Boost pressure vs air flow vs power
 

For the longest time ever I didn't quite understand why different turbos at same boost pressure produce different power.

Well, they don't. As long as engine restriction (and hence volumetric efficiency) doesn't change, same boost will make same power.

Boost pressure is a function of air flow vs restriction of the engine's intake path. The more restriction, the higher the pressure differential (boost pressure) needs to be to overcome the restriction.

The turbo's compressor is part of the intake path, and it has limits too. A T25, for example, maxes out at about 25lb/min. So if you put a T25 turbo on a V8 that flows 500cfm (~35lb/min), then you're effectively putting a fancy restrictor plate, since a T25 only flows 25lb/min.

Another simplistic example:
You have an engine/intake/turbo that can flow 30lb/min @ 20psi. If you port your head to flow better, you can now flow 30lb/min @ 16psi. If your turbo can flow 30lb/min @ 16psi, then you will make same power at 16psi as you could at 20 before.


So to recap:
The amount of power you make at a given PSI is the intersection point of your engine's intake restriction and your turbo's compressor map.

A really big turbo will not make 300whp @ 10psi unless your valves are size of hustler's anus.

Swapping a bigger turbo will make the intake path less restrictive so you will make a bit more power at same psi. Head porting will make the intake path less restrictive so you will, again, make more power on same psi.

A bigger turbo DOES NOT make more power at same psi simply because it "flows more". It flows more, if the restriction is less. If the restriction doesn't change, a bigger turbo will not make more power

not sure if trolling

I'm hoping it's copypasta from Miata.net.

I'm terrible at explaining my thoughts but no, not trolling.

bigger turbo doesn't automagically mean you make more power at same boost. A little bit, yes, because the turbo itself is less restrictive, but not 3x power at same boost.

Depends on the motor, and depends on the turbo, but possibru.

As long as we're doing copypasta:

In very basic terms, a "bigger" turbo will move more airflow with less heat generated during the compression stage. This is due to the compressor wheel being sized to move the appropriate amount of air within the "happy" compressor RPM range.

And, more importantly, once this air is combusted and turned into exhaust, a turbo with a "bigger" turbine wheel will spin at an appropriate RPM and will induce less power-robbing exhaust backpressure. The less backpressure on the exhaust side, the more air that can be pumped through the engine at any given boost pressure.

So you add up the effects of a cooler air charge with less back pressure and you can see why different turbos can make different power at different power levels.

It also explains why comparing turbos by comparing only compressor maps is meaningless. The turbine size/configuration is arguably more important.

Notice I put "bigger" in quotes because bigger isn't always better when talking about turbo tech/aero.

I agree and all I'm saying is unless you change the engine, the extra airflow will result in higher boost pressure - because boost pressure is a function of the engine's restriction.

That doesn't make sense either. When you put it like that, you make it sound like a GT40R would be forced to make more pressure than a 2554R simply because it's capable of more flow. This negates the point you made in your first post, which was negated immediately after by the 2nd point you made in your first post.

Thats exactly what I mean. Don't see how the original post negates anything. Want to quote the exact parts?

Sure, but it's not correct. It's CAPABLE of more pressure, but it's not being FORCED to create more pressure.


I missed a disclaimer in the first statement i was looking at. What's weird to me is that you're making a few true statements, then throwing a somewhat strange conclusion out of it.

Sky is blue, grass is green, therefor the moon is made of Swiss.

is that why your big turbo makes so little power?

No, its because I stuff 30psi of boost in a bone stock head @ 25* timing.

This is pretty much accurate I think.

Too often you hear claims that 5 psi on a 28r is "a lot" less power than 5 psi on a 35r, or something along those lines. Its just not true.
It comes from the understanding that at higher boost levels a larger turbo will make more power than a smaller turbo that is getting inefficient and has a small exhaust housing. This knowledge gets turned into a blanket statement thats used to explain to noobs that theyre stupid for asking "how many boosts psi can i haz on my stock native american?"

I don't think he is saying the opposite, just stating it the other way around.

He's not saying it the other way around, just backwards.

I have been thinking the same thing that Soviet is saying, IF we are talking about the MAP. If talking about the boost at the turbine outlet, then truely, there is no correlation between boost and power as there are so many restriction variables involved. But, in the manifold, there is pretty much just valves, turbine, and exhaust.

Yes, reversed cause and effect.

...didn't we already established in this thread that pressure is a measure of restriction? Oh, wait no.

The size of the valves being the same, the larger turbo WILL make more power per psi than the smaller.

If we are saying that a T25 can only push 25 lb/min of air at 30psi, but your large turbo can push 50 lb/min of air at 30psi;

then at 30psi, your turbo is compressing a lot more air, into the same space, than a t25 would. So at the same pressure, you have almost twice the available oxygen to burn and that right there should equal tons more torque.

The valves, rpms, cams only determine the mass rate of air--you're always displacing the same amount. But it's your turbo and the amount of air it compress that determines how many air molecules you can actually stuff into the cylinder at this rate.

All boost pressure is telling you is that you're stacking up a shit ton of air behind the valves/piston, but it cannot tell you how much air you're actually compressing; and it's not a function of the engine's restrictions. That would like be saying anytime you add pressure to a sealed vessel it's infinite pressure because the vessel has no openings.

https://www.miataturbo.net/attachmen...ine=1402341029

Theoretically, I think it would be like this for 2 different sized turbos at the same boost with similar efficiency at that boost level.

If the dyno were stopped at 6k rpm, the smaller turbo never becomes a restriction, and they make the same amount of power on the same amount of boost. But when the flow starts to move the small turbo off its efficiency island and its turbine housing becomes restrictive (6k+ rpm in this graph), the bigger turbo is making more power comparatively.
At 8k rpm, there is the same amount of boost in the IM, but the BMEP is lower for the smaller turbo because the air temps are higher and there is less exhaust scavenging due to high pressures in the exhaust manifold.

yes, heat efficiency is a big factor. i was trying to leave it out of it. obviously, the more colderer the charge, the more airer the boost. do not leave the island.

Above is also why I always say get the smallest turbo for the jarb.

At 30psi a turbo can flow a range of volume, in case of EFR6758, anywhere between 30 and 53lb/min
https://www.miataturbo.net/attachmen...1&d=1402341382

The actual volume it will flow depends on the restriction.

What i'd like to know is if there's actually a pressure point in which turbos become unhappy to the point that they consider and/or carry out suicide?

And yes, i need this data for a practical application.

I believe that point would be when you are spinning the turbo too high, which is going to be a combination of boost and flow.

Well... i'm talking more like... take a BP, then cut the flow in half. In Soviet's case, i'd be spinning his same turbo to the point that it would be making 60psi and still only 460whp. (Bullshit maths, but you get the idea)

Is this bad? Would it actually be spinning too high? Or would it spin the same, just with more pressure because shitty motor?

I just want to know how to know at what pressure amount a turbo will become asploded.

Look at the compressor map

https://www.miataturbo.net/attachmen...ine=1402343057

You may need compound turbos to get that kind of boost is something like a GTX cant do it.
Although, the turbo may survive going spinning above where its supposed to. I know DSM guys will push their little stock turbos way off of the compressor map. They like to set turbo records.

But what makes it asplode, exactly? Would pressure alone do it?

I just need a turbo that will handle 60psi or so. Nothing crazy.

Turbo speed. Thats what those thick lines are with the numbers at the far right end.

If you take a 2560 beyond 143405 rpm, Garrett says it will die.

Nooooooooo . Damn backspace killed my description. I never use backspace to go back on my browser. I hate whoever invented that.

So anyway.. since I wrote a pretty technical section on this bare with me as i use the wrong words like bigger and smaller and whatever to describe turbos and things.

Basically the gist is. Smaller turbos spool quicker right? When they spool quicker it causes more heat. If gas is held at a constant pressure then it will expand to fill a larger volume. Since a smaller turbo will spool quicker it will constantly be applying more pressure to the molecules. Larger turbo will have more dense air.

Think about someone boosting in Colorado or in Florida. Which of the two states gets more power out of the same amount of boosties?

I understand speed, but speed doesn't have a direct relationship with pressure by itself without bringing a motor (restriction) into play.

If the 2560 is on a big block, it might only make 5psi before it hits 143406rpm and dies. If it's on an F2T, it might do almost 30psi before it hits 143406rpm and dies.

If the motor is shitty enough, you could go off the pressure end of the chart while staying in the "safe" zone in terms of RPM. Is this still bad, then? (And yes, i realize that we'd likely be walking on the nasty side of the surge line.)

Following on Soviet's map post:

It's not what a turbo can do, but what is the operating point. The question is, at the flow required at a given engine RPM and torque, will the large and small turbo end up operating at about the same pressure and flow. I think they will, less a little for efficiency, and a little for IC and charge piping. The latter variables go away if we look at MAP only.

Comparing (2) maps.
2554 @ 25 lbs/min and 2.0 P/R 69% efficiency
2871 @ 25 lbs/min and 2.0 P/R 75% efficiency.

So the 2871 is 75/69 = 9% more efficient so it has that much advantage on the compressor side. This does assume equal pressure drops in the IC and other intake areas.

Err i re-read that. I don't think it would matter actually. If the car can't flow enough to over spool then it just gets hot and runs poorly... right?

That sounds awesome.

"Dear BW,

I'm writing to you to let you know that i've found a new and interesting way for your turbos to fail. I've been using an EFR with the turbo RPM logged so i know that it was not the RPMs that caused this particular example to fail.

I've attached a log showing what happens when you remove the wastegate from one of your EFRs and weld chains around your motor to keep the head attached to the block.

Yours Truly,

The Swanky One"

I edited it. I don't think It would get to that point. Since it can't flow you'd just end up with really hot exhaust gasses getting trapped and maybe some det? I don't think it's anything any of us will ever run into. Would be a humorous conversation though.

I'm not sure. All i know is that i've run a Toyota CT26 to 25psi+, and this turbo is known to be pretty much put to death at 15psi eventually on the motor it came on originally. IATs were still well within "good" range.

Given the power goals i'm looking at and how little this motor flows (double digits on the bench awwwwwwww yiss) pressure may very well be something i need to look at, since the acceptable answer isn't "oversize the shit out of your turbo."

I am in no way a turbo engineer but I'd imagine that the factors that determine the life of the wheel would be things like lubrication and heat. If it can't be cooled properly it's most likely going to suffer some heat failure and warp or whatever... causing some sort of eventual or catastrophic failure.

I'm glad that this thread makes sense to some people. I'm not crazy, I guess.

My turbine wheel sheared off the shaft while mostly intact, then rattled in the downpipe. The force was not enough to go through the downpipe, unlike in some cases

https://www.miataturbo.net/attachmen...ine=1402344908

https://www.miataturbo.net/attachmen...ine=1402344908

That's kind of awesome. What were the circumstances?

Not awesome from costing $$$ standpoint but intriguing.

Wow, I thought your turbine wheel failures were on Borg turbos. Did you have failures on Garretts and EFR's?

Actually... it looks like once you get into manly turbos, my question becomes a non-issue, since the pressure ratio scale goes square into "much testosterone" realm anyways.

https://www.miataturbo.net/attachmen...ine=1402346239

thats not mine, thats a diesel drift mercedes wagon from finland

My boost gauge. True story.

challengeaccepted.jpg

https://www.miataturbo.net/attachmen...ine=1402346771

BLOW'ER

Pretty sure i got it from a parted out M45 car.

WHAT DOES THE FUCKING COMPRESSOR MAP SAY? It has the turbo speed lines on it, go too much past the turbo's redline in any direction and kiss your shaft and/or bearings goodbye. :vash:

Just to clarify... you're saying that it doesn't matter if the turbo is pushing against a brick wall, it's going to take that S366 exactly 117119rpm to make 55psi?

And yes, i know that PSI doesn't matter, i know the bigger picture is RPM. I'm just wondering if there IS an actual pressure limit to turbos. That's all.

If its pushing against a brick wall you're going to have compressor surge and destroy the thrust bearing faster than over reving it.

These are the sort of answers i'm looking for.

So basically: Going to the left of the entire map = surge = hilarious noises = turbo sadness.

Those few are just crazy like you, that's all.
You're wrong, very very wrong.
Either that or I'm completely misunderstanding you or you're explaining it wrong.

Among other reasons, I'll give you a practical one from my experience: I remove a td04 from a subaru, and bolt in a vf39. Both turbos have the same inlet and outlet size on both compressor and hotside. The 39 just has slightly bigger wheels. I run 10psi on both. Literally all other variables are identical. With the 39 car picks up at least 30-40hp

Please don't go full retard and go down the path of that one moron n00b trying to explain to us that you can't "overfill the cylinders", cause it seriously looks like you're headed that way.

By your logic I can remove a gt2554 from a miata, bolt up a 3076r and as long as I don't exceed 10psi on both, they will both make 180hp.
That's not even close to true

Yes. And going off the top of the map without going off to either side is normally really hard to do, because thermodynamics and time and shit. But way over speeding it by going up and right isnt too hard, but there's no power there and most IWGs wont let you get over there anyways so again, it doesnt happen often. Because once you're over there EMP goes through the mother fucking roof.

I can't really Fathom any motor having a pressure ratio of say 2ish and flowing less than 5 lbs/min... You would have to be a complete retard to pair a turbo that badly. I'm pretty sure like 99% of turbo failure from going outside that range is not from being on the left side.

Haha, my thoughts exactly. I would like to add that I'm not sure you can fail a turbo by boost pressure alone. Flow wise, the turbo would end up being destroyed by other things first. Keep in mind, taking most compressors into the 60psi range(Pressure Ratio > 5) will either be far into the turbo killing surge region, or wayyy beyond the designed maximum turbo speed. Either way, it's not going to be pretty.

I don't mean to come across as rude, but this is nowhere close to reality.

The small turbo in and of itself will not cause more heat because of better spool. In theory, a small turbo could spool very quickly into its highest efficiency island, and therefore produce less heat.

Heat comes from inefficiency in the compressor. If a compressor was 100% efficient, all of the energy would go to the compression of the gas, and no energy would be lost due to heat.

I'm also confused at what you are referring to when you are talking about the smaller turbo constantly applying more force to the molecules. Your mixing and matching terms and principles from fluid dynamics, thermodynamics, and chemistry. Not in a good way, either.

beat me to it. well said.

The best video of turbo destroying spool up surge you can find on the internet is an RB26 with a GT35R on it, I think they're doing terrible things to it with a mustang dyno though to make it make the sound. I've also heard of people pushing 16gs on 2.5 subarus across the surge line on cold days.

It's rhetorical when we're talking extremes like that. And it would surge first.

In most cases boost pressure is a function of the exhaust path and not the intake path.
In regular turbo set ups the significant flow restriction in the system will be the hotside wheel. The exhaust manifold will be see about the same pressure as the throttle body and the exhaust gases are pumped out of the engine by the crank. If a larger hotside wheel is fitted then it will allow more flow to pass through it for a given boost level.

If you have a huge turbo or a really restrictive intake then boost pressure becomes a function of the engines intake path.

I did a bad job of breaking it down and I jumped all over the place.

Really in frustration from me losing my bit i typed about about turbo efficiency and temperature.

If you take your in temp at say 100degree cause you live in southern arizona and apply some vacuum lets say -1 then some discharge pressure at say 15 and your turbo is 75% efficient you will end up with....

~268 degree discharge temp. Lets say your motor is PERFECTLY paired and you have a sweet spot that your turbo is 100% efficient it would be running at ~226 with the same environment. That's a 50degree difference in temp. That's a HUGE difference in air density.

So as you can see the efficiency of the turbo plays a large roll in the intake temps. Now the amount of air the motor can flow also plays a nice roll in here. Based on your restrictions and motor size. The amount of air you can flow is the volumetric efficiency.

Because turbo compressor maps are simply the compressor maps they don't take into account the turbine side. Having the right turbine can change the efficiency of the compressor as well.

Edit: Anyway the idea is it isn't always about psi. Based on many variables you could get more power with a "larger" setup (same psi as "smaller" turbo).

Yep, I totally agree with what you said in this post. In general, you will probably make more power with the larger turbo, even if the boost is the same. I was just making sure that post was qualified. You guys reply so fast, my last post was probably unneeded due to what Leafy said.

Great discussion starter though. Soviet and 18psi you both have valid points, but your just looking at it from different perspectives.

So I guess nobody really defined efficiency. What makes a turbo efficient? Anybody have a good definition or explanation on turbo efficiency calculation?

Thread too long, didnt read.

Turbos pose a restriction on both the intake and exhaust side. Also bigger wheels move air easier.

End of story.

Dann