924Board.org

[Racing]

Welp.
Read your reply in the innovation post,and you wanted to shoot something with me.
I´m all ears.

Altho i´m not much of a "spill all guts guy",i´ll make an exception to plant a "ground zero" in my case as i haven´t really introduced myself yet.

40yr old ex race mech.
Used to make a living hauling around with the F3/F3000/Opel-Lotus aso gang around europe.
These days(since 7 yrs back) run a speciality shop here in Gothenbrug Sweden.
Have dropped all when it comes to circel track.
Used to work as a set-up tech and then as crew chief.
As circel track takes some real money and time,and i still like the heat of racing,i since the same 7 yrs compete from time to time in national Swedish dragracing.
Build an engine or two,and have done a lot of work as far as engine management.

Used to work a lot on Porsches in my earlier yrs,and then more to the point the 911,s.The 931 i own currently is my 4th over the yrs,and frankly it´s only seen duty as a winter beater so far-albeit about to change.

Apart from the Porsche i run several detroit irons(yeah well shoot me...dragracing you know ),and amongst those..
Here´s an example of my 495 cid CBB that has run in the mid 8;s so far.
Have put it in a streetlegal chassis this time out,and hope for mid 9;s being traction limited.
EFI is a beta Haltech that runs a modded Hilborn 65mm setup btw.


As far as the 931 combustion chamber,as i presume that´s what you wanted to get in depth on as far as thoughts and ideas,i´ve as stated run into some heavy duty problems when it comes to detonation.
Seing other more viable designs as far as bath tub chambers go,i jsut can´t get through my head how the stock 931 design´s supposed to work when you get into mid territory boost situations.
As of current we limit boost to 25 psi.

..and then the ball and court is all yours..

[John Brown]

Thanks. Essentially I hoped you would educate me on the shortcomings of the piston design you mentioned. From your comments it appeared you know your stuff (and now I know why).

6 hours later. Did some internet research here trying to come up to speed. Maybe I'll come close. Think your original question towards Porsche decision to use the Heron style in-piston combustion chamber. My reading would seem to indicate that it would have been mostly a cost decision driven by the design of the parallel valve stem single cam heads? Good enough for the power levels they envisioned?

What are the options for us now in terms of mods? Given that it's pretty common to have pistons made; what could be done with the piston to improve things?

Thanks
_________________
John
80 931 - #931 44Cup
99 Escalade - tows track cars
gone but not forgotten: original 924.org car - 82

[Racing]

Yeah,i guess that i´ve run into sort of a power "roof"- with the OEM design at least.

A heron design-at least the ones i´ve been into(VW-Volvo asf) all have in common that they produce squish with a completely flat head surface(cylinder head that is)
That,is something i can understand and relate to.
The main purpose of squish is to make the emulsion mass travel towards the spark plug on compression AND to create turbulence to keep emulsion in suspension.
Cool enough.

Now,we can also agree on that basically any and all sharp corners and such within a combustion chamber is basically just a leading point for detonation.
Cool enough.

As i see it the prob with the OEM Porsche design is that they have combined both the heron and the "common" way to create squish,leaving a combustion chamber that obviously gets "creative" and induce sharp corners nooks and dingleberrys all over the place.
In fact when you simply view the idea by putting one of the OEM slugs over the combustion chamber face,it´s so far fetched that at least i start shaking my head.From my experience there´s no WAY that design will tolerate especially much as far as high output goes.

What really gets my attention is that when i´ve seen other Porsche designs(most of the actually)i´ve always been left with an insight and feeling that...*WOW*
The guys at zuffenhausen/weissach know their crap.I´ll NEVER get into argueing that!
Or..put another way.
If a design like that was to come from some other schmuck company i´d just re-engineer and be done with it already.

What can be done you ask.
Well...if you look at other designs of similar capacity and supercharging-like the old Volvo 531 aso- they indeed run a similar shape HEAD but flat top pistons.(In other words they HAVE a combustion chamber within the head)
Ergo,none of us on here are die hard engine designers so why not take advantage of what´s proven.

What at least i have done this time out(have taken the OEM design CC engine apart 4 times since oct due to...)is to implement what i know.
Flat tops...
What i´ve done-being the cheap bastard i am-is to scrounge through the Mahle main catalogue to find what i need.
Now have in mind that my engine is a series 1,rendering 45mm compression height vs the series 2 -41mm.
What i found was a forged flat top mass produced slug for an 87mm cylinder and with a 39,6mm comp height when hotroding a VW bug.
Ring height is more nominal at 2/2/4 with a chrome top,moly 2;nd and common oil scraper.
Piston is "fairly" massive-seing it as a stock item- in the crown on 6mm;s.
Pin is 22mm,which by me is alright anyways as that mod had already been done for the GTS Kolbenschmidts that were the last ones to go AWOL.

So..what does that leave us as far as problems then-from a simply logical standpoint within the 931 engine?

What makes me think that i can get away with it is the downright tank like design of the block.Especially so in the deck area.
If you get to take a look at the deck of the 931 block you´ll see that it´s;
1/Have cylinder walls that would make ANY smallblock guy or whatever green of envy.
2/A reinforced area about 30mm down the bore where the dimensions are dowright ludecrous.I´d say they wall is about 10-15mm thick in that area.
3/Bore spacing is fairly tight.

What i´m doing in practice is using the first 5+mm of the bore for combustion chamber as that´s how far down the bore the slug is at TDC.
I´ve also minutely laid back the edges of the OEM CC in the head.

To make the "split"(where the head mates the deck of the block) stand up to the high pressures of a mid boost turbo engine is have same friend that runs engine shop cut me a head gasket out of solid tempered copper at a thickness of 2mm.
Another confusion in all of this is that when calculating "new"comp ratio the OEM head volume was speced at 20cc.
At least not so in my head-that speced out to 27,5-28cc;s.

Anyways.
I´ll end up with around 8:1.
All good in my book.

[Racing]

John.
Take a look at the "speaking of pistons" post in tech.
Found them last night while going through Peters site.
Could say it looks like we´re on the right track doesn´t it?

[Racing]

I´ll call Peter and ask him whos motor that is,and how modified it is.
Also ask for power figures and other relevant info.

[Zuffen]

where are you at now with your tinkering?


any pics
_________________
Bob Dodd - 924turbo@cox.net
931 1982, 944 1982 euro, 924S 1988SE, 93 968 tip 06 Silver Cayenne S, 06 Black Cayenne S

I have Way too many cars, parts for the 931,944 and 951

[Joes924Racer]

Hey bob isnt it cool to have racings talent here.
Thasnks Racing. Its pretty awesome...
_________________
1979 porsche 924 Na
1980 porsche Turbo 931GT Replica
Have u ever driven a turbo.

[Zuffen]

Yes it is...
I've wanted to play with enlarging the dish in the head a bit more and loosing the dish on the piston
_________________
Bob Dodd - 924turbo@cox.net
931 1982, 944 1982 euro, 924S 1988SE, 93 968 tip 06 Silver Cayenne S, 06 Black Cayenne S

I have Way too many cars, parts for the 931,944 and 951

[Racing]

Basically what´s writen above turned out to work IRL too.
Ie;the flat top 22mm piston pin Mahle works like a charm.