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Re: Supercharger (archive)

[ Follow Ups ] [ 5-series (E34) Message Board ] [ Msg. Board FAQ ]

Posted by Brock Fraser on March 02, 2000 at 00:22:44:

In Reply to: Supercharger posted by Luc on February 27, 2000 at 09:39:05:

Not to argue with you, but I disagree about a few things here:

The engine's mechanical compression ratio is 9:1, not 10.5:1. 6-8psi is a "walk in the park" on high octane pump gas at this level of boost without an intercooler. Even if it were 10.5:1, how could you claim that 7psi will detonate and 6psi is safe? This is pocket change, guy. When you try to run 20-25psi on pump gas, then we can talk about the knock limit. Using an intercooler at this level of boost is barely breaking even, unless the thing is gigantic like the one you have described, with virtually no pressure drop. I'm curious as to how you packaged such a thing in the front of the car... I'm sure it involved cutting, which the average "kit buyer" won't be thrilled about doing.... OR the 530 has a BUNCH more space since the engine's shorter.

Screw-type or twisted roots, as you pointed out, make more boost at lower shaft speeds... which is precisely what this car needs since it's cammed for the top end and has very weak torque response from the factory at low RPM's. We're talking street cars here, often equipped with slushboxes. You have the best of the centrifugals, having an ATI, but I'm not particularly fond of any of them. In fact, I don't like any superchargers very much as compared to turbomachinery, but this car really needs low RPM help without the expense of a variable geometry turbo.

I'm not sure what you are basing your potentials for power gains on... you may be right - only testing will tell on this particular application. You have to look at the whole package... not just the added manifold pressure, but the engine management, ducting treatment, and how the gas excange process makes use of the boost. When you start looking at the way the engine delta P changes, and the rate of heat release as a function of boost, afr, and timing, all the pieces have to come together. Did you change your valve events?

Corky's book is a pretty good primer on the subject for the people fresh to the craft. If you want something on the next level, try "Design and Simulation of Four-Stroke Engines" by Blair- it's outstanding.

Thanks for your input.

: I designed and constructed the supercharger system for my 530i V8: ATI Procharger P600B with giant intercooler (38"x14"x3") and a $300 fuel management unit, running 8 psi of boost, resulting in an extra 127 HP.
: If you would have some questions or need some information that could be helpfull on a sixcilinder, please email me.
: Some advise: if you are talking about a stock 535i with 211 HP and a compression ratio of 10.5:1 you really don't want to run 7 psi of boost, especially not with low octane fuel and without an intercooler. Not a single supercharger manufacturer or dealer offers a kit with more than 6 psi of boost. For this application is
: 7 psi a risky business (detonation).
: Why a screw type supercharger and not a centrifugal supercharger? This would be much easier to mount onto the engine and to hook up to the throttle body.
: The only real advantage of a screw type supercharger is its linearity. Boost pressure raises in relation with engine rpm.
: Even with 7 psi you will not obtain a power gain of 100-125 HP. No way with a basic kit without an intercooler. A realistic value with 6 psi would be 40-45 % more power, about 85-95 HP.
: Material costs can be well below $2500 for a basic kit (supercharger, fuel pressure regulator, pulleys, belt, hoses, brackets). Basic kits from companies like ERT and Dinan start at $6000.
: About the cooling fan: on my V8 I had a 1" long spacer made that went between the waterpump and the cooling fan clutch. This moved the fan 1" closer to the radiator and freed up more than enough space for the belt drive system. Maybe this will work on an inline 6 too?
: Get a copy of "Maximum boost" by Corky Bell. An up to date book on turbochargers and also very informative for superchargers.
: Luc.

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