Here is the quote I was trying to find earlier. From Andi about this:
Quote:
Originally posted by Sound Performance
You cant get surge at part throttle!! What you have is a pressure imbalance between the manifold and the IC pipes because your throttle plates are half open. This causes the BOV to open and close!! Any turbo car will do this with a responsive turbo and part throttle applications.
Lar
Lar,
This is incorrect. You absolutely, 100% *CAN* get surge at part throttle. Nothing to do with the BOVs, you can get pure and simple compressor surge at part throttle. In fact it's a LOT more likely to happen at part throttle than full throttle. Let's imagine a system where there is no BOV. Holding part throttle to make, say, 1/2 a bar of boost on your boost gauge, the turbo is making upto 2 bar of boost in the IC pipes by 6000RPM. (I've measured this myself w/ boost gauges before and after the throttle plate). So the turbo is making 2 bar of boost, but it's only flowing enough air for the amount of power you're making at 1/2 bar boost part throttle (not much air flow). Unless it's a REALLY flexible turbo surgewise, this will more often than now be WAY off the surge map and cause part throttle compressor surge.
Band-aid solution #1: a push-type BOV that opens freely at part throttle boost situations, to vent all that extra air that the part-closed throtte plate isn't letting by. This helps, but doesn't totally fix the problem... in fact my testing that showed 2 bar in the IC pipes at 6000RPM when I only had 1/2 a bar in the manifold at part throttle, was done with 3 bovs on the car (2 ssbov's and 1 blitz dd, and the blitz DD was fully open whistling like a mofo steady state during this test).
Band-aid solution #2: a non-surgy turbo, i.e. a very flexible turbo. My old T64 was very bad at surging at part throttle, because its surge characteristics were BARELY good enough for the 2jz even at full throttle. My PT67/T04R compressor wheel is so much more flexible that it never surged even at part throttle.
Best solution: Make sure that whatever boost control method is being used references IC PIPE PRESSURE rather than manifold pressure. Boost controllers that go by manifold pressure to control boost (i.e. ones like the Blitz DSBC, SBC-iD, etc) cause such part-throttle compressor surge to occur because they allow huge amounts of boost to build up in the IC pipes at part throttle. That's one reason HKS EZ and MBC cars are so much more driveable... if you ever drive an MBC car at part throttle through the rev range, and it has open wastegate, you'll notice that you can hear the wastegate open at high revs even when your boost gauge is still at slight vacuum or just 1-2 psi.. indicating you're already above 15psi in the intercooler pipes. The way I fixed this on my car (since I have a Blitz DSBC) was that I rerouted the head unit's boost sensor source from the manifold to the nipple at the bottom of the TB just before the throttle plate. This is in direct disagreement with the blitz directions, and yes it screws up the peak hold since it reads the boost spikes from throttle let offs.. but it makes the car SOO much more driveable because it now controls IC pipe pressure and prevents such compressor surge, not to mention stopping those awful spikes I used to get when going from part throttle to full throttle (i.e. modulating to exit a corner, then nailing it).
Sorry for the long post.... just wanted to clear this issue up.
Andi
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Andi Baritchi
