Originally Posted by MaxLTV

I'm too lazy to think for myself now, so can someone tell me if the graphs would be the same for PSE on and off if the valves are disconnected.

Originally Posted by Paul Graville

Best post ever!

Am I right in thinking the valves open a hole in the exhaust between the side mufflers and the centre muffler? I.e. one hole on each side?

Or do they restrict the pipe when closed?

If the former then we could expect greater gains still if the centre muffler has not been deleted.

Originally Posted by Mike in CA

Fascinating Matt. and thanks so much for the effort and thoroughness.

I was particularly interested in the 2 plots shown below; the part throttle-PSE on-valves connected (runfile 2) and the part throttle-PSE on-valves disconnected(runfile 7). Runfile 2 actually shows slightly higher peak HP (226 vs 218) and fractionally higher torque (227.94 versus 227.02) with the valves connected. A difference in peak rpm between those two tests?

Regardless, there doesn't seem to be the torque dip that my butt dyno detetected; perhaps it needs a calibration. You mentioned at the top of your post about the drone with the stock center muffler and the valves disconnected. I think the character of the sound with my stock system may have been misleading me; a lot more noise with disconnected valves but no real difference in performance tricked me into feeling that torque was degraded.

Bottom line, I won't disconnect the valves again as I prefer the stock sound, but at least it's good to know that there appears to be no performance penalty for those who want to do the mod. Thanks again for providing some clarity to all of this.

Originally Posted by bronson7

Being I'm not up to speed as many of you here, my question is why is the GT3 rated at 475 hp and 325 ft lbs. but the dyno reports on Money's car shows much less. What am I interpreting incorrectly?

Originally Posted by Mike in CA

The big difference is that these read outs are for rear wheel horsepower. I'm no dyno expert, but honestly, given the car's performance numbers I would have expected higher numbers even at the rear wheels. Someone correct me if I have that wrong.

Originally Posted by fastmd

Great post Money! All dynos are different, but how conservative did they say their dyno performs? Also, what is the current thinking on drivetrain loss with Porsche's and the 991?

Originally Posted by sunnyr

I am also curious about this. The max HP on the dyno charts seems to be ~380HP. Even with 15% drivetrain loss, the HP at the engine crank would work out to be ~437HP. Did the dyno guys give you a reason why it is reading so low?

Originally Posted by rockitman

I think the dyno jet is the issue. I seem to remember back when i did my 996.2-3 back in 2005 at AWE Performance in PA that Mustang Dyno's gave more accurate results. My car with a Europipe exhaust did 377HP at the wheels.

Originally Posted by 008

As he mentioned in the post the rears are also driving the front rollers which is a greater load. You can do a reverse negative run to calculate loss on dynojets but consistency between changes as opposed to ultimate numbers is more important. Was the throttle position sensor tapped for the various partial throttle runs so they were consistent? Have these been sae normalized? Any chance there was an intake air thermometer set between the runs to monitor loss fom heat gain?

Originally Posted by Tre

Mustang dynos actually read lower than DynoJets. Example, the 2009-11 GTRs returned ~430awhp on DynoJets and ~385awhp on Mustangs. In the case of Money's dyno results, a lot depends on the roller weights + the added resistance of using the front rollers. Since there haven't been many (maybe a couple documented) GT3s on a dyno, dyno techs likely don't have baseline numbers of what weights to use. Since dyno numbers are subjective to so many different variables, the important thing to look at are comparisons between the different runs.

Originally Posted by Money2536

Dyno Jets normally give higher results than Mustang/DynaPak dynos. I asked the guys at Titan what their experience has been with their dyno. "Does it give similar results to other Dyno Jets?" Their answer was yes, but we are dealing with an AWD dyno. AWD dynos show greater drivetain losses than typical rear wheel versions.

I wasn't concerned with Peak HP numbers, but remember that this is a rear wheel drive car driving an AWD dyno with a link to the front wheels. So we are easily seeing an additional 10-15% in losses due to mechanical disadvantage. Thanks to Porsche finicky ECU you must drive the front wheels at the same time as the rear of the car will go into limp mode. Heck, I got a traction control and parking brake error that took about 10 miles to go away.


These are SAE numbers. Realistically the car is put somewhere between 410-430 at the wheels, but I was more concerned with the comparison of connected to disconnected which we accomplished.

I hope that makes sense.

Originally Posted by bronson7

What process would Porsche then use to arrive at all their technical data? eg. hp and lbs ft, etc....

Originally Posted by Money2536

Theoretically Porsche would measure HP/TQ at the crank using an engine dyno. In other words, they have the motor out of the car in a room that measures output.

We are measuring output at the wheels. If it makes you feel any better, my M3 measured 430hp/460tq at the wheels on a DynoJet and my GT3 stomps it in a straight line.