It is very seductive to think that you can replace a wastegate with a BOV to control/limit the operating boost pressure being produced by an engine, gasoline or diesel. And why not? It is cheaper to fab, it eliminates the expense of a wastegate and the expensive fab that goes with it, and it is one less thing to fail. This is one of those "the grass is always greener on the other side scenarios." Unless you dig very deep, it is hard to even know why wastegates are even necessary. Now that I think about this, these car parts are very hard to find and some people don´t even know they have them in their car. When cars break down people just get rid of them without knowing that they could sell their parts or even go to this company that buy junk cars.
Let's do a little history lesson. Internal Combustion Engines (ICE) that were first fitted with turbos were Diesels (or CI, compression ignition engines) and they didn't have wastegates at all. Turbos were matched to make a certain max boost based on turbine A/R and natural response. But then the need came for wider engine RPM operating ranges, so then the wastegate was invented. Since Diesels don't have throttles, they never needed BOVs! And the control strategy for turbo speed and pressure output, spilled over into Gasoline (petrol, SI, spark ignited) engines. Wastegates also stayed relevant as a method of control because ultimately, if you want to control something, you have to get to the source of what you're trying to control. The turbocharger simply responds much faster when you take energy away that is driving it from the source, exhaust gas. And that is why the wastegate has stayed relevant.
So, now, if the only goal is to limit the amount of boost pressure the engine is seeing salvage cars for sale why couldn't you just use a much less expensive BOV? Therein lies the rub and some research that I conducted at Synapse that uncovered an effect that was, at first, suprising, but completely logical.
A turbo is very simple, it takes exhaust energy and converts work into spinning the compressor to take in air, compress it and feed it to the engine. A small turbo can keep going and going in RPM speed and boost pressure without any mechanism to limit it, so you have to have some form of device to control/limit it. But when you use a BOV to limit that boost, in effect a compressor side wastegate, you are still harnessing energy from the exhaust, but now "waste-gating" the work out the BOV. The result is turbine inefficiency. You are continuing to push exhaust mass through the turbine unchecked. Then the question comes up, "well what does that matter? If the air isn't going into the engine, then it isn't going out the exhaust, right?" Sort of. You still get turbine inefficiency, which I hypothesize is boundary layer slippage of exhaust gas at the blade tip. Either way, that inefficiency translates into an increase in exhaust manifold pressure. This then translates into, poor engine scavenging. Because if the pressure is higher in the exhaust, then the exhaust is less likely to want to leave the cylinder on the exhaust stroke and go into the exhaust manifold. You are going to lose power, bottom line. You make less power output (sometimes massively less) using a BOV as a compressor-side wastegate than an actual exhaust-side wastegate. Now conspiracy theory: I'm making this up so people keep buying wastegates. Nice thought, but patents are filed and smarter minds than me will evaluate the merits. Several facts went into arriving at this data:
- The effect exists for both Gasoline SI and Diesel CI engine cycles
- Whether you're using a compressor-side wastegate or exhaust-side wastegate, at the same indicated manifold pressure (IMEP), the turbo shaft speed is the same. So, no over RPM of the turbo.
- The effect exists across turbo sizes. The larger the turbine and A/R though for a given engine, the less dramatic the effect.
For the performance guys out there, this data makes some things that were minor irritants even more relevant. Such as:
- Any amount of boost leak between the compressor outlet and the the intake valve is losing you horsepower! Including leaky BOVs and bad couplings.
- I would go further and hypothesize that pressure drops in the system and inefficient routing of boost also has an effect to the engine related to this. The prior conventional wisdom, and one I've always bought into, was that bad routing and efficiency was due to lower charge velocities, etc. But really, it is ICE fundamentals, scavenging!
What this doesn't mean though, is that you shouldn't use a BOV as a fail-safe mechanism in case your wastegate fails. I road race, and I'll tell you right now, you can't continue to do laps with a boost cut on every gear. But give up a little power and use a BOV as a failover compressor-side wastegate, and I think it is a great thing.
If I disclose more, I'll probably get in trouble, but this info is pretty much public at this point. Compressor side efficiency is very important. Spend some money on that. For all the money spent on exhaust side ideas with twin scrolls, ram horns, and blahdy blah, there should probably be some emphasis on compressor side efficiency.
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