Accurate Approximation of Porsche 911 Engine Power Output

[dkirk]

When making modifications to the Porsche 911 engine, it would be nice to accurately estimate the increase in power output. As we are all aware, there are not many modifications that can be done to substantially increase output as the factory did an outstanding job in the design and engineering of this beautiful engine. Most of the P-car drivers I know (myself included) have performed the easily–implemented modifications…performance chip, catalyst bypass pipe, low-restriction air filter, and possibly an aftermarket exhaust system or low restriction muffler. In some cases, different camshafts are also installed. The changes will alter the air/fuel ratio for best power at full throttle conditions plus increase the delivery ratio (volumetric efficiency) for an improved power output. These effects, combined with an increase in maximum engine speed, are well-known performance boosters. There exists a straight-forward method of calculating these power increase predictions that I would like to present here.

To measure the power output of any internal combustion engine, a crankshaft dynamometer is the instrument of choice. This is how engine performance is measured in the factory development lab and is what determines the rated brake horsepower that is advertised. A chassis dyno would be the next choice – not as accurate (due to “guesstimates” as to transmission and driveline losses) but some would argue is more realistic in that power output at the driving wheels is measured. When we discuss engine power output though, it’s usually the crankshaft-generated figure we’re all interested in. I would like to present a method that works remarkably well in approximating the power output of a modified and well tuned 911 engine. This discussion only applies to the two-valve, air-cooled, naturally-aspirated 911 engines…pressure charged engines are not considered.

When comparing engines of similar type, a term known as BMEP (Brake Mean Effective Pressure) is most useful. BMEP is a back-calculated term, meaning it is determined from knowing the displacement of the engine, the engine speed (rpm), and the brake horsepower produced at that speed. For a four-stroke engine cycle, the relationship is as follows:



Where:

BMEP = brake mean effective pressure, psi
bhp = brake horsepower
D = engine displacement, in^3
N = engine speed, rev/min

BMEP can be thought of as the average (mean) pressure in the engine cylinders that produces a given power output at a given engine speed. It is a “specific” term, meaning that it remains relatively constant for engines of various displacements, speeds, and power outputs, as long as the engines in question are of similar mechanical design, and function using the same operational cycle (two vs. four-stroke cycle). This consistency makes BMEP a most-valuable term in comparing different engines to each other. BMEP is an indicator of how well an engine inducts and traps its air/fuel mixture, and how efficiently this mixture undergoes combustion to produce useful power output at the crankshaft. The consistency of BMEP over a wide range of full-throttle operation makes the term valuable for predicting:

• horsepower developed by increasing maximum engine speed
• horsepower produced by changing displacement
• both effects combined

The procedure used here was to examine representative 911 engine performance data from a relatively wide range of engine builds. From this information, maximum power BMEP values were calculated and compared for consistency. If consistency is indicated, accurate horsepower values may be back-calculated from this baseline BMEP. For acquiring data on various 911 engines, I relied on Bruce Anderson’s excellent book, “Porsche 911 Performance Handbook”. The author presents engine performance data in both tabular and graphical forms, thus allowing a representative sample of both stock and modified engine information to be assembled. The following engines were thereby selected for this study:

1. Stock 3.2 US version
2. Stock 3.2 Euro version
3. Stock 3.6 US
4. RSR 3.8
5. 2.7 Mod
6. 3.0 w/ Webers
7. 3.2 Mod SC
8. 3.2 SC Racing
9. 3.6 Mod w/ Webers
10. 3.6 Racing w/ MOTEC

Knowing the selected engine’s displacements and power produced at the specified engine speeds, the BMEP values were calculated using the expression given above. The plot of this BMEP data versus engine rpm is shown below:



It will be readily apparent how consistent the maximum power BMEP values are for the sample engines over the relatively wide range of engine speeds. Values range from a low of 150.9 psi (engine # 1) to a high of 185.3 psi (engine # 7) with an average being 166.3 psi. Statistically, this is a standard deviation of 14.0 psi, a remarkably low value for this sample, indicating the consistency of BMEP when related to engines of the same type, even though displacements and operating speeds differ greatly. Actually, a BMEP of 170 psi is quite typical of naturally aspirated, high performance, two-valve, hemispherical head 4-strokes utilizing exhaust and inlet system tuning to enhance volumetric efficiency. As an aside, the volumetric efficiency necessary to produce this BMEP is approximately 107% (calculating this is the possible subject of a future article) mostly being made possible by proper induction system tuning.

Now that we’ve established an average BMEP value of 166.3 psi with a standard deviation of 14.0 psi, we have all the information necessary to calculate the approximate brake horsepower output of our modified engine, and also a range into which the results will fall within. As actual engines always have a slight variation in power output, one compared to another, this needs to be accounted for. One standard deviation above and below the mean (for a normal distribution) accounts for 68.3% of the engines sampled herein. From experience, most engines of a given type would easily fall within this range and therefore, this was deemed the maximum deviation that one would encounter if actual dynamometer testing were conducted.

By rearranging the expression above to solve for brake horsepower, we have:



We now have all the information necessary to construct the following chart:



The results may now be plotted for the four engine displacements considered:









The plots clearly indicate that increasing maximum engine speed on a well-prepared 911 engine is the easiest way to gain horsepower. As an example, the 3.2 engine with modifications mentioned previously, would produce approximately 239.2 bhp @ 5900 rpm as indicated in the chart. By bumping up the rev limiter to 6600 rpm, the engine should then produce 263.5 bhp @ 6500 rpm, a 10% increase in power output. The other obvious power increaser is to raise the engine displacement. Using the chart, the modified 3.0 engine should make 241.7 bhp @ 6300 rpm. By increasing the displacement to 3.2 L, the engine should produce 255.4 bhp at the same rpm – a power increase of 5.7%. However, detailed and very careful engine preparation plus use of the best hardware available, our new 3.2 L could produce the upper limit horsepower of 276.9, an impressive 14.6% power increase.

Be aware that this predictive technique is only an approximate way of estimating the power output attainable from modified 911 engines. Don’t expect that just bumping up the rev limiter by 1000 rpm will yield the power increases shown. Turning engines up to higher speeds will obviously require valve gear and camshaft changes to allow high volumetric efficiencies to be maintained, commensurate with the higher mechanical inertia loadings imposed. Induction and exhaust system modifications would also be necessary to allow optimal tuning at these higher speeds.

To conclude – BMEP is a simple, yet effective term for predicting brake horsepower outputs of modified Porsche 911 engines using the typical modifications commonly performed. The average max power BMEP for 10 different build configurations is 166.3 psi, calculated from data obtained from a reliable source. From this figure, the brake horsepower output may be estimated knowing the engine displacement and operating speed.

[larrym]

OK - so my 2.7 dynos 125 at the rear wheels

- that gives me 85.7 BMEP per your formula

- now what dio i do with that number?

.

[racing97]

BMEP is not nearly as sexy as IHP (internet horsepower)

[racing97]

By the way excellent thread

regards

[weddinginshop]

thanks~~

[dkirk]

Thanks guys, I appreciate the comments.

Larrym – You can’t determine crankshaft BMEP from a chassis dyno. There are too many unknowns to account for. I believe most consider a 20% power loss to be an agreed-on value, but this is highly variable. Viscosity of the gearbox oil, tire pressure, a dragging brake, can all contribute to non-consistent and unreliable results. What I tried to convey was that if you use a BMEP value of 166.3 psi (based on an average of 10 test engines), you can calculate the approximate crankshaft brake horsepower of your modified 911 engine knowing the displacement and operating speed. Example - From the chart (or plots) for the 2.7 engine, if you’re turning up to 6100 rpm, you should be making approximately 210 bhp at the crankshaft.

On another thread, there is discussion about the possibility of developing 100 bhp/L from the 911 engine. It is possible, and using this methodology it is easy to calculate. Assuming the BMEP is held constant @ 166 psi, and knowing the displacement of the 4 engines in question, we solve for rpm. Result is that the engine needs to turn at 7800 rpm to produce 100 bhp/L.

[racing97]

100 hp per ltr on the competition engines never more than 105 VE on any two valve motors some as high as 190 BMEP and the larger the motor the BMEP drops off a bit.