compound or twins question

[truckwrench1]

sorry maybe a stupid question but have been surfing thru old posts and am finding that some people are running the smaller turbo on top of the larger. why?. would you not want the smaller turbo to help spool the larger one or am i just missing somthing. you would still see boost of the smaller turbo and then the big one would lite. what am i missing. ie he35 under a ht3b. the 35 would help build that LARGE turbo would it not?

[BC847]

sorry maybe a stupid question but have been surfing thru old posts and am finding that some people are running the smaller turbo on top of the larger. why?. would you not want the smaller turbo to help spool the larger one or am i just missing somthing. you would still see boost of the smaller turbo and then the big one would lite. what am i missing. ie he35 under a ht3b. the 35 would help build that LARGE turbo would it not?

SPEAKING IN GENERAL TERMS . ..

It helps to look at the sequential (proper terminology) turbochargers from two sides. The exhaust (hot) side, and the charge-air (cold) side. Further, the larger turbo is called the "Primary" or sometimes "atmospheric" turbo. The smaller turbo is called the "secondary".

The HOT side:
The hot, dense, high-pressure exhaust exits the exhaust manifold and enters the exhaust housing of the turbine of the secondary turbo. There, it expands and converts some of the energy of heat, into mechanical energy at the shaft (to be used by the compressor of the turbocharger).
The still quite hot, high-pressure gases then go to the primary turbocharger's exhaust housing where again, it expands and converts still more energy of heat to mechanical energy at the shaft (again, to be used by the compressor of the turbocharger).

- The hot expanding exhaust gases are the source of energy that power the compressors of the turbochargers no mater if it's one turbocharger, sequential, etc. The drop in EGTs that are commonly seen across a turbo illustrates this. 1200*F pre-turbo, 900*F post-turbo. That 300*F difference is the thermal energy converted to mechanical energy at the shaft.
- The use of turbochargers in general substantially increase the overall thermal efficiency of any engine as the energy of heat in the exhaust gases are otherwise wasted out the tail-pipe.


The COLD side:
The Primary turbocharger's compressor takes atmospheric air and compresses it to, let's say, 25psig. From there, the charge-air is fed to the Secondary, where it is still further compressed to 50psig.
The charge-air of the primary is of a larger volume (lower density) as compared to that same charge-air coming out of the Secondary (higher density), hence the Secondary having a smaller footprint. They both move the same original volume of air, it's just the increased density afterward results in a smaller apparent volume. (if that makes sense).



NOTE: Social terminology has confusing effects if not better understood. We DON'T run "Twins " on our trucks.
- Parallel turbos (true twins if you must) work side-by-side effectively splitting the work.
- Sequential turbos (what we use) have a larger turbo stuff a smaller turbo.
- Compound turbos work similarly to sequential turbos, but share a common shaft.

Hope this helps more than confuses.

[truckwrench1]

yes that does make sense but wouldnt the second in your comparison ( smaller ) if gated to say 20-25 lbs then plumbed in to the largeer capable of 60-70 lbs with already the 20-25 lbs lite the larger and allow it to spool please dont take this the wrong way trying to get it straight in my head and also if the large turbo is stuffing the smaller turbo what about overspeeding the smaller turbo if the larger is pushing say 50 lbs and the smaller effiency is only to say 40 lbs would that not get hot extreamly fast

[BC847]

Maybe this will help . . . . .

[Tacoclaw]

I believe I read somewhere that it's not that a turbo can't compress more air, it's that it can only draw in so much air. Our HX35s and S300s only have a 4 inch inlet, and even less at the actual wheel, so at atmospheric pressure only so much is able to get in to fill the void left by the turbo compressing air. The point of the larger charger is to force feed the air into the smaller charger, therefor removing the limiter holding it's airflow abilities back.

The large charger doesn't have to be run at much pressure, just enough to keep the smaller one fed, and once the smaller one has compressed air coming into, it doesn't have any feed problems and therefore doesn't have to work nearly as hard to gather the air.

This may be completely incorrect, or maybe even not what you were wanting to know, but once I read it in those terms I was able to figure out exactly how they worked any why such a large charger was used. It's no use having a smaller/equal charger feeding it, it'll just have the same feed problems as a single.

[JQmile]

Both smaller and larger turbos can be properly sized for up to 60psi each. The temperature in a compound setup is extremely high no matter which turbo you spin harder....

[1990]

Good info here

[truckwrench1]

ok thanks guys little clearer now i need to forget about the effiency at atmospheric pressure and look at effiency at pressure levels the large charger is producing

[ahale2772]

So its safe to say that the atmospheric turbo takes load off of the secondary?....essentially allowing our trucks to run a larger secondary exh housing? Is it ideal to get both primary and secondary similar in drive pressure and heat?

Also, has anyone ever observed pressue in the primary to secondary coldpipe? Is it low pressure such as single digits? (I know its high volume), just wondering what kind of pressure is expected from the big guy

[Tacoclaw]

While I'm no sequential expert, I believe your secondary charger is used to light off the primary, then in an ideal setup the secondary would then gate off and let the larger turbo actually move the air. It's still going to have a compounding effect, but I believe it's workload is reduced substantially.

You wouldn't want a big turbine housing on your secondary, because then you would basically just be spooling 2 large turbos.

[ahale2772]

yeah thats the confusing part seems like some guys consider the point of compounds to spool the secondary fast then get the big turbo to light...while others consider the primary the aid to the seconday, in order to make it more efficient.

not saying either is wrong, just who is right?

[truckwrench1]

i dont know but looks like kta, and a few others have it figured out, guess i will just have to follow there lead

[Tacoclaw]

Still talking COMPLETELY out of my ass here, but I see the small/large combo working for trucks that are driven on the road and used to pull trailers. You need the ability to spool down low on a truck that is used like this. Perhaps also for trucks that are tuned to put down big torque numbers.

The big/bigger combo I see being used on Dyno trucks tuned for horsepower numbers and pulling trucks that have 3 minutes a pull to sit and get them spooled. The big/bigger combo would unquestionably put out more, cooler air, but I would imagine it would be a pretty useless setup on a street truck.

[SNOOT]

Till you goto twins and try it for your self you really dont know what your missing.... I drove around with that dam 62MM S300 for almost three years and thought how could it get any better... Add a turbo and bam your temps go down not up during WOT runs and bam your boost almost doubles.... and BAM your poop pulls twice as hard and BAM you break more parts..

[swank]

Also, has anyone ever observed pressue in the primary to secondary coldpipe? Is it low pressure such as single digits? (I know its high volume), just wondering what kind of pressure is expected from the big guy

in my setup, i would see anywhere from 0-35 psi in the cold pipe as manifold pressure went from 0~60 psi. the cold pipe would be right around 0 until manifold got around 15, then start coming on. if i was standing on it, the primary would light a little earlier though.

also, i had a two probe intake air sensor/gauge. one right before and one right after the intercooler. the gauge read up to 400*F before the ic, and it would be pegged by about 40 psi. after, it was always no more than 20*F above ambient temps (no matter if it was -10 or 95).