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Old 10-21-2001, 12:05 AM   #1
mikeje
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Default Boost vs power ej22t w/2.5 heads

Boost by itself means very little in the overall power equation. If your heads flow better, less boost is required to force the same amout o O2 into the cylinder and at less pressure. The intercooler will be more effective since the intake temps are lower to start with.

Does anyone have a graph of boost vs power for a 2.2 or 2.5 engine at a specific rpm i.e 4000, 4500, 5000 etc? I am interested in how these variables relate.

Specifically, a ej22t block with 2.5 heads
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Old 10-21-2001, 12:23 AM   #2
Eric SS
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Although I don;t agree with the first part of your thread , I seriously doubt there is a chart that will show the graph of boost vs. power because of how many variables there are. I wish there was though

Eric
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Old 10-21-2001, 12:54 AM   #3
mikeje
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Eric:

Why don't you agree? My reasoning is as follows:

The better the heads flow, the more air and fuel (if metered right) will be forced into the cylinder at any given level of boost.
I don't know what the relationship is between head flow cfm and total air mass in the charged cylinder. For example, increasing flow by a measureable 25% cfm may or may not increase cylinder charge by 25%.

Therefore, boost is a secondary indicator of the amount of charge to the cylinders. High boost on restrictive heads will yeild a lower cylinder charge than a somewhat lower boost against high flow heads.

At lower boost, the induction temps after the turbo are lower and the intercoller starts cooling a lower temp air flow= lower temp at the intake valve.
What factors am I not including?
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Old 10-21-2001, 01:18 AM   #4
bsquare
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Valve timing, valve size, injector flow, ignition advance, ignition strength and duration, exhaust back pressure...

There are a _lot_ of things that can be different and have a significant effect on the final power output.


Ben
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Old 10-21-2001, 02:11 AM   #5
Eric SS
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I disagree because, although boost may be one factor in making power, it is probably the most important factor overall in making a forced induction car (Along with CR). Although many other things do help, a car with 4psi of boost if properly tuned, will almost never be able to make as much power as a car with 15psi if properly tuned.


I just think that it is a little bit of an overstatement to say that "Boost by itself means very little in the overall power equation." Especially when it comes to head flow. A very good flowing head in many instances will have less power than a head with less flow characteristics.

Reason being is because flow is actually secondary to cross-section of the heads in a performace engine. A head can flow awesome but if it's cross-section is to big (i.e it flows to much air for the motor), your engine will make very little torque and have horrible low end power. This is because the airspeed in the heads isn;t very fast and cannot properly fill the cylinders or cause enough vaccuum in the cylinder to help draw air in. Although the importance of this is reduced on a forced induction engine, it is still very important.

Also, just because a turbo is producing less boost, and at a lower temperature, it doesn;t mean that it will make more power with a set of good flowing heads. Each turbo is made to run at it's best effiecency at a certain amount of boost. A turbo that works best at 12psi of boost will take a longer time to spool up to 5psi than a turbo made to run best at 8psi therefor blowing off more boost and runnign than a lower than optimum impeller speed.
Again, just my $.02


Eric

Last edited by Eric SS; 10-21-2001 at 02:49 AM.
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