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Skimming Block Till Pistons Stick Out - Cr Calculation Help


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#1 BaronVonchesto

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Posted 14 June 2019 - 11:45 AM

So i'm fitting 998 +40 thou dished pistons and trying to calculate how much to skim my block to get a CR of 9.2

 

Using C-AHT88 head (23.5cc comb chamb vol according to minispares) & payen head gasket (2.8cc vol).

 

Bore rad: 32.78 mm

stroke = 76.35 mm

piston dish vol = 7cc

ring land vol = 0.75cc

head gasket vol = 2.8cc

comb chamber vol = 23.5cc

 

that gives a USV of 34.56 and SV of 257.74

Which means a CR of 8.46

 

at TDC the distance from the block face to the top of the piston is 0.15mm.

 

I made an excel file to figure out how much to skim but that tells me i need to take 1.05mm of the top to get a CR of 9.2.

 

The problem is, if i skim more than 0.15mm, then the piston will protrude through the block into the combustion chamber, meaning that the unswept volume will also change now. This also means that my excel file is no longer correct in giving me the CR.

 

if the piston protudes into the combustion chamber at TDC, then the combustion chamber volume will have reduced. but the complex shape means simple geometry cant be used to calculate the new comb chamber vol right?

so how on earth do i calculate how much to actually skim?


Edited by BaronVonchesto, 14 June 2019 - 11:49 AM.


#2 Cooperman

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Posted 14 June 2019 - 12:20 PM

You need to have the deck of the block skimmed so that the pistons come to between flush and 0.005" down the bores at TDC.
You then calculate what the combustion chamber volume needs to be and skim the surface of the head to achieve this.

#3 unburntfuelinthemorning

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Posted 14 June 2019 - 12:23 PM

Have the block skimmed anyway so the pistons are flush.  You'll need to then remove material from the head.

 

You want a CR of 9.2:1 so you need the formula:

 

          SV               =  USV

CR required -1

 

 

SV = 257.74   CR = 9.2

 

 

257.74     =   31.43

9.2 - 1

 

 

Piston dish, ring land, head gasket volume = 10.55

 

31.43 - 10.55 = 20.88

 

So you need a combustion chamber volume of about 21cc.

 

Basically you need to fill the combustion chamber with 21cc fluid and measure the distance from the face of the head to the fluid to work out how much material needs to be removed.



#4 gazza82

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Posted 14 June 2019 - 01:45 PM

Is there a limit on how much you can skim off that type of head? I know some it would break into the water/oil ways if you cut off too much.



#5 BaronVonchesto

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Posted 14 June 2019 - 01:50 PM

thanks for this guys! i'll bring my head to the machine shop and do just that.

 

slightly seperate question? will RON 95 run well on a CR or 9.2:1? based on various charts i've studied that seems to be about the limit you can push RON95 before predetonation is a problem.

 

i'm sure theres a sweet spot somewhere where you get the CR as high as possible without having to advance the ignition so much that you actually start losing power?



#6 BaronVonchesto

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Posted 14 June 2019 - 01:56 PM

also the minispares website says that valve lift is 0.475" (12mm), though it doesnt state what rocker ratio is used for this. I'm assuming it's the standard 1.3?

 

i'm using the "cooper" forged rockers and a EVO 01 cam.

need to make sure the valves won't make contact with the pistons



#7 BaronVonchesto

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Posted 14 June 2019 - 01:59 PM

Is there a limit on how much you can skim off that type of head? I know some it would break into the water/oil ways if you cut off too much.

 

that's a good question! I do not know!

 

it's cast from a late series A+ head. casting number is CAM4180



#8 GraemeC

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Posted 14 June 2019 - 03:20 PM

The other option is to use pistons with less (or no) dish



#9 unburntfuelinthemorning

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Posted 14 June 2019 - 03:41 PM

thanks for this guys! i'll bring my head to the machine shop and do just that.

 

slightly seperate question? will RON 95 run well on a CR or 9.2:1? based on various charts i've studied that seems to be about the limit you can push RON95 before predetonation is a problem.

 

i'm sure theres a sweet spot somewhere where you get the CR as high as possible without having to advance the ignition so much that you actually start losing power?

Standard factory 998cc cars in the UK designed to run on 95 RON unleaded had about a 9.6:1 CR.  With a longer period cam than standard you can go a little more.  Also bringing the pistons up flush to the block deck staves off detonation (even though the CR increases) by increasing squish and therefore turbulence in the cylinders.

 

With 95 RON fuel you're going to have to have the ignition timing retarded from optimum in some parts of the rev range if you're running a half decent compression ratio.  Often the 2000 to 3000 rpm range is where that can happen when operating at a wide throttle opening.


Edited by unburntfuelinthemorning, 14 June 2019 - 03:42 PM.


#10 BaronVonchesto

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Posted 15 June 2019 - 07:42 AM

It comes to 0.74 mm that needs to be skimmed, so ill just have it skimmed to 0.70mm.

Thats not too much is it?

#11 ACDodd

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Posted 15 June 2019 - 08:18 AM

The question is how thick is the deck on the head? Some were very thin, and any skimming causes sealed by issues.

My option woud be skim the block so the pistons pop out but around 0.005 to 0.008". Then skim the head to 21.5cc.

Ac

#12 BaronVonchesto

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Posted 15 June 2019 - 10:49 AM

The question is how thick is the deck on the head? Some were very thin, and any skimming causes sealed by issues.

My option woud be skim the block so the pistons pop out but around 0.005 to 0.008". Then skim the head to 21.5cc.

Ac

 

is there a way to measure this? and any guides to suggest if its too thin?



#13 ACDodd

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Posted 15 June 2019 - 11:00 AM

Look through the water passages on the deck face and see how thick they are. Also measure the oil way depth.

Ac

#14 Cooperman

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Posted 15 June 2019 - 12:49 PM

Best to use flat top pistons, then set the combustion chamber volumes to be correct, which may mean grinding material out from the chambers.

With dished pistons in a 998 getting a high CR can be an issue with skimming the head. Personally I have never liked pistons protruding from the block deck at TDC, although a few thou protrusion shouldn't be any sort of problem.

 

However, a CR of just over 9:1 should not be a problem so long as the pistons come right to the top of the bores and the chambers have not been excessively modified.



#15 BaronVonchesto

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Posted 15 June 2019 - 01:08 PM

at this point i'm not really keen on getting flat tops, i'll measure the oil and water ways if i can, though i did stick my finger in them an felt a fair bit of metal, so I think it should be okay, but best to measure it like AC dodd says.






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