parish8

iTrader: (6)

i am looking to have a technical conversation about compound boost and looking for any good links to threads that discus the technical side of compound boost.

all to often i see people posting up "the blower will restrict the turbos" and other often repeated ideas. sure it might be true but i have yet to find info with technical info supporting this.

sometimes i see in threads where people say they pulled the blower and gained power but once again no dyno charts or technical info to go along with this statement.

i have two ideas on the subject to get things started. first one is can the blower actually be a restriction? it is hard for me to see how it could be. if you compress the air to a denser charge and feed it into the blower wont the blower just take that charge and compress it further. it is like the compound turbo chargers on diesel trucks. the 2nd turbo would never be able to make 70psi on its own but if it is fed 35 psi and only has to take the charge pressure up another 35psi it can do that. i am looking for points of view on how the blower might be a restriction.

my 2nd thought on the subject is i bet most people pick turbos that are not ideal for the task. lets say you want 1000rwhp. a pair of 61mm turbos or even smaller should be fine. problem is when you think about what you want those turbos to do i think that would be way too small. you will want those things to flow 1000hp worth of air but at only 7psi or cut that in half so each one is flowing 500hp worth of air but at only at 7psi(1.5bar). a 61mm turbo is not going to be happy at all flowing that much air at that low of pressure. here is an old map from a 61. you can see at 50lbs of air and 1.5bar it is off the map(lots of heat)



now compare that to a pair of 88mm turbos. look where you are on the map at 1.5bar and 50lbs of air. much higher efficiency almost in the ideal circle. is it possible that the people with less than ideal results that are gaining power by pulling the blower just have too small of a turbo for the task?

[media]http://www.forcedinductions.com/_derived/Tseries.htm_txt_t88.gif[/media]

these are old maps, as far as i know there are no maps on the new stuff. that being said i doubt the low pressure high flow part of the map has improved much.

dmaxvaz

iTrader: (1)

Well, a buddy of mine has a twin turbo/blower ford gt, and it makes like 17psi compounded, and makes 1000 hp and totally useless under 100 mph on the street. Totally burns the tires off at 5psi. Its a hennesy built car, and think its got twin garrett 35r's, and would run better with just the turbos (no blower).

HexenLord

There are a few things to remember here. First is that roots style blowers are not nearly as efficient as turbos. Therefore adding a roots blower to a turbo setup actually lowers efficiency.

second, the blower needs to be able to draw in at least as much air as the turbo in order to not sacrifice peak power. if the turbo is cramming in more air than the blower can swallow, the problem is not that people are using turbos that are too small, they are using blowers that are too small (or turbos that are too big). In this case, you'll just be creating lots of excess heat.

Heat is a major issue here. The blower generated extra heat. It makes for some inflated PSI numbers. Hellion added a twin turbo kit to a supercharged cobra back in 08 and made 1100HP at 44 PSI. Those turbos could've made that power alone at a lot less pressure. a large twin screw could have made similar numbers on a similar curve.

And compressor maps are useless in compound boost applications. The turbo isn't creating nearly as much pressure for its RPM as normal because its being forced into a blower that swallows a lot more air than the engine normally would at any given RPM. Therefore you have to increase the effective displacement before plotting on the compressor map.

Compound boost has its merits for given situations. If you want a wow factor, twincharging wins. If you want a jack of all trades build, twincharging will make roots blower torque while sacrificing a small amount of the turbos peak efficiency. Then there's compound boost with supercharger/supercharger and turbo/turbo. Those are almost completely different discussions.

HexenLord

I know of a Ford GT using twin 35Rs that makes 1200 HP at about that same boost pressure and no compound boost.

AmishAssassin

iTrader: (2)

I think it would be interesting to instead of addressing the turbo output addressing the blower input. Most of the builds are with smaller roots style blowers and a decent turbo or twins feeding them I would like to see a T-88 feed into a Procharger D-1 you would still have the option of intercooling the air charge more than you would with a roots style blower. As far as the blower being a restriction, I would say no the expansion of the charge air is the wildcard. If you had a way to keep the air cool the whole way through to the cold side of the supercharger now that would be interesting, it would be a terror on the street. The biggest problem I see is the expansion of the air after it is compressed, not the fact that the blower is an actual restriction I did find this compound setup quite amusing though.
It seems to work well on small displacement motors.

slow67

iTrader: (2)

http://www.performancetrucks.net/for...histle-464822/

sbcgenII

iTrader: (1)

Small read.
http://www.autozine.org/technical_sc...duction_5.html

Mike@Diablosport

iTrader: (2)

There was a local GT500 that did a compound setup, made similar power with and without the blower as I recall, but went faster with just the twins...

http://www.mustang50magazine.com/fea..._shelby_gt500/
http://www.svtperformance.com/forums...llion-kit.html

HexenLord

That's not a compound setup.

hellbents10

iTrader: (10)

It works good for pulling stumps out of the ground and killing head gaskets.

parish8

iTrader: (6)

I do not agree with some of what you just said. :-)

The tvs blower is rated at "over 70% eff" the maps that you can find on turbos has them in the 60-80% eff range. Similar. I have read that the eff is compounded just like the boost and will create a lot of heat. Using this calc http://www.stealth316.com/2-turbotemp.htm you can play with various set ups and compare charge temps.

I did 90deg inlet, turbo only, 14.7 ambiant, 20.5psi, 70% eff and it spit out 310deg.

Now do the same thing but run it through twice. 1st time 90deg input, 14.7 ambient, 7psi, 70% eff and it spits out 183deg. 2nd time 183deg input, 21.7 ambiant, 13.5psi, 70% eff. It kicks out 318deg. Pretty much the same temp. This is all before intercooling. Both should give you 20.5psi at the valve.

On the blower size issue, typical compounded boost set ups require the first device to support 100% of the power you want to make. The 2nd compressor is usually much smaller since it is dealing with a very dense intake charge and only compresses it the next step.

Not sure why a compressor map is useless at least for the turbo. I have an idea on airflow and on boost level. Look at the map. My thoughts are this is the one area where everyone might be doing it wrong. You need a very large turbo to flow 60lbs of air at only 1.5p/r.

Sent from my iPhone

cajundragger

iTrader: (2)

keep in mind pressure is the resistance to flow. As I'm sure you know, a car that makes 600hp on 10psi adds a cam and free flowing exhaust and now makes 600hp on 4psi.

Psi doesn't = horsepower. Airflow, more specifically mass flow equals hp.

A PD pump(blower) is just a dP machine. Regardless (within reason) of density, the blower will move the same volume.

PD pumps can and do 'slip' when they start reaching their limits. you may feed 5psig on top the blower, the blower rotors grab some of that dense air and heat the rest. Its a choked flow scenario. At a given rpm, the blower can only flow so much CFM at a given backpressure reguardless of the intake pressure....so yes, blowers can limit the air flow. Its the partial pressures that surround the rotors that screw things up. You must look at a 'micro' level not a 'macro'.

To make twin charging work, you need a way to bypass the blower when the turbos kick in. I've seen several especially in the ecotec world.

parish8

iTrader: (6)

Let's say I want to cut a 1.4. 60' in a 4300lb car with a nearly stock feeling converter. It is going to take some tq to do that. Spraying a turbo set up off the line is my alternative. I know that works but it has to be kind of harsh on the head gaskets too.

cajundragger

iTrader: (2)

sorry but this is very wrong. turbos are not additive on pressure.

turbos are basically centrifugal pumps...or constant head machines( not to be confused with your buddies girlfriend)...to get more pressure(essentially head) out of a centrifugal pump the density needs to go up. The smaller turbo is just a prestage for the bigger turbo.

Like I said above though its not all about pressure, its about flow and density or mass flow. how much oxygen is getting in the cylinders...

also, using a procharger with turbos isn't real smart because the pro charger is still going to be laggy because its a centrigual pump and not a positive displacement(PD) pump.

parish8

iTrader: (6)

I have been searching this topic and have not found a good resource. My question, how do they get to 100+ and even over 200psi with compound turbos if the turbos do not multiply the boost that they are fed? As far as I know even a very good turbo maxes out around 50psi.

hellbents10

iTrader: (10)

4300lbs and a tight converter to a 1.4x 60ft is going to be very hard on parts to say the least!

I have tuned some stock converter AWD stuff with positive displacement super chargers that needed timing values close to 0* at the launch.

Mike@Diablosport

iTrader: (2)

i've always understood the compound reasoning to be the smaller turbo is to help it spool since the big turbo would be lazy as hell, but I believe the big turbo is basically on its own up top, and can/will support the final power numbers on its own.

parish8

iTrader: (6)

My understanding is that yes the first turbo needs to be large enough to support your power goals but with a diesel you can not get 1000+Hp without high pressure. That is where the 2nd/3rd turbos come in.

NemeSS

iTrader: (127)

imo,
the ic under the blower is the greatest restriction on a pd/turbo compound setup.
remove that screen the ic provides and it will free up some breating room.
im actually working on something similar.
im using a 2011 gt500 1.9 sc on my 408. I built the 408 to handle boost. 8.25:1 w arp main studs,hellfires,the works.
now I know the 1.9 is undersized for the 408, but my goal is to use along w a tc78.
a long duration cam, in the 240s. I believe the 408 will have no problem w pumping out exhaust volumr to spool any turbo w the sc help. but imo it will instantly tax the turbo dam near immediately. its probably way under sized. but I want to give it a go.
ideally I think a s47x would b a better fit.
I have the plenum to mount the sc, making the plenum lid, and making the tb adapter for a ls 90mm tb.
there will be no intercooler under the sc/lid.
that particular sc has a egr or a bore in the housing. I found out thats its possible to fit a injector in it. I found an injector that fits w a good seal w orings I used off something else.
kind of like a 9th injector.
im using a bank/bank batch fire ms efi system. so I can draw fuel from the main rails from the schrader valve ala wet kit for the 9th injector.
the 9th injector will mimic #1 or the driver side bank. it will supply fuel fuel time as the same cycle as the corresponding bank. it will also increase fuel pressure like the other injectors, since the fuel system is a rising rate return type.
the 9th injector will provide a source of cooling and fuel all the time. perhaps not as good as a ic but will come alive on top w alot of gas flowing. this will also make for a pre loaded fuel charge like a blow thru. the boost will have gas mixed in alredy as soon as it exits the discharge.
also it will have meth/h2o injection before the rotors on a psi switch which will also aid in coing,raise the ve, and provide fuel and higher octane. this is all just on the blower side of things.
this all makes sense in my head at least. but ive been wrong many times before. :/
I strongly believe the low dcr/scr of the motor w a large duration cam will not be a factor in throttle response. if anything it will aid in allowing the powerband to flatten out some and be controllable. and allowing the motor some breathing room. and not choke under all that pressure at higher rpm. a smaller cam would restrict it and raise the backpressure even more as it is.
like I said I may be wrong w everything. but im gonna give it a go.

ls1_chevelle

iTrader: (9)

I found this script.