mn_vette

Somehow the Servers lost my post and every time I click on it it goes to a quick reply something or other. But in my 18 hours driving boredom this weekend I spent a small time of it thinking about sequential charging of a motor using a roots or twin screw supercharger and a turbo. There was a post about a month back about an STI that did that and the idea has been rolling around in my head ever since.

To actually try this it would be benoficial to find a car that has a factory blower and an after market turbo kit, just to make things easier and not to be blazing too much new territory at once. Well, the Grand Prix GTP fall right into that category. I've been looking into it a bit and everyone that goes turbo on their GTP's pulls out the supercharger to do it. I've only really seen one example of sequential charging with these and it was considered very experimental.


SUPERCHARGER:
My first thoughts were that the supercharger would be a major restriction in the intake tract. But lets analyze how that works. The GTP has a 3.8L engine that runs ~7 psi of boost from the factory. The supercharger is hooked directly to the engine by pullies so it is always spinning. And since it is a possitive displacement blower it moves a given amout of air for every revolution. And at 7 psi, or 50% more air than the engine needs, the blower will pump 5.7L for every 3.8L the engine requires naturally asperated.

Lets look at the idle situation. When the car is idling there is a bypass valve that opens and connects the plenum before the supercharger to the plenum after the supercharger. This equaizes the pressure and causes no boost to go to the engine. So there has to be some extra airflow through this bypass valve, but how much. If the engine pulls 3.8L of air at a given vaccumm for every 2 revolutions and the supercharger is flowing 5.7L of air at the same vaccumm we will get 1.9L of air flow through the bypass valve, but it is flowing against the normal flow of the engine. it actually flows away from the engine an back towards the throttle body to keep the plenums equalized with pressure.

I believe the same exact thing would happen if you were boosting into the supercharger. The supercharger will still move 5.7L of the boosted air and 1.9L of it will flow back into the pre-supercharger plenum. The problem that I can see with this is that the air may eventually get heat soaked from sitting in the manifold for too long. But the supercharger does not become a restriction in the intake, and since it is not making any boost the parasitic loss is very minimal.


THE TURBO CHARGER:
Picking the right turbocharger would be essential to making this setup work. As with any turbo charger you need to pick the compressor and turbine sections seperately. Lets starting with the compressor side. Since the top end of the powerband is taken over totally by the turbo charger I would think that the same compressors used in the current turbo setups would work just fine as the overall boost and flowrates remain the same. From what I've seen people use compressors in the range of the 60-1 to the T66. http://www.turboneticsinc.com/compmaps.htm

Choosing the turbine side is a bit different though. Since the supercharger is creating boost to the engine it will create more exhaust going out the back which will help spool the turbo very nicely. Since the engine is a 3.8L running at 7 psi it will have just as much exhaust as a 5.7L engine. This will call for a much larger turbine then conventionally used on the 3.8's. From what I've read a .84 a/r P-trim turbine is about as big as you would want to go for a 3.8 and many people are using T3 turbines for them. This just won't do for our sequential setup. At the very minimum you would want to run a 1.15 a/r P-trim and probably a .96 Q-trim on the turbine. With the extra boost from the supercharger the turbo will have no issues spooling and once it is spooled you will gain alot of top end horsepower by have a much less restrictive turbine housine.


BOOST CONTROL:
At some point there needs to be a transition from the supercharger creating boost to the turbo creating boost. This transition needs to happen just when the turbo starts creating boost into the intake tract to the supercharger. If you wait too long you end up with the supercharger compressing already compressed air and the boost multiplies and this is very bad for the engine. If you transition as the boost in the intake tract reaches 1 psi then the transition should be safe, with possibly a small dip in boost for a fraction of a second. This trasition should be made by opening the bypass valve on the supercharger. You may be able to play with the exact transition point to keep the boost level constant, but that's too much math for this morning.

The amount of boost that the turbo charger runs does not need to be greater than the supercharger, you can have less acutally if you desire. As long as you take the boost reference for the wastegate. from the intake tube or the upper supercharger manifold and not what is actually going into the engine.


CONCLUSION:
Well, the benifits of a sequential charging system are the instant low end boost, the high end power from the "drag" turbo that you would run. The downfalls would be, well, the complexity and maybe the added weight. I think that this would be a great project to try if someone has an extra $10k or so lying around and they want a nice 11sec daily driver.

I hope these random thoughts of mine didn't bore you. Any comments or what not please let me know.

Baldturbofreak

Man, I have been trying to talk my dad into this forever.

I could hang 2 Tv-77's of the side of that thing, 2 5" downpipes,40 psi and alky. Sooner or later he will cave. He always does.

Bruce's car would be ideal for this. cowl hood and a small blower. Possibly one from a AMG that is clutched. and a switch back to the Big turbine housing. wicked.

redbullapril23

what i could see workin is when the turbo kicked in maybe have some sort or clutch system on the supercharger pulley like say the AC compressor for instance. so you have low Grunt and then the big turbo kicks in and you have upper power... now the hardest thing would be devoloping a sort or clutch system and allowing for a smooth transition.

just a thought. i like thinkin because most of the time i dont.
haha
ne wayz nice pondering
your friend
Sean

mn_vette

The clutch idea is good, but the problem is then you have to have some other kind of off the manifold setup and additional tubing. But with the Grand Prix example the roots blower is directly mounted to the intake the only way to get around it is through the bypass valve and I don't think you could get 400hp+ through that. You'd have to keep the blower running to do it.

Baldturbofreak

You sure? I think the blower willl free wheel just fine. the one in the above picture runs without the belt. the blower wheels around on it's own, a little sluggy but hey low comp/big cam. you could just not bypass at all. Roots blowers are pumps, not compressors. If you feed them compressed air, they dont really know it. if your just using say 7 psi from the blower (or less) just for the low end instant punch,( and let me tell you what, with a positive displacemnt blower it is faster than right now) and the other intercooled 10 turbo you wouldn't need any clutch, bypass or otherwise.

mn_vette

I didn't realize that you could get them to spin without the belt. I'm not sure if it would be worth it because you'd be causing it to sping weather it would be comming from the crank or from the turbo pushing the air past it. But the turbo would probably be more efficient depending on the turbo.

You could just not bypass the roots blower, but you would not be gaining the efficency from the turbo. And that is what you'd want for the top end. Not to mention the fact that you'd be creating alot more heat from compressing the blower and you start having more resistance on the supercharger pulley because of the compressing that its doing. It would also put the turbo in a bad place in its compressor map. Having to make it move so much air with so little boost would be bad. For the little thinking that it would take to reopen the bypass valve it would be worth it.

I just used 7 psi as a base because that's what the stock GTP's come with. And the turbo kits could be had for $3k and it already has a blower on it.

Baldturbofreak

There are many turbos that are very happy pumpin 6-10 psi. In fact they come from mostly detroit 2 strokes that are indeed feeding a roots blower. albeit not being used in the same fashion but non the less designed for very high flow and relatively low boost (one atmo).
Im not a huge roots proponent, my dad always has been so I have some experience with them. If you mix fuel above the blower, the Iat's arn't nearly as bad as you think. just as long as you don't ask the blower for high boost or spin it really fast. That's what the turbo is for
I will say this for the ol roots blower. NOTHING BEATS THE SOUND OF THAT DAMN BELT!!!!! don't beleive me? rent hollywood nights and you'll understand.

mn_vette

Ok, so lets spin that turbo with a 3.8L engine. I think we could probably cook a burger in the time it would take to spool it. Sorry, it's lunch time I'm getting hungary. I guess the point of the discussion was that using sequential charging would be fun, maybe not as much fun as twincharging like you are talking about. If I had a blank car to play with it would be good. I started thinking about this because I totaled out my daily driver and its time to get something else and I think this would be a fun little project to under take, even though that's a bad idea for my daily driver.

Baldturbofreak

I know folks that run giant snails on 3.8's they spool around 4-5k rpm. thats becuase it take that much rpm to make the required hp (lbs/min exhuast flow) to effect a boost condition in the system. If you took that same motor and hit it with a whiff of nos or put a supercharger on it it would make the HP (required lbs/min) at a much earlier rpm and effect a boost condition. Once effected, the mass flow within the system negates the need for the Squeeze anymore so you turn it off. Same thing with the blower.

I was rereading your post, if the bypass is behind the throttle plate how is 1.9l of air recirculating? Isn't the entire blower case under vacuum and therefore mostly devoid of air? It was my understanding that the bypass helps with the pumping losses by giving the air a direct route around the "Pump" section and therefore not make it(the blower) fight against the throttle plate, but rather freewheel in vacuum. this is how they claim a 1/3hp usage under cruise.

If the blower is set to casue an atmo to 7 psi differential, does it work any harder if it's being fed 7 and takes it to 14? seems like it's doing the same amount of work to me.


I think the solution would lie with a large bypass in the case and a dual port actuator. One that opened the valve under vacuum behind the throttle plate AND boost in front of it.
COnsider the following:
your cruising, you mash the throttle
bypass closes, 7psi is effected
rpms come up to 4k
monster rear mount begins to pump hard
boost starts to effect in front of the throttle plate
bypass begins to open
when 7psi in the intake pipe is achieved, bypasss is full open
7 in front, 7 behind the blower, it moves nothing, free wheeling jsut like under cruise. Air will find the path of least resistance.

The difference will be Mass flow.

But yep, if it were me Id twin charge instead. way more poop. way less dickin around with controls. the main reason those guys go turbo (other than coolness factor) is that to get high boost, you gotta
1)spin the hell out of that little blower which makes scotching mechanical heat.
2) use a much bigger blower that takes more power to turn
we all know your average 3.8 can stand 15+ psi intercooled on pump gas, GN's taught us that.

say for the sake of argument the temp delta from compressing 6psi mechnically (roots) is 60 deg. well if your intercooled 10 comin in from the turbo is only 115deg then the 175 manifold iat is still acceptable for pump gas. If you tried 16psi with the roots it would prolly be over 230. 16 with turbo alone would prolly be less then 175 but to get that you'll need a larger effiiecnt turbo with a big turbine and eqaully big boost threshold.
use both you get immediate punch, unbelieveable top end power, and the ability to vary boost from the seat, by MPH or whatever.
Think of it not like turboing a supercharged 6, think of it more like turboing a big na motor, as thats what a roots will make a motor behave like.

hmm, if you think about it, you reference the turbo to intake manifold pressure and if the belt slips, the turbo will automatically make up the difference.

I think you should do it buy a GTP or super rivey. get yourself one of those "monster stone t-70" 's and matchin wastegate and hang it up under the rear that way you'll only have $500-600 bucks in the whole thing. Then you can experiment with bypass mode and twin mode with the same hardware. The factory ecm's have boost cut in them so I don't think you can hurt it. Sounds like a really neat project

BAM92

There used to be a Renault rally car back in the 80's that used a blower and turbo sequentially. It made either 450 or 650hp from a 1 liter motor. I can't remember. I have the article somewhere in a box in the garage. I do know that my buddies supercharged 350z works just fine when the belt breaks. It is the Stillen screw type. You just loose the sound and extra hp when it does, but you can drive it around, no problem.

Baldturbofreak

Man this has really got me thinking. A lightening blower can be had for 250-400 on ebay. It would be all too simple to mill the top of one of my Lt1 intakes and weld a 1/2 plate to it and BAM super/turbocharged.
That would make my UTV-94 spool at least 1k earlier. the bypass on that blower isn't very large from the pics. so why not use the front of the LT1 intake as a bypass? So long as the air path is still behind the throttleplates your cool. Less parts. Just will need a cowl hood.

DAMMIT! I need to talk myself out of how cool that will be.

lcvette

BTF,

eureka, I figured out a way to make it work with perfectly smooth and linear transition and it would be SUPER simple!! make provisions for large reed valves underneath the blower and on the back side of the lt1 throttle body opening, both reed valves facing inwards as to seal the plenum. have two throttle bodies, one on top/side of the blower and route the turbo through the front of the lt1 manifold as normal, connect both throttle blades to work in unison with one another (shared linkage). you would need to determine the point at which the turbo begins making more boost then the blower and have the belt/ pulley clutch disengage the blower drive much like an A/C compressor does. this could be determined by a map sensor in the turbo outlet charge pipe right before the front 58mm lt1 style throttle body, when the charge pipe map sensor and the manifold map sensor are approaching the same pressure level have a delay switch to dictate how long to wait to disengage the blower drive so the transition is seamless.

When the car is at idle and drawing vacuum, it idles like stock, begin accelerating and the blower will begin making boost, the pressure in the manifold will keep the reed valve behind the turbo inlet throttle body closed until the turbo has reached spool and begins to make more boost then the blower, this is the point the blower clutch disengages and the turbo takes it from there sealing off the blower outlet under the twin screws with the reed valves. voila.. linear, simple, functional and effective, off the line boost from the screw blower and top end spool from a larger then normally used for the application turbocharger!. the increased airflow from the blower will help spool a larger turbo sooner and once the turbo is spooled it can support itself and maintain spool. you could pulley the blower way down since it would no longer be a direct drive by the crank shaft with the clutch and would go offline mid RPM and this would create more boost sooner, it should have no ill effect on the motor since it will be going offline once the turbo has taken charge and you would regain the power required to drive the blower at the same time. you could find the pulley combination that would max the blower by the spool time of the turbo and get max efficiency from the blower and have lots left for the turbo to do.. this could seriously work!

Chris

chuntington101

i love this sort of talk! really get the mind thinking!

here's what my thinking was!

a twin screew or roots moves a fixed volume of air per rpm right (well the twin screew will compress the air too but forget that for the mo)? so what happens if you put presure on the inlet side of the blower? does the supercharger just boost the boosted air? if it does then this wouls simplyfy things even more! all you would do is rig the turbo outlet into the S?C inlet! the presure control for the turbo would be taken from the pipe leading to the S/C.

is this theary total cr*ap or do you guys think it might work? it comes from industry. where you need really high air presures you run pumps in seriers! on pump feeding the high presure air into another pump and then into another pump!!!

oh to have the money to play around with something like this!!!

thanks Chris.

mn_vette

I think you stated the contrary already. Remember that this is a PUMP, not a compressor. The only reason that it creates boost is that the engine can't intake all the air that the PUMP is pushing at it. If you put the same exact pump onto a 5.7L engine, how much boost would you make? Zero because the engine will take in all the air that the pump will throw at it. Does that make sense?

At idle even though the there is vaccumm behind the TB plates the pump will still move 1.9L of air per 2 crank revs. This air will be vaccummizes(word?). Volume has nothing to do with pressure in this case. The power consumption comes from when the pump has to force the air into the lower manifold causing pressure or boost.

Imagine a bucket of water with an electric pump in it. How much energy does it take to circulate the water around the bucket. Now switch the discharge to going up 10 feet and discharging on the other side of a wall. You'll see the amperage of the pump shoot up substantially, but you'll be moving the same amount of water assuming the pump rpm stays the same.



That it right on the money!!



Here's where I have a problem with your logic. If you run the turbo into the supercharger you start boosting boosted air. So normally the 14.7 psi * 1.5 = 22.35 or ~7psi above atmosphere is how the equation goes for the supercharger. Now if the turbo is running 7 psi you have (14.7 + 7) * 1.5 = 32.55 or 17.85 psi. If you just wanted 15 psi you'd have 4.9 psi from the turbo. (14.7 + 4.9) * 1.5 = 29.4 or 14.7 psi.

But now instead the turbo doing all the work and the supercharger spinning in place you have the supercharger doing the same amount of work and the turbo idling along.



Yes, but if that belt catches again, the turbo won't be able to react quick enough to save the engine, you'd have one heck of a boost spike. Bypassing the blower would be the safe bet, or just suffer with belt slip.



I think it would be fun to do, but I've gotta get my turbo vette up and running. Anybody know where I can get a cheap T-76 turbo from??? That's the only thing that's really holding me up right now. I sold my other one on ebay because the turbine wasn't big enough.

Baldturbofreak

we'll have to agree to disagree there, In my head it's more like the water in the bucket is gone because I turned the faucet going to it off.
the pump is therefore pumping air(much less density) not water. just like a 30kpa idle situation. turn on the water going to the bucket and bam! geyser so long as the water going to our bucket is more than the pump can draw at that rpm. Just like our motor.

If it was as you described , the throttleplate would be after the blower and the blower would be passing off into the atmo via bypass.
On those cars and pretty much all roots the TB is before the blower.
Nothing to pump= no loss.
Maybe you had a different Idea of how you were going to imlement the blower, and thats how we differ.

Boosting boosted air is exactly what Im talking about with the twin charging. Instant roots torque, turbo upswing on top. This may end up on my vette. The more I think about it, the more I like it. I belive a lEaton from the ford 5.4 would make 5-6 psi on my motor down low. I'll make the other 20 with my turbo.

Baldturbofreak

Something to consider, everytime we go smaller on the top pulley in the above truck, idle vac increases. no bypass on that blower.

mn_vette

What do you mean that there is no bypass?? how does that work? What kind of blower is it?

Baldturbofreak

Thats a 6-71 GMC blower(from a diesel bus) prepped by BDS. the bypass feature seen on most new blowers is a recent developement.
Hotrodders have been putting the scavenge pump from 2 stroke detroits diesels (all blowers came from this application) since the 60's. heck, before the advent of the gilmer belt they used to use 3x chains like TonyDEE is doing.
Having the carbs/throttlebody atop the blower makes the mechancial difference in engine volume/pump volume (I believe dad's blower displaces 390in^3 per rev) "fight" the closed throttle plate. When we speed the blower up relative to crankshaft speed, it's pulling harder against the closed throttle plate at that same idle speed craeting more vacuum. Strange but true. It's not a huge difference, but I suspect that it's mellowed because at low blower rev's the seals leak internally. A smaller tighter sealing blower may make it more pronounced.
The advantage of it being on top is Instant response, reduced blower load at cruise (compared to swallowing a full load of air and pumping it into the atmo)
Then there is the cam Phenomenon. From what we have seen in the 3 different progressivley more radical grinds from comp (the latest being270/280@0.050 114+4LSA!), that you can take whatever cam you choose and that roots blower will move the powerband down 800-1k rpm.
I believe it has to do with the fact that with the roots, your not depending on the atmoshpere to push air into the cyl and having reversion push some back out, you are commanding charge into the hole without the possibility of reversion up to the point of manifold pressure. This happens at all rpm, also your denser gas is behaving more and more like a fluid.
Here's another thing we discovered, at low airflow but higher rotor speed (light cruise) the damn blower will centrifuge the fuel out and it puddles in the the bottom of the manifold. also the rear cyl run richer no matter what. This is largely becasue all our fuel is currently mixed atop. The blower depends on some of this fuel for cooling and lubrication, but we really need to make the switch to port fuel injection. Instead we'll inject alky/castor oil atop the blower and gas below. tangent to our discussion but interesting none the less.

mn_vette

Rick,

Where is the vaccumm gauge located at?? Do you have one on both sides of the blower. I would love to see the difference between the two during idle.

I would think that the vaccumm change would be proportional to the wot boost change. Rick, time to do an experiment.

Baldturbofreak

This was just below the throttle plate. dad and I were dbating on whether the 2 48mm throttle bodies were a restriction at wot. Ya know I never thought to compare above/below blower action. I was concerened about a different issue.
Unfortunately the Truck has been put to bed for the year(snowing right now) and has a dozen cars in front of it now. She wont wake up until april.
Normally it stays home, but the old man is building something in his garage ... a new 3/4 Ton chev van, cut in half behind the slider, all new body behind that with a dimondplate flatbed/5th wheel. It's superluxuried out and to top it all off has bags on all 4's so he can drop it on the frame to help mom (and my youngest brother, the Black Lab )get into the van (she's got arthiritis in both knees real badfrom running 10k's So does the dog from running them with her) or to help load up the trailer without getting out and cranking the handle.

You guys gotta see it. He calls it the "Gifford family Truckster" after national lapoons vacation. I'll take some pics.

One more thing about the roots, taking signal above the blower you can have a "normal" pcv system. Unless your gonna feed it boosted air.