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June 9, 2011 5:43 p.m. Taiden HalfDork

I consider myself to be well versed (enough) in turbocharger theory, but I'm completely in the dark when it comes to superchargers.

Does anyone know any links that do a good job of quickly teaching a turbo savvy-ish person about superchargers?

It seems to me a lot of people are running supercharger setups without an intercooler. I thought superchargers blew hotter air? I could be wrong here.

And what about the different types of superchargers. Roots, eaton, centrifugal?

And finally the basic question.. under low load cruise will a supercharger drag enough to effect gas mileage?
June 9, 2011 6:01 p.m. Taiden HalfDork

which reminds me

k20 + m62 + bolt ons
June 9, 2011 6:06 p.m. Will HalfDork

Any time you compress air it gets hotter. Not running an IC is just a function of either cost or packaging, but either way, I don't recommend it.

Roots and Eaton blowers are the same thing. They make great hp at lower RPM levels, but they're not efficient at higher RPM. That's the point at which centrifugals and twin-screws like Whipples and Lyscholms make great power.

If you have a bypass valve, gas mileage won't be affected at all with a blower.
June 10, 2011 6:46 a.m. Taiden HalfDork

other than overall packaging, what would be the strengths of using an m62 over an m90? usually with turbos you size them properly so they spool at the rpm range you need them most, but my (limited) understanding of roots blowers is that there is no "spooling". They are spinning as fast as they are going to spin at a given engine RPM all the time.

so do you increase boost pressure by going with a larger supercharger?

is there an increased amount of drag when cruising with a larger supercharger?

everywhere i read, turbos on paper are better than superchargers. more efficient at moving air, lower charge temps, ability to easily vary boost levels, less drag on the engine at cruise... yet there has to be a reason why superchargers are used. 'turbo lag' and low end torque?
June 10, 2011 8:04 a.m. ppddppdd Reader

Taiden wrote:

everywhere i read, turbos on paper are better than superchargers. more efficient at moving air, lower charge temps, ability to easily vary boost levels, less drag on the engine at cruise... yet there has to be a reason why superchargers are used. 'turbo lag' and low end torque?

Don't forget packaging, cost and ease of installation and removal.
June 10, 2011 8:19 a.m. z31maniac SuperDork

Taiden wrote: so do you increase boost pressure by going with a larger supercharger?

is there an increased amount of drag when cruising with a larger supercharger?

By changing the pulley on the supercharger, you control how fast it spins and how much boost it will produce.

I'm not as familiar with screw-type and such, but Centrifugal superchargers build boost with RPM, in a linear type fashion.

Someone please correct if I am mistaken.
June 10, 2011 8:30 a.m. Schmidlap HalfDork

Taiden wrote: other than overall packaging, what would be the strengths of using an m62 over an m90? usually with turbos you size them properly so they spool at the rpm range you need them most, but my (limited) understanding of roots blowers is that there is no "spooling". They are spinning as fast as they are going to spin at a given engine RPM all the time.

so do you increase boost pressure by going with a larger supercharger?

is there an increased amount of drag when cruising with a larger supercharger?

Other than packaging, the larger M90 is going to have higher intertia due to it's parts being larger and weighing more. I don't know if it would be enough of a difference to be noticeable.

You can increase boost by going with a larger supercharger or by spinning the supercharger faster (instead of 1.8 supercharger revolutions per engine revolution you change the pulleys and make it 2.1 supercharger revs per engine revs - note, these numbers were pulled out the air). There are limits to how quickly the supercharger can spin. You can spin it so quickly it breaks, though this is unlikely. The likely thing is that you spin it so fast that it can't efficiently compress the air, so you end up with really hot air or you lose boost. Likewise, spinning it too slowly won't put it in it's max efficiency area, so you might need to go with a smaller supercharger instead of the bigger one. If you're familiar with turbo compressor maps, a supercharger compressor map will make sense to you. Eaton posts their's on their website.

Eaton Supercharger Site

Bob
June 10, 2011 4:50 p.m. Will HalfDork

I believe a smaller blower pulley will make more boost, not a larger one.

A larger supercharger will not make more boost pressure, but it will move more air with each rotation, requiring less boost to move the same volume of air, thus producing less heat.
June 10, 2011 5:34 p.m. Keith SuperDork

Read the Forced Induction chapter of How To Build a High Performance Miata.

The reason you don't see intercoolers on superchargers as much is that it's harder to do right. Superchargers put the throttle body before the charger, so you've got greater throttled volume. Throw in some drag from the blower and now you're starting to have some hassles dealing with idle control. Air/air intercoolers add to this throttled volume, making things worse. So a lot of aftermarket supercharger manufacturers just close their eyes and ignore it. In fact, superchargers are less efficient than turbos at compressing air so they actually need the intercooler more.

I'm only talking about positive displacement superchargers here - Eaton, Roots, Whipple, twin screw, etc. Centrifugals act more like a turbo, but without that pesky boost down low Centrifugals need bypass valves, keep the throttle body in the stock location, etc.

Superchargers do affect gas mileage slightly. The bypass is a good step, but it doesn't completely compensate for the fact that you're spinning a mechanism around. We usually see about a 1-2 mpg drop in fuel efficiency with supercharged Miatas and a 1-2 mpg gain with the turbos.
June 10, 2011 6:12 p.m. Taiden HalfDork

Keith could you speak towards the differences you find in comparable supercharger and turbocharger setups on those 1.8L miatas.

I feel like a lot of people start out with the idea of going with a supercharger, and then scrap it for a small turbo setup.

I figure you're the person with the most experience doing something similar to what i'm aiming for. (Low end torque on a 1.8L 4 cylinder using forced induction on a nothing special motor (no VVT or VVL etc)
June 10, 2011 6:41 p.m. Keith SuperDork

The biggest difference is usually in the implementation. You've got to choose the right hardware - size of the blower/compressor - and mount it right. Get the right turbo and it'll have low end torque. Get the right supercharger and it'll keep pulling hard to redline.

It's actually harder to do a good supercharger system in a lot of ways, as you've got to get your belt alignment juuuust right and manage to transfer a fair bit of torque through the belt without resorting to massive tension. An MP62 spinning fast enough to make 10 psi of boost on a Miata eats up 36 hp - and that's all run through the belt. Then you have to deal with the driveability problems due to the big throttled volume, and now we get into intercooling. It's a lot easier to put together (or buy) a turbo manifold and downpipe.

However, superchargers are a bit easier to tune off-idle because they're quite linear. x rpm * y throttle = z boost, every time. They're also more resistant to detonation thanks to the low backpressure, which does help make up for the lack of intercooling somewhat.

Generally speaking, I like turbos better. They're just better mannered, with less vibration and noise when you're not on it. The shriek and instant hit of a super is a lot of fun, though. There's a hard edge to a supercharged car that you just don't get on turbos unless you're romping around at high rpm.

Here's a comparison you might enjoy. The Janel in the story is my wife. Yes, it's being used to sell FM turbos. But she's convinced, and don't you even think about taking her turbo away from her.
http://www.flyinmiata.com/turbos/janel.php
June 10, 2011 6:43 p.m. erohslc Reader

Also check out the Eaton supercharger site, has some tech info, maps, etc. Compared to a turbine compressor (turbo, centrifugal), most superchargers are less energy efficicient. Much of the extra energy winds up heating the air. So yes, an IC is highly recommended. But the boost level is largely independent of RPM, ie 7 psi from 500 RPM - 8,000 RPM. For part-throttle cruise efficiency, many setups offer a vac controlled bypass system that recirculates air around the compressor. And technical advances in manufacturing and materials have reduced the friction and sealing losses dramatically. So it all depends on what your goals and application are.