The roots supercharger has its place, but many hard-core track enthusiasts who have bought the new Z06 are finding that they have severe limitations.
A roots supercharger is great on something that needs low rpm torque. They are at home on something that needs a lot of grunt like a truck or rock crawler. Basically, they are good on anything that’s the complete opposite of a car that’s meant to be driven on a road course. This could be a problem since the z06 is supposed to be a track oriented car…
The problem with the roots supercharger is inherent to its design. The rotors within the supercharger are loose fitting and therefore allow for some backflow of air. As air is compressed, it gets hot. When this hot air flows black though the supercharger, it gets compressed again gaining even more heat. While this process is going on, this heat transfers to the rotors and supercharger housing heating the entire unit which further heats incoming air. Essentially, a feedback loop has been created and only gets worse with increased boost and rpm.
Much of this can be manged to an extent. One way to do this is to only produce small amounts of boost which reduces the backflow of air though the supercharger. Another way is to mount carburetors on top of the supercharger which passes fuel through them, like what old muscle cars and dragsters do. When fuel is passed though the supercharger, it acts as coolant and even helps vaporize the fuel (though, the rotors take a beating when this is done). Yet another way to reduce heat is to only make one pass with the car. By limiting the amount of running time, heat does not get the chance to build up.
Unfortunately, the z06 doesn’t do any of this but does have an intercooler which helps a lot; but, I think the limitations of the design are just too much to overcome for hardcore extended track use. GM might not have been too concerned with this because 99% of z06 owners are never going to track their cars. On the street, the roots supercharger will most likely be fine for most drivers since extended sessions of high RPM use are uncommon, therefore, never allowing the supercharger to build heat. But, it sure does make the car look bad when magazines test the car at the track and report about lost power and overheating.
I’m pretty sure the only reason why GM went with a roots type charger was because its cheap and easy to package. Centrifugal supercharger units might be affordable, but for an OEM, they are hard to package since they are not entirely self contained like a roots setup can be. While a roots supercharger can have the charger and intercooler as one package that bolts to the engine easily, the centrifugal type needs piping, an external intercooler, special brackets and a strange belt routing making installation complicated; and in the end, more expensive. The only other option for an OEM would be a twin screw type supercharger. This can have the same neat packaging that the roots type enjoys, but the units are more expensive because the rotors require precision machining.
I think a twin screw supercharger would have been GM’s best option, but another factor other than cost which might have prevented its use is availability. Eaton, who supplies the supercharges for GM is a large manufacturer who has been supplying roots type superchargers for many years to various OEM’s and has the capacity to meet high volume demand. On the other hand, I am not aware of any large scale operations producing twin screw style superchargers.
GM never posted any lap times from their Nurburgring testing. While I believe the car is very capable and has indeed proven very fast, I think the very long lap that is the Nordschleife might have shown some of the limitations of the car. Its only a guess, but I wonder if this is the reason for the lack of a lap time.
6 Comments to "GM needs to stop using roots superchargers."
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I assume the blower on the Z06t is overdriven. If this is the case, then underdriving the blower would keep the blower from spinning too fast at high engine RPM and loosing efficency. Maybe a variable drive pulley on the blower?
It is not over driven. This is just a problem with the roots design. I’m sure the engineers at Eaton and GM made sure to drive the supercharger within its efficiency range. GM had to know all of the shortcomings with the design, but they chose to go with it since they knew that virtually all of their cars will never see a track.
Roots style superchargers provide a specific type of performance feel. They can produce near instantaneous throttle response allowing for power significant power increase while allowing a new car to have a non-aspirated feel. Changing to Centrifugal or other types may not have drivability results looked for in the American public.
You sir, know nothing of physics let alone superchargers. Housing heating incoming air? Really? ever heard of compression? how about specific heat? Rotors with loose fittings? ever heard of teflon stripping ? how about eaton proprietary coating? Roots on zo6? ever heard of tvs? how about internal compression?
by your arrogant igornace!
You sir, know nothing of physics let alone superchargers. Housing heating incoming air? Really? ever heard of compression? how about specific heat? Rotors with loose fittings? ever heard of teflon stripping ? how about eaton proprietary coating? roots on zo6? ever heard of tvs? how about internal compression? STOP SHOVING YOUR HEAD UP YOUR ASS!
I think GM used the roots supercharger for a number of reasons:
1. As you say, there are probably no companies with production capacity large enough that make twin-screws.
2. Eaton has put an incredible amount of R&D into improving the Roots type out of sight. While I cannot provide a direct link on this, I came across a forum on the topic of an American made car (maybe a Pontiac? I’m not from the States so I wasn’t familiar with the car) – but it looked like a number of people had swapped to twin-screws while others had stuck with the Eaton TVS (Roots type) series (same as the Z06 uses) and they were finding that the Eaton was consistently making more power AND low down torque. Not only that, but porting the TVS (and whatever other popular mods people do to superchargers) had little effect on performance – hinting at the fact that it was already very well designed and engineered for max efficiency.
3. Centrifugal blowers, as you know, don’t provide much useful boost until 2/3rds up the rev range unless they are overgeared and “waste” (bleed off) excess air at higher rpm. Would this matter on a 6.2L V8? Probably not! But again it’s probably to do wtih production capacity, and they also tend to be pretty noisy (which I personally have no issue with!).