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I am slow on the draw,.........your exhaust has the CROSSOVER by the mufflers......I thought this was DISPROVEN to provide any benefit.....you , I am sure, have done your homework, how did you come up with this design...????
I am slow on the draw,.........your exhaust has the CROSSOVER by the mufflers......I thought this was DISPROVEN to provide any benefit.....you , I am sure, have done your homework, how did you come up with this design...????
I don't understand your question. This is a turbo car and after the infinity pipe cross-over it's only about reducing noise with minimum pressure loss. Pulse tuning is pretty much irrelevant.
A clarification: Although I did help with a **** ton of background research, simulations, and computations, John Kuhn ultimately designed and built that exhaust.
On paper the Autos may be more turbo friendly and some say they can handle more power than a manual. But as always in practical terms I am not so sure it is so cut and dry. What I found out through hundreds of dyno tests is the few autos I used for testing stuff, they can't seem to take the increased load of a forced induction system, repeatedly. It seems at or around dyno run number 20? they start to slip badly and I had at least 2 that wouldn't even move the rollers by this time. You gotta pull them off put a fan on them for a few hours due to the heat-soak and then try again. Those probably needed rebuilding, but there were never any other indicators in road use, and as far as I know they are still going, even to this day 10 years later. Scared the crap out of me the first time, thought I had taken out a tranny completely on someone else's 928! I had similar results on a few others and was wondering if anyone else has experienced this?
Have a great one,
Dave
Turbo cars tend to suffer a little more power interruption when shifting in the manual cars. Throttle closes as you shift, and both intake and exhaust flows fall precipitously. You get to wait for the turbine and compressor to spin-up again. Contrast with the autobox that lets you hold the pedal down.
In the later S4 and GTS cars, the trans causes ignition timing to back off during high-load up-shifts. It functionally backs off the power to unload the gearbox, all without changing flows through the turbochargers.
On paper the Autos may be more turbo friendly and some say they can handle more power than a manual. But as always in practical terms I am not so sure it is so cut and dry. What I found out through hundreds of dyno tests is the few autos I used for testing stuff, they can't seem to take the increased load of a forced induction system, repeatedly. It seems at or around dyno run number 20? they start to slip badly and I had at least 2 that wouldn't even move the rollers by this time. You gotta pull them off put a fan on them for a few hours due to the heat-soak and then try again. Those probably needed rebuilding, but there were never any other indicators in road use, and as far as I know they are still going, even to this day 10 years later. Scared the crap out of me the first time, thought I had taken out a tranny completely on someone else's 928! I had similar results on a few others and was wondering if anyone else has experienced this?
Have a great one,
Dave
Dyno testing heats pieces beyond the temperature they would ever get on the road. Almost no dyno that normal people have access to can blow 100mph air over the entire car, much less 40 mph air. (There are certainly dynos inside wind tunnels, right?)
The fact that these transmissions did not have to be rebuilt after these tests would indicate that the fluid was hot enough to "flash" at the torque convertor and the slip was there, not at any of the bands or clutches.
Dyno testing heats pieces beyond the temperature they would ever get on the road. Almost no dyno that normal people have access to can blow 100mph air over the entire car, much less 40 mph air. (There are certainly dynos inside wind tunnels, right?)
The fact that these transmissions did not have to be rebuilt after these tests would indicate that the fluid was hot enough to "flash" at the torque convertor and the slip was there, not at any of the bands or clutches.
Hi Greg,
Thanks for that! Yes I know it is from dyno heat soak, and try as we might with fancy fans there is no way you can recreate airflow like on the road. We could come close at just the bumper cover opening, no way on the whole car. Was wondering if any of the guys with power adders and autos have experienced any tranny overheat issues on dyno or road.
Yeah Greg, some even pee'd a little, but they all recovered perfectly which is a testament to them. I have had lesser cars/trucks do this before, and they were towed to the tranny shop.
"There are certainly dynos inside wind tunnels, right?"
NOT on my budget! LOL
Happy Sunday Greg,
Dave
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David Roberts
2010 Jaguar XKR Coupe - 510HP Stock - Liquid Silver Metallic
928 Owners Club Co-Founder
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The high tech solution for the chassis dyno cooling problem is a fan wall built specifically for chassis dynos. That's expensive, like a wind tunnel.
A low tech but reasonably affordable solution is stackable carpet drying fans. The amount of air that they move and the speeds at which they move them are surprisingly high. About 1/2 hp each, moving about 2000-3000 CFM per fan, and can be stacked into a wall.
We always ran dynos with 1 or 2 of those fans into the grille opening with the hood always closed. Had very few overheating issues with the engines. But we did have to change tactics and restrict autos to 15 runs before taking a break.
We always ran dynos with 1 or 2 of those fans into the grille opening with the hood always closed. Had very few overheating issues with the engines. But we did have to change tactics and restrict autos to 15 runs before taking a break. Cheers, Dave
The 928 automatic does seem to have a higher power loss than the manual, and all that lost power will turn into heat. If you get 15% driveline loss even just with 500hps, you're looking to cool 200,000 BTU per hour. That's a lot in my book, even with realistic air flow under the car.
Some of those fans can be stacked one way for storage and another way to be run in columns.
Still looking for G28.13 transmissions in good condition. Also, if someone has new (or used, but like new) fifth gear 928 302 905 33 part, I'm interested.
So some of these gears have pitting. What causes pitting in the 928 transmission gears? And if installing new gears, how to prevent those?
First, avoiding hydrogen embrittlement. My guesses about it will follow. This kind of pitting due to hydrogen embrittlement is caused by water in the transmission oil. To get rid of any water in oil, the transmission oil needs to be above minimum temperature and the transmissions needs an effective breather. The temperature of the tranny oil increases by about +100F when it runs thru the gears, so the minimum safe temperature for transmission oil is somewhere between 110-160F in the sump. From those temperatures, the oil between gears heats enough to boil out any water. The water vapor then needs to be vented out thru a vent on the top of the 928 S4 transmission, but I don’t think there’s actual dry air circulation in any of the 928 transmissions. Any ideas for a true breather that would circulate dry cold air in and wet hot air out while filtering out any particles?
Second, avoiding metal to metal contact. My guesses about how to avoid this ate the following. We need high viscosity oil at the right temperature between the gears at all times. High viscosity oil will give a stronger oil film, and oil spray bar will make sure there is enough oil at all times between the gears.
Third, filtering out any debris or foreign objects. If I’m adding an oil pump, then I want to get a inlet strainer in there to prevent pump damage and an outlet filter to get rid of any smaller particles that can pass the pump. I am guessing that this might be helpful.
Fourth, micropolishing the gears to reduce micropitting. The uneven gear surface can at points puncture the oil film and microweld the greats temporarily, causing micropitting. This may be a “small” problem, but micropolishing should help, by my guess.
Anything else one could do to reduce pitting? If I ever get my hands on all the gear sets, including the fifth gear, I’d like to not have them go bad in a preventable way.
If someone needs to get rid of some fifth gears, part number 928 302 905 33, I will gladly accept them! Send them to me and I won’t complain.
08-19-2018, 02:07 PM
