924Board.org

[arunas]

Checked with ideola's compression ratio calculator:

Volume in head 19.5cc (personaly unchecked)
Gasket bore 88.0mm
Gasket compressed 0.046 inch (1.684mm)
Deck height 231mm (my block)
Cylinder bore 86.5mm
Stroke 84.4
Rod lenght 144mm
Piston compression height 40.8 (my euro specs 924 N/A)
Volume in dish 14.5 (personaly unchecked)
Results CR=8.7:1

Checked with ideola's compression ratio calculator.
Isn't to good to be true?
Do euro spec pistons have the same volume in dish and compression height as us specs?

[ideola]

I don't think 40.8mm is the correct compression height. You have to take HALF of the wrist pin diameter (12mm), and then measure the distance from the top of the wrist pin bore to the crown of the piston. AFAIK, *all* of the NA pistons have the same compression height. The difference in CR came from changing the dish volume in the piston. With all of the other specs you plugged in, the CR will only be 7.8:1, and on top of that, you end up with less than ideal quench.

If you want higher compression 931 configuration, just use S2 Euro-spec 931 slugs (nominal 8.5:1 CR); with the 231mm deck height, that gets your CR up to 9.2:1, and you have a much better piston profile than using the NA slug.
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erstwhile owner of just about every 924 variant ever made

[arunas]

28.8 + 12 = 40.8

[arunas]

My goal is to use N/A pistons and cut top of block to achieve CR = 9:1 - 9.3:1 , to make it cheap and avoid problems of getting parts if it will brake. S2 pistons wuold be not that bad, only thing i don't like is short side skirt and REALY tall crown for nothing...(or to make piston unstable in bore), compare to N/A piston.


931 (S1) piston VS 924 N/A piston

[ideola]

You think that's a short skirt??? Modern piston designs have much shorter skirts than that for weight savings.

And the tall crown is not for nothing, it's to provide proper quench for the turbo-charged application, which is ideally less than 1mm. If you cut the block too much, the piston end up proud of the deck at TDC, and that's not a good thing for the head.

If you want 9:1 CR, I say stick with the proper turbo piston, skim the block to 231 (but no lower), and run a thinner Cometic. While OEM rings aren't available, we now have TotalSeal rings available on individual order basis for very reasonable price.

On the other hand, give it a go and see how it works out!
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erstwhile owner of just about every 924 variant ever made

[ideola]

_________________
erstwhile owner of just about every 924 variant ever made

[arunas]

Ideola: And the tall crown is not for nothing, it's to provide proper quench for the turbo-charged application, which is ideally less than 1mm.
Diamond, and other new generation turbo charged engine (as example audi AEB) piston doesn't have tall crown like 931. I agree with short skirt ''style'', but why in conjunction with so tall crown....


I think part of piston between red lines is worthless.



My opinion: as high piston pin is in piston, that much piston is beter design. (sure not at all) That's visualization of that what i want to say,
IF WE HAWE OLD STYLE SAME SIZE PISTONS

Hope that pictures will compensate my poor language-communication skills

[arunas]

Ideola: If you cut the block too much, the piston end up proud of the deck at TDC, and that's not a good thing for the head.
If i use N/A pistons, i need to cut only 0.5mm, to get CR~9.2:1. So my block deck clearance will be still as much as 3.5mm. Eeven if my rod bearings will totaly fail, piston at TDC will newer touch head.
What do you think about 40,8mm compression height, is it rare or something?

[ideola]

[arunas]

Sure i will check volume of head and piston before i touch my beefy block

[arunas]

Euro 924 N/A volume in piston dish 19cc
Euro 931 S1 volume in piston dish 47cc
Euro 931 S1 volume in head combustion chamber 21.5cc
(i used medicine 24cc syringe, organic glass plate, water, and a little bit of sealant)
Had no leaks for sure.

I opened another euro 924 N/A engine and found that block deck height is same as my 931 231mm. Piston compression height was also 40.8mm. Deck clearance ~4mm. Engine had original head gasket, and hadn't any signs that it was opened before.

With compression calculator i have got CR=7.7:1 for 931, that's plenty right.

But for 924 N/A results were CR=11.0:1 I am confused...

[leadfoot]

With both the turbo and N/A both using the same block,rods and crank the pistons have to have different compression heights, as the N/A sit down in the bore and the turbo are just about flush.
Do your n/a pistons have the right part number?
Stu
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1981 ROW 924 Turbo -
carbon fiber GT mish mash
LS1 conversion in progress...