Why would a person who is going to turbo their car reduce the compression ratio if reducing the compression ratio reduces power and efficiency? I understand that higher compression ratios are harder to have w/o having detonation or misfiring but wouldn't that be counter-intuitive to having more performance and especially higher efficiency?

 

Not necessarily. It really depends upon the turbo that you're using and it's efficiency levels at a given amount of pressure. This is why Subarus can be efficient at 8.8:1 with a smaller turbocharger that is more efficient at higher pressure ratios to get the effective compression their looking for. It is because that smaller unit has its max efficiency at 2.0 pressure ratios or higher. While a Honda that typically usesa with a LARGER turbocharger that is efficient at a lower pressure ratio would not want to be at 8.8:1. You must look at the setup and power level that you're trying to use as a whole; both the engine setup (including static compression) and the turbocharger used (which may vary) together. For most B-series Honda T3/T04E/S BW 200 series owners, over 9.0:1 is best. many of us run over 9.5:1-10.5:1, but we make sure tuning is spot on so that detonation isn't an issue.

 

So the compression ratio of the car is a function of how efficient a turbo is? That if you had a turbo that worked great at any amount of boost, people would be keeping their compression ratios at stock?

 

There are many factors, but generally you want the compression ratio and boost pressure to yeild a lower dynamic compression than the fuel you plan on using. You wouldn't want 10:1 with 20psi with pump gas, while it could be easily done with another fuel.

Many Honda guys go with higher compression because they're scared to get a new MAP sensor to go with more boost. Instead they get a giant, lazy turbo, and pretend that higher compression actually helps spool it up. If you look at many big turbo/low boost setups, the compressor map for their turbo is actually LESS efficient at 5-10psi vs 15-20psi.

Another way to look at it is the benifits of compression and boost. More compression makes the combustion process a little more efficient. More boost stuffs more air into the cylinder, leading to more combustable mass. In most cases, more combustable air/fuel creates much more power than a more efficient burn.

 

Oh ok, your post makes sense. So if you're going with fuel economy, best solution is to try to get the highest compression ratio and if you're just looking for performance, a turbo is more suitable and easier to take advantage of..

 

Well, somewhat, but fuel economy was not really the point of the higher compression. What we're saying is that it is combination of the right turbocharger in its efficient range with a good compression ratio together to get to a particular effective compression (or as HiProfile puts it, dynamic compression).

I don't think that many honda guys go with higher compression because of a new map sensor. that's like spending 500 bucks to save only 50. Many people like the higher compression to assist with certain sized turbochargers and final drives to get that ultimate response out of the car, and use little timing, with great fuel.. Like the Tony the Tiger (he seems to be pharoah to this phenomena). To get the best of all worlds, look to a higher compression ratio (highest is risky) WITH the right turbo for the need.