That's actually a pretty interesting idea that I've never thought of. The only problem I can see is if you keep the MAF. If so, you would be letting out metered air and it would run very rich, just like if there were an actual boost leak. If you run an aftermarket computer using a MAP sensor that might actually work.Mettler wrote:Points taken mtcookson, just a couple of points in your essay that I'd like to query...
You comment that you need to set up a centrifugal to be boosting peak desired psi at redline, and if set up to do so earlier in the rev range you'll blow the motor. Wouldn't that be easily fixed by installing an adjustable pressure valve in the intake (in fact, the perfect place would be the frost plug in the side of the plenum chamber) to bypass through to the exhaust ? That way maximum boost pressure is always set at the valve, and you can overdrive the supercharger to boost harder, earlier.
The valve itself might have to be a pretty good size to let out enough air. You might be able to use a standard wastegate but I would definitely do careful testing to be for sure.
On the overdrive you have to be careful of one thing... if you are able to control the boost where you want it, you have to becareful not to overspin the compressor. I'd leave enough room to where you were at least a couple thousand RPM below maximum. You'll have to calculate the pulleys and such but you should be able to get quick response if setup correctly.
Quote »Further to the overboosting problem... if a roots blower pumps 'x' volume of air per revolution, then as it speeds up, isn't it pumping an exponentially higher quantity of air as the RPMs increase ? How is this any different if you are only wanting to get 6-7 psi into your engine, and you're getting that straight off idle ? How do you prevent overboosting with one of these ?[/quote]I'm not 100% sure, but I think basically the roots supercharger pumps enough air into the intake to keep up with the engine and provide a constant 6-7 psi if you set it at that. Now... roots superchargers actually tend to lose a bit of boost on the top end, unlike a screws supercharger.
Roots and screws superchargers also have recirculation valves used for when you're at idle so that the supercharger doesn't pressurize the intake while idling. I'm not sure if they're used while running, I'd say probably not... but that could be a potential other way to control it.
Quote »As for fuel efficiency... wouldn't a roots type blower provide the worst fuel efficiency out of three engines, each of which is equipped with a different type of forced induction ? And also the worst reliability, since the air entering the cylinders is significantly hotter ?[/quote]Compared to stock, I'd say fuel economy would go up a little bit. Generally when you add on stuff like a supercharger you'll modify quite a few other things increasing the overall efficiency of the engine. When cruising, the recirculation valve would keep there from being positive pressure in the intake so there wouldn't be any load on the engine from the s/c.
The roots superchargers themselves are actually quite reliable. As for reducing the reliability of the engine... at low boost I'm sure it would last quite some time just as if it were to come stock with one. It basically depends on the maintenance and such of course. If you make a lot of power, it'll be just like any other setup, it'll reduce the reliability accordingly.
Basically... at low boost, the engine will probably last longer than you'll want to drive the car with any of those setups.
Quote »Where does the aftercooler go on the roots blower that's been set up for the VH engines ? (The one that Wes can supply.)[/quote]It would basically be sandwiched between the supercharger and and the intake manifold. If I remember correctly, that setup uses an M90 and there could be a good chance one designed for a Grand Prix GTP could work.
Quote »Lastly, surely an engine that already produces a significant HP & torque output with a good set of extractors (340+ HP at the flywheel) in a lightweight car with good gearing and a 4.375:1 final drive ratio, is going to have mean as acceleration out of corners in almost any gear, even BEFORE supercharging it ?[/quote]Essentially... but its more about constant power so to speak. Peak power means nothing compared to having constant power. Say for instance you have an engine that peaks at 400 hp but only averages 150 hp from 1 rpm to 7,000 rpm. Take another that peaks at 200 hp... but makes that 200 horsepower from 1 rpm to 7,000 rpm (200 hp average). The second engine is clearly going to be faster, even though the peak is lower.
This is essentially how the roots, screws, and turbo is going to produce power compared to the centrifugal (if doing a standard setup).
Newer normally aspirated engines that have variable valve timing and all the new and fun technology stuff tend to have a very flat torque curve, which is very nice to have (its like the constant power I was mentioning above). With a centrifugal setup though, since boost is linear, the power will be peaky. It would be same with a huge turbo. They make a ton of peak power, but nothing down low. That's where the dyno queens come from. Tons of power, but not all that fast.
(Hopefully I didn't make that sound too confusing... I just about confused myself typing that. )
STS's dyno comparison kind of shows what I'm trying to say: http://www.ststurbo.com/remote_mounting___
Quote »I ask all these questions because I'll quote up the price difference in parts & fabrication for a centrifugal & the roots supercharger from here, and weigh up the pros & cons, and make a decision from there !
[/quote]That'd be the best way to do it.
