[Racer x]

There seems to be some confusion about compression ratios and how to calculate them. I'm offering up my years of experience for your perusal. Keep in mind, this is my opinion, and that there is some disagreement among engine professionals about what is, or what is not, important.

1/ STATIC Compression Ratio (also known simply as Compression Ratio)

This is defined as: clearance volume + swept volume, divided by the clearance volume. This relationship is easily calculated from measurements the average enthusiast should be able to determine. This is a theoretical number, useful for discussions during beverage consumption. It is also useful as a comparitive guideline for possible changes in C/R. It has no place in attempting to evaluate performance or output between differing engine types or displacements, although these comparisons MAY be useful in determining who pays the bar tab........


2/ EFFECTIVE Compression Ratio

This is typically defined as: clearance volume + effective volume, divided by the clearance volume. This relationship is more difficult to determine. The effective volume of the cylinder is whatever volume the engine "sees" when the intake valve closes. The effective volume is determined by relationship of the build geometry, (such as: rod length/stroke ratio; wrist pin offset; etc.) the intake valve actual closing point (in crankshaft degrees) and the volumes calculated for the static compression ratio. For normally aspirated racing engines with a long duration camshaft (or camshafts), this number is always lower than the static compression ratio.

There are a number of programs & websites out there, that are available to help calculate effective compression ratio. You must know your camshaft specs to calculate effective compression ratio. Although also theoretical, this number is more useful, in my opinion. This is the number you want to use to judge basic octane requirement for fuels, changes to camshaft timing(s), valve events and rocker ratios, and changes to build geometry such as rod length, wrist pin offset, stroke, bore diameter Vs stroke for displacement limitations, etc.

Keep in mind that typical performance camshafts delay the intake valve closing point until well after BDC. This is done to maximize inlet charge inflow, also known as "ramming" or "ram tuning". This is a compromise between the "ram tuning" and "maximizing" the swept volume. It is important to realize that once the crankshaft passes BDC, swept volume is lost at a rate dependent on the build geometry, thereby lowering "effective" C/R. Trade-offs of this nature can be negligible or severe, depending on how long the piston is effectively "parked" at or around BDC. Statements that long(er) rod/stroke ratios are always of benefit, without specifying an engine type and rpm range, are clearly simplistic. And yes, it is possible to have a rod which is too long. Good engine engineers/designers/tuners evaluate these variables during the design phase.


3/ DYNAMIC Compression Ratio

This is typically defined as the effective compression ratio multiplied by the volumetric efficiency% @ whatever rpm range (usually the peak bhp range) is being studied or optimized. This is usually modeled based on dyno bhp output results, or back calculated from the resultant BMEP figures from dyno output numbers. Typically, volumetric efficiency percentage can exceed 100% on well tuned race engine combinations. You need to be aware of whether your engine is capable of 115% V/E (Cosworth 4 valve x 2 ohc) or whether it can only achieve 50% V/E (Hello Briggs & Stratton flathead.....)

Modeling your engine from hard dyno data is the best way to accurately determine dynamic compression ratio, V/E% and BMEP. However, the programs which model these types of data are much more expensive, much more subjective and require much more experience to utilize properly.

4/ To make things more confusing, sometimes the definitions of effective C/R and dynamic C/R are interchanged........


So what does all this mean? Some examples:

Well, if you are only calculating "static" C/R, you are only giving yourself part of the picture. For instance: 12.5/1 "static" C/R with 320 degree cams may only be 8/1 "effective" C/R. But, combined with 110% V/E = 8.8/1 "dynamic" C/R.

OR: 14/1 "static" C/R with 240 degree cam(s) may give 11/1 "effective" C/R. Combined with 90% V/E = 9.9/1 "dynamic" C/R.

A common complaint I encounter is: My C/R is 14/1. Why are my bhp numbers so low? What's wrong with your dyno? When I calculate the BMEP for poorly performing engines, it invariably points to either:

A/ A parts selection error in the design phase. (ie: too much "camshaft", carb, etc. And difficult to correct quickly & cheaply.)
B/ A parts selection error in the tuning phase. (ie: exhaust too large/long/short, etc. Easier, possibly cheaper to correct.)
C/ A huge opportunity to improve state of engine's tune. (ie: "These are the jets/timing/etc, my friend runs." Easiest & cheapest to fix.)
D/ The "Triumvirate". A combination of all 3 of the above......... (Don't laugh. This is more common than you might think.)

The bottom line is: If you actually have 14/1 C/R, (and usually when I calculate/measure your C/R, you don't......), and your BMEP is 165psi, there is a problem somewhere.......

Well, we haven't even talked about supercharging yet.....and yes, it is possible to have 65% V/E, bolt on a huffer, and have a BMEP of 235psi. I'm going to leave this for a separate topic at some later date.

So, it turns out that the 4-stoke Otto cycle of internal combustion, which we were taught in high school & college, is a gross oversimplification. Depending on who you want to believe, performance internal combustion engines have as few as 5 separate "events" or as many as 7 significant "events"...........


Confused? Then I suggest this:

Determining "effective" & "dynamic" compression ratios is an area where "modeling" build geometry specs, ahead of ordering expensive parts, can payoff big-time. BUT, you must know what you are doing as opposed to just punching numbers into a program. I find that modeling helps to avoid poor decision making, due to lack of information. And, without hard numbers you are just guessing. The net result is: If you are just guessing, that makes you: Inspector Clouseau.

For the best results I suggest you:
A/ Model your engine during the design stage,
B/ Be realistic about your output goals, depending on engine type,
C/ then dyno your engine, to optimize output,
D/ then analyse the results of the dyno session,
E/ Make changes accordingly.

Final thought. Modeling programs that cost $39.95/$99.95 or even $199.95 are useful for the limited functions they calculate. Programs that cost $499.95 to $999.95 have more features and are therefore more complex and perhaps more useful, IF, they are used properly. Big-time professional race teams, aftermarket Mfg's and OEM's spend hundreds of thousands, if not millions, on their computer simulations, models and their engineers......... Caveat emptor......