ptuomov

Yes, the top ring width makes a big difference to the critical rpm in terms of piston ring flutter.

In my opinion, one should pick the engine redline rpm first and then size everything to be consistent with that redline rpm. This is true for piston ring width, too. I want the largest ring width that can be used, given the chosen redline rpm, especially for a turbo engine that produces a lot of hear. This is because the piston is to a large extent cooled by the piston ring transferring heat from the piston to the bore and a wide ring does that more effectively.

Very narrow piston rings transfer less heat, and I observe car factories going with piston oil squirters with narrow rings. Using oil cooling for the pistons in the 928 engine without other modifications is in turn problematic, because there's already too much oil on the bore walls. Piston oil squirters would likely add to that problem, and consistent with this the factory quickly eliminated the piston oil squirters after trying them out in the early '87 models. With, say, dry sump system and/or crankcase vacuum, however, thin top rings and piston oil squirters would probably work well in a 928 engine like they work well in many other race/racy motors.

Fortunately, we have other ways of making power than high rpms with the blue engine!

ptuomov

The blue engine differs from the stock ’87 S4 engine in some subtle and some not-so-subtle ways. One area in which there are differences is in breathing. The breathing improvements to the turbo system upstream of the MAF have been discussed in this thread previously, bigger pipes, compressors, filters, intercoolers, etc. There are also some breathing improvements downstream of the MAF in this blue engine. I've posted on some of those changes earlier, but here's some more.

The main intake manifold casting has been ported. The porting process was originally developed by the builder of this engine and has been replicated at least in two intakes, possibly more. It’s my understanding that the porting process focused on balancing the flow between different runners by improving the runners that flow the worst as cast. I don’t know all the details, but what I do know makes me fairly confident that the intake manifold is ported effectively and cost effectively. (Cutting and welding etc. this manifold would not be cost effective, in my opinion.) My technical input/influence on the main intake manifold casting was limited on picking the color that approximately matches the compressed air pipes…

A photo of the ported intake manifold main casting on the blue engine:



A photo of a similarly ported intake manifold on another engine (not mine):



That manifold on the other engine (not mine) has the cool color scheme of "bare-looking" tanked magnesium alloy intake manifold. I couldn't figure out how to get the boost pipes the same color, so my intake manifold is "boring" silver... The engine in the above photo is one bad-a$$ normally aspirated motor, by the way.

The throttle plate is stock size. In other ways, too, the S4 throttle body element is essentially in its stock form. As discussed elsewhere on this board, at very high flow rates air may be having difficulty turning up one of the oval up pipes. In the future, it might make sense to increase the throttle plate size to slow down air speed and port this element to improve that turn. I believe that this throttle body element and the awkward turn on one side also has some implications beyond just resonance effects on whether one wants to run the intake manifold flappy open or flappy closed at high rpms. A purist would perhaps say that the whole intake has the throttle plate in a wrong spot and is missing seven of them...

There’s one plenum spacer under the driver-side intake manifold plenum plate. It’s there to add some room for air to turn into the runner #5. Others, including Todd Tremel, have used just one spacer, too. Other than giving air a little more room to turn to runner #5, the increased plenum volume also slightly lowers the rpms of the flappy-closed torque peaks. The plenum spacer is removable, if one were to make it permanently installed by epoxying or welding it to the intake manifold main casting, one might be able to port even more room around the runner #5 inlet trumpet.

The flappy is retained as operational. There are some legitimate questions about whether that flappy is necessary with a turbo motor given that it adds complexity and two potential boost leaks, I guess we'll see.

The heads were ported by a person that the builder of this engine christened “the Guru”. That saga had us referring to the whole head-porting project “the Head Case”. The builder of this engine is not responsible for those heads, except in some ancient cultural sense of becoming responsible for a person after saving its life! ;-) The result is a set of fourth casting revision heads with new stock size valves (Ferrea, probably slightly inferior to the factory original vales that Porsche put in the late eighties, but also probably good enough for this engine.) In terms of flow, they probably flow somewhat better than the stock heads at low lifts, effectively extending the camshaft duration. Although the heads didn’t turn out exactly the way I hoped, and perhaps more importantly I now realize that I should've not hoped for what I was hoping for then, I think the heads should be fine. More on those heads in the long "Porting and polishing by committee" thread.





Then there’s probably the biggest change to breathing, namely the camshafts. Those are Elgin 65-6 profiles ground on modified S3 cores and heat treated / hardened afterwards by gas nitriding. The cam specs are listed earlier in this thread at https://rennlist.com/forums/928-foru...l#post13294122 . With the higher low lift flow of the ported heads and longer duration of the new cams, this is a “racier” motor now than what the stock '87 S4 motor was. I think Roger Tyson and a couple of other people are running those cams in their cars (including the bad-a$$ normally aspirated motor in the above photo), and by my understanding are very happy with the results. It’ll be interesting to see how these cams work in a turbo engine.

Strosek Ultra

I am working on a BMW six stroker engine having a bore of 85mm and 1.0mm top rings. Wonder if the simple calculator can be applied to that much smaller bore as the weight of the top ring will be much less than for a ring of same thickness intended for a bore size commonly used for V8 engines? The stroke is 90.5mm and con rod length 135mm (the rod length is limited by the height of the engine block). This is something theoretical for Tuomo to have a look at.
Åke

ptuomov

The bore size cancels out in the formula, so that simple calculator you linked to should apply just as well to the smaller bore engine.

Another thing that cancels out is the ring depth inside the piston. If you make the ring deeper, it gets heavier. But at the same time, the top area that is exposed to the cylinder pressure grows proportionally. The end result is a wash.

ptuomov

Someone asked a question about the compression ratio.

The blue engine has 8.6:1 static compression ratio. Below is an explanation of how one ends up with 8.6:1 starting with an early S4 ('87 in the case).

The heads are about 37-39cc stock. The porting and slightly different valves brought those to 40cc after skimming the heads

The 1.4mm factory gasket has a 102.2mm opening. The head gasket opening adds 11.5cc.

Also, the piston is 0.25mm in the hole, which adds 2cc.

The piston dish is 28.0cc. The early S4 piston has very thick top and relatively big fish at about 23.6cc. There's ample room to take off 4.4cc or 12.3g of aluminum from the piston top. The piston dish design is a compromise between strength in terms of gas loads, strength in terms of heat flow from the center to the sides, combustion chamber compactness, and stock like squish. The later S4 piston with a thinner top and smaller dish would have required a different dish design.

A repost of the piston photos, one of which I copied from Tony's post:






If starting with a later S4/GT piston that has a smaller dish and thinner top, I'd probably give up on some of the squish and go with a circular dish. Not quite as extreme as this 9ff modified 944 piston but close:



In my opinion, the squish area becomes less important as boost goes up. That's because squish is there to generate turbulence in the charge, and I believe (but don't know for sure) that there can be such thing as too much of a good thing in terms of turbulence. A spherical/circular dish with a symmetric constant width squish band will still give some squish effect, directing charge up from the front and back and down from the intake and exhaust sides. For high boost motor, that might be a reasonable solution if starting with the late S4/GT pistons.

ptuomov

To fit the sewer pipe compressor inlets, things needed to be moved a bit. One thing that had to yield was the exact alternator position. This in turn means either a new custom belt or an idler roller back wrapping the stock belt. For now, the system has this idler pulley in the back of the alternator belt:



Version 2.0 of the idler roller for the alternator belt:

ptuomov

The stainless steel exhaust going back on for the dyno break in.

This exhaust system version is called "Silent Night":

BauerR

Is that 3"?

ptuomov

Yes, "Silent Night" is 2x3" with two Borla XR-1 Sportsmans and no cross-over.

Sound is a matter of personal preference. This exhaust sounds a bit like four Harleys driving in a line on a dark road with only the lead bike having its headlight on.

BauerR

I'm not concerned about sound. I'm going with Kuhn's turbo setup too and was planning on going dual 3"

ptuomov

"Silent Night" works in the sense that the turbine outlet pressure is low. 99% of power production of a turbo exhaust is right there, low turbine outlet pressure.

Strosek Ultra

It is commonly considered that dual 3" exhaust is good for 700+ hp.
Åke

V2Rocket

re: pistons, there's a 944 guy in Europe who took stock 944S pistons and shaved 1mm from the top of the crown without touching the bowl/dish at all. so now piston sits 1mm below deck (aka no squish anymore) and it dropped compression from 10.9 down to 9.6 or so with otherwise stock 944S head and head gasket. that engine lives on 1.4 bar boost/400whp steadily.

944S pistons have a 13cc dish with a 42cc cylinder head of very similar design to 928S4 head.
so with your 25+ cc piston dish you could get your 8.x:1 CR without digging out the center of the piston, but just "shaving the edges"

ptuomov

One could do that, but is that the best one can do?

I'm thinking that there are a couple of things that matter:
1) minimum distance of the top from the ring groove.
2) the ability of the piston crown shape to hold gas loads
3) the ability of the piston crown shape to flow heat to the rings
4) any squish if desired
5) compactness of the combustion chamber

The first says that you should probably leave enough material closer to the bore wall. The second says that you should leave enough material around the wrist pin boss areas. The third says you can have the piston crown a bit thinner from the center than the sides. The fourth says you probably want at least a small squish ring for turbulence. The fifth says the center should be lower than the edges. I'm thinking that one should be able to do better dishing the piston while leaving the edges of the crown untouched than just shaving the piston flat.

For what it's worth, the builder of the engine blue designed the dish in the blue engine and it was implemented by a firm that modifies pistons. To the extent I'm able to judge, it's a good compromise between heat flow, strength to gas load, ring-land strength, squish, and compactness. But my ability to judge the design is questionable to say the least.

FredR

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]?

Rgds

Fred