Twin Turbo 928 fixed and back out there terrorizing the streets!
#1426
Captain Obvious
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Joined: Aug 2003
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From: Cambridge, Ontario, Canada
V2Rocket's post says, what that dude did to those pistons works really well and is proven. I would not cast is aside because another theory sounds better.
Personally I don't like removing material from the middle of the piston.
Personally I don't like removing material from the middle of the piston.
#1427
However, part of the fun for me is trying to understand what works and why. Why don't you like removing material from the middle of the piston? The factory removed material from the center of the piston, but not so much from the edges, in 1988 when they decided that they needed to make the skirts thicker for S4 pistons.
#1428
Captain Obvious
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From: Cambridge, Ontario, Canada
REBCO's been dishing pistons for a long time, and probably a lot more of them than that "the dude" has. I'm confident that those pistons were dished in a way that is a good compromise.
However, part of the fun for me is trying to understand what works and why. Why don't you like removing material from the middle of the piston? The factory removed material from the center of the piston, but not so much from the edges, in 1988 when they decided that they needed to make the skirts thicker for S4 pistons.
However, part of the fun for me is trying to understand what works and why. Why don't you like removing material from the middle of the piston? The factory removed material from the center of the piston, but not so much from the edges, in 1988 when they decided that they needed to make the skirts thicker for S4 pistons.
#1429
There isn't much support, other than the thickens off the piston top, at the centre. The factory spec was set to have X amount of material in the centre, now you remove a bunch and then add a ton more load, seems no logical. The edge of the piston is a lot closer to the skirt where the load travels straight down to the wrist pins. So my eyeball engineering tells me to take the material from the edge before the middle. Maybe Porsche decided to ditch the idea of turboing of the 928 and then went with a thinner piston centre. That seems more probable to me.
In any case, the blue engine pistons were modified in a way that took no material out right at the center! There's around four cm circle in the middle with all the metail retained. The material is removed mostly from the areas that are on top of the large pin bosses. I can think of a way that would have removed the same amount of material while keeping the piston as strong. The '87 early S4 pistons are a luxury starting point for dishing turbo pistons, the late S4 pistons would have been slightly trickier.
#1431
It was loud with that "Silent Night" exhaust system. Not droning, but loud. I drove 1000 miles in one day with that exhaust.
The problem (to the extent there is one) is that our street, a little cul-de-sac, is the steepest grade in our town. So I have to give it some gas to get up. The lights come on in neighbors houses when I do that.
Wonder what the neighbors are going to think about the bigger cams with more overlap?
I've got some modular exhaust plans in case the road trust starts proposing votes on my 928.
We can add 2x3" -> 4" Y in the rear with a short 4" magnaflow muffler in location of the stock pumpkin. The would combine pulses and add muffling.
in addition or separately, the middle two components can be replaced with a single oval 4" X that has an integrated muffler and two magnaflow mufflers.
Some combination of those might work both in terms of sound suppression and low back pressure.
Additional components that may make it to the two new modules (I've got a shorter tip muffler, too):
The problem (to the extent there is one) is that our street, a little cul-de-sac, is the steepest grade in our town. So I have to give it some gas to get up. The lights come on in neighbors houses when I do that.
Wonder what the neighbors are going to think about the bigger cams with more overlap?
I've got some modular exhaust plans in case the road trust starts proposing votes on my 928.
We can add 2x3" -> 4" Y in the rear with a short 4" magnaflow muffler in location of the stock pumpkin. The would combine pulses and add muffling.
in addition or separately, the middle two components can be replaced with a single oval 4" X that has an integrated muffler and two magnaflow mufflers.
Some combination of those might work both in terms of sound suppression and low back pressure.
Additional components that may make it to the two new modules (I've got a shorter tip muffler, too):
Last edited by ptuomov; 01-13-2017 at 08:49 AM.
#1432
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Joined: Feb 2011
Posts: 2,249
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From: Mostly in my workshop located in Sweden.
On a NA engine it would be even louder. The turbos do act as a kind of silencer. However, I hope Tuomo has understanding neighbours. Well I do not think he will be driving the uphill road to the house very often as the car mostly will be in the garage of Herr Kuhn.
Åke
Åke
Last edited by Strosek Ultra; 01-10-2017 at 12:41 PM.
#1433
On a NA engine it would be even louder. The turbos do act as a kind of silencer. However, I hope Tuomo have understanding neighbours. Well I do not think he will be driving the uphill road to the house very often as the car mostly will be in the garage of Herr Kuhn.
Åke
Åke
Brings up the plea:
It's kind of loud, but not as loud as the scream of this four year old's mother when she saw me taking her up the top of Mt. Mansfield to ski down:
Last edited by ptuomov; 01-13-2017 at 08:49 AM.
#1434
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Joined: Feb 2011
Posts: 2,249
Likes: 511
From: Mostly in my workshop located in Sweden.
Fair point, but these old photos show that the current exhaust might be too loud on the day no even for John Kuhn's habitat:
It's kind of loud, but not as loud as the scream of this four year olds mother when she saw me taking her up the top of Mt. Mansfield to ski down:
It's kind of loud, but not as loud as the scream of this four year olds mother when she saw me taking her up the top of Mt. Mansfield to ski down:
Åke
Last edited by Strosek Ultra; 01-10-2017 at 11:24 AM.
#1435
Adding an upstream cross-over (before the pipes divorce for main mufflers) will be a minor-event or a complete non-event in terms of power production for a turbo car. However, it may improve sound by canceling the pulses.
Currently, the two pipes emit a small-big-small-small-big-small note but in off beat fashion:
Passgr: BB-B--B-BB-B
Driver: --B-BB-B--B-
If I add an upstream cross-over, then they'll hopefully blur to:
Passgr: BBbBbbBbBBbB
Driver: bbBbBBbBbbBb
Completely combining the pipes would probably result in something like:
Combined: BBbbBBbbBBbb
Divorcing the pipes again into unequal lengths, and then combining them again, will shift the notes slightly. It's going to not be anything like 90-degrees of crankshaft rotation, but it should soften and cancel all sorts of higher frequency waves and make the note sound lower.
#1436
I wonder what dB level it takes to go from "nice neighbour" to "noisy A-hole"?
My system makes a "pleasant noise" - my wife who cares nothing about the 928 whatsoever was in a local shopping centre car park one afternoon - she heard a car accelerating hard about 1km- said to our daughters "that sounds like your Dad's car" - it was!
I should imagine with that "PA system" you are proposing to use your neighborhood might become another war front- just make sure you have brick proof windows!
Rgds
Fred
My system makes a "pleasant noise" - my wife who cares nothing about the 928 whatsoever was in a local shopping centre car park one afternoon - she heard a car accelerating hard about 1km- said to our daughters "that sounds like your Dad's car" - it was!
I should imagine with that "PA system" you are proposing to use your neighborhood might become another war front- just make sure you have brick proof windows!
Rgds
Fred
#1438
Captain Obvious
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Joined: Aug 2003
Posts: 22,846
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From: Cambridge, Ontario, Canada
Got any pictures of the mufflers you've made?
#1439
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Joined: Feb 2011
Posts: 2,249
Likes: 511
From: Mostly in my workshop located in Sweden.
Sorry I do not have any pictures of finished mufflers but here are some pictures taken during the manufacturing process. At the time I made mufflers for several engines. The three largest diameter mufflers as well the oval ones are for 928.
Everything is made from stainless steel. The cores are wrapped with a layer of stainless steel woll, the rest of the cavity is filled with Basalt-wool a kind of rock-wool that come in big bags from a German supplier.
http://www.dbw.de/index.php?lang=en&hm=2&lm=1
Åke
Everything is made from stainless steel. The cores are wrapped with a layer of stainless steel woll, the rest of the cavity is filled with Basalt-wool a kind of rock-wool that come in big bags from a German supplier.
http://www.dbw.de/index.php?lang=en&hm=2&lm=1
Åke
#1440
https://goo.gl/photos/Wm4nbX34Tq5k5qaG6
That video has the car with the "Silent Night" exhaust on and I'm just feathering the throttle open a tiny bit, as you can see from the rear wheels retaining traction.
Sorry I do not have any pictures of finished mufflers but here are some pictures taken during the manufacturing process. At the time I made mufflers for several engines. The three largest diameter mufflers as well the oval ones are for 928.
Everything is made from stainless steel. The cores are wrapped with a layer of stainless steel woll, the rest of the cavity is filled with Basalt-wool a kind of rock-wool that come in big bags from a German supplier.
http://www.dbw.de/index.php?lang=en&hm=2&lm=1
Åke
Everything is made from stainless steel. The cores are wrapped with a layer of stainless steel woll, the rest of the cavity is filled with Basalt-wool a kind of rock-wool that come in big bags from a German supplier.
http://www.dbw.de/index.php?lang=en&hm=2&lm=1
Åke
If you make your own mufflers, you'll have one meaningful advantage over the rest of us who use high-flowing off-the-shelf SS mufflers. You'll probably be able to use the available space under the 928 much more efficiently. Much like with intercoolers, with mufflers there's no substitute for volume.
Last edited by ptuomov; 01-13-2017 at 08:51 AM.