A new thought for twins - Compound Turbos? Maybe?

[TitaniumTT]

Originally Posted by antman0408

Boost Logic has been working on a kit for the supras, you should search supraforums and see what they have done or give them a call. They probably have the most knowledge of a compound kit on a gasoline engine.

Thanks Anthony, I'll do a little searching around.

Originally Posted by RotaryProphet

The big turbo sucking through a small turbo is the same as a big turbo sucking through an inch and a half inlet; the big turbo is going to pull vacuum on it as soon as it starts to spool, and then the smaller turbo is just a restriction.

What you'd want is a sequential system with a reasonably large turbo for the top end, and a small turbo to create boost down low; that's what I'm working on with my sequential controller. It uses electronic controlled valves to control the secondary turbo and the wastegate, and allows a small turbo to boost to say, 14 PSI almost immediately, and allows the big turbo to make as much boost as you care to let it when it spools. And since the wastegate is electronic, it doesn't even start to crack until the second turbo is at full boost.

Would you mind sharing your idea for this? This is something else that I'm considering. Without doing any math, the sizes that I'm looking at are GT25/28 for the small, and a GT3071-R for the large. My thoughts for this type of system are

Exhaust - an equal length mani, each with a wastegate and an internally gated turbo just to give the turbo the chance to vent out as much exhaust energy as possible. The two individual gates will plumb back into the exhaust AFTER the second larger turbo. The larger turbo is also internally gated.

Intake - First, I'm undecided if the intercooler will be a dual inlet or not. For simplicity and to give the first turbo the least amount of space to pressurize, I believe the turbo's will be merged as close to the first turbo as possible and an electonic cut-out will be utilized. So, the first turbo is plumbed strait to a merge collector. The second turbo is plumbed to the merge collector as well but inline is an electric cut-out to keep the first turbo from back-spinning the second. There also needs to be a vent to allow the turbo to spool without creating a TREMENDOUS amount of exhaust backpressure.

Control.... well I'll be using a Motec M800 so the control's are virtually limitless. I'm thinking there needs to be a 3d map setup such that when MAP = the charge pipe for the seconday turbo, the valve opens and the vent closes. Easily accomplishable. Two different WG controls but the closed loop being on the larger turbos internal gate and the WG on the mani's being run off duty cylce at a preset limit to control "absolute boost"

Thoughts?

[RotaryProphet]

Originally Posted by TitaniumTT

Would you mind sharing your idea for this? This is something else that I'm considering. Without doing any math, the sizes that I'm looking at are GT25/28 for the small, and a GT3071-R for the large.

Certainly. My initial design is something like this (and keep in mind, this is a budget buildup to even see if it's plausible, so the parts combination is all whatever I could find in a junkyard; I'm currently using a T3 turbo off of some 4 cylinder, and a Masterpower 70mm T4.

The exhaust manifold is based on a standard, short, fat runner design that merges into a T4 divided flange, in about the same position as a megan racing 13b manifold. About halfway between the engine and the T4 flange, we run a second, somewhat smaller pipe off of each runner, and route it to a T3 flange, in a position to make mounting as easy as is possible in a setup like this. At roughly the same position that the T3 splits off from the main runners, we split on the other side, route the pipes out toward the exhaust, and mount the electronic wastegate. I'll get back to that.

So basically, each runner of the exhaust goes three places; a wastegate, a small T3 turbo, and to a T4 turbo, which you'll note has exhaust priority. In my setup, the wastegate pipes come off of the top of the runners, between the exhaust and the intake, and run down towards the downpipes. The T3 runners go from underneath, and are routed up towards the front of the car, to mount the turbo somewhat in front of the T4, and higher up. It's also worth noting that neither turbocharger is internally wastegated.


The only thing of note in the intake system is a pair of valves on the turbocharger outlets. Basically, you use a reed valve on each one to prevent back flowing through either turbo. This could happen through the T4 when the T3 is doing it's initial spool up, or from the T3 if you choose to run more boost from the T4 than the T3 is capable of pushing.

As for the control, I'm an embedded systems programmer by trade, so I chose to build my own microchip driven control box. In it's current form, it's dead simple, with just two stepper motor outputs to control the two exhaust valves (we'll get back to those), and two MAP sensors, one connected to the outlet of each turbocharger. The capacity is there to input RPM, throttle position, gear, or whatever else, and to map the boost just like a full fledged boost controller.

In the down pipe for the T4 turbocharger, there's an electronic valve. It's basically a throttle body that's driven by a stepper motor, and designed to deal with the kind of heat that we're going to see in a rotary exhaust. The part I'm working with now was designed to route jet engine exhaust, actually. This valve, when closed, prevents any exhaust from flowing through the T4's exhaust turbine by blocking the outlet. This is preferable to blocking the inlet, as the temperatures are not nearly so extreme as in the manifold.

There's another valve located where the wastegate would otherwise be, and functioning as it.

To help you visualize how all this comes together, here's a play by play.

You floor the engine at low RPM. The control box reads vacuum on both turbo outlets, and closes both the turbocharger select valve, and the wastegate valve fully. This routes all exhaust through the smaller turbo. Once the primary turbo shows that it's reached it's pre-set full boost limit, the system begins opening the turbocharger select valve. The system will gradually open this, working much like a wastegate, releasing any exhaust pressure not required to run the T3 at capacity through the T4 turbocharger, which gradually brings it up to speed.

Once the MAP sensor for the T4 charger reads that -it- is at full boost (some time before the turbo select valve is fully open, generally), the system begins opening the wastegate valve in the same way, metering it exactly to keep the boost solidly where you want it. It also fully opens the turbocharger select valve, if it wasn't fully open yet.

Anyone familiar with how a wastegate works knows that as a control device, they do a terrible job, as they waste energy by opening before the turbocharger reaches full boost; by using an electronic valve for a wastegate (which has been done before, but not very often on road cars, usually due to cost), we keep the wastegate closed until it's absolutely necessary to open it to prevent over boosting. By doing this, we pick up significant torque during the engine spooling phase.

With two properly sized turbochargers, there's no reason you couldn't have the rock solid straight-line torque numbers you're looking for with a system like this. A small T3 can spool before 2000 RPM on a 13b, but I would investigate a slightly larger one that spools by, say 2500 RPM if I were planning to run high boost numbers with a relatively large secondary turbo. This would help keep the transition between the small and large turbos smooth and unnoticeable from the driver's perspective.

In a way, the smaller turbo selection is more important, because the goal of the entire system is to run the small turbo into it's peak efficiency range, and then keep it there by gradually moving work to the larger turbo. This will help the low end torque numbers by keeping heat out of the incoming air charge. Then you just need to select a secondary turbo that, when coupled with the primary, will leave both of them in their peak efficiency at about 75-85% maximum revs.

I know I've already got a book here, but it's worth looking at pros and cons versus a single turbo setup.

As far as pros go, you've got the very quick spool up of a small single turbo, the high end efficiency and power of a large single turbo, and a very smooth power band and seemless transition. Operationally, the sequential setup takes it every single time.

The list of cons is entirely logistical. It requires two turbochargers, a custom manifold (although, I'm hoping I can start producing these once I get it up on the engine dyno and start tuning, and can put out some numbers of my kit vs a single turbo vs stock twins.), some not inexpensive valve hardware, an electronic control box, and let's face it: a f*cking plumbing nightmare under the hood.