sequential twin turbo with a 2.5 block?

ok, so I've been doing alot of daydreaming when I am driving and I thought I would share my visions... :rofl:

So, if i took a stock WRX, and pulled out the 2.0 block and replaced it with a 2.5 it would increase the amount of air the engine pumps, right? well, what if I took the stock turbo and left it right where it was. The little turbo would probably spool up at around 2000 rpm's with the added volume... the problem would be that it would tire out by 4000 rpm or so

Now, what if I took a larger turbo, maybe a 20g or larger, and mounted it directly after the stock turbo.

So, here is where it gets interesting. The stock turbo is set to 10+lbs boost. it would reach 10+ lbs by around 2300-2500 rpm, but it would start to lose peak boost probably around 1500-2000 rpm later.

now the catch would be to set the larger turbo to 15+ psi.

the smaller turbo would spool off of the larger block, but the bigger turbo would spool using the exhaust energy created by the smaller turbo. the larger turbo would spool up about 1500 rpms after the smaller turbo and hold bost until redline.

now, the trick would be to fine tune it so when the larger turbo goes over 10+psi, the wastegate for the smaller turbo would open (cause its set to 10psi), releasing the exhaust pressure into the larger turbo. This would cause the larger turbo to continue to spool up until its max psi.


I'm not going to talk about suporting mods like fuel, ECU, and other bolt-on. I' more interested in the theory.

now, I know that all of this is a bunch of talk, but what I want to know is if this is even feasible. If its not feasible, what am I missing? it seems like a bunch of welding and tuning to me.

 

The first turbo would need a large external waste gate to really work. Both pipes from 1st turbo go to 2nd turbo. Plumbing would be difficult, heavy & hot. But the theory has already been proven in production cars. Most of those cars ended up going a big single aftermarker turbo's for big HP though.

 

If you're plumbing a small turbo into a big turbo, what keeps the smaller one from acting as a serious bottle neck?

Consider how the RX7 (and Supra?) sequential turbo systems work: you have two turbos, and one is closed off (both the exhaust and the air side) through use of valves. In other words, turbo A gets to play by itself. As RPMs and boost builds, the valves will open to allow some exhaust gas to start spooling turbo B and once the pressure is enough start opening the valve allowing air to flow into the intake. So you go from having one turbo running to having two... you'll see a dip in power when it starts bringing turbo B online, but that's the price you pay. You basically end up running the turbos in parallel at higher RPMs.

These systems are fairly complex, and I'm not sure you'd want to futz with it unless you have a LOT of knowledge and fabrication skills... not to mention the tuning.

 

The 1st one is supposed to have a BIG external wastegate. That's why I said it had to be external. The pressure drop won't be as great with a 2nd turbo behind it, but it's easier than computer controlled valves...

 

Why not just swap in the TT setup from the Legacy Blitzen EJ20TTs?

Oh yeah because it's really complex, expensive, and not really worth it IMO.

Twin turbos are nice but they are overly complicated. The only case that I see them working well is in v6/v8 engine configurations. Our 4 cylinder motors are pretty small to try to spool two turbos. I like where you're going with the 2.5L block but the added .5L as well as the AVCS if you wire it up do wonders to reduce lag.

 

Its fine to speculate here

Couldnt comment much on if that would work. But for fun.

Here is how the kit works on the ej20tt.

Twin turbo operation

The two-stage operation of the twin-turbochargers means power is fed progressively, with only the primary turbo operating at low revs.

The secondary turbocharger operates at the mid-rev range and above.

Both turbochargers have high gas pressure when operating in unison.

Primary stage

The primary turbocharger, located at the rear passenger side of the engine, provides manifold boost pressure primarily for low to medium engine speeds, although it is active across the entire operating range.

The Engine Control Module (ECM) regulates boost pressure generated by the primary turbocharger by regulating the opening of the wastegate.

The secondary turbocharger does not operate in the primary stage, as the exhaust control valve is closed.

The primary turbocharger supplies pressurized air to the intake manifold via the intercooler, which reduces the air temperature from 120-130 deg C down to 70-80 deg C under normal operating conditions.

The cooling of the intake air increases air density and helps reduce combustion chamber temperature.

Intermediate stage

At medium engine speeds, around 4,000-4,500 revs, depending upon engine load, the exhaust control valve is partially opened. This causes the secondary turbocharger to begin rotating.

The ECM regulates the amount that this valve is opened and controls vacuum applied to the diaphragm of the control valve.

Any pressure generated by the secondary turbocharger during this preliminary phase is redirected to its inlet side.

The ECM uses a differential pressure sensor to measure the difference in boost pressure generated by the primary turbocharger and the secondary turbocharger.

Secondary stage

When the ECM determines that the engine operating conditions are suitable, the relief valve is closed and the exhaust gas control valve is fully opened.

By measuring the pressures via the differential pressure sensor, the ECM determines the correct opening of the intake control valve. When this valve is opened, pressure from the secondary turbocharger is supplied to the intercooler.

Boost pressure control during this phase is regulated via the primary turbocharger's wastegate


I believe the method you are visioning would create some serious backpressure issues.

You could drop the tt in a car, someone around here is working on it, if they feel like commenting. Sure you could also upgrade the turbos, since the ecu controls the exhaust diversion valve, there is most likely a way to work with it.

/paste lol

 

The Impreza L I bought is getting a twin setup. Check my website next week, I should have some info about it up.

Matt