l28ett manifold designs
All images are exactly to 1:1 scale, there is no deviation in any of these parts from their real-life counterparts. I spent months drawing these extremely accurate models, so please point out anything that might be considered even slightly different from factory specifications. Also, I’m colorblind. The color scheme is pretty much just so you can tell the different parts from each other.
Since I got so many questions about my l28ett design, I decided I’d write up the basics of how this will work. Bear in mind, this is different from a standard twin turbo setup, or a sequential turbo setup. I started writing it up, then quickly decided that words just aren’t adequate to describe this and broke out a pencil and some paper. I drew up how it would all work, then realized that was just as confusing as words, so I sat down in front of my drawing tablet and I’m gonna walk you through it now.
Base design concerns
Here’s an l28et block and head. It has 6 cylinders inline with each other (instead of assembled in a v with 3 on each side), and 12 valves (meaning 12 ports). The 6 holes on top of the head are intake ports, the 6 on the bottom are exhaust. I was a little creative with the valve cover, and it’s decidedly not to scale, but I don’t really care. I’m explaining something here, not re-engineering an engine. This engine is a little abnormal because the status quo of engines is known as ‘crossflow’ meaning that air goes in one side and out the other. Crossflow designs make turbochargers a little more complex to route, but that’s a solved problem and we aren’t going to worry about comparing the two. Just understand that the intake and exhaust are on one side of the engine, and the spark plugs are on the other.
Tube steel is the best choice for exhaust routing, since it can withstand heat, is relatively easy to work with (the term is called welding) and doesn’t cost a whole lot. A mild steel manifold will most likely cost around $100-$150 in supplies for a hobbyist like myself, and makes the front end of the car significantly lighter and also allows exhaust to be routed in an efficient way. I’m basically taking the front three exhaust ports and collecting them into one, which feeds one turbocharger, and collecting the rear three ports to feed the other turbocharger. The collectors also have a tube that runs between them, with my wastegate in the middle of that tube. Let’s explore the wastegate idea a little more, it’s the most important part of any turbocharging setup and often gets ignored.
A wastegate is simply a pneumatic valve that opens to let exhaust gases through. It’s engaged by air pressure, which is available as soon as the turbochargers start to spin. When the turbochargers have enough air in the compression turbine side, the wastegate opens allowing exhaust to bypass the turbochargers, and they are no longer powered (exhaust gas is what spins turbochargers). Without a wastegate, turbochargers would spin out of control and some part(s) of your engine would just explode under the air pressure.
When my wastegate opens, exhaust gas from both halves of the exhaust will be allowed to escape, rather than having to push the turbocharger around before going out the back of the engine. This allows me to control the air pressure going into my engine, and the cross-pipe keeps both turbos relatively in sync.
Normally, both turbochargers would pull air from the front of the engine. My engine bay is long enough that I can pull air from the rear with the rear turbo, and from the front with the front turbo, and the exhausts of each can merge efficiently and seamlessly between each other.
This is also where the non-crossflow design comes into play. Instead of having lots of bends and ductwork to get my compressed air to go into the intake manifold, my compressed air points straight up. If only an intake manifold existed that could take advantage of that…
I’ll just have to make an intake manifold. All I need to do is make a plenum, then 6 runners that point toward the ports of the engine, and cut two holes in the bottom of the plenum. 50mm and 60mm throttle bodies are readily available, so I can use two: one for each turbocharger outlet.
This provides instant throttle response. Turbo lag is slightly present, but pressing on the gas pedal will give a jolt like no other turbocharged vehicle is capable of.
Delivering fuel to this is as easy as drilling the correct holes for an injector at each intake port of the engine, then installing a fuel injector there. I have a great set of injectors from osidetiger, and pallnet has excellent fuel rails available, so this would be a simple process. However, the stock Nissan electronic control unit would be completely confused by this setup (don’t tell anyone, but it’s confused by pretty much any setup, including the stock setup from the factory). Megasquirt is a great alternative for controlling fuel electronics and newer versions can control boost levels, so I’d go with that.
There are some notable issues that I can see, just looking at this design. The one that really matters is heat. A turbocharger generates quite a bit of heat, and throws out compressed, heated air. Two turbos do … double that. Normally, a turbocharger is routed into an intercooler (like a radiator, but for cooling air instead of radiator fluid). This design has no room for an intercooler, so it throws a lot of hot air into the engine (that’s bad).
The proximity of exhaust and intake is always an issue, and that’s compounded even more when there are two turbochargers stuffed directly under the intake manifold. There isn’t a whole lot that can be done about this.
The main problem with all this heat is called “detonation“. Detonation occurs when your air and fuel mixture is hot enough that it explodes too early. If this happens a few times, it’s nothing to worry about, but if it happens as a regular part of driving, you won’t be driving very regularly at all. To combat detonation, you can add extra fuel to the intake mixture, which is called ‘enrichening the circuit’. It’s not very efficient, but helps cool down the inside of the engine.
Another option is to run coolant through the intake manifold. This is a significant amount of work, so I’d suggest settling for the next best thing: running coolant through the throttle body. The 240sx has a 60mm throttle body that’s perfect for this application, and has coolant lines installed on it from the factory. Both these techniques will go a long way toward preventing detonation, but let’s take it a step further.
Higher octane fuels are resistant to detonation. That’s their only advantage. They don’t make your car faster or burn cleaner or make the car gods love you any more than they already do. It’s not a bad idea to run higher octane fuels on any high-compression or turbocharged engine. It costs a bit extra, but if you’re reading about making an exhaust manifold and intake manifold for your two turbochargers… you’d be willing to spend a little extra here and there.
Adding water or a form of alcohol (nitrous oxide or methanol are two popular examples) before the fuel is mixed in will enrich the circuit and drastically reduce detonation. If you end up going this route, add another injector or two between the throttle bodies and set up megasquirt so they fire alcohol into the compressed air whenever a ‘knock sensor’ is triggered. You can also add a heat sensor into the intake manifold for this.
One more way to reduce detonation caused by excess heat is to reduce boost. Opening the wastegate will immediately drop pressure and power in the engine, and will stop the increase in temperatures, but won’t do anything to cool down the engine if it’s already in a dangerous state. This is best used as a preventative measure, so reduce boost BEFORE your combustion temperatures are excessively high.
The final, and most important thing you can do to prevent detonation is to minimize ‘heat soak’. The hot air in your exhaust and turbos generates a lot of heat, and that heat wafts up to your intake manifold, making the gases inside even hotter. An aluminum plate with a fire blanket or another type of insulation as a barrier between the intake and exhaust is a must.
I don’t see the twin turbo setup as being worth the work. While cool, I won’t really benefit from this design. I can push as much air into my engine as it can safely burn off with a single t3/t04e turbocharger, and it’s easier to use an intercooler with it as well. I won’t have the instantaneous throttle response the dual-turbo setup would offer, but I think I’ll be perfectly fine driving across the country with just one turbo. My goals are only 300hp or so, and that’s very easily attainable with a good turbocharger and an accurate fuel delivery tune.