Discussion and questions related to the course Engine Building Fundamentals
Hi All, I know my chamber CC and can work out the volume above the top ring, and I know the compressed thickness of the head gasket and I know the compression ratio I want to be at.... but with all this, how do I work out what piston dish size I need? Or do I just have to work it out with the different dish sizes available and just choose the closest to my needs? The things I know right now are: 38cc - Chamber volume 81.42mm - bore 88.9mm - stroke 0.041 - compressed head gasket (others are available)10.8:1 - target CR (between 10.5 and 10.8) I have to buy pistons next... but I just don’t know enough to say which are the right ones for me... Any help is greatly appreciated
Usually pistons manufacturers state CR for particular sets. If that is not an option and you need custom pistons to be made you can use online CR calculator like this one - https://www.summitracing.com/newsandevents/calcsandtools/compression-calculator there are plenty of them on Enternet.
Those types of formula won't work.
It's relatively simple in principle, but a bit of a pain to work out in practice and aas I'm a bit busy I'll write it out for you tomorrow. You will also need your piston deck distance/clearance.
In the meantime, you may find formulae for compression ratios on-line and, if you're reasonable competent in basic math's, you'll figure out how to put the dish/dome on the right with everything else on the left where the calculation takes place.
compression ratio = (swept + unswept)/unswept.
Swept volume = piston area x stroke = 52.07 * 8.89 = 462.9 cc
Unswept volume = chamber volume + (gasket cylinder area * compressed thickness) + (bore area * deck clearance) + valve reliefs (if applicable) + dish OR - dome = 38 + (52.07 * 0.104 [assuming gasket matches bore]) + (52.07 * 0.05 [assuming piston 0.5mm down bore at TDC]) + 0 [unknown, assuming zero for now for valve notches] + ? [value to be determined] = 46.02CC +/- volume of dish/dome.
So, nominal CR is to be, say, 10.8, so the formula will be -
10.8 = (462.9 + 46.02 + ?)/(46.02 + ?) which can be re-written as
10.8(46.02 + ?) = 426.9 + 46.02 + ? or
497.0 +10.8? = 472.92 + ? which can be further simplified to
24.1 = -9.8?, so that gives
? = -24.1/9.8 = -2.46 cc dome = 2.46 dish.
However, those calc's were for illustrating the process, less deck clearance will mean more dish to compensate, large valve reliefs will require less [shouldn't be a factor the B series engine].
Normally, as Shota said, unless you're building a rather unusual variation of a common engine (stroked/destroked for example), and you have quite a large range for CR in mind, you would normally use the nominal CR the piston manufacturers give for their pistons.
However, with your particular engine, I would use a flat-top piston, tailoring the deck height and gasket thickness to get the best quench for the engine.
Thanks guys, I have had a good long chat with the machine shop and we have worked out the following on my engine when using flat top pistons...
With a zero deck height we calculate a 11.6:1 C/R
With a .030” deck height we get a 10.7:1 C/R
I can get a .027thou (compressed) cometic multi-layer head gasket (HG), allowing for a little loss due to how lumpy the cam is I reckon we will get 10.7 or 10.8... which should be just right for running on premium unleaded (98ron) pump fuel...
It really is amazing the thickness of a head gasket makes such a big difference... I wonder how many people do all the calculations an then just throw on any gasket... not realising they are throwing away compression... I can get a range of HG from .015 though to .100 for my car and there is a full copper gasket that is even more...
Have a great Christmas all!!! :)
A similar thing is the number of 'experienced' engine builders, who REALLY should know better, who use very thick grease coatings to seal CCing plates to heads, when the lightest smear should be optimum. A thick coat can easily hold the plate a half mm, sometimes much more, off the head face and, if we take your head as an example, that would be 2.5 cc of inaccuracy right there - makes it difficult to get it down to 0.5cc tolerance, doesn't it? Large bore engines are even worse, but percentage wise, do have more tolerance.