Compound Questions?
 

Can someone explain exactly how the compound turbo set up works?

1. What is the job of the top/upper/smaller turbo?
- From what i understand it is used to for low end power and to get the motor going and spool up the big one.
2. What is the job of the lower/under/bigger turbo?
- From what i understand it is the hp maker on the upper end of the rpm. IT goes into effect when it overcomes the amount of air the smaller turbo is flowing. From that point on it is what is making the boost one sees on the guage.

Is this even close?

Please be detailed in the answer

Thanks, Adam

The top turbo will give you the low end response, as the bottom one is spooling. In a proper setup suited for the application, the bottom one will be spooling and ready in lower rpm as a larger than stock single. In full swing the bottom is making about 25 psi, then the smaller top turbo is taking that 25 psi and doubling it or more. Thats what gives you the gauge psi.

I think you basically have the idea. The reason for the two turbos is not so much for low end then high end. The reason is that a single compressoe can't effectively make high pressure ratios (over 4:1). So if you need high multiples of air pressure, it's more efficient to split the work up into two stages. You can run two individual turbos at lower pressure ratios and gain both efficiency and operating range-- it's both bigger and smaller than a single turbo would be.

One way to think of twins is that your are "turbocharging a turbo". You are using the bigger turbo to turbocharge the smaller one.

The sizing of the respective compressors and turbine housings is a very important element of a good compound turbo setup. Like single turbos, you have setups optimized for response and lower power (ideal for towing), big dyno power on fuel only, nitrous setups, and those that try to be jack-of-all-trades.

While a good compound setup will have earlier spoolup and more power than a somewhat comparable single, they still have a limited operating range and must be tailored to the application.

JMO

Most street trucks use the top turbo (secondary) for most driving situations. Its like normal driving with a little bit of extra air to help fuel the motor (lowers egt's). Once you build enough exhaust mass flow, you open the wastegate and bypassing the secondary turbo, some. This will lite the primary harder, pushing in more air flow (CFM and boost). It will act like your running the bigger turbo and will increase the size of your turbo compressor map and will help improve the overall compressor efficiency through the map. Like Hohn said, find a turbo that has a Pr ratio of 4:1 (70-80psi of boost) with 110 lbs/min of flow. You can but it will be a dog on spool-up and probably need 2000 rpms before she will start to come on very hard. Try driving that beast in traffic. The smaller secondary fixes the spool-up and will make the truck more drivable. Its kinda like eating cake with lots of frosty icing on top.

So if there iis postive pressure on the smaller turbo it then takes that pressure and doubles it?
- Does this occur at all rpm and boost levels or just when the smaller one is at its most effective point (As it would be in a effciecy island on a compressor map)

It depends on where the turbo is operating on the compressor map. You could 3:1 ratio or more (depends on turbo compressor speed). It could be less than 2:1 ratio it depends. Most of the time the primary just add 1 to 5psi until the wastegate on the secondary opens. There almost no energy left in the exhaust after the secondary to push a big turbo to push boost. Once the secondary wastegate opens, that where the twins comes to life.

im not too sure that the 25 psi and 25 psi to make 50 psi is completely accurate. one way to test this, which i have never done is to measure the boost between the primary and secondary turbo as well as the final boost number to see what the secondary is actually adding to the equation. i am more inclined to believe once the wastegate is open on the secondary turbo, the primary is making almost at the initial opening and close to all the boost. the secondary is still screaming because it has to move the volume of air that the engine is consuming but it is already compressed air from the primary. there is not need to compress already compressed air right? does this make sense to anyone else? or am i just blowing smoke out of my "ahem".

i meant to say the primary is making most of the boost at the initial opening and almost all the boost at higher boost levels.

Ok so the wastegate system on the secondary (smaller turbo) is routed to the big turbo right? So the waste gate goes in front of the secondary on the exhaust side (external wastegate right)- that exahust pressure is used to spool up the primary charger becasue it makes it as if the primary is directly mounted to the exhaust manifold right? So since the primary has been spooling up and providing some boost it is close to spool up- then the secondary wastegate opens and lets the exhaust go from the manifold to the primary causing the to light and produce boost.

Is this the logic of them some what

Thanks Adam

yes, you seem to be fairly correct in your assumptions. even if you dont have an external wastegate and you only utilize the wastegate on the housing (ex. stock wastegate on a hx35) you are still allowing the exhaust gases to bypass the secondary turbo in order to have more energy to move the primary turbo. it also prevents you from spinning the secondary turbo too fast and having a big kaboom problem on your hands. one thing i reccommend, that i will be doing when i build my twins with my stock hx35 is boring the wastegate port to a larger diameter in order to move more exhaust gasses through it at times of high rpm and high boost. a 14cm housing may help lower egts as well but they are another $450. i am going to build mine with the stock 12cm housing and upgrade to the 14 when time and money allow me to.

Ok so you do not neccesarily have to route the exhaust gas from the wastegate to the second turbo via extra piping right? This would mean it is just letting exhaust gas out into the atmosphere right.

I guess my question is how the wastegate actually acts to let the exhaust flow past the turbine of the secondary charger?

Thanks for the help
Adam

Compressor wheel curves are based on rpm speed line. The faster the turbo spins the more pressure ratio it builds. The faster the turbo spins (Rpm), more flow and pressure it builds (increase in pressure ratio)

This^^ is all part of the delicate balancing act of sizing the two turbos. If the top turbo is too small, it blocks the exhaust flow a bunch and you don't get good gasflow to the big turbo without bypassing a lot with a big wastegate.

But if you go too big on the top turbo, you lose the earlier reponse that is a good reason to go with twins to begin with.

There's still a good bit of energy left in the exhaust to drive the turbo, but it's just not enough to get such a large turbo going. The small turbine is not that efficient, converting probably less than 1/3rd of the exhaust energy to work. The rest goes out the pipe. A second, large turbo has a chance to recapture some of the wasted energy. Unfortunately, the big turbo takes a LOT more juice to get rolling.

By now you'll see the problem Mike is mentioning. Going from driving a small turbo with 100% of the exhaust drive energy to driving a larger turbo with only 2/3rds of the available exhaust energy makes it hard to get the bigger turbo fully up to speed before the top turbo becomes very restrictive.

That's why wastegate is really a must-have for twins, imo. You need to able to account for trying to stuff the airflow from a large compressor (the primary) through the turbine of a smaller secondary. Without the wastage, a compound setup could easily explode the small turbo by overspeeding it.


If you stagger the pressure ratios on the turbines (and hence compressors), you can get more favorable results. For example, If you have 27lb/min heading to the top turbo at 900*F and 43psia, the flow "expands" at the larger turbo because it's at a lower pressure.

So, while the 27lb/min at the top turbo comes in at a PR of 3:1 and TIP of 900*, it might leave the top turbo at 600* and a PR of 2:1 going into the larger Primary turbine.

This is where the "corrected" gas flow comes in, because the expansion of the gasflow as it dropped in pressure from 3:2 to 2:1 has increased the mass flow from 27lb/min to 34.3lb/min, even at 300* cooler temps.

Put another way, the more pressure, the less flow (think in terms of pressure as resistance to flow, or restriction).

A perfectly efficient turbine would extract all energy from the gasflow available from heat and pressure (enthalpy), meaning that outlet of the turbo would be cold and at atmospheric pressure.

So going a little bigger on the small turbine housing and a little small on the big turbo housing can help to improve efficiency and help the turbos operate more in harmony instead of fighting each other.

JMO

You're right that 25psi + 25psi does not equal 50psi. You need to look at your pressures in absolute terms. All the previous numbers are gauge numbers, so add another 14.7 psi (if we are at sea level) to your numbers to get absolute. So now our numbers look like 39.7psi + 39.7psi still doesn't equal 64.7 psi. Now lets look at pressure ratios. You always compare absolute to absolute numbers. Our gauge pressure of 25 psi is 39.7, compared to our atmospheric pressure of 14.7psi. 39.7/14.7 equals a pressure ratio of 2.7:1. Now if our goal is to get to 50 psig from the secondary charger, then we need to look at our absolute discharge to our absolute suction. Discharge would be 50 psig, 64.7 psia. Suction is at an elevated pressure from our primary, which is 25 psig, or 39.7 psia. 64.7/39.7 is a PR of 1.6:1. If you took the PR's from the primary at 25psig and used that for both, you'd be looking at a combined of 7.3:1, which would work out to 107 psia, or roughly 93 psig.

The charger I'm using, a GT4202, has a max mass airflow of about 95 lb/min. To feed all of that air into a 359 CID engine at 3000 RPM would require a pressure of 82.5psia. The GT42 is capable of a PR of 4.5:1, which works out to 66 psia. At max mass flow, the PR is 3.5:1, or 51psia. We obviously need more pressure to feed all this air into the cylinders. Thats where the secondary turbo comes in. To compress the air further to a usable pressure for use. At the max mass flow on the big charger, at the higher PR's, its not very effecient. If we keep the mass flow up, and lower our PR, we can make the primary work not as hard, and pass more of that work onto the secondary, we can keep them both happy near their effeciency islands. The less RPM we have, the more we need to compress the air to get it all in the cylinders. At 2000 RPM, we would need a pressure of 124psia to get all 95 lb/min in. At 4500 rpm, we only need 55 psia, which would be doable as a single turbo.

Tate, good post. Yes its complex but sometimes you have to put the science aside and just do it. Everytime you learn something new.

You also need to think about is your drive pressure. Every time you increase suction pressure you turbocharger requires more HP to compress the air. There will a point like Hohn stated that you need bigger turbine housing or bigger turbo. With so many choices, there lots of custom tweaks that you can do just to sweetin up how the twins work just abit better