GUNDIE

Need some help here it doesnt make sense to me.
But I went in for a tune today on the new setup and its hitting 18psi by 5500 rpm and creeping up from there.

The turbo is brand new with a 12-14psi internal wastegate, shouldnt the wastegate be opening within that range and keeping the boost Down? Currently bypassing the boost solenoid so theres no interference between boost and wastegate.

Shop is suggesting I need an 8-10 psi wastegate but I cant understand why that would matter, feel like if this was a common problem with this turbo there would be more posted on it but I cant find it or Im looking in the wrong place

Setup below
Kouki s14 SR
Tomei manifold
Gt2871r-15 .64
Link ecu
Haltech solenoid
550cc injectors

spooled240

This is a common problem on larger internally gated t25/t28's. The problem is the larger compressor wheel and turbine wheel with the small back housing and internal wastegate. There is a lack of flow through the wastegate. Some options would be:
-port the wastegate
-run a larger .86 back housing
-go external wastegate
-introduce some restriction on the exhaust

My suggestion is to go external by modding the manifold or getting a "t25 plus" housing

GUNDIE

To you and the guy below I have a buddy running the hks wastegate around 14psi making around 300 which I?m down with, I don?t need a ton of power. I do have a s15 wastegate which should be around 8psi would that solve the issue or should I just buy the hks? And thank you both for the input 👊🏻

Another option is head gasket studs bigger injectors and E85 lol but I want to actually drive this build for a bit

spooled240

The issue isn't the actuator, it's the flapper design. It's small in diameter and not angled in a way where the exhaust would flow through it efficiently. You could try running a softer spring, but its not a real solution. The crack open point will be at a lower psi so youll lose some spool and then the boost will just spike to 14. Pretty lame. I would port it for more flow.

slider2828

I agree with spooled that man knows what he is talking about....

You can try also an HKS wastegate, but going external is the best way to control it.

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Abeo

I have a chinese 2871 clone that was creeping bad, I ported the housing to match the manifold as well as the flapper (not the flapper opening, but the flow path to it). My 12 psi wastegate is preloaded, I run a boost controller off the ecu, and my boost is rock solid up to 20 psi (pings after that).
I doubt a genuine Garrett turbine housing casting is as bad as mine was, so +1 on porting.

GUNDIE

Ended up ordering an external wastegate tial mvs, getting it welded on now so cross my fingers this solves it.

Still cant wrap my head around why it cant maintain a low boost level and continues to climb, from reading up on it it seems hit or miss on people have issues with it

jr_ss

Known and common issue on these turbos. You?ll have better response and control with the external.

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Kingtal0n

exhaust gas takes up more volume the hotter it is.

Therefore EGT has a direct influence on the gate behavior. It is likely you are seeing a wide range of results due to a wide range of EGT and post turbine exhaust system configs

to fix this issue and keep the internal gate may be as simple as reducing the EGT via tuning or water injection,
plus a superior flowing downpipe and exhaust (cutout system or larger diameter tubes)

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.

Kingtal0n

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.

spooled240

One major variable you're overlooking is the design of the turbine housing and internal wastegate valve itself. It's positioned in a way where faster exhaust gases will not flow through it efficiently. Wastegate position is very important in many long tube turbo manifolds where the gate needs to be positioned in a way where its partially facing against the flow of the exhaust. The same is true for turbine housings where the exhaust is flowing at an extremely high velocity in one direction. Its like trying to catch water from a stream with a cup perpendicular to the direction of flow. If you look at the EFR turbos, the internal wastgate is also facing the entry of exhaust gas for optimal wastegate flow.

Kingtal0n

none of that matters when we hold the mass flow of the engine constant

If you have two identical examples with identical mass flows and turbine shaft speed, and you upgrade the downpipe or post turbine pressure drop efficiently, the only thing that will change is the wastegate fraction of ejected volume. The lower pressure means more exhaust can flow through (any type) of gate, whether internal or external or merged or divided etc... makes no difference, the flow rate will increase, which causes the gate to close slightly compared to previously, which means it is no longer "working" as much as previously. So if it was at 100% previously now it will be 99% or less, you gain gate effectiveness with no change to power, shaft speed, turbine behavior. Even though the post turbine pressure is lower the turbine never speeds up because we are manually controlling the shaft speed as a constant instead of a variable.

I get what you are saying, i.e. any change to the post turbine or pre-turbine will influence power, turbine shaft speed, rate of change, there are myriad variables that will wander around. But because there are innumerable molecules and pressure gradients in play in such a complex situation, for engineering examples, modelling, and classical physics in order to determine which variables are dependent and which are independent (If I wanted to create a dynamic equation and a matlab circuit to model this with a computer) we would be interested in mathing out individual examples while holding many variables as constants in order to see which variables influence which aspects. And so we lock the volume flow rate and external ambient temperature and pressure as constants and see that the only change in a post downpipe situation with an infinite number of potential downpipe sizes (i.e. 1.001", 1.002", 1.003", ... all the way to 5.0" or 10.0" of downpipe) will result ONLY with a change in wastegate potential volume ejection and not much else, no matter what type of gate or plumbing is involved. e.g. we don't need to guess whether 1" downpipe is better than 5" to understand that there will be some improved volume flow rate of any wastegate situation with the larger downpipe series.

Kingtal0n

I want to try and say it more simply, lets see

The mass flow rate of an (combustion) engine is calculated by comparing pressure, volume, and temperature to get some rate of airflow in mass per unit time which closely correlates with engine power.

If you ask "pressure from where?" I will answer that the mass flow of air into the turbo has to equal the mass flow out of the exhaust port, just like with current into an electrical circuit, so what we really have is a set of partial differential equations which represent the pressure at EVERY POSSIBLE MOLECULE In the entire engine, easily calculable but spread apart by boundary conditions.

Luckily I don't need to write or solve any math to apply our universal law that mass in has to equal mass out. If we do something, anything, to the exhaust which will influence one of those major variables: static and 1st 2nd 3rd differentials of pressure, temp, & volume then the mass flow will also change, perhaps in a number of non-linear ways.

By making the assumption that larger diameter is going to increase flow rate we are essentially saying that engine power is increasing, which implies the turbocharger shaft is speeding up, because there is no way to get any additional mass flow rate through a turbocharged engine without altering the temperature of the air incoming to the turbo, or adjusting the pressure of the air entering a turbo (elevation is the only way to do that unless you are compound turbo/boosting which is way off discussion topic), or changing the volume of air pumped by that turbo which is only possible via shaft speed. The critical idea here being that turbochargers always flow a specific volume number at some specific shaft speed no matter what the temperature or pressure is, in classical physics.

For this to be a fair comparison of stock downpipe vs upgraded downpipe, I feel that shaft speeds (and turbo inlet air temperature) should be identical. You may not feel that way and so you are also right in your own way if so.

I suggest that we bring shaft speed back to the original shaft speed by adjusting the wastegate properly, and the boost will become lower than it was before, due to the larger downpipe and WHAM you've just solved the boost creeping issue with a larger downpipe upgrade because turbine mass flow rate will be the same as it used to be with the stock downpipe (engine will make the same power) but pressure in the downpipe will be lower because of the larger downpipe, and the gate will have all that extra downpipe volume and reduced pressure to eject exhaust gas into.

Hope it starting to make sense !


okay I see now how to say it with one sentence:
"Install larger downpipe, reduce boost pressure, engine will make the same power as before but boost won't creep"

Kingtal0n

thank you for taking the time to respond, read, and think about this.

Although in the real world such principles seem obsolete (we just buy a turbo and use a tial and call it a day) its good practice to re-visit these sort of problems.

I make this quick in paint, to show what I am saying instead of just a wall of text



Note the gate shares the same volume as the turbine. So when the downpipe volume grows larger, and the turbine isn't using any of the extra space (same wheel speed with even less pressure) all that extra volume can be used by the wastegate.


... And just FWIW. None of this really matters to a MAF car. Boost isn't measured by a maf sensor, only mass flow is measured by the MAF. SO only cars with MAP sensors care about the actual boost number.
In other words, if the engine overboosts from 12 to 18psi, the MAF has no clue there is any over boost condition. The maf does not care about that. The only thing that matters is whether or not the MAF can support the mass flow, and whether the injectors and ECU can fuel the engine properly at that mass flow. To put it another way, 12psi can make the same power as 18psi (same mass flow) due to differences in intake/exhaust restrictions, and to a maf sensor in that situation, 12psi = 18psi, it can't tell any difference.

If there is some issue with fueling that mass flow (lets say the boost and mass flow is climbing too high for comfort of the fuel injectors),
the easiest 'fix' (instead of upgrading the injectors) is to use a smaller air filter, or add a restriction plate to the inlet.
This would be similar as a restriction in the exhaust (smaller diameter downpipe) without the negative influence on exhaust gas temp and pressure that a small downpipe volume creates.

Kingtal0n

This is just for fun. from fluid dynamics


water is a fluid, like air, the equation holds for air, water, any fluid

Vr is velocity relative to the blade, so Vnozzle - Vblade = Vrelative

The equation setup to find the Force on the blade inside a control volume from knowing the mass flow rate and relative velocity of the fluid to the blade.
To find the force on the blade what we need is the velocity of fluid leaving the nozzle, velocity of the blade, mass flow through the turbine, area of nozzle, density of fluid and angle of the blade.

Force on the blade is equal to the difference of (mass flow times velocity)out - (mass flow times velocity)in.
mass flow is density of fluid times volume flow rate.
mass flow is also density times velocity times area.

Since mass flow out = mass flow in, all that is changing is the velocity of the fluid when it strikes the blade and redirects with some angle, which causes a force on the blade that we are interested in.

On the left hand side we convert mass flow into (density * area * velocity) and factor it out, leaving one more velocity inside the ( ) allowing us to deal with the difference in relative velocity of the stream of fluid by taking the cosine of the angle where it strikes the blade (angle is given as a variable alpha).

This example also shows how we can neglect negligible information for control situation, such as the influence of gravity and change in velocity due to volume, and some surface forces that don't need to be considered in this example. And how important mass flow is for our calculations of moving fluids, how we can use mass flow in so many different ways to make life easy when trying to figure out what will happen with fluid systems. If we have mass flow then technically we have the fluid density, velocity, and area it is flowing through, which is a lot of useful information packed into 1 term. So for example if the area is changing, we can find new velocity and density depending how we setup the equation.

https://www.grc.nasa.gov/www/k-12/airplane/mflow.html

pacotaco345

what in tarnation is going on in here.

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Kingtal0n

it just goes to show that two trolls can't occupy the same place at the same time without some consequence

spooled240

I think the bots are back..or maybe Steven Hawking

I just want to get back to talking about blacktops instead of blackholes

Kingtal0n

Funny thing about black holes. The universe is full of them. There are so many black holes you can't count them.







So uh, ... they aren't that difficult to create.


IMO If protons have a half-life it means they decay. Which means from one instant to the next- they are different. Always different as moving forwards in time. That is why nothing material can become a constant. Our contrived so called "physical constants" In physics are really infinite repeating decimals that are constantly changing but so far out in the decimal that to us it seems constant, in our and our sun's relatively short lives.

jr_ss

King, stop doing this to every thread. No one wants a dissertation to read through every time a topic comes up that you think you know better. Say your piece in a condensed version and move on.

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Kingtal0n

Luckily I don't care what you say and after having caught you in a lie I have lost all respect for you

good day

jr_ss

Caught me in a lie? I?d love to know what lie that was. Wait you mean my 2L motor not making 370ftlbs of torque because your mathematical equations say it?s impossible? Haha ok...

And do you honestly think I care what you say or think?

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spooled240

Jr, did you double check your math? King did prove that a t25 wastegate will be more effective with a freer flowing exhaust because the derivative of the square root of a snail with a half life of exactly 3 years that poops everday at 12 noon = 3, therefore a SR cannot make 370ftlbs. I mean maaaybe if the snail poops at 230, but thats wishful thinking..

jr_ss

😂🤣

My formula must have been off.

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jedi03

rofl i miss zilvia posts!