Just cant get my head around it....

[fingersnfumbs]

So..
Im trying to work out a compression ratio for a proposed build.
Its a Cosworth N/A.

Using the online summit calculator at say ''for domed pistons use - and for dish use +''

Now, Im using Wossner N/A Pistons but are they a dome or are they a dished piston.?

https://www.shopbhp.com/products/wos...ed-pistons-set

On the Wossner website it say +11cc which makes them a dish I would have thought? but surely a + indicated adding volume i.e a dome.

https://www.summitracing.com/newsand...ion-calculator

Specs are as follows:

Wossner 92mm Cosworth n/a Pistons. flush with top of block.

93mm diameter head gasket ?

92mm bore

76.95 stroke (standard)

4x4 Cylinder head. Chamber volume of 41cc

Any help most welcome.

[Enea]

they look dome from the picture, and if thery're not turbo, no reason to have dish pistons.

For calculation, it should be - 11cc, as you are taking away voulme from the chamber. Try both, If they're dome pistons the CR must go Up, dish it goes down.

[katana]

Clearly Domed and i'd guess that the Wossner +11cc refers to the dome equating to 11cc ie. greater than a flat top piston.

With the Summit Racing calc., you probably have to deduct that 11cc as it comes out of chamber capacity in swept volume calc - hence the minus?

[Graham]

are you sure about 41cc in the head? it sounds too small, with an 11 cc dome your going to have about 14.1 cr

[rallyrob]

I take it that the 11c dome takes into account the valve cutouts?

[Graham]

I take it that the 11c dome takes into account the valve cutouts?

i would of thought so

[fingersnfumbs]

are you sure about 41cc in the head? it sounds too small, with an 11 cc dome your going to have about 14.1 cr

Thanks all.

Agree Graham, it dosnt seem much head chamber capacity.
I had new seats inserted and new valves which means the valves are a bit proud. Engine shop told me the chambers are 41cc.
Perhaps I need to check myself.

[Forest_rallying]

Thanks all.

Agree Graham, it dosnt seem much head chamber capacity.
I had new seats inserted and new valves which means the valves are a bit proud. Engine shop told me the chambers are 41cc.
Perhaps I need to check myself.

You firstly need to CC the head and not just take the word of someone or you’re never going to get. an accurate reading of your compression ratio.

The simplest way to find what the piston dome is going to be is to lower your piston 10mm down the bore, seal round the top of the piston, seal a piece of Perspex over the top of the bore and measure with fluid how many Cc’s It holds. If your pistons were flat tops I have worked the volume out to be 66.5 CC. With your bore size. But seeing as you have domed pistons your volume will be less than that, for example it may be say 56.5 CC, so you deduct the smaller figure from the larger one and that gives you the volume of the dome if that makes sense, which would be 10CC. That figure then needs to be deducted from your chamber volume when working out your compression ratio.

[Forest_rallying]

So..
Im trying to work out a compression ratio for a proposed build.
Its a Cosworth N/A.

Using the online summit calculator at say ''for domed pistons use - and for dish use +''

Now, Im using Wossner N/A Pistons but are they a dome or are they a dished piston.?

https://www.shopbhp.com/products/wos...ed-pistons-set

On the Wossner website it say +11cc which makes them a dish I would have thought? but surely a + indicated adding volume i.e a dome.

https://www.summitracing.com/newsand...ion-calculator

Specs are as follows:

Wossner 92mm Cosworth n/a Pistons. flush with top of block.

93mm diameter head gasket ?

92mm bore

76.95 stroke (standard)

4x4 Cylinder head. Chamber volume of 41cc

Any help most welcome.

This isn’t the same engine by any chance that had Woosner pistons and Newman stage one cams that you were having all the noise trouble with back in May in another thread is it?

[fingersnfumbs]

Thanks for the Info FR. I will give that a go.

This is another Engine.
The other one did indeed have tappet noise issues, eventually cured by thinner oil would you belive.