Forcefed86

iTrader: (4)

Whoa ton of misinformation above. Makes my head hurt. I’ll give it a shot…

All a turbo does is multiply atmospheric pressure.

You are dealing with pressure ratios and absolute pressure on compound setups. Yes, a compound setup will “multiply” aka compound the air resulting in higher manifold pressures dividing the work between the two turbo chargers at a specific ratio.

For example…

Lets say we have a typical single turbo diesel rig maxed out at say 40psi on the boost gauge. Take that 40psi and add 14.7 to it to get your absolute pressure at sea level. Total pressure would be 54.7psi absolute. Take that 54.7 / 14.7 and you get a 3.72 pressure ratio multiplier. So we will say that at a 3.72PR this turbo is maxed out.

If we were to add another larger turbo to this setup (sized properly and compounded) at a 2.2 pressure ratio the following would happen.

The large turbocharger is running at lower low boost and multiplies its 14.7psi inlet pressure by 2.2 times and sends out to the second (smaller) charger at 32.3psi absolute.

Now the small turbocharger will receive 32.3psi absolute (17.6lbs on the boost gauge) at its inlet and multiply that 32.3 by 3.72 just as it did before, but now the pressure output will be 120.16psi. The small turbo will still be spinning at the same speed it was maxed out at previously, but the manifold pressures more than double!

Atomic

iTrader: (37)

Exactly...it also easier to think in terms of density ratio once you find out what your final pressure ratio will be.

cajundragger

iTrader: (2)

I need to fix my previous post, the big turbo is a prestage for the smaller one.

its a density thing. Someone with more real world experience can probably elaborate, I can only explain based off the thermodynamic/fluid mechanic aspect.

the smaller turbo is going to build boost quick(we know this). While the big turbo spins 'slow' the air is just pulled through it for a slight restriction but still way better than the big turbo alone.

When the big turbo starts to hit its effective range, its output instantly multiplies the smaller turbos output not because the added intake pressure into the smaller, but the increased density going into the smaller turbo. This changes the map of the smaller turbo completely as its not pumping air at atmospheric conditions.

the big turbo 'compounds' air into the smaller, keeping the smaller in its map by moving a larger amount of air at basically a constant pressure ratio. as in without gaining much back pressure in the exhaust, the mass flow rate will continue to climb. so at max effort, both turbos work together to achieve a higher pressure ratio(boost) than either could do alone. The big turbo does not take over and the little just spins, they both work together at max effort.

You are correct to say it takes both turbos to make super high boost, but its wrong to assume one turbo makes x amount and the other makes x amount and together they make 2 times x amount. Remember, we are dealing in volumetric and mass flow rates. boost is only created because the flow rate is 'too much' from the turbos and excess is building in the intake.

Whats interesting to think about but possible, is the 'max' boost an engine can actually utilize. Eventually, you can keep adding boost to the intake and the power output will stabilize and start dropping. Cylinder pressures would probably be too severe for us to ever see this in the real world though.

to make ultra high boost, I could speculate but I don't really know. Like I mentioned, I'm a theory person. I remember a duramax that had quad turbos and made ~205psi of boost. To make a turbo make more boost you can increase the density of your pumped fluid(compounds) and/or find a way to increase the pressure differential. Maybe after burn diesel or propane in the exhaust header? You'd need a way to increase exhaust pressure without using the internal combustion pressure.

HexenLord

Complete apples to oranges misrepresentation.

70% efficiency and 90% efficiency doesn't make them similar.

Also, you were wrong right off the bat assuming pressures are additive. They are multiplicative based on pressure ratios of boost pressure/atmospheric.

Also that calculator doesn't apply since pressures are multiplicative and not additive when compounded. The calculator you are using for heat calculations is nothing more than a guide that doesn't include ANY variables. It doesn't differentiate between superchargers, turbochargers, whether center sections are oil cooled only or also water cooled, whether the intercooler is air to air or air to water, and on and on and on. That is not a 'real world' calculator.

This guy seems to understand that fact perfectly.

parish8

iTrader: (6)

your high level of confidence does not seem warranted.

what difference does it make if an intercooler is water or air? the eff rating of the unit and pressure drop across the unit is all that matters.

what difference does the means of boost matter when looking at compressors? you have an airflow, pressure and eff.

oil cooled or air cooled bearing? really?

i understand the pressure is multiplied and the numbers i used represent that. 7psi turbo, 9psi blower, 20.5psi at the valve. i used 7 psi turbo and 13.5 psi blower but the blower will really have a 9psi pully or a 1.6p/r which turns out to be 13.5psi when you account for the 7psi inlet pressure.

RonA

iTrader: (1)

My truck used to like the primary making about 45psi running against the wastegate and a total boost at the manifold of between 68 and 72psi depending on the day. I would think a large wastegated single running at around 10psi feeding a blower and seeing under 20psi at the manifold would be better than the blower by itself, but not better than 20psi from the single turbo with no blower. Would spool better with the blower though.

cajundragger

iTrader: (2)

The compressor type matters. You can't compound blowers but you can compound turbos.

Pressure ratios are ambiguous for implementing compound setups...But they do explain how they work. Also a turbo that has a 3:1 ratio as a single will not have a 3:1 compounded.

Atomic

iTrader: (37)

Turbo compressors are constant mass devices (with respect to engine rpm) while blowers are constant volume devices (with respect to engine rpm).

The calculator you use is fine, its just using the isentropic (perfect) compressor equation with an compressor efficiency input (this is what you read off a turbo map). Everyone knows air gets hot when you compress it, even if you do so with 100% efficiency. The difference in air temperature of the turbo output at a given pressure and when you do it isentropicly (perfectly - no losses) is how you define the compressor isentropic efficiency which is whats on the compressor map.

With a blower, it is moving a constant volume of air per revolution (see the picture below)

Notice how the VOLUME of air is about the same from 6psi to 21psi.


And you get an isentropic efficiency of something like this:


A roots blower gets less and less efficient with more boost...keep in mind this does not mean it doesnt make more power, it just doesnt compress it as efficiently. A roots blower only makes boost because the volume of air its moving is some amount higher than the engine will consume NA. The excess air gets compressed while its stuck between the blower and the intake valves. Blowers do not make very good compressors.

Bringing this back, the engine really doesnt care about how much boost you run, it cares about the mass of the air getting in the engine. More air means I can burn more fuel releasing more energy and I get more power, very simple. If you feed a blower boosted air, it will still move the same volume of air, but it will be moving more mass because the air going in is more dense. Think of it like a bucket. A bucket will only hold lets say 5 gallons. That bucket doesnt care if its filled with water or filled with feathers, its only going to move 5 gallons at a time, but the mass of the stuff coming out of it depends on what you put into it. If you put in air that is twice as dense, it will move twice as much mass but the same volume.

This whole volume discussion is important because the turbo moves a constant mass of air, it doesnt care how much volume that is. You can figure it out from the compressor map, volume=mass/density. You can read mass right off the chart. You can find the density ratio by using a chart like this:



So for example, lets say you have that turbo from the OP at a pressure ratio of 2, and mass rate of 40lb/min. This is a density ratio of about 1.5. Now you are using a tvs1900 with a pulley ratio of 3/7 (blower/crank) and you want to know what a 6liter engine is doing at 6000rpm. At that speed, the blower is spinning (7/3*6000=14000) and based on the chart above means its moving 850cfm. Now since its being fed by the turbo it is actually 850*1.5 = 1275cfm. For the engine airflow it will be (364*6000/3456)*.75=474cfm [(CID*rpm/3456)*ve)].

So we are feeding it 1275cfm and it normaly ingests 474cfm so using boyles law you are going to have 14.7*1275/474 = 39psia in the intake or about 25psi of boost using the turbo at a pressure ratio of 2 and a 9psi blower pulley. However, remember we are still only using as much mass of air as the turbo is giving us, which is 40lb/min.

Moral of the story here is you can make plenty of boost twincharging, but it doesnt mean you will make more power which is why "turbo only" setups generally do better. You have to account for the isentropic loss of the blower, drive power of the blower, and a VE loss of the extra piping and intercooler under the blower. The advantage is you have the blower making power before the turbo spools up so your overall average power will be higher initially. If you are going to twin charge, you need a big turbo to make it worthwhile.

Using sequential turbos is easier since you can just keep it in terms of mass and not worry about volume. Basically you want both turbos to be in their efficiency range at the same time, which gets tricky. Diesel guys dont care because they want as much boost pressure as they can get because most run huge excess air. One of the turbos on diesels in sequential supplies enough air mass, they just need it at a higher pressure.

To answer your question directly, the blower really isnt a restriction, but it does rob power in terms of less efficient compression and required drive power so a turbo-only setup will do better on the same boost level, as long as the turbo isnt maxed out to begin with. You get extra boost with the blower, but it comes at a high price. Case in point, if we are using the above example and twin charging, the turbo is at 76% and blower is at 68% for a "compression efficiency" lets call it of (.76*(1-14.7/25)+.68*(14.7/25))=71%. If we just ran the turbo at 25psi (pr=2.7) it would be 75-76% still. Keep in mind we need about 45hp to drive the blower (http://www.tbssowners.com/forums/showthread.php?t=21976) which is a pure loss. Of course there is no free lunch with more turbo boost as backpressure increases and engine ve suffers somewhat, but this loss, I would wager, is less than what you get by using the blower.

As far as reading material, I really enjoyed this book, especially the history section

Hank Peabody

iTrader: (2)

That is awesome, that line applies in alot of threads on the internet!

Forcefed86

iTrader: (4)

Heres a chart my buddy linked me to. that may apply more closely to your setup. I believe it was twin 61mm turbos on a supercharged (oem charger) 2003 cobra.

This information was calculated on a spreadsheet.

Atomic

iTrader: (37)

Did he record that chart from observation or calculate it?

Forcefed86

iTrader: (4)

Calculated. I was wrong on the turbo, it was the hellion twin 61mm kit for the 03 cobra.

parish8

iTrader: (6)

i like where this thread went. great info, thanks!

RonA

iTrader: (1)

If the blower acts the same way as a secondary turbo, the added benefit will be faster spooling of the primary turbo due to the increased exhaust gas coming out of the motor. My 360 inch diesel would spool A GT5533 Garrett primary up to 5 or 6 psi at 2000rpm. Without the Gt4094 working as a secondary increasing the exhaust gas output I doubt a 5533 would lift the gauge off the peg by 2000 rpm. I hope you try it. I've got a little MP112 on my car and have wondered what it would be like to throw a turbo in front of it.

parish8

iTrader: (6)

that works with the numbers i have been using. 7psi of turbo boost on a 9psi blower and i have been using 20.5psi finial seem like the right amount of boost on a 6.2L lsa

atomic 6

Great post ATOMIC,

I think that your post would reinforce what Jim is saying, a larger turbo on a factory(lets say LSA) setup would only yield the output of the Large turbo(fine with that), but would give the instant torque needed for a great launch in a heavy car, not previously possible on just the large turbo, without all the other measure it takes(and we have done) to get that launch.

Would there be draw backs to this setup, yes, but are they so bad they make it a futile exercise? No, I think.

I think we learned a lot about pressure ratio, with the maxed out turbo on the Fairmont, A denser air feeding that turbo( -200 DA) made it possible to get more from a 70mm turbo, than we ever had(evident from ET, and MPH)

One problem I see would be the rarified air between the blower inlet, and the turbo outlet. That could reduce the normal output of the blower at the lower RPMs into the engine, until the turbo compressor "catches up"

The way I see it is, the blower is just a pump, and that is not going to work well with rarified air feeding( less than 100kpa), and conversely will work fine with compressed air(more than 100kpa) feeding it.

Very interesting thread.

Atomic

iTrader: (37)

You are correct blowers are just pumps, more specifically roots blowers were originally and are still manufactured as low-pressure pumps. Used extensively in HVAC and industrial settings, but the ones are cars are more like a combined roots/screw than anything else, but I digress.

Blowers are very sensitive to pre-rotor losses, and I imagine one wont be happy having to suck air through a turbo, intercooler, and a an extra few feet of piping. Of course this is a moot consideration once the turbo is spooled, but the blower will have trouble breathing until that happens.

I agree it has advantages, but those kind of disappear if you build it for drag racing. There are a lot of ways to launch with a turbo spooled up so lag is basically gone. However it would be a very fun DD to mid power setup; low end of a blower with the top end of a turbo.

parish8

iTrader: (6)

Atomic, I am not flowing how you came up with the 59% eff. If I run the numbers through the calc twice I end up with the same temp as running them through once. If the eff is compounded resulting in a lower eff than just the turbo isn't the boost also compounded resulting in a higher p/r? I do not think I am being very clear here but you probably understand my question.

parish8

iTrader: (6)

My Lexus had a t76 rear mount on a 5.7l engine. It had a pretty long intake tube. I could stab it from an idle and it would go strait to ~100kpa or whatever the pressure was that day. I am not sure what this proves since the blower is going to want alot more air than that but it is one example where I was worried about pre boost performance but didn't have a problem.

Atomic

iTrader: (37)

I had a slight math error and corrected it last night if you look back at my first post. Its basically a weighted average based on the air flows. Its actually (.76*(1-14.7/25)+.68*(14.7/25))=71%. As the blower does more of the work it gets closer and closer to the blower efficiency. Obviously 4% isnt that much of a difference, but when you consider it also has a dead loss of around 45hp its a little worse.

The pre-rotor losses I was refering to in the above post will show up in terms of volumetric efficiencies.

Fundementally, the turbo compressor is a compressor, which means it increases air density only. The blower is a pump, which means it moves a volume of air at a low pressure. Turbo = high density, low volume. Blower = low pressure, high volume.