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How to determine CR

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Hi Andre and all

We have this project and I need to determine the CR. Even tough I am fairly experienced with the topic, I am wondering if there is a more scientific approach to calculate compression ratio, rather than working based on solely experience.

-6 cyl. inline,displacement 5000 cc, bore and stroke 101 mm

-Coated pistons

-Target hp 1400 - 1500

-Rpm 9000

-Fuel E 85

Thanks in advance

I take it you are trying to decide what CR to run in a new build?

I'm not sure if there is a scientific method that can be applied although if someone has one, I'd love to hear about it. My own technique has been developed through simple experience of building engines with various compression ratios and testing them. The CR will depend to an extent on the boost pressure you intend to run as well as obviously the fuel.

On methanol and E85 if I had no prior knowledge with an engine, I would be going with around 10:1. Your power aim isn't that high for a 5 litre engine so I'd assume you won't be running extremely high boost levels in which case you could even consider going a little higher. The reality is that the gains you get from a higher compression ratio start to become less significant the further you go. What I mean here is that you get to a point of diminishing returns. The biggest improvement is often in the off boost performance and ability to spool the turbo faster.

To put it in perspective I've made over 1000 whp from a 4G63 Mitsubishi on methanol with 8.5:1 CR and 11.0:1 so the CR is not always the ultimate deciding factor in what power you can make.

I'd say that CR is highly reliant on compressor efficiency at a particular region/window of the compressor efficiency map. So it all depends on the turbo you want to run.

CR is like pretty much any mechanical choices you could make on an engine, it's all about compromise.

Over the compressor efficiency I would add at least the camshafts and their timing.

But, keep in mind that what matter is the pressure in the cylinders during the combustion process (that's what generate the torque and so the power) and in a turbocharged application, once you get the boost, the CR have only a small influence on that pressure.

The CR doesn't increase the cylinder feeling, the boost does.

Twice the volume once compressed create a lot more pressure than the same volume compressed a little bit more.

As for a scientific method, or at least a scientific approach, it does exist.

But it is scientific (understand a bunch of formulas impossible to read), and I'm currently still trying to decipher it to get a more "simple" equation.

I think Ludo just summed up what I was attempting to say quite nicely. With a turbocharged engine, the boost is predominantly responsible for the power we can expect, not the CR. Particularly as we move to larger turbos and higher boost, the CR has a much smaller effect on performance. This is why I mention that the biggest effect is usually off boost.

Of course fuel and its octane rating is important, but since the question was about CR with E85 I think it was obvious.

One last things that is also worth mentioning, is application.

Even if you're looking at the same specific engine and power, if it's on a drag / time attack / drift / or rally car, you're looking at a totality different behavior.

On drag racing for example, it is all about full power at WOT (very narrow power curve), so you wouldn't mind sacrificing the bottom end in which case CR is not really relevant, but on a rally car it's absolutely the opposite.

E85... i forgot... comment removed.

In that case you're right... the only real determining factors i would say - boost, head design/efficiency and application.