21 replies to this topic

[MacFreuden]

Hi all, currently building my 1380cc engine to go in my 75 mini and am struggling to figure out the CR accurately, I am aiming for 10.5:1 . the configuration I’ve got is...

73.5mm Powermax 9cc dished pistons
Standard con rods so 81.28mm stroke
Gasket is BK450 so 3.3cc compressed volume
Piston depth at TDC 0.98mm as the deck has been skimmed but not sure how much?
Cylinder chamber volume is 21.7 / 21.8 / 22.1 / 22.6 so average I used was 22cc

I used an online calculator as I kept getting a CR of 81 something which is obviously wrong but that gave me 9.7 CR.

I’d like to get 10.5:1 as from reading I have seen it to be commonly used for fast a road set up, the cam is the minispares evolution one (998cc - 1380cc fast road cam) and standard rockers.

There are some scratches / imperfections on the cylinder head face so I was thinking to have this skimmed to raise the CR and give good head gasket mating face... how much would this need to be skimmed to raise the CR to the desired amount? Is this the best way to raise the CR and also how do the piston ring clearances factor into the equation, I am re-using the old ones as they have only been run in really and so am unsure on there spec. Oh and it’s a 12G940 head,

Any help or prods in the right direction would be great thanks!

[Spider]

I come up with a figure of 12.72:1

 

<Edit: Ignore this !  Some numbers missing >

Edited by Moke Spider, 02 December 2019 - 07:00 AM.

[MacFreuden]

Hmmmmm, I got 81 or so from this...

Swept volume / total compressed volume

Cylinder volume + clearance volume + piston volume + gasket volume + chamber volume /
Clearance volume + gasket volume + piston volume + chamber volume

Have just realised my error I wasn’t dividing the cylinder volume by 1000 so was partially working in cubic mm (or something) not cc.. but I get a CR of 10.8 not 12.72 ??
I would still like to skim the head due to the scratching, so how should I then raise the CR back to the 10.5 or so if my calculations of 10.8 are correct and not Mokes at 12.72, would enlarging the combustion chambers be best?

[Earwax]

When you are doing the same calculation over and over but keep getting different results, go back and check everything.  Are you sure your pistons are .98mm down the bore? - (1 mm =40 thous)

[Rorf]

I would skim the block to have the pistons level with the deck first, then work on the head from there. What is the thickness of the cylinder head?

[dotmatrix]

Arent you forgetting the volume above the piston and ringland volume?

[carbon]

For 10.5 CR the total compression space per cylinder would be 36.3 cc

 

Your volumes are roughly:

- dish in piston = 9 cc

- 1mm above piston = 4 cc

- gasket = 3.3 cc

- combustion chamber = 22 cc

 

So a total of about 38.3 cc which I reckon gives 10.0 CR

[MacFreuden]

Thanks for the replies everyone,

Earwax... I think so, I am going to triple check just found it difficult to do accurately with a vernier, but am going to try with the DTI tho, unless theres an easier way?

Rorf... wouldn't that reduce the total volume and therefore CCs reducing overall power and it would no longer be a 1380? The deck has already been skimmed too so its in a good condition for the gasket as well.

Dotmatrix... no I havent included the ringland volume as i couldnt find online how to find it or how to use it in the formula I jotted down above, how would it fit into it?
Am also using the old piston rings so dont have a datasheet for them.
The volume above the piston I called the piston clearance.

[MacFreuden]

Ahhh thanks Carbon you're right I had the Volume the above piston incorrect at 1cc for some reason.
38.3 is bang on 10CR, after skimming the head and reducing the combustion chamber (was thinking a 20 thou skim should be enough) what would be the best way to get a CR of 10/10.5 again?

[dotmatrix]

I would skim the block to have the pistons level with the deck first, then work on the head from there. What is the thickness of the cylinder head?

 

why? I would only skim the block if it needed a skim to be flat. I only see pro's with having the pistons a bit bellow the block top

[Earwax]

The theory is the the best squish occurs when the edge of the piston is very close to the block. ( various thoughts on spread of flame - eg flat topped pistons vs head chamber mirroring piston cut out etc).

 

Regarding rorf's suggestion of skimming the block - (which i agree with) - given the bore is nearly at max before re sleeving is required (and then only is if it has been offset) there is not much point to having a safety margin of .98mm /40 thous to allow for future reskim  of the block. On a race motor i would allow 5 thous down the bore for one more reskim - but you could leave more. (you would just need to adjust your ' piston clearance' measurement to less than 4cc

[nicklouse]

Rorf... wouldn't that reduce the total volume and therefore CCs reducing overall power and it would no longer be a 1380? The deck has already been skimmed too so its in a good condition for the gasket as well.

Dotmatrix... no I havent included the ringland volume as i couldnt find online how to find it or how to use it in the formula I jotted down above, how would it fit into it?
Am also using the old piston rings so dont have a datasheet for them.
The volume above the piston I called the piston clearance.

would not change the volume of the engine one bit. think what is the engine volume.

 

 using the ringland info for a standard 1275 engine  (which is too small) i get 10.1 : 1

 

ring land gap is the space between the piston and the wall down to the top ring. so quite easy to work out if you can measure the piston.

[MacFreuden]

 

Rorf... wouldn't that reduce the total volume and therefore CCs reducing overall power and it would no longer be a 1380? The deck has already been skimmed too so its in a good condition for the gasket as well.

Dotmatrix... no I havent included the ringland volume as i couldnt find online how to find it or how to use it in the formula I jotted down above, how would it fit into it?
Am also using the old piston rings so dont have a datasheet for them.
The volume above the piston I called the piston clearance.

would not change the volume of the engine one bit. think what is the engine volume.

 

 using the ringland info for a standard 1275 engine  (which is too small) i get 10.1 : 1

 

ring land gap is the space between the piston and the wall down to the top ring. so quite easy to work out if you can measure the piston.

 

Thanks for the replies guys,

 

I understand what you mean now about it not changing the size of the ccs of the engine now, a rookie mistake, I am still not keen on the idea of reducing the block deck down so much however, especially when its all assembled and in good nick.

Where did you get the info for the ringland on a 1275? I found there to be some on 'calverspecialtuning' but that's the only website that seems to mention it ?

 

I've had a re-re-re-measure of some things ...

 

V Gasket (GEG300)     3.4cc

V deck to piston            4.242cc  (I have a gap of 1mm above the piston to the block deck)

V combustion chamber 22cc

V Piston Dish                9cc

V Swept cylinder          344.864cc

V Ringland                   0.8162cc  (I worked this out by working out a ratio of how much larger a 1380cc is to a 1275cc engine is, so 344.864 / 318.295 = 1.083, this multiplied by 0.75cc equals 0.8162cc, alternatively I also used some feeler gauges and the largest that would fit was 0.305mm, which gives a ringland volume of 0.424cc, however I don't believe this to be as accurate)

 

so by my logic this comes to a CR of...

9.74 when using the 0.8162cc ringland volume or...

9.83 when using the 0.424cc figure

 

There are probably a few minor errors elsewhere and so I'd take this difference in CR as negligible but I guess its good to know its negligible right?

 

Out of interest, how much in terms of performance how much does a different CR of 9.5 / 10 / 10.5 to one actually give you ?

Edited by MacFreuden, 01 December 2019 - 10:46 PM.

[nicklouse]

ringland can not be scaled as it depends on the distance from the top of the piston to the top ring and not all pistons are the same.

 

what it is (others please correct me if i am wrong it has been a while) you are trying to measure the space between the piston and the bore down to the depth of the top ring.

 

top of piston to ring distance X

diameter of piston 73.5mm

cylinder bore Ymm

 

so you need to do this and take the piston volume from the cylinder volume

 Capture.JPG   19.86K   0 downloads

where is X is h for the two different diameters.

[Earwax]

What performance difference from 9.5 to 10.5 -

I have no direct experience with one engine but i have a road engine 1310cc running 9.75 and it runs okay but not entirely spirited - it has a 266 cam and serves me well as a weekend run about - lots of noise and gear changes but probably would be beaten by all the modern cars out there.  I have a 1360 road motor at 11.0CR and a 286ish cam  and it is spirited, can win its fair share of traffic light duels, but still wont beat a golf GTI -both run Premium 98RON . So back to the question - some difference but not a lot by itself - what it does do is give your headroom before pinking could possibly endanger the engine . I think somewhere in 9.75-10 range suits most road car needs.

 

There is an approximation for head shaving ( eg shave 10 thous to lose a cc-- NOTE this is NOT the ANSWER JUST an example)  but failing that- go back to your calcs and use different figures for head chamber volume and plug into the formula until you get your desired CR.   ( eg 21 cc - might give 9.9, 20 might give 10.2 etc)  then invert your head chamber and fill the chamber with that volume of water ( valves in sparkplug in (vasoline used to seal)  using a beaker or syringe and take a reading of how much you need to shave - - I just use the bottom of a vernier and a straight edge- top of the water to edge of the cylinder head . Again replicate these calcs before skimming  ----- and just as an aside on rebuild always check you have valve to piston clearance