Could anyone give me some education on twin turbo setups?

Is the secondary turbo normally smaller and main purpose is to spool up the primary faster?

How do you calculate boost #s w/ twin turbo setups?

Only reason I am asking is I have a buddy with acouple Murkers w/ Garret T3 turbos and he offerd to give me one just to mess around with and I just figure'd it'd be neat to experiment w/.
Nothing out of control, just somthing fun.

So could you give me some education on these setups, how they work, how they're setup, ect?

 

alot of info on this subject just search twins more than likely you'll find what you're looking for

 

Basically you have a primary and secondary turbo. They are not twin turbos by definition but actually sequential turbos. The primary is the larger of the two and is typically a s400-ht3b-b2 sized turbo. The secondary is the smaller and typically a hx-35, b1, s300 or similar.

The primary compresses the air to 15-30 psi (for an average sized set of twins) then the air is compressed further by the secondary to 45-65 psi.

I think a T3 would be too small for twins on one of these trucks, smaller than a hx35 IIRC. You would not want to use the 35 as a primary turbo as it is too small. There are other ways of using the T3 but you probably don't have the time or money to waste on it, and it probably wouldn't be worth it anyways.

 

I like to think of twins this way: you are "turbocharging" another turbo.

Just as the small turbo is capturing exhaust energy to power a compressor that force feeds the engine, the big turbo is capturing exhaust energy and powering a compressor that force feeds the small turbo.

The benefits are similar to multistage air compression: more efficiency and less heating. In other words, it's more efficiency to have two separate compressors operating at 2:1 pressure ratios than it is to have one single compressor operating at 4:1 pressure ratio.

Also, the pressures multiply, not add. So if you have each of your two turbos running at 3:1, then the overall compression is 9:1, not 6:1. This is why twin turbo setups can make such high boost pressures.

Putting numbers to this, I can take a stock turbo that's efficiently making 28psi of boost from just atmospheric pressure (regular single turbo).

But if I add a "compound" turbo to it that that can force feed this turbo with 28psi of boost, then I end up with 112psi of boost total-- at least in theory. Moreover, the small turbo is now operating at the same efficiency making 112psi of boost as it was before making just 28psi of boost. In reality, the numbers don't look that good because there are lots of thermal losses and a host of variable to account for, but the basic premise is valid.

The small turbo doesn't really spool the big turbo-- the big turbo "amplifies" the small one. Let's assume for simplcity that the small turbo is operating at a 3:1 pressure ratio, or roughly 28psi of boost. The small turbo "sees" a 5.9 L engine. But in order to provide 3 times more air to the engine, it ingests about 3 times the displacement, or about 18L of air.

As a result, the big turbo "sees" an engine of about 18L displacement, or right around the size of a huge Class 8 semi engine.

Turbos only see masses of air, and what temperature they are. They don't care what engine they are bolted to, or if they are bolted to another turbo. A turbo can't see the difference between a 4L engine spinning 2000 rpm and a 2L engine spinning 4000 rpm.

So using a semi-tractor turbo on a CTD works because the big turbo "thinks" that it's still attached to a semi engine, based on the massflow of exhaust.

Theoretically, you could add ANOTHER compound turbo (triples!) to your rig, and that turbo would "see" a 54L engine. Obviously, you run into practical limitations and the law of diminishing returns kicks in very quickly.

jh

 

Very nice reply

 

Ditto, I'm somewhat of a diesel n00b, but I've been tinkering with gasser turbo cars for a number of years and I have to agree that a Garret T3 (or even a T4 for that matter) has no place on our trucks. They are just too small... unless you want to run a triple or quadruple turbo so that you have absolutely no smoke what so ever. Give it a try and let me know how it works.

All joking aside, I wouldn't use anything less than the HX-35 for my small turbo.

 

In reality, your total boost will end up being about 2x what you are seeing in the cold pipe(primary boost). Most setups will run between 30 and 35psi off the primary 60-70psi overall. I try to use as large of a top charger as I can stand so that I can move the air from the primary without having to to go over the 2x figure.