Originally posted by John Maher on the Cal-look Lounge, May 2010
How much effect does compression ratio have on the BHP?
For maximum power output and engine efficiency, you should use the maximum CR your engine can safely handle. Major determining factor is the fuel you intend to use. Higher octane fuel is more resistant to detonation.
I think in terms of an engine as having three compression ratios…. Static, Dynamic and Effective.
1. Static CR is the mathematical ratio as measured on the assembly bench. Determined by deck, chamber volume, piston notches/dishes (if applicable) and engine capacity.
2. Dynamic CR…. you arrive at this by calculating piston position at the point the intake valve closes following the completion of the intake stroke. In theory, this is the point at which cylinder pressure changes from negative (intake) to positive (compression). The intake valve usually closes between 50° to 70° after BDC. If you measure how much stroke is left before the piston reaches TDC, input this stroke figure into your normal CR formula. Due to effective decrease in stroke (the piston is already on its way up the bore when the intake valve shuts), Dynamic CR is always lower than Static CR. The longer your cam’s duration The later the valve closes), the bigger the difference between Dynamic and Static CR. Rod length also plays a small part in the equation because different length rods will alter the piston’s position at a given valve closing.
Take a street engine as an example… for best efficiency and good power output on regular fuel, you probably want to run Dynamic CR somewhere in the region of 8:1. A stock (short duration) cam has a dynamic CR fairly close to it’s Static CR, because it closes the intake valve sooner than a long duration cam (i.e. closer to BDC), therefore Static CR won’t be a million miles away from your static figure of to 8:1. Fit a longer duration cam eg FK10 and you now find Static CR needs to be up around 9.5 to 10:1 to achieve a dynamic CR close to 8:1.
There are various DCR calculators on the web but there’s another factor at play which is much more difficult to pin down….
3. Effective CR is the term I use, others may call it something different. It’s a way of better estimating the actual CR the engine actually ‘sees’ at any given point in the rpm range (talking about wide open throttle testing here). Yes, Effective CR changes throughout the rpm range, generally maxing out at peak torque.
Static and Dynamic CR don’t take into account what happens under running conditions. Both formulae assume the intake charge stops entering the cylinder when the piston hits BDC on the intake stroke. They also assume the cylinder gets a 100% fill i.e. 100% volumetric efficiency (VE). Few engines (n/a) achieve 100% VE.
E.g. on the dyno, say an engine achieves 90% VE @ 6000rpm and Dynamic CR was mathematically calculated as being 8.0:1.
What the engine actually ‘sees’ is… Effective CR = Dynamic CR x 90%.
ie 8.0 now becomes 7.2:1
In other words if my plan was to hit an Effective CR of 8.0:1, I have to raise Static CR, which in turn raises Dynamic CR.
E.g. a low CR engine fitted with an FK89 under performs – dynamic CR is way too low for the system to function efficiently.
A race engine with a well developed tuned length induction and exhaust system can easily exceed 100% VE by optimising manifold lengths and exhaust primary pipes. Engines of this type manage to ‘ram’ high pressure intake charge into the cylinder such that it continues to pile in after the piston has hit BDC and starts it path upward on the compression stroke. Ideally the intake charge will keep coming in up to the point the inlet valve closes. An optimised engine can be tuned to push VE above 120%. Using the same example engine as above, Effective CR now becomes…
8.0 x 120% = 9.6
That’s all a long winded way of coming back to what I said at the start…. there’s no simple answer.
To summarise…. the milder the cam, the lower the CR (Static CR).
The wilder the cam, you need more CR.
Higher octane fuel can tolerate more CR.
Intake and exhaust systems should also be taken into account because they have a major influence on VE, which in turn effects Dynamic CR etc etc.
In the real world you either stick with a tried and tested combination, relying on the experience of others, or you experiment on the dyno if it’s a unique combination and you’re looking to exploit the engine’s fullest potential.
Having said all that, minor increases in CR make very little difference to power output. It’s very much the law of diminishing returns.
If for whatever reason you’d built a 2276cc engine with FK89 and set CR (static) at 8.0, you’ll see a healthy increase in power by stepping up to 10.0
However, the difference between 10.0 and 10.5 will be very small. The exact number to aim for will be a judgement call based on all that stuff I discussed above.