kahlveen

Today I did a quick test with the stock unshimmed wastegate. With the full intake manifold pressure delivered to the wastegate (no boost controller) I was only able to reach only 5 psi boost. Then with the signal completely blocked I could get to at least 18 psi (I'm not sure how high it could go, but even on race fuel I didn't want to let it go any higher).

I'm not sure if I'm thinking about this correctly, but it seems that with all of the intake manifold pressure delivered to the top port of the wastegate, it's only taking 5 psi plus whatever the exhaust manifold pressure is to open the wastegate valve. I'm guessing that the exhaust manifold pressure should be somewhat close to the resulting intake manifold pressure. Maybe a bit higher since there's surely not a 100% efficient transfer of energy. But even if there was 5 psi from the intake manifold and and additional 10 psi opening pressure from the exhaust pressure (half efficiency), this would only be 15 psi to overcome the spring and open the wastegate. If this is the case, then how can the wastegate with no intake manifold pressure signal resist enough exhaust manifold pressure to produce upwards of 18 psi intake manifold pressure (36 psi exhaust manifold pressure if we continue to use the half efficiency assumption). There must be some other factor or factors I'm not considering in this. Maybe someone can point out the obvious flaw in my reasoning here...

beebone

I'm not sure about the mechanism of the boost built up, but I'm pretty sure if no boost controller in between, the boost should be very low.

In my case, I installed EBC to the WB, when the EBC turned off or set in 0%, the boost shows 0.3bar only, which is equal to your intake manifold direct contacting to WG with cylinder valve bypass.

kahlveen

Any engineers out there? I know there are some folks here who really understand wastegate function. I thought I did, but the results of this test are confusing to me. Again, in single port configuration with no signal from the intake manifold, the only force keeping the wastegate valve closed is that of the spring. As exhaust pressure approaches and this force, the wastegate will open to a point of equilibrium. I don't see how 18+ psi can be achieved with the stock wastegate that many knowledgeable folks here agree has a fairly weak spring. In addition, the fact that only 5 psi intake manifold pressure is achieved when supplying this full pressure to the opening force of the wastegate (connection to top port) further evidences the fact that the force of the spring is not enough to prevent opening until 18+ psi of exhaust/intake pressure is attained.

I'm thinking that the answer may have to do with how the two pressures are acting on the spring. Is it that the 5 psi supplied to the top port acts on a smaller surface area and therefore supplies more force to overcome that of the spring than 5 psi exhaust manifold pressure acting on the greater surface area of the valve? My understanding of physics is a bit rusty. Can anyone one tell me if I'm on the right track here.

TurboTommy

Well, I wondered about this myself.
The only thing I can think of is that the intake manifold pressure on top of the wastegate diaphram is acting directly on the valve with a fair amount of area; as opposed to the exhaust pressure entering the backside of the valve tangically (spelling?) with alot less area acting on the valve.

Rogue_Ant

Thomas is correct. The diaphragm in the wastegate acts like a lever for manifold pressure to open the wastegate.
But even so, the stock wastegate spring is quite weak, and enough exhaust pressure will open it after a certain amount.

kahlveen

Yeah, I was just forgetting that the same psi applied to different surface areas will exert different amounts of force. It seem like the stock wastegate is particularly sensitive to intake manifold pressure applied to the top port (must have small surface area in relation to the valve), but this wouldn't necessarily mean that the spring is weak. If the pressure is applied to a very small surface area, the force on the spring can be quite large. The fact that without any pressure applied to the top port, the wastegate can allow for 18+ psi intake manifold pressure shows that given the strength of the spring and the valve surface area, the exhaust pressure may need to be quite high to open the valve. Of course I suppose the valve could be opening partially and still allow 18+ psi to be achieved although more slowly than if it was completely closed.