2 bar BMEP) out of a 4 valve unit and quite hard to achieve in practice.
The very best two valve I’ve seen approach perhaps 80/Bhp/litre from the 911/993 Porsche RS but this was a twin plug affair. A more realistic and achievable figures are like the Chevy Vette C5 engine and the BMW M20 325i E30 unit-for two valves they produce around 68-70 Bhp/litre. Obviously you can compromise low speed and mid range to up this figure- but getting above 80/litre is good going! The best specific torque you’re likely to see is 93.9Nm/litre (11.8 bar BMEP) or 69 lb ft per litre- that’s good going!'
I'm actually going to shameless rip-off/quote some of my previous posts and copy and paste here.
BMEP stands for Break Mean Effective Pressure.
Think of BMEP it as specific torque- or torque per litre- that's all.
It allows you to compare how well an engine produces torque for the capacity, so you can compare submarine engines (say) with Fiat Cinqecentos!
wheeljack888 said:
Textbook equation for BMEP is:
BMEP (Pa) = n * P / D * N
or because P = T * 2 * pi * N
BMEP = n * T * 2 * pi / D
where
n = no. of revs per power stroke (2 for 4 stroke)
P = Power (watts)
D = displacement (m^3)
N = revs / sec
T = torque (Nm)
BMEP (Pa) = n * P / D * N
or because P = T * 2 * pi * N
BMEP = n * T * 2 * pi / D
where
n = no. of revs per power stroke (2 for 4 stroke)
P = Power (watts)
D = displacement (m^3)
N = revs / sec
T = torque (Nm)
Worked example:E36 3.2 litre M3 engine makes 350 Nm of torque at 3250 rpm .
The engine capacity is 3.201 litres.
So 350/3.201=109.34
109.34 X 0.1257= 13.744 bar BMEP.
And finally, to put things into perspective, I've seen the ignorant comparing pushrod Chevy V8 engines with twin cam 4 valve per cylinder VCT engines- thinking that GM haven't been trying- which is as pointless as comparing a Turbo charged engine with a naturally aspirated engine.
To be a bit fairer let's NOT compare boosted engines with naturally aspirated
The next thing should be to seperate home modifed cars from production cars by the manufacturers, the manufacturers are far more constrained (with emissions and durability etc etc), and there's alot of unscrupilous "pub talk"- with people claiming to get 100/Bhp / litre from naturally aspirated 2 valvers all the time!
This post will focus on the most difficult of all: Production naturally aspirated cars design to be sporty road units- NOT race engines.
The most difficult task is to obtain high BHP/litre from a Naturally aspirated engine (like BMW and Honda) and THEN achieve good BMEP (or specific torque at low speeds) Like BMW have with the E36 3.2 litre S50 M3 engine. Infact I would rate the E36 M3 3.2 litre way above in terms of achievement over the honda S2000, no other car can achieve close to 14 bar BMEP at 3250 rpm and over 100 Bhp/litre at 7400 rpm-such incredible range. (The new CSL is even more impressive as it achieves 14.3 bar BMEP and 360 Bhp from a 3.2 litre)
I’d say that in a production type of engine it is very very difficult to get 100 Bhp per litre from a 4 Valve. It’s also pretty impressive going getting 105 Nm/litre (77lb ft/litre)(13.2 bar BMEP) out of a 4 valve unit and quite hard to achieve in practice.
The very best two valve I’ve seen approach perhaps 80/Bhp/litre from the 911/993 Porsche RS but this was a twin plug affair. A more realistic and achievable figures are like the Chevy Vette C5 engine and the BMW M20 325i E30 unit-for two valves they produce around 68-70 Bhp/litre. Obviously you can compromise low speed and mid range to up this figure- but getting above 80/litre is good going! The best specific torque you’re likely to see is 93.9Nm/litre (11.8 bar BMEP) or 69 lb ft per litre- that’s good going!'
Mr Whippy said:
Very nice.
Any chance of a more complete equation though
Quite odd getting metric Pa BMEP values and then your referring to Bar BMEP values wrt to the M3 etc.
Just to save people getting confused anyway
Also, what does BMEP suggest then? A more complete picture of the engine's specific output apart from bhp/litre?
Are revs irrelevant then? Surely it's pretty flawed in itself if like lots of modern engine's they are "de-tuned" to give flat torque outputs, then the peak torque may not be indicative of the engine's actual ability. 3.2 VR VAG lump for example... Phaeton, R32, A3 3.2 and 3.2 TT all have varying outputs for example...
Just curious why people use BMEP.
BMEP = n * T * 2 * pi / D
All constants except T and D, and thats basically BMEP is proportional to T/D = Nm/litre?
Dave
Any chance of a more complete equation though
Quite odd getting metric Pa BMEP values and then your referring to Bar BMEP values wrt to the M3 etc.
Just to save people getting confused anyway
Also, what does BMEP suggest then? A more complete picture of the engine's specific output apart from bhp/litre?
Are revs irrelevant then? Surely it's pretty flawed in itself if like lots of modern engine's they are "de-tuned" to give flat torque outputs, then the peak torque may not be indicative of the engine's actual ability. 3.2 VR VAG lump for example... Phaeton, R32, A3 3.2 and 3.2 TT all have varying outputs for example...
Just curious why people use BMEP.
BMEP = n * T * 2 * pi / D
All constants except T and D, and thats basically BMEP is proportional to T/D = Nm/litre?
Dave
As indicated in the post it's an indication of Specific torque for comparison purposes, it's all written up there.
Bar BMEP is a common convention that's well known-it's not desined for laymen- more for technical folk, who usually know what other comparable engines in the field produce- but it can be an eye opener when enthusiasts use it to on their favourite sports cars. No more different using bar over Pa than using Bhp rather than KW, and BHP/litre. It's just a convention that people are familiar with over here- Americans often use PSi, nothing to get excited about or split hairs about, just do the conversion
Who's informed you that the 3.2 litre VW lump was tuned for low speed torque? VW themselves?
Nothing flawed about using BMEP as a measure as long as it is used in context, i.e comparing what BMEP other competitor engines make at 1000 rpm, then at 2000 rpm then at 2500 rpm etc. Quite simple really.
What kind of engine dyno was that measured on?
Was the dyno calibrated to DIN72000? (even if the engine dyno was calibrated to DIN72000 there is always dyno to dyno variation- if the dyno is rated at a much higher output than the engine being tested- it's generally accepted than there will be less accuracy.
Was that measured with EEC correction factors, SAE or DIN?
Was the mapping checked for MBT ignition/DBL and mixture for LBT/or catalyst protection?
Was the dyno calibrated to DIN72000? (even if the engine dyno was calibrated to DIN72000 there is always dyno to dyno variation- if the dyno is rated at a much higher output than the engine being tested- it's generally accepted than there will be less accuracy.
Was that measured with EEC correction factors, SAE or DIN?
Was the mapping checked for MBT ignition/DBL and mixture for LBT/or catalyst protection?
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