20 replies to this topic

[jonb_5]

Hi

I am looking to get a cylinder head but would just like to understand the whole process a bit more.

What does the cc mean, i.e. 21.6cc.

How is this calcuated? Just wondering if it would be suitable for my engine.

I have 1330, 296 cam, omega pistons, the block has been decked 10thou.

Thanks

Jon

[orcadian]

Hi Jon,
The 'cc' you refer to is the volume of the combustion chamber in cubic centimeters (cc) The only accurate way to measure it is to fit the valves and a plug and lay a piece of clear perspex (sealed to the head with a bit of grease) with a small hole in it over the combustion chamber and then drip oil from a burette (remember those from school chemistry?) through the small hole until it is full and then see how much the graduated burette went down - this is the combustion chamber volume. It helps if the head is tilted slightly (not your own head, silly!) and the filling hole on the higher side of the chamber.

Now you have to resort to sums (don't tell me - you were off sick when they did that!):

Bore AREA (bore diameter squared multiplied by 3.142/4) times stroke will give you the swept volume

If you think what is happeneing when the engine is running, the swept volume PLUS the combustion chamber volume is being squashed into the final combustion chamber volume when the piston reaches the top of the stroke, so to calculate the compression RATIO, add the swept volume to the combustion chamber volume and divide by the combustion chamber volume.

If there are any really pedantic folk reading this, then if you want absolute accuracy, you also have to calculate that tiny bit of volume from the top of the piston down to the top ring face - forget it you don't need that level of accuracy.

Anyway, you can find the swept volume of your 1330 engine and using the info supplied by the head seller, find out what compression ratio it will give on your engine, but then decide for yourself if it will be suitable for you.

Hope this helps,
Regards

OOPS!

Again, just for the pedants - you also need to add the volume of the head gasket (bore squared times Pi/4 times head gasket thickness, to the combustion chamber volume - makes a SMALL difference to the ratio

Edited by orcadian, 28 September 2009 - 10:40 AM.

[lrostoke]

cc stands for cubic centimeters, its a measure of volume.

The only true way to find out what yours is, is to have the head with the valves facing up, a piece of clear perspex placed over the combustion chamber with a hole in the perspex and fill the chamber this creates with a measured amount of liquid. However much it takes to fill the chamber is the CC .

Thats a simplified version.

Or see the better version posted before I'd finish

Edited by lrostoke, 28 September 2009 - 10:26 AM.

[Cooperman]

Head gasket volume for a 1275 and bigger is 4 cc. For a 998 it's 2.7 cc
Don't forget that for every 0.010" the piston top sits down the bore from the block deck at top-dead-centre you have an additional 1 cc of combustion volume. I've seen pistons as much as 0.050" down the bore. Also the piston dish must be included into the combustion chamber volume. On the Hepolite 21253 piston (the most popular for the 1275 range) it's 8.4 cc.

[chazzybaby]

um... am i being dumb here but the head gasket I put in my 998 mini said on minisport that it had a compressed volume of 4cc... did i get the wrong one? or do they do a low compression head gasket for this engine? or is this a mistake on their part?

[Cooperman]

Hard to tell without the part number of the head gasket.
The basic copper-faced 998 gasket is 2.7 cc-ish. I measured it myself using a used gasket to ensure the thickness was correct.

[Spitz]

cc stands for cubic centimeters, its a measure of volume.

The only true way to find out what yours is, is to have the head with the valves facing up, a piece of clear perspex placed over the combustion chamber with a hole in the perspex and fill the chamber this creates with a measured amount of liquid. However much it takes to fill the chamber is the CC .

Thats a simplified version.

Or see the better version posted before I'd finish

I would ad one note to that....make another small hole in the plexi for the air to escape while squeezing in the measuring fluid. ( I use ATF )

[Dusky]

Bringing an old topic back : I don't really get the bore volume.. What would be the bore volume on a standard 998?

[Cooperman]

Do you mean the swept volume? The 998 is a 4-cylinder engine.

The swept volume for one cylinder of a 998 is 998/4 = 249.5 cc

[Dusky]

I dont know really, I got a compression calculator in an excel file, it asks for : bore volume,gasket volume,head volume and engine volume

[Gremlin]

I dont know really, I got a compression calculator in an excel file, it asks for : bore volume,gasket volume,head volume and engine volume

Thats sounds like a quality file, where did you get it from, they are probably talking about the swept volume when they say bore volume, and probably the volume above the piston when they say engine volume

[Dusky]

Hmm,when I use that numbers I get a comp ratio of 1..

This is a screenshot of the file ( if anyone wants it, I'll upload it.. got

it from the minifinity forum..)

Edited by Dusky, 25 December 2013 - 10:55 AM.

[carbon]

Dusky,

 

If you can get hold of a copy of the January 2014 'Mini Magazine' there is a 2-page 'Calver's Classroom' spread on Compression Ratios (page 94 + 95). This covers most of the CR topic questions.

[Cooperman]

To try to explain in simple terms, the geometric compression ratio is the ratio between the total volume in one cylinder including the head chamber when the piston is right at the bottom of its stroke (BDC) compared to the volume in the combustion chamber on top of the piston when it is right at the top of its stroke (TDC). Volume is measures in cubic centimetres (cc's) just as the engine size is always in cc (except in the USA).

 

So what are these volumes?

 

The bore volume when at BDC is the capacity of the engine divided by 4 (e.g.: 998/4 = 249.5 cc. This is called the swept volume). To this must be added the volume of any dish in the piston (the dish volume), the volume of the head gasket, the volume in the head chamber and the volume between the piston top and the block deck when the piston is at TDC. Call this 'TV' (total Volume)

 

To work out the CR you add together all those volumes then, separately, add together the dish in the piston, the gasket volume the head chamber volume and the volume between piston top and block deck level. Call this CV (Compressed Volume)

 

Divide the TV by the CV and that is the compression ratio.

[Dusky]

Thanks, got it now. I used milimeters instead of inches