I totally disagree! I'm not talking large amounts of difference, but on my supercharged V6 4runner, before I got a new exhaust my boost levels were higher. Once I got the higher flow exhaust, the boost levels dropped. It may be magnified more with a boosted motor, I don't know for sure.

Chris

 

Boost is simply a measure of the backpressure... On my 03 Cobra, when we change ANYTHING to do with the Intake tract or exhaust on the car it affects the boost pressure. Just because you are seeing lower boost levels on your 4-Runner now doesn't mean it is making less power. Just means there isn't as much backpressure on the manifold side of the blower... Still flowing the same amount of air (probably more).

 

I don't know for sure, so I guess let's leave it at that! All I know is that with my stock exhaust, due to the backpressure, my low end power was definitely higher. Once the new exhaust was put on, the low end grunt I once had was gone for an increase in upper end power.

Chris

 

Chris, you actually got the right ideas but are relating them to the wrong sources. On your Forerunner you decreased backpressure and yes you did lose some low end torque, but increased some higher RPM power. This in no way relates to compression. compression is simply a measurement of the amount of air/fuel that can get squeezed between the cylinder and the combustion chamber. Compression Ratio is not a measurement of Power, compression ratio can produce more power in the right combination. I would recommend any of the Car Tech books for guys wanting to learn more about engines in general, some of the books are make/model specific, but the basics are quite sound and very easy to read. Cheaper than getting a degree in mechanics! godspeed

 

high flowing exhaust has nothing to do with compression.. go back to the principles of 4-stroke engines...intake,combustion,power,exhaust... these events happen seperatley therefore exhaust wouldnt have anything to do with comp..it might affect the cylinder heat tho...lower it

 

AMEN!

 

I stand corrected, and I appreciate you guys keeping me in line. Have you guys ever tested going from a stock exhaust to an aftermarket exhaust and how your fuel octane level might be able to decrease, depending on the effectiveness of the exhuast?

Chris

 

That would be a great test to do! I have not done it, the easiest way would be to run the bike hard and have an infrared temp gauge at the ready and take a reading, let the bike cool down, and put on a the newer exhaust and repeat the test keeping everything the same except the exhaust systems, I would imagine on a really free flowing exhaust that there would be as much as 10-15 degrees difference in chamber temperatures, that could be enough to allow lower octance ratings.

 

Not only that but it will produce more power too!

Chris