Deciding on the boost level attainable has a lot of different factors. One would be octane of the fuel. Another is static compression ratio. Combustion chamber design also plays a big role in this. Also the size of turbo, and maximum engine power before failure has a role in all this also.
Some engines can handle a lot, such as the 2jz, in stock block form has been known to handle close to 1000hp and over at times with some reliability. The RB26 has a reputation for being able to handle these power levels, but in no where near stock form. The 2jz is a beast, it is built like a desiel truck engine. It has huge bearings, big bolts, a lot of meat throughout the whole block.
The type of fuel is the first limiting factor in the amount of air you can push. It is directly related to compression also. They go hand in hand. An easy way to tell if your pushing it too far, is when you have to start to drop timing instead of holding static or increasing timing, while power is also increasing. If you find mbt at 1 bar increase boost to 1.2bar and all the sudden need to decrease timing by 6 degrees, you have met the limit of your fuel/compression combo. If you can increase boost leaving timing the same or within say 2 degrees of same with no hint of knock, you can turn it up, until you reach one of the limiting factors from above.
The size of the turbo, or more the efficiency of the turbo in your boost level also has a large factor in this. The less efficient the turbo is at the boost level you are at, the more heat energy is needed to run in and put in to your engine because of it. Most stock turbos are usually too small for the application in the first place. They are made for response and have no real consideration of what the car actually needs to run properly. Some are undersized because of limitations placed on the designers by car makers. Whether it is a number not to go over, or a number to stay away from so it doesn't get in the way of the more expensive cars, they are always told to stay in check.
Limits of the engine is by far the least of most peoples concern. I have seen a few blow over the years because of this, but this is the exception not the rule. I would venture to say over 90% of engine failures in turbo cars are from knock damage. Whether it is a broke piston, or melted cylinder this by far is the most common. Which brings us back to the first part of this. You must be careful with the static compression/ fuel settings. One bad thing is that it will be a different number for every engine type. Which makes my job all the more interesting.
Scott is the owner of
Stec Motorsports, he has been in Motorsports for 12 years
