compound numbers
- Thread starter B18B1LS1
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At 70psig the overall PR is ~5.76:1. With 25psig from the first stage you have a first stage PR of ~2.7:1.
So 5.76/2.71 = 2.13 or an equivalent second stage boost of ~16.6psig my man.
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Got it. Unit-less numbers can be difficult to decipher.
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B18B1LS1
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At 70psig the overall PR is ~5.76:1. With 25psig from the first stage you have a first stage PR of ~2.7:1.
So 5.76/2.71 = 2.13 or an equivalent second stage boost of ~16.6psig my man.
Charles,
Had a question for you that I hope is not too much of a hijack,
I pulled up two compressor maps off of Bullseye' s sight that are probably close to what I'm thinking of using to make the same sort of pressures (roughly 70# boost).
Using a S476 primary and S258 secondary the PR numbers look great. The PRs are right in the sweet spot of both maps (If anything a little higher PR would be good on the secondary). Looking at the LBS/Min of air moved. it's roughly 70# for the primary stage (2.71 PR) and 40# for the secondary stage (2.13 PR)
Are the LBS/Min for 2 compounded chargers roughly additive? I have seen calculations where people started with desired horsepower and could make charger choices based on the amount of air required.
Thanks
You bring up a VERY good point. Something I have thought about myself a good bit, especially when I was initially sizing my chargers as you seem to be doing now. Ask yourself this.... how can you have 70lbs/min coming down a pipe and then only have 40lbs/min going down that same pipe after it goes through another compressor? Where did the other 30lbs of air physically go? Are we to believe that over 360 cubic feet of air literally went poof and dissapeared? 30 lbs of substance went goodbye? Anyone familiar with the equation E = MC^2 can attest to the fact that we're talking about a LOOTTTA energy if that kind of mass just up and dipped out.
No matter how you slice it.... the second stage is moving every bit as much mass flow as the first stage. No way around that, period. Unless your intercooler is a nuclear reactor anyway... This is because mass flow is always constant at all parts of the system from first stage inducer right to the intake manifold inlet (assuming no additional mass is added via water injection or similar). What is not constant throughout is Volume flow.
Which leads to the inevitable posting of a somewhat arrogant remark on my part...
The compressor maps are wrong.
Although it is preached at you from all angles that mass flow (most often in lbs/min) is the correct measure of compressor outflow, as you very well see for yourself, it's simply incorrect.
Here's my rationale:
The compressor moves an amount of air per rev @ ___rpm largely based on it's own size (holding blade design/shape constant). This holds true so well, that we even have competition classifications based solely on the inducer size of the compressor (2.6, 2.8, 3.0...). This is because a given compressor can only move as much air as it can physically "grab". For any given shaft speed I can't see any compelling reasons to believe that the wheel cares what the air density is. Now as density increases I could see that additional turbine power may be required to accelerate this denser media, but I don't see the wheel itself caring in terms of efficiency, or in terms of shaft speed required for a given volume flow.
The problem occurs when you notice that while your second stage map might stop abruptly at say 50 or so lbs/min, you may very well be pulling in 100+lbs/min across your first stage compressor. Being that mass flow is maintained (else physical laws be damned) you now have a serious conflict here.
In my mind, the solution is simple. This whole "mass flow is correct" deal, and the practice of producing compressor maps in units of mass flow is simply incorrect.
The first thing to do when you see a compressor map is to immediately convert it from mass flow to volume flow. Good old CFM units will prove very practical, and highly useful. I cannot say the same for mass flow units.
I can't explain how lbs/min is correct, when my GTP38R is moving around twice the maximum mass flow listed on it's map each and every day sitting just downstream of the GT47.
However..... you will find that the volume flow rates are SPOT ON! As this is what the wheel is actually doing. Grabbing a given volume of air per rev @ ____ shaft rpm.
That's all I've got.
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TheBigNasty
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So basically you get the mass flow of the large turbo but with a pr it would never produce.
Yes
This is why IMO, so many people fall short on sizing the first stage because they think the second stage will somehow supplement the first. When the truth is, you're not going to be moving any more air than you've got first stage compressor to support.
And to go along with that, some people seem to want to attribute ____hp to ___ turbocharger, completely independently of whether or not it's being utilized as a single or as a first stage in a compound setup. When they do this, they are oblivious to the fact that while any given charger might make ____hp as a single (at a X:1 PR), that same charger may be completely off the map at the exact same compressor flow, when operated at a PR of 1/3 to 1/2X:1 as will likely be the case when run as one stage in a multistage setup.
It takes a bigger wheel to make ____hp at 1/3 to 1/2 the PR. I would advise anyone that thinks otherwise to go check out your nearest compressor map, run to a spot at a PR of ~4:1 and a spot near the right of the map as would be the case for a common single charger, then at the same flow rate (power potential) drop down the map to ~2:1 and see what kind of efficiency you're at...... if you're even on the map anymore.
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Big Blue24
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I think mass flow is a great way to map turbo compressors. When calculating compounded compressors, it seems second stage compressor's map would need to be adjusted with some sort of a correction factor to reflect the higher in density air at it's inlet.
I do, however, agree that the total airflow capability of a twin compound turbo setup is almost 100% dependent on the first stage compressor's flow.
It seems that the only way the second stage could affect total output would be based on how much restriction it creates therefore raising or lowering the pressure ratio of the first stage putting into a different efficiency range.
For example, a twin setup with hx30 over S475 might make 60psi total boost with 40 psi coming from the S475 and remainder from hx30 stage. The S475 would have to blow through the small hx30 turbo which would increase pressure ratio with little additional mass flow. The higher pressure ratio or boost would come from heat from working at a less efficient part of the map.
Next consider a twin setup with S366 over S475 making 60psi total boost with 25 psi coming from the S475 and remainder from S366. This set would make a little more airflow because S475 would be working more efficiently at 25psi because the S366 would have more suction instead of creating a bottleneck. The S366 would not increase mass flow, it would just efficiently increase the pressure ratio so the 60psi boost would be at lower temperature and therefore have more mass of air molecules.
I do, however, agree that the total airflow capability of a twin compound turbo setup is almost 100% dependent on the first stage compressor's flow.
It seems that the only way the second stage could affect total output would be based on how much restriction it creates therefore raising or lowering the pressure ratio of the first stage putting into a different efficiency range.
For example, a twin setup with hx30 over S475 might make 60psi total boost with 40 psi coming from the S475 and remainder from hx30 stage. The S475 would have to blow through the small hx30 turbo which would increase pressure ratio with little additional mass flow. The higher pressure ratio or boost would come from heat from working at a less efficient part of the map.
Next consider a twin setup with S366 over S475 making 60psi total boost with 25 psi coming from the S475 and remainder from S366. This set would make a little more airflow because S475 would be working more efficiently at 25psi because the S366 would have more suction instead of creating a bottleneck. The S366 would not increase mass flow, it would just efficiently increase the pressure ratio so the 60psi boost would be at lower temperature and therefore have more mass of air molecules.
I just want to ask why? Why would you want to use an incorrect unit of measure and then make up some kind of correction factor when volume flow is right on the money, and truthfully what the wheel/shaft speed is determining in the first place? The mass flow numbers are a "corrected" value set to begin with based on actual outlet flow at some given atmo condition. Possibly at STP. Whereas, CFM is CFM, at any temp/pressure.
What is appealing about measuring a compressor's outlet flow in lbs/min? I'm not attacking your thoughts, I'm just curious what appeals to you about it, and how it helps you with your calculations because I have yet to find a use for mass flow values.
TURBOLVR
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How do you determine exact compressor compatibility in a compound set-up?
In other words, how much air can flow through how small of a hole? Can you run a 488 with a HY30? or conversely a 465 under a 366? Whats the best formula for determining good compound sizing?
In other words, how much air can flow through how small of a hole? Can you run a 488 with a HY30? or conversely a 465 under a 366? Whats the best formula for determining good compound sizing?
First of all....
While a first stage compressor might draw in, in excess of 1500CFM, at the same time it may only have 700CFM leaving on it's discharge. Pressure ratio and compressor efficiency determine the extent to which the outlet flow will be reduced.
And always.... keep in mind the fact that it is a compressor. Obviously if it compresses the air, the volume on the outlet must be less than that of the inlet. Hence the volume flow rate will also be less on the outlet than on the inlet.
In just the same way, volume flow entering an intercooler will be higher than the volume flow leaving. This has nothing to do with restriction, it has to do with compression. In the case of an intercooler, thermally driven compression.
And lastly....
Say you have a 5.9L engine, and we'll say that's 360 cubic inches exactly. Lets say it's turning 3000rpm, and it's ranking a Volumetric Efficiency of 90%.
360 CI @ 3000rpm equates to 312.5 CFM. 312.5 CFM x 90% (VE) yields an actual intake flow of 281.25 CFM.
So no matter what your turbochargers do in terms of air pressure, the actual flow through that engine will never be anything other than 281.25 CFM at that rpm as long as things like valving and sealing remain constants.
This should make it clear now that in that case, the 700 CFM outlet of the first stage would then become a 700 CFM flow inlet of the second stage... Then, the second stage might have say 300 CFM on it's outlet, which goes into the intercooler, and you would then have the 281.25 CFM leaving the cooler and heading toward the engine.... which is displacing the same old 281.25 CFM it always was, turbo or not.
The only thing the turbocharger(s) and intercooler(s) do is determine how much each of those cubic feet of air weigh.
The whole point of that is just to show that volume flow is not constant throughout the system. Mass flow is. Which is precisely why a compressor map utilizing units of mass flow isn't giving you an honest picture of what that wheel really cares about.
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May I ask then how you plan to size two turbochargers without knowing the flow rate of air entering the engine, nor the flow rate from one charger to the next or anywhere else?
Secondly....
How do you plan to determine how two compressors will match up without comparing the flow demands of each to their respective compressor maps?
You just plan on using calipers to determine compressor flow?
Help me out. I tried to give a good, detailed explanation and you're killing me here.
Charles, what kinda of power are you making, and what do your egts look like? Trying to build a 600 plus hp tow rig and would like to have your opinion on charger size. I don't want to end up with two turbos litteraly fighting each other. Harmony is what i like lol.