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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.
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.
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.
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.
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.
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.
When skirts started cracking, they took some excess material from the top and put it to the skirts. I think it's just that they oversized the whole piston assemblies, judging based on weight alone.
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.
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):
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
Last edited by Strosek Ultra; 01-10-2017 at 12:41 PM.
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
Fair point, but these old photos show that the current "Silent Night" exhaust might be too loud on the day no even for John Kuhn's habitat:
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:
A large size rear straight through muffler with dual 3" perforated cores and stuffed with sound absorbing wool will do the trick reducing the exhaust sound to an acceptable level. The stainless mufflers I have made are diameter 230mm and length 400mm.
Åke
Last edited by Strosek Ultra; 01-10-2017 at 11:24 AM.
A large size rear muffler with dual 3" perforated cores and stuffed with sound absorbing wool will do the trick reducing the exhaust sound to an acceptable level. The stainless mufflers I have made are diameter 230mm and length 400mm.Åke
One additional thing to note. Currently, the system has no cross-over whatsoever.
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.
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!
A large size rear straight through muffler with dual 3" perforated cores and stuffed with sound absorbing wool will do the trick reducing the exhaust sound to an acceptable level. The stainless mufflers I have made are diameter 230mm and length 400mm.
Åke
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
Tuomo, Apologies if I missed anything but with regard to the gas flowing of the inlet manifold which two cylinders flowed the most and which two the least [if such is known]?
It differs with rpms and gets in my opinion complicated really fast. The simple flow restriction of runners #5 & #8 that you can detect in the flow bench only comes into play at high rpms when the flow demand of the engine is very high. At mid range rpms, the long center runners are likely to fill better because of better tuning, regardless of the flow bench results. At lower rpms, the 180-degree exhaust blowdown interference into cylinders #3, #4, #5, and #7 both reduces the cylinder filling and increases knock. At high rpms, the 90-degree exhaust interference that hurts cylinders #1 and #6 has the similar effect. So in my opinion, it's both the intake and the exhaust that have a significant impact and all effects are rpm dependent. Goes without saying that these are all just opinions and my attempts to make sense of the world.
Originally Posted by FredR
I wonder what dB level it takes to go from "nice neighbour" to "noisy A-hole"?
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.
Originally Posted by Dave928S
I noticed you used the small font .... Passgr: BB-B--B-BB-B
Driver: --B-BB-B--B-
Which totally convinces me that it won't be loud when you get on it ....
Actually, I wanted to use the constant width font to line them up correctly, but of course the font has to be smaller because of all these clever silencing ideas of reinventing the wheel square!
Originally Posted by Strosek Ultra
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
I'm all for making America great again, but I don't think we're going to make our own mufflers here... ;-)
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.
01-09-2017, 06:56 PM
