[Guest]

"The first “practical” turbocharged road car eventually appeared in 1975, that’s the Porsche 911 Turbo 3.0. To reduce turbo lag, Porsche engineers designed a mechanism allowing the turbine to "pre-spin" before boosting. The secret was a recirculating pipe and valve: before the exhaust gas attains enough pressure for driving the turbine, a recirculating path is established between the fresh-air-charging turbine's inlet and outlet, thus the turbine can spin freely without slow down by boost pressure. When the exhaust gas becomes sufficient to turbocharge, a valve will close the recirculating path, then the already-spinning turbine will be able to charge fresh air into the engine quickly. Therefore turbo lag is greatly reduced while power transition becomes smoother."

Got that from here:

http://autozine.kyul.net/technical_school/engine/tech_engine_3.htm

Just posting it. Can't really discuss the effectiveness of it, only way is for someone to try.

 

[65vette396]

If you have ever driven an old 930/911 turbo you would realize that porsche engineers weren't really at all concerned with lag. Althought the valve is correct. All of the 930's I have ever been in owned all went from no boost at all to leaving the earth in about 4000-4100 RPM. Probably due to the valve that you mention. However, I don't think it helped lag, just seemed to make it feel more violent when it finally came on boost. Drew

 

[turbosupra]

with the improvements in turbine design and impeller wheel design, as well as bearing design, i don't see this being practical?

at almost any rpm above idle, modern turbochargers will make some boost, and bleeding that off boost is going to end up in a loss of torque, not a gain, IMO

i do see that being a very good idea in auto drag race cars that have a high stall, and it takes them a long time to build 3X.xx pounds of boost, while sitting at the line, and trying not to over heat the converter

"The first “practical” turbocharged road car eventually appeared in 1975, that’s the Porsche 911 Turbo 3.0. To reduce turbo lag, Porsche engineers designed a mechanism allowing the turbine to "pre-spin" before boosting. The secret was a recirculating pipe and valve: before the exhaust gas attains enough pressure for driving the turbine, a recirculating path is established between the fresh-air-charging turbine's inlet and outlet, thus the turbine can spin freely without slow down by boost pressure. When the exhaust gas becomes sufficient to turbocharge, a valve will close the recirculating path, then the already-spinning turbine will be able to charge fresh air into the engine quickly. Therefore turbo lag is greatly reduced while power transition becomes smoother."

Got that from here:

http://autozine.kyul.net/technical_school/engine/tech_engine_3.htm

Just posting it. Can't really discuss the effectiveness of it, only way is for someone to try.

 

[Guest]

Actually... have you ever seen how the sequential setup on 3rd gen RX-7s work? I don't know how the MkIV sequential setup works, but the FDs setup was probably one of the most sophisticated out there. It's also failure prone.

How it works is that up to 3k RPM, it's mostly just the primary turbo spinning, there's a bit of exhaust directed to the secondary, but not much. Then from 3k-4.5k, more of the exhaust is directed to the secondary turbo. There's two valves to put exhaust to the secondary turbo, the pre-control valve and the main one. Right now the pre-control valve is all the way open. Like that Porsche, a valve is open so it's directing the air from the compressor outlet into the air intake system.

Now, what's really funny is the crossover at 4.5k. The pre-control valve and main valves are both open, so it's getting all the exhaust energy it's going to. However, it's still not going to be up to speed, so what happens is that the recirculating outlet closes an instant before the outlet to the engine opens. So during this transition, there's no airflow, and the compressor surges! Yes, by design. This lets it get up to speed a lot faster, so when it does let the boost from the second turbo go into the engine it... well, there's a noticeable spike on dyno plots right around where the crossover happens, let's just say.

Lot of people don't like it, because of that sudden spike... hurts controllability. Plus, all this crap is really complicated... lot of people just say to hell with it and go singles.