Compression Ratio’s – Up close with the calculations

I’ve been spending some time reading up on compression work and volumetrics regarding an engine and how it performs. This is in reference to a book that I bought a while ago, titled:

“Forced Induction – Performance Tuning”- A practical guide to superharging and turbocharging.  amazon link : click here

This will be a rather technical post to allow fellow enthusiasts like myself to step into the engine building process, hopefully clarifying some of the mysticisms of engine building beyond just spending that hard earned cash of yours.

I have some CP-carillo pistons purchased several years ago sitting in the dust, ready to be put to good use. CP uses a technology called “MIL” which takes the traditional piston and uses a series of analysis, identifying how much cooling, how much material is required to produce an efficient piston for each application. This saves them money as much as it creates a better product for us.

I’ve had the CP pistons for quite a while now, attempting to blueprint the piece. I will soon do my own thermal analysis on the pistons, as well as volumetric performance to measure the efficiency of the engine from which I am going to build. The specifications of the pistons I’ve purchased are:

SCY300    KA/Turbo   3.524in-89.5mm (bore)    CR-9:0:1     CH:1.399

The KA24De is spec’d at 2.4 L (2389cc) with each of the 4-cylinders rated at 597cc respectively. One of the reasons I went with a replacement piston is due to the popular belief that it is the weakest component of the KA. Due to the shallow depth of the ring seat on the piston, the pistons are a common component of the combustion chamber which fails when novices like myself apply forced induction. The CP pistons change the KA’s compression ratio (CR) from 9:5:1 to 9:0:1 and this difference in air density and volumetrics should be measured. Why put a part in your car when you don’t know what it does? other than rumoured performance gains?

The Combustion Chamber Volume of a 9:5:1 measures the total combustion chamber volume (CCR) at 70.24cc where as the resulting CCR from the CP pistons enlarges to 74.63cc. A race prepped engine aims to have all pistons equalized with a similar or same CR and volumetrics within 0.1cc difference.

CP also has its own bore specifications which allow the pistons to function at their best. The short block will need to be honed to fit these larger diameter pistons. I’m sure that each piston manufacturer will provide specific instructions, in this case, I will need to find an engine machine shop which does ‘plateau honing’, with an angled honing process of 35-40 degrees. This may be an engine honing standard, but I’ll be sure to clarify this as I progress. It also specifies a certain roughness of the hone and what kind of honing tools should be used to achieve the specified finish. Be sure to discover all of the necessary technical information for your engine components to ensure that you deliver the best results.

I only have half of the information I require to make an appropriate assumption as to how the engine will perform. The rest of the calculations must be calculated after the headwork has been completed, and also, after the short block has been decked. Only then will I have some parameters for calculations. The formula’s for these calculations can be found in the book as mentioned.

This is a rather anti-climatic post but progress is progress.


– Billy


About awdaltima

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