Ok.. Please dont laugh at this basic question.. but I need to know the answer!

It's driving me LOCO! :crazy:

If your cruising at 3000rpm (I know thats a little high) with a car with an ATI or Vortech supercharger.. I'm guessing you wont have any boost unless the car is full throttle.

How is it possible to not have boost at 3000rpm during normal driving, but have boost at 3000rpm at full throttle? Since its belt driven, isn't the blower speed always the same at a given RPM? And.. does this mean the the blower is always working the same amount whether your full throttle at 3000rpm or just cruising?

Any explanation would definately help me sleep! :sleep:

Thanks! :lurk:

 

You know the saying; there are no stupid questions here, just stupid answers.

The key to your question is something called a blow-off/bypass valve. It has a intake manifold vacuum activated plunger that closes when there is no vacuum (vacuum drops in the intake at WOT when the TB is open, equalizing pressure on both sides of the throttle plate). It sits somewhere between the blower outlet and the throttle body and vents out charged air when the throttle plate is not open enough.

You’re correct in assuming the blower moves about the same volume of air at a given RPM, but it is not really putting any load on the belt unless the blow off valve is closed. Just like any pump, it’s pretty easy to spin as long as the air going out is not being limited to a specific flow.




[Modified by QuickSilver2002, 7:49 AM 5/26/2003]

 

Hi QuickSilver,

Thanks for the response! :cheers:

It sure sounds to me like the blow off/bypass valve will be venting A LOT of air at 3000rpm and higher?

Do you think the longevity of a blower is then affected by the size of the pulley? If you want more boost, it's ALWAYS spinning faster with a smaller pulley?

Just curious.. and I'll soon let this topic vanish into the archives... :leaving:

 

Smaller pulley = more air moving at a given RPM and more load on the crank/belt at a given RPM. However, the blower internals are usually the least of your worries. The head units are precision engineered and designed to spin very fast. It’s nothing like a motor and there is very little resistance/heat when the air is being vented.

The problem is the load the motor and belt will take when you are actually in the boost.

Here is something to think about and I'm not exactly sure how accurate the #s are. A N/A motor producing 500hp is only putting the strain of producing 500hp on the internals. A supercharged 500hp motor is actually putting a strain of about 600hp on the internals due to the fact that about 100hp is being taken to spin the blower. A turbo would also put some additional strain, but it would be much less since it is only adding backpressure on the exhaust side.

The smaller pulley will always result in more boost and power, but will also put way more strain on the motor. It would probably not cause any additional wear or strain under normal driving conditions when the blow off valve is open.

 

Excellent info! :cool:

You bring up two interesting points.. I wonder how much hp is taken just to spin the blower at normal cruising speeds, and then what % of hp is needed for the blower per lb of boost? Or I wonder if its just a straight hp #? :confused:


[Modified by MyFirstCorvette, 12:19 AM 5/27/2003]

 

OK.. Now I'm more confused! :cry

I just spoke with a friend of mine that has an ATI Procharger on his naturally aspirated 350cid Monte Carlo.. When he first installed the kit, it did not have any type of blow-off/pop-off valve and he only got boost at full throttle? :confused:

H E L P!! :eek:

 

Seems odd, but I guess it is possible. Centrifugal superchargers produce flow in a non linear fashion. I think it generally accepted that boost builds exponential to engine speed on a centrifugal. This means the flow really goes up fast in the top of the rpm range, but initially rises real slow. As an example, about 7psi at 6k would only be about 1.7psi at 3k. It’s not 3psi like most would think. Also remember that boost is typically measured in the intake, so it is possible to have boost in the inlet tract without seeing any on the gauge. In fact, you can have vacuum on the gauge and boost in the inlet tract. It all depends on how much air the throttle plate is letting in.

Centrifugal superchargers are not susceptible to back spin like a turbo, so it is conceivable that you could run one without a blow off valve. The unneeded air would just get bled back out the blower through the impeller. It would hurt fuel economy and I really doubt you could do this on a fuel injected car, since surging and metering reverse flow with the MAF would be a big problem. It might work ok on a carbureted setup, but I honestly have no idea.

Anyways I think it is safe to say that you want a blow off valve with a centrifugal supercharger. It helps to eliminate the parasitic effect of the blower while under normal driving conditions, but also eliminates surging when the throttle plate is suddenly closed under boosted conditions.

 

I seriously doubt that spinning my blower is robbing my engine of 100 horsepower. I know it takes some, but one hundred sounds a long way off the charts.

Mike M

 

I think he meant that the numbers were mostly just an example.

 

Yea, the #s are just an example that I read somewhere. It would be interesting to know the real #s (ie hook up an externally driver blower with the same cfm). 100hp seems like a high # at first glance, but when you consider 690 crank hp, that is only about a 14% loss to drive the belt. A quick calculation I did says that would be 87 lb-ft of torque to drive the pulley at 6000 rpms. That seems possible, considering some people have belt slippage on the dyno. It's interesting to think about, but there is really nothing we can do except build the bottom end stronger when we turn up the boost.


[Modified by QuickSilver2002, 6:35 PM 5/27/2003]

 

Here's another brain twister..

When you say 87ft lbs of torque... is that at a steady RPM or accelerating? I bet it takes a lot more power to increase the speed of the blower than it does just to turn it at a constant RPM.

;) <- headache