Since I don’t have nearly as many projects as Jeb http://forums.corvetteforum.com/show....php?t=1450689

I have time to mess around with my car.


For a street car, throttle response and low end power makes for a really fun car to drive. So I trying to figure out if using a small turbo for low end power and a big turbo for top end can be made to work together?

Looking for info or comments on a twin turbo system, where one turbo is small and the other is big. For instance, instead of each turbo providing 50% of the air flow, the smaller one would provide say 30% and the other 70%.

Just as an example. I can buy a ball bearing turbo built for a 2.5L engine, and another turbo sized for a 4.5L engine.

Then using the electric exhaust cut out valves Summit sells, I can force all of the exhaust thru the smaller turbo up to a preset boost of say 5 psi. Then the exhaust cut out valve would open and allow exhaust to both turbos.

However, I might have the cut out valve not completely closed, but open just enough that some exhaust will always go to the big turbo to make sure it is spinning when the time comes for it to kick in.

Anyone ever tried this?

Also, when both turbos are making boost, would the big turbo over power the smaller one, and force air back into the compressor side of the small turbo?

 

It's called a sequential turbo system, and some of the import cars have had it from the factory.

Yes, the larger turbo will feed into the smaller one. If you think about it, the intake, intake plumbing, vacuum lines, and a the compressor side of the small turbo would all see the 15+ PSI from the big turbo. You would have to build a system that completely blocked the intake tract from the small compressor once XX boost is reached.

Sequential is tough, even the auto makers ended up shying away from it. It can be done though, so let us know if you decide to tackle it!

 

The really crappy thing about it is that you would really want to even out the back pressures on both banks so you would need to connect the sides and then put them in to the two turbos. Good luck figureing out that plumbing nightmare.

You might be better off seeing if you can make some kind of a variable a/r turbine. Or something simpler like a butterfly valve that blocks off half of a divided turbine.

 

Was he talking about sequential ( one feeding the other ) or keeping them parallel, one on either side, of different sizes?

 

Thanks for the comments guys. Right now this is just in the "wonder if it would work" stage

In response to a couple of comments, I would join the exhaust before the turbos, it just doesn't seem right to have unequal back pressure between one bank of cylinders to the the other. Also, the compressed air from the turbos would feed both sides of the engine thru a common TB & plenum.

Wasn't really thinking along the lines of one turbo feeding the other. But both turbos feed the engine, a small turbo to spool up fast to provide fast throttle response, and a large turbo for top end power.

I would use a butterfly valve in the exhaust after the exhaust pipes from both sides of the engines are joined. All of the exhaust would feed the small turbo for quick throttle response, until it reached say, 5 psi in the intake, then the butterfly valve would open so both turbos are fed exhaust.

Seems like I could use something like a blow off valve to make sure the big turbo never pushed air backwards into the small turbo?

 

with the right ball bearing TT it just isn't needed. using a different size turbo on one bank vs the other won't work.

however, a true TT with different sized turbos routes the exh to the small turbo, and then cuts to the big one. huge hassle and just not the way to go IMHO.

one drive in my TT Z06 or my big single Turbo buick and you won't complain about the lag. some is really needed as the torq swells so quick you have NO way to hook it.

go with the ball bearing twins of balanced and correct size for your application and you won't have any lag worries. the modern stuff really is that much better.

 

:lol

I killed a couple of those projects just for spite. Sold the GT350 and the 455SD Poncho...Now I've just got the three Vettes, the Jeep, and the El Camino to deal with... Oh, and about ten Dana 44 builds...
-Jeb

 

sure would have scored that SD TA I still have a soft spot for those cars and owned a bunch of 70-72s and one 74 SD. they are nice rides!!

I also like the Turbo 89 TA. one really nice one is for sale right now that's calling me.

 

I see a few errors here.. one major concern would be using different sized turbos.. if you do use different sized turbos, the smaller would need to be shut off from exhaust energy as soon as it reached its max impeller speed or you would end up destroying the little turbos from over spinning them. so you would need to have valves not only on the exhaust side but also on the intake side.

this system is really kind of just out of place on a V8 engine when proper turbo sizing and exhaust housing size can be used for almost non existant lag and still breath plenty good for top end.

if you really want to increase spool, slap a 50 shot on the setup and it will bring the turbo online instantly as soon as it activates. think once you start trying to build this thing you will realize just how quickly you run out of realestate under the hood. but on a larger engine bay it might be feasible but again....why??? if just to do it and say ya did I understand that completely but thats ALOT of man hours and money to prove it could be done and realize it didn't need to be done. just my take on it... but I would be readily available for input if you decide to give it a whirl.

Chris

 

Thanks Chris. You are correct, it is getting way too complicated.

I may look into the 50 shot of NOS. Good thing this board has a search function.

 

i have been giving this some thought for a while, chris and I talked about it.
It will require the turbo's to be in parallel, one smaller than the other. The larger of the two (or now that I think about it, it really can be the same size) has to be shut off on the LOW side of the turbine. A nice electric slide cutout will do. This will drastically reduce the metalurgical requirements of your system, as the control valve will be after the temp/pressure drop.
the other part of the system will be a 1 way valve in the compressor inlet to prevent backflow when on the small turbo.

It works like this
floor it- small (relative to engine size) turbo spools up wicked fast (prolly at part throttle)
at a predetermined level (say 12psi + window switch of 3500 rpm)
apply 12 v to the open position on the exhasut cutout.
As turbine flow in the second turbo increases the wastegate will shut itself down to lend flow until both are running paralell in pressure ratio. The one way valve in the compressor inlet will operate without outside interference and being on the inlet side will prevent the turbo from going into surge.
You MUST be sure it has a positive seal in the compressor to prevent it from sucking oil into the intake when it's building pressure.
I ran the math- it should work fine. the only concern would be the duty cycle/speed of the exhaust cutout.