Critical compression

[colsy 0]

Hi all,

 

When looking at turbo charging a N/A engine, compression ratio is a influencial factor, my question is what compression should be aimed for?

 

I realise that the turbo provides a pre-compression of the charge, and therefore when the crank strokes, the actuall compression is actually larger due to this pre-compression. I have been told if running 10psi boost to reduce compression on a N/A unit to 7.5:1 (area) i.e the neglecting the pre-compression, I calculate (assuming ideal gas laws)that the actuall compression of charge, taking into acount the pre-comprssion from the turbo, then the secondary compression from the piston stroke would be more like 12:1 which to me seems to high....would the engine not suffer excessive pre-ignition?

 

Any thoughts.

Cheers,

Col.

[flusted 0]

i think 9:1 or 8:5.1 is a good figure to aim for

[colsy 0]

Cheers flusted,

 

By this you mean the comression the piston does on the allready compressed (by the turbo) air/fuel mix? not the overall compression.

 

Cheers,

col.

[Lizard Racing 0]

In reality 7.5:1 compression will mean the car will drive like a dog off boost (Cossies, 5 Turbo etc.). Generally the newer the engine the more forgiving the engine, allowing slightly higher compression ratios to be used (BMW, Hondas etc). I used to run 1.5-6bar with 9.3:1 compression on my 12v. The higher the compression the more efficient the burn.