Question about roots chargers and torque

[Breadfan]

First this is intended to be a technical facts discussion, don't want to get into a roots vs. centrifugal thing...but this is something I've been wondering for awhile and haven't found a good technical answer for.

I know Roots chargers are good for developing low end torque. At the same time, I hear most of them aren't getting into boost until 2500rpm or so.

So my question is this...if you're not getting into boost until 2500rpm or so, then how is torque in the low rpm ranges boosted so much? Just curious for the technical resons why a roots charger makes so much low end torque.

I'm sure you're making some boost even at the low RPM's, maybe that's enough. Still, I'm mainly wondering why the design of the roots allows for such high low end torque.

Thanks!

[MikesMerc]

I know Roots chargers are good for developing low end torque. At the same time, I hear most of them aren't getting into boost until 2500rpm or so.

So my question is this...if you're not getting into boost until 2500rpm or so, then how is torque in the low rpm ranges boosted so much? Just curious for the technical resons why a roots charger makes so much low end torque.



The "not getting into boost until 2500 rpm" is not correct.

Most roots blowers are making significant boost right off idle depending on how the blower is pullied. One of the advantages of the roots blower is that the manifold is always seeing some boost. So you have boost available at the crack of the throttle. Although some boost is bled off with a by-pass valve that relieves the pressure when the throttle is closed (or not open far), there is still a small amount of "charge" in the intake manifold at all times.

For example, the Eaton M112 on my Trilogy kit makes 8 PSI at 1600 rpm, 10 PSI at 2000 rpm, 12 PSI by 2400 rpm, and max 13.5 PSI by 2800. Now, I have a smaller pulley which accounts for bigger boost numbers than the stock trilogy kit, but the same principles apply. The stock kit can make 5 PSI at 1600, 6 PSI at 2000, 8 PSI at 2400, and 9 PSI by 2800.

That low end boost is what produces the low end TQ :)

[BillyGman]

...if you're not getting into boost until 2500rpm or so, then how is torque in the low rpm ranges boosted so much? Thanks!Everything Mike stated above in his post applies, and in addition to that let me also point out to you that making a ton of torque by 2,500 RPM's as you've pointed out above^IS considered "LOW-END" torque since even the factory stock torque converters have a 2,400 RPM stall speed, which means that the total torque of the engine isn't even applied to the transmision and therefore to the rest of the drivetrain until 2,400 RPM's anyway.

And keep in mind that with the stock Marauder engine(non-supercharged), the amount of torque at 2,500 RPM's is much less, just as it it is with other types of superchargers besides the roots type. With the roots supercharger, the onset of boost, as well as the total amount of boost isn't as dependent on engine RPM's as it is throttle position. I have the Trilogy/Eaton roots supercharger with the standard 9.5 PSI pulley on my Marauder, along with the factory stock throttle body, and the bypass valve doesn't fully close until about a little past the one third throttle position. So the supercharger doesn't get into the boost mode until just before the half-throttle position, regardless of what gear the transmision is in, or what RPM the engine is at. I hope this helps your understanding.

[jdando]

Ummm boost. I need boost.

Thanks for the explanation guys.

jeremy

[Jerry Barnes]

Mike and BillyG are correct. This is why we went with a Roots type supercharger for our kit. The car is very heavy and needs torque, depending on throttle position, very early to move the car quickly. Big torque, immediately was our goal and I think it has been very successful. There are several dyno charts comparing the roots vs other types of superchargers. These charts will be very useful in answering your question.

Thanks

Jerry

[RoyLPita]

Ummm boost. I need boost.

Thanks for the explanation guys.

jeremy

I need it too. Oops, need to get the MM 1st. I had the pleasure of trying out a "Trilogized" MM. I didn't get heavy into but I went to about 3400 RPM in 2nd gear and it flew with no boost. It still had it there w/o going all out.

[Breadfan]

Thanks guys! I had figured there might be some boost very early on, didn't expect it to be so high at the lower rpms though! I was thinking if any boost at all it'd be 1-2psi.

That was a great explanation! :)

[sailsmen]

The Roots has more surface area and moves more air at lower RPMs. The Roots was originally designed to pump air.