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Old 04-10-2009, 04:42 PM   #1
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Default Boost vs timing

OK, I'm just starting to go over my data here. I'm going threw my scans from last year and I was running a 3.3" pulley with a MAP psi reading of 26# at max RPM. I have just finished swapping out my top end with a ported LIM and supercharger. While doing this I also dropped pulley size to a 3.1" pulley. The general rule is, each .1 drop is about 1# of boost. So stock boost is about 7psi, a 3.3" pulley is .5" smaller, so about 12 to 13psi. 26-14(air pressure)=12psi of boost. Now boost is really back pressure, and that causes heat, heat causes KR, and that robs you of timing.

So following those rules dropping another .2" should bring my MAP psi up to 28# or 14 pounds of boost. But it didn't. Porting the LIM and supercharger actually allows the air to move more freely, there for less back pressure. I have 27PSI at the MAP or 13 pounds of boost.I lost a pound just due to porting.

So far I have added 1* of timing and still no KR, about to add another degree. Might be able to add a total of 3 or 4 degrees. Part of this is due to my intercooler, but some is the porting. Each degree of timing that is added is about 5 more HP. So if a simple ported LIM can gain you 2 degrees of timing, you just got 10 more HP.

Keep in mind that adding a degree of timing will net you more HP then dropping a .1 in pulley size. HP is not all about pulley size. It a balance of the most timing you can support, and the smallest pulley you can run that will allow that timing.
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Old 04-10-2009, 07:24 PM   #2
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I had said in a way earlier thread I thought you would lose a little boost pressure when porting. Just from my experience in using roots blowers in industrial non motor applications. But is flow enhanced? Trading more flow for a little less boost might not be a bad trade off?? Crunching numbers off a tuner gives you baselines to work from but isn't it going to take real world comparisons (track times) to know whats really working?
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Old 04-10-2009, 07:35 PM   #3
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I'm with Venom on thinking that with the PSI drop, volumetric air flow will increase. Namely because of the porting. If you move air through a smaller opening, velocity increases with pressure and volume decreases. Seeing that you've opened the LIM and SC, you have a larger opening you should have more volume and less velocity and pressure.
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Old 04-10-2009, 07:41 PM   #4
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This is all very true. I didn't loose any flow, I'm still moving the same if not more air it'* just not boost stacking. Boost stacking hurts, this is why you don't want to run a pulley that'* to small. Most pressure means the supercharger needs to work harder to maintain that pressure and that robs you of HP, not to mention the higher pressures increases the air temp causing KR which also robs HP. As I said, the timing you can support, and the smallest pulley that will allow you to hold that timing.
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Old 04-10-2009, 07:56 PM   #5
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That'* what I was kicking around in my head. More flow without stacking the boost. The positive flow outweighs the negative pressure. In an industrial application, it meant more bubbles in plating solution tanks. LOL With a car you cant just go down the ladder and check the tank. I got better volume by increasing piping diameter and had the benefit of an variable pulley. Just cranking the pulley didnt do the job. I had reached a point where I couldnt physically cram more air done the system given the diameter of the piping through the roof. Just adjusting the pulley alone caused heat and excessive blow off, not more bubbles. Make sense?
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