as most of you may know (and I recently discovered), increasing head gasket thickness or increasing deck clearance on a quench designed motor can actually lead to increased detonation...some motor builders claim anything over .060" total squish (gasket & deck clearence) can SEVERELY INCREASE detonation....http://www.speedomotive.com/Building%20Tips.htm

potentially bad news for the stroker crowd trying to lower compression for a bit of extra FI HP...

this only leaves reducing the stroke/volume while keeping the same compresson ht or using a much larger dish (50cc) to lower compression for FI motors starting out at 10+:1 compression (strokers etc)....so much for just "slapping in" a lower compression ht piston....yep, even the angles count....

anyone know what the total squish/deck clearance limits are for the 928 or if it is squish sensitive?

as always, thx for any and all of your squishy/sensitive comments...

 

Rob the 16 valve head / piston is probably much more sensitive due to the offset location of the spark plug and flat top pistons . The 85-86 32 valve has a medium dish but with a pentroof (nearly hemi head) with the spark plug in the middle . The 87> piston is almost fully dished (no squish ) and spark plug centered on a shallower pentroof (Hemi) . The 32 valve is a crossflow head with big ports and valves and alloy to resist detonation ........ this is not your father's Oldsmobile ...

 

I must be 1 of the few that didn't know..... sounds counter-intuitive to me.
How would thicker hd gasket/ lower compression lead to detonation?

dan z

 

The link Rob provided in the initial post has a very good explanation.

 

thx jim & lag (yet again!) don't feel bad zman - although I occasionally can correctly parrot what I've been told or read, even w/an engineering degree - HP motor design is riddled w/counter-intuitive phenomena...theory and religion get thick, and even a slight materials change can often cause dramatic & surprising results...

thats why when there is a generally accepted theory in motor building, it may not apply to every make, model or combination of stuff - hence my sensitivity question on the 32V w/S4 heads....some motors are more "resistant" than others due to materials, design, etc...

I just wondered if anyone knew the limits for this particular situation, ie had run ok w/.060" or more deck clearance on an S4 head w/104mm bore, etc...

In fact, I suspect that unless the head gets opened up to match the extra piston surface area, and the dish gets re-matched to the head for an OEM type total combustion chamber profile, more squish and all kinds of weird stuff CAN result - it's just determining where the theory meets the metal that is usually the big question....

probably will just hog out some more on the pistons & call it a day, although some top 928 builders seem to think it would be ok for a bit more squish volume via more deck clearance...would be a lot easier...

overall lowering the piston ht/increasing deck clearance/bigger head gasket is not the theoretically ideal way to drop compression due to combustion/compression pulse inefficencies - although some builders claim better turbulance/mixing w/more squish volume...lots of little trade-offs only can be settled on a dyno or w/lots of testing...

although entertaining at times, meddling w/Dr. P's stuff, a stray cat, or mother nature is never considered "safe" fun...

 

So, what is the quench height on the 16V/32V/S4 engines?

 

0.0 is the number I've seen banted around for 928 deck clearance, S4 comp ht is 2.2047", rod is 5.9095"+/-, stroke is 3.106"/throw=1.553", or 9.6672"/245.415mm total deck height if the clearance is 0.0...

have to look at gasket thickness (.039"/1mm std), gasket dia, and area (dish/chamber dia - piston dia) to figure squish volume...

it's different btwn stock S4 and a custom 968 and S4 head combo...guys claim to be doing it, but details are engine builder trade secrets at times...been tricky to determine who's strategy is best, and there just haven't been a lot of these built either (ie over 100/500/1000 etc)...

using the much touted 95.25" stroke, w/104mm, 1.428" 968 comp ht pistons, it's possibly looking like I'd need a 6.35 rod, w/the 3.75"/1.875 throw for the 9.66 deck ht, but - I WAS thinking about offsetting the wristpin a bit or a shorter rod/stroke to lower the piston for less compression...

still awake? then just dig out the calculus book for some old fashion volume under a curv junk, unless someone has it handy or it really just won't matter...or you fall asleep re-visiting integration calcs...

lots of junk to wade through on this one...or just hand lots of money over, close your eyes and hope you don't get a 30 hour mtbf monster paperweight...

 

No easy answers ... yet ... til Rob figures some of it out ;-) Well I don't have it in me to calculate that schtuff ... at least not without my handy-dandy doctorate, which I'm not about to go get

 

I've read 45/1000" quench pad clearance as a minimum rule of thumb. Start swinging bigger slugs longer ways and I'd worry about the piston kissing the cylinder head. I also read that keeping this clearance as little as possible is EXTREMELY important in detonation control.

 

just to be clear, that was not specifically aimed at a 928 rule of thumb. (We all know these engines are made out of special alloys not available to other manufactures.)

 

Lifted from another discussion "Quench, or squish area is typically the flat area on the top of the piston that's almost level with the top of the block deck. It must have a corresponding flat area on the deck surface of the head to qualify as quench. " This was my point from a much earlier post the 32 valve engine the 85-86 has almost ZERO quench area the 87> even less as the piston has a much bigger dished out area . This discussion has little to do with the Porsche 32 valve engine because there is almost no Quench !! Why ? in my opinion , not NEEDED the centrally located spark plug and 4 valve cross flow head eliminated the need to shove all the gasses to one side just to be near a spark plug .

 

add that to your head gasket thickness. As Jim said, with so much dish in the piston the squish (in the S4 atleast) isnt nearly as critical as it could be if you were dealing with more flat surface area . However, to be safe and to leave "well enough alone" when i machined my pistons i kept the same amount of squish and just altered the slope of the dish to increase the volume slighlty. If my calcs are right i should be around 8.75:1

HTH

 

thx guys! this is just so much better than talking to myself in the bathroom... tony's work/pics are always a big help & SOOO cool... (no, I don't bring those into the bathroom though)

did some research on my own - apparently, w/S4 heads, and w/my 104mm, "stepdown" Finite Element Analysis designed pistons (FEA: let the supercomputer do the work!) - there is NO squish (I know Jim, you've been trying to pound that into my thick little head from the beginning - but I'm SOOO slow sometimes)

apparently, unlike dad's olds or even Dr P's 32V successors, the perimeter of my forged JE has an angle - eliminating squish while creating more turbulance/atomization & the "stepped down" flat bottom promotes better flame travel and complete combustion - guess the quench thing got handled somehow... (nope, I didn't test them yet or figure ANYTHING out- I just regurgitated what JE, KB, mahle and manley say)

don't worry tony - I don't think dished pistons will stop working all of a sudden & shortly after I get my motor running, I'm sure someone will verify your design is actually better - after the FEA guys upgrade their software - just like it always rains after I wax my car...

http://kb-silvolite.com/index2.php has some good examples & helpful engine modeling calculators....

also "discovered" some cool stuff about piston design improvements since our 928's were built - http://www.babcox.com/editorial/ar/eb40354.htm
an interesting read for anyone considering an overhaul....

thought it was odd that although mahle was mentioned as a leading piston designer - they only offer OEM stuff for the 928...(maybe THATS what the custom group does for $275/slug?)

sure gives me a lot more respect for state of the art HP builders (ie keith black, greg brown, phil threshie, etc)...probably should stop 2nd guessing them to death - someday...

as a side note from my travels, I "discovered" that some OEM mahles had offset wrist pins - presumably to aid w/thrust/side loading... but some motors have had the same pin offset on both banks - so 1 side will have a "proper" offset, and the other side will be just the opposite....think I'll address that one w/bushing offsets and centered wrist pins...

interestingly enough, I haven't read or heard of this being an issue in terms of premature piston failure - but it might explain some scuffing issues...and it does help the case to upgrade the pistons/bore during an overhaul or prior to going boost crazy (if less friction, more power, better heat dissapation, and saving 3200 grams (pistons:453g vs 750g OEM & rods 600g vs 700g OEM - per cylinder!) of recprical mass doesn't get your attention).... of course, with tony's setup, I'd have $$$ to burn in vegas...

as always - thx again!

 

That is not the only function of the squish area. For example, one of it's most important jobs is to provide mixture homogenization which mininimizes localized hot-spots/lean-areas which can flash just before the controlled flame-front reaches it; the 'flashed' area then collides with the controlled flame-front. This phenomenon is called detonation.

This statement is flat-out wrong because the 928 32v head does have a squish area. Just because it may not be as large as a 2-valve wedege head doesn't mean that it's role is diminished; if anything, it's role is even more critical due to it's smaller size, and thus having to do more with less.

Ford with their kick-*** 4.6L 32v engine has constantly tweaked the squish areas on their various heads they have produced over it's current 9 year lifespan. Some of them resemebled clover-leafs as the squish followed the contours of the 4 valves, while others resembled the 928 32v where it didn't follow the valves.

On a side-note, people remark and say that they would consider dropping a Chevy engine in their 928; however, I think a more interesting candidate would be the 4.6L 32v Ford. The STOCK 2003/4 Ford Cobra engine is good for 1000 hp at the crank from the factory; iron block (..closed deck, siamesed cylinders), steel crank, Manley rods, forged pistons. All of that straight from the factory with a modern programmable computer to boot, all one has to do is up the boost!

 

Rob the offset is really just to limit piston slap and it is not a large amount and as you point out one bank is "backwards" . It is not significant in my opinion .......... not worth fixing .