Are you sure it wasn't the M28.45/46 pistons instead of M28.43/44 pistons? I didn't think the 12:1 S4 head/S3 bottom combo would do well on pump gas.

 

Yes. It appears that with actual 93 pump gas, that would work fine.

And not only did it live just fine normally aspirated, I think it run thousands of miles turbocharged before developing ring problems. EZK hit the retard limit on hot days with boost, I hear. Pistons look fine:

 

Impressive!

Modified S3 cams too?

 

Yep, heads are modded 16V ones from my 81 and the block is a 5.0 from an 85-86.

I could use the factory position of the CPS on the 5.0 block as one input and then use the hall sensor out of the distributor as another. It should work with an aftermarket ECU.

 

I have terrible hearing, from shop noises and years of shooting. However, I can still hear piston slap in '85/'86 engines, quite easily.

Worth noting I originally ran my 104mm engines at the middle of the clearance specification and was able to hear piston slap quite easily, when these engines were cold. I've "adjusted" my piston to wall clearance tighter to help keep my engines quieter.

Other engine builders tell me that aluminum engine blocks "ring" much more than cast iron blocks and piston slap is much easier to hear and more difficult to differentiate from knocks.

....I've always just assumed if I can hear that slap, a knock sensor will be able to hear it, also.

Maybe knock sensor technology has evolved since 1987 (although they continue to use the same sensor), but I read that the '87 pistons had to be modified to eliminate some of the piston noise for the knock sensor system to properly function.

 

And the "ring damage" came from........

I build and tune quite a few 928 engines....more than the one or maybe two anyone else, here, will ever build and tune.

I comment on threads like this, to share part of my knowledge/experience. People are welcome to pay attention to my vast experience....or ignore it. "No skin off of my back!"

A 10.5 to 1 naturally aspirated stroker engine will max out the knock sensor system with stock ignition timing and 91 octane fuel. Period. End of story.

I'm going to guess that the 12 to 1 engine with 93 octane fuel had all 9 degrees "removed" by the knock sensors most of the time it was running.

Supercharging that engine without removing any ignition timing would be stupid. I'd submit that timing was retarded when the supercharger was installed and the compression ratio still "killed" the engine.
The only chance that engine had of survival would have been if it was turned for 116 octane or E85.

 

This is very interesting. When I acquired the ST2 kit I had to rely on the EZK system to tell me what the limits were. What I found was that the system seemed to be somewhat inconsistent in that when I pulled a little timing after recording a knock event and then reversing the change I would find no knock event. This made me wonder whether the system could resolve knock from other noise correctly. Then of course in the literature Porsche advise that a certain number of knocks are "acceptable". All this makes me wonder if they expect false knock events and if so how does one optimise timing without a dyno? My original assumption was that the knock events I registered may be because I was running 95RON on a motor tuned for 98RON. Then I was advised that having a faster exhaust might require less advance so another curved ball as it were. Now I have 98 RON available I am thinking of simply throwing 3 degrees of advance into the system and rely on the EZK to pull it back a tad if needed.

Bottom line I have heard motors knocking but on my 928 even though I have registered knock events I have never detected any sound to indicate such was actually taking place. I am sure I am leaving something on the table as it were through being a bit on the conservative side.

 

Just when I'm pretty sure I know a bunch about timing and knocks, I get thrown a huge curve. I recently built a 5.8 engine with aftermarket cams. I generally tune cars, until they knock, and then back down the timing in the areas where they are knocking to a very low knock count. (I try to keep my total knock count under 25 in 10,000 combustion events, especially when "stock" rod bearings are used.) I tuned this engine in this manner. This particular engine, with the pieces I had assembled it with 'felt" lame....it just had no "snap", on the top end. The dyno confirmed my "butt"....the engine fell on it's face, on the top end. We increased the timing and ran it again. Still no top end. We finally, in desperation, reduced the timing 10 degrees....everywhere. Grabbed the whole map and hit the "big" reduction button a bunch of times. Instant top end power....huge gain. (And a loss of power in the low and mid range.) Huge surprise. This particular engine hated ignition timing, on the top end, but loved timing in the low and middle range.

You just never know....

 

I my case oh so true!

ST2 helped confirm what I suspected in that I know squat about tuning motors! That I managed to get a noticeable improvement mid range was something I was quite proud of but top end just does not have that "snap factor" you refer albeit we are talking about little more than a stock 5 litre build in my case. Still, ST2 is very easy to operate and a powerful tool with Jim's Sharkplotter software and it enriches the ownership experience quite considerably IMHO. That tuners invariably resort to the dyno to optimise custom tuning tells its own story.

The trouble with anything that is "infinitely adjustable" is that there is one right combination and "an infinite number minus one" wrong ones!

 

FYI I'll add this as an option, I use this one and it doesn't require a cam sensor, works great on my Twin Screw Super Charger at a bit over 9 PSI, I have one Knock sensor installed, the unit has a vacuum input to set Boost level activation too.
It can also work with N2O, but have yet to try it

http://www.jandssafeguard.com/VampirePage/Vampire.html

Dave K

 

On the original question, I'm convinced that the S4 knock detection system works with S3 pistons.

In terms of compression ratio and ignition advance, I believe that there's an optimum given the fuel (and charge motion generated by the cylinder head and piston). With a very high octane fuel and no knock, higher compression ratio (really, higher expansion ratio) makes more power. The relationship is non-linear even without the knock limit binding, in that an increase in compression ratio from 8:1 to 9:1 gives a greater power increase than an increase in compression ratio from 9:1 to 10:1. Then, when we run engines on pump gas which are almost always knock limited short of some sort of parts mismatch, the increase in compression ratio will force one to reduce the ignition advance. This reduction in ignition advance causes the charge to burn later than ideal, the ideal burn in some sense being an instantaneous burn right after TDC. At some point, the efficiency gain from a little more compression becomes the same as the efficiency loss from having to retard ignition because of the knock limit, and that's the optimum. I think that with good 93 octane pump gas, headers, 100mm bore, and the right cams, by my guess this optimum isn't too far from 12:1 for a normally aspirated 928. If one reshapes the intake port in a specific way (so called "tumble port"), published research (not my practical experience) suggests one could go even higher in compression ratio. With the "91" octane California **** water, the optimum compression ratio is lower, as it is with exhaust manifolds and short intake cams.

I don't think anyone would intentionally build a hot-rodded forced-induction engine with 12:1 compression ratio, that's something that only car factories would do to reduce fuel consumption. It's called Miller cycle and the valve timing is very different from typical valve timing.

For a 5.0L five-speed turbo 928 S4 running on 93 octane pump gas, I'm thinking that the optimum compression ratio is about 8.5:1. 8.5:1 compression ratio will allow one to run a lot of boost (about 20 psi) with sensible ignition timing such that the manual transmission becomes the bottleneck. Lowering compression further to increase boost would make the car less nice to drive in transitions while it would increase the torque to the level where the transmission can't handle it anymore.

 

Interestingly, one of my goals is to use a twin screw supercharger on my 89 with a set of custom intake cams (intake valve open 15-20 degrees beyond BDC during the compression stroke) and convert it to a miller cycle combustion.

The increase in power and thermal efficiency are tempting.

I also intend to boost the engine I'm building for my 81 at some point in the distant future.

It has a 85-86 shortblock with 951 rings, flycut pistons, 81 sump and pickup, along with modified US 81 heads (bowl blended to 100mm, port matched to a euro S2 manifold, 951 intake and sodium exhaust valves).

It's complete overkill for a NA engine, but I want it to last. This is also the engine I want to put knock sensors on that prompted this thread.

 

You could have solved this entire issue.....

When I build '85/'86 5.0 engines for use with 2 valve heads, I always cut the new valve reliefs so that the 5-8 pistons end up with the original exhaust reliefs on the top, when the pistons are installed.

This allows the pistons to be installed with the offset problem corrected and eliminates the piston slap issue, completely. if you'd done this, knock sensor programming would be possible/much easier.

If you've already modified your pistons, without taking this into consideration, buy 4 more pistons and have them modified so they have the correct offset.

 

Let me toss out my logic about why I disagree and see if any of what I think makes any difference to your opinion:

1. Knock sensor "ability" clearly existed in 1985/1986. (Was used on production engines for the 944 Turbo engines, in 1986.)
2. No knock sensors on the 1985/1986 928 engines (I believe because of the problem of differentiating piston slap from knocks.) Note that the ignition timing is very conservative on the 1985/1886 928 engines because they had no knock sensors (Which is why PorKen was able to "crank" a bunch of extra timing into some of these engines andincrease the power output.)
3. Knock sensor system adapted for use in 1987 928 engines.
4. Porsche redesigned the pistons in 1987, so the pistons all had the correct offset. Just out of interest....after 9 years of production of the 928 engine, does anyone have any reason why Porsche would spend money and "bother" to do this, for any other reason other than reducing piston slap? Anyone? Anything?

 

As I wrote earlier, that accidental use of 86 S3 pistons in an 87 S4 turbo conversion engine completely satisfies me on the question whether the 87 S4 knock system works with 86 S3 pistons. It does.

As to why Porsche decided to do what they decided to do, reading dead people’s minds is difficult. It seems to me that all the 944/928 models got knock sensors about the same time, the turbo first in 1986 and normally aspirated engines the next year. There was no piston change for the four cylinder engines.

From the engineering perspective, S3 has equal length intake runners while S4 has unequal length runners to package the intake manifold into a smaller physical envelope. That would be a good reason to have cylinder specific knock retard.

As far as piston slap goes, there are other reasons to make the engine quieter than helping the knock sensing system.