I have been reading the forum a lot lately because I am hoping to get a 7 after a few months. I am very interested in the technical aspects of the rotary engine and turbocharging.

I have noticed that people talk about turbo sizing and that a turbo that is capable of more CFM, will provide the same HP numbers as a smaller turbo with less boost. Is this true?

Also, I am used to the piston engine, and from what I have seen there, is that boost is boost. It seems that even with a turbo that flows more, 6lbs is 6lbs and the only other thing that make a difference, is intake charge temps. One reason I say this, is because of positive displacement superchargers. If you get a larger supercharger, it is just going to raise the boost at the same RPM because of higher airflow. It wont be able to run less boost and more power. Am I wrong?

Please tell me if I am wrong about any of this, and explain why? I want to learn as much as I can. I have a book on turbocharging, but it only covers piston engines. Thanks.

Kenton

 

pressure difference what causes air flow. if you have 6 psi of boost your above the ambient air pressure by 6 psi. VE the effiency of an engine to suck in a measured volume over the actual volume of the engine. an egine with a greater VE will consume more air at 6psi than one with a lesser VE. with more air you can burn more fuel and make more power. with a small port you will have a greater pressure drop and you wont flow as much air. example if you have a turbo with a pipe on the back of outlet of the compress, and say that pipe has a 1 inch square hole in the back of it. you will not have to flow as much air to create 6 psi of pressure in the pipe, say you have the same pipe but with a 2 inch hole in it. your going to flow a lot more air to hold 6 psi.

hope this answers your question as im burnt out on thinking the act this morning

ps. about the turbo being bigger and making more hp is not true it depends on the effieceny of the compressor side of the tubo. small turbos generally heat up the air more when they compress it than a larger one and you wont be able to make as much power before the engine will want to detonate

 

I understand about flow restrictions and pressure, but it seems that on if you put a higher flowing turbo on a rotary, it will generate more HP than a smaller turbo at the same boost. I am talking about identical engines too. I also understand about compressor efficiency causing heat and less power.

Why is it that if you were to run a GT35/40 at 12psi, you could get the same power out of dual GT35/40's with like 8psi. Those two pressures are just examples and I realize that they are not correct.

Are the turbo's just not able to flow the same amount of air that the engine is capable of?

Kenton

 

You've got to consider that larger turbos put less back-pressure onto the motor. SO... the motor won't be using as much of it's energy to push the exhaust gasses out through the turbine... and you'll have more power at the wheels.

The trade-off is big housings (big A/Rs) don't spool as fast.

 

if the turbos couldnt flow the air they wouldnt make the boost. pressure is cause by restriction to flow

 

That is what I thought. That just makes it harder for me to understand how a bigger turbo with more airflow will make more power at the same pressure or less. Is this a characteristic that only rotary engines have?

Kenton

 

bigger turbos flow more air because they have less back pressure. Having less back pressure in you system makes your turbo flow more air to maintain the same boost level.

 

There is nothing special about the rotary engine. pressure and volume are not the same thing. Take a 1 inch tube, 1 foot long, and pressurize it with 10 psi. Now take a 6 inch tube, 1 foot long, and pressurize it with 10 psi. Obviously the 6 inch tube will contain more air. More air equals more HP.

 

I understand that volume is different than pressure.

Like I was saying about a positive displacement blower, you get a bigger one that will move more air per revolution, and you will get more airflow at the same rpm making your boost pressure increase. It is pushing more air into the same sized space so there is more pressure.

What I dont understand is when I see people saying "I got XXX hp from a T78 at 13psi" and someone else says "you would have to run at least 20psi on XXX turbo to get that much power".

I also understand that some compressors are more efficient than others making the charge cooler and more dense.

Kenton

 

Kenton: I think you understand everything better than you think you do!

You have to take into account many different things. Some deal with our perception of how much boost we are actually running. First off where is the boost guage mounted? Is it registering pressure at the turbo outlet or after all the pressure robbing restrictions such as the intercooler, throttle body, etc. I control my wastegate by picking up the signal after the turbo but I use the boost guage hooked up after the throttlebody inside the plenum chamber. In truth 6 psi is still 6 psi no matter how you do it. This only explains the pressure of the air not how much air there actually is or what its temperature is. If power were based solely on pressure then 6 psi from one turbo would yield the same results as 6 psi from another. You already knew this.

When someone claims that they made so much horsepower at a certain boost level you have to take into account how free flowing the exhaust was, how long the intake and exhaust timing are, how cold the intake charge is, how much pressure loss through the intercooler, you get the idea. When someone claims that more boost is needed to achieve that horsepower level then that is what THAT person needed to achieve the same results with their particular setup. Some people can barely get 250 rwhp at 14 psi while others can exceed 400 rwhp at 14 psi. Too many other factors for anyone to actually claim that something is possible or not.

The reasoning behind a larger positive displacement blower is that yes you do get more air displacement per revolution. However when people move up in size they may do it for one of two reasons. The first is that they want to run more boost and the blower they are using can't supply that level without heating the air to an overly excessive level. The second is because they want more power at the same boost level. When the switch is made to a larger blower then they just simply use a larger pulley to slow the blower down and achieve the same boost level as the smaller one. The horsepower gain is due to the fact that the larger unit heats up the air less than the smaller one.

There are other things to take into consideration also but I think you already knew all of this and just confused yourself!

 

So all the crap I have been thinking about for all this time is only related for the most part to compressor efficiency? All about heat? Well crap, no I feel like a moron.

The above question is obviously only relevant if we are talking about setups that are identical besides the turbo. I guess I have just been overthinking all of this.

Kenton

 

you have to consider pressure loss through the system where the measurement was taken and the amount of porting on the engine, exhaust and fuel management like rotarygod said

 

rotarygod- I'd say it's a combination of compressor efficiency (heat) and turbine size/design (exhaust flow). it's not completely accurate to compare superchargers to turbochargers. The compressor sides are similar but a supercharger does not have an exhaust side. The turbine side of a turbo can be considered the bottleneck of the entire system. That is why larger tubine housings tend to make more top end power than smaller turbine housings. The more exhaust flow, the more volume the compressor needs to flow to maintain the same boost pressure. I'm sure the ports become the limiting factor at some point, but you get the idea...

sbertolone- there are a lot of factors to take into consideration, but I think kenton's question was more concerned with why one turbo flow more than another at the same boost pressure, all else being equal.

 

PSIxCFM=HP and thats the rough of it. You add one of the two you get more hp, you subtract one of the two you get less hp.

 

Not exactly. You forgot to mention intake charge temp. This is why it's so important to choose a compressor size that fits your needs. Too small of a compressor will heat up your intake temps, decreasing HP. This is also one of the reasons upgrading your turbo can give you more HP at the same boost pressure.

 

So basicaly by increasing the size of the turbine housing, you are increasing the VE of the engine? This creates a situation where you need increased airflow to maintain the boost pressure? That sounds good.

The gist of my question was why one turbo could run less pressure and have more power than a smaller turbo running more. I thought that a certain amount of airflow against an engine was going to creat a set amount of boost. But as I see from the posts above, the larger the turbine housing, the more air the engine can flow.

Kenton

 

Thats why I said "the rough of it" air temps do also play a major roll as well as tuning but I just figured we all knew that.

 

Yeah if you decrease the exhaust back pressure (larger a/r) your VE will go up.

You also have to take in account when you run a larger a/r your rpm band will go up or your peak VE will be at a higher RPM. 300lb/ft at 5500rpm is a lot less hp than 300lb/ft at 6500rpm.

 

please explain to my why a lower exhaust pressure will preoduce a better ve i thought ve was only dependent on how effiecent an engine is at sucking in air and silver 7 if you have an engine at 10 psi of boost with turbo A which is a small turbo and switch to turbo B with is large its still going to flow the same amount of air. because pressure is caused by resistance of flow

 

this is true if you just go with a larger compressor wheel. But if turbo B has a larger turbine then it will reduce back pressure in the exhaust system and flow more air then turbo A. The ability for an engine to suck in and get rid of air is directly related to the intake and exhaust system. You have to look at VE of the whole system and not just the engine.

 

It is really a complicated phenomenon in reality, but I have two easy-to-imagine ways that backpressure reduces the amount of fresh air that gets into the combustion chamber:

#1: Rotary engines have a lot of overlap (intake and exhaust are open to same chamber at same time). Any pressure in the exhaust is going to fight the fresh air coming into the chamber. More backpressure means less flow.

#2: If your compression ratio is 9:1 and the exhaust backpressure is twice the intake pressure, you would only get a 4.5:1 ratio of fresh air to exhaust gas. (Okay, its way more complicated than this, but this is easy to imagine and hard to refute.)

-Max

 

It finaly all makes sense to me.

 

It is not all about compressor efficiency and heat. It is all about VE. If your turbine introduces less backpressure, there will be a higher VE. The VE determines what volume of air will be pushed into the chamber at a given pressure.

Sometimes it's easier to look at extremes. Take a hypothetical, but not very useful engine that has a VE of 0%. This engine will, in effect, have no exhaust openings. At 6lbs, NO AIR will be introduced on the intake 'stroke'. The chamber will already be full. The same will be true at any boost pressure. This is actually a gross simplification but it should illustrate the point.

A turbo that introduces less backpressure will give a higher VE. Therefore, at a given pressure, a larger volume of air will be introduced. Air needs to be removed to make room for new air. The amount of exhaust that is removed has an impact on the amount of intake that can be forced in at a given pressure.

Max, I think you have the overlap statement backwards. Rotary engines have comparitively little overlap. The rotor has swept past the exhaust before the intake opens. This is what makes the rotary engine a good design for alternative fuels.