Tune with a downpipe.....why?

[bmx045]

No, I simply have a problem with people making blanket, inaccurate statements.

 

But I have even more of a problem with people trusting their motors to off the shelf maps without extensive logging and a WBO2. The hysterical irony here is that it's probably safer to run the stock tune (with corrected boost control) with a downpipe, than an OTS map without proper logging. IMHO. Please explain why if I'm wrong.

.

 

The stock tune with stock dp knocks enough as it is, adding a dp will only exacerbate the issue with the quicker spool, and holding boost for longer. The entire timing table needs to be readjusted for the new spool time and prolonged boost.

[turbodog]

.

 

The stock tune with stock dp knocks enough as it is, adding a dp will only exacerbate the issue with the quicker spool, and holding boost for longer. The entire timing table needs to be readjusted for the new spool time and prolonged boost.

 

But what if you were to adjust boost control so boost onset and level is similar to stock?

[bmx045]

Then whats the point?

[dr_sharp]

Then whats the point?

Argument with turbodog fail.

[turbodog]

Then whats the point?

 

That the motor doesn't fail? I for one have never broken a subaru motor.

[NSFW]

Of course, to get maximum benefit from a downpipe an ecu tune is the best choice.

 

That's the main thing, with an LGT. There aren't a lot of people who want a downpipe but don't mind leaving that power on the table. And MBCs are kind of annoying. (However PTFB is not harmful, contrary to what 95% of NASIOC seems to believe.)

 

Also note that the load axis on the factory fueling and timing tables only extends to about 2.5 g/rev, and my car (catted DP, 3" midpipe, 2x2.5" Y-pipe, stock muffers) was able to pull about 2.75 g/rev. If you go catless with aftermarket mufflers, you'll likely get even more airflow.

 

It's not a big deal with the fuel table IMO, but the problem with running past the edge of the timing table is that the ECU stops tapering timing when load increases beyond 2.5 g/rev. I suspect you'd knock but I'm not about to go try it. You could limit boost to avoid running past the edge of the table, but like BMX said, then what's the point?

[turbodog]

That's the main thing, with an LGT. There aren't a lot of people who want a downpipe but don't mind leaving that power on the table. And MBCs are kind of annoying. (However PTFB is not harmful, contrary to what 95% of NASIOC seems to believe.)

 

Also note that the load axis on the factory fueling and timing tables only extends to about 2.5 g/rev, and my car (catted DP, 3" midpipe, 2x2.5" Y-pipe, stock muffers) was able to pull about 2.75 g/rev. If you go catless with aftermarket mufflers, you'll likely get even more airflow.

 

It's not a big deal with the fuel table IMO, but the problem with running past the edge of the timing table is that the ECU stops tapering timing when load increases beyond 2.5 g/rev. I suspect you'd knock but I'm not about to go try it. You could limit boost to avoid running past the edge of the table, but like BMX said, then what's the point?

 

Ok, but is that at increased boost levels? Because at factory boost levels I don't think you would be pulling any more air flow.

 

I guess the point is, that you should not run a MBC or otherwise increase boost levels on the stock tune? Or allow a boost creep situation? Because that's what sounds dangerous here.

[NSFW]

You might flow a little more air due to the reduced backpressure, but staying with the stock boost curve will probably keep you within the stock tune's safety margin.

 

I say "probably" because I am just guessing - I have not tested this and I am not aware of anyone who has. You might be the first person ever to install an aftermarket downpipe while making a conscious effort to prevent any additional boost or power.

[BAC5.2]

Ok, but is that at increased boost levels? Because at factory boost levels I don't think you would be pulling any more air flow.

 

I thought you understood the relatively loose correlation between boost and airflow?

 

What determines airflow across an exhaust driven turbine? Going from a 2.5" flat-flange downpipe to a bellmouth 3" pipe, you see a massive jump in volume right after the turbine. And what happens when you evacuate air into a relatively larger chamber? Pressure drops.

 

And what is an indicator of turbine efficiency? The inlet/outlet pressure ratio. Drive that ratio up (by driving the outlet pressure down), and the turbine efficiency jumps. A jump in turbine efficiency means a jump in compressor efficiency. And that means a drop in temps, and an increase in mass-flow across the compressor.

 

Even at the same boost, you'll almost undoubtedly flow more air at every point.

 

Since airflow and pressure ratio are nearly linear (for the narrow band of pressure ratios we are talking about), you COULD calculate the efficiency change with a downpipe, then tune boost to reflect that change. Then you'd more closely approximate "stock" output, but with an aftermarket downpipe.

 

You can see this manifest by comparing a tuned stage-1 car against a tuned DP only car. Overlay the plots, and you'll notice that the stage-2 car makes more power EVERYWHERE, not just once boost is increased above stock levels.

 

I guess the point is, that you should not run a MBC or otherwise increase boost levels on the stock tune? Or allow a boost creep situation? Because that's what sounds dangerous here.

 

Not exactly.

 

You CAN, though you should not, increase boost on the stock tune. The ECU has provisions for air-flow overages (remember that the car is designed to operate in climates significantly colder than wherever it is you live). You COULD put an MBC on the car and add a psi or so and make a little more power without the ECU dumping on your parade.

 

You simply shouldn't, and you certainly cannot do that with an aftermarket downpipe (for the reasons I just outlined). Why? Because the closer you get to that critical point where the ECU no longer has provisions for increased airflow, the closer you get to an anomaly causing a hiccup that destroys the engine. Like NSFW said, it's not so much that the ECU will shit it's pants, but that timing will no longer taper.

 

Now, you COULD combat this with a very sophisticated boost control setup. You could use an off-the-shelf electronic boost controller, and then fine tune boost to MAF readings to get somewhere close to stock. Or you could build an arduino EBC that would reference OBD information and tune boost automatically.

 

But doing that would cost you more than the $200 it would take to tune the car properly for an aftermarket downpipe. If you DON'T want to make the typical stage-2 power, simply tell your tuner what your plans and goals are. It's entirely likely that you can achieve your goals by simply tuning the otherwise stock car (a professionally tuned stage-1).

 

If your goal is to not blow up a Subaru engine, then installing an aftermarket downpipe and trying to keep it safe by merely manipulating boost is absolutely NOT the way to achieve that goal.

[turbodog]

And what is an indicator of turbine efficiency? The inlet/outlet pressure ratio. Drive that ratio up (by driving the outlet pressure down), and the turbine efficiency jumps. A jump in turbine efficiency means a jump in compressor efficiency. And that means a drop in temps, and an increase in mass-flow across the compressor.

 

Actually, this is not correct as far as I know. Even though the turbine and compressor are joined by a shaft, this compressor itself is still an air pump - for a given boost level, ambient temp/pressure, you should see the same air mass and shaft RPM. Keep in mind you have a set of valves in gas path, completely separating their flow most of the time.

 

Reduced back pressure will of course increase pressure ratio at the turbine. The wastegate will need to open further, reducing back pressure even more (hence the need to port wastegates to stop boost creep).

 

The main impact is in volumetric efficiency, AKA "the efficiency with which the engine can move the charge into and out of the cylinders", specifically on the out. You would see minor impact on where the compressor operates due to valve overlap and reduced heat to the turbo overall, but this is minor.

[SSpeed]

This is going to be fun, telling BAC5.2 he's wrong.

 

FYI, I tried a downpipe on a stock tune while I was messing around trying to pass emissions. I had significant knock as boost came on (long before hitting overboost), I also had a couple of backfires almost immediately when stepping on the gas, before boost came on. It was repeatable, I didn't care enough to prove anyone wrong to keep trying to see why.

[BAC5.2]

Actually, this is not correct as far as I know. Even though the turbine and compressor are joined by a shaft, this compressor itself is still an air pump - for a given boost level, ambient temp/pressure, you should see the same air mass and shaft RPM. Keep in mind you have a set of valves in gas path, completely separating them most of the time.

 

The compressor is an air pump, driven by the turbine. The efficiency of the turbine directly impacts the efficiency of the compressor. That's why turbine dynamics are so critical to turbocharger efficiency, and why you can't just arbitrarily spec a turbine, it's design is paramount to the performance of the compressor.

 

This is clearly evident when comparing different turbine wheels with the same compressor. Performance can differ VASTLY between the same compressor with a different turbine.

 

Reduced back pressure will of course increase pressure ratio at the turbine. The wastegate will need to open further, reducing back pressure even more (hence the need to port wastegates to stop boost creep).

 

Not exactly. You have to think of the fluid dynamics of what happens on a wastegate. If you drop pressure behind a wastegate, you decrease the ability for the actuator to accurately control boost. Look at wastegate design, and you'll see that it's an essentially a siphon tube. As exhaust gas blows across the orafice leading to the flapper, there is a pressure drop inside the turbine housing in the wastegate portion. With a stock DP, there is relatively low pressure behind the wastegate flapper as well (a flat flange effectively turns the wastegate area into a siphon area of substantially larger volume than the one inside the turbo (which is important, because gas velocity is slower on the exit side of the turbine). That makes it relatively easy for the wastegate actuator to control the flapper. The spring closes the wastegate (which is somewhat close to equilibrium), and pressure in the actuator opens it. Nice and easy.

 

When you get rid of the low-pressure behind the flapper by opening up the space behind it (via a bell-mouth), you tip the balance of pressure to holding the valve closed. This means that the flapper is effectively being held shut by the higher pressure behind the flapper than there was before. This causes boost spiking, because the flapper is now much more difficult to open, and thus requires higher actuator pressure to begin to do so.

 

Porting the wastegate is an attempt to increase the pressure on the housing side of the wastegate, and help restore that state of quasi-equilibrium.

 

The impact is in volumetric efficiency, AKA "the efficiency with which the engine can move the charge into and out of the cylinders", specifically on the out.

 

You should know better than to quote wikipedia, man!

[sumfoo1]

^ completely correct.

[seabass07]

Damn, I missed this yesterday. If you want your engine to knock, then go for it. Now if you were too cheap to buy a tune and too lazy to learn how to do it yourself, then you could just reduce timing and zero out wgdc and boost targets. If someone was dumb enough to gut or replace their downpipe and then remove all benefits from it by detuning everything in a ridiculous way, then they should be ridiculed endlessly.

 

OP, if you're asking is what actually needs to be changed in the tune for a downpipe, that's an intelligent question that has already been answered in this thread. But I think that's giving you waaay too much credit!

[turbodog]

The compressor is an air pump, driven by the turbine. The efficiency of the turbine directly impacts the efficiency of the compressor. That's why turbine dynamics are so critical to turbocharger efficiency, and why you can't just arbitrarily spec a turbine, it's design is paramount to the performance of the compressor.

 

This is clearly evident when comparing different turbine wheels with the same compressor. Performance can differ VASTLY between the same compressor with a different turbine.

 

Again, no. The efficiency of one does not effect the other. Efficiency is a characteristic expressed by their operating parameters. The only way to make either more efficient (or inefficient), is to physically change the part. If you want to say that they are operating in a more efficient zone well then yes. Look at the graphs.

 

Of course you have to have a properly sized turbine to match your compressor and engine performance goals, that's turbos 101. Want to get into A/R's, or have you never run a larger turbine housing?

 

 

 

 

Not exactly. You have to think of the fluid dynamics of what happens on a wastegate. If you drop pressure behind a wastegate, you decrease the ability for the actuator to accurately control boost. Look at wastegate design, and you'll see that it's an essentially a siphon tube. As exhaust gas blows across the orafice leading to the flapper, there is a pressure drop inside the turbine housing in the wastegate portion. With a stock DP, there is relatively low pressure behind the wastegate flapper as well (a flat flange effectively turns the wastegate area into a siphon area of substantially larger volume than the one inside the turbo (which is important, because gas velocity is slower on the exit side of the turbine). That makes it relatively easy for the wastegate actuator to control the flapper. The spring closes the wastegate (which is somewhat close to equilibrium), and pressure in the actuator opens it. Nice and easy.

 

When you get rid of the low-pressure behind the flapper by opening up the space behind it (via a bell-mouth), you tip the balance of pressure to holding the valve closed. This means that the flapper is effectively being held shut by the higher pressure behind the flapper than there was before. This causes boost spiking, because the flapper is now much more difficult to open, and thus requires higher actuator pressure to begin to do so.

 

Porting the wastegate is an attempt to increase the pressure on the housing side of the wastegate, and help restore that state of quasi-equilibrium.

This is somewhat correct. However by your logic, a flat plate downpipe would always provide better boost control, which does not seem to be correct.

 

There is a massive pressure drop across the turbine - through the wastegate there is little pressure drop. Thus you have high pressure exhaust stream blasting out the wastegate into the lower pressure post turbine exhaust stream. A flat plate is only going to hurt flow through the wastegate in all situations.

 

I understand your point about the siphon effect pressure drop of the flow perpendicular to the wastegate orifice, but unless you can point to something that proves it conclusively, I don't believe it out weighs the loss of backpressure from removing the cats. If you ran a stock catted downpipe with a bellmouth, it would probably control boost fine, and a flat plate catless would not control it well at all.

 

In a severe boost creep situation I assure you the flapper is wide open, and in fact can be completely detached and still creep.

 

Have you ever personally ported a wastegate or seen it done? I have. On a catless car. In addition to the hole being opened slightly, the internal leading edge of the orifice is rounded over, forcing much more flow out the wastegate. This works exceptionally well and is IMHO necessary on any catless subaru running in cold temps, tune or not.

[BAC5.2]

Again, no. The efficiency of one does not effect the other. Efficiency is a characteristic expressed by their operating parameters. The only way to make either more efficient (or inefficient), is to physically change the part. If you want to say that they are operating in a more efficient zone well then yes. Look at the graphs.

 

Of course you have to have a properly sized turbine to match your compressor and engine performance goals, that's turbos 101. Want to get into A/R's, or have you never run a larger turbine housing?

 

I'm sorry, but you are just not correct here. The efficiency of the turbine directly impacts the efficiency of the compressor. The compressor is an air pump, driven by another air pump.

 

This is somewhat correct. However by your logic, a flat plate downpipe would always provide better boost control, which does not seem to be correct.

 

That's not my logic at all. That's my logic as to why bellmouth downpipes cause unpredictable boost control in otherwise untuned vehicles. Isn't that what you started this post for?

 

There is a massive pressure drop across the turbine - through the wastegate there is little pressure drop. Thus you have high pressure exhaust stream blasting out the wastegate into the lower pressure post turbine exhaust stream. A flat plate is only going to hurt flow through the wastegate in all situations.

 

I'm not entirely sure that's correct, especially in situations of otherwise stock vehicles with only downpipe modifications. The moment we bring other configurations or other variables into it, EVERYTHING changes.

 

If you put a bellmouth downpipe on an otherwise stock vehicle, you will undoubtedly experience unpredictable boost control.

 

Anything beyond this, is a completely different argument.

 

I understand your point about the siphon effect pressure drop of the flow perpendicular to the wastegate orifice, but unless you can point to something that proves it conclusively, I don't believe it out weighs the loss of backpressure from removing the cats. If you ran a stock catted downpipe with a bellmouth, it would probably control boost fine, and a flat plate catless would not control it well at all.

 

We are talking about removing cats now, too?

 

I'm sorry, but I can't follow your argument if you continue to change parameters. If you want to twist the story until it finally falls in line with your argument, then more power to you. I'm not interested in quibbling.

 

In a severe boost creep situation I assure you the flapper is wide open, and in fact can be completely detached and still creep.

 

That's correct, it's a flow problem through the wastegate orafice... Like I said.

 

Have you ever personally ported a wastegate or seen it done? I have. On a catless car. In addition to the hole being opened slightly, the internal leading edge of the orifice is rounded over, forcing much more flow out the wastegate. This works exceptionally well and is IMHO necessary on any catless subaru running in cold temps, tune or not.

 

I have, and that's exactly what I said in my post where I talked about porting the wastegate to increase flow out of the wastegate orafice by decreasing the lower-pressure effect caused by perpendicular flow...

[BarManBean]

It's pointless, BAC. I commend your efforts though...

[bmx045]

I don't know where BAC gets his patience from, I really don't...

[.Catalyst.]

I'm glad BAC is settled in after moving! Seeing his knowledge challenged makes for great threads, because he is that smart, and isn't relying on internet lore to back his claims. Rather his input is well rounded and contiguous in nature - not choppy bits of pasted text trying to argue for the sake of arguing.

[BAC5.2]

I appreciate it, all.

 

I'm not really interested in continuing this topic, though.

 

I vote case-closed and /thread.

[turbodog]

I'm sorry, but you are just not correct here. The efficiency of the turbine directly impacts the efficiency of the compressor. The compressor is an air pump, driven by another air pump.

 

You just straw manned me here.

 

Yes of course they are directly connected, by a fixed shaft. Same RPM. Function is directly related. But as it operates across different rpms and P/R's, each side will follow it's own efficiency curve which is a non linear relationship. Of course you could mathematically express the relationship.

 

Going back to my original point here, if you are to make the same boost in the same ambient conditions, the compressor will operating at roughly the same RPM and efficiency. HOWEVER, due to the decreased backpressure (higher turbine pressure ratio), more flow will have to pass through the wastegate to maintain a constant boost pressure. (not accounting for valve overlap)

 

 

 

That's not my logic at all. That's my logic as to why bellmouth downpipes cause unpredictable boost control in otherwise untuned vehicles. Isn't that what you started this post for?

 

I most certainly don't believe the whole siphon flow flat plate deal unless you want to cough up some proof. A theory and a BS theory at that.

 

 

I'm not entirely sure that's correct, especially in situations of otherwise stock vehicles with only downpipe modifications. The moment we bring other configurations or other variables into it, EVERYTHING changes.

 

If you put a bellmouth downpipe on an otherwise stock vehicle, you will undoubtedly experience unpredictable boost control.

 

Anything beyond this, is a completely different argument.

 

We are talking about removing cats now, too?

 

I'm sorry, but I can't follow your argument if you continue to change parameters. If you want to twist the story until it finally falls in line with your argument, then more power to you. I'm not interested in quibbling.

 

That's correct, it's a flow problem through the wastegate orafice... Like I said.

 

I have, and that's exactly what I said in my post where I talked about porting the wastegate to increase flow out of the wastegate orafice by decreasing the lower-pressure effect caused by perpendicular flow...

 

You can't have this discussion without talking about catless. It's by far the worst case scenario in decreased backpressure, resulting in maximum turbine pressure ratio. The standard wastegate cannot flow enough to cut down P/R, nor can the stock boost control act aggressively enough. That is the real problem with boost control here, not the bellmouth. Hence the need to alter boost control.

 

You just made a pretty bold claim back there about the flat plates flowing better, and having more consistent boost control. Back it up or retract.

 

And it's orifice, btw.

[dr_sharp]

Back it up or retract.

Oh snap! No he diunt...

[BarManBean]

You can't have this discussion with...me because I'm an idiot...It's by far the worst cast scenario.

 

Why don't you try writing in complete sentences before pointing out spelling mistakes just so you get the last word? You act like a child.

 

And before you say it--yes, so do I. The difference is that I mean to do it on purpose.

[bmx045]

And it's orifice, btw.

 

he's all sorts of nazi.

[seabass07]

he's all sorts of dumbass.

 

fxt