Ok are there any turbo theory experts out there that can set me straight?
I read some time ago that if, all other things constant, you flow a certain volume of air, you make a certain amount of horsepower. The corelation I have is 1 lb/min of air is about 9.5-10.5 HP at the crank. Hence if your turbo's max flow rate is 30 lbs/min, you could make anywhere from 285-315 at the crank.
Does this rule seem to hold true, or is it to vague to generalize from?
Sorry one more thing:
The air flowing from a turbo is constant per a certain boost pressure on that turbo, regardless of what engine or turbine it's attached to right?
oldskool wrote:Ok are there any turbo theory experts out there that can set me straight?
I read some time ago that if, all other things constant, you flow a certain volume of air, you make a certain amount of horsepower. The corelation I have is 1 lb/min of air is about 9.5-10.5 HP at the crank. Hence if your turbo's max flow rate is 30 lbs/min, you could make anywhere from 285-315 at the crank.
Does this rule seem to hold true, or is it to vague to generalize from?
That's a fair approximation, but it is nowhere near reality. BSFC, VE, and turbo efficiency all play a major role in the grand scheme of things. If a company says their turbocharger has a 30lb/min wheel, that's about the max you "should" get out of it safely. It is possible though to run a higher boost pressure, outside of the compressor's intended range, end up running at a lower efficiency, but still make more power. Typically, that involves spinning the compressor blade faster than you probably should.
turbobygarrett.com has good tech info with a little better approximation to the problem.
oldskool wrote:Sorry one more thing:
The air flowing from a turbo is constant per a certain boost pressure on that turbo, regardless of what engine or turbine it's attached to right?
This is far from the truth...hp is function of the airflow...and this goes back to VE, displacement, turbo efficiency, boost pressure, engine speed, exhaust manifold pressure, etc. The otto cycle engine is an already complex system...you add a turbocharger, and things get even more complicated.
I have no signiture
air flow = hp
that's where the old saying there is no replacement for displacement comes from... bigger motors are capable of flowing more air.
Turbos stick it to the man and say screw that rule... they let you flow more air than your motor is capable of flowing NA.
1lbs/min = ~10bhp... this "rule" is true to an extent. It really just depends on the motor but it is a good rule of thumb.
For example... the M45 can flow around 22.4lbs/min at 5psi. The blower has about 18.5hp parasitic loss when doing this.
If we do the math...
Flow - 22.4lbs/min x 10 = 224hp
Parasitic Loss - 18.5hp
224hp - 18.5hp = 205.5hp
Which is roughly what GM rates the factory ld9 supercharger at... it's actually 190 I think... but you get the idea... air flow will give you approximate hp values. That's only varies by like 8.15% and I think boost pressure is lower than that when you install the supercharger like 4.5psi or something... so it gives you a pretty good idea.
Whalesac wrote:
oldskool wrote:Sorry one more thing:
The air flowing from a turbo is constant per a certain boost pressure on that turbo, regardless of what engine or turbine it's attached to right?
This is far from the truth...hp is function of the airflow...and this goes back to VE, displacement, turbo efficiency, boost pressure, engine speed, exhaust manifold pressure, etc. The otto cycle engine is an already complex system...you add a turbocharger, and things get even more complicated.
I meant to say the air flowing from a given compressor is constant per certain boost. It may make more or less power on a given engine, or spool differently on a different engine, but x PSI out of turbo Y flows Z lbs/min right? I'm talking strictly about airflow, not HP. Is this still off base?
base it off of cfm really. when i had my stock motor in i turned it up to 13 psi. then took it off for the new motor. put the new motor in, left it on 13 psi and it did not like it because i didnt even think about the increased cfm and it was more like 25psi.
BuiltNBoosted wrote:base it off of cfm really. when i had my stock motor in i turned it up to 13 psi. then took it off for the new motor. put the new motor in, left it on 13 psi and it did not like it because i didnt even think about the increased cfm and it was more like 25psi.
but isn't what your talking about affecting the flow through the TURBINE which is affecting the COMPRESSOR flow. 13PSI from your compressor is still flowing x CFM or lbs/min however you want to measure it, regardless of what else is going on. That's what i'm trying to understand - picture a compressor wheel by itself...
true 13 psi is still flowing more cfm. thats why when someone asks, can i run my motor at 10 psi? well that depends on how much air you are flowing. i may be reading backwards, but the turbine is feeding the rest of the system, yeah i guess the less restrictions the easier it can flow. the more free flowing the rest of the system is the more cfm and more lbs/min.
oldskool wrote:Whalesac wrote:
oldskool wrote:Sorry one more thing:
The air flowing from a turbo is constant per a certain boost pressure on that turbo, regardless of what engine or turbine it's attached to right?
This is far from the truth...hp is function of the airflow...and this goes back to VE, displacement, turbo efficiency, boost pressure, engine speed, exhaust manifold pressure, etc. The otto cycle engine is an already complex system...you add a turbocharger, and things get even more complicated.
I meant to say the air flowing from a given compressor is constant per certain boost. It may make more or less power on a given engine, or spool differently on a different engine, but x PSI out of turbo Y flows Z lbs/min right? I'm talking strictly about airflow, not HP. Is this still off base?
Still no.
PSI and compressor rpm tell you how much air is flowing. Consequently, those two parameters also map how efficient the compressor is. Unless you have one of the newer technology turbochargers that measure angular velocity, there is no way to trully tell, and so you have to make apporximations. As was mentioned,
horsepower ~ airflow
Also Note,
Torque ~ VE
Let's take a simple example example: Let's assume you have perfectly flat torque curve (VE constant) from 3000RPM to 6000rpm and make 250ft-lbs of torque during that band.
If you spool to 10psi at 3000RPM, you will make 143 HP
If you spool to 10psi at 6000RPM, you will make 286 hp
You make twice as much power at the at the 6000RPM engine speed because you are filling the cylinder with the same mass of air (VE) as you did at 3000RPM, except you are filling the cylinder twice as fast, so for a given period of time, you are moving twice as much air,
So, if it takes 30 lbs/m of air to flow at 6000rpm, then at 3000rpm your engine (and compressor) will only be moving 15 lbs/m.
Does that help at all?
I have no signiture
also many people get confused with air flow and psi
pressure and air flow are not the same
for example: (using water)
If you have a garden hose that is at 10 psi and a fire hose that is at 10 psi the amount of flow between the two is very different. Being that the fire hose will be flowing way more water.
The same is true for turbos. A t3/t4 60 trim at 10 psi will flow way more air than a t25 at 10 psi.
Just thought I would clear that up a little as people who are not familiar with turbos always refer to everything in psi which basically means nothing unless you specify the turbo that is being used.
i didn't write this but read it in an article i was researching.
As a rule of thumb figure a 12.5 bhp increase for every pound of boost increase.
Why do I say 12.5? The answer is simple. Some superchargered systems are more efficient than others. Whether it be the actual blower or turbo that is being used or if the speed at which the blower is being spun. 1 pound of boost increase can range anywhere from 10 to 15 hp. The best thing to do is take the average to get the best round about estimation. So if your planning on going from a 3.6 pulley to a 3.3 pulley and expect a 2.5 psi increase. Take the 2.5 psi and multiply it by 12.5. 31.25 bhp increase would be a safe estimation of increased power. This is important to know because now you can figure out if your fuel system can support the extra 31.25 bhp without any modifications.
kevo1586 . wrote:also many people get confused with air flow and psi
pressure and air flow are not the same
for example: (using water)
If you have a garden hose that is at 10 psi and a fire hose that is at 10 psi the amount of flow between the two is very different. Being that the fire hose will be flowing way more water.
The same is true for turbos. A t3/t4 60 trim at 10 psi will flow way more air than a t25 at 10 psi.
Just thought I would clear that up a little as people who are not familiar with turbos always refer to everything in psi which basically means nothing unless you specify the turbo that is being used.
What you're talking about is analogous to two different sized compressor wheels, for which if you look at his post, he certainly wouldn't disagree.
oldskool wrote:I meant to say the air flowing from a given compressor is constant per certain boost. It may make more or less power on a given engine, or spool differently on a different engine, but x PSI out of turbo Y flows Z lbs/min right? I'm talking strictly about airflow, not HP. Is this still off base?
I think Ryan is stuck on the concept of how a given compressor wheel can flow different amounts of air for a given pressure. The answer is pretty simple though...you spin the compressor wheel faster.
I think his concept of air flow is probably a common misconception by many. People get caught up in single value benchmarks (i.e. max horsepower, max torque, 30lb/min wheel, etc.) that it makes it hard to comprehend the more import parameter, which is the complete engine response, and especially supercharged (turbo and/or belt driven) engine responses. I mean this by no offense to you Ryan...sorry if it did.
My advice, Ryan, read the turbobygarrett pages 5+ times, read (or reread) your copy of "Maximum Boost" or "Street Turbocharging" and get comfortable with how to read a turbo map. If it helps, they're technically 3D plots (imagine being in a helicopter and looking down at a mountain... compressor efficiency = elevation of mountain at different points).
I have no signiture
Ah, i wasn't even thinking about compressor wheel speed.
Ok i guess this is what a compressor map gets at, but if you plotted airflow vs compressor RPM and kept the boost pressure constant, the plot would increase to a peak value, and then taper off? Or would it remain constant and just produce hotter air?
That's correct. For a given pressure ratio, the plot would look something like this...
While this graph isn't exact (it's a simple y=3*ln(x) plot), you can see that as you spin the wheel faster, there is a point of diminishing returns.
I have no signiture
I have an example from experience..I went from a smaller t3 super 60 to a to4b style turbocharger. The turbine side was all the same, but the compressor housing and wheel were MUCH larger. I had a 7.35lb spring in my wastegate, but due to the increased air flow the pressure would build up to about 9-10lbs at full boost.
Don't forget your elevation base psi adjustments.
14.7 psi at sea level
and the higher you go the less base PSI you have
like around here its 5000 above sea level and we have
about 12.7 PSI.
So adding that into your equations could change your graph plots from 1.96 as far over as 2.16 on most T3 style .60 ar trim units
and GM LN2 and LD9 motors the poor head flow gives you a 60-70 VE most of the time.
So you 1lbs per 10HP is more like 7HP/per Lbs
I used Garrett's website when I need to do the math and when I was working out the details on a T-25 trim for a guy:
The math came out the 22Lbs of air at a air/fuel ratio of 12:1 running 13lbs of boost made about 205 HP at 5000RPMs.
Anything over the 13lbs of boost took the engine outside the turbos effecincy range
I did another for a giant T04 ar.60 trim that I bought for my car and it was just funny on the graph:
at the same lbs/ a/f ratio numbers this massive turbos ar.96 exhaust trim needed 5000RPM to make 8lbs of boost for the same
205-210HP target, but anything under 3kRPM's and the turbo made only about 1.5 lbs. so as you can see the lag and spool time
made this really bad for street but with alot of mods it would be easy make well over 300HP with the Rev limiter over 7000RPMs.
I was thinking that if I dropped this beast under the hood I would only make big boost under heavy throttle and save a little mpg off the boost.
But I spoke with a fe guys that work on S10s for drag racing and they all told me that with the setup I would never be able to control the
amount of power that would come on that late in the power band and I would most likely break either a axel or a destory my valves and springs.
So I'm selling the T04 and looking for a T3 .60 trim my math tells me that I would spool up as fast as 2800PRMs and top off at 14 psi to make my target 220HP.
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