Turbo boost build up without increase RPM...

[curtis73]

Originally Posted by dellguyz

A little question, in rally car (like WRX and EVO) racing in WRC, there always a "bird whistle" like blow off valve sound come out from those car. What's this type of BOV exactly? I have heard many brands of BOV (HKS, Blitz, Sard...) the sound are absolute not in this kind.

A BOV is a secondary way of controlling maximum pressure. If you are at maximum boost (wide open throttle) and then go to no throttle, the turbo pressure spikes (gets really high) because the turbo is at max effort and you shut off its only outlet. The pressure spikes can cause the turbo to slow down alot with all that extra pressure. The BOV leaks extra pressure and allows the turbo to stay spinning faster, so its ready for boost the next time you open the throttle.

It also acts as a safety. If the wastegate fails, it could over-boost the engine and cause damage. The BOV will leak off extra pressure and prevent damage.

On the street, a BOV is not necessary.

[dellguyz]

To Curtis:

in my case, the wastegate should be triggered if i shut down the turbo timer, is it? I just wonder is it the boost limit in my turbo timer setting is much lower
than the car's wastegate working pressure....

[INF3RN0666]

I don't know if this was discussed earlier because I didn't get a chance to read the whole thread, even though it sounds very interesting. Turbo chargers aren't all designed EXACTLY alike. The turbo I saw on my friend's porsche 911 only spins up after the throttle is past a certain position. So the turbo doesn't engage all the time. The exhaust moves freely without spinning up the turbine. So I would assume that his turbo has some form of a transmission that disengage the turbo at low throttle. Others have a waste gate which is purely mechanical and the turbo is providing boost at all times except when the turbo starts providing too much pressure in which the waste gate lets excessive air out. Right?

[curtis73]

I'm going to copy this thread over to our forced induction forum where it might get some better responses.

[KiwiBacon]

Originally Posted by INF3RN0666

I don't know if this was discussed earlier because I didn't get a chance to read the whole thread, even though it sounds very interesting. Turbo chargers aren't all designed EXACTLY alike. The turbo I saw on my friend's porsche 911 only spins up after the throttle is past a certain position. So the turbo doesn't engage all the time. The exhaust moves freely without spinning up the turbine. So I would assume that his turbo has some form of a transmission that disengage the turbo at low throttle. Others have a waste gate which is purely mechanical and the turbo is providing boost at all times except when the turbo starts providing too much pressure in which the waste gate lets excessive air out. Right?

I'd bet the porsche simply has a larger turbo that requires more airflow to get it pumping air.
Below the airflow needed, it'll basically just idle around, providing a small restriction to airflow but producing no measurable boost.

[INF3RN0666]

Yeah I don't know much about turbos but I do know that my buddy's '86 Porsche 911 is notorious for turbo lag. I'm not really sure why the turbo doesn't spin up from regular driving, but it really only kicks in when the peddle is basically on the floor. He once told me "your steering wheel better be pointing straight when the turbo kicks in, or you're dead!".

[curtis73]

Originally Posted by INF3RN0666

Yeah I don't know much about turbos but I do know that my buddy's '86 Porsche 911 is notorious for turbo lag. I'm not really sure why the turbo doesn't spin up from regular driving, but it really only kicks in when the peddle is basically on the floor. He once told me "your steering wheel better be pointing straight when the turbo kicks in, or you're dead!".

Older air-cooled porsches needed to use bigger turbos. Smaller turbos which create lots of low-end boost make screaming hot pressures at higher RPMs. In an air cooled engine, that's no good. Since its a race-derived car anyway, they opted for the larger turbo that creates boost at higher RPMs to be more compatible with the air cooled engine.

Compare that with the Chrysler fashion turbos of the 80s. They were tiny turbos that produced copious boost at low RPMs to make the driver feel that instant boost, but they choked off high RPM flow and made hot, inefficient air. Since they were water cooled, they could take the heat, but they were designed to fulfill a buzzword market niche.

And, not to nitpick... but don't confuse boost threshold with turbo lag. Turbo lag is how long it takes the exhaust to overcome the inertia of the turbine. Basically it means how long it takes to get the turbo spinning after teh engine is producing enough exhaust to move it. Boost threshold is basically the time it takes from when you mash the pedal until when boost starts being produced. Porsche engineered their cars with a high boost threshold, but that shoudn't be labeled at turbo lag.

[KiwiBacon]

Originally Posted by curtis73

And, not to nitpick... but don't confuse boost threshold with turbo lag.

That's a pet peeve of mine too.

[dellguyz]

Originally Posted by KiwiBacon

For an engine, air in through the throttle has to come out the exhaust.
At part throttle you're restricting the air in, which restricts the amount of exhaust out.
When you open the throttle (at any rpm) the amount of air into the engine greatly increases, this produces more exhaust which drives the turbo to produce even more intake air (boost).

Increasing rpm also increases the airflow (if you're comparing similar throttle position).

To Mr Kiwi and Curtis,

so if the RPM is low and I open the throttle big, lots of air intake, lots of exhaust will generated and drive the turbo to build up pressure, but doesn't the big throttle and the engine can consume this pressure and balance at this "lots of air intake" condition??
how about in high RPM? can the pressure generated by turbo fully consume by the engine?

[KiwiBacon]

Originally Posted by dellguyz

To Mr Kiwi and Curtis,

so if the RPM is low and I open the throttle big, lots of air intake, lots of exhaust will generated and drive the turbo to build up pressure, but doesn't the big throttle and the engine can consume this pressure and balance at this "lots of air intake" condition??
how about in high RPM? can the pressure generated by turbo fully consume by the engine?

At low speeds the engine will consume all the turbos air without creating any boost. But once you reach the airlflow that the turbo needs to create boost, it will pump more air, resulting in higher pressure (boost) in the intake system.

You can feel this point quite well when driving a turbo car.