37 replies to this topic

[MRA]

Good, you may also like to get some plastic sheet fixed up in the roof to stop dust and worse from dropping on your engine, I also always cover a work in progress (WIP) engine or engine parts.

Thanks for the OBV

I think that what people are saying is that the figure has to be right, ie, you said you measured it and found it was 22cc's, however if you measured it and found it was 26cc's then it may not have worked, the CR would possibly be too low for your application, unless you are building a turbo of course..

[minilee94]

okay I i will do I have plastic sheeting about and yeah I see what your saying I will ring bill again tomorrow and have a chat about Cr thank you for your help

[Cooperman]

Getting the C.R. correct to what a person wants is probably the one thing which causes some difficulty to a new engine builder.
However, C.R. calculation and measuring is an exercise which needs to be mastered and it is a really basic task. That's what we on here can do to help.
Although most will already know this, probably including minilee94, the static (or geometric C.R.) is calculated by dividing the total volume in the combustion area with the piston at bottom dead centre by the volume with the piston at top dead centre (TDC).
So take the volume swept by a single piston between TDC & BDC (the SV), which is the engine capacity divied by 4 and add the piston dish volume plus the gasket volume plus the volume in the top of the bore between piston top at TDC and block deck level plus volume in gasket plus volume down the sides of the piston until the top ring level. Call that the Total Volume (TV).
Now take the total volume (TV) just measured and subtract the swept volume i.e. capacity divided by four. That figure is the combustion volume at TDC (CV). Divide the TV by the CV and that is the compression ratio.

[Mini and Puck]

So, kind of hijacking this thread, but not wanting to start a new one, I have the following figures.

1293cc block divided by 4 = 323.25cc

Volume in the block at tdc to the deck of the block = 10.8cc

Head gasket BK450 = 4cc

Cylinder head volume of one chamber = 22cc

So the calculation is as follows:

323.25 + 10.8 + 22 + 4 divided by 10.8 + 22 + 4

I get the answer 9.78. Now for my Omega Pistons, with a SW5 cam, I am thinking that I need around 10.5CR. The only variable, since I have assembled the engine less the head, is to further skim the head. But reducing it to 20cc only brings the CR to 10.28, and I'm reluctant to skim the head without some expert opinion on whether this would be a good step. Any ideas?

[carbon]

What fuel octane are you planning to use?

 

I've not used SW5, but MD266 in a 1293 is getting bit marginal at over 10:1 and really needs 99 octane. SW5 is shorter duration than MD266, so with higher octane you may have to retard ignition to avoid pinking.

 

But there are plenty of SW5 cammed motors out there, others may be able to comment further on optimum CR

[Mini and Puck]

I'm hoping to be able to use standard 95RON unleaded, as it's an unleaded head. When I can, I always use the highest octane I can get. I do a lot of motorway miles, so I need it not to get too hot, yet still get good power. I'm hoping for 80bhp.

[carbon]

You may be better staying with CR of 9.78 and see how you get on...

[ACDodd]

You definitely do not want any more compression with that cam and 95 Ron fuel. If it were mine I would be running 9.5:1.

Ac