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Boost or Compression

22 February 2001
---Boost or Compression....clicky clicky

Which one is more beneficial and how..I have heard of people lowering the compression of their pistons so that they can boost higher with..well just say their turbo..Isnt pressure pressure, whether the turbo is making it or the piston is making it..what is the advantage...the only thing I can think of is that you would loose low end-because of lack of compression, until the turbo kicked in and would maybe gain top end due to increased boost...someone explain please..

Peek-a-boo...I'll smoke you!!
Okay, this is the short version, in no particular order, so please don't be too picky.

If the piston is making the compression, you have a couple of important limits. First, going past 14:1 or so does not gain extra power, as you hit the point of diminshing returns. This sets an absolute limit on the amount of air you can actually get into the cylinder. For example, you could not double the amount of air going into an NSX motor by increasing compression. A turbo could easily double the amount of air.

Second, there is little you can do to manage the heat generated by compression. Water or alcohol injection work, but are not proactial for the street (in most cases, in my opinion).

Turbos give you a great deal of control over the amount of compression (or more specifically, the volume of extra air added) and can allow you to control the temperature of the air going in to a great degree.

So, why lower the compression? Again, to over simplify, it is to let the turbo do its work better. If you compress and cool the air as it enters the manifold, you can get a lot more power than if you do all of the compressing in the cylinder. And, you can add a far greater volume of air, which means more ponies.

How much you lower the compression is a trade off. The more you drop it, the less power you have before the boost comes on. Some motors designed from the start for turbos have compression as low as 8:1. Personally, I think 9 or 9.5 to 1 is a good compromise. The compression on my car worked out to 9.6 to 1.