I have been curious about finding a way to get big hp out of our motors, while at the same time minimizing turbo lag. Essentially, there are at least two routes that can be pursued, and these are twin-charging and compound turbo charging.

EDIT EDIT - Sequential turbo charging is another available option.

EDIT - I'll leave the twin-charger information here just for references purposes, but superchargers made for our cars do not seem to be adequate to make a twin-charged system worthwhile. Superchargers large enough to move enough air to make it worthwhile appear to be too large and not designed to fit our cars. If anyone knows of a supercharger to fit our cars that makes decent boost below 3000 rpm, please post to let us know about it. It probably will need to be a roots style S/C.

In a typical twin-charged setup, the airflow out of a turbo charger compressor is channeled into a super charger compressor. The supercharger always provides a level of boost, but it is the boost at low rpms that we are most interested in. Once the turbo charger spools up, then you get boost from both. The total boost ratio (Bt) is a product of the boost ratios from the from the turbo charger and the super charger, i.e.:

Bt = Btg * Bsg

where Btg is the turbo charger boost ratio and Bsg is the super charger boost ratio. Remember, boost ratio is (absolute pressure)/(intake pressure). Absolute pressure is boost + intake pressure. E.g., 14.7 psi of boost has an absolute pressure of 28.4 psi. The boost ratio os 28.4/14.7 = 2.

There are some issues with twin-charging, the foremost being cost (you have to by both a turbo system and a super charger system) and fit (it all has to fit in your engine bay). Nonetheless, it has been performed successfully on Nissan platforms. Here is a 350Z with a roots style super charger: http://www.turbomagazine.com/feature...ged/index.html and here is a twin-charged GTR: http://www.tremek.com/forum/car-pict...arged-gtr.html


With compound turbo charging, the compressed air from a large turbo charger is feed into the compressor of a smaller turbo charger. Below is a sketch I made of a compound turbo charging system.

Advantageously, the smaller turbo will spool up quickly, thereby providing boost at low rpm, and when the larger turbo spools up, you again get a compounded boost ratio, i.e.:

Bt = Bbt * Bst

where Bt is total boost ratio, Bbt is the boost ratio of the big turbo charger and Bst is the boost ratio from the small turbo charger.

Although compound turbo charging does work well for high boost applications with cast iron blocks/heads, at this point it is not practical for our motors because presently available turbines are designed to efficiently flow maximum air up around a 2.5 pressure ratio. So, if you series two turbos, each operating at a 2.5 pressure ratio, you end up at a pressure ratio of 6.25, or 5.25 Bar of boost. That is 77 psi of boost, which is way, way too much for our motors. If, however, turbo compressors were available that would efficiently maximize air flow at a pressure ratio of around 1.6 - 1.8 (0.6 - 0.8 bar of boost), then compound turbo charging would become a viable option for us.

In summary, if someone wants big HP with minimum turbo lag, there are no good options right now for our motors. If someone wants to change over to a cast iron block/head(s), e.g., the RB26 motor, then compound turbo charging starts to become an attractive option.

Here is a sketch of a compound turbo charger system:



There would be a significant benefit by using an additional intercooler placed between the two compressors. The easiest way to make this fit is to used a small air-liquid heat exchanger betwen the compressors and pump the liquid to a front mounted cooler, maybe something like an oil cooler.

 

You gotta love the sketch

I'm thinking about twin FP GT35 or GT30 HTA's in my build in the future, more power and less lag.

 

i always found compound charging very interesting, its found on many cars from factory like diesel BMW's, ive driven my neighbors and it works really well.

 

Terry,
Have you seen SP's quick spool valves?

They also have a crazy twin sequential setup for the supra with bigger twins, but easier to plumb the exhaust on the 2jz....I know larry has ideas about stuff like this too.

Tom

 

Yes, it is a good solution. The key is having the right compressors. Unfortunately, the compressors posted on Garrett's website are not suitable for the VQ motor. Their efficiencies are very low at the pressure ratios that we would need for our cars.

I have. They look good too. I would love to see a dyno plot on a VQ motor using them.

I looked at sequential setups too, but then there is valving to worry about. One idea I had is to run the turbines in series like in my sketch. Then, instead of running the compressors in series, run the compressors in parallel. The compressor outputs on each side of the motor can be merged, but a butterfly valve and BOV would be needed for each pipe before the merge point. That starts to get complicated.

A compound turbo system definitely would be cleaner, and is a well proven concept. The problem is finding the right compressors for our motors. At least one, but preferrably both, of the compressors needs to acheive high efficiency at low boost. I tried to find a matching pair of Garret turbos that would work, but I didn't find any where the results would be worth the effort.

 

Guys check out boostlogic compound turbo set up! Good vid too, Click on bottom left vid!

http://www.boostlogic.com/index.php?...d=36&Itemid=43

 

That is wicked!

 

The only problem is i don't see "down low" boost for superchargers.

Here is a log from my vortech system with a 2.87" pulley. pressures are as follows:

2990: 2.76psi
3500: 3.71psi
5040: 7.54psi
5880: 10.13psi

The guys running gt35r turbos have max boost by 3200rpms. How is a supercharger that takes 5880 rpms to get to 10psi going to provide faster boost than a turbocharger that can hit 10-15 (depending the boost level set) by 3200 rpms?

I might be confused, but personally i think my supercharger takes way longer to build boost than any of my friends' cars that have turbochargers. Am I missing something?

 

For a twin-charged system, you would want something much bigger than a Vortech. Something that is up to around 8-10 psi of boost at 2800 rpm would be nice.

 

Here is some technical information on selecting turbos for use in a compound turbo system, followed by an example.

Both turbo chargers need to support the mass air flow (MAF) that is required for the desired hp. Determining the capabilities of the larger turbo is relatively straight forward - look at the turbo's compressor map and find where the pressure ratio of the large turbo (Bbt) and MAF intersect. On the map, Corrected Air Flow corresponds to MAF.

For the small turbo, determining the MAF capabilities is a little more tricky because the compressor maps are generated at atmospheric pressure (1 Bar) and at 25 deg. C (298 deg. K), and the MAF capabilities of a compressor changes in accordance with both of these parameters. So, in order to determine the MAF capabilities of the small turbo in this particular setup, a MAF correction factor (MAFcorr) must be generated so that we can still use the published map. The equation to do this is:

MAFcorr = MAF * ((Tin2/Tref) / (Pin2/Pref))^0.5

where

Tin2 is the temperature in deg. K of the intake charge into the small turbo compressor (from the large turbo compressor), Tref = 298, Pref = 1, and Pin2 is the absolute pressure of the intake charge into the small turbo compressor in Bars (boost pressure in bars + 1 bar).

Tin2 = Tin1 + (Tin1 * ((Pin2/Pin1)^0.15) - 1)/eff

where Tin1 is the intake air temperature into the large turbo and Pin1 is atmospheric pressure, and eff is the efficiency of the large turbo at the pressure ratio/MAF point on the compressor map.

EDIT: I found varying references for the value of the exponent for computing Tin2. This value has a lot to do with the volume of the tubing etc. after the turbo. I set this equation up in a spread sheet, and adjusted the value of the exponent to output a value that matches the I/C intake temperature as posted by Sharif in XKR's previous build. I will do more research on this, but I'll use 0.15 for now. I have updated the analysis below based on this.

To convert from deg. C to deg. K, add 273 to the deg. C reading. To convert from deg. F to deg. K, K = (F + 459.67)*(5/9). Alternatively, just use this conversion web site: http://www.onlineconversion.com/temperature.htm. Also, remember that the air flow has not yet been cooled by the I/C.

The MAF correction factor and boost ratio of the small turbo charger then can be plotted on the small turbo's compressor map.


Example #1

Displacement: 3.5 L
Desired power: 1000 whp
Setup: Quad turbo (compound turbos for each bank)
Computed MAF: 54.5 lb/min per side
Computed Boost (above atmosphere): 36.2 psi (2.5 Bar)
Intake Air Temp: 77 deg. F, (25 deg. C, 298 deg. K)

Bt = 2.5 Bar + 1 Bar = 3.5 Bar

To divide the boost up evenly, let's try:

Bbt = Bst = 3.5^0.5 = 1.87

Based on the compressor map for the GT3582, it will not flow 55 lb/min at a pressure ratio of 1.87 with adequate efficiency. So lets try a GT4088R (yes it will be hard to find a place to fit this, but let's ignore that for this example).

At Bbt = 1.9, the GT4088R flows 54.5 lb/min at 70% efficiency

Tin2 = 298 + (298 * ((1.9^0.15) - 1)/0.70 = 349 deg. K

MAFcorr = 54.5 * ((349/298)/(1.9/1))^0.5 = 42.7 (use as lb/min value in published map)

So, for the small turbo, we need a turbo that flows 42.7 lb/min at a 1.9 pressure ratio. The GT2876 just makes it. It will flow 42.7 lb/min at about 65% efficiency.


In contrast to example #1, if the turbos were available with larger compressor inducers (e.g., higher trim values), then we could use a GT3582/GT2871 combination. This would give us the hp of the GT3582 with the turbo lag of the GT2871.

At Bbt = 2.3, the GT3582 flows at 73% efficiency.
Bst = 3.5/2.3 = 1.5
Tin2 = 365 deg. K
MAFcorr = 39.7

Although the GT2871 will flow 39.7 lb/min, not at a pressure ratio of 1.5. At that pressure it is beyond the choke line. With a 56 Trim, 0.60 A/R, it needs about a 1.8 pressure ratio to get to the required air flow. With the right compressor blades it could get there at a pressure ratio of 1.5, though. I'm speculating that a trim in the range of 60 - 70 might work, but that is just my guess by comparing the 48 and 56 trim maps.

 

what supercharger built for our vehicles offers this?!

The most i've seen so far is out of pharmD's t-trim that puts out 17psi. They are rated a little higher. v7 ysi i think will put up to 22psi, but that's at max rotation, not anywhere near that at 2800rpm.

 

Perhaps it will need to be a supercharger that is not necessarily designed for our cars. Let me look to see what they used in the GTR.

EDIT: You are right. The superchargers that are available and that will fit in our cars tend to suck. There are bigger ones out there that will give 8-10 psi as low as 2000 rpm (e.g., Kenne Bell), but they don't fit our motors and, even if they did, they would look ridiculous on our cars.

EDIT EDIT: Scratch the twin-charger setup idea unless you use a roots style S/C with decent boost, if you can find one.

 

so where did you learn all this stuff? I build NA race bikes so all this turbo info and pressures are new to me. I'm not sure of a school that has adequet information on it.

and would those kenne bell superchargers boost higher in a lower rpm range due to the lower peak rpm of the motor? I think those are also root screw type chargers and not our centrifugal style.

anyone have a pressure/rpm map of a stillen charger with the highest psi pulley on it? they provide more down low torque so maybe they boost sooner.

i would love 8-10 psi at 2k rpms!

 

The Kenne Bell superchargers are roots style, and they are huge...

EDIT: I just tried to find boost vs rpm map for the Stillen S/C, but could not find one. There are some other roots type S/C's, like ORC, but still no info.

As far as learning, I learned the basics in Thermodynamics during undergrad. Although I was an EE major, I really liked Thermo. Whenever I get interested in something, I just start researching the Internet, going through my old books, etc. Once I get stuck on something, I usually can't let go until I find the answers I am looking for.

 

I think in order to do a twin charged setup properly you would need a roots type blower, not a centrifugal one. There just isn't enough room under the hood for this type of setup.

Guys with Grand Prix GTP's run this type of setup. It is easy for them because the car is already supercharged, so all they do is add the turbo. Basically full boost at 1500rpm. You do need two BOV's though, so it would be:

Turbo...BOV...Supercharger...Intercooler...BOV...to TB. That is a ton of stuff for an already cramed engine bay.

Here is a nice setup:

 

Cool, thanks for the info. That looks like a nice setup. I just read the sign - 1600 hp

Do you know how much boost the S/G's for our cars provide below 3000 rpm?

 

No, unfortunetly I don't. Most being centrifugal superchargers, I would suspect very litle boost. I know with my old car (I made my own supercharger kit) with an Eaton M62 blower, I would get full boost at 1200rpm. The TB was after the supercharger so that makes a difference as well. All OEM root style setups have the TB mounted to the S/C intake.

Here is an OEM twin charged setup, it gives you a much better idea of how the BOV's/plumbing works:




I was actually thinking about doing/fabricating a twin charged setup, but ended up doing a single turbo. You would need something like an M90 for the 350z, and mounting it anywhere but the intake manifold is just not going to happen. If I had some time, I bet I could figure something out, may have to remove A/C but nothing is impossible.

 

I meant to ask if you know how much boost the roots style S/C's for our car produce at 2800 rpm. I got to typing and forgot to put "roots style" in the question.

Nice find with the drawing. It definitely looks like a roots style S/C is the way to go.

EDIT: I think the reason they are using the control valve is because they have the S/C before the T/C. When the T/C spools up, it needs to bypass the S/C. That won't be an issue if you put the T/C first. I'm not sure why they didn't do it that way, but I'm sure there is a reason. Maybe they have the S/C connected to a clutch so that they can stop spinning it after the turbo spools up. That would increase the overall efficiency of the system.

 

Sorry but I don't. Maybe there are a few guys (that Dough hasn't scared away yet) with the good old Stillen kit, maybe they can chime in.

Again though, with the Stillen the TB is mounted to the supercharge intake, so it would not be an optimal setup. You really want the TB after the supercharger pressure side. With a roots style blower the BOV/Bypass valve is always venting (a ton of air). So as soon as you step on it the valve closes and you ge instant boost.

With the TB before the supercharger there is a delay, so any input from the Stillen comunity would not really be aplicable.

 

^^^^

Good information. So really, we need a roots syle S/C that does not sit on top of the motor. I'm not sure if any are made off the shelf like that, but that would be nice.

EDIT: A screw syle S/C MAY even be better than a roots style S/C.

Still, I like the compound turbo setup, if we could only get the right compressor blades...