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Custom tuning w/ By Design - lots of power - need new clutch

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997 Turbo / GT2 2006Ė2012 Turbo discussion on the 997 model Porsche 911 Twin Turbo.

Custom tuning w/ By Design - lots of power - need new clutch

 
  #31  
Old 06-27-2018, 07:12 PM
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Originally Posted by A418t81 View Post



My car was dyno tuned on TopSpeedís dyno. I know precisely how much it makes, haha. Thereís a thread here with all the dyno charts.

https://www.6speedonline.com/forums/...o-warning.html

Boost levels didnt change post intercooler install. I actually had the load targets/boost lowered in my maps once I got the big intercoolers because the car was making enough torque to slip my upgraded clutch slightly on my hottest E85 map. Thatís too much for my liking on the factory motor. My solution was to lower boost down until airflow values were similar to my pre-install logs which resulted in 2-3 psi less across the board depending on RPM.

E85 explains a lot. But I wanted to say a few things about the dyno jet dyno after years of experience working on them - STD correction means STD as in sexually transmitted disease - depending on air temps and altitude the power is raised - the best correction is none and smoothing 5 is another typical dyno jet method to make a dyno sheet look smoother

Main thing is that you are happy as no one drives a dyno sheet but I think too many people are fixated on dyno sheets in the performance world. No one drives a dyno so that is why I prefer to log and drive my car on the street - have not had my 997 on the dyno yet
 
  #32  
Old 06-27-2018, 07:15 PM
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LOL I love it!
No correction to be clear and a nice thing is the comparison with all data in as a constant and set up for the current conditions. Most are in line with each other and make sense. E85 is awesome on these cars 089!
 
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  #33  
Old 06-27-2018, 07:34 PM
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Originally Posted by 08957 View Post
E85 explains a lot. But I wanted to say a few things about the dyno jet dyno after years of experience working on them - STD correction means STD as in sexually transmitted disease - depending on air temps and altitude the power is raised - the best correction is none and smoothing 5 is another typical dyno jet method to make a dyno sheet look smoother

Main thing is that you are happy as no one drives a dyno sheet but I think too many people are fixated on dyno sheets in the performance world. No one drives a dyno so that is why I prefer to log and drive my car on the street - have not had my 997 on the dyno yet

I agree on the CF. I actually own my own dyncom 1800x with eddy current brake and have it just for fun. The point of dyno was two-fold. One the car was exclusively road tuned and this was back when I was my own stock turbo E85 guinea pig. Nobody really had any actual data on how much power the car was making. I had an idea and figured 610 whp.

Two: I was looking to push the tune further. Again, since this was the infancy of E85 tuning and I was doing this basically on my own, we didnít know where MBT was regarding timing on a hot E85 map. It is easy to advance past MBT on E85 without realizing it unless you have a dyno to verify power gains after additional adavance added.

Finally, this dyno has had 100s of GTRs on it in every sort of tune. It was an excellent way to evaluate the power potential of this platform again a known monster platform with bolt-ons. The 997.1 actually makes a bit more power with less mods than an equivalent GTR on E85 and factory turbos.
 
  #34  
Old 06-27-2018, 08:06 PM
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Originally Posted by A418t81 View Post



I agree on the CF. I actually own my own dyncom 1800x with eddy current brake and have it just for fun. The point of dyno was two-fold. One the car was exclusively road tuned and this was back when I was my own stock turbo E85 guinea pig. Nobody really had any actual data on how much power the car was making. I had an idea and figured 610 whp.

Two: I was looking to push the tune further. Again, since this was the infancy of E85 tuning and I was doing this basically on my own, we didnít know where MBT was regarding timing on a hot E85 map. It is easy to advance past MBT on E85 without realizing it unless you have a dyno to verify power gains after additional adavance added.

Finally, this dyno has had 100s of GTRs on it in every sort of tune. It was an excellent way to evaluate the power potential of this platform again a known monster platform with bolt-ons. The 997.1 actually makes a bit more power with less mods than an equivalent GTR on E85 and factory turbos.
How is your driveability / cold start with the E85 over time ?

Any disadvantages to using E85 on the 997?

Have you ran into any problems?

I note by the way the reason why I know about the STD trick is because all my dyno "records" where always made on STD mode with a dynojet. Just the same way there is a lot of manipulation on Vbox numbers (people taking out seats - spraying intercoolers with NOS before runs etc) there is a lot of manipulation in the tuning world on dyno sheets

I am glad that you tuned your car mostly on the road - I have little use for dynos other than comparing parts and setting up timing curves for MBT
 
  #35  
Old 06-27-2018, 08:27 PM
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No cold start issues. No real drivability issues. Occasionally there is a lean backfire when it is warming up, but every big injector 997 with the stock ECU Iíve been in eventually does this even on pump gas.

I change the oil every 3k or 1 year since E85 acidifies the oil.

This motor and turbo combo on E85 wants to make A LOT of torque. We actually pulled 4 degrees of timing from the midrange and did our best to keep boost down in this area as well and it was still making 700 wtq. Frankly, the torque comes in somewhat explosively. The car is somewhat violent on the hottest E85 mapping. I have an E30 map thatís a lot more enjoyable to actually drive the car around with and it is in the 600-610 whp range. Torque is still really impressive, but not enough to upset the car and cause traction issues in second gear.

I donít care about ďdyno records.Ē I care about results. This car ran a 5.9 60-130 on the 625 whp calibration with no BS. No weight reduction, no funny business. I had my groceries in the car actually lol. It was 30 deg outside, which helped. This still appears to be the 997.1 6MT record best I can tell.
 
  #36  
Old 06-27-2018, 08:39 PM
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Originally Posted by A418t81 View Post
No cold start issues. No real drivability issues. Occasionally there is a lean backfire when it is warming up, but every big injector 997 with the stock ECU Iíve been in eventually does this even on pump gas.

I change the oil every 3k or 1 year since E85 acidifies the oil.

This motor and turbo combo on E85 wants to make A LOT of torque. We actually pulled 4 degrees of timing from the midrange and did our best to keep boost down in this area as well and it was still making 700 wtq. Frankly, the torque comes in somewhat explosively. The car is somewhat violent on the hottest E85 mapping. I have an E30 map thatís a lot more enjoyable to actually drive the car around with and it is in the 600-610 whp range. Torque is still really impressive, but not enough to upset the car and cause traction issues in second gear.

I donít care about ďdyno records.Ē I care about results. This car ran a 5.9 60-130 on the 625 whp calibration with no BS. No weight reduction, no funny business. I had my groceries in the car actually lol. It was 30 deg outside, which helped. This still appears to be the 997.1 6MT record best I can tell.
Thanks for all the great info and for being a pioneer in running the E85 on these platforms

Certainly a great 60-130 time

Were you able to get the injectors in without dropping the motor out? I am looking and it seems very tight for my fat hands - the AMS intercoolers were hard enough for me

I would like to run the 60 - 130 mph tests here but they will result in jail time in this area and the need for multiple Court appearances - anything over 100 mph in this area and you have serious problems

My best bet is to go to the track and try to run the testing there - sadly without a cage they will give me only one pass before they throw me out


Living near NYC is not the best place for enjoying high speeds
 
  #37  
Old 06-27-2018, 10:48 PM
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Originally Posted by A418t81 View Post
No cold start issues. No real drivability issues. Occasionally there is a lean backfire when it is warming up, but every big injector 997 with the stock ECU Iíve been in eventually does this even on pump gas.

I change the oil every 3k or 1 year since E85 acidifies the oil.

This motor and turbo combo on E85 wants to make A LOT of torque. We actually pulled 4 degrees of timing from the midrange and did our best to keep boost down in this area as well and it was still making 700 wtq. Frankly, the torque comes in somewhat explosively. The car is somewhat violent on the hottest E85 mapping. I have an E30 map thatís a lot more enjoyable to actually drive the car around with and it is in the 600-610 whp range. Torque is still really impressive, but not enough to upset the car and cause traction issues in second gear.

I donít care about ďdyno records.Ē I care about results. This car ran a 5.9 60-130 on the 625 whp calibration with no BS. No weight reduction, no funny business. I had my groceries in the car actually lol. It was 30 deg outside, which helped. This still appears to be the 997.1 6MT record best I can tell.

BTW - what exhaust do you have on the car and do you run any cats? Thanks
 
  #38  
Old 06-28-2018, 10:31 AM
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I have an AWE 3Ē catted exhaust on my car. It works very well, but I wouldnít say itís the best sounding exhaust Iíve heard on these cars.

The injectors were put in the same time the motor was dropped to do the clutch, and a ton of preventative maintenance / pinning the coolant pipes, etc. If you already have soft aftermarket silicone turbo inlet pipes the injector job isnít as terrible, but with the factory hard inlet pipes it is downright torture. I honestly have zero idea how anybody could do it period with the factory inlets in the car. There is zero room.

My advice is to wait until you do the clutch and add the injector and inlet pipes then. The inlets make a significant difference in flow particularly up high in the rpm range and at higher boost pressures.
 
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Old 06-28-2018, 11:52 AM
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Originally Posted by A418t81 View Post
I have an AWE 3Ē catted exhaust on my car. It works very well, but I wouldnít say itís the best sounding exhaust Iíve heard on these cars.

The injectors were put in the same time the motor was dropped to do the clutch, and a ton of preventative maintenance / pinning the coolant pipes, etc. If you already have soft aftermarket silicone turbo inlet pipes the injector job isnít as terrible, but with the factory hard inlet pipes it is downright torture. I honestly have zero idea how anybody could do it period with the factory inlets in the car. There is zero room.

My advice is to wait until you do the clutch and add the injector and inlet pipes then. The inlets make a significant difference in flow particularly up high in the rpm range and at higher boost pressures.

I have a cat myself - I went with the Europipe as they seemed to have about the largest cats

I am curious if you had any thoughts about what effects the high flow cats had upon your power output at these levels?

I am not that familiar with this platform but it would seem that the turbine housing looks like the biggest restriction on the exhaust side

After having a few performance cars without cats I don't want to deal with the smell and the fumes anymore. The 100 sport cats in the Europipe delete all the smell, fumes and noxious emissions. I feel like this is a luxury GT and should not have an exhaust like a Honda - while I do want to make it very fast
 
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Old 06-28-2018, 12:14 PM
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08957 yes there is same power there however its a function of airflow and the resultant delta pressure you get as you know. I made estimates for another member here using Europipe with upgraded VTGs and I think I was spot on based on the difference in performance he sees on the track vs. a similar car but with catless Europipe.

Assuming the same geometry, we can model it fairly easy. Its harder to model lets say your Europipe vs. a Kline catless. Too much geometry change for 0D.


With the VTG we would see the same behavior A418t81 saw swapping his intercoolers. If you target the same power it would be at a lower pressure or more open vane position. This will drop the pumping work gaining you BSFC back. Or you can use all that fuel again and make more power lol.

I should probably add, if the catalytic is large enough which is what Europipe is after, they should support higher flow. Ultimately you will find some limit. There is a member here with a 996 turbo (Powerhound), nice 3" exhaust with large cats however I don't recall the brand of them which did a back to back showing just run to run variability (no cal changes ). During that test he was in the 600whp range I think. Obviously a 1khp car would show gains taking the cats off etc etc.
 

Last edited by M3 Maestro; 06-28-2018 at 12:30 PM.
  #41  
Old 06-28-2018, 05:03 PM
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Originally Posted by M3 Maestro View Post
08957 yes there is same power there however its a function of airflow and the resultant delta pressure you get as you know. I made estimates for another member here using Europipe with upgraded VTGs and I think I was spot on based on the difference in performance he sees on the track vs. a similar car but with catless Europipe.

Assuming the same geometry, we can model it fairly easy. Its harder to model lets say your Europipe vs. a Kline catless. Too much geometry change for 0D.


With the VTG we would see the same behavior A418t81 saw swapping his intercoolers. If you target the same power it would be at a lower pressure or more open vane position. This will drop the pumping work gaining you BSFC back. Or you can use all that fuel again and make more power lol.

I should probably add, if the catalytic is large enough which is what Europipe is after, they should support higher flow. Ultimately you will find some limit. There is a member here with a 996 turbo (Powerhound), nice 3" exhaust with large cats however I don't recall the brand of them which did a back to back showing just run to run variability (no cal changes ). During that test he was in the 600whp range I think. Obviously a 1khp car would show gains taking the cats off etc etc.
Looking back to 2015 Mitch from Cobb did the best method of testing exhaust / turbo efficiency - by installing a boost gauge on a copper coil - here is the thread

https://www.6speedonline.com/forums/...eets-more.html

In that thread he was developing the Cobb E85 Maps and made 601 whp 636 TQ on stock turbos on E85 with a GMC exhaust with cats

The back pressure measured with just 20 psi of intake manifold boost was nearly 50 psi of exhaust manifold boost pressure

Needless to say with an excess of 30 psi of pressure at the exhaust side of the turbine the evacuation of the combustion is not very efficient

Of course I think the test in that case was missing a comparison of pre and post turbo pressure drop which would be more of an indication of the efficiency of the exhaust relative to the turbo itself. The differential between the two pressures - pre and post turbo is what gets your turbo spooling.

According to Europipe the maximum backpressure on a stock 997.1 exhaust is .44 bar or about 6.4 psi

The Europipe Stage 1 system according to Europipe cuts that down to .18 bar or about 2.6 psi

I would assume that these figures from Europipe were achieved on a totally stock car making about 480 crank hp - I will check with Stef on how these figures were obtained

In any event going to a straight pipe out the back from a Europipe Stage 1 would gain you about 2.6 psi off the 50 psi measured at the turbo which is not that significant which is why we notice that just adding a complete exhaust system over a stock one does not yield that much of a power improvement - Europipe claims 18 whp gain for its exhaust which goes from 6.4 psi to 2.6 psi - so I would theorize that taking off the exhaust completely on a stock car would be good for about another 10 whp or so vs. a good cat back system

It seems to me that the most significant restriction on our cars is the turbine wheel and the turbine housing which if you look at the chart provided my Mitch of Cobb shows what we feel when driving the car that the stock turbos begin to fall of dramatically around 6300 rpm as the back pressure in the exhaust manifold begins to rise

This being the case, raising the boost would only increase the back pressure in the manifold which is approaching very high levels and creating heat and ineffective discharge of spent combustion gasses between ignition events

What remains to be seen is what effect high flow 100 cell cats have upon the back pressure

The fact remains that the Porsche has the exhaust set up in a poorly configured location due to the engine location and has a lot of additional sharp bends that would otherwise be avoided in a conventional set up. The cats no doubt are very close to the turbos and a great source of heat

As I have said, I have previously owned many very fast street cars with no cats and open dump tubes and there is certainly a smell that becomes associated with having that kind of catless system

Thus car on my mod path I have liked the fact that the Europipe has not subtracted in anyway from the Porsche experience and only enhanced the car. The materials and construction being superior to the OEM unit and the cat working just as well as a stock one with no odor or smell what so ever even on MS109 fuel

What I am not sure about is whether or not I can achieve my desired power objectives while maintaining the cats and to what extent the back pressure and power loss will increase as I am increasing my power production with cats

At some point I may have to get rid of the cats totally

I guess the ideal solution would be to run a bypass valve system of some kind but most of the ones I have seen are bypassing the mufflers and not the cats in a Porsche there is not a lot of room between the turbo discharge and the cats

Back in the day many turbo import street racers would run a totally stock exhaust with a by pass valve to throw off the police and the competition - not to mention I would not be surprised if this kind of sound so close to the engine would not set off false knock on a 997
 
  #42  
Old 06-29-2018, 07:26 AM
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08957 I know the plot you are referring to, I even saved it since it was such good information (thanks Mitch). I'll expand some more on this subject since its my favorite and what I do for a living. For the following "we" means us turbocharger development engineers in the turbo OE industry.

In reference to the turbine wheel or housing being too restrictive this is not the case on the VTG. We oversize turbine A/R on VTGs, its rare that the turbine housing on a VTG is the flow limiting part of the stage. If you find one, someone screwed up . The turbine wheel is actually a high flowing turbine wheel. If you look at the design, its a very tall blade, high trim, little wrap telling you its meant for flow and not extracting work. Perfect for gasoline like this application. As you start looking through more datalogs keep an eye on vane position (reported out as turbo duty cycle because its actuator position). You will not find a datalog with a car at full load running at full open vane. This is tell you the turbine stage has more flow left in it. I fully understand most peoples experience is with a wgt or fixed geometry and where the comments of the turbine being restrictive comes from. With VTG its a little more complex. We can go deeper here but I'll leave it here for now.

Now lets cover the turbine inlet pressure Mitch reported because here is where its gonna get interesting for gearheads like ourselves. The reason you are seeing turbine inlet pressure skyrocket is because of high vane closure. High vane closure = "I need more turbine power to hit the desired compressor operating point". What goes into turbine power? The ingredients are turbine inlet temperature, turbine efficiency, and the easiest ****, turbine expansion ratio. Driving the vanes closed you are achieving higher expansion ratios, plotted on a turbine map it would like like you are picking on the fly a smaller and smaller A/R (for illustration purposes) as you increase engine speed. So why are we consuming so much turbine power? The compressor operating point is in the dumps. We are running the stock compressor in choke where efficiency is tanking consuming the turbine power. Back in high school this took care of itself on my friends Eagle Talon I use to mess with . Yeah we would hit 22psi but by redline we were down to 15psi since its a fixed flow turbine and the compressor running in choke was dragging that turbine down. There was no VTG to add the required turbine power to keep it at 22 psi.

On upgraded VTGs we run at a much better vane position and our turbine inlet pressure will not be that bad. I included a plot of my car with that same E85 file on stock turbos vs. when I switch to the larger 63.5mm compressor. You can see the huge change in vane position. And exactly as you stated, since now I'm at a better engine delta pressure, I'm effectively increasing my VE by being able to evacuate the cylinder more efficiently.

In the end its not turbine flow that everyone is blaming...its really turbine efficiency. Because of the turbine efficiency of this particular stage, we can only run the vanes so far open and keep a matched point to the compressor power desired.

Now to address the exhaust restriction deal. The affect on turbine inlet pressure is higher than you estimating. The reason is expansion ratio. If you turbine is running at a 3.5 expansion ratio, you would be multiplying your exhaust restriction times the expansion ratio and that's how it would affect your turbine inlet pressure. That's why we see huge gains when someone running 6 psi of backpressure goes to a free flowing exhaust. Moving from 6psi backpressure to 2psi gains you a 14psi drop in turbine inlet pressure. Its a significant number.

I used Europipe's published numbers to model the exhaust restriction curves as a function of exhaust flow (horsepower). I don't want to post these cause I don't want to start a pissing match I can tell you my prediction is accurate. Again if we use the same exhaust geometry its an easy calc. I can tell you the exhaust restriction you are at today if I know your airflow numbers and AFR.
 
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Old 06-29-2018, 10:46 AM
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Originally Posted by M3 Maestro View Post
08957 I know the plot you are referring to, I even saved it since it was such good information (thanks Mitch). I'll expand some more on this subject since its my favorite and what I do for a living. For the following "we" means us turbocharger development engineers in the turbo OE industry.

In reference to the turbine wheel or housing being too restrictive this is not the case on the VTG. We oversize turbine A/R on VTGs, its rare that the turbine housing on a VTG is the flow limiting part of the stage. If you find one, someone screwed up . The turbine wheel is actually a high flowing turbine wheel. If you look at the design, its a very tall blade, high trim, little wrap telling you its meant for flow and not extracting work. Perfect for gasoline like this application. As you start looking through more datalogs keep an eye on vane position (reported out as turbo duty cycle because its actuator position). You will not find a datalog with a car at full load running at full open vane. This is tell you the turbine stage has more flow left in it. I fully understand most peoples experience is with a wgt or fixed geometry and where the comments of the turbine being restrictive comes from. With VTG its a little more complex. We can go deeper here but I'll leave it here for now.

Now lets cover the turbine inlet pressure Mitch reported because here is where its gonna get interesting for gearheads like ourselves. The reason you are seeing turbine inlet pressure skyrocket is because of high vane closure. High vane closure = "I need more turbine power to hit the desired compressor operating point". What goes into turbine power? The ingredients are turbine inlet temperature, turbine efficiency, and the easiest ****, turbine expansion ratio. Driving the vanes closed you are achieving higher expansion ratios, plotted on a turbine map it would like like you are picking on the fly a smaller and smaller A/R (for illustration purposes) as you increase engine speed. So why are we consuming so much turbine power? The compressor operating point is in the dumps. We are running the stock compressor in choke where efficiency is tanking consuming the turbine power. Back in high school this took care of itself on my friends Eagle Talon I use to mess with . Yeah we would hit 22psi but by redline we were down to 15psi since its a fixed flow turbine and the compressor running in choke was dragging that turbine down. There was no VTG to add the required turbine power to keep it at 22 psi.

On upgraded VTGs we run at a much better vane position and our turbine inlet pressure will not be that bad. I included a plot of my car with that same E85 file on stock turbos vs. when I switch to the larger 63.5mm compressor. You can see the huge change in vane position. And exactly as you stated, since now I'm at a better engine delta pressure, I'm effectively increasing my VE by being able to evacuate the cylinder more efficiently.

In the end its not turbine flow that everyone is blaming...its really turbine efficiency. Because of the turbine efficiency of this particular stage, we can only run the vanes so far open and keep a matched point to the compressor power desired.

Now to address the exhaust restriction deal. The affect on turbine inlet pressure is higher than you estimating. The reason is expansion ratio. If you turbine is running at a 3.5 expansion ratio, you would be multiplying your exhaust restriction times the expansion ratio and that's how it would affect your turbine inlet pressure. That's why we see huge gains when someone running 6 psi of backpressure goes to a free flowing exhaust. Moving from 6psi backpressure to 2psi gains you a 14psi drop in turbine inlet pressure. Its a significant number.

I used Europipe's published numbers to model the exhaust restriction curves as a function of exhaust flow (horsepower). I don't want to post these cause I don't want to start a pissing match I can tell you my prediction is accurate. Again if we use the same exhaust geometry its an easy calc. I can tell you the exhaust restriction you are at today if I know your airflow numbers and AFR.
very insightful observations.

what are your thoughts on the "clipping" of the turbine wheels that Tial Sport does on its VTG compressor upgrades? It would seem from what you are saying the compressor wheel is the limiting factor not the turbine

thanks
 
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Old 06-29-2018, 11:38 AM
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I'll be brutally honest even if may offend some of my colleagues lol.

Clipping in general when we do it in OE its NOT for flow. Its done to get a higher natural frequency out of the wheel. One of the many reasons for a speed limit on a turbo is the blade excitation. We can get into conditions where the excitation is so strong the deflection we see has strains high enough causing cracks and eventually a blade off event. Yes, the blade or section of the blade will come flying off. We can fairly easily predict when this will happen using some rules of thumb and a microphone to record the frequency of the blade when we "ring" it.

So what happens when you have to use what is already available and are not allowed time or funds to make a new wheel? You clip it! It will ring at a higher frequency ultimately letting you run a higher speed. So if you see wheels being clipped in the OE space...its for speed not flow (most of the time). This issue happens on both compressor and turbines...we can clip both compressor and turbines (typically higher clip angle).

The VTG again is a little more complex than a wgt or fixed geometry. On the wgt or fixed geometry the turbine tongue is what excites the wheel as the blades goes past it (pressure wake). In the VTG every vane is exciting the wheel. We have to do multiple things to make them survive in that environment. I'll stop here.

In the aftermarket you typically see clipping as a means to drop turbine inlet pressure because you gain flow (small gain, not life changing lol). You usually don't realize all the gain because you are paying an efficiency penalty at the same time. But now we go back to my earlier post...the housing and wheel usually is not the flow limiter on a vtg stage. Again I'll stop here.

OK so what do I think of clipping this specific stage. TiAL does it, my Protos are also clipped etc. I think its fine...aftermarket does it looking for the flow gain..ain't happening on this stage. However some of them may know, some may not, by clipping they bought speed margin allowing you to drive to higher pressures.

997 GT2 versions of this are clipped and have a bigger A/R etc etc...eh <shrugging shoulders>. The clipping is for the higher speeds allowing more boost and the A/R tweak is really just the resultant A/R from making a straighter turbine inlet (EVEN if the engineer who did was trying to get flow also, speed is main). The GT2 A/R is a small change. I know the maps are not public so I can not comment any further. For those of you who have GT2 turbine stages...yeah nice to have. For the cost($$$) I would spend my cash elsewhere is all I will say if you are looking.

There is a ton of detail on this subject and many of us have written papers on it. If any of you have access to SAE papers you can find plenty on this subject.
 

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Originally Posted by M3 Maestro View Post
I'll be brutally honest even if may offend some of my colleagues lol.

Clipping in general when we do it in OE its NOT for flow. Its done to get a higher natural frequency out of the wheel. One of the many reasons for a speed limit on a turbo is the blade excitation. We can get into conditions where the excitation is so strong the deflection we see has strains high enough causing cracks and eventually a blade off event. Yes, the blade or section of the blade will come flying off. We can fairly easily predict when this will happen using some rules of thumb and a microphone to record the frequency of the blade when we "ring" it.

So what happens when you have to use what is already available and are not allowed time or funds to make a new wheel? You clip it! It will ring at a higher frequency ultimately letting you run a higher speed. So if you see wheels being clipped in the OE space...its for speed not flow (most of the time). This issue happens on both compressor and turbines...we can clip both compressor and turbines (typically higher clip angle).

The VTG again is a little more complex than a wgt or fixed geometry. On the wgt or fixed geometry the turbine tongue is what excites the wheel as the blades goes past it (pressure wake). In the VTG every vane is exciting the wheel. We have to do multiple things to make them survive in that environment. I'll stop here.

In the aftermarket you typically see clipping as a means to drop turbine inlet pressure because you gain flow (small gain, not life changing lol). You usually don't realize all the gain because you are paying an efficiency penalty at the same time. But now we go back to my earlier post...the housing and wheel usually is not the flow limiter on a vtg stage. Again I'll stop here.

OK so what do I think of clipping this specific stage. TiAL does it, my Protos are also clipped etc. I think its fine...aftermarket does it looking for the flow gain..ain't happening on this stage. However some of them may know, some may not, by clipping they bought speed margin allowing you to drive to higher pressures.

997 GT2 versions of this are clipped and have a bigger A/R etc etc...eh <shrugging shoulders>. The clipping is for the higher speeds allowing more boost and the A/R tweak is really just the resultant A/R from making a straighter turbine inlet (EVEN if the engineer who did was trying to get flow also, speed is main). The GT2 A/R is a small change. I know the maps are not public so I can not comment any further. For those of you who have GT2 turbine stages...yeah nice to have. For the cost($$$) I would spend my cash elsewhere is all I will say if you are looking.

There is a ton of detail on this subject and many of us have written papers on it. If any of you have access to SAE papers you can find plenty on this subject.
If your Proto turbos and the TiAL turbos are both clipped then it seems at least they are apparently reliable which is my main issue.

It seems that your understanding of turbos is very in depth and that is a great resource.

I only know what i do about turbos from reading Maximum Boost years ago and from experimenting with hundreds of different turbo combos over the years.

On this particular application my choices are limited by my objectives to maintain a good safety margin on the stock rods and keep is as stock like as possible.

I am following your set up to a large degree although I may have to rethink the exhuast now that larger turbos and E85 are in my plans
 

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