Jump to content
SAU Community

Power Versus Turbo Choice

Branelly

By Branelly,
in Engines & Forced Induction

 Share

Recommended Posts

Branelly Enthusiast

Branelly

Posted
Posted

I was originally under the impression that turbo choice wouldn't effect power as long as the psi its putting into the engine is the same.

For instance two turbos, 1 reaches full boost at 3000rpm. The second which is slightly bigger reaches full boost at 3500rpm. Both are putting the same psi into the same engine. Are they both going to make the same power at 6000 rpm, because both are putting in the exact same amount of boost in at that time?

Hope you understand what im getting at, and can clear it up for me.

Link to comment
https://www.sau.com.au/forums/topic/300803-power-versus-turbo-choice/
Share on other sites
STATUS Grand Master

STATUS

Posted
Posted
I was originally under the impression that turbo choice wouldn't effect power as long as the psi its putting into the engine is the same.

For instance two turbos, 1 reaches full boost at 3000rpm. The second which is slightly bigger reaches full boost at 3500rpm. Both are putting the same psi into the same engine. Are they both going to make the same power at 6000 rpm, because both are putting in the exact same amount of boost in at that time?

Hope you understand what im getting at, and can clear it up for me.

no, think of turbos as pumps 15psi from a truck size turbo is going to pump alot more air into the engine than a small lawnmower turbo pumping 15psi

Lukas 33 Proficient

Lukas 33

Posted
Posted
Hmmm weird, i just immediately thought that once the psi equals your set level your wastegate will open, there fore its not going to matter what your volume is.

In other words more volume into space = more pressure.

Your going to need to do a fair bit of reading. No offense.

SECURITY Grand Master

SECURITY

Posted
Posted

say for example you have 2 pipes, both flowing @ 15psi. one pipe is 50mm, the other is 100mm thick. you are getting more air out of the 100mm pipe than you are out of the 50mm one.

this is the same for turbos. the bigger the turbo, the more air it will push out. a bigger turbo can produce the same flow at a lower psi than a smaller turbo can produce at its highest psi. for example, a standard r33 turbo will cause a car to produce 200kw @ 12psi because it can flow a certain amount of air at 12psi. a larger turbo, for example a gt35r can produce the same 200kw but at a lower psi (say 7psi) because it can flow the same amount of air at that psi. at 12psi on the gt35 will see you pushing out almost 300kw because it can flow much more air at 12psi.

cars run on air and spark (basically). if you have more air flowing through the motor you can produce more power. it doesnt matter what the air pressure is, its the flow of air that will effect performance (ofcorse having lots of flow and high psi will produce lots and lots of power if your car can produce enough exhaust - but thats a whole other issue.

not sure if that makes sence - but i tried lol.

Lukas 33 Proficient

Lukas 33

Posted
Posted
say for example you have 2 pipes, both flowing @ 15psi. one pipe is 50mm, the other is 100mm thick. you are getting more air out of the 100mm pipe than you are out of the 50mm one.

this is the same for turbos. the bigger the turbo, the more air it will push out. a bigger turbo can produce the same flow at a lower psi than a smaller turbo can produce at its highest psi. for example, a standard r33 turbo will cause a car to produce 200kw @ 12psi because it can flow a certain amount of air at 12psi. a larger turbo, for example a gt35r can produce the same 200kw but at a lower psi (say 7psi) because it can flow the same amount of air at that psi. at 12psi on the gt35 will see you pushing out almost 300kw because it can flow much more air at 12psi.

cars run on air and spark (basically). if you have more air flowing through the motor you can produce more power. it doesnt matter what the air pressure is, its the flow of air that will effect performance (ofcorse having lots of flow and high psi will produce lots and lots of power if your car can produce enough exhaust - but thats a whole other issue.

not sure if that makes sence - but i tried lol.

Yeh thats a pretty good explanation. If I didnt already know that Im sure I would know that now :D Like everyone has said think of it as thought air in a pipe or water going through a pipe. A uni students definition. If you have beer bong with a straw at the bottom your going to need a lot of pressure in it to drink your bourbon in under 5 seconds. However if you had some garden hose on there the bourbon will flow into your mouth a lot faster and with a lot less pressure. Hence the idea of using a bigger hose when you have a 1L + beer bong.

Nah but seriously securitys and trents definition are good. Mine is just coz I have some spare time at work :)

Lithium Grand Master

Lithium

Posted
Posted

I sortof agree with the above explanations, however strictly speaking I'd say the engine is the "hose" bit of that explanation - the turbo is something different. The turbo has the capacity to pump a given amount of air at a given amount of pressure, but the engine has a natural capacity to move a certain amount of air at atmospheric pressure.

Any more air than that and the engine starts providing the turbo with resistance, which is (partly) where boost comes from. Pressure also increases with heat, so if the engine is able to breathe more than a turbo can move at a given pressure - then yes, you are likely to get more power on the same boost with a bigger turbo. If the turbo is able to pump more than the engine can breathe at a given boost level however, then going to an even bigger turbo will probably not make a lot of difference...

JonnoHR31 Grand Master

JonnoHR31

Posted
Posted

exactly, boost comes from the turbo pumping air against the backpressure of the motor, hook a turbo up to a pipe leading into thin air and it'll flow shitloads of air but make no boost. so with bigger cams and manifolds etc you'll flow the same amount of air on less boost with the same turbo aswell. the reason bigger turbos generally make more power is because of less restriction on the exhaust side and less effort from the bigger compressor to flow the same air, hence less heat, which means less pressure. also why heat makes such a big difference for turbo cars, we use boost as a control of airflow, so the amount of air flowing at that same boost level varies depending on the air temp.

MrStabby Grand Master

MrStabby

Posted
Posted
exactly, boost comes from the turbo pumping air against the backpressure of the motor, hook a turbo up to a pipe leading into thin air and it'll flow shitloads of air but make no boost. so with bigger cams and manifolds etc you'll flow the same amount of air on less boost with the same turbo aswell. the reason bigger turbos generally make more power is because of less restriction on the exhaust side and less effort from the bigger compressor to flow the same air, hence less heat, which means less pressure. also why heat makes such a big difference for turbo cars, we use boost as a control of airflow, so the amount of air flowing at that same boost level varies depending on the air temp.

That's the best explanation i've read. I used to wonder why there wouldnt be a direct correlation between airflow and boost pressure, as i had thought that the engine would provided a fixed amount of resistance against a given flowrate. The difference is that we're talking about mass flowrate not volume flowrate. The bigger turbo = less temperature at the same pressure so more mass for the same volume. Is that it?

Lithium Grand Master

Lithium

Posted
Posted

Its not quite that easy, but as an overly simplified view of it - yes. This kind of thing is why we have compressor maps showing percentage efficiency at different flow/pressure levels. There are cases where smaller turbos have higher compressor efficiency at equivalent flow rates - this can be but isn't strictly at the higher pressure ratios/lower flow levels and could partly answer why some smaller turbos can be perceived as torquier.

Bigger compressors tend to make and hold their efficiency at higher flow rates, which is why they tend to be capable of making more power at lower boost. The turbine flow comes directly into it as well, as the easier the exhaust can leave the engine - the easier more fresh air/fuel mixture can enter the engine.

Branelly Enthusiast

Branelly

Posted
  • Author
Posted

I think im slowly learning something here, keep it up and i might retain some of it.

So basically boost (psi) is more a measurement of the air that's left before the engine uses it. If the engine can use the air the boost is lower, but the flow is the same or more.

@ Lukas, Yeah i understand basic turbo info. But still a little (a lot) clueless about compressor maps.

Kwyjibo Rising Star

Kwyjibo

Posted
Posted
I think im slowly learning something here, keep it up and i might retain some of it.

So basically boost (psi) is more a measurement of the air that's left before the engine uses it. If the engine can use the air the boost is lower, but the flow is the same or more.

I don't think thats quite right. 'Boost' or the PSI is a measure of the pressure in the manifold piping created by the air. The measurement is Pounds per Square Inch so at normal atmospheric pressure its applying no force, but as you compress/pressurise the air it starts pushing on the inside of the manifold and piping. Basically the reason for this is that you now have forced a higher volume of air into the space than the atmospheric pressure at whatever altitude you are at. By increasing the air pressure(therefore overall density of air) you can force more air into the engine, without having to make larger pipes or increase port size.

So by putting in a larger turbo you are able to compress more air, it might only go to the same 'PSI' but its actually filling up a larger volume with the same compression.

An easier example to understand might be a water pump, a small water pump might be able to move around or 'pressurise' a volume of say 10. If you put that into a system with 20L you would need a bigger pump to pressurise the system to the same level. So the overall PSI might be the same but you are actually moving a much larger volume of water.

I'm by no means an expert, this is just what I've gathered so if anyone thinks I'm wrong please let me know.

R31Nismoid Grand Master

R31Nismoid

Posted
Posted
So by putting in a larger turbo you are able to compress more air, it might only go to the same 'PSI' but its actually filling up a larger volume with the same compression.

Careful :)

Not filling a larger volume.

The volume of the piping/plenum/cooler is always a constant.

It is the amount of air flowing at that given pressure that differs, measured in lb's/min.

One turbo will flow 40lb's/min of air @ 15psi.

The other will be 52lb's/min of air @ 15psi.

Kwyjibo Rising Star

Kwyjibo

Posted
Posted
Careful :)

Not filling a larger volume.

The volume of the piping/plenum/cooler is always a constant.

It is the amount of air flowing at that given pressure that differs, measured in lb's/min.

One turbo will flow 40lb's/min of air @ 15psi.

The other will be 52lb's/min of air @ 15psi.

Good point. Am I correcting in saying that if you increase the amount of air you can compress that you could fill a larger volume at the same PSI with a larger turbo where a smaller one may struggle?

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
 Share
Go to topic listing


  • Similar Content

  • Latest Posts

    • suko
      J50 clock date suddenly reset to January 1

      By suko · Posted

      Well, I can recommend the partial AV system translation CD I ordered from Car Audio Workshop in NZ. Whilst it didn't address the date issue, it has conveniently translated on-screen menu items into English, and now allows the GPS-received time to be offset in hours rather than minutes, so I can display Eastern Australian time accurately ( and bump it by another hour when daylight savings starts ).
    • GTSBoy
      Baron's R34 GT-R build thread - car in Japan for 5 years -_-

      By GTSBoy · Posted

      Yuh, if it's 45°C outside, my car is driving in it.
    • soviet_merlin
      Baron's R34 GT-R build thread - car in Japan for 5 years -_-

      By soviet_merlin · Posted

      I'd be curious to hear more. Otherwise, have you driven a modern x-trail? I wonder how it compares. Here in Australia they are/were popular for rentals and fleet vehicles. I have been in some and my impression was they are bad. But, this may have been very different in the 2000s at a good trim level. Twenty years is plenty of time to make the model worse. I do very much agree with the 2 silver cars in the garage approach. But, not driving because it's too hot would not leave a lot of time in the year for many Australians. I don't think you need to worry too much unless the car has actual issues with overheating. 
    • BaronOBeefdip
      Baron's R34 GT-R build thread - car in Japan for 5 years -_-

      By BaronOBeefdip · Posted

      Back again. I returned to Japan in Jul/Aug to spend time with the car on my birthday and remind myself what all the sacrifice and compromise is for. It happened to line up with the monthly morning meet in Okutama, which I have been wanting to go to for a long time. It's a unique event at a unique spot with really rare, interesting, and quirky cars. It's where all the oldheads and OGs gather. The nighttime scene at DKF certainly has its place and should be experienced if you're into cars, but there's too much bad attention and negativity around it now. IMO the better time is Sunday morning at DKF or Okutama; it's more chill and relaxed. I'm glad I was finally able to go, but not sure it's worth the drive from all the way from Nagoya immediately the day before, unless I was already staying in Tokyo for the days right before the meet, because you have to wake up quite early to make it in time. Funnily enough though I didn't drive the car all that much this trip because it was just too damn hot. While there were zero issues and running temps were nominal and the A/C was strong, RBs already run crazy hot as it is. Sure, it took it all like a champ but something about driving these cars in the ridiculous heat/humidity bothers me and makes me feel like I'm asking too much of it. I'm just me being weird and treating the car like a living thing with feelings; I'm mechanically sympathetic to a fault. Instead I was mainly driving something else around - a KX4(silver) 2001 X-Trail GT, that I acquired in May. There's a few different flavors to choose from with Xs, but visually it's the Nissan version of the Honda CR-V. Mechanically it's a whole different story as this, being the top-trim GT, has an SR20VET mated to a four-speed auto and full-time AWD! It was a very affordable buy in exceptional condition inside and out, with very low mileage...only 48k kms. Most likely it was owned by an older person who kept it garaged and well-maintained, so I'm really happy with how it all worked out. It literally needs zero attention at the moment, albeit except for some minor visual touch-ups. I wanted something quirky, interesting, and practical and for sure it handily delivers on all three of those aspects. I was immediately able to utilize the cargo and passenger capacity to its full extent. It's a lot of fun to drive and is quite punchy through 1st and 2nd. It's very unassuming -in the twisty bits it's a lot more composed than one would think at a glance- and it'll be even better once I get better tires on it(yes, it's an SUV but still a little boat-y for my liking). So...now I have two golden-era Nissans in silver. One sports car and one that does everything else; the perfect two-car solution I think👍 The rest of the trip...I was able to turn my stressed brain off and enjoy it, although I didn't quite get to do as much as I thought. I did some interesting things, met some interesting people, and happened into some interesting situations however, that's all for another post though only if people really want to know. Project-wise, I went back to Mine's again to discuss more plans and am hoping to wrap that up real soon; keep watching this space if that interests you. Additionally, while working in the tormenting sweatbox that is the warehouse, I was able to organize most of the myriad of parts that my friend is storing for me along with the cars, and the 34 has a nice little spot carved out for it: And since it can get so stupid hot in there, that made it all the more easy -after I was standing there looking at the car and said 'f**k it'- to finally remove all the damn gauges that have mostly been an eyesore all this time. Huzzah. The heat basically makes the adhesive backing on the gauge mounts more pliable to work with, so it was far less stressful getting this done. I didn't fully clean it up or chase the wiring though; that will happen once I have the car in closer possession. Another major reason to remove all that stuff is to give people less reasons to get in my car and steal s**t while it's being exported/imported when/if the time comes, which leads us to my next point... ...and that is even though it's time in Japan is technically almost up since it's a November car and the X would be coming in March, I'm still not entirely sure where my life and career is headed; I don't really know what the future looks like and where I'm going to end up. I feel there's a great deal of uncertainty with me and as a result of that, it feels like I'm at a crossroads moreso now than any point in my life thus far and there are some choices I need to make. Yes, I've had some years to consider things and prepare myself, however too much has happened in that time to maintain confidence and everything feels so up in the air; tenuous one might say. Simply put, there's just too much nonsense going on right now from multiple vectors. Admittedly, I'm struggling to stay in the game and keep my eyes on the prize. So much so in fact, that very recently I came the closest I ever have before to calling it quits outright; selling everything and moving on and not looking back. The astute among you will pick up on key subtext within this paragraph. In the meantime I've still managed to slowly acquire some final bits for the car, but it feels nice knowing there's not much left to get and I'm almost across that finish line; I have almost everything I'll ever want for my interpretation and expression on what it is I think an R34 should be. 'til later.
    • Bwagz
      Engine model

      By Bwagz · Posted

      Thanks for that, hadn’t used my brain enough to think about that. 
×
×
  • Create New...