Poor math student seeks tutors - Wrist Twisters
 
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post #1 of 20 Old 05-06-2009, 07:46 AM Thread Starter
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Poor math student seeks tutors

Ok. I suck at math. Algebra kicked my ass. And I think it's because they generalized when they said "you'll use this an infinite number of ways, places."

Fine. But something more grounded, say, internal combustion engines would give me a carrot and a framework to work within. Feckers.

Ennywho, starting with basics: How would one figure the correct cfm carb to use on a 350 cid engine? I said basic, didn't I ?

So, 1 cubic foot of space equals 1,728 cubic inches.

and if a 350 at 1000 rpm equals 350,000 ci of air,

350,000 divided by 1,728 equals 202.546 cfm necessary for it to breathe, yes?

I know squeeze factors in at some point particularly if 5000 rpm is the new number. 1,750,000 ci converted to cf is 1,012.731.

I'd be wheezing myself if I needed that much air. And this is where I go


One general question: What does "oversquare" mean ?

My guess is either the stroke is longer than the bore or the opposite.
whereas a "square" engine would be 75mm stroke * 75mm bore. yah ?

I intend this to be a place for all to bump up their figuring skills.

Thanks in advance for anyones help.

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post #2 of 20 Old 05-06-2009, 07:52 AM
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Forget all that mumbojumbo...Assuming it's stock or even mild for that matter. A 600cfm will suite it just fine.

Alluminum or cast intake?

Either way Edlebrock makes a very good, user/tuner friendly 600cfm carb. They come in many options...Manual Choke, Electric Choke and so on and you can find adapters to mate up to what ever intake your running.

Check your local Pep Boys (if you have them) and if not...see summitracing.com


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post #3 of 20 Old 05-06-2009, 07:56 AM
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Yes, oversquare means bore is larger than the stroke, or short-stroke motor.
Undersquare means stroke in larger than the bore, or long-stroke motor.
Square is just as you said, equal bore and stroke measurements.

Can't help ya with the airflow math.

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post #4 of 20 Old 05-06-2009, 08:01 AM
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Quote:
Originally Posted by McCrimmon View Post
Forget all that mumbojumbo...Assuming it's stock or even mild for that matter. A 600cfm will suite it just fine.

Alluminum or cast intake?

Either way Edlebrock makes a very good, user/tuner friendly 600cfm carb. They come in many options...Manual Choke, Electric Choke and so on and you can find adapters to mate up to what ever intake your running.

Check your local Pep Boys (if you have them) and if not...see summitracing.com
also
check jegs.com



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post #5 of 20 Old 05-06-2009, 08:11 AM Thread Starter
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Sorry, Mc Crimmon. I'm interested in the math. I don't necessarily have a chevy to toy with, I'm just using it as a jumping off point. If you were just being funny, HA ! you got me. (Aluminum. Tunnel ram, in fact. heh)

However, did have a friend with a rebuilt tree fitty. Wound up a 360 I think.
Had an edelbrock cam/manifold RV combo that worked pretty well considering it was moving a late 70's era 1/2 ton 4X4 short box. Started witha 650 holly.
went to a 750. I like the smaller carb better. The big one "felt" like it gave up
a gob of low end for a smidge high. I dunno. What's a butt dyno anyway, right?

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post #6 of 20 Old 05-06-2009, 08:14 AM Thread Starter
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Quote:
Originally Posted by sugs View Post
Yes, oversquare means bore is larger than the stroke, or short-stroke motor.
Undersquare means stroke in larger than the bore, or long-stroke motor.
Square is just as you said, equal bore and stroke measurements.

Can't help ya with the airflow math.
Thanks sugs !

Bore is bigger = oversquare
stroke is bigger = undersquare

Got it.

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post #7 of 20 Old 05-06-2009, 08:23 AM
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As for carbs....

A big 2 barrel can feel much stronger than a big 4 barrel.
The 4 barrel open primaries before secondaries.
The 2 barerel gives you all she's got immediately.

The big factor tho... can your engine handle it all at once. If not, you may be wasting tons of money in fuel with a two barrel.

The size of the primaries makes a big difference when comparing 4 barrels...

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post #8 of 20 Old 05-06-2009, 09:07 AM Thread Starter
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Still, is my basic math correct in figuring the minimums and the maximums of flow necessary based on the engine being a "350" ?

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post #9 of 20 Old 05-06-2009, 09:15 AM
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Quote:
Originally Posted by Lemonhead View Post
Still, is my basic math correct in figuring the minimums and the maximums of flow necessary based on the engine being a "350" ?
My only thought...doesn't each cylinder only breath in once every 2 revolutions? So for every revolution, only half the motor is breathing in? If that's true, you would need to compensate by reducing your in^3 by half.

However...I don't know that for sure and it is just my guess. Someone who knows internal engine workings better than me please speak up and tell me if I'm wrong.

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post #10 of 20 Old 05-06-2009, 09:30 AM
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Intake stroke
Exhaust stroke

You are correct sir.

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post #11 of 20 Old 05-06-2009, 10:18 AM
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Quote:
Originally Posted by Lemonhead View Post
Sorry, Mc Crimmon. I'm interested in the math. I don't necessarily have a chevy to toy with, I'm just using it as a jumping off point. If you were just being funny, HA ! you got me. (Aluminum. Tunnel ram, in fact. heh)

However, did have a friend with a rebuilt tree fitty. Wound up a 360 I think.
Had an edelbrock cam/manifold RV combo that worked pretty well considering it was moving a late 70's era 1/2 ton 4X4 short box. Started witha 650 holly.
went to a 750. I like the smaller carb better. The big one "felt" like it gave up
a gob of low end for a smidge high. I dunno. What's a butt dyno anyway, right?
More then likely is was bored .030" over making it a 355 ci unless he went the 400 crank route plus .030" over which would've made it a 383 stroker...Now those are some reving beasts


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post #12 of 20 Old 05-06-2009, 12:17 PM
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Quote:
Originally Posted by cmurphy84 View Post
My only thought...doesn't each cylinder only breath in once every 2 revolutions? So for every revolution, only half the motor is breathing in? If that's true, you would need to compensate by reducing your in^3 by half.

....
I was looking for this factor.

Your (lemonhead's) final figure of ~203 CFM is twice what is actually needed at 1,000 rpm with a Wide Open Throttle (WOT).

~102 CFM is actually the maximum the engine will breathe at 1,000 RPP with a WOT.
Multiply that by 5 for a redline of 5,000 RPM and the engine will want to draw ~510 CFM at redline with a WOT.

McCrimmon suggests that a 600 CFM carb will do just fine, and by golly, that is pretty close to the max possible air the engine will breathe; it even exceeds that figure by 15%-20%.

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post #13 of 20 Old 05-06-2009, 12:34 PM
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To determine the correct size carb for a particular engine you will need to calculate the air demand by multiplying the displacement by RPM, then dividing by 2 (4 strokes intake only every other revolution) and convert from cubic inches to cubic feet by dividing by 12 ^ 3, or 1728. This will give a base figure which will then need to be adjusted for the volumetric efficiency (VE) of the engine, a measure of the amount of air drawn in versus the displacement, which throws a bit of a monkey wrench in the works as VE changes with engine speed: with ports, valve sizes, cam, exhaust, and intake designed for optimum VE at the horsepower peak (arbitrarily 5,000 RPM) the VE can be as high as .95 or 95%. This will be a bit lower on stock engines. Those same components at 2,000 RPM may generate a VE in the 75% range due to lower port velocities and attendant fuel drop out.

At any rate, once you have the flow base figure simply multiply it by the assumed VE and you're in the ball park.

Let's crunch some numbers and see what happens.

Flow at power peak:
((350 C.I. * 5000 RPM / 2) / 1728) * .95 = 481.05 CFM

Flow at cruise:
((350 C.I. * 2000 RPM / 2) / 1728) * .75 = 152 CFM

Technically, a 1 barrel carb flowing 500 CFM would work, but GOOD LUCK getting it off the line due to the extremely low port velocity at low RPM being very bad at picking up fuel and keeping it atomized.


From this, it should be evident that a "spread bore" carb, with smaller primaries than secondaries, would be the way to go.


Of course you could always fit 8 individual carbs or 4 Webers, each bore flowing 63.3 CFM, but that's a bit much.

Well, that should give you the general idea.

Rob

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post #14 of 20 Old 05-06-2009, 02:48 PM Thread Starter
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Yup. Forgot that nifty little 4/2 factoid I did I did. Still nice to know the base is good, so

if you'd be so kind as to hold my hand, erm, let's work it from the top down
factoring in our venturi size(s) only. Then we'll do the manifold then heads etc., please. Mind you this does NOT need to be done all in one day so If you gotta run, by all means. I appreciate the help, and thank you all.

Venturis: Have a number which denotes what ? Then it's a matter of adding all venturis together (sticking with a 4bbl), right?

Here's where it gets complicated. Air has to speed up to pass through a smaller opening, so what is the physics number for air accelerating ? Is there one like gravity?

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post #15 of 20 Old 05-06-2009, 04:36 PM
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Sounds like you have a homework project on your hands.

Aren't you supposed to do the work yourself?

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post #16 of 20 Old 05-06-2009, 05:09 PM Thread Starter
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Well, kinda. As I'm doing this at home on my own.

Yes. But with nothing more than basic math skills, I need a lot of help to build on that. And as mentioned in my first post, the variables were too variable for me. Using something I'm familiar with, an application, will help me understand higher math. In theory at least.

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post #17 of 20 Old 05-07-2009, 12:57 AM
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Flow and pressures through a venturi can be calculated fairly precisely as long as the shapes and contours can be accurately described mathematically. It takes some relatively simple math to get a basic answer: http://en.wikipedia.org/wiki/Bernoulli's_principle#Compressible_flow_equation I was going to write it all out, but am basically too lazy.
All of this flies right out the window when you encounter the smaller variables such as gas temperature, moisture content, boundary layer behaviour, eddies and turbulence, the effect of intermittent flow and inertia, sonic wave activity, etcetera. You're in the calculus domain to get an accurate description of the system, and that's just for a simple venturi.

When you throw the massive number of variables inherent in port shapes including branching for multiple valves, the valve guides and stem, and the valve head which when open is never really stationary with respect to the seat, and is therefore a dynamic variable -- at least it's relatively quantifiable -- you are in the super computer time domain if you want an answer in a reasonable time. Even then, fully describing the characteristics of so dynamic a system mathematically is a daunting task without a considerable amount of data to support your efforts -- such as practically every manufacturer has from countless hours of measurements taken from operating engines.

Better by far is to do a flow test and measure the actual pressures and flows in the complete system. With modern solid state microminiature sensors and fast computers you can get an incredible amount of data while the engine is running.

Rob

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post #18 of 20 Old 05-07-2009, 01:24 AM
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And of course all your math goes out the window once Barton slaps a fiberglass scoop on there and increases your VE 500,000%

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post #19 of 20 Old 05-07-2009, 04:43 AM
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Quote:
Originally Posted by justintyme73 View Post
And of course all your math goes out the window once Barton slaps a fiberglass scoop on there and increases your VE 500,000%
how'd i get pulled into this??

innocent i tell you i'm innocent..



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post #20 of 20 Old 05-07-2009, 06:05 AM Thread Starter
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Rob, are you trying to scare me? Because it's working. OR maybe I'm barking up the wrong tree and should just go buy Algebra for dummies. I will check out that link though.

Sonic wave inteference ? No wonder so many guys just pick their parts slap em together and then adjust accordingly. Sheesh.


VE increase of 500,000% ? Yeah barton would do that.

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