RSAmerica.net

On Sat 17th July '04, around 20 members of the PCA San Diego Region met at The Dyno Shop in Santee CA - around 15 miles east of downtown San Diego with the sole intent of answering the “how many horsepower?” question.

There were 2 RS Americas in attendance and they were both owned by members of the RS America registry, and naturally, both owners were wondering who had the most horsepower .......

The two owners were myself Keith V of CA and Bob B of CA with his Guards Red example. (Click on name links to view their respective web pages on www.rsamerica.net).

PRE-TEST EXPECTATIONS.

The figures quoted by Porsche for the stock RS America maximum horsepower and Torque are 247 hp @ 6,100 rpm and 228 ft-lb @ 4,800 rpm respectively. As a general rule of thumb, the estimate of horsepower lost from the flywheel (where the factory measurement is taken) through the drive train to the rear wheels is approximately 15%.

Therefore, for an actual horsepower of 247 at the flywheel, the dynamometer would be expected to read somewhere in the region of (247 x .85) =

          209 hp at the rear wheels.

VEHICLE MODS

In order to put the power output from any given vehicle in perspective, it is important to know exactly what modifications have been done to it so the results shown are not taken to be typical for a stock example of the model.

The Fly Yellow RSA has had the following modifications embodied that affect the horsepower output:

The Guards Red RSA has had the following modifications embodied that affect the horsepower output:

TEST PREPARATION.

When the time came for the RSA to be tested it was driven into position with its rear wheels on the dyno roller and then webbing straps were attached to the chassis at one end, and fixed to the floor behind the car at the other. Then the task of connecting up data sensors began.

The sensors consisted of an electrical pulse measurement to monitor the engine RPM and an exhaust sensor to measure the air/fuel mixture.

Next came attaching the webbing straps to both the front wheels. Chocks were also used at the front wheels to ensure that the car stayed in place throughout the dyno run (this is a good thing).

Somehow it looked like the car was about to be tortured, and at this point with all the expectant onlookers, the subject cars owner tends to go very quiet and often take on the air of a someone who just “might” be having second thoughts.

The guys doing the testing seem oblivious to the car owners concerns as they go about the process they have done many times before without the slightest concern. The tension increases as they step through their checklist and start warming the car up.

LET THE GAMES BEGIN

Once the wheels are turning and the roller under the rear wheels starts to spin, things start happening pretty quickly. Fourth gear is selected and after a very brief pause, full throttle is applied.

As the wheels spin faster and faster the white PORSCHE lettering on the brake caliper became visible through the spokes.

Most of us have never experienced the joy of standing right next to our Porsches when they go up to over 130 miles an hour and believe me, the first time you experience it, it can be quite daunting. Due to being at low revs in 4th gear, the car seems to labor slightly and then begins to accelerate slowly at first but then rapidly gaining speed and noise. As you listen to the growling exhaust note and the engine whirring smoothly, you can clearly define when it reaches around 4,000 rpm and “comes on the cam”. The engine begins to whine, tire noise increases and the acceleration is markedly more rapid than before. At this point the owner of the car being tested will often beam with pride.

That “magnificent air cooled wail” that motor journalists extoll as being “pure 911 music” takes on a very different feel when instead of it disappearing behind you as you take off like a scalded cat, it builds to an ear splitting crescendo just a few feet in front of you.

The sound is truly remarkable, it builds and builds to the point when you wonder if your car is going to be alright, and then ...... it continues mercilessly on. On the flat smooth rollers with only a small knurl on their surface to provide grip and avoid tire spin, tire noise seems to be accentuated. At this stage, onlookers are smiling and nodding approval and the decibels climbing unstoppably. There may even be an occasional thumbs up from a grinning bystander. Then, as the sound becomes so loud observers start covering their ears, their grins seem to fade as rapidly as the roar continues to grow. Right at about the point when I felt I could stand no more and I was seriously considering dragging the tester out of the car before the whole thing exploded into about a million small-but-very-expensive pieces, the dyno tester guy did something truly remarkable......

I don't think I would have been more surprised if he had disappeared in a flash of light and a puff of smoke. As I anxiously watched the frantic proceedings, in front of an audience of about twenty Porsche fanatics, as the car passed through around 5,000 rpm with the RS America at full bore and the sound almost hurting your ears ................. he yawned!!!

I will never be sure if it was a real yawn, or if this was a staged symbol of coolness at the critical point in the testing, but whichever it was, the few of us that saw it were truly impressed!

Below is a link to a 20 second video of the dyno test for the yellow car. Even at full volume on a good set of speakers, it does not begin to show the extent of the noise from the dyno run. If you watch very closely when the car approaches 6,000 rpm and 132 mph, you can just make out the front suspension shifting as he gets off the gas. From this point on, the majority of the sound is tire noise.

RESULTS

Factory figures

For the Carrera 2 and Carrera 4 Porsche shows the horsepower at a maximum of 247 hp SAE-Net at around 6,100 rpm. The torque is shown on a different scale as it is calibrated in Newton-meters, but measures around 310 Nm which equates to 228 ft-lb @ 4,800 rpm. It is critical to note that the graphical curves show represent power measurements taken at the flywheel and as a consequence do not reflect power losses though the drivetrain.

It is interesting to summarize the torque characteristics of the engine as the curve shows two discrete steps. The first being in the 1,500 to 3,000 rpm range at around 260 Nm, and the second being between 4,500 - 6,00 rpm at around 310Nm.

These two plateaus can be clearly seen when interpreting the curves of the yellow car below, however, when the red cars curve is interrogated, it can be seen that the torque curve displays a gain in overall max torque, but loses the first plateau at around 2,000 rpm.

Yellow RSA

So, with 231.7 hp available at the rear wheels, assuming a 15% power loss through the drive train, it is a reasonable assumption that this engine puts out around 273 hp at the flywheel.

Guards Red RSA

It is interesting to compare the two cars torque curves. The yellow car has a much flatter torque curve. The red cars torque curve starts at an angle which it maintains as the torque gradually climbs to its peak.

SO WHY THE HORSEPOWER DIFFERENCE Between the two RSA's?

The difference of almost 20 hp and 19 ft/lbs torque are to attributed to the modification to the exhaust system of the guards red car - specifically the Fabspeed cat bypass pipe.

Since this dyno test, I have purchased a Fabspeed cat by-pass pipe and fitted it to the yellow car - which makes the car illegal for street use sooooooo ........ of course I replace the stock catalytic converter if I am about to drive on the street because that's the right thing to do officer!

I have heard stories about naughty people who fit a cat bypass pipe all the time and only re-fit the catalytic converter for the smog check which is due every two years, but that would be illegal ........ right?

Just for reference, removing the catalytic converter and replacing it with the by-pass pipe takes around 25 minutes with a cold car - including jacking the car up to take off the LH rear wheel for better access, and refitting the wheel and lowering the car back down.

As a final verification, I am planning to re-visit The Dyno Shop here in San Diego on Sat 23 April '05 and intend to invite as many RS Americas as possible from the San Diego area. This has a potential for being a great event as there are around 20 RSA's in the San Diego/South LA area - so watch the “Latest Breaking Site News” and “Member Articles” pages of this website!

Even if no other RSA's can make it, I will go back and re-test my car and see exactly what difference the Fabspeed cat bypass made to the rear wheel horse power.

Many thanks to Sean C and Steve G for their camera work - their images truly made the article.