Calculating Compression Ratio

Bear with me on this one, I'm still learning. Spent a good few hours today trying to get the compression ratio calculated its looking like 12.95:1 which is on the high side. I have calculated manually and used a few online calculators with varying success and answers! I did get one spitting out the same answer as me. Would machining the pistons be the best way to reduce this? If anyone is interested enough see my calculations below and check if I've made a mistake:

Rough Engine spec - Ford 1.6 16v (Sigma) from Caterham - PEC Rods, CP Carrillo Pistons, Ported head, Piper 285 Cams, Jenvey Throttle Bodies

Swept Volume = Area of bore x stroke

Bore 78.98mm (measured)

Bore area 3.142 x 39.492 = 4882.46mm2

Swept Volume 4882.46 x 81.4 = 397432.24/1000 = 397.43cc

Head Gasket Volume = Area of gasket bore x thickness

3.142 x 402 = 5027.2 x 0.35 = 1759.52mm3

Deck clearance volume = Deck Clearance x area of cylinder bore

0.47 x 4882.46 = 2294.75mm3

Clearance volume = combustion chamber vol + head gasket vol

+ Deck clearance vol – piston dome volume

30700mm3 (Measured with Burette) + 1759.52mm3+ 2294.75mm3

- 1510mm3 (Measured in cylinder with Burette 5mm down from TDC 28.3cc– flat top equivalent (bore 78.98 with stroke of 5.47mm = 26.79cc) 28.3-26.79=1.51cc))

= 33244.27mm3

Compression ratio = swept vol + clearance vol / clearance vol

397432.24 + 33244.27 / 33244.27 = 12.95:1

It looks overfilled with numbers to me. Try calculator attached - i made it very simple to work with for myself.

Piston dome volume cell is negative by default. In my calculator 22 means the dome volume is -22 cc.

Thought I'd commented on this?

As Shota said, there are a lot of digits there - if you just use CCs instead of cubic mms, the numbers will be MUCH smaller and easier to grasp.

I think you have the dome volume measurement process correct, but using 28mm instead of, say, 1.00 cm does make the calculations even more complex, as does having all the sub-unswept in the same line, it works but introduces the risk of finger trouble. Place the piston(s) 1.00 cm (10.0 mm) down the bore, use some vasaline, or similar to seal the piston the bore, carefully cleaning off and above the piston, and measure the volume, then take that from the calculated bore volume - a simple piston area if using cm.

All in all, though, 12.95:1 is in the region I would expect - depending on the fuel available to you where you live, and the camshafts (there are three different 1600 twin-cam engines listed by Piper, not sure which one you have?) which affect the nominal dynamic compression, it may be perfectly reasonably for you to use it. It may need a little timing pulled at the bottom end - but as cylinder filling will be poorer with the cam's, it may not - then you would still have the higher rpm benefits

It may be worth a call to Piper, they may be better able to assist on recommendations?

If you do decide to lower it, a common method is the piston machining, to remove some dome material - with some piston designs this isn't a problem, as the dame is "added"material, but some piston designs are "hollow", with the underside of the pistin crown being relieved where the 'dome' is, to lighten it - normally this is fine because the material is still thick enough to take the loadings, and they're uncommon as it needs a special forging die and/or additional machining. If you look underneath it should be obvious if yours are using that process.

The machining does cost money, though, and will need further work checking the same amount was removed on each, which may mean hand fettling the domes. Plus you will need to re-balance them, etc.

Another option, if it's close, is to use a thicker head gasket - with some engines there's a range available, but if not, it may still be cost effective to have a couple or three (spares) custom made.

Oh, depending on the specific engine, this bit should be noted "Note: Additional machining to the tappet bore is required with this profile to allow the camshafts to rotate."

Thanks Shota and Gord. Usefull info.

I double checked all my numbers and though long and hard about each calculation and realised that although my piston does have a domed top the valve reliefs are so large that it is classed as a dish overall after measuring it. I had my Calculations minus the piston dome when it should have been plus as it is a dish. Ended up at 12:1 (11.998 to be exact) which is pretty close to where i wanted to be.

Everyday is a school day!

Awesome!