This question is directed more towards people experienced with twin turbo setups or any kind of super/turbocharged setup. I want to run twin superchargers as opposed to twin turbos. If you decide on how much boost you want to run would each supercharger be able to spin at a slower speed to create the same amount of boost in the intake manifold. I think they would be able to. Example: If you wanted to run 25 psi boost, you could run two superchargers at 12.5 psi each which means that they would be more efficient and create less heat and draw less horsepower off the crankshaft, but still create the required boost in the intake manifold. It might also provide more cfm airflow. Please give me your input. Thanx

 

You might get a little more boost, but you will not get the double.

With a centrifugal supercharger you set up pulley so you get max boost at max engine RPM. If you use two of the same superchargers in paralell they will be able to flow twice the air at the same boost. If you run them in series ( output from one to input on the other ) you will be able to get twice the boost and the same amount of air.

Look at this graph:


You want to travel in the center of this graph. If you want a pressure ratio of 2.1 then you should set up the pulleys for 45.000 rpm and have an engine that uses 600-1000 CFM.

If you have an engine that requires 1200-2000 CFM to get 2.1 in pressure ratio then twin blowers in paralell is a good idea.

 

Actually, I believe that you are mistaken because boost is a measure of restriction. Parallel superchargers might not produce double the amount of boost, but if they flowed twice the cfm in airflow it would still create boost because you can only force a certain amount of air into the engine before you create boost. It is true that you pulley the supercharger for the redline but if you pullied both superchargers for the max boost then you would have more boost than you wanted.

 

Look at the graph again ...



Lets say that the comressor spins at 35000 rpm. Follow that red line. It is almost horisontal. You will get almost the same boost at 300 CFM and 800 CFM. If you switch from one supercharger to two you will still have to spin them almost as much to get the same boost. You only need two if one does not frow enough CFM's at that boost level.

If you put them in series you can reduce the RPM to keep CFM and total boost constant. You will also move away from the surge-line.

 

Found a twin supercharger:



http://www.vortechsuperchargers.com/marine/torque.html

V-12 engine with 860 cubic inches ...
1,400 to 1,600 HP on premium gasoline ...

 

Hmmm, I'm sure having the headers touch the intake tubes is good for the ole volumetric efficiency.

 

I could see twins working well on a large cubic inch application where an intercooler is not practical (as inlet air temp would be lower with twins) and they also look mean as hell. Two superchargers in series would be animalistic....the boost would be multiplied instead of summed. Im sure heat would be a major issue there though.

 

It was explained to me that twins aren't really worth it unless your going all out on the hp scale.........that extra blower is just causing that much more drag on the crank. Its not practical for street use considering the boost that some of your street kits provide.

 

JoBy, I'm just curious as to how much experience you have with forced induction other than that vette that is pictured in your sig. I have to correct you again, you are looking at a compressor chart for a single supercharger. Look at it this way, an engine consumes x amount of air at atmospheric pressure, a supercharger creates more airflow than the engine can consume, the airflow fills the engine up and any additional airflow has to be pressurized in order to be put in the engine. This is where boost comes from, a supercharger will not create boost if there is no restriction. If you hook up a supercharger with an open end and spin it with a belt all it will create is a strong wind. My theory is that if you use twin superchargers you will not have to spin each of them as fast to create the same amount of airflow which would create less heat and would not draw as much HP to turn the blowers even though you are spinning two. This would make the whole system more efficient.

 

I don't work professionally with cars or superchargers if that is what you mean. I have done a lot of reserarch.

You are right that a restriction is needed to build boost, but you also have to consider the type of supercharger. An increased restriction can reduce flow and/or increase boost.

With displacement supercharger, like Roots or Eaton, you can use two in parelell and run half the RPM on each.

This is a graph for Eaton M112 supercharger.



When the supercharger is in its normal operating range ( with flow restriction ) the inlet flow is propotional to the supercharger RPM. If you change the flow restriction you change the boost, but the inlet flow is constant.



A centrifugal supercharger is an open design and the inlet flow is not propotional to the supercharger RPM. In the normal operating range boost is mostly a function of the supercharger RPM, not the flow restriction. If you change the restriction you will change the inlet flow, but the boost will be more or less the same. When the restriction is so low that you get outside the normal range the boost starts to drop. The efficiency of the supercharger also starts to drop at this point.

 

It's not a question of CFM, it's a question of work and energy. Flowing twice the CFM through the same opening only doubles pressure for an INCOMPRESSIBLE fluid such as water. Since air is obviously compressible the amount of work it takes to compress air to say 5 psi is constant. For instance if you are compressing air into a volume that is already at 5 psi it will take the same amount of energy as if you wanted to compress air into a volume that was at 0 psi. If you run two blowers in parallel at 5 psi a piece you will successfully cut the flow of each compressor in half, and double the amount of work needed to compress the air.

However, the amount of air that can be flowed through the opening of the compressor at ambient pressure is directly related to the size of the opening since ambient air will only travel at the speed of sound. Flow is equal to the density times the velocity times the cross-sectional area of the opening. In this manner you can determine the size of the blower opening for the mass of air you wish to ingest. So, only use two blowers if the inlet of the blower is restrictive.

 

Ok, but then i ask you, what is the advantage of using twin turbos over a single turbo, other than faster spool-up, and therefore less turbo lag? Or even quad turbos? I am not a fan of turbos on large displacement engines because I think that you lose more power than you think because of the backpressure created. Superchargers are more responsive to boost increases and produce lower EGTs. Turbos make power to a point until the increase in backpressure overcomes the increase in pressure. If it always takes the same amount of energy to compress the air to say 5 psi then why have different size blower housings? I have seen dyno charts of single turbo vs. twin turbo applications where everything else is equal and the lines on the graph are almost identical. In my opinion that shows that twins must be more efficient than a single compressor because you use two smaller compressors. By the way I am not talking about using this in an application running 8-10 psi, I am talking about running 25-30 psi. I am concerned about the efficiency it would provide at the higher boost levels.

 

The compressor part on a turbo is identical to the centifugal supercharger, so boost is propotional to the turbo RPM. The RPM is dependent of engine load ... That makes it a bit more complicated.

I don't see any real performance benefits with twin turbos. One big one can do the same job. But there are other things to condsider too. One thing is the exhaust. It gets VERY hot and if you use one short header on each side the heat emitting area will be minimized. It is probably easier to make room for two small turbos than one big. If you use one big turbo it will probably be located on one side, so the exhaust will have different length from the left and right head.

I have compared a few dyno runs from turbos and supercharged cars. The turbo cars usually have a very flat HP curve. The turbos spool up to max boost at quite low rpm, and then they limit the hp at that level. tourqe peaks early and starts to drop. As the tourqe drops and RPM increases the HP stays almost at the same level. On a centrifugal supercharged engine the boost increases with RPM and it keeps the tourqe up where it would start to drop on an N/A engine. The tourqe is flat and HP increases with RPM. The turbo and centrifugal supercharged engines have a very different feel.


25-30 psi ... That is a lot ...

To maximize efficiency you should look at the Compressor maps. For the Vortech S-Trim max efficiency is 72% between 25.000 (4.4 psi) and 35.000 (8.8 psi) rpm if you use 500 to 700 CFM.

To get 30 psi you need a pressure ratio of 3 ( off the chart for a single S-Trim ... )
If you use two in series they need a pressure ratio of 1.75 each. That is below 40.000 rpm. With this setup you can use 450 to 1000 CFM. That is probably way low for your application. The engine will have to be very restrictive not to flow more at that boostlevel.

I have never seen a twin supercharger setup with the superchargers in series ... When it starts to build boost it will increase FAST with the rpm. The setup would probably need one intercooler between the superchargers, and one after too.

Perhaps you should use four of them instead!

Two in series to build boost below 40.000 RPM, and then another two in series on the other side to get the total air-flow required ... Probably too much plumbing and $$$


I think that it would look cool with a twin supercharger setup like a twin turbo, but it might not be the best setup unless you need a huge amount of air at a relative low boost.

 

Was just doing a little more research and found that if you are choosing a turbo to run and you decide to run twin that you just need to divide your final pressure ratio in half to come up with the proper set of turbos to replace the single turbo. You use two turbos doing half the work to equal one large single. Just a little food for thought. :hail:

 

Sorry to but in, but wasn't there a twin supercharged IROC on an episode of Popular Hot Rodding TV? It might be worth traking this guy down and see what his experience has been.
Just my $.02