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Selecting a compression ratio

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This seems like an elementary question but it has been difficult it get an answer with the level of detail needed.

I am building a 300ci inline 6 Ford motor. Cylinder head is complete and cam is selected. Fuel and spark will be controlled by mega squirt 3. Coil near plug and knock sensors will be used. Next step is placing an order on custom pistons.

Using Engine Analyzer Pro and the specs from a stock motor, I have a pretty good insight on what to expect (if I use it correctly). This program is showing that increasing compression and less spark lead than the optimum burn angle will provide a good performance increase.

Is this a good idea? Anything else I should be considering?

Without experience, you will need to test to determine what fuel octane will be required to support that compression ratio. I think you can use premium pump fuel up to 10 - 10.5:1 (modern head /piston design sometimes goes as high 12:1, but is often knock limited, so you can't get the performance you expect).

As always - it depends on engine application, what's wanted from the engine.

Here is an example how it can be worked out ( again, depending on what the engine is put together for).


Hi David, thank you for the quick response. I should rephrase my question.

Is it better to build a motor that uses the optimum ignition advance and pick a safe compression for a given fuel? Or retard spark at peak VE which will allow a higher compression to be used for the same fuel?

The idea behind option 2 is the increase in thermal efficiency will carry the torque curve farther as VE in on the decline.

In my experience raising CR makes na engines prone to overheating. If that issue can be solved then it's definitely beneficial to raise CR. But in most cases there is a limit to it dictated by engine design and even installing of bigger radiator doesn't help much to reduce engine temperature enough to get the full advantage of increased CR. So conservative CR and optimum spark advance is more common way to go from what I've seen...

PS I've built my na engine with 12.5:1 static CR although initially i was planning to make it around 14:1 as the camshafts i got aren't that aggressive... Had i different cams with longer duration I would have chosen higher static CR since the engine is built for drag racing and overheating wouldn't be an issue as it's working at high loads only about 12-13 seconds.


I will watch that video here shortly. My “increase in compression” would only be from 8.8:1 (stock) to 9.5:1.

It's difficult to find "one fits all" answer as obviously all engines react differently to increased CR but in your case it seems to me that raising CR from 8.8 to 9.5 should be a piece of cake with no downside at all providing that pump gas of 93 octane ratio would be used.

As Shota said - "As always - it depends on engine application, what's wanted from the engine."?

Do want a bit more mid-range torque for long climbs/towing? Better fuel efficiency? manual or auto' transmission? What fuel do you want to use? Etc!

Depending on the camshaft selected - shorter duration high lift for lower rpm cylinder filling/torque or higher rpms, you should be able to use 9.5:1 all day and all night, possibly on lower octanes if the fuelling and ignition is correct.

You said the head has been "done" - if it's been "ported", especially with a longer duration camshaft, you may need/be able to go a bit higher because of the poorer low rpm efficiency.

I can give all the information but it will not be in metric measurements. I was intentionally a little vague on the engine specs because I wanted to know others general theory’s and techniques rather than caught up in the details. Some people stick to a known dynamic compression ratio that’s close to stock. While some never look at such a value.

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