We recently had a US Spec 991 GT3RS in the shop and the owner was gracious enough to let us get a baseline on our dynapack hub dyno, test the catted system and install and test the race headers on his car.


991 GT3RS at Dundon for Race Header install

We've tested the 991 GT3RS previously on a friends DynoJet dyno in 40F ambient temps and saw the car made about 420-430 whp on that dyno.

We hooked the RS up on the Dynapack, put the car into Rolling Road mode with the factory PIWIS2 computer (to allow the car to run in RWD only without pulling power) and starting warming her up.

Did the first few pulls and got 396whp-400whp. Ambient temps were fine in the 50-60degree range. We logged Intake air temps and they started at 95-100F and came down to around 80-85F by the end of the pull. This slightly elevated air intake temps by themselves should have been 4-5whp tops due to air density changes. So we started digging a little deeper.

The air going through the radiators from our fans was escaping the top duct on the front fenders and that hot air was going into the fans we had setup to blow air into the side ducts. Still the air being 85-95F shouldn't kill 20-30whp...

We decided to pull the air ducts out and connect them so the air ducts face the ground and have the fans blowing directly into them pulling colder outside air directly into them.

What we saw was both interesting a bit surprising...


Dundon GT3RS Testing: Intake temps at 85-95F vs intake temps at 65F, same car, stock exhaust.

Dashed Line is 65F intake air temps, solid line is 85-95F temps

The car gained 20+whp and 20+ftlbs almost everywhere with cooler intake air, vs 80-90F intake air. As I said the air density differences between 85-95F (0.07157 lb/ft3) air and 65F air (0.07540 lb/ft3) air is only about 5%.

We also noticed the car wasn't adding any fuel like a 997 GT3RS does when the intake temps start to rise a bit, it actually limits cylinder pressure/torque.

We talked to a source at Bosch about this and they told us that it's purely emissions based, that to limit emissions Porsche isn't adding fuel as the intake temps increase they're limiting torque output.

The other thing to consider is Torque and HP gains reported need to be scrutinized, as most on here do anyway as you can make some pretty astounding before and after's with the car when dyno's aren't shown and details are given...

Here's some examples

Below is a comparison of the 991 GT3RS with Dundon Race with 65F IAT's vs Stock at 85-90F. The ambient temps of the dyno and correction factors are the same for the 2 runs, so if someone was busy or just didn't care to look at the intake air temps, they would make the conclusion that Dundon Race headers make 50whp. This is highly misleading as the car is limiting torque output for the stock run (possibly unbeknownst to the dyno tech) and not for the Race Header run.



991 GT3RS DMS Race at 65F Intake Air Temps, vs Stock at 85-90 intake Air Temps

In our opinion, this is the proper way to report it, where the conditions are as matched as possible between the runs, even though it makes the gains lower.



991 GT3RS, Dundon Race Headers, 65F Intake Air temps for both runs


The 991 GT3RS is a sophisticated monster governed first by emissions and second by performance. Knowing this helps us to understand a little better the performance of these awesome cars!

 

Jamie. This was really fascinating and very informative to read. Thanks for sharing, and thank you for your honesty. Looking forward to meet you on the 5th. Mark

 

Very interesting info.

 

My guess is that the GT3 and GT4 are the same way. And this is why I skip tracking during the summers...car performs so much better in colder temps. My intake temps at Chuckwalla a few weeks back were 120 on the GT4 and I could feel the power getting sucked out of the car.

 

We didn't see this effect on the GT4, but will retest, when we had it on the dyno we had the fans isolated, but it was warmer here at the same time. We'll be doing some final tweaking on the GT4tune with the final production headers just checking and will see what we see then...

 

I could feel the power sucked out of the car in the afternoon session when my intake temp was around 120 (per Solo DL) and the morning session when it was around 100. It may not be to the same extent as the RS though.

 

Interesting!
All I neef is one freezing cold Sebring morning
But tomorrow will be anoth 80F plus day!

 

I'm sure it was down on power especially at 120F Intake temps, that's when most cars will try to protect themselves to not melt the cat's, especially on the track! Honestly a water/meth injection kit at those intake temps might help!

 

How about a 50/50 mix of 93 and 100-103 race fuel.


Wonder if it pulls timing based on ambient temps or internal temps and or detonation sensors.

 

It's not detonation, or timing being pulled, what's really gonna blow your mind is its actually running less timing when it makes more power. We believe it's adjusting cam timing or the fuel injection timing (DFI can do that) to limit cylinder pressure, working on a slick way to log this to bear it all out...

Also we've run 105unleaded in the 991 GT3 before and there was no difference in power. These cars don't run a lot of ignition timing and also don't have issues with detonation or riding the knock sensor so to speak...

 

Incredible thread, very informative. Thank you for the education!!

 

When you did the Dyno on the stock GT4 what numbers did you get??

 

Thanks, as we learn more we'll be sure to share, a performance based tune goes a long way with these beasts, but alas must wait until the warranties are done before we really start exploring!

The answers you seek are here!
https://rennlist.com/forums/gt4/9495...group-buy.html

 

Interesting information. Thanks for posting!

 

No wonder I'm so slow here in Texas.
Good info!