Willie

Surge valves are highly recommended with superchargers. In fact, they're included in kits for our cars. But I've never heard of needing one with a turbo. So hypothetically, if I were to replace the supercharger with a turbo, would a surge valve still be necessary? To avoid any confusion, I am not talking about BOVs.

Willie

SMOKNZ

Not needed!

Willie

Thank you. Why? Does it have anything to do with the wastegate?

Willie

SMOKNZ

No the wastegate has nothing to do with it, it is usually closed during normal driving anyway thus all the exhaust is going thru the turbo.

The reason why? Don't know exactly, but due to the fact that the turbo isn't driven directly by the crank, when your cruising around the turbo is pretty much freewheeling and isn't producing any boost, while the supercharger may be producing some boost (pre-throttlebody if no bypass valve was present) at cruise rpm.

I've had my turbo discharge disconnected while idling the engine before and it doesn't move much air. I've had a blower discharge disconnected while idling and the damn thing was blowing tools and everything out from under the car that thing was moving some serious air. This difference is probably the reason for the surge valve.

my $.02... anyone have anything better?

Bill

Willie

No the wastegate has nothing to do with it, it is usually closed during normal driving anyway thus all the exhaust is going thru the turbo.

I'm very new to turbos so please bear with me. The way I read your statement is if the wastegate is closed, all the exhaust gases bypass the turbo. Is this correct?


....when your cruising around the turbo is pretty much freewheeling and isn't producing any boost....

I didn't know this.


....while the supercharger may be producing some boost (pre-throttlebody if no bypass valve was present) at cruise rpm.

Ahhhh, this explains it!


I've had a blower discharge disconnected while idling and the damn thing was blowing tools and everything out from under the car that thing was moving some serious air.

Absolutely. I have done the same thing. It's amazing how much air blows out!

All this makes complete sense. I'm getting ready to make the change and was wondering whether I should remove my two surge valves and install a BOV. Thanks!

Willie

SMOKNZ

No the wastegate when closed is not bypassing the turbo, it's all going thru the turbo, but you have to think, how much air is actually going thru the turbo at cruise idle anyway. The throttlebody is near closed, and the acutal volume of gass being sent out the exhaust is a fraction of what you see at WOT. The turbo's are spinning, but not at a speed that will produce any boost.

Absolutely install a BOV with your turbo setup. IT will prevent surge of the compressor. Take a look at any compressor map and then imagine what happens when (at any pressure ratio) you suddenly lift off the gas and now you can just about negate any airflow for that given pressure ratio, you will be past the surge limit for sure.

mn_vette

And what does surge acutally do? Will it hurt the turbo or is it just a performance thing?

SMOKNZ

Most compressors have a stability limit that is defined by a minimum flow rate on a pressure-rise-versus-flow-rate characteristic curve. Surge margin refers to a margin of safety between the normal operating point of the compressor and the stability limit. Exceeding the surge limit can result in rapid flow reversal ocillations which can be damaging to a turbo, or any centrifugal compressor for that matter of fact. Some say it helps the turbo keep spinning between shifts as well, as the flow reversals of surge will slow the turbine down fast.