Technical combustion chamber volume?

[yiorgos]

Does anyone know the volume of combustion chamber that is within the head of the FIRE 1.1 spi engine and whether it is different from the 1.2 mpi aka P75?

To put a context to this, would a P75 head need any skimming to retain the compression ratio of the 1.1 engine?

Thanks

 

[fingers99]

It'll be fine.

 

[yiorgos]

Hi fingers99,

I am sure it will work, but my guess is far from optimally:

Both engines have 9.6 CR.

This means, that the 1.1 engine should have compustion chamber volume ~32.25ccm and the 1.2 ~36ccm.

Thus, installing the 1.2 head on a 1.1 block would result to a CR of 8.7, and this is lame.

This is assuming that both engines have identical piston crowns and distance from the crown to the head in TDC.

So, we have to get rid of 3.75ccm/cyl, which means 1.3mm head skimming would be required, maybe more if the squish band is taken into account.

I will have to re-check my calculations just in case I have done a gross error, and whether this amount of skimming is possible (!)

Y

 

[fingers99]

The only way of being sure -- even on brand spanking new heads -- is to measure with a burette and a plate. Should be plenty of stuff on you tube on how to do that.

Note though. that most gas flowing will actually reduce CR (static CR really isn't the holy grail of tuning) and that different thicknesses of HG are available.

All I can add is that, visually, there's bugger all in it.

I think your calculations are likely to be off by quite a margin.

 

[Cento186]

A P75 engine has a compression ratio of 9.8 rather than 9.6 of the P60 and I know people have fitted a P75 head to an 1108 without issue so I think your calculations may be off.

As I understand it the heads are the same anyway (Aside for the port sizes/stud pattern) so surely the compression ratio will be the same? (I'm not sure where the extra compression comes from between a P60/75)

That amount of skim is fine as well, mines had 1.5mm taken off.

 

[yiorgos]

C55:
CR=9.6=(1108+CCV)/CCV => CCV=129 i.e 32.25ccm /cylinder

P75
CR=9.6=(1242+CCV)/CCV => CCV=144 i.e 36ccm / cylinder

Hybrid (!):
CR=(1108+144)/144 => CR=8.69

Note though. that most gas flowing will actually reduce CR (static CR really isn't the holy grail of tuning) and that different thicknesses of HG are available.

True, if by gas flowing you mean unshrouding valves inside the combustion chamber. Also as I understand, the cento and 866 cams differ mostly on lift and not on duration, which reduces effective CR.

I am not saying it would not work, obviously people have done it with good results.

What I am saying is that it seems to me totally worth it spending £50 or so skimming the head. even when not warped just to retain stock CR

 

[fingers99]

C55:
CR=9.6=(1108+CCV)/CCV => CCV=129 i.e 32.25ccm /cylinder

P75
CR=9.6=(1242+CCV)/CCV => CCV=144 i.e 36ccm / cylinder

Hybrid (!):
CR=(1108+144)/144 => CR=8.69

The maths behind your figures is utterly and totally incorrect! (Where I can work it out. "=>" equals more than? Ma che!)

Try this

or

this with added nice diagrams.

 

[yiorgos]

The maths behind your figures is utterly and totally incorrect! (Where I can work it out. "=>" equals more than? Ma che!)

No, it means that the equation on the left is equivalent to the ones on the right.

i.e solving 9.6=(1108+CCV)/CCV for CCV gives the result CCV=129

True I did not take into account the gasket thickness, this should be subtracted (in cm, times π*3.5^2) from CCV. Assuming identical gaskets it does not change the essence of what I am saying though.

Apologies for the obscure symbols, this is the best I could do with vBulletin ;-)

Thank you for your comments

 

[fingers99]

Are you actually measuring the combustion chamber volume and deck height or inferring it on the basis of published figures?

Anyway, the books give P55 9.6:1, P75 9.8:1.

You can probably run anything up to 10.5:1, given that you have control over spark advance, fuelling and some way of detecting knock.

 

[yiorgos]

correcting for the CR of P75, which I misread in a very very dirty Haynes, to 9.8, the volume at TDC above each piston is 35.3cm^3

Thanks for pointing this out to fingers99 & Cento186, it would have escaped me.

More once I have the C55 head out ...

 

[fingers99]

Still not sure if you're measuring the volume or calculating it from the manufacturer's CR figure.

Often the manufacturer's figures for CR are out by quite a way.

 

[biggelmo]

Hi, am I correct in thinking that the basic total compression capacity (piston bowl+head gasket+cylinder head chamber) at top dead centre can be worked by the following.....

Engine capacity (1242cc) devided by number of cylinders (4) = 310.5cc Per cylinder.

310.5cc devided by 9.8 (Compression Ratio) = 31.68cc total compression capacity.

 

[fingers99]

Only very roughly. The sane way to do it is by measurement of volume with a burette and plate.

To look at it the other way -- if you're just building an engine, you can mingle heads and blocks providing the head is not skimmed past the limit marks with impunity (use the gasket for the block!).

If you are semi-serious, you need to measure the CR. Any decent tuning book will give details as to how.

Talking 8v here -- 16v has the issue of being an interference engine.