eclou

To my knowledge only 1 6speeder has done a non-eddy Dynojet run on a 997tt. Despite the fact that the shop had never run a 997tt before, they decided to run it the same way they did 996tt's - they disconnected the AWD differential and ran it as a RWD car. Unfortunately, they did not account for the newer electronic stability/braking systems and the car literally cooked the rear brakes while trying to "stabilize" the front and rear wheel speed imbalances. IIRC he actually warped both the rotors, vaporized the brake pads, boiled over the brake fluid and was left with an unusable car. He fought with Porsche over warranty work on this for weeks without ever understanding that it was the dyno procedure that killed the brakes - instead he had the notion that Porsche had built him a defective car!

mw911

Hello TB993tt,
I think I can add some information to the thread....
I was at Manthey several times (and at the Sportec Dyno - a Bosch) with my 993 tt rwd measuring and comparing the different set ups I had at that times. (K16/24 based or -currently- K24/26 based, both kits done by SHK here in Germany. One has to know that SHK has a close connection to Sportec.)

[QUOTE=TB993tt;4742806]

Bottom line is it seems that around 700-720NM (515-530lb/ft) is the limit of grip for the rear wheels - I know Jean's car had the same issue....
The operator tried about 5 times but the torque was too much he resorted to using part throttle through peak torque then nailing it at higher revs -eventually he managed to get me a peak power number, the part throttle run took 23 seconds

On my last visit at manthey the power run came out with the following:

Ansaugluft-temperatur: 17.3 C
Schleppleistung 83,8 kw (which is more than you had recorded but less than the Gemballa)
Radleistung(RWHP): 316 kw
Motorleistung(Engine horse power): 399,8 kw
corrected engine horse power: 418,7 kw
torque (NM) 728,8

At my run there was no slippage on the rolls, that might be a result of the fact that I do not have anything like "Zero-clearance" turbos and thus the peak torque is a little later (4865 rpm) tha in your (or Jeans) car I guess.

From several comparisons using my old set up (K16/24based) I know that the Bosch dyno in general measures 8 to 10 percent more engine power (in my personal experience) and also more torque.
This would be in line with a dyno comparison published in sport out several years before. Those guys having been at the locations of the two dynos testet in that sport auto issue do know (so do I) whose dynos were compared...

Both companys using the bosch or the maha dyno respectively declare that they are measuring the correct horse power, the other one measuring too low or too high respectively. I personally believe in the maha technology....

However using both dynos there are "sources for error" If I might term it (The positioning of the "Ansaugluft-Temperatur" sensor and the detection of the "Schleppleistung")



The answer lies in the Ansaugluft Temperatur - this is the air intake temperature. The Gemballa has an IAT of 75 degC. My IAT was 10.5degC....

Because of the very high Gemballa number that engine was given another 88hp as a correction for the inlet temp and it got 136 hp from coast down losses compared to my (ultra efficient 993tt ) 90hp.

Now that 10degC intake temp measured for my car was courtesy of the big pipe blowing cold air (from outside) directly into the airbox which is where the temp sensor was sitting - where did Gemballa put their sensor ? In the post intercooler intake tract ? that seems most likely....

The question is, is this "cheating" - I would say probably not (unless the Maha handbook specifically states where that temp should be read from) but isn't it amazing how it can effect the results and how Gemballa can show a 700hp engine which would probably read more like 570hp if measured a different way - and of course (presuming the Gemballa had grip on the dyno) if my car could have been run at full throttle it would certainly have pulled much higher torque number at the wheels than the Gemballa......


Several years before I had a long story comparing different MAHAS on my -at that time still N/A - 993. Stummel does know this story we had a lengthy thread on one of our german forums, unfortunately the thread is completely lost due to a server crash in 2005.

To make things short: The difference between 3 different MAHA 2000s was between 270 ps and 320 ps. The comparison of the different dyno-sheets revealed that the main variation was due to differences in "Schleppleistung" and "Ansauglufttemperatur"....

At that time I concluded that : (1) one should only use the "Radleistung (RWHP) for comparison issues between dyno runs on Mahas or between different cars tested on Mahas and (2) one should check performance on the road (e.g. Vmax or acceleration using driftbox technology which was not avaliable/popular in germany at that time).

Please appologies for my english...

Michael

TB993tt

Michael
Thankyou for posting, very interesting information.

I guess your 728NM will be right on the limit of tyre slippage...

Why would the Schleppleistung (coast down losses) vary so much ? OK maybe the Gemballa was "massaged" but yours lost 23PS (25%) more than mine during coast down. They put a set of GT3 mark 1 wheels , fairly narrow (with I guess 265 or 285 tyres) would that make a difference ? Did they go through the ~30 minute warming up procedure with yours ?

It seems the Rear wheel hp mantra applies to Mahas as well then does it not ?

Stummel

Michaels car is a narrow body, maybe it also has a different gear box with more loss?

Felix

I've surfaced back in the UK. It was great to see tb993tt's awesome car in the flesh as well as its seriously committed (or should he seriously be committed?) owner and to meet Stummel again.

Manthey's Maha installation (as opposed to the dyno itself) seems to have a shortcoming in that it only has a strap running through the door opening from one side of the car to the other which keeps the car from driving out of the rollers but does very little to pull it down.

eclou

Thanks Michael for your input, and for giving us your second post in 3 years!!!

I think this highlights the fact that any dyno is subject to variations in testing output, which are affected by the information input into the testing - "garbage in/garbage" out so to speak. 50 PS is quite a large % variation on a naturally aspirated 993. That is even much wider than the variations I have seen on the maligned Dynojets when I was testing my old E36M3 using 4-5 different facilities.

eclou

Are you kidding me? This is what I would consider to be a major testing flaw. TB do you recall them using any ratcheting straps on the front and rear chassis, perhaps even on the tow hooks or points? I would not allow my car to be tested in such a negligent manner as with a single cross strap. No wonder the cars are not getting any traction. Looking back at the photo clearly supports this. The will be almost no preload on the suspension this way. The perpendicular tie down points are not axially resisting the force of the thrust!




This also explains why the tires are overheating and blowing up. Jean if your R-comps can survive an enduro or even a sprint in Dubai at 40+C and now blow up, why would one think that a 30 minute pull on a dyno without the extreme loads of braking and cornering forces would kill your R comps? It is because these tires are probably slipping the entire time on this Manthey setup. Do the cars have that distinct smell of burning rubber after the runs? The years of experience there seem to have ignored some very basic principles of physics.

TB993tt

Hey.....I resemble that remark
Good to meet you also
To this and Eclou's remarks above - Do we really think Manthey have not tested the best way to run these cars ? Were they just being lazy ? (baring in mind they spent 1 hour prepping the car for the runs)

Doesn't strapping the back on the car down to press against the rollers cause even more stress which would increase the liklihood of a blow out ? Do we not think they may have BTDT with their MM700 996tt

eclou

Toby, I would be very disappointed if they strapped down my car in such a whimsical fashion. Not only would the results be less than accurate, but here in the US they would not be able to operate in such a unsafe fashion. If that 1 single strap were to fail, your car would have been launched into the wall ahead. Take a look at how Stephen straps down his cars



The are not only straps but also safety chains in the rear to load the suspension and axially resist the thrust. In front of the car are more straps as well . Even on my 250 PS M3 we would never use less than 6 straps on the car.

Also suspension loading is not going to overheat a tire the way that slippage/friction will, just as most of your lorries blow out tires on emergency braking.

Felix

I looked carefully; there were no other tie-down points in the room. Perhaps Stummel and I should have leant on the striker plate of the rear lid while you filmed!

mw911

Yes I think so

Yours

Michael

dbf73

Jean, you mentioned the tire change - what kind of tires did they put on? Street or true race slicks?

mw911

eclou,
from my point of view, not all variation in these measurements was by chance. I - at that time - had a 3.8 n/a motor build (by a german tuner unknown in the States or in Britain; the aim was 320 PS).
The first dyno run on his MAHA revealed 285 PS, I told him that this would be very disappointing. Then he changed something in the engine compartment and did a second run revealing 320 PS. He told me he had modified the air/fuel ratio.
I did not trust in these measurements and tested the car on two other MAHAs (and on a Bosch) afterwards. On the other MAHAS the car produced 285 and 270 Ps respectively.
I addition, the vmax of the car on a straight autobahn was compatible with 285 but not with 320 PS.

I do not want to report the subsequent weeks and months (and the correspondence etc.) but as a result, I changed the tuner and bought a used 993 turbo engine.

Michael

TB993tt

I have spoken both to Maha UK and a top Porsche specialist who has a lot of experience in dynoing high powered 911s on his Maha

Firstly, Gemballa cheated with the IAT ! also the Gemballa coast down losses are higher than they would expect and my 90PS are lower.

The UK Maha operator said that when doing power runs with high power Porsche turbos the car has to be in 5th gear by the time 50kph has arrived so they pass 50kph in 5th gear and then feed the power in gradually usually not giving it WOT until 4000rpm has passed otherwise the tyres will slip (and he says they use all sorts of strapping) so effectively getting a mid range torque curve of over 720-750NM is not going to happen however a good operator can measure your peak hp (as long as it is not over 680 @ 6500rpm

So on balance I think Manthey did the best job they could and they did measure my maximum power at 6770rpm and got it right

Felix

They were street tires; there was a concern that R compound tires might overheat. The tires that tb993tt arrived with were street (Pirelli P Zero rossos??) tires as well and they were also changed to Manthey's own. Perhaps they would much rather shred one of their own tires rather than leave the customer stranded?