brake specific air consumption
Thread Starter
TECH Apprentice
Joined: Nov 2016
Posts: 371
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From: switzerland
brake specific air consumption where can i find data on how much air mass is needed to produce certain hp? preferably for lsx engines.
there is a lot of info on bsfp and it could be calculated from that, but im looking for measured bsac.
there is a lot of info on bsfp and it could be calculated from that, but im looking for measured bsac.
TECH Junkie
Joined: Dec 2012
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From: Iowa
1 lb/min=10hp
That has been the rule of thumb for prettymuch any 4 stroke gasoline engine for a long time.
+/- your slush factor due to efficiency.
Camshaft, compression, valvetrain, etc etc etc all factor in...so it might be .8=10hp or it might be 1.2=10hp
That has been the rule of thumb for prettymuch any 4 stroke gasoline engine for a long time.
+/- your slush factor due to efficiency.
Camshaft, compression, valvetrain, etc etc etc all factor in...so it might be .8=10hp or it might be 1.2=10hp
TECH Senior Member
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From: Central Cal.
Quite possibly because it is not needed like measured fuel demand is. Air is inhaled as needed, while fuel is the part of the equation that needs a delivery system, and hence consumption must be calculated in order to ensure that a delivery system of adequate capacity is provided.
TECH Junkie
Joined: Dec 2012
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From: Iowa
Also...because if you want an accurate measurement for YOUR setup...you're going to need someone that had an identical setup.
If someone measured their 9:1, 325CI, stock 706 head, 218 degree cam in Denver, CO and it needed 1.1 lb/min for 10hp
That's not going to represent the same air need as your 12:1, 360ci, ported head, 240 degree cam combo in Tampa, FL
Because both engines are air pumps...just wildly different ones. They're not going to be equally efficient therefore the airmass consumption per hp isn't going to be equal.
There's a reason the ballpark figures are in place.
That 1lb/min for 10hp is accurate enough to usually estimate turbo size, get you in the right neighborhood for CFM on a cylinder head, get you into the right series carburetor (650/750/etc)
If you could give a little more detail as to why you necessarily need an exact air measurement...and what you're needing it for...you would probably get better help.
If someone measured their 9:1, 325CI, stock 706 head, 218 degree cam in Denver, CO and it needed 1.1 lb/min for 10hp
That's not going to represent the same air need as your 12:1, 360ci, ported head, 240 degree cam combo in Tampa, FL
Because both engines are air pumps...just wildly different ones. They're not going to be equally efficient therefore the airmass consumption per hp isn't going to be equal.
There's a reason the ballpark figures are in place.
That 1lb/min for 10hp is accurate enough to usually estimate turbo size, get you in the right neighborhood for CFM on a cylinder head, get you into the right series carburetor (650/750/etc)
If you could give a little more detail as to why you necessarily need an exact air measurement...and what you're needing it for...you would probably get better help.
Thread Starter
TECH Apprentice
Joined: Nov 2016
Posts: 371
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From: switzerland
yes, obviously. but it just might be, that bsac is a better parameter than bsfc and might be measured more accurately. while bsfc varies depending on afr, bsac would stay the same. and btw, bsac is independent of volumetric efficiency. no idea why we live with +/- 20% (and therefore contradicting) estimates when choosing a compressor, for example. it could also serve to validate bsfc and afr data on the dyno.
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TECH Senior Member
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From: Central Cal.
Actually, if it were so pertinent to measure BSAC, all engine builders would do it. The reason not to? Air is FREE.
You design air handling systems to be as free flowing as possible to get as much of the free stuff in as possible, while designing the engine to use as little fuel (which costs MONEY!) as possible. Fuel management is far more vital and complicated than air management.
There are many engine builders on these forums, and I can guarantee more time is spent by them on fuel management and measurement than is spent on air measurement.
By the way, fuel, being a noncompressable liquid, is far easier to measure than air, which is extremely variable in density. This is a major reason BSAC is NOT a better parameter than BSFC. Fuel properties are more constant than air properties when it comes to scientific measurement.
You design air handling systems to be as free flowing as possible to get as much of the free stuff in as possible, while designing the engine to use as little fuel (which costs MONEY!) as possible. Fuel management is far more vital and complicated than air management.
There are many engine builders on these forums, and I can guarantee more time is spent by them on fuel management and measurement than is spent on air measurement.
By the way, fuel, being a noncompressable liquid, is far easier to measure than air, which is extremely variable in density. This is a major reason BSAC is NOT a better parameter than BSFC. Fuel properties are more constant than air properties when it comes to scientific measurement.
Last edited by G Atsma; Jul 15, 2020 at 01:40 AM.
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Verdad Gallardo TECH Junkie
Joined: Dec 2012
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From: Iowa
BSAC Independent of volumetric efficiency?
What?
In addition to ALL the reasons G Atsma stated above...there is another simple reason BSFC is used... because fuel makes power.
You measure the energy per MOL of fuel. You HAVE to burn fuel to make power. Energy per litre of pump 91 vs methanol vs e85 vs c16 etc etc all vary...but we know what they are and we can run the math.
FYI...if you want to try this on your own...go for it. Educate everyone else.
It's been done for about 4 decades now in automobiles with MAF sensors. a tool that measures air mass (to the best of its ability) then uses that airmass to deliver an appropriate amount of fuel.
If you want to get 4 different engines. Lets say a 4 cyl, an inline 6, a junkyard 5.3, and a spunked up race spec 5.3...measure the airmass for each while being run on a dyno. Compare that airmass to hp.
I'm gonna doubt what you say that the BSAC would not vary.
What?
In addition to ALL the reasons G Atsma stated above...there is another simple reason BSFC is used... because fuel makes power.
You measure the energy per MOL of fuel. You HAVE to burn fuel to make power. Energy per litre of pump 91 vs methanol vs e85 vs c16 etc etc all vary...but we know what they are and we can run the math.
FYI...if you want to try this on your own...go for it. Educate everyone else.
It's been done for about 4 decades now in automobiles with MAF sensors. a tool that measures air mass (to the best of its ability) then uses that airmass to deliver an appropriate amount of fuel.
If you want to get 4 different engines. Lets say a 4 cyl, an inline 6, a junkyard 5.3, and a spunked up race spec 5.3...measure the airmass for each while being run on a dyno. Compare that airmass to hp.
I'm gonna doubt what you say that the BSAC would not vary.
Thread Starter
TECH Apprentice
Joined: Nov 2016
Posts: 371
Likes: 18
From: switzerland
see, thats my idea exactly. to have data on how bsac varies between different operating points and different engines. it would be closely related to engine efficiency, while bsfc is obscured by a/f ratio.
i was asking if such data was available and not if i should be concerned about it. thanks.
i was asking if such data was available and not if i should be concerned about it. thanks.
TECH Junkie
Joined: Dec 2012
Posts: 3,614
Likes: 1,326
From: Iowa
You got the data, you just didn't like the answer for some reason...1lb/min = ~10hp
If you want more specific than that...start measuring. But you'll be wasting your time, unless you're writing a paper for a class (in which case you're wasting your professors time) or unless you're trying to do something for SAE/EPA in which case...you should be smart enough to not come lean on LS1 tech and random internet dudes
There's the same rule of thumb for BSFC to help size injectors and fuel pumps and carburetors.
The ballpark is enough for what its practical use is...estimating which components will be best for the combination.
Past that, you're not going to need to really know either BSFC or BSAC...you'll be tuning a VE table or a MAF curve to get the engine to perform as desired.
P.s. you'll find that one of the best ways to realize BSAC for an engine will be to measure BSFC...then work the math to get to BSAC (accounting for AFR)
If you want more specific than that...start measuring. But you'll be wasting your time, unless you're writing a paper for a class (in which case you're wasting your professors time) or unless you're trying to do something for SAE/EPA in which case...you should be smart enough to not come lean on LS1 tech and random internet dudes
There's the same rule of thumb for BSFC to help size injectors and fuel pumps and carburetors.
The ballpark is enough for what its practical use is...estimating which components will be best for the combination.
Past that, you're not going to need to really know either BSFC or BSAC...you'll be tuning a VE table or a MAF curve to get the engine to perform as desired.
P.s. you'll find that one of the best ways to realize BSAC for an engine will be to measure BSFC...then work the math to get to BSAC (accounting for AFR)
TECH Senior Member
Joined: Jun 2016
Posts: 22,291
Likes: 3,616
From: Central Cal.
You got the data, you just didn't like the answer for some reason...1lb/min = ~10hp
If you want more specific than that...start measuring. But you'll be wasting your time, unless you're writing a paper for a class (in which case you're wasting your professors time) or unless you're trying to do something for SAE/EPA in which case...you should be smart enough to not come lean on LS1 tech and random internet dudes
There's the same rule of thumb for BSFC to help size injectors and fuel pumps and carburetors.
The ballpark is enough for what its practical use is...estimating which components will be best for the combination.
Past that, you're not going to need to really know either BSFC or BSAC...you'll be tuning a VE table or a MAF curve to get the engine to perform as desired.
P.s. you'll find that one of the best ways to realize BSAC for an engine will be to measure BSFC...then work the math to get to BSAC (accounting for AFR)
If you want more specific than that...start measuring. But you'll be wasting your time, unless you're writing a paper for a class (in which case you're wasting your professors time) or unless you're trying to do something for SAE/EPA in which case...you should be smart enough to not come lean on LS1 tech and random internet dudes
There's the same rule of thumb for BSFC to help size injectors and fuel pumps and carburetors.
The ballpark is enough for what its practical use is...estimating which components will be best for the combination.
Past that, you're not going to need to really know either BSFC or BSAC...you'll be tuning a VE table or a MAF curve to get the engine to perform as desired.
P.s. you'll find that one of the best ways to realize BSAC for an engine will be to measure BSFC...then work the math to get to BSAC (accounting for AFR)
GM was even kind enough to provide the effects that varying spark advance, cam position, E85 and even EQ ratio have on it. Run the old TQ*RPM/5252 and you're on your way...
Those specific numbers are an output of a response surface model, but the torque values used to generate the response surface coefficients are from thousands of hours of engine dyno effort.
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The closest thing I know of to measuring BSAC came from doing a dyno pull while logging MAF. It was in the ballpark of that 1 lb/min per 10hp quoted above. A bit lower, but since it was a chassis dyno there were drivetrain and tire losses getting in the way.
For the average DIYer that's probably as close as you're going to get to measuring it.
For the average DIYer that's probably as close as you're going to get to measuring it.