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Dynamic compression ratio for an engine running on ethanol

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I'm building a VW 16V engine for drag racing. It will run with a modified GTX42 on about 4-4.5 bar of boost and will be able to rev to 10.5k rpm. Fuel will be at least E85, more likely E100. This is my first engine running on ethanol and so I'm very unsure what dynamic compression ratio I should choose. Can anybody here help me with that question?

Best regards


What exactly are you referring to? Dynamic compression ratio meaning isn't always clearly defined.

Are you talking about the compression ratio from the piston/combustion chamber design? Or the effective compression ratio according to valve timing? Or the charging efficiency from boost as measured by the airflow compared to standard conditions?

You can run high compression pistons but you need cold plugs because ethanol tends to preignite more.

Hey, thanks for your reply. I mean dynamic comp ratio according to valve timing.

Is this a fixed cam engine or variable timing?

Is this a fixed cam engine or variable timing?

Yes, it’s fixed without variable cam timing.

I think we need to look at this another way. You can't design a piston or camshaft from scratch or pay for one-off parts. It sounds like you are trying to buy off the shelf parts, like the GTX42 turbo. What parts are available that work with your engine and fit your budget? What pistons and what cams?

There is absolutely no part that is off the shelf. The turbo is based on a GTX42 but it is modified. The cams will be made only for my engine, the pistons, crank, rods..., too.

But before designing a piston it is essential to know what comp ratio the engine should have and this is where I’m standing now and what I don’t know.

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