can someone please explain how compression ratio works... what i mean is...

why put higher compression ratio?
why put LOWER compression ratio?
what ratio do you put on a turbo car?
what ratio do you put on a supercharged car?
Why would this benefit from it? why would it NOT benefit from it? (lower or higher)

ive looked around but all it explains is how compression ratio works and how to calculate them. not really going into details on why put xxx amount of ratio on a turbo/super charged car. Maybe the dezod guys can chime in on this one?

 

well... too many people can go into more detail than me but granting you know what compression ratio is...

1. Higher compression ratio would be found on N/A cars because you want to compress the air/fuel mix more because there is nothing forcing air in, just atmospheric pressure.

2. Low compression ratio is found on forced induction (i.e. turbo and super charged) because since air is being forced into the cylinder, you would want more volume at BDC to force as much air as humanly possible into the cylinder.

I hope I helped. If not, well it makes sense in my head. Hehe

 

In a forced induction vehicle, you want to cram as much air in as possible without seriously hurting your engine with predetonation (where the fuel ignites before it is supposed to).

Let's say you have a 10:1 compression ratio on a forced induction car (10:1 is really high for FI). We want to cram "X" amount of air in and then it's corresponding fuel so that we get a good combustion process. Well, when we compress X amount of air with 10:1 the pressures and thus temperatures rise to very high levels inside the cylinder. This has the potential to cause the fuel to ignite before the piston is ready to be pushed, and thus could severely damage your engine.

So then we could modify our internals to run say 8:1 compression ratio. We're still compressing X air and fuel and now we're doing so in confidence, because the pressures and temperatures inside the cylinder aren't as extreme as before. If you're looking for the ability to run more boost with FI, lowering your compression ratio is an option.

This is a very simplified view of it. Combustion theory is very complex and nothing can be done with the compression ratio without affecting several other things in the process. I hope this at least helps.

Good questions.

-Tide