Macca from Rennlist's Review of Dundon Race Headers on 991 GT3

I had the DMS race headers fitted to the GT3 whilst I was in NZ for a week earlier this month. I should note I also had the KMS filters supplied by Dundon Motorsports, fitted at the same time on their recommendation.

I have put around 500km on the system including a day of fast back road driving through some twisty roads with a group of 8 other Porsche® enthusiasts (a couple of 991 GT3s and a GT4).

I must say the DMS Race headers are very exciting. After almost 3 years ownership of the 991 GT3, picking up the car and driving off again really made me feel like the day I first got the car all over again!

The car for the first 45-60 seconds, cold idles very loud. The valves on this system are much larger and better quality than the ones Porsche® use so they may take longer to build up pressure. Im not sure.

At idle it seems to me that the exhaust is 15-20% louder than it was before (when the car was in "normal" exhaust mode) and 10-15% louder than it did was in "sport" exhaust mode.

On the run 1000-4000 rpm with the DMS valves shut - it is again around 10-15% louder than before on sport exhaust and 15-20% louder than normal.

VERY nice indeed, and sends a message through the seat that feels very good with the slightly louder noise in the cabin.

On "sport" exhaust mode its 20-25% louder in the first 1000-4000 rpm, probably a bit much for inner city cruising if Im honest – there is no resonance as such but the vibration has caused a rattle in the rear compartment (maybe the cage or my camera bracket) around 2000-2500 rpm which is likely a function of the harmonics. Its possible its the glass vibrating or something like a loose speed nut, but its only occasionally, not all the time, so Ill get behind there sometime and make sure everything is buttoned up properly.

Regardless I would normally start the car in "normal" exhaust mode anyway and drive it around town like that only using the "sport" exhaust mode when Im thrashing back roads or at the track.

From 4000 rpm regardless of the exhaust setting the sound is the same (essentially the button only makes an audible difference between 1000-4000 and on idle).

Id say from 4000-7500 its around 15-20% louder than before with "sports" exhaust. In stock trim, my view was sports exhaust only made a 10% difference from 4000 rpm".

Above 7500 its say its 20% louder but takes on a different timbre – sounds REALLY nice inside and outside (I also got a chance to hear the exhaust from outside when driving behind my GT3 in the GT4).

The workmanship of the exhaust is impeccable – it fitted right up first time and caused no CEL (Jamie said it wouldn’t because my car is ROW variant). Fitting was billed at 6 hours, this was with rear bumper and lights removed and included clearing any codes and a road test. I believe next time around this will be a 4 hour job for them.

The tech at Porsche® was very impressed and all the mechanics took a look. The 321 SS is lovely and the welds perfect. The factory side mufflers are gone and with them and the cat - so almost 40lb weight difference and a huge amount of extra space under there!

You keep the same factory third muffler to ensure no resonance and make maximum power (so Dundon found on the dyno).

After 500km I can tell you there is no increase in black soot on my rear bumper or any noticeable difference in the smell of the exhaust really.

Does it feel more powerful? YES in almost every way at every rpm!

Its really hard as I did not do a back to back having flown in and not having driven my car properly for 6 months prior – but I have recently driven a stock 991 GT3RS fairly hard and I can say it feels strong as that engine all over. From 3500 rpm the car feels “meatier” under throttle, sounds much better and sends a message through the bucket seat of its intent.

I cant be precise untill I have the car on my local tracks next (in November), but right now Im without doubt the car is making all the 35+ wbhp and 20+ wlbft that DMS and other independents claim.

It’s a real screamer too in the last 500 rpm. It feels like the difference on a traditional car of leaving the hand brake on slightly then discovering that its on a fraction, and releasing it! Very responsive too - the engine spins up/down fractionally quicker than before IMO.

If you were wanting to re invigorate your 991 GT3 ownership experience I would say definitely fit it. My wife hasnt yet noticed from outside (after idle) or around town. Its only a little more than the Sharkworks but it’s a different sound as it conveys the additional power across the rev range and to me feels very similar to the 991 GT3 RS engine in this regard. I have no doubt its making at least RS power now, possible a fraction more, but it spins up faster than that engine and goes on for longer (400 rpm) too. Just delicious.

Im very happy with it. In terms of affecting the cars outright performance, as such, I cant really say till I've been on the track, strip or Dyno with it - but Ill eat my hat if the car isn't measurably quicker in all situations. On a fast back road run for 150 miles it felts quicker for sure but in terms of lap times I don’t know how this will translate – Im thinking 4 tenths over a 1 minute track lap?. The DMS has given the car a different dimension and is one less reason not to rush out and upgrade to the next GT3....HIGHLY RECOMMENDED - Im a very satisfied customer! This is not a cheap product and infact the most money I have ever spent on an exhaust set up (and its only headers), for any of my cars ever. That being said its a quality 321SS unit that should last the life of the car, it is a direct fit for 991 GT3RS, 911R and likely the forthcoming 991.2 GT3, so there is a large secondary market if I should sell the car or I can take it with me to my next GT acquisition...and there is also a GT4 set for those who like to row their own gears on track!

Thanks Jamie & Charles for your excellent product, your excellent service and all your help in getting these to me. I look forward to your GT3/4/RS exhaust tips next! 

P.S. On the day of our fast road drive I swapped keys with a friend (GT4) so I could hear my own exhaust. I wrote a comparison review on the GT4 found elsewhere on this board. I have pasted his feedback on my car below. He and I traveled 4000 miles to 6 track events in March so spent alot of time on road and track together so he knows what my car sounded like before and after even better than I do!

"....Yes an awesome day for it today. My first time driving a GT3 and one hugely exhilarating experience. Mark, your car sounds so much better now than over last summer. I thought I might get to hear my car from the outside for the first time when I got you to go in front of me, but it was pretty difficult as I couldn’t stay with you while trying to drive “quietly” in your car to hear it..."

P.P.S. When your tech removes your centre box (3rd muffler) ensure they let the nuts on the bolts soak in penetrating oil for a while and torque them off slowly to avoid snapping the factory 3rd muffler studs. These are notorious from snapping due to the heat cycles they have endured and I believe are very difficult to repair...