Skimmed pistons

[S2central.net]

Good point Marco - its only a matter of time until someone here has such a seriously damaged head that it could be cut open thru the chambers to see wall thickness. Thats the stuff that scares me with future thoughts of porting and polishing the head for increased flow capacity.

Paul

[S2central.net]

Marco - help me with my maths here - my values disagree with yours, so I must be making a mistake or forgetting something stupid - electronic systems is my bag.

The basic formula I have for compression ratio is (Unswept volume plus cylinder volume) divided by unswept volume. Maybe that is too simple for this

Assuming bore and stroke of 81.0mm and 86.4mm respectively, I get cylinder volume of 445.22.

To derive unswept volume needed to achieve CR = 9.3 I get 53.65cc
and for CR = 9.0 (with the metal gasket) I get 55.65cc

If I add 4.45cc of chamber volume to that figure I get a CR of 8.41.

What have I missed ?

I do have a book on engine blueprinting that covers this, but its ages since I read it.


Paul

[Pisobiker]

You are absolutely right Paul.

Did the math again and it is exactly what you say.
I used part of an excell sheet with much more complex math I also used for other purposes. Now need to dig out where I screwed up the formulas in it

For dropping to 8.7:1 from 9.0:1 it only takes ~2.1 cc negative volume increase.

......


Marco

[Doug_RS2]

http://www.s2forum.com/forum/showthread.php?t=2530&highlight=fao+lee

Erm - I think I did my calcs wrong for reducing our CR - I calculated 4.2cc removed from the piston so we would be at 8.7:1 but I used the 9.3:1 instead of 9.0:1 - Ah well - maybe Mine, Marks and Sals cars will be running closer to 8.4:1 CR instead of our estimated 8.7:1

I need to check if JE took 4.2cc off or reduced our CR to 8.7 - maybe all the skimming of our heads might take some of this into account.

I'm confused and been thinking about this too long - it works anyway, although a bit lumpy off boost.

[Doug_RS2]

Marco - I think you might have used 9.3:1 instead of 9.0:1 like I did?

[Pisobiker]

Marco - I think you might have used 9.3:1 instead of 9.0:1 like I did?

hmmm,
sheet now works, and if using 9.3:1 it takes 4.18 cc to get 8.7:1, so I must have mistyped a number of something like that...

Anyway Doug, taking off lets say 0.3mm total from head and block and running 81.5mm bore, the 4.2 cc removed from your pistons should keep engine in the 8.7:1 range


Marco

[Doug_RS2]

Anyway Doug, taking off lets say 0.3mm total from head and block and running 81.5mm bore, the 4.2 cc removed from your pistons should keep engine in the 8.7:1 range

I knew that Thankfook MarkH might not kill me now

[Mark Halligan]

Doug,

no worries, and remember I had almost 0.5mm taken off of the head so it wont be a problem.
Besides yours goes well enough, at least it did when I was in it .

Cheers,

Mark.

[S2central.net]

Ahhh - so I'm not going insane then. Thats a relief... I've now found the blueprinting book and it confirms my formula with a more convoluted one that itemises unswept volume as chamber volume (in the head), dome/relief volume (on the piston), head gasket volume and deck height volume.

I assume we have a deck height of zero - have never measured it.

I agree with Marcos estimate on the revised CR with 0.3mm off the head reducing chamber volume by 1.5cc which counteracts the excessive 4.2cc removal on the piston to give a CR of approx 8.7.

The only way to be sure of course is to burette the bores with pistons fitted and use the '1 inch method' to calculate exact amount of CC in valve relief and dish profiles.

Paul

[S2central.net]

Mark - I reckon you are nearer to 8.9 - approximating 2.6cc reduction in chamber volume with 0.5mm skimmed from the head. Also assumes 81.5mm bore like Doug's.


paul

[S2central.net]

Just remembered Mark - you buretted your chambers didn't you - what were the results before and after head work ?


Paul

[Mark Halligan]

I assume we have a deck height of zero - have never measured it.

Funny you should ask that Paul, as I have noticed that part of the pistons is higher than the rest and slightly protrudes above the block deck, well it does on mine (I had the barest minimum taken of off the deck). I've used Marcos pic to show where. Presumably this is for squish and forces the charge into the bowl and chamber areas of the pistons and head?
I've checked the standard pistons and these too are higher at this point.

Cheers,

Mark.

[audiman]

Right, now IM REALLY confused. I see that Marcos first figures were wrong. I can see that Paul got them right(well done). For those of us that dont have the money for the JE's, what is the plan of action with a std piston then. IS it Dont take off the outer edge? How much deeper should the dish be? What diameter should it be? Does it stay central, or offset?

[sal_s2]

Where does it say that the rs2 has a cr of 9:1?
AFAIK the rs2 has the same cr as an s2 9.3:1

http://rsmodels.net/index.php?mode=cars&car_id=11

[S2central.net]

Tom - have you got burette equipment ? I'll happily help you calculate some CR info for the stock pistons but without the burette to establish the baseline then we are struggling a bit.

Sal - thats a damn good question. I always understood that all the 20V I5 turbo engines had a CR of 9.3 but somebody (Mihnea or Marco) told us that when the heavier duty metal gasket was introduced it effectively reduced CR to 9.0 by nature of being much harder to compress than the no longer available paper gasket. The theory seems to hang well in practise as its known that the metal gasketted cars can handle more ignition advance and boost than paper gasketted ones. This is a positive side effect of having lower CR. Only way to prove this is to measure the dimensions of used gaskets (assuming they don't relax much after being removed to skew the results).

Got a feeling this topic will be hot for a while - we need a burette, a wadge of plexiglass and a tub of vaseline to sort this out quickly.

Paul