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Old 07-31-2011, 10:55 AM   #1
sempersubi
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Default Rally car question

Probably gunu be something that im gunu slap myself for asking but in the subaru rally America cars what is that bird chirp sounding noise that happens when they let off the gas? used to think it was the anti lag but i have never heard that noise from a normal STI or any car for that matter besides the rally cars
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Old 07-31-2011, 11:08 AM   #2
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It's either a blow off or bypass valve, depending on the car.

And what the hell is a gunu?
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Old 08-01-2011, 07:16 AM   #3
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Quote:
Originally Posted by sempersubi View Post
Probably gunu be something that im gunu slap myself for asking but in the subaru rally America cars what is that bird chirp sounding noise that happens when they let off the gas? used to think it was the anti lag but i have never heard that noise from a normal STI or any car for that matter besides the rally cars
It is called compressor stall , it when you have the throttle closed and there is no where for the air to go and it slows the compressor wheel to stall a bit causing the chirping noise .
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Old 08-01-2011, 07:21 AM   #4
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compressor surge. Also called turbo flutter or (incorrectly)wastegate chatter.

Last edited by MFR Sweep; 08-01-2011 at 07:43 AM.
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Old 08-01-2011, 07:23 AM   #5
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Waste gate chatter is something different .

That is when you have an internal wastgate that does not seal properly or when it is on boost it bleeds off the boost erratically also has a metallic sounding noise.
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Old 08-01-2011, 07:43 AM   #6
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that is true, but people incorrectly call it wastegate chatter. i should have noted that i guess.
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Old 08-01-2011, 09:02 AM   #7
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Gunu = going to lol. So the verdict is compressor surge?
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Old 08-01-2011, 09:17 AM   #8
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Or it could just be a sequential blow off valve. Really depends on the car.
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Old 08-01-2011, 11:49 AM   #9
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OK mr sempersubi...you see the roof scoops on top of all the rally cars? Those let cool air in for the crews (no A/C in race cars) but often these scoops will literally scoop in birds.

so that noise you hear is the birds excaping.
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Old 08-01-2011, 11:51 AM   #10
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(no A/C in race cars)
Speak for yourself
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Old 08-01-2011, 12:33 PM   #11
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Speak for yourself
When are you bringing your new beast out again to play?
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Old 08-01-2011, 12:53 PM   #12
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This summer has been crazy between other racing and broken cars. I haven't even touched it yet, but I figure if I put some effort into it over the course of the next month, I should be ready for LSPR.
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Old 08-04-2011, 01:07 AM   #13
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Quote:
Originally Posted by sempersubi View Post
Probably gunu be something that im gunu slap myself for asking but in the subaru rally America cars what is that bird chirp sounding noise that happens when they let off the gas? used to think it was the anti lag but i have never heard that noise from a normal STI or any car for that matter besides the rally cars

Here's a pretty good description from Garretts website:
"Surge is most commonly experienced when one of two situations exist. The first and most damaging is surge under load. It can be an indication that your compressor is too large. Surge is also commonly experienced when the throttle is quickly closed after boosting. This occurs because mass flow is drastically reduced as the throttle is closed, but the turbo is still spinning and generating boost. This immediately drives the operating point to the far left of the compressor map, right into surge.


Surge will decay once the turbo speed finally slows enough to reduce the boost and move the operating point back into the stable region. This situation is commonly addressed by using a Blow-Off Valves (BOV) or bypass valve. A BOV functions to vent intake pressure to atmosphere so that the mass flow ramps down smoothly, keeping the compressor out of surge. In the case of a recirculating bypass valve, the airflow is recirculated back to the compressor inlet."

Surge although technically bad is unavoidable if you want the kind of response rally drivers want.
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Old 08-04-2011, 01:12 AM   #14
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Originally Posted by logic246 View Post
It is called compressor stall , it when you have the throttle closed and there is no where for the air to go and it slows the compressor wheel to stall a bit causing the chirping noise .
Slows the compressor wheel until it stalls? What stalls, the wheel? How does that make a noise?
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Old 08-04-2011, 01:28 AM   #15
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the noise is the air going through the turbo backwards.
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Old 08-04-2011, 08:18 AM   #16
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Quote:
Originally Posted by Andy Bernard

Slows the compressor wheel until it stalls? What stalls, the wheel? How does that make a noise?
Here.

The noise isn't good.

You don't want it to happen on your car.

Even though you think it sounds cool.
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Old 08-04-2011, 09:59 AM   #17
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They use a surgeless type of compressor housing. It happens during decelleration/braking. On those cars they rarely lift of the gas pedal to keep the throttle plate open and the turbo spooled. With the sequential transmissions they are using there is no need to use the clutch other than to take off so keeping one foot plated on the gas is very easy. This is why the brakes glow so often as well because even while braking they are on the throttle.
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Old 08-04-2011, 10:02 AM   #18
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Quote:
Originally Posted by greycar View Post
They use a surgeless type of compressor housing. It happens during decelleration/braking. On those cars they rarely lift of the gas pedal to keep the throttle plate open and the turbo spooled. With the sequential transmissions they are using there is no need to use the clutch other than to take off so keeping one foot plated on the gas is very easy. This is why the brakes glow so often as well because even while braking they are on the throttle.
Not necessarily true.

Some drivers like an aggressive brake pad , hence the glowing disks . All Rally driver do not left foot brake.
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Old 08-04-2011, 10:12 AM   #19
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Originally Posted by greycar View Post
This is why the brakes glow so often as well because even while braking they are on the throttle.
this statement could be turned around quite accurately...glowin brakes result from braking even while the other foot is mashed on the gas

stages with lots of short straights followed by tight corners also breed cherry red rotors, esp on high end turbo cars
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Old 08-04-2011, 11:00 AM   #20
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If you block off your bypass valve on any turbo car it will make a little bit of chirping but the main reason the SRT cars (Antoine's Evo X is even louder) is super loud is due to the restrictor design in combination with the design of the compressor wheel. Subaru uses an early variation of the TR30 WRC garrett from a Step II mitsu wrc (but a different TR30 on the rallycross cars) and Antoine uses a more recent TR30.
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Old 08-06-2011, 03:56 AM   #21
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For those of you asking what specific sound he is talking about, watch this, you can hear it a ton every 15 seconds.
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Old 08-06-2011, 06:24 AM   #22
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Quote:
Originally Posted by RB5 Clone View Post
OK mr sempersubi...you see the roof scoops on top of all the rally cars? Those let cool air in for the crews (no A/C in race cars) but often these scoops will literally scoop in birds.

so that noise you hear is the birds excaping.
LOL!!! Winning!
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Old 08-08-2011, 09:45 PM   #23
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Quote:
Originally Posted by greycar View Post
They use a surgeless type of compressor housing. It happens during decelleration/braking. On those cars they rarely lift of the gas pedal to keep the throttle plate open and the turbo spooled. With the sequential transmissions they are using there is no need to use the clutch other than to take off so keeping one foot plated on the gas is very easy. This is why the brakes glow so often as well because even while braking they are on the throttle.
Have you ever actually watched a rally drivers pedal box? "rarely lift"? what a load of misinformation. We use trail braking a fair amount to help stabilize the car at speed, as well as on both pedals in the corners sometimes, but that's not why the brake glow. They glow simply from the massive amount of heat generated while on stage where your either flat on the gas or hard on the brakes. Picture driving from 0 to 100, then slamming on the brakes, repeated, for 5-20 minutes non-stop. Also, unlike a track car we do not have the same type of airflow, or oversized brakes/rotors to dissipate the heat (gravel events require a 15" rim keep in mind)

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Old 08-09-2011, 07:09 AM   #24
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Have you ever actually watched a rally drivers pedal box? "rarely lift"? what a load of misinformation. We use trail braking a fair amount to help stabilize the car at speed, as well as on both pedals in the corners sometimes, but that's not why the brake glow. They glow simply from the massive amount of heat generated while on stage where your either flat on the gas or hard on the brakes. Picture driving from 0 to 100, then slamming on the brakes, repeated, for 5-20 minutes non-stop. Also, unlike a track car we do not have the same type of airflow, or oversized brakes/rotors to dissipate the heat (gravel events require a 15" rim keep in mind)

‪Petter Solberg WRC Rally Italy 2003 Onboard pure sound FOOT CAM‬‏ - YouTube
WRC cars do have the airflow needed to direct air to the wheels , so do track cars and they still glow.
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Old 08-11-2011, 07:04 PM   #25
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WRC cars do have the airflow needed to direct air to the wheels , so do track cars and they still glow.
I was saying the same type, ie not nearly as much. Wasn't saying there wasn't airflow...

Also, if you compare something like group N brake components (like most of us in the US run, 4/2 pots) compared to big brake track kits, there is so much more surface area to dissipate heat in a track version.

We run 11" x .95" thick rotors in the front, compared to say the wilwood track kit, which runs 12.8" x 1.2". That's a lotttta extra heat dispersing capacity.
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