The way it is now, you may have a problem. When your small turbo is all spooled up and you suddenly have a valve close off the exhaust inlet, you'll have a turbo that is still spinning at 30,000 RPM - it will create a vacuum between the closed valve and the still spinning inlet turbine. I don't think that would be good, could set off a chain-reaction engine failure on a big scale.

It would make more sense to have the exhaust of the smaller turbo directed toward the inlet of the larger turbo, and then use a diverter valve to bypass the small turbo and altogether once your target boost level is attained. The difference here is that the diverter valve would be placed after the exhaust outlet of the small turbo, so when you close it, that surge of pressure is all directed toward the larger turbo's inlet, possibly helping it spool even quicker.

 

that could lead to you stalling the small turbo before the big one is up and running properly! also dont like the idea of forcing red hot exhaust gasses back the wrong way through the turbine! its bad enough for the turbo on the inlet when that happens!!!!

if i was doing it i would run a nice small ish front mount (say a 67 or maybe 70). now where the collectors joind and just before the turbo i would install a bypass valve that would totlaly (well mostly) bypass the samll turbo and dump into the outlet of the samll turbo. this is basically like a second weastgate!

then i would run the small trubo exhaust and baypas gasses back to a large (GT42R) turbo rear mounted.

now you would need a BIG weast gate up front! one (or two ) that could deal with the GT42R on full song!

now the inlet would be similar but in revers! so you have the main inlet at the rear, that then runs upto the front small turbo. inbertween the small front turbo and the intercooler you would run another bypass vavle that would poerate at the same presure as the exhaust one.

to operate the 2 bypass vavles you would monitor presure in the inbertween pipe from the big turbo to the small one (cold side). once possitive presure wss reached in that pipe( ie when the big turbo was take the load not the little one) the vavles would START to open!! thsi would then bypass the smaller turbo but still flow enough through it to keep it upto operating speed so when you came off the big turbo power band it was still there ready and waiting.

what you think???? no electronics wouls be needed and would all be manual. need about a million miles of boots lines thoguh! lol

thanks Chris.

 

Guys cMon think! why use 2 different sized turbos, use two smaller ones they will spool quicker and use awastegate to control the boost pressure, No lag time and constant boost. Or better yet stick with your larger turbos out a nitrous jet on her *** and spray it til it spools to where you want it. Thats what we do on all the turbo outlaw cars.

 

Larger turbos can produce a higher CFM at a given boost level, which means they can produce more power, but they take long to spool which limits their effectiveness to the high end of the powerband. Find a way to make a big turbo spool quickly and you get the best of both worlds. Sure, you could use nitrous to compensate for lag, but that's a shortcut I would not take for my own car.

 

This is something that seems to get argueed quite often..

Do 2 smaller T4 turbos really spool up faster than a T6?

Excluding the impeller, with 2 T4's you are using 4 cyl's through a single 3" pipe to spin up the smaller 5.9 square inch exhaust housing vs 8 cyls through two 3" pipes merged into either a 4 or 5" pipe going through a 9.2 square inch exhaust housing.

Sure roatating mass becomes a factor on both the exhaust housing side and the impeller side.


Plus the idea behind the small big turbo is to take advantage of spool at 2k rpms up. My car is not a drag car. Using N20 to spool a turbo for open road racing probably isn't the best idea. Coming off a corner at 5500rpm and having a PT91 spool all of a sudden probably would be a bad idea. But keeping the rpm's down to say 3k in a corner, letting the small one spool quick and roll over to the PT91 probably wouldn't shock the tires as much.

 

LIL SS thats a great point! racing drivers HATE turbos because of their dilivery! with a normal turbo its nothing then bang! loads of boost! and with a big one its nothing, nothing, nothing, BANG **** loads of boost!

you ask a racers how he likes his turbo car set up and he will say give me more low down! this way you open up the rev range and give the car a more consistant feel!

instead of being on and off it more like an NA car. this lets you drive it smoothly and thus you become faster!

think about the weight distribution of your car when going through a corner.

the entry the weight is thrown forward. this helps the tires bight and give them grip. but you dont want to much or you overwhelm them! then you hit the ape and start to open the throttle out the corner. this means the weight is shiffted back on to the rear wheals, giving you more grip at the back, but making the front go light! now if you hit full boost (big turbo) half way around the exit to a corner you are going to push wide and go off! end of! with a small pregresive set up you will control the car on the throttle and then give her full on the exit! thus making your driving smoother and thus faster!

Chris.

 

ok, cfm and inlet outlets sizes and all this other stuff, doesnt as 120lb blond weigh the same and a 120lb brunette. 20 lbs of boost is 20 lbs of boost no matter how much cfm your pushing. Of course the larger cfm turbos maybe able to produce more up top but isn't the smaller turbo's abilty to spool down low and still be able to put out enough up top really worth looking into. you can only cram so much air into and engine anyways.

 

It doesn't work like that. The goal of forced induction is to cram as much air AND fuel into the engine per cycle to increase power. If you have a one turbo moving 1000 CFM at 10 PSI and another smaller one that does 500 CFM at 10 PSI, which do you think will make more power at 10 PSI? CFM is a measurement of the quantity or amount of air, and that is what determines how much more power you can make. PSI is a measure of pressure, and a smaller turbo will need higher pressures to move the same volume of air as a larger turbo. PSI really has nothing to do with power output, and techincally we shouldn't be using PSI as if it were some universal factor that applies to all engines equally.

 

ummm... PSI determines how much air the engine will ingest period. If a large turbo is able to pump 1000 CFM at 10 PSI and a smaller turbo can pump 500 CFM at 10 PSI and the engine only ingests 450 CFM at a given RPM it doesn't matter which turbo is used assuming the compressor wheel and turbine efficiencies are the same. Really with forced induction you don't use CFM because volume is not a constant, you use mass.

When using multiple turbos you want to utilize turbo with the more efficient compressor and turbine for the mass flow at a given rpm. This results greater charge density due to the reduce intake charge temperature as well as lower backpressure. Generally a smaller turbo is used at the bottom end because it spools faster and has a higher efficiency for a lower mass flow.

 

its funny how he gave a open ended example (that obviously assumed that the engine could use 1000cfm and 500 cfm @ 10 psi) and you corrected it with another open ended statement that is also incorrect unless specific other assumpsions are made... and then you followed it with an example that was as flawed with assumptions as his, and it didnt even re-enforce your inital statement...

 

Aren't you describing the bi-turbo setup that Ausi S4's used to use?

 

the Audi just ran two indopendent turbos like any normal twin set up i thought????

Chris.

 

You can have two turbo running at the same time(like porsche) and you can have one smaller turbo for lower RPM and one more bigger turbo for higher RPM(toyota supra)

There is the sequential twin/bi-turbo setup (959, Supra) vs parallel (996TT, Audi RS6) twin/bi-turbo setup.

Either way there doesn't seem to be too much naming consistency.

B