Sure, let's take a generic engine that needs 45lb/min air mass flow at peak load and engine speed.

I drew generic lines, but you will get an idea of where the compressor is operating as the engine speed is increased and air flow required is increased.

The GT35 is a good match, and this is what a good match will look like. The line is where you turbocharger is operating as the flow requirements are increased. At peak, the compressor efficiency is around 79%



This is a generic line I drew for the GT45. With a larger turbo, the compressor will lag down low until it spools up, where it will climb very quickly. At peak, the compressor efficiency is around 74%



In this particular case, the smaller compressor is a better match, and the efficiency is 79% at peak, compared to the larger GT40 compressor which at peak is 74% efficienct. Although these lines are arbitrary, you can still see how it is possible, depending on how well of a match a turbocharger is to an engine, to get a higher compressor outlet temperature with a larger compressor. (Lower compressor efficiency = higher compressor outlet temperatures)

Kevin

 

Thanks for the excellent and informative post Kevin. I see exactly what you are talking about and it further explains why compressor maps give us useful information about a turbo which is too involved for a hobbyist (such as myself) to calculate. It actually makes sense that a larger compressor could require more gas energy to accomplish the same job because the compressed intake charge acts with more leverage against the turbine via the longer compressor veins, even assuming the same compression ratio. Gotta love them turbos

 

I agree with most of what you said except for the last part. If we are talking about two different size turbos operating at the same boost level then the opposing force imposed by the intake charge is the same, regardless of temp. The reason for this is that PSI is the measure of force exerted within the entire intake as well as the face of the turbo and intake ports.

A larger turbo will move more air but will have less of a mechanical advantage since the air opposes with greater leverage (due to fin surface area further away from the shaft center). So while a large turbo has a RPM/heat advantage it has to work for it!

Ofcourse it's a game of trade-offs any way you look at it.

 

Allthough this is thw two turbos I wanted to use for an example, I don't understand your lines. Does it not imply the GT35R is using a small turbine housing? It should have the largest housing and the GT4R the smallest.

With the largest housing on the GT40R, doesn't that map show it would move 60/bls of air?

Anyhow, shouldn't the examples stay at 15lbs of boost to be more realistic for the most of us.

 

Thats not a very good comparison. I understand the "theory" of what you are attemping to suggest but in actuality that rarely happens if both turbos are a decent match for the hp range you are trying to accomplish. To give you an example of what I mean, at 45cfm yes you are within the efficiency of that tiny GT35, with the GT45 the chances of running that minimal amout of cfm at wot without surge would require a very specific application because as you can see 45cfm barely makes it on the chart.

Yes, it is always best to select a compressor that is efficient within the hp range you are trying to make. But your example rarely happens in real life unless someone is comparing between a small turbo designed for thier application with a large turbo designed to make twice the power. When two compressors are similarly sized and designed or similar hp range its really going to get more into blade design, ect as to which one ends up being best. At that point you are really too close to tell off a map which is best. The maps are based on free air so everything little things changes the map. You have a dirty filter on it, you effectivly changed the map, you different ar's you effectivly changed the map, ect.

Anyway, I think your example is a good way to show why its best to select a turbo that is efficient within the power range you are trying to make but its not all that great of a example in a discussion about why larger turbos are more efficient. It might confuse some people more than help them.

Here is a good rule of thumb....select a turbo that is designed for your power range, most likely there will be a selection of them, the larger one will probably be more efficient makeing the power with the lease amount of heat.

But to be honest, you guys are really really over analizing this stuff. If you pick a turbo that is designed for your hp range you will be fine. The difference between 2 turbos that are designed to be efficient for the same output range is going to be minimal and whatever differences there are can be adjusted by other factors like manifold design, IC design, ect.

Stephen

 

The pressure inside your IC piping that you are reading on your boost gauge is independant from the pressure inside the compressor housing where the air goes from the wheel exducer and into the diffuser and volute. The pressure Im refering to in my example is the pressure at the diffuser and volute.

My point is that when you have 2 different sized turbos both making the same pressure on your boost gauge that isnt a reflection of the pressure inside the housing just past the exducer at the diffuser and volute.

 

Aw headache.

 

If you read the previous posts you would have known that Kevin was just giving an example to prove the fact that it is possible for a larger turbo to generate more heat at a specific CFM than a smaller turbo. He never intended this to be an application-specific recommendation.

With all due respect, you posted incorrect information above and now you are saying that fundamental explanations are not necessary.... If you would have taken a few minutes to read previous posts you would have seen in detail why what you said cannot be right, but instead you read the title of the post and just post away w/ incorrect info and then criticize the posters for taking the time to post info to help others understand.

I bet 80% of the people skimming through this thread, many of which are very knowledgeable, still think a larger turbo flows a larger volume of air at a given boost level. This is a fallacy that is carried on by those who don't understand the difference between a test bench and an engine, or are just too lazy to read and actually attempt to understand the formulas and explanations posted earlier.

Can you just call tech support and ask for a turbo recommendation? Ofcourse you can, but that's not what this thread is about. The point is to actually understand, in detail, why turbos behave the way that they do. You can treat systems on your car as black boxes or you can try to understand them, the choice is really up to the individual.

 

?? I didn't say that the pressure was in the housing.... Yes that pressure which we are both referring to acts on the exducer wheel as I don't believe a magic vaccuum exists where the exducer introduces the new air to the diffuser....

 

You said......"If we are talking about two different size turbos operating at the same boost level then the opposing force imposed by the intake charge is the same, regardless of temp. The reason for this is that PSI is the measure of force exerted within the entire intake as well as the face of the turbo and intake ports."


I am saying that the operating boost level starts in the outlet of the turbo. The pressure inside the turbo at the exducer of the wheel, diffuser, and volute are not the same as the operating boost. The opposing forces I was talking about that affect the compressor efficiency are inside the housing from the exducer to the volute.....not the operating boost pressure that you see in the intercooler system or at the intake ports, ect. They are different and it all changes based on diffuser and volute design which has a different effect with every compressor wheel.

Heck even from to turbo to the engine it changes because of pressure drop at the IC.

If you are comparing 2 different turbos at 10psi "operating boost" they will have different pressures inside the housing from the exducer, to diffuser, to volute......point being that based on what my original post was discussing the operating boost is irrelevant for the most part.


The vacuum is at the inducer. The air flows down the vanes and is compressed at the diffuser where it creates a very high pressure area then the pressure is dropped as it goes thru the volute untill it is operating boost at the turbo outlet. The smaller the turbo, the smaller the diffuser and volute, and the more flow you try to cram thru it then it creates more and more pressure causing the wheel to become more inefficient because the wheels has to work harder to overcome that higher pressure which generates more heat.

 

The foregoing is correct. Simply put, the compressor wheel generates VELOCITY PRESSURE and it is the job of the of the volute to efficiently convert this energy into a usable static pressure [at the outlet] which represents real boost. Further to the point, there is a real and definite relationship between the wheel, diffuser, volute and [wheel] speed which will determine the size, shape and location of the efficiency islands; note that manifold boost pressure will only affect the instant operating point location on a given island--it will not change the characteristics of the island itself.

 

I'm stretching my knowledge a bit here, so I'll have to look into this, but it seems that static pressure is acting on the compressor wheel by through intake backpressure, though I do for this moment (until further notice) concede that dynamic pressure may result in a different pressure reading at the wheel - as the 02 molecules are being thrust forward providing some amount of inertial energy that is lost as heat until they hit the brick wall that is the static charge- (I will have to check on this, or perhaps you can back it up with some sources/formulas).

The catch that I empose on the above statement, however, is that OPERATING BOOST can occur anywhere that static charge presents this brick wall, which is not necessarily beyong the volute. As long as there is significant backpressure (when boost is stable and flow is sufficient), the PSI reading measured at the volute should be the same as the IC inlet, the only difference should be the velocity which is dependent on the diameter of the orifice through which the specific volume of molecules are traveling. It seems that energy would need to magically disappear (after the heat has already been created) for this not to be the case.

Good call stephen. I should have mentioned before that a pressure drop does occur at the IC due to a change in charge density, which is common knowledge.

Boost, flow and VE make power. To say that the pressure measured at the intake manifold means nothing is a bit misleading as this is where you are best able to measure what your intake ports are seeing.

At this time I do not really disagree with Stephen's last post (though I still disagree with the first one) or anything that speed of light had to say. Perhaps you can now elaborate on how these islands change when the turbo is slapped onto an engine and provide a bit more evidence for your previous statement.

Thanks for your posts gents.

Rob

 

I wish I had a online document or something to link you too but I dont. My first post was pretty much saying the same as my last one....all of the other posts after the first one were just going into further detail explaining what I was saying in the first one. I was "attempting" to explain some of the concepts behind turbo efficiency so people might understand why larger turbos with larger wheels, diffusers, and volutes typically make more power at the same pressure. But I know I'm not all that great at explaining things. Thats why I really just stopped posting on the board much, sometimes it just becomes too much of a time consuming burden that I cant fit into my very busy life.


To this comment you had........."Boost, flow and VE make power. To say that the pressure measured at the intake manifold means nothing is a bit misleading as this is where you are best able to measure what your intake ports are seeing."..........Im sorry if my comment came out sounding confusing, I didnt mean to suggest that operating boost is irrelevant to making power, it is very important. I was just saying its somewhat irrelevant as far as the specific scope of turbo efficiency that I was refering too and that it wasnt the pressure that I was refering to in my first post.

Great discussion guys!
Stephen

 

I hear ya! I'm currently finishing up an all nighter, which is always fun. I'm going to go upstairs take a shower and wake up a bit before finishing my work and rushing off for a meeting. I really do appreciate everyone's post, even if knit picked you a little .

have a good one,
Rob

 

Yea, I know the feeling man....If I can give you (and everyone else) some advice.....dont let this place dominate your time. Its easy to sit down for 30 minutes which ends up being a hour, 3hours, 5hours, ect. Its very easy to let RX7's and this place dominate your life. That is what happened to me, finally it was breaking me down and affecting other parts of my life and I had to just put it all down and take a step back untill I get the rest of my life back in order. Its amazing the much impact a car (and the time you spend on it) can have on the other areas of your life.

Take it easy man!
Stephen

 

good thread; thanks Rob.

 

Well said there Stephen! I'm trying to stay way these days...

 

All of this info has been amazing and HIT ME IN THE HEAD WITH A HAMMER PLEASE wonderful, but what I want to know is that given all this info and physics, science, math and whatever else, has any two RX-7s sat on the same dyno, at the same altitude, in the same wind tunnel, with the same body modifications and one with a large turbo running 17 psi and one with BNR Stage 3 turbos running 17psi and the large turbo put out more hp? If not then are all these explanations based on theory and what should happen given mathematical equations and physics?

Just curious?

Brad

 

Check my posts on the "all things staying the same" theory.

Since the manifold, turbo (compressor and turbine), and presumably down pipe would be different it's an inaccurate comparison.