Why hasn't anyone applied the sequential theory to a twin setup for these trucks? You know, one small one to spool up fast, and a second big one to keep it moving.

 

I don't know much about turbos, but I have seen sequential turbo's in very high boost applications. Maybe it would work well with an LSX and 50 psi?

Here is a breakdown of turbo design;
http://en.wikipedia.org/wiki/Twin-turbo

 

Probably for the same reason most of the guys with factory sequential setups convert to a single turbo: reliability. But I could think of a lot of other reasons why no one would want to tackle a project like this (cost, plumbing nightmare, valves that have to handle the high temps, electronics or other means to make it work, etc)...

I know the Supras that came from the factory with sequential turbos had problems with the smaller turbo breaking all the time (bypass valves wouldn't close fast enough and allow high pressure air to "revert" back through the smaller compressor's outlet. The resulting "surge" would snap the shaft in the smaller turbo). Every guy I knew that wanted to make power would ditch the sequential setup in place for a big single turbo.

If you're doing two power adders (and trying to eliminate the turbo lag), you might as well just do a super-turbo setup with a supercharger for the low speed boost and clutch out/bypass the supercharger at the high speed. It would be simpler, easier to execute, and most likely work better and be more durable. VW has a very successful car in Europe now that has a "twincharged" 1.4L gasoline engine that gets 40+MPG and still makes something like 170hp... People who I've talked to who have driven this car say the clutching out/bypassing of the supercharger is seamless and it just feels like you're driving a much bigger engine. It's much more responsive than a turbocharged engine and has excellent acceleration from a stop and no lag to deal with when trying to pull out in front of moving traffic.

I've also got a friend that did a "twincharge" setup on a Honda and was pretty successful in SCCA racing (using an Eaton M62 supercharger with a turbo). He would crush all of the turbo cars out of the corners even though peak power of the other turbo cars were supposedly much higher. But then again, last I talked to him he was thinking of switching to a single power-adder (supercharger no less) just to simplify things.

 

Oh, I had no idea that there was so many control elements that have to be in place for that to work properly.

I suppose if one of the "better funded and highly educated" (ahem...cough...sponsors...cough) could design something like that, it'd be pretty cool. That is, if they could make it reliable. But, I guess that's what this forum is for, learning new stuff. Thanks man.

 

Just a question...

In a sequential setup, which turbo is first (closer to the exhaust manifold)? The smaller one, I would think.

If the smaller one was inline before the larger one, once the larger one started to spool, the small compressor would just have to free wheel, in theory.

Any thoughts? (no trying to kick the dead horse, just tring to figure things out)

 

I can't find the link, but I have seen the pictures. Back when I had a diesel Ram there was a guy in Texas that had 3 turbo's on his Ram for a total of 120 lbs of boost.

 

I suppose there are a couple configurations that could be considered "sequential" turbocharging.

The configuration I had in mind was the one Toyota used on the Supra. There are valves in place that direct the flow of gases so that the smaller turbo is spooled at low RPMs and then at a higher RPM the valves are switched so that the small turbo is isolated from the exhaust/intake gases. The larger turbo then takes over. So, in effect only 1 turbo is in use at any given time.

One of the turbo manufacturers (I think it was Garrett) developed an integrated sequential turbo that handles the switching from one turbo to the other within itself and was demonstrated in a Porsche a few years back to get the lag down. If memory serves correctly, they were able to knock the lag time down to around 1 second (maybe less) which isn’t bad, but still no match for a positive displacement supercharger. I saw a picture of the thing and it looked like a human heart, although inevitably simpler to install than plumbing two separate turbos together in a sequential setup.

What 408Sleeper is talking about is really called "compound" boosting in which each compressor empties into another compressor. It's really just a matter of plumbing the oulet of one compressor into the inlet of the next. This is really effective at reaching very high pressures and you see this done on big diesel engines used in tractor pulls. It works since each compressor is compressing the air at the same ratio as the previous one. This also works with both turbochargers and superchargers...

 

I thought they took and put the small turbo on the exhaust, spun it up then dumped the air from the compressor into the "exhaust" of the larger turbo, then that compressor side went to the engine

Engine exhaust-turbo one exhaust in--shaft--compressor one out---turbo two exhaust in--shaft--compressor two out--engine intake

Maybe on some applications?