49 replies to this topic

[Crazy Canuck]

Which is better: turbo or supercharger? I was thinking about that last night and came to the conclusion that superchargers were better because you get instant access to the additional power it provides. With a turbo you need to get the engine speed up before you get any extra ‘juice’ – turbo lag. Then I thought to myself: ‘If superchargers are better then why are they not used very often? Why are turbos more popular? Do turbos have another advantage that I don’t know about? Perhaps there is a fuel consumption difference or turbos provide more power per unit of air.’

If anybody can help shed some light on this I would be very appreciative.

CC

[Darren Galpin]

Superchargers are driven from the engine, so there is less power available to produce motive force. A turbocharger works off the exhaust gasses, so you get the power "for free". However, as the supercharger is an engine driven pump, you can get more low end power from a supercharger. The turbocharger requires the exhaust gasses to be above a certain speed before reaching their operating speed, so tend to provide more top-end power. I think.....

[carlos.maza]

Crazy Canuck:

Basically, a supercharger is driven from the engine's crankshaft, so there is a power loss involved, but yes, you get instant access to the additional power it provides.
With a turbo charger, the power that moves the compressor comes from a gas turbine, which uses the exhaust gases from the engine, which is a "free" energy that otherwise is wasted into the atmosphere.
Regarding the "turbo lag", modern technologies tend to reduce it. For example, in the F1 Turbo era, all the manufacturers used twin turbos (less inertia) to compensate for this.

Hope it helps

Carlos

[Crazy Canuck]

So neither is “better” but they have particular strengths and weaknesses, just like all things in design it’s a compromise.

Carlos, with respect to reducing turbo lag I’ve heard of another technique where the turbo fan continues to spin even if the motor is idling. I’m not sure if this is true or how it’s done but a friend of mine told me that they use this idea on World Rally cars.

Do you know which device provides more power per unit of air? Is that even a relevant specification?

CC

[MPea3]

i believe some of the l;ancia rally cars of the 80's had both super & turbo chargers, and that the super charger gave the engine immediate boost without lag and at low engine speed, then phasing out as the turbo boost pressure climbed.

[marion5drsn]

Belt, gear, chain or any other mechanically driven superchargers have one advantage over turbine drive superchargers and that is they almost instantaneously pressurize the intake manifold over atmospheric pressure (1 ata or 14.7 p.s.i.a.). However the turbo has the big advantage in thermal efficiency since it uses the usually wasted energy in the exhaust gages to drive the turbine. This was the big reason that Lockheed P-38 Fighter of WW-2 had such a long range as opposed to fighters using the Rolls-Royce Merlin engine, which has a gear driven supercharger. Mechanically driven superchargers use lots of shaft horsepower to pressurize the intake manifold. The second big negative with mechanically driven superchargers is the vibration problem. V-8s with 180-degree cranks are very prone to failure due to the Secondary Shake. The most famous case is the WW-1 Hispano-Suiza with bevel gear and pinion wheels. V-10s with properly balanced cranks at 72-degree bank angle and crank pin spacing should not have this problem.
The turbine driven supercharger was invented by the French in 1918 by the French Gateau (sp) and tested on the French fighter SPAD. How would you like to fly an aircraft made of wood and canvas and painted with aircraft “dope”? A fire looking for a place to happen! This must have made for a very nervous test pilot. M.L. Anderson

[david_martin]

In rough terms and for a given engine, the power increase produced by a mechanically driven supercharger is directly proportional to the engine speed, whereas the power increase produced by an exhaust turbine supercharger is proportional to the square of the engine speed. Hence turbochargers are better suited for high engine speeds and mechanical superchargers for low engine speeds. That was the reasoning behind the Lancia Delta Integrale engine already mentioned, which had both and used the a clutch on the supercharger drive to cut the supercharger out once the turbo boost began to build up.

The turbo lag reduction system mentioned was pioneered by Toyota and works on a very simple principle - if the engine is not making enough exhaust gases to keep up the turbine speed, then make some by another method. Additional fuel is injected into the exhuast manifold and allowed to burn - producing gases to keep the turbine speeds up. That is source of the characteristic banging and popping you hear from WRC engines on trailing throttle.

[Bluehair]

I always felt that if a centrifugal supercharger were geared to spin as fast as a turbocharger at its maximum rpm, that it would be as good as the turbocharger. For one thing, the exaust would be less restrictive, and there would be one less turbine to spin ( 1/2 the weight to rotate). In addition, you wouldn't have heat transferring through the shaft from the exaust.

I have never seen a decent comparison, in which both the turbo and super chargers are spun at the same rpm, have the same size impeller, and both have the same intercooler.

Furthermore, the energy that turbos provide isn't totally free. That implies that the exaust flows around the turbine without restriction. Try disconnecting the boost side turbo (no boost allowed into the intake), and you'll see that a na engine makes more power with nonrestrictive headers than by spinning a turbine.

[pa]

Most of the lag encountered by users of earlier turbocharged engines is now substantially offset by the use of "variable vane" turbochargers. These devices temporarily alter the pitch of blades spun by exhaust gases, thereby allowing the turbo to spin up far more quickly.

[Yelnats]

Blue Hair. The centifugal supercharger (Are there any in use now?) did indeed spin at some remarcable RPMs in the early fifties when it enjoyed a brief popularity, the F1 BRM being the most significant example that I am aware of. But it suffered from all of the turbochargers disadvantages and had a few of it's own.

As the boast from this device was roughly proporional to the square of the engine RPMs, it would generate insignificant power at anything less than 2/3 of maximum rpms and was almost undrivably powerful from that point upward. The centrifical supercharger was enourmously sucessfull in fighter aircraft in WW2, as these engines were fairly constant speed devices but were spectacularly unsuccessful in race cars due to their unlinier response and lack of low speed tourgue.

All the superchargers I have seen recently are the positive displacement Roots/Vane type which produce a boost in direct proportion to the engine RPM so are most the drivable superchargers but anable to match the high end boost of a turbocharger unless massivly huge, witness the units on American dragsters and try to imagine one of these in a F1 car and you can see the problem.

[Crazy Canuck]

Great stuff! You guys are a real wealth of information, thanks a lot. I guess that Toyotas technique of generating extra gases would certainly increase fuel consumption. This wouldn’t be an issue on shorter rally stages but surly would be detrimental in an endurance rally like Paris-Capetown.

Bluehair – your points are intriguing and remind me that a coin always has two sides. I would also be interested in a comparison of similar sized superchargers and turbos.

Variable vane turbos – wow! I imagine these units would be very difficult/expensive to maintain. Is the pitch change controlled with some form of mechanical actuation? Sounds like a mechanics nightmare! Small mechanical systems that rotate at high speed in an high temperature environment.

CC

[MRC]

I would have to agree with the previous statement that a turbocharger is not free power. If you put on a turbo, you still have to add more fuel to maintain your chosen AFR. You also pay in backpressure.
A turbo does not have to be inferior to a supercharger on the low end of the rev range. A turbo having a higher pressure ratio for a lower mass flow rate, will provide better low end charging, but you are more likely to choke out on the high end, and thus performance will suffer on the high end.
The VATN turbos usually do not have any real reliability issues. Often, a pressure actuator (like a wastegate actuator) is employed to move the vane ring. Some are even totally enclosed units, regarding oil.
Over the years, there are have been many ways in which people have tried to reduce response time for turbos. One of the more interesting is what Saab is doing recently. They have a actuated throttle (fly-by-wire). When at low TP's, and under certain conditions, the throttle will blip open just briefly. This keeps the turbo spooled up, moreso than staying at one TP. This was shown to me by a friend of mine working at Ricardo, for which they were doing road tests on the car.
Also a few years back, somebody was working on a centrifugal supercharger that had a friction driven planetary gear train inside to spin the impeller faster. Don't know if they ever came out with it for production.
David_martin, are the WRC guys actually injecting the fuel straight into the exhaust manifold, or are they just cutting spark to a cylinder?

MARK

[pa]

Originally posted by Crazy Canuck
Variable vane turbos – wow! I imagine these units would be very difficult/expensive to maintain. Is the pitch change controlled with some form of mechanical actuation? Sounds like a mechanics nightmare! Small mechanical systems that rotate at high speed in an high temperature environment.

CC

Actually, variable vane turbos are available as kit bolt-ons for street cars, which means they're simple and very reliable. They don't even violate most warranties, and they're 50 state legal.

http://www.mechtech-...dy_z3page3.html

[marion5drsn]

Both mechanically driven and turbo driven superchargers are centrifugal! I don’t know of any supercharger at this time that is not centrifugal, the only exception being the Roots, which is seldom if ever driven by a Gas turbine. The Rolls- Royce Merlin V-1650 was Centrifugal as was the Allison V-1710 plus the D.B-601. All the automobile engines except for the Buick and the Lexus are turbo/centrifugal. Their, (Buick/Lexus), Roots superchargers are mechanically driven.
The amount of horsepower consumed by a turbo is very small compared to a mechanically driven centrifugal supercharger. The horsepower used by the Merlin was in the region of 200to 250 h.p. 1/10 of the output of the engine. What this discussion is really about is that Roots superchargers are good for all around h.p. increase and the Turbo are good for sustained high-speed output. The problem of variable speed Turbo/Centrifugal is cost and reliability, I noticed their lack of mention of cost of a variable vane type..
M.L. Anderson

[JForce]

One of my friends brothers has a Supercharged Toyota Levin, with a turbo as well. The engine management cuts the Supercharger as the turbo kicks in. They overlap, but the power throughout the rev range is insane....

[pa]

Originally posted by marion5drsn
I noticed their lack of mention of cost of a variable vane type..
M.L. Anderson

They're under $5k.

[Yelnats]

The question of reliability in turbochargers has been effecitivly solved with modern metals, lubricants and cooling systems. My 1989 Dodge Shadow Turbocharged 2.5 litre four has accumulated 330k and the turbo bearing is still tight and consumes no oil.

The Turbo vanes themselves (variable or not) are not a reliability issue as I have never seen one fail from that cause. Older turbos used to cook the lubricants in the turbo bearing when driven hard and shut down immediatly but proper attention to design details which may include heat sinks, water cooling and in some cases external fans directed on the turbo after shutoff has insured a long trouble free life for a modern turbocharger.

[Bluehair]

Clearly, roots-type superchargers have many different characteristics than exhaust driven turbos. Generally superior to roots-types, are the screw-types, which are a topic in and of themselves. This is not the comparison that I long for...

My concern is the comparison of the extremely popular mechanically driven centrifugal supercharger (Paxton, Vortec, Pro-Charger, etc.) vs. that of the turbochargers that are being offered for aftermarket street use. I doubt seriously that a typical Paxton or Vortec would draw more than 30 horsepower from the crankshaft to produce 25 psi. for most applications (it's driven off the same belt that turns the AC and alternator). Now, to get the same 25 psi from turbos, wouldn't you waste nearly the same horsepower by restricting the exhaust? Turbo guys are always preaching about their efficiency. In this example, the efficiency of the turbocharger does not impress me (30 or less, horsepower wasted turning the belt vs. ?? horsepower choking the exhaust).

The argument then turns to the way these things produce boost. If both types were set up to produce a maximum of 25 psi at 7000 rpm, would the turbos produce more boost at 3,000 rpm? 5,000 rmp? How much more? I realize that turbos "spool up" at a different rate than a belt driven impeller, so the turbo guys must be under the impression that the spool up rate will increase at a better rate than that of the engine-rpm-linked belt driven types. THAT MUST BE THE CASE!

Perhaps the turbos become far more favorable at higher boost levels. Only then can I see a real problem with the mechanically linked compressors. At these levels, the turbocharger's efficiency may become far more noticable.

Another thing that the turbo guys are always advertising is something that should not be limited to turbos: INTERCOOLERS. Intercoolers can, and should be used in both applications, so that should shoot down that argument.

The one thing I do like about turbos is this rally car stuff I keep hearing about!! If they are keeping the turbos spinning fast all the time, then there is a definite advantage. Can we buy this stuff for our street cars? Turbos win that one hands down!

[Wolf]

Can anyone tell me why the crankcase isn't utilized on 4 stroke engines in a way it is done with most 2 stroke engines (utilizing piston movement to compress air below it)? One would think it would do quite nice...

[Darren]

What became of Hitachi's experiments in electrically-driven (i.e., not driven directly by the engine but microprocessor controlled to match engine speed) turbos?

[Crazy Canuck]

Originally posted by pa


Actually, variable vane turbos are available as kit bolt-ons for street cars, which means they're simple and very reliable. They don't even violate most warranties, and they're 50 state legal.

http://www.mechtech-...dy_z3page3.html

cool link. it clarified something for me though; initially I thought variable vane referred to the vanes on the prop, however the description implied that the vanes are variable intake vanes used to throttle the exhaust gases.

CC

[MRC]

Mr. Anderson, there are more supercharger/trubocharger variants in use today than simply roots types or radial compressor types. Bluehair mentioned the screw type supercharger, which is used in the 3.8l supercharged Pontiac Grand Prix (if I recall correctly). VW has also used their "G-lader" charger in recent past. While this looks like a radial compressor, it moves in an eccentric pattern and charges the air in a similiar fashion as the screw type chargers. Also, larger marine engines usually employ axial or combined flow compressors. Axial compressors are superior in regard to insentropic efficiency, but can not provide large pressure ratios for a single stage (unlike radial compressors), and are more sensitive to blade to housing clearances, not to mention more expensive. Also, Roots blowers and screw type superchargers are often employed on high output race marine engines, some of these being available in regular production. One other thing, roots and screw type supercharges have pretty shitty peak insentropic efficiencies (~50-60%); radial compressors can get peak isen. eff. up to ~80%. Multistage axial's can be above 90%.

With regards to comparing radial compressors, one crank driven and the other turbine driven, I would have to say that it is difficult to estimate the power consumption of the turbine. It is not insignificant, in my view. At high loads and speed, there is more than enough gas energy to drive the turbo, and thus the wastegate is actuated (except some VATN setups). From this the power developed to the turbine is decreased, but a decrease in back pressure also occurs. The shaft driven compressor engine does not have the higher backpressure, but is obviously dealing with increasing mechanical losses, which are not linear with speed. This is one of the reasons that the turbo can generally claim a higher efficiency at higher load and speed. As far as the question of what compressor drive configuration would make more boost, I think that given a constant load and speed, and equivalent compressors, compressor speed, same cam timing, etc, etc, I don't see how they would be making any real different pressure ratios or boost. Obviously exhaust pressures would be different.

Wolf, sounds like a good idea, but only as long as you seperate some of the undersides of the cylinders (for multi cylinders). Wouldn't work for something like your typical four banger, unless you seperated #2&3 from #1&4. Along this note, some manufacturers have done some research on(and I believe are employing) gapless piston rings. However, the interest was not in improved power output, but less leakby of the blow-by gases from the case, into the combustion chamber side of things, during the intake stroke. I don't really get this, as blowby gases are fed back into the intake anyway. Claimed lower HC, was what I was told. So the previous might just be another reason.

MARK

[Croaky]

For an explanation of anti-lag systems, check out
http://www.rallycars...s/bangbang.html
It's pretty clever, making the mixture really rich and seriously delaying the spark so fuel's still burning when it gets to the exhaust. The link above reckons a normal car exhaust system would last 50 to 100km before being destroyed by an anti-lag system...

[Hellenic tifosi]

I understand that purpose of the ALS systems is to keep the turbo spinning while the driver is off the throttle. However, the article says that The dump valve will evacuate the pressurized air coming out of the turbocharger while the inlet manifold is closed thus allowing the turbine not to stall and avoiding possible damage to its bearings. Can someone explain?

[david_martin]

Basically, during trailling throttle with low exhaust gas volumes, the turbine is spinning mainly due to its own momentum. The difficulty arise because the turbo is attempting to pump air against the closed or partially opened inlet manifold. This produces a rapid increase in delivery pressure and resistance acting against the impeller which decelerates (and in the worst case stalls) the turbo. The dump valve opens the delivery side of the turbo impeller to atmospheric pressure, which eliminates this pressure build and reduces the loss of momentum of the turbine, as well as eliminating unhealthy torque transients on the impeller drive spindle and bearings. The turbo spins at higher speeds for longer, and boost pressure can be recovered more rapidly when acceleration resumes.

[Hellenic tifosi]

So the waste gate is applied on the other side of the turbo! Also, this means that after market waste gates let the air go out later than the normal ones?

[Christiaan]

Did you all know that this was one of the first topics we discussed at the birth of the technical forum? We debated about this right when the Tech Forum was a wee lad (I think on the day of its inception) and came to the conclusion that for high end power a turbo is better because it draws no power from the engine nd at high revs is self sustaining. For low end power a supercharger is better because it does not need to gain momentum like a turbo does (turbo lag). There are interesting Turbo/Supercharger combos that I have seen which give the best of both worlds. I can only find the one link right now one Keonig's Ferrari Testarossa conversion.

Personally, I am a fan of low inertia Turbos where multi-stage turbocharging and intercooling techniques can be employed. The only car which has such is the Bugatti EB11 but I'm not sure. The principle is basically instead of using one big turbo on say a flat 4 engine, use two small ones (better four smaller ones) instead. The inertia of a smaller turbo is much lower so you get less turbo lag. If you connect the turbos in series and have an intercooler after each turbo you can compress the air with less work and get a higher air density output. The complication is of course the cost and also I think this setup is very very delicate.

Unfortunately the thread with the original debate seems to have grown legs but this is one of the more interesting ones I found.
check out this thread

[Croaky]

When a privateer team went to Le Mans with Bugatti EB110s, the things were chewing up turbochargers every few hours. I'm pretty sure they ended up retiring. I think you might be right about them being a bit delicate.

[david_martin]

Originally posted by Hellenic tifosi
So the waste gate is applied on the other side of the turbo! Also, this means that after market waste gates let the air go out later than the normal ones?

Don't confuse the waste gate, which is basically a bypass valve that controls the volume of exhaust gas supplied to the turbine, with the dump valve. They are two completely separate things that have totally different functions.

[Yelnats]

Theoretically, the turbocharger can have advantages over a mechanically driven Supercharger at low speeds because it's RPM is not proportional to crank shaft RPM. So the ideal turbocharger could be designed to deliver an overboast at low RPM's to flatten out the power delivery curve and taper off as RPM's rises so as not to overstress the engine.

Of course this is not what takes place with most turbochargers now but with the introduction of variable vane and multiple turbochargers we are begining to see this flatter power curve being implemented. These newer designs not only reduce the turbo lag but provide a step towards the idealised situation I've outlined above.

I forsee the time when automotive engines will be routinely equiped with turbochargers and even the constant speed engines proposed for hybrid electic/gas vehicles would benifit from the reduced engine sizes permitted by this technology. The writing is on the wall for mechanically driven superchargers because as designers begin to understand the charicteristics of the turbocharger it will displace them for most applications.

[Nathan]

Note the RPM levels in which peak torque was made. If Im not mistaken, is it not true that the most air going into the engine is when peak torque occurs???

Supercharged:

Pontiac Grand Prix GTP
3.8-liter V6
Horsepower: 240 hp @ 5200 rpm
Torque: 280 ft-lbs. @ 3200 rpm

Jaguar XK8 Silverstone
4.0-liter V8
Horsepower: 370 hp @ 6150 rpm
Torque: 387 ft-lbs. @ 3600 rpm

Aston Martin DB7
3.2-liter I6
Horsepower: 335 hp @ 5750 rpm
Torque: 361 ft-lbs @ 3000 rpm

Mercedes SLK Kompressor
2.4-liter I4
Horsepower: 192 hp @ 5300 rpm
Torque: 200 ft-lbs. @ 2500 rpm

Nissan Frontier SC
3.3-liter V6
Horsepower: 210 hp @ 4800 rpm
Torque: 246 ft-lbs. @ 2800 rpm

Turbocharged

Audi A4 1.8T
1.8-Liter I4
Horsepower: 170 hp @ 5900 rpm
Torque: 166 ft-lbs. @ 1950 rpm

Porsche 911 Turbo
3.5-liter I4
Horsepower: 415 hp @ 6000 rpm
Torque: 415 ft-lbs. @ 2700 rpm

Subaru WRX
2.0-Liter F4
Horsepower: 227 hp @ 6000 rpm
Torque: 217 ft-lbs. @ 4000 rpm

Nissan 300ZX Twin Turbo
3.0-liter V6
Horsepower: 300 hp @ 6400 rpm
Torque: 283 ft-lbs. @ 3600 rpm

Volvo C70
2.3-liter I5
Horsepower: 236 hp @ 5400 rpm
Torque: 244 ft-lbs. @ 2400 rpm

[Yelnats]

Nathan, Peak tourque occurs when the engine achieves it's peak volumetric effeiciency, ie when the cylinder is filled to the maximum extent. But the point "that the most air going into the engine" is always at a higher rpm because the volumetric efficiency falls at a slower rate than the increased pumping volumes due to the rising rpms. The point at which the maximum volume of air going through the engine is somewhat above the point at which peak power occurs and just below the point at which it bursts.;)

[palmas]

variable geometry turbos are quite comon in everydaycar (mainly turbo diesel engines). The geometry variation is not in the impeler itself, but on the "wings" that guide the air to the impeler.
The secret for a good (low backpressure, high air flow) turbocompressor is the intercooler...with just the limitation of aero drag.
I personaly think turbos are much better than all the other kind of compressors, because they use less power, they can be at top pressure from early rpm (unlike those that will only achieve top pressure at top rpm). Also the proliferation of turbos makes them quite cheap and tested in all environments (even industrial with 8600h/year, 100% capacity). Many of the turbos disavantages can be solved redisigning the exhaust system.
Sorry i CANNOT EXPLAIN BETTER, BUT THIS IS ENGLISH...

[Hellenic tifosi]

Excuse for bringing back such an old topic but after looking back at it, I don't think that i fully understood the purpose of the waste gate. Why should it exist, since the turbo is free to spin with the dump valve open? Also, why is it so loud on WR cars(esp the Subaru), while it is barely noticable on production cars?

[mhferrari]

Supercharger-pump driven by engine
Turbocharger-turbine driven by exhaust gases.

[JForce]

Originally posted by Hellenic tifosi
Excuse for bringing back such an old topic but after looking back at it, I don't think that i fully understood the purpose of the waste gate. Why should it exist, since the turbo is free to spin with the dump valve open? Also, why is it so loud on WR cars(esp the Subaru), while it is barely noticable on production cars?

I believe its so loud on the WRCars due to the fact that they run high boost, so the pressure release is a lot louder. This is the same on modified cars, I used to have a Supra, and when the boost was wound up, it was pretty loud on pressure release.

[JollyRoger]

Just a musing from somebody who knows little about turbo's etc, but am I riht in thinking that often the problem is created by an inability to create enough boost or more simply put do people tun into limits on how much boost they can apply?

If they do, then would the use of turbo AND supercharger tech's make it possible to increase boost that extra bit?

[david_martin]

Originally posted by Hellenic tifosi
Excuse for bringing back such an old topic but after looking back at it, I don't think that i fully understood the purpose of the waste gate. Why should it exist, since the turbo is free to spin with the dump valve open?

The waste gate is a normally closed valve on the exhaust side of the turbo charger which is designed to control the volume of exhaust gases so as to limit turbine speed, and hence turbo boost. Waste gates perform a function largely aimed at engine protection and fuel consumption management. Once boost pressure reaches a certain level, the waste gate opens to bypass some of the exhaust gases to maintain a constant maximum boost pressure. The sound you hear when the throttle is closed on a turbo engine is the waste gate closing, as the exhaust gas volume falls, the turbo slows and the boost drops below maximum pressure. To use a crude analogy - think of it as an accelerator control for the turbine. Variable boost turbo systems have what is essentially a variable waste gate - one that either opens earlier or later to control the maximum boost the turbo delivers.

The dump valve, on the other hand, is designed to stop the turbo stalling against the closed engine throttle. On an turbo with a dump valve, two actions occur when the drive gets off the throttle. Firstly the waste gate closes as the boost drops from a lower exhaust gas volume. Secondly the dump valve opens to reduce to resistance to motion on the impeller pumping against the closed engine throttle. To extend my crude analogy further, think of it as a clutch which is engaged to provent the turbine from stalling. The two valves serve completely different functions which are largely independent.

[Hellenic tifosi]

David, it is all clear now, thank you very much for your help.

[desmo]

Beautifully done David, thanks for the excellent explanation!

[Pistone]

Hmm, not 100% right...

The wastegate is a spring-loaded normally closed valve which is opened by the pressure in the intake system, on newer cars there is usually a control valve of some sort that activly regulates WG actuation pressure. If there is no electronic regulation the WG opens as soon as the intake pressure overcomes the initial load of the spring, the spring can be set at diffrent loads by making the WG actuator rod shorter or longer. The valve is not open or closed, it opens more if the boost level is higher.
The main function of the WG is to bypass the turbine so some exhaust gasses don't pass by the turbine. This is done to keep the turbone from turning too fast and cause too high boost levels.

The sound you hear when the throttle is closed on a rally car is NOT the WG closing, it's the dump valve. The reason why you hear it mor in a rally car compared to a street car is that in a rally car they dump the air straight into the atmosphere but in a normal car it is recirculated into the intake system just before the turbo.
The WG will of course close as well but only because the intake pressure drops when the dump valve is opened. not the other way around.

The dump valve opens when a pressure drop over the throttle occurs, i.e. if the throttle is closed. The DV opens and lets the pressure out. This gives tree benefits.

1. It lets the turbo 'freewheel'. Because there will be no pressure differential over the compressor the energy needed to spin the impeller is near nil.

2. When the throttle is closed it creates a pressure pulse, if this pulse is allowed to travel back to the impeller it can cause severe damage to it. The DV eliminates this pulse.

3. Air has mass. If the throttle is closed the flow will reverse and travel back against the impeller. If you open the throttle again the flow must be reversed again, reversing the flow uses a lot of energy and causes lag. If a DV is used the flow will not reverse in the first place.

[Yelnats]

Pistone, Excellent! This explaination makes sense and explains the unusual sounds made by WC Rally cars on overrun and the graduated regulatory function of the waste gate before the turbo.

I have not heard a sound from a CART turbocharged engine that compares to the WC Rally cars and wonder if this means that they don't dump turbo over-run gasses to the atmosphere? There is no fuel in these gasses so economy is not the answer. Any ideas?

[Pistone]

Thanx Yelnats.

I can only speculate here...
The sound from these cars is extremely loud. I think that the sound is there but since it is so much lower then exhaust and intake sounds it's difficult to hear it.
I guess they turn down the volume or dampen the mic otherwise the recording would just be distorted.

BTW. Where can I find those WC Rally Cars? Do you have a private rally championship in your bathroom.;)

[bugeye]

Hi guys,

The old memory is not what it used to be, but I have a Turbo SAAB (and a Merc Turbo Diesel) and the SAAB uses a turbo bypass valve. One additional consideration IIRC is that the air from the bypass valve is passed back into the system (on this car) right before the turbo which also helps keep the turbo spooled up. Also, I think it needs to be kept as a closed cicuit as the air may have already been metered by the FI, though I could be wrong on that count...Also, I (think) mine is activated by the relatively simple mechanism of vacuum. When the throttle plate shuts, the vacuum rises and it opens the valve...what I have not been able to figure out is wether it works both ways, is there a condition which would cause the system to suck air around the turbo? ie. If your turbo craps out, would it give you better limp home capability?

[Pistone]

bugeye: The bypass system is the more accurate name for the "dumpvalve" I was talking about in my previous post. on your Saab the valve is opened at a pressure differential of 0.2 bar across the throttle, the valve is a small spring loaded piston that moves when a pressure differential is introduced.
This valve is supposed to be kept shut so the air cannot go past the turbo. If the turbo "craps out" it will not block the airstream, it just won't produce boost.

[bugeye]

Yes, but if the turbo froze solid (something I hope never to experience) wouldn't the resulting increase in vacuum as the motor tries to suck air around the turbo open the bypass valve?

or is it only a differential between the two sides of the throttle plate which opens the valve.....

One thing which happens on the car when the bypass valve goes belly up is that the car will stall when you are in traffic and get on the gas for say 10-20 yards only to have the car in front of you slam on the brakes.....The only thing I could think of (besides something to do with the IAC in the Feul Injection System) was that the following happens does anyone know the real reason? Here's my working theory..

1. Get on the gas....turbo spools up...
2. Slam throttle shut and onto brakes.
3. Pressure wave builds up and literally bounces off throttle plate, heading back towards turbo (keep in mind bypass valve is inop in this scenario)
4. Being a wave, as it receeds it creates a brief vacuum near the throttle plate which stalls the motor.

[Pistone]

Originally posted by bugeye
Yes, but if the turbo froze solid (something I hope never to experience) wouldn't the resulting increase in vacuum as the motor tries to suck air around the turbo open the bypass valve?

or is it only a differential between the two sides of the throttle plate which opens the valve.....

Exactly! Only differential between before and after the throttle.

One thing which happens on the car when the bypass valve goes belly up is that the car will stall when you are in traffic and get on the gas for say 10-20 yards only to have the car in front of you slam on the brakes.....The only thing I could think of (besides something to do with the IAC in the Feul Injection System) was that the following happens does anyone know the real reason? Here's my working theory..

1. Get on the gas....turbo spools up...
2. Slam throttle shut and onto brakes.
3. Pressure wave builds up and literally bounces off throttle plate, heading back towards turbo (keep in mind bypass valve is inop in this scenario)
4. Being a wave, as it receeds it creates a brief vacuum near the throttle plate which stalls the motor. [/B]

When you slam the throttle shut there will be a strong vacuum on the downside (flow-wise) of the throttle anyway so that is not why the car stalls. I have no dump/bypass valve on my two turbo cars (one Saab and one Alfa) and they don't stall.
Check for vacuum leaks in the plenum/intake. Why does the bypass go belly up? Does the hose from the plenum fall off, because that could cause this behaviour.

[bugeye]

yeah, your probably right, the failure of the functionality of the bypass valve is likely irrelevant (though it doesn't help probably) and it is more likely that the when is fails, the diaphram inside goes and consequently you get a vacuum leak our of the vacuum hose which actuates it...the funny thing (Funny queer, not funny ha ha) is that other plenum vacuum leaks do no't cause the same behavior....oh well, suppose it doesn't matter anyway as everything works fine now. (well everything in the engine compartment anyway ;-) sorry for draggin everyone down this rathole.....

[Pistone]

The reason why you don't get the same symptoms when you get a leak in the bypass as when you get a 'regular' plenum leak is that if you have a bypass leak, it leaks measured air but when you get a plenum leak it is unmeasured. This does not apply if your car is running a speed-density system i.e. mapsensor instead of AFM/MAF.
Which Saab model do you have?

[bugeye]

I have a 9000 CD-T,

when the bypass vavle goes, the vacuum which is used to activate the valve (the wee little hose, operating the valve which connects the large hoses) will loose vacuum and let in unmetered air, but I think the reason the behavior is different is that there's probably a solenoid or some other control which only turns on the vacuum (so to speak) to operate the valve under certain considitions, so you have I guess an "intermittent" vacuum leak...if that makes sense...

Anway, kindof fun to ponder...thanks for the help..