Engine Output Analysis
Here's an analysis of some engine output data that I obtained from the back pages of Evo magazine.
This is simply the power output divided by the engine capacity. There are no obvious trends apart from around 80BHP/L being higher end of typical for a normally aspirated (na) engine and turbo engines having more. Obviously due to boost.
Much more informative is the (brake) mean effective pressure. This is power
divided by engine capacity and then divided by rpm at which that power was made.
bmep = power / (L * rpm)
Since L*rpm represents the nominal air flow rate through the engine, then bmep
represents the power produced per air flow rate. This represents how well the
engine is using the air flow to make power.
Another way of looking at it is to make the substitution that:
power = torque * rpm
mep = torque * rpm / (L * rpm) = torque /L
So bmep is giving the torque to capacity ratio at max power. A constant
multiplier is needed depending on what units bmep is in.
Looking at the chart below, then the na engines now have a much closer
agreement. Somewhere between 10 and 14 BHP/(L rpm/1000) is normal. So if you
have a 2L engine and want to make max power at 6K then the power would be
between 12*10 to 12*14BHP which is 120 to 170BHP. This doesn't mean that you
will make that much power, but those numbers are what a typical engine
makes.
A 20XE on a 196 kit is making about 196BHP at 6500rpm say. That's a bmep of
about 15. About 7% better than the typical engines in the graph.
Come to think of it, when talking about bmep....
P = 0.193*T*(rpm/1000) if the units are lb-ft and BHP.
=> P/(rpm/1000) = 0.193*T
So if the bmep graph values were divided by 0.193 then they'd be in units of
lb-ft per L.
Taking a typical bmep value of 12.5,divide this by 0.193 and I get 65. This is a
tiny bit lower than the typical specific torque value of 70lb-ft/L. So at peak
power the engines are usually making about 65/70 of peak torque, which is about
10% down. This is typical.
Notice how the na engines all agree closely. Between 60 and 75 lb-ft per Litre is typical. So anyone claiming to make more than 150 lb-ft from a 2L engine is acting suspiciously. It is possible to make up to 190lb-ft from a 2.0 engine. This requires a high compression ratio, race fuel, suitable cam shafts and carefully tested exhaust manifold design.
Looking at the XE on throttle bodies, since the increase in bmep was about 7%, then you could wager that around a 7% increase in torque would be possible with throttle bodies. That would take torque from 150 to 160lb-ft. Sounds about right.
Finally, a quick look at 0-60 time versus power to weight. I deduced a long
time ago that the time to reach a certain speed from rest, would roughly vary
by:
T proportional to speed^2 / (power to weight ratio)
By comparing this to the actual data from the Evo listings I found:
T(s) = 0.28 x (speed in mph)^2/(power in BHP/weight in tonnes)
This curve is shown in Purple in the last chart. It's not a bad result; reality
very roughly follows with expectations.
