So basically you double your HP at 14.5psi but you make different power at an 8:1CR then 10:1.So my question is, at either compression ratio are you making more or less than double the HP and if so how much?

 

So I did a ton of research and have come up with my own hypothesis. I see that the theory of 14.5psi giving you douple your hp comes from the fact that we live in 14.5psi of atmosphere (roughly). So if you boosted to 14.5 psi you would essentially be doubling the amount of air into the engine and essentially doubling your HP. Of coarse with the upped boost you would be raising air temperatures which would loose a small amount of HP, but in a very generalized manner of speaking you are roughly doubling your HP. Some people like to say 15psi instead of 14.5psi to double your HP to make up for that bit lost with increased air temperatures. But again, we are focusing on a generalization.

In regards to compression ratio, the consensus seems to be that in the relm of compression ratios like 7.5-12.1, for every point, it's a 2% change in HP. For instance, if you have a 100hp 10.1 CR engine and raise the compression to 11.1, you would end up with 1.2hp more. When you are at 13.1 CR or higher though, the percentage decreases and can get as low as 1% or lower as you go farther in compression.

With all that said, lets put it into a senario. Lets say you have an LQ4 running about 300rwhp, or about 350hp at the motor. You throw a turbo on it (with a few supporting mods) and boost it to 15 psi. You now have 600rwhp or about 700hp at the motor. But because of boosting you need to lower compression so you bump the compression down from 10.1 to 8.1. That's 2 points or 4%. So, 4% of 700hp is 28hp or roughly 30hp. So you end up with 670hp at the motor or about 570rwhp.

That's the best guestimate I can come up with. I know a bunch of people could shoot a thousand holes in this theory about how if this and this happened then it wouldn't be exactly 2% and blah blah blah. But if you look at all the information you start with, like the 2% theory and the 14.5psi doubling your HP and set them as your foundation, I bet it would be astonishingly close to a real world dyno pull. If I were to guess it would be about 30hp + or - from an actual pull on a dyno.

 

Whatever that guys says ^

 

Keep in mind also that more and more hardcore setups are running higher compression in boosted motors(quite a few of the turbo guys here are) the reason being it spools the turbo better and off boost power is better. Everything else being equal the only problem with doing this on the street is the low octane fuel you will find at 99% of service stations. So if you go this route its either C16 or E85/98. You will most likely have to get a 55 gallon drum to store at home if you choose C16 or E98. If not you are left with the option of lowering compression and using a band aid like methanol injection.