suckho

Does anyone here have first-hand experiences of using Wössner 10.5 compression ratio pistons in stock SC engine with original K-Jet and original cams?

Any problems expected? What are the differences to stock? Any special adjustment/mods needed?

My car is late RoW (930/10, 204hp) so it is 9.8 CR stock. I am looking for a set of replacement pistons and I am now wondering if those Wössner pistons would be a good match. They only sell 10.5 and 11.5 CR models, not the original 9.8 CR. Mahle sells original, but you can only buy a full set of cylinders and pistons.

AndrewCologne

AFAIK Woessner Pistons with 10.5:1 CR do come with symmetrical domes? IIRC with single plug you should stuck with wedge shape/dome pistons cause the flame front would be suboptimal ... well at least at high revs.
Maybe others will correct me.

suckho

They seem to look like this:

AndrewCologne

yes, the almost symmetrical ones.

i.E. the 10.3:1 Max & Moritz ones come with wedge shaped domes.
https://www.fvd.net/us-de/1001030320...lzen-22mm.html

suckho

Got my set of Wössner pistons from Design911. They look so good!

AndrewCologne

Lets see if/how beside good looking they will perform well at all revs in you setup. Keep us posted.

suckho

Yes, exciting times! Sure, I'll post my first-hand experiences when it is up and running again.

suckho

First smokes taken already.. Tomorrow we'll know more. Exciting times!

suckho

Here is how it runs now after rebuild. Sounds ON!

Porsche 911SC - Test Drive after engine rebuild


The engine specs are:

• 930/10 (RoW, 204bhp)
• Dansk pre-74 stainless steel dual-in-single-out exhaust
• Wössner 10.5:1 CR pistons (stock 3.0 95mm bore)
• Stock cams
• Stock Bosch K-Jet injection and ignition
• Fully rebuilt: crank&rod bearings, heads, valves, etc.

AndrewCologne

Really good video for sharing your rebuild result!

As these Pistons come with 10.5:1 compression, ...

- what are your ignition timings? Same curve as before?, means distributors mechanical and vacuum advance with 930.10 specs?
- Cam timing at 0.9-1.1 (930.10 factory default) ?
- What gasoline are you running? 98 RON?

suckho

Thanks! Timings are set to factory spec. No tweaking to curves either. Mixture was checked/adjusted by my mechanic using broadband lambda while test driving it. I don't have numbers on that.

I always run on 98E5 (RON) (or similar, e.g. Shell V-Power). No problems at all!

AndrewCologne

Must be a noticeable stronger pull with that 10.5:1 CR.
Just always listen for possible detonations with that CR – but maybe everything matches just fine and you now got what your work deserves: i real fun making engine and car. :-)

Greets to Finland

Porschelarry

Thanks for the video, appreciate the information. Sounds great.

suckho

Some initial thoughts after the rebuild with these new pistons.

• Starts beautifully, both cold and hot.
• Engine runs very smoothly and idles beautifully.
• Sound is smoother, more even, and some mechanical noises are gone (1)
• When lifted on second gear, not that excessive pops and burps in exhaust anymore, but still some (2)
• Pulls beautifully on full rev range, responds to throttle really well.
• Biggest differences on lower/mid rev range rather than on high/peak revs (it was no slough before on WOT)
• The only problem is minor jerking when on low gear and pulling on low stable revs in low speed (3)

(1) There was some rattle like noise on lower gears (mainly second) on engine braking. Also, valve train sounded a bit noisier before. These might be related to the broken valve spring in cyl#4 intake.
(2) Expected, acceptable and even desired on pre-74 sport exhaust without pre-muffler. However, again that broken valve string and deposits in heads and valves might have caused some extra unburnt fuel leaked into the exhaust?
(3) Somewhat expected, and the engine has always been very sensitive to K-Jet adjustments in this sense. Need to experiment a bit more on different ignition timing/mixture settings next spring.

It would be really interesting to take the car on dyno now to get the actual numbers. Also, I am somewhat tempted to try Classic Retrofit CDI+ ignition box.. Not sure if it would even further improve it, especially on lower revs outside the happy zone?

In general, I am really happy about the end come of this rebuild! Of course I can't comment on reliability of the Wössner pistons (be back in 10 years or 200000 km later?) but based on the initial 350km I can already recommend them as an worthy option if you need to replace your pistons to stock car. That said, probably you'll get good results and similar significant improvement to worn-out engine even with the standard compression Mahle pistons as well. But the 10.5 compression not a problem here.

PS. Couple of remarks about the rebuild.

The car is 38 years old (1982) and has 261000+km in odometer. I have some information that this car has had (at least some) valves replaced in circa 1990 (due to money shift) in professional service, but can't confirm that info. Otherwise the engine has not been opened before. I have owned the car for 7 years. I have known that it needs rebuild (at least heads and valves) soon, but the final trigger was the piston failure in cyl #2. Luckily, piston fail didn't cause any other damage and all pieces were found from the bottom of the crankcase. Anyway, now it was the time to bite the bullet and do a full rebuild. The only clear flaws in addition to the broken piston was the broken intake valve spring in cyl#4. Otherwise everything was in good working order, though pistons, valves and heads were already really dirty of deposits. Valves were also worn out and their seals were hardened and misplaced. Crankshaft and connecting rod bearings were all original and in perfect order. My mechanic told that the engine could have been put back together with the original parts. Besides the Wössner piston set, only new parts in addition to the normal service parts (gaskets etc) were the new valves, guides and their seals and the new bearings to crank, rods, and ims. Heads were blown clean and machined. Engine tins were also blown clean and painted.

AndrewCologne

Yezzz, ... it would be very interesting, ... especially the torque curve how it has been changed along the rev bandwith
AND here a dyno report is the best invest ... as beside performance report its a good approach to check the whole rev range for possible occurring detonations cause of a risky CR of 10.5:1!


Its a very good reliable most modern CDI alternative which mainly gots its benefit with higher voltage sparks at high revs where the OEM version from Bosch ends up with a voltage drop, ... here you should get your own opinion and check reports from others in the www regarding how CDI+ improves performance, and especially if its worth in your case if your focus is on the mid rev ranges. ... just get in contact with Jonny from Retrofit, .. a nice guy who will answer your questions.
Another plus is that with CDI+ you can adjust the ignition curve via software, BUT in your case with that high compression ratio I would use such an adjustable ignition only to optimize the spark advance/retard in a way with focus on combustion optimization for better engine life rather that on even more power.

If you will order from Retrofit directly you should be aware that now in the next weeks you as EU member still won't suffer from additional taxes and VAT which will come up from Jan 2021 when UK has left the EU. But I guess Retrofit also has partnerships with other distributors in Europe where you could order from.