Eclou

Leave behind this personal BS, it is not your business whether I have a fast 993TT, I rent a 997TT or I am supposedly good driver. I did not use any of this to support any of my arguments, I don't need it.

You seem so focused in impressing your audience with internet cut and paste replies that you don't even bother reading the poster's comments, if you did you would not be answering so off topic or interpreting my statements so out of line consistently.

FYI, when a few guys and myself started the datalogging/performance benchmark and debate 2 years ago, most tuners were laughing about it. Now you are posting pictures to show me what they look like and what each one of them does and lecture me about them?

Stephen K. the offer still stands. I am talking about graphing a full dyno chart (no peak HP estimates like trap speed) based on GPS road acceleration, the REAL torque on the ground across the RPM range, I would share all the workings and formulas with you to be transparent and you could later compare to chassis dyno charts.

Rmrmd1956, no one will provide you with VTG maps, but they only flow reliably 550BHP

 

Find someone to loan me one and I will do it. I will also run what ever test we need to. What we get we will post good or bad.

 

Jean,

It is very simple. Your source is probably correct - that the VTG turbo is limited to 550hp, BUT only at the VE they modeled. Once the VE of the engine and the efficiency of the turbo was changed, that contention is no longer valid. The tuners achieved this by removing backpressure. Can you not understand and acknowledge the implications of this?

 

Stephen

You might be able to find one much easier than me. Just to be clear I am not challenging anyone here, I am talking about any stock 997TT AX22 runs in 4th gear or any other modded ones if you are interested in the data for experimental purposes.

Eclou,
I am glad we are back on a technical aspect, thanks. I agree with you re. the efficiency of the engine, a 997TT engine has an adjusted BMEP of 174 based on my calculations and data available from the factory. I am attaching a spreadsheet I posted sometime back as well calculating adjusted BMEP between Porsche engines. This shows how much more efficient is each engine vs another, at atmospheric pressure (no boost). (Sorry for some reason I cannot post the chart!)

The 997TT engine is about 5% more efficient than the 996TT engine (BMEP 167) with all the improvements that they did to them as far as mapping, intake, heads and exhaust are concerned, and 12% more than the decade old 993TT aircooled engine (BMEP 156).

I would not think that anyone can improve that efficiency by more than another 5%, if he spends a very substantial amount of money working on the internals, heads etc..

N/A engines are a totally different ballgame. By looking at this chart one understands why GT3 based TT engines are so powerful.... (Markski)

The typical exhaust, intake modifications will not yield more than 1-2% logically speaking. That is a 1-2% incremental input from the turbos as a result of improved BSAC or VE/BMEP.

 

Jean,

since I cannot see your charts, I am not sure how you are coming up with a BMEP of 174. I calculated the 997 GT3 to have a BMEP of 205 based on the Porsche TUV torque figures, and would be impressed if there is 17% difference in BMEP between the 2. At stock levels of boost, the effective BMEP of the TT motor equates to 342. As you have alluded, naturally aspirated power is somewhat linearly related to VE changes, whereas FI power output can see much more dramatic improvements.

Also, by BSAC are you referring to BSFC?

 

Eclou

I have loaded the chart differently. The turbo engines are adjusted for atmospheric pressure in order to compare apples to apples. This was done for comparison with 993TT engines, which I used as the base.

Yes the N/A engines are much more efficient than turbo engines because of different intakes, larger ports, twin plug, throttle body, exhaust differences etc.. My numbers match yours as you can see on the GT3RS.

BSAC, Brake Specific Air Consumption is the amount of air needed to produce one HP by the engine. BSFC is more commonly known, but is related to fuel consumption, the lower the number the better unlike BSAC.

Efficiency improvements can be very limited unlike what the majority thinks..

 

Eclou & Jean,

Wonder why I have this feeling you 2 know a thing or two about turbo engines? (Stephen too but of course that's expected.)

I am a "mere mortal" not blessed w/ such extensive knowledge. Just reading this thread even though it's 'above my head," partly because I need to decide between a 997tt or 997 GT3, partly because I find it interesting. I do have a couple questions that I vaguely think is related to your discussion.

From your experience, is it true VTG has failed to eliminate, or even reduce, turbo lag? Does VTG generate more heat than the traditional turbo? If so, has VTG created a potential problem without providing any benefit?

My friend has the new BMW 335 turbo, and his butt dyno shows nearly no turbo lag. If true, why can't Porsche do what BMW does?

 

A translation of this thread would be helpful to us idiots. something like VTG for Dummies.

 

Chris,

LOL. I don't think these guys can help confusing "us"; they simply know too much.

Anyway, hope the following explanation helps. The reason I asked is in that paragraph, Porsche stated heat IS a problem with the VTG technology. If it doesn't reduce lag, then I am just wondering what it is good for. http://forums.beyond.ca/showthread/t-109329.html


Introducing the new model, Porsche will be presenting the world's first turbocharged gasoline engine with variable turbine geometry (VTG). This technology featured in turbocharged diesels since the '90s ensures a significant improvement of engine flexibility and acceleration particularly at low engine speeds. So far, however, much higher exhaust gas temperature of up to 1000° C in comparison with the turbocharged diesel has always proved to be an insurmountable barrier. But now Porsche has solved this problem in close cooperation with Borg Warner Turbo Systems by using high temperature-resistant materials from space technology wherever required.



The core features of the VTG system are the variable turbine blades guiding the flow of exhaust gas from the engine in exactly the direction required on to the turbine wheel of the exhaust gas turbocharger. The principle of variable turbine geometry thus combines the benefits of a small and large exhaust gas turbocharger all in one – a combination ensuring both very good response and high torque at low engine speeds as well as superior output and high performance at high speeds. And the supreme level of torque is now maintained consistently throughout a much wider speed range.

 

For a VERY simplified explanation of VTG, I would use an analogy compare a small turbo vs a big turbo. Let's just forget about turbines and compressors altogether. The small turbo would be like a paper windmill, the big turbo would be like an electric windmill. The paper windmill will spin/spool up very quickly with just a light blow of air across it



The electric windmill will take alot of air blowing across it before it starts to turn, thus the spin/spool time is much slower

The paper windmill obviously can only generate a fraction of the power of the electric one, but it can do so with very light breezes. The same occurs with turbos, the smaller turbo can generate less power but can do it at lower exhaust flow (rpms). The VTG theory is to create a big turbo that can behave like a small turbo at lower rpms and vice versa. The vanes change orientation so that the "windmill" is transformed in size as the force of the wind increases.

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 new theory as to why VTG did not deliver, and if my theory is correct, it explains how both sides (Porsche and Jean vs tuners and me) are correct:

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 another analogy, I would compare the VTG control of boost pressure to controlling flow in the garden hose with the water valve. 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 just moving your thumb across the end of the hose.

2)The Maha dyno 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 Jean and Porsche are correct.

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 tuners and owners driving observations are correct.

So to answer, is VTG all BS? No. The TT certainly builds boost faster than the 996TT, but the 1950 rpm dream will be achieved by only a few under special circumstances. The dramatic reduction in backpressure may actually aid the responsiveness of the vanes and allow VTG to closer parallel the lab conditions.

 

Eclou, just want to clarify that I take no sides, I certainly agree with many engine builders and tuners very often. Todd Knighton for one is a true inspiration.

Proof is that my car is the furthest from being a Porsche purist's car.

I am not too hung up about lag in general, I simply drop one gear when I need it, I believe in 4k+ RPMs. A good measure of lag is to check out some magazine tests and compare in-gear acceleration from a roll with the 996TT. Similar weight between both cars should yield somewhat comparable data.

VTG should work well, it might be somewhat overmarketed I agree.

 

Wow, eclou (& Jean), thanks for taking the time to post such detailed responses. BTW, I am deeply moved that you are so careful in your reply, but you really didn't need to post pictures of the paper & electric windmills.

Kiddind aside, as mentioned I am trying to decide between turbo and GT3, so far leaning towards turbo because I don't plan to track and this is a daily driver. I saw your signature, yeah that bit about the "'07 997 GT3 4/07 order." Are you not happy w/ the turbo and are switching to GT3?

Thanks and regards,
Can

 

Jean,

the very fact that we are posting on the same board confirms that we are on the "same side," and as Porsche owners we obviously aren't too disappointed them either.

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.


Cannaga,

I signed up for both cars a long time ago expected to get only 1. I am taking the GT3 on euro delivery this summer as a vacation. I expect to keep the turbo, but if the GT3 is impressive enough I may consider keeping that one. I figure I win either way.

 

There's also the fact that people mean different things by 'lag'. Some mean how early boost builds up ie whether it is at full boost at 2000rpms or not and others (more correctly I believe) mean how long the car takes to build boost at a particular rpm level once full-throttle is applied.

For me, I have no interest in boost or torque at 2000rpm, thats for diesels and that why my car has 6 gears, so I can use the right one at the right time. That said 'in extremis' no boost until very high rpms is a real pain, I had a T88 equipped Skyline GTR that made no boost until 5500rpm, so cruising in high gears was a pain as full throttle in 6th at 50mph resulted in almost no change in speed.

Back on topic, I'm more interested in lag by way of throttle response i.e. if I'm at 4000rpm and no boost and I go WOT, how long does it take to produce full boost. This does matter on a track and also in a real world overtaking situation on single lane roads. Oh and I'm talking about gears 3/4/5/6 where there is sufficient load to build boost, unlike 1st and 2nd where you're virtually through the gear on the road before it's had time to build full boost.

This type of lag is a real issue for track driving where it's about throttle application out of corners, or on roads where you're timing an overtaking manouevre (as most of our roads in England are single lane each way).

So - has anyone measured how much quicker the VTG turbos at spooling to full boost at given rpms, or are they not? ie does VTG turn a turbo engine into a n/a one in terms of throttle response?

Guy

 

What kind of HP is Protomotive acheiving from the 997TTs?