Here is another side I think should be considered.

If you have 2 turbos, and they can both supply the airflow needed to hold whatever boost you wanted untill redline, but throwing on the "bigger turbo" made a difference we would also have to take into consideration internal or externally gated. If it was internally, was the increase of power made through the wastegate expelling more backpressure, or was it the higher flowing turbine that made the difference. Also wich one would be more preferable? the higher flowing turbine or the higher flowing wastegate? Then wouldnt we be back to volume vs volacity?

But if the bigger turbo was added to an external gate system then the added power was obviously from the higher turbine correct?

Not exactly. First off, internal or external gated shouldn't make any difference in performance. The job of the gate is to bypass the turbine when you reach your desired boost level. Turbine size/capacity is unrelated to the WG.

Turbine size affects VE and generally where your torque peaks. Assuming the turbine is paired with a suitable compressor that delivers the desired airflow, and we have maintained a reasonable turbine to compressor ratio, then chosing a turbine is a matter of personal performance and drivability preference. A smallish turbine will spool fast and peak at lower revs, while a bigger turbine will take longer to spool and moves your peak further up the RPM range. In most cases it's a tradeoff. But as I stated before, you can have more turbine than your engine can use so you end up running out of VE before you reach the maximum capacity of the turbo. On the flipside if your turbine is too small it spools very quickly, but it chokes off VE in the upper rpm range. Neither one is good or bad, just a matter of what you want from your application.

As for wastegates, the right wastegate for your application is the one that wastes at the proper boost level you set. Some turbos are available with internal gates, others are not. Some turbos with internal gates don't function very well up to or over certain pressures, as is the case with the GT32. But I've also seen some external gates experience boost creep for one reason or another. I haven't seen any specific evidence to suggest that an external gate is better or worse than an internal.

 

Not exactly. First off, internal or external gated shouldn't make any difference in performance. The job of the gate is to bypass the turbine when you reach your desired boost level. Turbine size/capacity is unrelated to the WG.

Turbine size affects VE and generally where your torque peaks. Assuming the turbine is paired with a suitable compressor that delivers the desire airflow we want, and we have maintained a reasonable turbine to compressor ratio, then the chosing a turbine is a matter of performance and drivability goals. A smallish turbine will peak at lower revs, while a bigger turbine will move your peak further up the RPM range. In most cases it's a tradeoff. As stated before, you can have more turbine than your engine can use so you end up running out of VE before you reach the maximum capacity of the turbo. On the flipside if your turbine is too small it spools very quickly, but it chokes off VE in the upper rpm range. Neither one is good or bad, just a matter of what you want from your application.

As for wastegates, the right wastegate for your application is the one that wastes at the proper boost level you set. Some turbos are available with internal gates, others are not. Some turbos with internal gates don't function very well up to or over certain pressures, as is the case with the GT32. But I've also seen some external gates experience boost creep for one reason or another. I haven't seen any specific evidence to suggest that an external gate is better or worse than an internal.

There are two main advantages of the external gate.. firstly, the turbine outlet is far less turbulent as it isnt exiting into a large chamber housing both the turbine and wastegate. Exit is smooth from the turbine... which results in slightly lower exhaust pressure pre-turbine (due to better post turbine flow/lower pressure... much like running a larger dump pipe)

Secondly, waste gasses can be plumbed back into the exhaust further down where temperature/pressure of the exhaust gas is lower.. therefore a) creating less exhaust restriction and b) making for a much smoother join of the two pipes... as opposed to both of them 'spewing' into one awkward chamber as with the internal gate

Thirdly.. which isnt always done.. you can easily run a screamer, therefore not plumbing gasses back in at all and reducing exhaust restriction

-Tristan

 

This is one really good read... I have to sit down another day and read through this entire thread.

 

Not exactly. First off, internal or external gated shouldn't make any difference in performance. The job of the gate is to bypass the turbine when you reach your desired boost level. Turbine size/capacity is unrelated to the WG.

Turbine size affects VE and generally where your torque peaks. Assuming the turbine is paired with a suitable compressor that delivers the desired airflow, and we have maintained a reasonable turbine to compressor ratio, then chosing a turbine is a matter of personal performance and drivability preference. A smallish turbine will spool fast and peak at lower revs, while a bigger turbine will take longer to spool and moves your peak further up the RPM range. In most cases it's a tradeoff. But as I stated before, you can have more turbine than your engine can use so you end up running out of VE before you reach the maximum capacity of the turbo. On the flipside if your turbine is too small it spools very quickly, but it chokes off VE in the upper rpm range. Neither one is good or bad, just a matter of what you want from your application.

As for wastegates, the right wastegate for your application is the one that wastes at the proper boost level you set. Some turbos are available with internal gates, others are not. Some turbos with internal gates don't function very well up to or over certain pressures, as is the case with the GT32. But I've also seen some external gates experience boost creep for one reason or another. I haven't seen any specific evidence to suggest that an external gate is better or worse than an internal.

Your correct when you say it SHOULDNT matter. But if you look at it from a standpoint of this turbo made more at the same boost level.

If anyone read the thread i posted about backpressure a few days ago we know we need the most velocity w/o causing backpressure.

That being said if you WERE to run lets say a smaller turbine with a higher flowing wastegate(no difference to internal or external for argument sake) You could keep the high velocity by the turbine and waste the extra backpressure that would be created.

On the other hand you could run a larger turbine but now you have decreased velocity for volume were it will take longer to spool, you wouldnt need a bigger wastegate.

So my next question comes in as what would be the best pressure or if any between the head and turbo. I would think same as the intake side or slightly lower?

If i am wrong please correct me I am just trying to get it straight in my head.

 

backpressure is prob the best/easiest way to fix this up.
i'd use the example of the Supra TT
why do the owners not keep the twin turbo and crank the boost?
part of it is prob. because the backpressure caused by the smaller turbo at higher boost.

 

and a higher flowing wastegate would fix this problem correct? then you could run a smaller turine with a higher flowing compressor

 

and a higher flowing wastegate would fix this problem correct? then you could run a smaller turine with a higher flowing compressor

NOPE!

Size of turbine should be independent of WG. As for external WG, I generally prefer them, but on MR2s an external WG is anything but straightforward. I've some very poorly designed WG ports that flow poorly and cause boost creep issues. Also, the routing and integration of the dump tube back into the exhaust can be at a bad angle and causes turbulence and flow problems also. This is by no means a no-brainer.

Again, turbine and compressor should be sized properly and within reasonable ratios. As a general rule compressor to turbine ratio should be no larger than 1.25:1, and should be as close to 1:1 as possible. Big compressors on small turbines just create backpressure and lag. As discussed throughout this thread, you need to pay equal attention to both sides of a turbo.

 

As a general rule compressor to turbine ratio should be no larger than 1.25:1, and should be as close to 1:1 as possible. Big compressors on small turbines just create backpressure and lag. As discussed throughout this thread, you need to pay equal attention to both sides of a turbo.

Hmm, that's an interesting statement, and one that I don't agree with.

What dimension are you comparing here? Inducer or exducer? And how have you come to the above conclusion?

 

woops, i was 2 pages ago

 

I normally don't post on this forum, but my roommate asked me to. When you get down to the physics of the situation, you can actually express power output/potential as one of the following three terms: volume, pressure or temperature.

This is because everything is governed by the Ideal Gas Equation:

PV=nRT

P = pressure, V = volume, T = temp, n & R are constants

Rearranging this equation, you will get PV/T = nR

NOTE: The following example has nothing to do with turbo lag, which has to do mostly with factors like inertia of the wheels in the turbo and intercooler system volume

For the example of two turbos making different power at 10 psi, consider (PV/T)1 vs. (PV/T)2 since the constants will cancel out.
Obviously, P1 = P2 and we're assuming T1 = T2, so these cancel out of the equations. Now we're comparing the volume flow rate of the two engines, which directly relates to horsepower.

This leads you directly to the fact that the VE of the engine HAD to increase to make more power. Where this gets complicated is that changes made have a circular effect, i.e. increasing VE by going to a larger A/R turbine housing will allow you to run a larger compressor, which will flow more air at the same pressure ratio. But then you're moving more air through the engine, which will affect turbine performance...

VE changes are the easiest way to think about horsepower increases in an engine, but like I said, you can also think of it in terms of pressure and temperature. You are extracting energy from the exhaust gases by expanding them across a turbine. It all relates back to the Ideal Gas Equation and how you arrange it.

 

Ok, let make a more full comparison. Everything will remain the same except for the turbos.

Turbo one is smaller. Has a smaller turbine and compressor wheel. It will hold 15 psi to redline.

turbo 2 is bigger. It has a larger compressor and better flowing turbine. It can also hold 15 psi to redline.

when switching from turbo 1 to turbo 2 you gained 20hp(or tq wichever you like to use, but in the end it made 20 more) What was the reason?

I would think the inreased power came from the higher flowing turbine of turbo 2. Turbo 2 did make more power but it sufferes from a slower spool rate than turbo 1.

Why was turbo 1 making less power, due to the extra backpressure it had between the head and turbo. So you had less VE.

What if you only re did the wastegate so the extra backpressure from turbo 1 could be released, would you gain the same 20 hp? Now you would have to consider that the excessive exhaust backpressure is gone and higher VE is achieved will the compressor side still be efficient to hold that 15 psi to redline. If it did still hold the 15 psi untill redline you would have a turbo that still spools the same, and would win the race.

If it did not hold the 15 psi then obviously work needs to be done on that side.

I know you said you want the 2 wheels to be as close to 1:1 as possible but why? Is it possible to use the idea i just wrote and lets say you needed twice the size compressor, but kept the small turbine with a larger better flowing wastegate that was up to the job. Would you have a killer turbo, or am I still missing something?

 

93turbo - I'm not sure I'm following your point. Reducing backpressure is always a good thing, period. So an external gate vented to atmosphere, or a divorced dp with the downtube vented to atmosphere are likely going to produce better results. A 3" exhaust with mandrel bends and minimal bends will produce better results than a 2.5". Etc, etc, etc. When it comes to wastegates, a clean and direct flow from the manifold to the WG and a straight flow out of the WG will also be best. As for sizing the WG, many people making upwards of 300WHP are using 38mm Tial, and some people use bigger.

Now, if you swap to a bigger turbo and got 20HP more than before, it could be for a number of reasons. First, where is the extra 20hp in the rev range? Did the extra power affect spool? If so, was that a tradeoff you are comfortable with? If so, great. If not then you may have to reconsider.

For the most part when I size a turbo I try calculate compressor air flow rates at the desired pressure for my specific application. Then I look at the turbines available for that given turbo. Some turbos don't have turbine options, but Garrett and others do provide various turbine options. From experience and feedback I can usually detemine the general size of turbine that will produce reasonable results on my specific setup, which I use as a starting point. For example, I know a .64 to about .69 AR on Garrett GT turbos is what works for me. Any bigger and lag will be greater than I like, any smaller and flow will be limited up top.

Now, as described in various threads the VE of the engine can power a certain size range of turbines, but that range is generally not going to cover the entire rev range from 2000 to 8000 RPM at maximum efficiency. The highest efficiency rate will likely only give you 4000rpm (at most) of good strong boost pressure. So when you size the turbo you need to determine where you want your power to develop (torque curve). If you size it smaller, you develop spool earlier, peak earlier and then you'll begin to fall of up top. If you size it too big you get lag and you may even run out of VE before the turbo reaches max flow capability. It all depends on what your driving preference is. This is why at least one turbo manufacturer I know of created the variable AR turbine housing, with the theory being that you can adjust the turbine flow to you specific application. SCC tried this on their MR2 project but with less than stellar results. Assuming you aren't going to invest in a VATN turbo, then you should look at the efficiency rating of the turbine and you should look at the pressure graph and try to find a turbine where the pressure ratio within 10-15% of the compressor ratio you intend to run.

As for turbine to compressor ratios, my research from reading various sources suggests that when the compressor is significantly larger than 1:1 to 1:1.25 you get to a point of diminishing returns and rather than making more power you actually impact VE and hurt spool. So as a general rule of thumb I try to avoid turbos that are outside that threshold.

 

The point that isn't being made is that 15 psi or any boost figure alone means nothing. A larger compressor moves a larger volume of air at a given pressure ratio than a smaller one. The downstream effect of moving more air through the engine is that the VE must increase. You theoretically could wastegate more air and keep the turbine size the same while still increasing VE, but keep in mind that any air increases into the engine will be seen downstream by the turbine. That is why you typically will see compressors and turbines increase in capacity at the same time, because one side affects the other.

 

This is why at least one turbo manufacturer I know of created the variable AR turbine housing, with the theory being that you can adjust the turbine flow to you specific application. SCC tried this on their MR2 project but with less than stellar results.

That was Aerodyne, correct?

The concept is very interesting, but I suppose the turbo, even with a variable A/R, might have been completely unsuited to the application. SCC, especially back when they were building the MR2, has made a lot of questionable engineering decisions.

 

What if you only re did the wastegate so the extra backpressure from turbo 1 could be released, would you gain the same 20 hp? Now you would have to consider that the excessive exhaust backpressure is gone and higher VE is achieved will the compressor side still be efficient to hold that 15 psi to redline. If it did still hold the 15 psi untill redline you would have a turbo that still spools the same, and would win the race.

Is it possible to use the idea i just wrote and lets say you needed twice the size compressor, but kept the small turbine with a larger better flowing wastegate that was up to the job. Would you have a killer turbo, or am I still missing something?

Thats a really good question. Im all about what I get in the last 3K RPMs, so I dont really care about spool or anything, but its an interesting idea to have the spool of a smaller turbo (large turbo with a low AR), but have the flow of a larger turbo due to less backpressure in the manifold (increased VE through the use of a larger wastegate. Although since I want an ATS TD06, a larger turbine wont cost me anything, but a larger wastegate isnt doable with half of the kit's parts.

 

The point that isn't being made is that 15 psi or any boost figure alone means nothing. A larger compressor moves a larger volume of air at a given pressure ratio than a smaller one. The downstream effect of moving more air through the engine is that the VE must increase.

How did you work that out? How can the turbo move more VOLUME when the volume of the system (ie pipes from turbo, through TB and manifold to the engine) are the same, and the capacity of the engine hasn't changed?

Read my post earlier. It's a misconception that bigger turbos flow more VOLUME of air, they flow more MASS of air (ie VOLUME x DENSITY = MASS). Increase the density and you get more mass flow through. To increase the density, the air has to be cooler.

 

How did you work that out? How can the turbo move more VOLUME when the volume of the system (ie pipes from turbo, through TB and manifold to the engine) are the same, and the capacity of the engine hasn't changed?

Read my post earlier. It's a misconception that bigger turbos flow more VOLUME of air, they flow more MASS of air (ie VOLUME x DENSITY = MASS). Increase the density and you get more mass flow through. To increase the density, the air has to be cooler.

Re-reading your post again, I can see where you're going, I just merely wanted to say that just changing the compressor on it's own does not increase VE - the compressor is merely capable of pumping a larger mass of air for a given pressure ratio.

 

I hope this helps a bit with visualising on how improving VE affects hp.
Since we know that by choosing a larger turbo we move the VE to the right.
So lets say it we have one turbo pushing 200 pounds of torque at 7k and another turbo pushing 200 pounds of torque at 7.5k. Using the hp equation we get.

Turbo 1 - [(200*7000)/5252]= 266.56 HP
Turbo 2 - [(200*7500)/5252]= 285.61 HP

Thats a difference of 19.04 HP just by shifting the torque graph 500 RPM to the right. I see that its sometimes clearer to just do the calculations.

Big compressors on small turbines just create backpressure and lag.

Hey sflmr2 i hear this a lot but i never understand or could i find a thread explaining why this is so. I do believe its true but i was never able to logically understand why. Maybe its something simple but i still would like to know.

 

Hey sflmr2 i hear this a lot but i never understand or could i find a thread explaining why this is so. I do believe its true but i was never able to logically understand why. Maybe its something simple but i still would like to know.

If you read this thread carefully, dude, you'll see that at least a couple of people have explained why this happens. It's a long read, though, but worth it.

 

It's a long read, though, but worth it.

Very true.

And on a side note. Ive learned so much more from this forum than SupraForums. Sure you learn more about different cars and such, but I would have never learned the importance of Torque and HP, why larger turbos make more power, how water injection works, what detonation is, and so many more things. Since I was focused on the MKIII Supra before the MR2, I spent alot of time in that forums... and all I learned was how to do everything as cheaply as possible lol. Used 550 injectors, a Lexus AFM, and an SAFC are what about half of the modified MKIIIs have lol. But anyways... I just felt like saying that heh, kindof a thanks and congrats to the MR2OC I guess hehe.

 

I hope this helps a bit with visualising on how improving VE affects hp.
Since we know that by choosing a larger turbo we move the VE to the right.
So lets say it we have one turbo pushing 200 pounds of torque at 7k and another turbo pushing 200 pounds of torque at 7.5k. Using the hp equation we get.

Turbo 1 - [(200*7000)/5252]= 266.56 HP
Turbo 2 - [(200*7500)/5252]= 285.61 HP

Thats a difference of 19.04 HP just by shifting the torque graph 500 RPM to the right. I see that its sometimes clearer to just do the calculations.

Hey sflmr2 i hear this a lot but i never understand or could i find a thread explaining why this is so. I do believe its true but i was never able to logically understand why. Maybe its something simple but i still would like to know.

Horsepower can be deceiving. Torque is really the key to making power. As a matter of fact there is a very good discussion on this subject in another thread...
Click Here

In the example you gave both turbos are making the same torque. Also, it is a flawed example because you're comparing one turbo at 7k to another at 7.5k. Suppose the 7k turbo drops to 190WTQ at 7500, then your WHP would be 271 (still lower, but higher than 266 - despite lower torque!). Conversely, if the 7.5k turbo is making 190WTQ at 7K then it would only be making 253WHP. So, which turbo makes more power?

Since we can't see the rest of the torque curve, it's impossible to know what the performance of these cars would be outside this very narrow range from 7000-7500. The HP number doesn't really tell us anything meaningful. For example, one could assume that the car that makes 200WTQ at 7000 rpm is accelerating faster from 3000-7000 rpm. Then from 7000 to 7500 the other turbo goes faster. Given a choice, which would you choose, the one that goes faster between 3000-7000, or the one that goes faster between 7000-7500? I know what my choice would be!

Selecting a turbo based solely on a peak horsepower number is impractical and will likely be a big disappointment.

Through research, test drives (I recently wrote a review of Bruce's GT28RS in this forum), and discussions with several other MR2 owners I've concluded there are no perfect 2000-8000 turbos out there. I've come to believe that a turbo's power producing range is limited in practical terms to about 3000rpm. Once you realize that, then it is a matter of selecting which portion of the rev range you would like the most torque. For me and my driving style I prefer low to mid range. For someone who drags their car they may prefer mid to upper. It's a tradeoff either way.

 

when switching from turbo 1 to turbo 2 you gained 20hp(or tq wichever you like to use, but in the end it made 20 more) What was the reason?

Sorry sflmr2 i see the flaw in my examples and i should have quoted 93turbodeuce because i was trying to show how easy it was to have a big difference in hp while having 2 turbos capable of keeping the same boost through redline with the idea of one having a better VE in the higher powerband.

So a better example would be something like where Turbo S and Turbo B can hold 15 PSI to redline of 7.5K
Turbo S would make a lot less torque compare to Turbo B at redline because even though the compressor side of Turbo S is capable of flowing enough air for the engine to consume at 100% VE. Its because the turbine side of Turbo S would reduce the VE of the engine due to excessive backpressure. So lets say by redline Turbo S reduce the VE down to only 65% of VE. While the Turbo B turbine side is still flowing pretty well and is keeping the whole system VE at 90%.

So just putting random down some random numbers here
Turbo S - [(160*7500)/5252]= 228.48 HP
Turbo B - [(200*7500)/5252]= 285.61 HP
So lets say at 7.5 was Turbo B peak torque and Turbo S peak torque was something like 250 ft-lbs @ 5500
Peak TQ for Turbo S - [(250*5500)/5252]= 261.81 HP.

If im thinking straight here this is where most of the misconception happens when people just hear the peak hp number and not listen to where it occurs at and they automatically assume that this turbo is better.

I know this is just looking at the redline point and peak torque here and it doesnt serve much purpose for the real world application. But i just kinda want to see if my logics are making sense.

To dinomic - i tried rereading this whole thread but i still cant find out why would changing a compressor on lets say a CT26 would create more lag. I can see why it doesnt benefit much just by upgrading the CT26 compressor side but i dont see why it would hurt spool time... Maybe you can help me with a post number so i can read that with more attention.

 

To dinomic - i tried rereading this whole thread but i still cant find out why would changing a compressor on lets say a CT26 would create more lag. I can see why it doesnt benefit much just by upgrading the CT26 compressor side but i dont see why it would hurt spool time... Maybe you can help me with a post number so i can read that with more attention.

I'm not dinomic, but I'd like to take a swing at this. It makes sense that whenever the compressor wheel is increased in size, and the turbine stays the same, that it will take a little longer to get that larger compressor wheel spinning. That larger wheel is grabbing and pushing more air on each revolution and that takes more power in the shaft to accomplish. However, since the exhaust energy turning the turbine wheel hasn't yet increased to keep up with the extra demand, the turbine faulters and struggles until the exhaust energy slowly increases, eventually providing the energy to allow the turbine to spin the compressor at the required speed to provide the desired boost pressure. The turbines struggle to catch up causes the lag.

Add to that the increased weight of the larger wheel that the turbine is trying to spin, and you can see why both of these things affects the turbine's ability to do work in a timely manner (turbine efficiency). These factors are why Garrett recommends a compressor to turbine wheel ratio range of 1.1:1 to 1.25:1 to achieve both good spool characteristics and power.

Bruce

 

** closed due to length

 

Continued here:
http://www.mr2oc.com/showthread.php?t=124499

<-- thanks Scott -->