Potential negative factor with third muffler bypass systems
03-22-2015, 01:55 AM
#32
Instructor
Thread Starter
04-29-2015, 10:29 PM
#35
Rennlist Member
^
I spoke to the owner of Hoopumpers shop a few weeks ago,
"they are not in the parts bus".
he told me to fab a piece of alum rear of the factory pan, done.
I spoke to the owner of Hoopumpers shop a few weeks ago,
"they are not in the parts bus".
he told me to fab a piece of alum rear of the factory pan, done.
04-29-2015, 10:48 PM
#37
+1. That's what I'm really after too. I've done some high speed runs, near top speed, and I would not want to lose any downforce from stock. There have been some reports in other threads about squirming on the brakes at higher speeds. Wondering if there is a correlation with that and by-pass install.
04-29-2015, 11:42 PM
#38
Rennlist Member
+1. That's what I'm really after too. I've done some high speed runs, near top speed, and I would not want to lose any downforce from stock. There have been some reports in other threads about squirming on the brakes at higher speeds. Wondering if there is a correlation with that and by-pass install.
I also did not notice the rear end getting light at the end of the back straight, which I usually got up to 150-153 mph.
I think the squirminess with hard braking can be mitigated with a smoother but firm application of the brake peddle, which results in less upsetting of the balance of the car.
04-30-2015, 08:50 AM
#39
Three Wheelin'
Absolutely, positively no difference after many runs into the 170s, you can show me charts, graphs, and engineering reports and I'm telling you all this car is not losing downforce with bypass installed. If anything this would show itself during some 70-100 mph sweepers (and again doesn't feel a lick different vs stock muffler). At higher speeds the engine tray and its slight pitch up towards back is providing 99% of the whatever downforce affect is present (not much) and the air is simply moving too fast to be trapped up in the bypass area void......I believe the squirminess that many have posted is all in the way one is transitioning to brakes at high speeds, jabbing hard late upsets car during weight transfer especially if car isn't tracking dead straight nothing crazy but it shows itself occasionally and its no different with bypass installed. FULL DISCLOSURE: All this feedback was giving to me by my buttocks after track days, and no wind tunnels were used in this analysis!
04-30-2015, 03:15 PM
#40
Nordschleife Master
Absolutely, positively no difference after many runs into the 170s, you can show me charts, graphs, and engineering reports and I'm telling you all this car is not losing downforce with bypass installed. If anything this would show itself during some 70-100 mph sweepers (and again doesn't feel a lick different vs stock muffler). At higher speeds the engine tray and its slight pitch up towards back is providing 99% of the whatever downforce affect is present (not much) and the air is simply moving too fast to be trapped up in the bypass area void......I believe the squirminess that many have posted is all in the way one is transitioning to brakes at high speeds, jabbing hard late upsets car during weight transfer especially if car isn't tracking dead straight nothing crazy but it shows itself occasionally and its no different with bypass installed. FULL DISCLOSURE: All this feedback was giving to me by my buttocks after track days, and no wind tunnels were used in this analysis!
04-30-2015, 04:06 PM
#41
Rennlist Member
I just ran 3 days at COTA after installing the GMG center delete. I noted squirminess on high speed braking before with the stock exhaust and I noted it a little while breaking into turn 12 at COTA. So no difference.
I also did not notice the rear end getting light at the end of the back straight, which I usually got up to 150-153 mph.
I think the squirminess with hard braking can be mitigated with a smoother but firm application of the brake peddle, which results in less upsetting of the balance of the car.
I also did not notice the rear end getting light at the end of the back straight, which I usually got up to 150-153 mph.
I think the squirminess with hard braking can be mitigated with a smoother but firm application of the brake peddle, which results in less upsetting of the balance of the car.
Absolutely, positively no difference after many runs into the 170s, you can show me charts, graphs, and engineering reports and I'm telling you all this car is not losing downforce with bypass installed. If anything this would show itself during some 70-100 mph sweepers (and again doesn't feel a lick different vs stock muffler). At higher speeds the engine tray and its slight pitch up towards back is providing 99% of the whatever downforce affect is present (not much) and the air is simply moving too fast to be trapped up in the bypass area void......I believe the squirminess that many have posted is all in the way one is transitioning to brakes at high speeds, jabbing hard late upsets car during weight transfer especially if car isn't tracking dead straight nothing crazy but it shows itself occasionally and its no different with bypass installed. FULL DISCLOSURE: All this feedback was giving to me by my buttocks after track days, and no wind tunnels were used in this analysis!
04-30-2015, 04:22 PM
#42
Amen guys! I've trying my hardest to keep my mouth shut on topics like this. This making tiny modifications to insignificant things being a detriment to performance like deleting the rear center muffler is getting crazy. Of couse you won't notice a difference. The RSR cars don't have some giant tin can on the back of the car. They have a straight pipe.
it would be interesting to know whether the turbo has more or less downforce than the GT3. I made a very high speed run in the turbo. Once you get up near terminal speed, every bit of stability counts.
04-30-2015, 04:56 PM
#43
The RSR has an undertray covering the straight pipes, and much better downforce in general. Kind of apples and oranges. Just sayin'. Your point is well taken though. Personally, I was curious to hear input from those that have tried it both ways. As always, rennlist comes through.
it would be interesting to know whether the turbo has more or less downforce than the GT3. I made a very high speed run in the turbo. Once you get up near terminal speed, every bit of stability counts.
it would be interesting to know whether the turbo has more or less downforce than the GT3. I made a very high speed run in the turbo. Once you get up near terminal speed, every bit of stability counts.
Total downforce of 65 lbs at 130mph is referenced here.
Found IT.
Summary, per Petevb:
"From Sport Auto's Supertests, downforce at 124 mph:
991S: 4.4 lbs front/ 0 lbs rear (spoiler extended)
991 GT3: 40 lbs front/ 64 lbs rear
991 Turbo S: 44 lbs front/ 88 lbs rear"
Last edited by Bartron; 04-30-2015 at 05:11 PM. Reason: more info
04-30-2015, 06:28 PM
#44
There was a thread in the last few months where someone had posted actual numbers from a German test. I seem to think it was in a thread related to the Crawford Wing.
Total downforce of 65 lbs at 130mph is referenced here.
Found IT.
Summary, per Petevb:
"From Sport Auto's Supertests, downforce at 124 mph:
991S: 4.4 lbs front/ 0 lbs rear (spoiler extended)
991 GT3: 40 lbs front/ 64 lbs rear
991 Turbo S: 44 lbs front/ 88 lbs rear"
Total downforce of 65 lbs at 130mph is referenced here.
Found IT.
Summary, per Petevb:
"From Sport Auto's Supertests, downforce at 124 mph:
991S: 4.4 lbs front/ 0 lbs rear (spoiler extended)
991 GT3: 40 lbs front/ 64 lbs rear
991 Turbo S: 44 lbs front/ 88 lbs rear"
04-30-2015, 07:52 PM
#45
If you're above ~120 mph I think the aero point is worth considering.
Based on both tests and some behind the wheel impressions the aero balance of the GT3 is probably too forward, meaning in stock form it can use all of the rear downforce it's got.
This will probably be particularly true if your car has steel brakes, which according to Sport Auto these give the GT4 (same brakes) about 13 lbs more front downforce due to a more open escape path for air from the wheel well. So particularly for this corner case- used at very high speeds, with steel brakes- I'd personally start to think about it.
On a related note, I'd be very interested to hear from anyone who has complained about high speed balance, particularly high speed nervousness, oversteer or "snap oversteer". If that's you, are you running steel brakes or PCCB?
Based on both tests and some behind the wheel impressions the aero balance of the GT3 is probably too forward, meaning in stock form it can use all of the rear downforce it's got.
This will probably be particularly true if your car has steel brakes, which according to Sport Auto these give the GT4 (same brakes) about 13 lbs more front downforce due to a more open escape path for air from the wheel well. So particularly for this corner case- used at very high speeds, with steel brakes- I'd personally start to think about it.
On a related note, I'd be very interested to hear from anyone who has complained about high speed balance, particularly high speed nervousness, oversteer or "snap oversteer". If that's you, are you running steel brakes or PCCB?