since its been slow around here

[UncleBob]

here's a little concept playing for you guys.

As everyone knows, "sequential turbo" designs are an attempt to find the best of both worlds, that being fast early spool, and also a large turbo that has a high peak capability which maximizes HP potential without choking the engine.

The problem, of course, is the complexity required for sequential turbo'ing. Some have come up with unique ways of getting around this problem, using a single turbo. Such as variable housing turbos. Aerocharger, Garrett, and some diesel custom turbo's have done some attempts at this, some are way overly complex and fragile (aerocharger) and others just didn't hit it off too well.

But! I found a somewhat simple, cheap way of recreating the same effect. All it requires is a tangient exhaust housing turbo, and an extra external waste gate. How you ask?

Well, a tangient housing turbo, is an exhaust housing that is split into two seperate runners. They are theorized to be more efficient (ever so slightly)....but, what happens when you block off one of the inlets? It might be a surprise to some, but you halve the AR....this makes it spool roughly twice as fast as it would of other wise.

So its quite simple, run your manifold all into one side of the housing, then on the other side, run it to an external waste gate and from there, run the external waste gate to the other side of the tangient housing.

You have the added bonus of tuning this also. You can set when the "secondary" opens up, how hard it hits, etc. It creates a pretty adjustable situation.

"sounds good on paper" you say....well, I did just that on one of my turbo'd motorcycles. I had a hard time finding a small enough tangient turbo, because they are usually reserved for much larger turbo's. I found one though....off the turbo'd MR2 is a CT26, and fit the bill nicely. I had the compressor replaced with a T3-super 60 trim and slapped her on there.

Unfortunately, my math on the expected boost/RPM's wasn't quite right, and I'm a little disappointed in the end product, but the idea definitely works though. With the external waste gate spring removed, so that both inlets are fully opened, I hit 10psi boost at 10.5K RPM's. With the spring in, and no boost reference, I hit 10psi at 6.5K RPM's.

I was aiming more for 4K....but thats life unfortunately.

But a couple pics, although its hard to see the waste gate work, because I had very little room to work with, so its somewhat hidden.


With the radiator in place:





and with the radiator out of the way.....

[534BC]

I think I can see it working good, You must have a very large waste-gate to be able to run half the engine thru it. Let me rephrase it to make sure I get it. I've done a number of custom turbo jobs over 10 years ago and always had to just "live" with the sacrifice of exhaust housing sizing. Most of my set-ups were truck/tractors so was no real problem. On my street vehicles I had simply sized too small and then lived with an exhaust gate dumping pressure to control over-boost.

You have in a sense installed an exhaust housing that it sized way too small forcing the whole engine thru it yet kept a normal size compressor (very good) . when boost approaches the setting of 10 pounds it starts in effect increasing the size of the housing until a level of 12 or 15 or whatever the max open pressure is until the exhaust housing is in effect too large now (good for max power)

I have to thionk some more about this, only thing I see is if the gate is big enough it is not exactly a nice smooth trip to the turbine, but who cares ?

I like it so far and curius when you get it all worked. Its a good concept, It is simple I can't think why no-one had this.

[534BC]

How's the housing split? Does one side use the outside and one use the inside? My turbos with two chambers I think used about a half a circle each.

[UncleBob]

Originally Posted by 534BC

I think I can see it working good, You must have a very large waste-gate to be able to run half the engine thru it. Let me rephrase it to make sure I get it. I've done a number of custom turbo jobs over 10 years ago and always had to just "live" with the sacrifice of exhaust housing sizing. Most of my set-ups were truck/tractors so was no real problem. On my street vehicles I had simply sized too small and then lived with an exhaust gate dumping pressure to control over-boost.

You have in a sense installed an exhaust housing that it sized way too small forcing the whole engine thru it yet kept a normal size compressor (very good) . when boost approaches the setting of 10 pounds it starts in effect increasing the size of the housing until a level of 12 or 15 or whatever the max open pressure is until the exhaust housing is in effect too large now (good for max power)

I have to thionk some more about this, only thing I see is if the gate is big enough it is not exactly a nice smooth trip to the turbine, but who cares ?

I like it so far and curius when you get it all worked. Its a good concept, It is simple I can't think why no-one had this.

you discribed it exactly correct. I'm currently using a 40mm external waste gate, which is actually (IMO) plenty for this project.

If you think about it, the bends and turns aren't super important in this situation, because one side of the turbo is very streamlined, the other side of the turbo is main relieving excess pressure. Although it would be optimal to have both flow perfectly, you will still get extremely good flow through, at the very least, half the turbo, so you add the two together and its not too bad.

Thats basically what this design is, the external waste gate acts as a pressure regulator for the exhaust manifold, and turns that extra pressure into HP while its at it.

I've had this system working for some 3000 miles, but like I said, I'm disappointed with the boost point, so I will be going back to a "regular" turbo that is "too small"....If there was a better selection of tangient housings of this size, I would try finding a better suited turbo to boost where I wanted it to, but its hard when you're looking for a turbo for a 1L high performance motor

The other issue, which is mostly bike specific, is the weight. Tangient housing turbo's are extremely heavy. The turbo alone weighed more than my entire old system (manifold, exhaust, turbo combined). Adding 25 pounds to a bike is a much bigger deal than on a car.

But....I will keep this turbo around. I KNOW I will find a pet project on a car that it will work perfectly on. If I had a bigger engine, such as a 1.3L bike, I know it would have been perfect also.....but thats just how it goes....

[UncleBob]

Originally Posted by 534BC

How's the housing split? Does one side use the outside and one use the inside? My turbos with two chambers I think used about a half a circle each.

Here's a pic of my CT26 before I had the work done (thats why its all nasty)



the smaller turbo is my old turbo, a T25. The two openings snail around all the way to the turbine and stay seperate.

[534BC]

I really like the concept, it is very well thought and simple. It really could be incorporated with a valve right in the wall between the two exh passages where a type of valve was controlled by intake boost. It would have to open the two passages and close another at the same time. Too bad I don't play turbos any more, I really enjoyed it.