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I have been trying to get to the bottom of this topic for years and have never been provided a factual and truthful answer on the 993 GT2 EVO straight pipe design.
Why did all cars and teams run this design which tapers right at the tips from a 3” inch pipe to a 2”inch outlet “tip”?
I always thought this looked really funky and have never seen other straight pipe designs like it, however, everyone ran the same design...
Does anyone have a factual answer? Why taper instead of keeping a consistent 3” inch pipe at the tip?
I believe the taper is the keep the velocity going as the gasses cool
Interesting perspective. I can see that as a natural rise of heat creating a venturi effect after the thin layer of watervapor at the bottom of the cooling stack begins to rise and increase speed. However the design of the cooling stack is also highly focused on structural integrity, wind sheer factor etc.
Not sure how much impact or if any at all of that theory comes into play with the GT2 exhaust pipe, gasses being under extreme pressure forces post turbo and for such a short distance of 12" inches or so.
The best suggestion I've heard yet, just not sure if that is actual engineering reason or decision the teams made behind running these straight pipes opposed to a simple consistent 3" inch tip on exit.
Found in another forum, no confirmation of the logic and pipe vs tip may also come into play. And GT2 being not NA. Also a possible reduction in sound, but do not recall that being a thing back in the 90's.
Well an old theory used by racers for along time now is that smaller tailpipes actually create more power on NA cars. Here's why when reducing the size of the tailpipe i.e 2.5 cat back and 2.25 tailpipe the "bottleneck" event actually creates a vacum (not a really strong one) this event increases what is called the scavaging effect (when the exhaust's ability to expel gas is greater than the amount of gas being produced). Your car will perform like it actually has more flow because of this than if you were to put on bigger tailpipes i.e. 2.5catback and 3 tailpipes. If you look at all the cars running in the Fastest Street Car Shootouts you'll see that 90% of them are runing this type of setup although on a larger scale. This is a tried and true method and the only reason exhaust manufactures put those large tips on is simply cosmetic. Let's face it who wants to run around with a single 2" tailpipe.
Last edited by Kein_Ersatz; 09-21-2021 at 04:27 PM.
Found in another forum, no confirmation of the logic and pipe vs tip may also come into play. And GT2 being not NA. Also a possible reduction in sound, but do not recall that being a thing back in the 90's.
Another interesting possibility, thank you for posting. That said, in my mind (I'm not an engineer) I don't feel like getting the exhaust gasses to flow is really an issue here on a 3" diameter by 10-12" pipe right off the turbo. That is probably as free flowing situation for an exhaust as possible. I could be completely wrong, just amazing after all these years, I still can't find one factual reason or response from anyone in the know..... why they designed the exhaust specifically like this for the GT2 EVO race cars.
***The only reason this matters to me is I have the straight pipes above, however can't stand the looks of them aesthetically on the car.... I would like to cut off the tapered end and replace with the uniform 3" outlet. However, if there is a logical and important reason to the initial design, I would love to know and would not modify them at all.
Pinching the exit speeds the flow and creates a low pressure area at the exit
In a n/a application a pinch like that is added to a long taper, this reverse cone adds an additional reflected rarefaction signal to be reflected back up the pipe where if timed right it creates an additional low pressure zone at the exhaust valve when it's open, This extra signal smears out and extends the time that the low pressure stays at the exhaust valve. If there is sufficient valve overlap in addition to sucking spent gas out more efficiently new mixture is also sucked into the cylinder creating a supercharging effect. This effect is a double edged sword because at a lower rpm a high pressure signal is placed at the exhaust valve when it is open, the timing of these signals depends on the geometry of the pipe and temperature of the gas
My guess is that on a turbo charged motor there is also a signal reflected back to the turbo exhaust exit point, this will help pull extra gas through the turbo, the timing which is dependant on the pipe geometry and gas temperature is to be such that the part of the torque band you want to be enhanced is so affected
Pinching the exit speeds the flow and creates a low pressure area at the exit
In a n/a application a pinch like that is added to a long taper, this reverse cone adds an additional reflected rarefaction signal to be reflected back up the pipe where if timed right it creates an additional low pressure zone at the exhaust valve when it's open, This extra signal smears out and extends the time that the low pressure stays at the exhaust valve. If there is sufficient valve overlap in addition to sucking spent gas out more efficiently new mixture is also sucked into the cylinder creating a supercharging effect. This effect is a double edged sword because at a lower rpm a high pressure signal is placed at the exhaust valve when it is open, the timing of these signals depends on the geometry of the pipe and temperature of the gas
My guess is that on a turbo charged motor there is also a signal reflected back to the turbo exhaust exit point, this will help pull extra gas through the turbo, the timing which is dependant on the pipe geometry and gas temperature is to be such that the part of the torque band you want to be enhanced is so affected
This doesn't apply to a turbocharger exit. N/A engine exhaust ports produce pulsed flow as a factor of valve opening/closing events. This pulsed flow results in compression and expansion waves (effectively lower vs higher pressure) which also reverberate when they meet an area of restriction such as a collector. When timed correctly these compression/expansion waves can help scavenge a port. The exit of a turbocharger is not pulsed as the turbine removes these compression/expansion waves.
Pinching the exit does speed the flow of gas but does not necessarily decrease pressure. You may be confusing things with the Bernoulli effect where in fluid dynamics as the speed of flow of fluid increases, pressure decreases. Fluids are generally incompressible and this doesn't strictly apply to compressible gases.
This doesn't apply to a turbocharger exit. N/A engine exhaust ports produce pulsed flow as a factor of valve opening/closing events. This pulsed flow results in compression and expansion waves (effectively lower vs higher pressure) which also reverberate when they meet an area of restriction such as a collector. When timed correctly these compression/expansion waves can help scavenge a port. The exit of a turbocharger is not pulsed as the turbine removes these compression/expansion waves.
Pinching the exit does speed the flow of gas but does not necessarily decrease pressure. You may be confusing things with the Bernoulli effect where in fluid dynamics as the speed of flow of fluid increases, pressure decreases. Fluids are generally incompressible and this doesn't strictly apply to compressible gases.
Interesting, so what is your theory here with this application specifically?
This doesn't apply to a turbocharger exit. N/A engine exhaust ports produce pulsed flow as a factor of valve opening/closing events. This pulsed flow results in compression and expansion waves (effectively lower vs higher pressure) which also reverberate when they meet an area of restriction such as a collector. When timed correctly these compression/expansion waves can help scavenge a port. The exit of a turbocharger is not pulsed as the turbine removes these compression/expansion waves.
Pinching the exit does speed the flow of gas but does not necessarily decrease pressure. You may be confusing things with the Bernoulli effect where in fluid dynamics as the speed of flow of fluid increases, pressure decreases. Fluids are generally incompressible and this doesn't strictly apply to compressible gases.
There are 2 sorts of waves in any exhaust
1)periodic pulses from the exhaust cycle which I agree a turbo intervens w/
2) acoustic resonances which are reflected back up stream, each transition generates one of these acoustic signals which like the gas pulses consists of a series of compressions and rarefactions, unlike the one way downstream flow these acoustic signals do go upstream where they have the expected results, a rarefaction at the right place and time sucks additional gas downstream a compression at the wrong place and time impedes the flow. The only difference between a turbo and n/a motor is where this takes place, the n/a motor wants a rarefaction at the exhaust valve when it is open, this cannot happen in a turbo motor because the turbo intercepts the reflected signals but the turbo stills reacts to the presence of a rarefaction or compression at the turbo exhaust exit
In a compressible fluid faster flow always creates lower pressure, exhaust gases are compressible fluids
Here's an example of a tunable aftermarket exhaust(Burns Stainless) that has an additional venturi in addition to the end pinch, these additional venturis come is different sizes to enhance turbo exhaust flow at different rpms and loads
There are 2 sorts of waves in any exhaust
1)periodic pulses from the exhaust cycle which I agree a turbo intervens w/
2) acoustic resonances which are reflected back up stream, each transition generates one of these acoustic signals which like the gas pulses consists of a series of compressions and rarefactions, unlike the one way downstream flow these acoustic signals do go upstream where they have the expected results, a rarefaction at the right place and time sucks additional gas downstream a compression at the wrong place and time impedes the flow. The only difference between a turbo and n/a motor is where this takes place, the n/a motor wants a rarefaction at the exhaust valve when it is open, this cannot happen in a turbo motor because the turbo intercepts the reflected signals but the turbo stills reacts to the presence of a rarefaction or compression at the turbo exhaust exit
In a compressible fluid faster flow always creates lower pressure, exhaust gases are compressible fluids
OOhh! A technical debate. Me likey! Rennlist wont know what hit them Bill!
That being said, acoustic resonances won't have any measurable effect in a pipe length that is effectively a little over 12 inches. Plus resonant frequency will alter according to gas flow rate so even if it was a miniscule amount of help scavenging the gas flow at certain rates of gas flow, it will impede flow at others. It's been well proven for eons that for turbocharged vehicles, any form of backpressure is detrimental to overall power output. A GT2 will make more power with the GT2 style exhaust removed compared to them being attached all things being equal. Tapering the exhaust down from 3 inches to 2 inches WILL reduce power compared to running a straight 3 inch pipe all the way.
From my vague memory, the real reason GT2 Evo pipes taper is because of FIA homologation regulations. Regs dictate that exhausts can be modified and catalytic converters and mufflers are able to be removed. The only stipulation is that exhaust pipe size remain the original diameter and that the exhaust must exit at the standard location. The GT2 Evo style pipes are designed with this in mind. They route exhaust gas to the standard tailpipe locations in the most efficient (shortest) way possible to reduce exhaust weight and then taper down to the stock exhaust pipe diameter.
09-14-2021, 11:39 AM
The Truth - 993 GT2 Straight Pipes
