dumb rally question

[01_el_tiburon]

whats that cricket sound you here when the rally cars lay off the gas ?

omar

[Race8100]

I'm pretty sure it's the tires chirping on the surface of the road under braking. i've wondered the same thing for a while.

I'm ready to be proven wrong...

[01_el_tiburon]

No way mule, its not tires, its engine churping, not rubber, i think its turbo but not sure why, how ..

[duncan661]

it's either their blow off valve or their wastegate making that sound...but you're right, they both relate back to the turbo

[Chase 2]

The chirping you hear when they back off is the tranny.

[01_el_tiburon]

The chirping you hear when they back off is the tranny.

Never would have guessed tranny...interesting....

omar

[duncan661]

The chirping you hear when they back off is the tranny.

that's an interesting answer...how is it the tranny?

[Chase 2]

My bad, I should have said I think it’s the tranny. Now, that I’ve been thinking about it, I'm not that positive it's the tranny, but by the sound presented in the in-car cameras sure sounds like its coming from the drive train to me. The sound you hear is like that you hear from straight cut gears. Turbo related waste gate and blow offs sounds are more of a popping/gurgling noise.

[Alex Paterson]

Blow off valves from what I understand.

-Alex

[ShaneR]

Blow off valves from what I understand.

-Alex

I thought it was blow off valves to, real high pitch squeek from the pressure they run and the short time they are off the throttle?

[KitRacer]

I remember reading on here a long time ago that they injected fuel into the exhaust and lit it off or something to help keep the turbo spooled??? Could this cause the sound

[Uncle_Bob]

Kitracer, you just about hit it on the nail. One of my team members is the crew chief on a pro rally Subaru at which I also help.

This is how it works on the our Subaru:

When the driver lifts his foot from the gas pedal the ignition timing is altered with sometimes 40° or more of delay (retard) and the intake air and fuel supply mixture is made richer. The inlet butterfly is kept slightly open or an air injector is used to maintain air supply to the engine. This results in air/fuel mixture that keeps getting in the combustion chambers when the driver no longer accelerates. The ignition being delayed, the air/fuel mixture reaches the exhaust tubes mostly unburned. When the spark plug fires, the exhaust valve is starting to open due to the ignition delay mentioned above. Additionally, the exhaust temperature being extremely high, the unburned fuel explodes at the contact of the exhaust tubes. Luckily the turbo sits right there and the explosion keeps it turning, otherwise it would slow down since its intake, the exhaust gases, is cut-off.

[Chase 2]

OK, If you guys say so, thanks for straighting me out, it sure sounded way too mechanical (again more like sraight cuts gears whine). I aready knew about the timing/unburned fuel blow off which is what uncle bob has described. But doesn't that tend to be more of a gurggle/popping sound which you can always hear from outside the cars? Again from inside the car the "chirping" isn't as smooth and constant as a gas flow noise would be expected to produce. Guess I'll just have to listen closer next time.

[.CAL.Photo.]

I believe gurgling and popping is what you hear from NA engines mostly, but a turbo exhaust also does that although it is at low revs. I was told that the chirping is a result of a combo of unburnt fuel and the BOV sound flowing thru the exhaust.

Uncle Bob~I believe what you described is also referred to as a "misfiring system" in import tuning/racing circles.


CORRY

[Uncle_Bob]

The chrip chirp sound is known as ALS which stands for Anti-Lag System and is an engine management technique that allows to minimize the turbo lag time.

[BitorPerformance]

the high end turbos used in these applications use a computer controlled B.O.V in order to keep a constant pressure thru the tunning of this it helps create a better throttle response for the on/off throttle dirving they must do in certain sections helping reduce the turbo lag, ive only heard one rice rocket with this type of system they are very expensive.