I'm looking for some advice on compression ratios and piston dish cc's for a 4.6 modular that is going to be built for racing with a single turbo. I'm brand new to turbo's so I'm trying to do a lot of research and planning before I start buying the wrong stuff and I'm having a hard time finding much on race engines, found plenty of street stuff but this will be race only. I'm used to NA engines and we've always ran a lot of compression. I was thinking 12 to one, to much or to little? We are planning to run either methanol or e85 also. The car will be ran in 275/true 10.5 heads up races.

 

Even race only I wouldn't go no higher than 10-11:1 max, its not necessary with a turbo.

 

The high cr will really only help off boost, and in transition, but there is much more power to be made with boost than there is with cr. I agree with the above post, run enough to work for your intended usage off boost, and be able to up the boost level if you choose.

 

Thanks for the advice guys.

 

I like 10-10.5:1.

 

Think of available space for air charge. On a NA engine you compress charge tighter to get a bigger bang [more power] On supercharged [blower or turbo]applications you use a lower CR which leaves more space that you can but a bigger charge into effectively raising your CR . You only limits become parts and pressure or boost. Becoming a variable displacement engine. Boost also helps overcome flow, cyl heads intake and cam restrictions.

 

....but, a forced induction engine will not move, or use a larger volume of air (flow) than its n/a counterpart, the power increase comes from air mass (density). The air is not being forced in, just what is ingested is more dense. The mass, or density is what drives up cyl pressure, and a lower cr allows more air mass with a given fuel, or cyl pressure.