BMEP - why it is the best way compare engines

BMEP - why it is the best way compare engines

BMEP - what is it and what does it tell us?

 

So what is BMEP an abbreviation for? Perhaps it's British Member of the European Parliament, or maybe Brakes a Massive Enterprise in Pastries, or maybe even Big Motors Equal Power.

 

Well, I can safely say that it is none of those. In fact, as we're in the UK, I really think we ought to call them engines not motors.

 

So the actual meaning of the catchy BMEP, is Brake Mean Effective Pressure. So what is that?

 

Sometimes when you aren't writing for a fee, you can simply post the Wiki answer as it's generally pretty good.

 

“The Brake Mean Effective Pressure is a quantity relating to the operation of a reciprocating engines and is a valuable measure of an engine's capacity to do work that is independent of engine displacement.[1] When quoted as an indicated mean effective pressure or IMEP (defined below), it may be thought of as the average pressure acting on a piston during the different portions of its cycle. “

 

 

So although it clearly gets higher with a greater horsepower per litre output, the beauty of BMEP is It does not discriminate against engines of a lower peak horsepower output, and even a lower horsepower per litre.

 

Let's put it into perspective with a few examples.

 

The Honda Type R Civic engine is generally regarded as a fairly incredible thing.

Producing 197 hp from 1998cc it makes 98.6 bhp/litre.

 

The Peugeot 106 Gti Makes 118hp from 1587cc - 74.4 bhp/litre.

 

The FIAT Punto HGT Makes 130hp from 1747cc - 74.4 bhp/litre


 

This puts the Honda in a lead by 32.5%

 

So are these two little Latin cars lagging way behind? Surely with the Honda making that much more per litre you would think they would be miles behind. Not when you look at BMEP.

 

The Honda makes 178.8psi

The FIAT 171.1 psi

The Peugeot 166.5 psi

 

So what gives the FIAT the edge over the Peugeot, and why is the Honda only 4.5% higher than the FIAT?

 

The reason is the BMEP lets you look at the real engine performance. If you like, the “money” behind the engine. The Honda engine does indeed employ the revolutionary VTEC system, this raises the output at high rpm. However, due to Honda wanting an excellent marketing tool, the switch to theVTEC is made all the more exciting, by employing very inefficient cam design when off VTEC lobe. This reduces the BMEP of the engine. So what you can see is that the headline figure of the VTEC engine makes it look like the Honda could beat the FIAT by 30% or more. In reality though the BMEP tells the truth as does the flying Kilometer – the Honda 27.8 secs the FIAT 29.9 secs. Making the Honda only 7 % faster than the FIAT. Slower obviously, but close enough to allow qualifying in the same race if they were running in a series like F1.

 

So what the BMEP tells us, together with the specific output – is how big the overall bang will be. If you look at the Honda HP per litre in isolation, it looks unassailable, but in reality it could make a much higher BMEP. This would be achievable by running camshafts with both higher lift and longer duration ,when the engine is not in VTEC mode. In addition a higher compression ratio would help as well. So how does the FIAT make more BMEP than the Peugeot if the Specific output is the same? Once again we must look to the beauty within the BMEP equation. We know that compression ratio is a key determinant in engine torque output per litre. One wonders further then how the Peugeot is at 10.8:1 and the FIAT 10.3:1.

 

The answer is in the variable valve timing on the FIAT. Invented by Alfa Romeo and FIAT engineers, on the later twin spark engines, this system advances and retards the inlet cam according to RPM. By changing the inlet opening cam position from advanced to retarded, both the nature of the burn, and the RPM range of the engine are changed. The retarded position gives good torque and stability of operation at low rpm. Then as revs reach a mid point, advancing the inlet opening point creates a strong boost in mid-range torque. Then at the high end of the rev scale, retarding timing allows the engine to rev out better.

 

EVO Magazine tested the 106 as the faster car around a circuit, even though the Punto HGT won their slalom test against the Mini as well. However, what BMEP shows us is that you must not under estimate the speed of the vehicle, which has a high BMEP ratin

 

Performance data supplied from Carfolio, with thanks for your excellent website. If ever you needs stats just type the car followed by Carfolio.

thanks to wikipedia for the definition of BMEP.