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Calculating Air Flow

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Calculating Air Flow

12-21-2018, 08:07 PM
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Calculating Air Flow

I've been told the amount of airlfow required for a given horsepower is a 1.5x factor. More plainly, each horsepower needs 1.5CFM throughput. How was this standard determined? Does this guideline assume a given Volumetric Efficiency (VE) and if so, what is that number? I ask this because I would assume an engine with a higher VE would move less air for the same horsepower. Also, as a general rule, do smaller engines have a higher VE?
12-22-2018, 08:19 PM
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A higher VE engine and a lower VE engine move the exact same amount of air for the same horsepower.

A 346 making 500 hp has higher VE vs a 427 making 500 hp.

If a 346 makes 500 hp, the 427 is going to make about 620 horsepower given identical VE.

VE has more to do with the intake, heads, and cam than the bottom end.

It is generally harder to achieve higher VE on larger engines than smaller engines, since the heads do not always keep up with displacement. Typically this is countered by increasing cam duration with displacement.
12-23-2018, 04:07 AM
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Originally Posted by Darth_V8r
A higher VE engine and a lower VE engine move the exact same amount of air for the same horsepower.
I think I understand. The 346 and 427 are moving the same air at a given HP and that means the 1.5CFM/HP is more or less a constant. Is there anything we can do to bend that constant so less air is required for more HP? I've noted the LCV and LKW Ecotecs have high VE, what is stopping them from being used as performance platforms given their light weight? A ~300lb. four with a turbo that makes a long flat 400ft/lb torque and ~450HP seems more doable and desirable in a Miata than stuffing in an LS. Do the Ecotecs share the same footprint and mounting as an LS?

Last edited by yldouright; 12-23-2018 at 04:32 AM.
12-23-2018, 11:58 AM
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The Ecotec is a 4-cylinder engine around 2-2.2 liters, usually mounted in a front drive car. It and the LS engines have nothing in common except for being gas-burning internal combustion engines.

Last edited by G Atsma; 12-23-2018 at 06:34 PM.
12-23-2018, 12:06 PM
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Reduce friction and parasitic losses, increase combustion efficiency. Then you will make more power with less airflow.
12-23-2018, 12:47 PM
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Originally Posted by KCS
Reduce friction and parasitic losses, increase combustion efficiency. Then you will make more power with less airflow.
Things like low tension rings...?

I can think of stuff inside the motor like ceramic bearings, micro polishing the crank journals, cam journals.

Beyond that, micropolish the rear, transmission, and throw in a CF driveshaft, superlight rotors, super skinny tires with hard compound. You get the idea.

Maybe an aluminum truck bed?
12-23-2018, 12:50 PM
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I'm not sure this guy knows what he's talking about. Asked if LS and Ecotec shared the same footprint and mounting??
12-23-2018, 04:16 PM
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@G Atsma
I never said I was an expert, just a guy with lots of questions The LKW and LCV Ecotecs are closer to 2.5L and put out ~200HP NA. I didn't know these were front wheel drive. If they are, please disregard all those related questions.

KCS
Do any of the friction reduction methods described above reduce the 1.5CFM/HP constant? How much lower can this ratio go in your experience?
12-23-2018, 06:33 PM
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Originally Posted by yldouright
@G Atsma
I never said I was an expert, just a guy with lots of questions The LKW and LCV Ecotecs are closer to 2.5L and put out ~200HP NA. I didn't know these were front wheel drive. If they are, please disregard all those related questions.
FYI, Ecotecs vary from 2.0 to 2.5L, with NAHP varying from 145 to ~200, as you say, with the greater majority going into front drive cars, as most are these days. Google can be your friend too.
12-23-2018, 09:03 PM
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Originally Posted by G Atsma
I'm asking here because there was no information on the topic doing an internet search. Google is not anyone's friend but if you had done a search before inferring my questions about two specific Ecotecs that weren't FWD as FWD and idiotic, you probably wouldn't have thrown that shade my way. I use DuckDuckGo, everyone should
12-23-2018, 09:08 PM
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I NEVER inferred any question as idiotic, and never would. You read that into it. I spoke of Ecotecs in general, and did not even know the two you referred to were RWD. My point was a little homework would have avoided this whole fiasco. Google WOULD have helped you with all we discussed here. Never heard of DuckDuckGo....
12-24-2018, 02:42 AM
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Originally Posted by yldouright
I've noted the LCV and LKW Ecotecs have high VE, what is stopping them from being used as performance platforms given their light weight?
If you use a turbocharger to get high power from a small displacement engine, your powerband typically gets narrower. My Subaru for example made 377whp that last time it was on a dyno, but it needs to be over 4000 RPM to feel like a 377whp car. If you're driving around at 2500 RPM and you stomp on the gas, it feels like maybe half that much power. It's a lot of fun if you keep the rev up, but not many people are happy with that compromise.

With a bigger turbo it might make 500whp, but then you'd have to keep it over 5000 RPM to really enjoy it. I enjoy the car a lot as it is, but a powerband that starts at 5000 RPM sounds kind of annoying even to me. Then again, if you're focused on drag racing, performance under the top couple thousand RPM does matter, so it's reasonable to go even bigger, build the engine to rev to 9000, get a turbo that wakes up at 7000, etc, etc. There are some fast four-bangers out there.

Also, turbocharging means extra complexity for the exhaust and intake plumbing, and an intercooler (usually), all of which means more stuff to go wrong, and a more crowded engine bay.

There are lots of different ways to make power, they all have their pros and cons.
12-24-2018, 02:44 AM
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Originally Posted by yldouright
Is there anything we can do to bend that constant so less air is required for more HP?
Different fuels - race gas, E85, methanol..

Originally Posted by G Atsma
Never heard of DuckDuckGo....
http://lmgtfy.com/?q=duckduckgo
12-24-2018, 06:53 AM
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Originally Posted by G Atsma
I spoke of Ecotecs in general, and did not even know the two you referred to were RWD. My point was a little homework would have avoided this whole fiasco.
I guess I misunderstood your meaning when you said I didn't know what I was talking about
Now that you've been informed the two Ecotecs we're talking about are RWD, does anyone know if the mounting and footprint mate with those of the LS? I've read the Ecotec developed out of the engines used by Opel, do they use the Opel footprint and mounting? This information is going to be critical in any swap project.

NSFW
Are you saying the 1.5CFM/HP ratio applies only to unleaded gas and more energy rich fuels have a different constant? If this is so, what is the octane that number is based on and what is the CFM/HP with pure E85?

Merry Christmas for the next twelve days!
12-24-2018, 10:33 AM
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Originally Posted by Darth_V8r
Things like low tension rings...?

I can think of stuff inside the motor like ceramic bearings, micro polishing the crank journals, cam journals.

Beyond that, micropolish the rear, transmission, and throw in a CF driveshaft, superlight rotors, super skinny tires with hard compound. You get the idea.

Maybe an aluminum truck bed?
Yep, low tension rings is definitely one. The valvetrain as well, especially in all out drag race stuff where spring pressures are over 1000lbs open.

Originally Posted by yldouright
@G Atsma

KCS
Do any of the friction reduction methods described above reduce the 1.5CFM/HP constant? How much lower can this ratio go in your experience?
It depends where you measure the HP, but yes, they can. If you measure HP on an engine dyno, then reduction in parasitic losses in the drivetrain won't show for obvious reasons. On a chassis dyno, you can then see the benefits of a more efficient drivetrain.

IMHO, the CFM/HP thing is old school thinking. The CFM numbers are typically measured on a flowbench at a pressure differential of 28" of H2O, which is much lower than what an engine actually sees. At the higher pressure differentials, the air will move a lot faster. What that means is that you can have two different cylinder heads flow 300cfm exactly, but if one cylinder head has really bad ports that will go turbulent at the higher air speeds, then it will make less power than the other head that can tolerate the higher airspeeds. On the dyno, the bad head will lose power rapidly shortly after peak torque. The better head won't and will continue making power as the engine speeds climbs.
12-28-2018, 12:10 PM
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KCS
Thanks but what should we use in place of CFM to zero in on what size engine we need? It seems CFM is still useful for crank dyno numbers, the other parasitic losses from drivetrain, rolling resistance, etc can be factored. I've been told an average number is 0.85 of the crank HP number for a RWD and 0.70 for 4WD to estimate WHP. Having both dyno numbers would be useful locating parasitic loss. Since I have your attention, I've been thinking about diesel engines and their cylinders. Most are undersquare and non-interfering to increase compression but assuming an interference valvetrain and a shaved down head to make up compression, can you still create combustion with an oversquare stroke?
12-28-2018, 01:11 PM
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Originally Posted by yldouright
KCS
Thanks but what should we use in place of CFM to zero in on what size engine we need?
BMEP or Brake Mean Effective Pressure, which is a function of engine displacement and torque output. Naturally aspirated engines on pump gas seem to make around 200psi at peak torque and about 170-180psi at peak horsepower. If you know how much power you want to make and at what RPM, you do the math and see what engine displacement will get there without too much BMEP.

Originally Posted by yldouright
Since I have your attention, I've been thinking about diesel engines and their cylinders. Most are undersquare and non-interfering to increase compression but assuming an interference valvetrain and a shaved down head to make up compression, can you still create combustion with an oversquare stroke?
Yeah, I don’t see why not.
12-28-2018, 03:05 PM
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Originally Posted by yldouright
NSFW
Are you saying the 1.5CFM/HP ratio applies only to unleaded gas and more energy rich fuels have a different constant? If this is so, what is the octane that number is based on and what is the CFM/HP with pure E85?
I don't know what octane rating the 1.5 figure was based on, but the ratio of CFM to HP definitely depends on the fuel used. That's why E85 is so popular. Tune a car with pump gas, get X horsepower, then retune with E85 or race gas, you'll typically get more than X horsepower - same airflow, but with more advanced ignition timing made possible by the fuel's increased resistance to detonation.
12-28-2018, 03:33 PM
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KCS
Right, I thought so too but wasn't sure. I guess you might need to have a wider pattern of direct injection to cover the greater cylinder area but if the compression is the same, I suspect the heat would be too. Same heat, same ignition. The only wrinkle I see there is the difference in heatsinking. All the calculators for pressure assume the 150.8 constant, will that number also apply to diesel?
01-12-2019, 12:02 PM
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Originally Posted by yldouright
. All the calculators for pressure assume the 150.8 constant, will that number also apply to diesel?
I can’t think of any reason why it wouldn’t.