Killing misconceptions

[crankbender]

I wanted to repost this so that people could see it even if they didn't read the dyno posts. I have heard alot of people talking about a bigger turbo making more hp at the same pressure...this isn't exactly correct and I hope this helps show why. You need to get a turbo that does what you want and no more! Don't get a big turbo if you don't need it. You will just hate the lag.

First off i want to set one thing straight that many people seem to be confused on. Your turbo does not have a hp rating. They don't make hp, they flow air. The difference between turbos is how much air they can flow at a certain speed, pressure, and how fast they can turn efficiently. If you want to compare your turbo you have to see what pressure you are at and what temp. When the turbo starts to overspool they add more heat developed through mechanical loss to the intake air. Also they will max out at a certain flow rate.

If you can get a steady pressure at the intake and the same temp it doesn't matter what turbo you are using (except for exhaust back pressure). all of the turbos we use have similar back pressures at reasonable RPMs. Therefore your turbo will compare based on what temp the air at the intake is...get a temp gauge there, it will help you alot.

Don't let people say you have to buy their turbo based on how much hp it is rated for. Ask them for a flow at a certain pressure and what kind of temps it is going to be producing at that flow rate.

Ask them to look at an exhaust housing. How big is the "nozzle" that directs the air onto the wheel? that will dictate how fast you spool (along with moving mass) and what kind of back pressures you will have.

Remember the loss to having a turbo is that when your exhaust valves close you still have alot of used gas in the cylinder because the exhause is usually around twice the pressure of the intake air. that means if you are running 20psi when the exhause valves close you still have the air in the cyl at 40psi and that is used air.....this is why ball bearing turbos are nice they spin easier so require less back pressure.

This may not make alot of sense but basically what i am saying is that if you are looking at a car with a similar exhaust and you have the same engine internals, intake temps, and boost you will make the same hp (roughly) even if your turbos are slightly different. Don't let the turbo companies confuse you bigger turbos do NOT automatically make more hp at the same boost pressures.

 

[L2RTSiAWD]

But most of the time a turbo with bigger turbine and exhaust wheel will flow more air at the same boost as a smaller turbo. Compare larry's Big28 to my Muttlv2 and tell me how my turbo will not flow more air then his at any boost pressure.

Edit: If you didn't know my turbo has a 50 trim compressor wheel and T3 turbine wheel.

Later,

 

[crankbender]

They can't flow more air at the same boost level....yes they will if they arn't attached to an engine. The engine has a fixed displacement and at any given RPM it will have the same air flowing through it reguardless of the turbo(assuming the same back pressure). Bigger turbos CAN flow more air at the same pressure but they will NOT attached to an engine...there the engine is the restricting not hte turbo (assuming both turbos can flow that ammount of air.

Don't believe that hype that a bigger turbo will increase performance even at lower pressures...It just isn't true. They CAN decrease intake temps which will increase performance but they are not guarranteed to do so.

 

[crankbender]

take this little example

2000cc (displacement)
0.5 (because it is a 4 cycle)
7500 (Rpm)
0.00003531 (feet^3 per cc)

2000*.5*7500*.00003531=264.525 CFM

Now This is independent of pressure...for that you have to change the intake(of the turbo) CFM and the turbo output mass but not CFM

How can i put more air than this through the engine at that RPM? This doesn't include loss from pressures in the cylinder or that it doesn't have an infinity to 1 CR. So actually less than this is flowing through the engine.

Now you may ask why can't a T25 keep up it puts out more than that at 16 psi but can't maintain 16psi at 7500RPM? Well the air comming out of the T25 is extremely hot. As it cools it will become denser. So the 250 CFM that is comming out of the turbo may only be 200 CFM after the IC...not to mention pressure loss from the piping making it so that you have to maintain about 18 psi at the turbo where the T25 will only put out like 225 CFM. (if you have very high temps at the turbo and a good IC you could possibly have to flow double what the engine is using at the turbo...That is where turbo efficiency comes in.

As you can see you can have TOO BIG of a turbo.

 

[IPT]

Everything you are talking about is simply FLOW right?? To allow the turbo to flow more air something must be done to the engine to make it. Or lets say put a bigger turbine wheel in the turbo. That makes for less backpressure that will increase the flow through the motor.. Tell me if I am wrong here... Or install a set of cams. That opens up more flow through the motor and allows the the turbo to flow more...

I have been battling a case of compressor surge at low rpms in the tall gears, why because the turbo moves too much air at that rpm for the motor to flow. This is were careful turbo selection comes into play. You should look for a turbo that can not only move lots of air in the upper rpms range where the motor ingusts more air, but also has the ability to flow not as much in the lower rpms to stay away from the surge line.. A GREAT example of a compressor wheel that has broad flow charateristics if the 50 trim wheel. I can handle not flowing much air in the lower rpms (to stay away for the surge line), but also has the ability to flow the needed air in the upper rpms ranges.

I think this all makes sense...

 

[swordfish]

Insofar as the laws of mathematics are certain, they do not refer to reality; and insofar as they refer to reality, they are not certain.


jeff

 

[Ludachris]

I think the biggest problem is that many people want a huge turbo in order to go fast, yet don't necessarily choose the right size for their setup. They automatically assume, "since this one can handle up to 460hp, that's what I'll get" - even though they likely aren't going to have the rest of the supporting mods needed for 460hp.

I myself am seriously starting to look for my first turbo upgrade and have been doing a lot of research. I've been asking questions here and there trying to get an idea of which one will work best for what I want to do with the car. It always comes back to planning your goals.

 

[Ludachris]

and by the way Crankbender, thanks for sharing this with everyone.

 

[Atari007]

Thanx for the info crank! It makes sense to me!

 

[crankbender]

It all came down to seeing some posts that reminded me of a discussion I observed one time. A bunch of guys were talking about doing a turbo swap and were looking for the biggest turbo to run 13-14 psi on. everybody swore that a bigger turbo would make more hp because it flows more...but they wouldn't listen to the fact that it can flow more but won't when attached to an engine. Needless to say they spent 2000 dollars for something that would have been better accomplished by keeping the 14b and upgrading the IC.

We are all here to help eachother out and what better help can you get than how to correctly tune your car on less money. The more money you save on this part the more you can spend on another=faster car and more mods.

I appreciate the thanks and you are all welcome...If you need any info on what else can be done to help the car out just let me know.

BTW those of you looking for cams I wouldn't go there until I was in the 12's. Way back when (many years) one of my friends racing partners had his dsm in the 12's and wanted faster times so he dropped 6k on a motor built up from scratch...ended up dropping .2-.3 off his time. The stock 1G engines respond alot better to allowing them to breathe (external parts) than they do to internal engine work. This is probably because they are running at such high pressures at this point that even tho the valves could be bigger and open for a little longer they don't need to. Basically IMHO before you go out and spend that much on a part make sure you have it tuned to the max...you may find out you are getting air to the intake at 200 degrees and lowering that will increase performance alot more than getting a very small ammount more of that hot air into the engine. Why don't I ever hear of people pulling the water line that they have going to their TB? It heats the air you just put the IC on to cool...

 

[crankbender]

Sorry if this is spamming but I had something else to add that I didn't really touch on.

Many people replace their IC and piping to decrease restriction and gain more power. The standart assumption is that you will get more boost to the engine. This is untrue. You should be reading your boost/vacume after the TB. Anything that you do before the gauge and you don't change what the gauge reads does not change the boost the engine sees. Your static point for boost in the intake system is where you have it tapped for the wastegate(should be as close as possible to the TB). If you change anything before this you don't change the boost seen by the engine because that is your limiter. You may ask where does the power come from then? Well simply by decreasing the restrictions in the piping you decrease the pressure on the turbo end of the intake...this sounds counter productive doesn't it . This however is great because the turbo adds heat to the air based on the pressure on it's exhaust to the intake piping not what pressure the engine sees. By decreasing pressure here you are effectively lowering intake temps which lets you get more oxygen to the car even at the same boost pressure! just make sure your fuel is upped slightly to match.

I am not preaching and not assuming any individual doesn't know this. I am stating this for those that do not know this info and not trying to offend those that do.

 

[IPT]

The pressure on the exhaust side can be changed with the turbine wheel right???

 

[crankbender]

There is alot more in there than just the wheel so it is not an absolute but usually it will change the backpressure a small ammount. But this is not to say that bigger is better. Usually your pressure is regulated by the efficiency of the turbo...that is to say that the pressure loss through the turbo is directly related to boost pressure. Once you are at full boost (where you have it set not the max for the turbo) your wastegate will regulate the exhaust to keep that ammount of air flowing through the turbine and keep it spooled. The wastegate does this by bleeding off air to regulate the pressure. If the wastegate did not do this and the turbo continued to see more pressure as you went up in RPMs then you would creap...that is more pressure=more air pussing through=more spool=more boost=more exhaust pressure=more air pussing through=more spool.....it just goes on forever till the turbo is at it's max flow. You don't necessilarily have to up your turbine wheel size to get lower back pressure you just have to make your turbo more effiecient (wheel designs, bearings, balancing, wheel size, etc). A larger turbine wheel is usually more efficient because you have more surface area to push on and it takes alot of force to get that compresser wheel moving and keep it there. however a larger housing and turbine wheel will allow more gas to just slip through and not spin the turbo (why they don't spool at low rpms even at wot. Cruising speed for your turbine wheels is usually 6-8k RPMs but at boost you can see 140,000RPMs.... yeah that is alot of zeros). Also a larger turbine wheel will increase the mass that needs to be brought up to speed.

Hope that answers your question
Anybody want to double check me on this one?
I am fairly certain on it but it all depends on design of the housing vs the wheels, the wheels themselves, and the bearings. And unfortunatley I am not nearly as up to date on wheel designs as someone at garret for example may be.

*edited part* I am not completely happy with this post as I am not an expert on wheel designs, but I will talk with some of the people I know who are pretty darn knowledgable on the subject to make sure I am not fibbing here (give me a few days...sorry for the delay). Also if lud and larry will allow I will try and give garrett a call as a site rep to talk to a tech rep and get their version of this skinny. *end of edit*

Hope lud and larry read this

 

[swordfish]

Larger turbos do make more hp at higher boost levels due to efficiency alone. Example: Imagine these two turbo have the same hot side, only the compressor is different. Lets say they are both TD05H turbines in a a 7cm housing. One has a 16G compressor the other a 20G compressor. Lets imagine the compressor housings are the most efficient for the wheel thats in it. The 16G @ 30 psi will not make as much hp as the 20G @ 30psi based on aerodynamics and compressor efficiency alone. Above a certain pressure the 16G will begin to cavitate and cause turbulence whereas the 20G will have a cavitation threshhold above that of the 16G. This is where the idea that bigger turbos make more power comes from, because at different pressure ratios and higher levels of boost bigger turbos do make more power. Its all aerodynamics. Thats why you can take a garrett wheel that physically looks like a mitsu wheel (same size) and at 30 psi for example the Garrett wheel will out perform the mitsu wheel. The garrett wheel has a much better aerodynamic design and thus is more efficient.

Two compressor wheels may be able to flow the same amount of air for a given amount of boost but one may be able to make more hp for a given amount of boost. The more efficient compressor will have an air charge that is more dense thus being more oxygen rich and therefore having the potential engery (more oxygen for combustion) to produce more hp.

jeff

 

[crankbender]

Since you didn't mention temps and I specifically did I will assume that they are at the same temp. This being the case please explain to me how the air at the same temp and same pressure is denser.

If you can get a steady pressure at the intake and the same temp it doesn't matter what turbo you are using (except for exhaust back pressure). all of the turbos we use have similar back pressures at reasonable RPMs. Therefore your turbo will compare based on what temp the air at the intake is...get a temp gauge there, it will help you alot.

PV=nRT (n and R are constants and you can use this to make an equation if you are using the same gas in two different situations).
P1V1/T1=P2V2/T2

Now P1=P2 and T1=T2...how can V1 not = V2???

What you are saying is not possible...

As for them being more efficient and therefore not adding as much heat and possibly having less back pressure I will agree with you there (I have already stated that). But what about before the 16g cavitates and when the 20g can't get up to it's efficient RPM range? Here the 16g is better plain and simple.

Bigger turbos do not automatically produce denser air. Their compressor efficiency allows them to produce cooler air at higher pressures because they are designed for it. but they have alot more lag. It all depends on what you want to do with it. Which was the point in the first place.

Check out this link
http://www.hahnracecraft.com/hahn/parts/turbo.htm
You will notice that until around 250+hp (they use hp which is crazy but whatever) the super 16g is more efficient than the super 20g...and you know they would rather sell the super 20g so i doubt they are lying about that.

Turbines are designed to run in a specific RPM range. below and above that they loose efficiency. I am not saying use a 16g to run 30psi what I am saying is get a turbo to match what you want to accomplish. To do this you need to know what kind of intake temps you will have with each turbo.

If anybody out there has any info on their intake temps please let us know your setup and your temps.

 

[IPT]

I was talking to Dave at DSM PErformance one time about outlet temps on turbo. He told me about the testing they did on the original mutt turbo. He said that they got a reading of 300 degrees from a 14B at 20 psi and on the mutt they where getting around 200-240 degrees at 20 psi. Now the mutt moves more air and had cooler outlet temps..

 

[crankbender]

If they both were at 20psi they were not putting a larger volume of air through the engine. Now the temps mean they were getting denser air so they were getting more mass. This is the important reading but just saying air can be a little confusing.

Thanks for the info. But was this on a car? What were they limiting the flow at? Why did they choose 20psi? 20psi is high for a 14b...I am not going to deny that. But what if you don't plan on running 20psi?

I have not said try and push a little turbo past it's limits I have said pick a turbo that matches your limits.

On the subject of mutts here is a quote from the site linked below:
http://www.dsm-performance.com/catalog/mf_levels.php

Ah, the real question. It certainly all depends what you are looking for out of a turbo, and being realistic will give you the best results. All too often, people call looking for ETE53s for cars they drive on the street with basic bolt-on mods. Bigger is NOT always "better" if you have to run 12psi for the next year until you have the proper mods to turn up the boost safely.

We rarely recommend anything larger than an ETE32 for a decently modified daily driven car and the ETE42 for the even more highly modded street cars. Anything larger is strictly geared towards making power without much regard to driving to work.

Bigger isn't always better is what I am saying. I am not saying smaller is always better.

 

[IPT]

They where on race gas and at the track. That is why the boost was high. They were testing the intercooler at the same time.

Turbo matching will make or break your set up... Just try putting a T-66 on a most stock car and see what happens. Compressor surge big time. Things just won't be happy.

There is a good point. Everything needs to be balanced so that everything works together and is happy.

Have you ever noticed that once you get past a point the bigger turbo seem to be not as big of a step as the one before. example gone from "X"3 to "X"4 makes 40 hp more,but from "X" 4 to "X" 5 it make 28 hp more. It gets harder to make that last bit of power.

I think I got a little off subject.

 

[crankbender]

about the off subject but that is cool...send me an icq.

31386393

However; what you said about finding that perfect fit turbo to car is exactly the point I intended to make.

 

[swordfish]

As for them being more efficient and therefore not adding as much heat and possibly having less back pressure I will agree with you there (I have already stated that). But what about before the 16g cavitates and when the 20g can't get up to it's efficient RPM range? Here the 16g is better plain and simple.

Hey I never said a bigger turbo is better for someone who is going to run 18 psi all the time.

As far as the temperature goes, if a 16g is frantically spinning trying to make 30 psi the charge temp is going to be much hotter than that of the 20g both before and after the IC therefore making the charge of the 16g less dense and less oxygen rich when compared to that of the 20g. So even though P1 and P2 are the same it doesnt mean the temps are going to automatically be the same when talking about efficiency.

I'm not arguing with you about what turbo people should buy. Personally I could care less what they spend their money on. People who are never going to run in the higher boost ranges have no need for a large turbo and will have less area under the curve. Just like a small 16g @ 12 psi will more than likely have a greater area under the curve than say a TD07 25G @12 psi. By the time the 25G spools, the small 16 has peaked and is holding a flat torque curve and has been making hp for 1200 -1500 rpms.

jeff

 

[crankbender]

I think we are in agreement here...The post was not ment for those looking at 30psi at all. most people who even dream of going that high have done their research on which turbo to run.

there are some ranges that are not so clear tho. Say the 18-23 psi range. Here the supporting mods on the car make a big difference. You may be better off if you spend less on the turbo and more on the stuff to make it work efficiently.

That is all I was saying I just gave a little info to support why. I never said:
a smaller turbo will always be better.
Push your turbo beyond it's limits.

I answered some questions on why bigger turbos will not necessilarily flow more air and tried to give some info to use while deciding on a turbo.

And you can't say that 1 turbo will have higher temps before and after the IC than another will. That is unless you specify what ICs they are using. I can make a big 16g run hotter before the IC and then run colder after the IC if I get the extra 600+ bucks i saved to have a better cooling system. Of course this is dependent on boost pressure. It is all about tuning with the right mix of parts...

Hell I plan on running a 20G+ at 24+psi with intake temps at or below ambient air temps and EGTs in the 1450+ range. There is alot you can do to an engine to make it run what you want. Which is why I wanted to let people know that their decisions on turbos big or small is very important. Don't let people fool you that a bigger turbo is better!

 

[jdmawd]

crankbender:
do you think a intake temp gauge should be used in tuning because i have been told it should be

 

[crankbender]

Yes of course it should. Just knowing your intake pressure doesn't really tell you how much air you are getting in. To know that and in turn what kind of power you can expect you need to know the temps. Also intake temps play a big role in detonation. If you know they are high that is a good place to work on to decrease detonation.

Hope this helps

 

[IPT]

Where can you get a gauge to measure air temp??

 

[NJGSX96]

Originally posted by IPT
Where can you get a gauge to measure air temp??

Any speed shop should carry an air temp gauge. Just plumb it into your intake. That, or you could use the logger to get it.