[Kingtal0n]

Originally Posted by spooled240

A freer-flowing exhaust would just make the problem worse. The faster-moving exhaust would take the path of least resistance which in this case is the larger turbine wheel and not the wastegate flapper. This is common problem on FD RX7's when a full turboback is installed. Porting the wastegate helps, but in many cases you need to keep some restriction in place like the catalytic converter to prevent boost spikes on the small factory wastegates.

This is indeed a deep subject but I don't mind going over some
I am sure the problem you state exists, but the reason is not obvious.

On an OEM turbine outlet both the gate and post turbine region is shared. So they are at the same pressure, there is no differential between the gate outlet and turbine outlet, and pressure is a vectorless scalar so it has no direction either. If we neglect the kinetic energy of exhaust gas, we can safely assume that exhaust gas will "see" the pressure difference from pre-turbine to post turbine and choose the gate pathway as it is less resistance than going through a turbine, causing turbine wheel speed to stop accelerating or slow down, in theory. And it does up until a certain point of total flow volume where the gate is "full" and the turbine begins to look like an easier path.

The variable we must take into account however is the temperature drop between turbine and downpipe.
The turbine extracts energy from exhaust gas and the temp drops, and as it cools it also takes up less volume. In contrast, Exhaust gas passing into a gate does not experience any temperature drop , it merely seeks to equalize the pressure from high to low and full temperature exhaust gas is ejected post turbine, causing a rise in pressure there and putting a cap on flow rate, limiting power. This is why we never want to USE a wastegate if we can avoid it and why giant turbines with tiny wastegates are preferable for top end racing (drag racing stuff with high boost and flow).

What this really boils down to is, any gated exhaust gas is going to negatively impact the post-turbine exhaust volume, increasing the pressure and temperature post turbine. This is why using a tiny turbine and a huge gate won't net you ultra fast spool and a very powerful top end, even though to a novice it looks like we can just use the smallest turbine possible and gate the remaining volume to keep boost in check and get the best of both worlds, it won't work in practice.

To better understand lets look at your example and try to make sense of it.
Installing a large free flowing exhaust system does one major thing, it can drop the pressure of the post turbine region, which makes both the gate and turbine more easily to flow of course. It will have a positive effect on both gate and turbine, likewise engine flow rate will increase, which will increase exhaust gas volume at the same boost pressure, which increases turbine wheel speed when all conditions are held similar. The gate diaphragm can't detect flow rate, only pressure, so when flow rate increases at the same pressure (such a when the ambient air becomes colder sometimes) there will be a similar volume flow rate going into the intake manifold but a much higher volume of exhaust gas will be ejected due to the increase in total engine mass flow rate at the same old pressure which opens the gate. That is why it may seem like a larger downpipe causes further overboosting in that situation, when in reality what you've done is improve both the gate and the turbine efficiency so the engine is able to flow more total mass with a lower pressure in the exhaust system and the gate was never compensated to bring the mass flow rate back down to where it was before the upgrade to take advantage of the increased efficiency and superior boost control at that mass rate (better than before). This is why I suggested to increase the post-turbine exhaust volume previously and I hope you can see why it makes perfect sense in theory if we do everything right.

In other words, the engine may seem to be overboosting due to the reduced pressure post turbine, but in reality it is overboosting because of an increase in overall mass flow rate and horsepower output.
E.g. If we adjusted the diaphragm such that it would open at slightly less pressure, thus bringing down the engine's mass flow to the previous rate (before the downpipe upgrade) we would find that the engine can make slightly more power than before (or similar power at least) and has slightly better boost control (less overboosting with the larger exhaust system).

But since nobody ever does that, they always leave the original spring/diaphragm which cannot regulate mass flow rate, it can only respond to pressure, the mass flow rate overwhelms the gate more easily because although the intake pressure is the same , exhaust pressure, volume and temperature is higher.


This can be re-visited a few ways, I'll say it different ways to be super clear
1. By working backwards and taking careful measurement of the turbine wheel speed and engine flow rate, we will see what really happened. The engine efficiency improved and is now trying to eject more exhaust gas volume and have more power than before at the same boost pressure, since the gate spring never changed and it can't tell flow rate from pressure (the definition of more power = more flow but not necessarily more volume or pressure, flow rate in mass and flow rate in volume are two different things and the turbine wheel & wastegate diaphragm doesn't care about mass it only can see volume and pressure). So now there is a higher volume of higher temperature exhaust gas to "get rid of" and the gate is the same size as always so it has even more trouble regulating boost.


The larger exhaust doesn't cause anything with a "path of least resistance" since as we already discussed the post turbine and post gate pathways are both expressed in identical pressure units for an OEM style turbine outlet. i.e. resistance is always going to be lower in the gate than going through the turbine because they both share the same destination but the gate is just an open hole where the turbine is a solid, flat surface (the blade surface that exhaust gas strikes is a "wall" to the gas molecules). The capacity for flow rate in units of volume can become much higher going through a turbine because even though path resistance is higher, the cross section area is also much larger. so pressure rises more quickly as the gate jams up with unwanted exhaust volume than when going through a reasonably sized (large enough) turbine.

2. Review what we know about intercooling. If we intercool we lose power because an intercooler is a restriction. Any extra length of pipe is going to cost pump head, reducing volume flow rate of the turbo compressor. Whether the temperature drops or not has nothing to do with this fact, so it doesn't matter if we install a long pipe or an intercooler the same thing always happens: additional volume costs flow rate.
So how do intercoolers seem like they add power?
When the gate is referenced to the intake manifold, it cannot "see" the compressor outlet pressure. Thus when we intercool and drop the intake manifold pressure, now the gate can see the drop in pressure and demand more from the turbine, increasing power.
So the extra power comes from the TURBO compressor wheel speed, NOT the intercooler. The intercooler saps some pump head but the drop in temperature and pressure allow the gate's reference to call for additional shaft speed thus increasing compressor pump volume flow rate.

This is a useful analogy because a similar situation is occurring in the exhaust system when we play with downpipe, post turbine pressure. Again the gate cannot "see" the flow rate or exhaust pressure, only intake pressure. thus any increase to engine mass flow rate at the same pressure (such as when we install a larger downpipe) will also coincide with an increase in turbine wheel speed and wastegated volume. So it isn't fair to say "the higher flowing exhaust causes overboosting". Instead we should be saying "the higher flowing exhaust is increasing engine mass flow at the same intake manifold pressure which is what the gate is referencing so now there is a higher volume and pressure of hot exhaust gas being ejected which is causing overboosting because we never adjusted the gate diaphragm to correct the mass flow rate back to where it was before the upgrade to make the comparison fair".



3. Using a common equation,
mass flow through turbine = (mass flow) * (sqrt(temperature)) / (pressure)

Pressure being in the denominator will reduce mass flow rate as it climbs.
Likewise pressure dropping to smaller numbers will improve mass flow through turbine.

We already know that the more exhaust gas that passes through the turbine rather than the wastegate, the lower temperature and smaller volume our post turbine exhaust gas will be. Thus it will be able to gate more effectively since post turbine and post gate share the same pressure scalar destination. We can also conclude that pre-turbine and pre-gate pressure and temperature should be higher since now the turbo shaft is moving faster the engine mass flow rate will increase and more power can be produced at the same intake pressure.


To try and put it into words one more time,
When a larger downpipe causes MORE overboosting it is because engine mass flow rate increased and was never compensated for.

Example:
sr20 gt2871r internal gate setup lets say it gates at 13psi, overboosts to 15psi and has a stock downpipe. Lets say the mass flow rate is 35lb/min and shaft speed is 50,000rpm at some specific point.

Now we install a larger downpipe and suddenly experience an overboost from 13psi to 17psi, it got slightly worse because the turbine and gate is very small so this is a unique situation for small turbos we can fix.
When we check the flow rate we see that at the same point we measured previously, mass flow rate is 38lb/min and shaft speed is 60,000rpm. The engine picked up 30 horsepower due to the higher shaft speed but the overboost is annoying our customer who wants it to stop for some reason.

To fix this now, we need to adjust our gate to achieve the similar previous target of 35lb/min and 50,000rpm, whatever boost pressure that is will be lower than before due to the downpipe upgrade. Lets say this turns out to be 11psi. Now, the engine will hold a steady 11psi with 35lb/min and 50,000rpm shaft speed, no more overboosting. The downpipe upgrade "fixed" the issue once we adjusted our gate properly.
In other words, the engine is now making the same power as before, with less boost pressure. Since power is the same, shaft speed is the same, we didn't alter our intercooler or ducting or air filter or intake manifold so it would not change. The pre-turbine pressure and temperature is also the same because engine output has been fixed to the same point as previously and the same turbine and gate is in use.

Only our post-turbine exhaust gas pressure has been reduced, which improves the turbine efficiency and gate efficiency, reducing or eliminating the boost creep with the similar mass flow rate and shaft speed as before. This will slightly increase engine power at the same mass flow rate (maybe 1% or less) which is negligible so we don't need to mention it- but an improvement to efficiency anywhere will have that affect.