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    VTG - theory vs reality

    In my experience with the stock TT, the VTG never delivered as promised. The boost never registered on the gauge or on my backside until 3k rpms. It did not rapidly peak at 1950 rpms as I expected from the Porsche torque plots, and never even reached the levels of boost advertised either. I never saw over 1 bar boost even in Sport mode. At one point, I theorized that Porsche simply programmed in a boost limiter until break-in was achieved on the odometer.

    I have a theory as to why VTG did not deliver, and if my theory is correct, it explains why Porsche's claims are not observed in real world driving.

    1)In the new VTG turbos, there is no more wastegate - no internal nor external - because boost levels are controlled solely by the VTG vanes. The vanes vary the effective size of the turbine in order to control boost. My theory is that the VTG vanes cannot react quickly enough to engine load at low rpms the way a mechanical sprung wastegate can. Using an analogy, I would compare the VTG control of boost pressure to controlling flow of water in the garden hose with the valve on the wall. I would compare the wastegate control of boost to controlling the flow in the hose by putting your thumb over the end of the hose. Adjusting the water valve control will produce less rapid changes than the instantaneous effect of moving your thumb across the end of the hose.

    2)Some dynamometers can hold an engine at any constant rpm and measure its power output vs loading. If Porsche engineers put a maximum load on the TT engine and then slowly let the rpms increase from off idle, the VTG delay would be invisible, and we might see full boost and max torque at 1950 rpm. This would explain why Porsche's testing show the engine producing max torque at such a low speed.

    3)The problem is that in the real world, the only way to duplicate that would be to test the TT in 5th or 6th gear and make it accerate from a near crawl either up a steep hill or towing a boat - the load on the unladen car on a flat surface is not enough to hold back the car so that it blows past 1950 rpms before the VTG can react. Recall that VTG is a technology borrowed from diesel trucks. Those engines do not rev rapidly, and are subject to massive loads. The diesel VTG vanes don't need to be quick at all. This would explain why the owners don't aren't able to reproduce the torque punch at such low rpms.

    4)Since adding an aftermarket exhaust to my car, the torque and boost characteristics of the engine have become much closer to the Porsche charts. Boost onset is much faster, and is observable from as low as 2k rpms. The full boost will arrive as low as 2300-2400 rpms. I believe that by removing a substantial amount of backpressure, the VTG vanes can actuate more rapidly.

    Re: VTG - theory vs reality

    Which exhaust did you add - cats and headers too? Any CEL issues with just the exhaust mods? Thanks.

    Re: VTG - theory vs reality

    I used the AWE unit - 200 cell cats. No headers - I don't think they are worth it. No CEL issues whatsoever, even after 10 track days.

    Re: VTG - theory vs reality

    very comprehensive analysis of the obvious problem that is encountered by those that are actually using their tt as intended ..thx

    Re: VTG - theory vs reality

    Feel the same with my Fabspeed exhaust & Evo headers. Reduced backpressure seems to improve response with less of a delay/punch type effect, but more a linear/push response.

    Mike

    Re: VTG - theory vs reality

    Quote:
    eclou said:
    I used the AWE unit - 200 cell cats. No headers - I don't think they are worth it. No CEL issues whatsoever, even after 10 track days.



    I made the same experience using the Cargraphic exhaust, the throttle response is slightly better because apparently the chargers build up boost much faster from lower rpm figures. You're right, headers are a waste of money for the Turbo, they actually add only a bit of "sound".

    Btw: I've been told by somebody close to Porsche that the VTG chargers aren't actually the problem but the throttle setup. Porsche could easily offer a more aggressive throttle setup but apparently they're afraid that customers could run into serious issues.

    I know a tuner who actually changes the throttle setup and I was able to testdrive a car (Cayenne Turbo, not 997 Turbo) and I was very impressed. BUT: this setup comes at a price in the Cayenne Turbo, you need to be very careful because if 2.5 tons are sponteanously accelerated, you need to be very fast with the brake.

    Re: VTG - theory vs reality

    Quote:
    RC said:
    Quote:
    eclou said:
    I used the AWE unit - 200 cell cats. No headers - I don't think they are worth it. No CEL issues whatsoever, even after 10 track days.



    I made the same experience using the Cargraphic exhaust, the throttle response is slightly better because apparently the chargers build up boost much faster from lower rpm figures. You're right, headers are a waste of money for the Turbo, they actually add only a bit of "sound".

    Btw: I've been told by somebody close to Porsche that the VTG chargers aren't actually the problem but the throttle setup. Porsche could easily offer a more aggressive throttle setup but apparently they're afraid that customers could run into serious issues.

    I know a tuner who actually changes the throttle setup and I was able to testdrive a car (Cayenne Turbo, not 997 Turbo) and I was very impressed. BUT: this setup comes at a price in the Cayenne Turbo, you need to be very careful because if 2.5 tons are sponteanously accelerated, you need to be very fast with the brake.


    RC, this may sound like a novice question and I suppose it is given my limited mechanical understanding... but nonetheless here goes. Is a change in the throttle set up necessarily a ecu remapping or can it be accomplished another way, without potentially voiding the warranty? I intend to eventually tune my car and probably will go with Ruf. I'm just unsure of what the tuning will look like...so forgive me if the answer to the question is obvious. Just trying to learn more.

    Re: VTG - theory vs reality

    I made alot of calculations of the efficiency of the TT with stock exhaust vs aftermarket, and at normal boost vs 1.2bar. It was very clear that major improvements in efficiency were made through the exhaust mods, but that the overboost pushes the turbo into a less efficient state.

    The weakness in my opinion regarding the throttle body/intake system is that it is composed of 2 intake tracts (MAF, compressors, intercoolers) that siamese at the throttle body and then divert again at the intake runners. It invariably introduces significant amount of turbulence and heat that might be avoided by using 2 throttle bodies . Unfortunately, packaging that might be impossible.

    Re: VTG - theory vs reality

    Thanks Eugene. If it wasn't practical to have two throttle bodies, would simply increasing the size of the single throttle body improve the situation or add to the turbulence?

    Re: VTG - theory vs reality

    The problem isn't just the throttle body, because behind it the air will flow smash into a "T" intersection where air will then flow right or left into each intake manifold.

    Re: VTG - theory vs reality

    If thats the case is it logical to conclude that minimum benefit accrues from putting in a replacement intake system like twin cone filters?

    Re: VTG - theory vs reality

    any intake system that might be able to improve power would definitely have to be shielded from the heat of the engine bay. Unshrouded filters might actually lose power.

    The mods for the TT should be first focused on reduction of backpressure and exhaust gas temperatures. That should be the foundation established prior to any other mods.

    Re: VTG - theory vs reality

    In terms of the suspension, did you make any changes to your car in this area?

    Re: VTG - theory vs reality

    I just bought the Techart lowering springs. I plan on doing the install this weekend if I can sneak into my garage for a few hours. Lowering the center of gravity of this car should help handling at high demand situations. The only concern I have is whether the understeer/oversteer characteristics will be improved or worsened. Since the car has non-adjustable sways, I have considered using an adjustable rear unit from the GT3.

    Re: VTG - theory vs reality

    Thanks again. After you have some experience with the change in suspension, I hope you will do a review.

    Re: VTG - theory vs reality

    Quote:
    eclou said:
    I made alot of calculations of the efficiency of the TT with stock exhaust vs aftermarket, and at normal boost vs 1.2bar. It was very clear that major improvements in efficiency were made through the exhaust mods, but that the overboost pushes the turbo into a less efficient state.
    ....


    Pardon my ignorance, but how did you calculate the different efficiencies between the different setups? Also can you please post the findings?

    Re: VTG - theory vs reality

    There is a quick method of calculating relative efficiency of a motor through its Brake Mean Effective Pressure (BMEP). Formula is (max engine torque x 150.8)/ C.I.D. For a turbo car, the BMEP needs to be adjusted (divided) by the boost in absolute bar. The BMEP can be compared between vehicles to determine relative efficiency or within the same car to determine efficiency gained through various modifications. Here is an excerpt from another board which I posted

    Quote:
    I went back to calculate from my own cars to compare stock vs modified exhaust. The figures are from factory specs, inertia dyno testing, and AWE's published data

    A)E36M3 US spec (stock exhaust) tq 225, CID 192, press 1, BMEP 177
    B)E36M3 (2.5" exhaust no cats) tq 232, CID 192, press 1, BMEP 182 (+2.8%)
    C)E36M3 (cams, intake, headers) tq 246, CID 192, press 1, BMEP 193 (+9%)

    D)944 turbo (stock exhaust) tq 243, CID 151, press 1.75, BMEP 138
    E)944t (3" exhaust no cats) tq 325, CID 151, press 2.08, BMEP 156 (+13%)
    F)944t (K27/6, ported head) tq 374, CID 151, press 2.2, BMEP 170 (+23%)

    G)997TT (stock, normal) tq 457, CID 219, press 1.8, BMEP 175
    H)997TT (stock, sport) tq 505, CID 219, press 2.2, BMEP 158 (-9.7%)
    I)997TT (stg I, normal) tq 487, CID 219, press 1.8, BMEP 186 (+6.2%)
    J)997TT (stg I, sport) tq 535, CID 219, press 2.2, BMEP 167 (-4.5%
    K)997TT (stg II, normal) tq 522, CID 219, press 1.8, BMEP 199 (+12.5%)
    L)997TT (stg II, sport) tq 583, CID 219, press 2.2, BMEP 182 (+4%)

    Looking at this data, you can see the naturally aspirated M3's efficiency did improve from removal of exhaust restrictions. Comparing results (A/B vs D/E) the 944 turbo though saw almost a 4x greater benefit in efficiency vs the M3 when compared with no internal engine modifications. Even when the M3 is tested with internal valvetrain mods and exhaust, the % efficiency improvement is still less than the benefit to the turbocharged engine without valvetrain mods (C vs E). Add the valvetrain mods to the 944t and then the gap grows even bigger (C vs F). Removing backpressure has compounding benefits for turbocharged cars since the VE of the engine, the VE of the turbo, and the VE of the engine+turbo are all improved.


    This data is would suggest that the stock turbo (K26/6) in the 944 turbo is still well matched to the engine at boost levels up to 2.2bar. The 997TT's VTG is becoming noticeably less efficient when pushed from 1.8 to 2.2 bar, but it is still able to generate impressive numbers.



    Re: VTG - theory vs reality

    Eclou

    Lovely!

    What were the BMEP results of the tests you made on the TT (they were not shown above I think? ), stock vs. aftermarket exhausts, that made you determine that the reduction of backpressure is the most important thing to do on our cars to gain efficiency?

    Re: VTG - theory vs reality

    The figures for G&H are stock, I&J for exhaust mods only, and K&L for exhaust and ecu mods on the TT. Backpressure reduction yielded a 6% improvement in BMEP at regular boost levels

    Re: VTG - theory vs reality

    I do see it clearly now!

    Certainly it is honest truth to say that turbocharged engines benefit from less backpressure, the limit being our dear noise and emissions regulations.

    So the real comparison is between G&H and I&J above since K&L is irrelevant (modified chip). Am I correct in understanding that you are comparing Weissach numbers, which are likely understated as we know (G&H), against a tuner's (I assume "AWE" is a tuner?) roller dynamometer (I&J)and the difference is only 6% which could clearly be equipment margin of error or methodology. Are these exhausts sold by the same tuner who is posting the comparative data? With all due respect to the gents there.

    What I am trying to say is that we might not want to take the results out of proportion and undermine the effectiveness of these VTGs that are delivering afterall very strong acceleration results.

    Re: VTG - theory vs reality

    Yes,

    the results are only as good as the data, and as you point out can be subject to inaccuracies because of differences in dynos and methods of testing. The G&H and I&J do account for data taken from 1 dyno measuring stock vs exhaust on the same car - same car, same dyno, so the results should have some more validity.

    Re: VTG - theory vs reality

    Quote:
    eclou said:
    Yes,

    the results are only as good as the data, and as you point out can be subject to inaccuracies because of differences in dynos and methods of testing. The G&H and I&J do account for data taken from 1 dyno measuring stock vs exhaust on the same car - same car, same dyno, so the results should have some more validity.



    A question Gene - don't think I've seen a definitive answer anywhere - does the Sport Chrono Turbo option provide the same or similar re-mapped throttle response as the Sport Chrono Plus on the non TT 997's? Thanks.

    Re: VTG - theory vs reality

    Quote:
    jeffbco said:
    Quote:
    eclou said:
    Yes,

    the results are only as good as the data, and as you point out can be subject to inaccuracies because of differences in dynos and methods of testing. The G&H and I&J do account for data taken from 1 dyno measuring stock vs exhaust on the same car - same car, same dyno, so the results should have some more validity.



    A question Gene - don't think I've seen a definitive answer anywhere - does the Sport Chrono Turbo option provide the same or similar re-mapped throttle response as the Sport Chrono Plus on the non TT 997's? Thanks.



    Yes.

     
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