Max compression ratio to boost


Staff member
Deal, Kent UK
A3 1.4 TFSI 150 COD
What do you think the safe limits are when it comes to compression ratio and boost? Obviously we are talking generic theory here but I think it could prove to be an interesting discussion.

Points I'd like to bring to the table are the modern innovations such as direct injection and better knock protection allow car makers to run much higher levels of compression with boost without risking detonation.

Higher octane fuel also plays a part here but are the general rules of thumb a little too conservative.
There's a massive amount of variables tbh and it varies from engine to engine. Without getting into engine dynamics, of course mapping and octane levels are very important and will allow more boost on a higher CR build.

Whereas before it was common place to build and engine to 7:1 or 8:1 CR and run big boost to achieve the numbers, now even car manufacturers are releasing turbocharged engines with 10:1 CR like the 1.0 Ford Ecoboost for example. They get better results with less boost and drive better both on and off boost. The days of low CR high boost are long gone for road cars.
^^^ Agreed totally ^^^

Turbocharged engines are designed from the gun to be turbocharged. Very different to 'adding' a turbo.

Some poeple seem to think that turbos are aftermarket bolt ons.

Not the case at all

Ford's Ecoboost 1.0 and 1.6 and good examples.

As is BMW's 528i which now is a 2.0 litre four cylinder
Yes engines work best with turbo(s) when designed as such to start with.

Not sure about lc high boost being dead though although it certainly applies to small engines ( and probably always did)

So out of interest what counts as low cr and high boost and why is this so much worse than the other way around ?

Maybe smaller engines can only do things that way

Fact is if you have are running a small engine you wont spool a big turbo up regardless of the cr - its volume of air that counts.
So a smaller turbo is the only option and that tends to means low boost .
This in turn means higher cr s are possible . So by all means use them in that instance.

But the same doesnt neccessarally follow with larger engines.
^^ Twin turbocharging can get around this very effectively. if the smaller turbo-supercharger gets things going initially there will be enough mass exhaust flow to spool up a larger compressor.

Still think that the best turbocharged engines are those designed to be turbocharged from the ground up.
The problem with twin turbo engines when is " series " is the lag before the second one come in . It is very difficult to match the compressor maps , just try driving a twin turbo legacy to appreciate how bad it can be.

You can go down the route of twin matched turbos coming in together .
This makes spooling up easier and you still get the sum of the power of both when on full boost. This will only work with larger engines but when done right is extremely effective.
I was kind of optimistically thinking that if a 1 litre engine is breathing for 2.2 litres worth of engine that there will be 2.2 litres worth of new gas flow.

Multiple turbo charging seems to work better with diesel engines, possibly because of the high compression and throttle-less operation perhaps?

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