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Old 12-07-2007, 09:08 AM   #1
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Default Will it work?

I have an idea and would like to hear your thoughts about whether or not it will work....

I am in the process of installing a coldside MP62 in my car. Not that it matters, but I am running a MSPNP, 460cc injectors, and progressive water injection.

I am starting off with a 115mm crank pulley, which should put boost levels around 13psi.

What I would like to do (if possible)....

I would like to run a 140-150mm pulley and limit the boost to 15psi. So yes, I want to be like you turbo guys.

The bigger pulley would give me much more HP and TQ on the bottom end, which is where I want it being that my car is used for autocross 90% of the time.

My theoretical solution....

Put an adj. air pressure regulator or adj. blow off valve on the intake manifold. In theory this should do what I want. The question I have is what to use. Will a small industrial pressure regulator threaded into a 1/8 NPT port work? or Do I need the size and volume of a blow off valve that is able to flow more air quicker? Running a 140-150mm pulley, I shouldn't hit 15psi till 4-5k rpms and the ramp up will be linear. Being that the ramp up is linear, in theory, a small regulator should work. It will slowly open more and more to modulate the boost. But this is thoery. I have a 1/8 NPT port free on my manifold and it will be easily tested, so obviously it will be my first plan of attack.

Another question is how much is this going to heat up the air? If I am pushing 20+ psi out of the blower and then bleeding off the excess, I know the manifold temp will be higher, but how much higher.

As you know the downside to the coldside is controlling heat. I really hate to start digging myself a hole.

I know this is a turbo forum, but I am over here because you guys are much more knowledgeable about FI Miatas than the general Miata forums. Please don't bash my set up and tell me I should have bought a turbo. I did some serious research and test drove many FI Miatas before choosing my path. I feel I choose the best path for the way my car is and will be used.

What are your thoughts?
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Old 12-07-2007, 10:48 AM   #2
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I would think a controlled boost leak like that would put a lot of additional stress on the blower. Probably not as much as if it was a turbo though. Since the amount of boost is ultimately a funtion of RPM and pully ratio.

Could you work something like a wastegate actuator on the blower's bypass valve? I'd be concerned that a regulator would be somewhat course right at the threshold.
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Old 12-07-2007, 11:04 AM   #3
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I wouldn't try to control boost in your situation. However, if your gonna do it anyway, your gonna need a port bigger the 1/8 pipe. Something around 1-1.25" is more like it. I'm gonna do something similar with my SC setup and I'm going to try to use a bov in the intake pipe controlled by the EBC in MS. It would just be controlling a BOV instead of a wastegate. Seems like it should work, but I haven't tested it yet. The difference is I will be running the blower unthrottled, so I have to have something that will dump air when the throttle slams shut, as the SC will continue to pump air.

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