Altitude and Carburator size selection and VE

[Polyhead]

So... Math says my car will flow around 440cfm, but I'm thinking that's at sea level and at 4000+ feet of altitude, it's going to move a LOT less.  So I should probably select a smaller carburator right... but then again the drop in atmospheric pressure makes the carburator less effecient... so maybe I need a bigger one... but then again that means even slower velocities in the intake stream... so maybe it's the same... This is making my head hurt.

If someone could help me wrap my feeble mind around this....

[bbarn]

Ice water inter-cooler and a turbo or two to make your own atmosphere and the altitude becomes a lot less relevant! 

[RichFox]

For my 320 CID GMC 6 cylinder I used three 500 cfm two bbls. Wide open throttle is way different than cruising Dairy Queen.

[Polyhead]

For my 320 CID GMC 6 cylinder I used three 500 cfm two bbls. Wide open throttle is way different than cruising Dairy Queen.

True, but if the velocity gets to slow in the carb then you simply can't get jets big enough in the thing for it to work.

That said, if that's the case and it worked for you, then I probably should look into 1000cfm or so.  You probably have considerably better VE than my engine will have for this season.  Probably spun it faster than my engine as well.

[Milwaukee Midget]

A lot of this is going to depend on what you're going to use for a manifold.

You probably have considerably better VE than my engine will have for this season.

Back calculating, it looks like you're anticipating ~ 80% VE and 6,000 RPM, no?

If you're looking to run this year, I'd go with the dual plane you already have and any functional 4 barrel over 600 cfm you have lying in the corner of the shed.  It will likely be a lot closer to sorted out than anything you might want to try and craft between now and then.

August or September is going to arrive a lot sooner than you think.

Realistically, I'd tune if for where you're at and bring along enough spare jets to go one and two settings leaner, and read your plugs every run.

[Polyhead]

A lot of this is going to depend on what you're going to use for a manifold.

You probably have considerably better VE than my engine will have for this season.

Back calculating, it looks like you're anticipating ~ 80% VE and 6,000 RPM, no?

If you're looking to run this year, I'd go with the dual plane you already have and any functional 4 barrel over 600 cfm you have lying in the corner of the shed.  It will likely be a lot closer to sorted out than anything you might want to try and craft between now and then.

August or September is going to arrive a lot sooner than you think.

Realistically, I'd tune if for where you're at and bring along enough spare jets to go one and two settings leaner, and read your plugs every run.

Um.. no, was wishfully hoping for 85% VE and 5500rpm (but I bet bends a pushrod first!)  Chrysler claim 92% VE on this engine, but they don't say at what RPM.  I say they are full of it with those log manifolds...  Then again, the torque numbers pan out for that.  It's all guess work at this point.

The thing is, all my hot rodding gizstuffs (gizzards and stuff) was left in Indiana when I moved to oregon... and none of my family seemed in a big hurry to box stuff and mail it out to me... Soooooo I've got nothing laying around.  I'm starting with 0 stock of go fast goodies.  I took a nice long break from cars all together.  Haven't driven in like... 15 years.  Not out of any other reason than I was just busy doing other stuff.  Exploring my new surroundings out here in portland.  Another story for another day though.

So with nothing on the shelf to use, I was looking at the edelbrock Thunder AVS carburators.  Small engine, big carb, get something with an air valve secondary.  I was going to use a 500CFM carburator, which, at 1" mercury is right on the money.  At .8" it's kind of small.  But then Richard Ehrenberg tells me I'm mental if I don't get the 800CFM model.  But I don't know if he was considering the 4000ft of altitude or not.

I could eat peanut butter and jelly for another month and buy one of each, and the calibration kits for each (they aren't the same kit... for whatever reason)

But when you start pushing the numbers for dropping to .3 inches I start needing something rated closer to 976.55cfm.  The AVS has a fairly limited tuning range as far as maximum jet size goes, and I don't know if I could get jets big enough in an 800cfm carb to not go way lean... and it wouldn't get me to .3inches anyway, more like .4 or .5.  So now I'm thinking, why am I not just getting a 1050cfm 4150 holley for racing with?  I'm pretty damn sure I could get jets big enough for that, and if I didn't have what I needed on hand SOMEONE at the track would. 

It's my understanding as you reduce vacuum you have to go bigger in jet size to make up for the decrease in "signal" at the venturi due to the drop in velocity.  Less velocity means less fuel flow, less fuel flow means running lean, really tough on hypereutectic pistons.  But then altitude means running rich, which means leaning it back out... so.. I don't know, maybe its ok?

But then there is the altitude question...again... could I even get a 1050cfm carb to start and idle well enough to limp the car to the line and get her underway before the plugs fowl up and the thing shuts down?  Will I have enough throttle control left to have any hope of controlling wheel spin through first.  I'm confident it'll work when pinned to the firewall, but anywhere else...   Does anywhere else even matter out there?  At sea level i'm sure I could make it work... but at 4000ft, the vacuum numbers should be signifcantly lower... I can't find the math on how to work that out anywhere.  I can get barometer readings, but I don't know how to translate that into cfm used.

I'm going to just go pull some hair out and stop thinking about this for a bit.

[WOODY@DDLLC]

Maybe this will save some hair.........

CFM will be the same but density will change - each cubic foot will have less air molecules so less fuel and less HP.   

The new jet diameter is equal to the original jet diameter times (new density/original density)^0.5 [raised to the 0.5 power]
So if you have a Ø0.1000 jet at 100% air density and B'ville gets you 82% air density then your new jet is Ø0.0906 [Jet area is 82% of original.]

Remember that formulas are only a guide as reality can will be slightly more complicated since carburetors are a volume solution for a weight problem.

This calculator has a jet correction routine and other useful routines for plotting various schemes to raise HP by lowering the weight of your wallet  :
http://www.amazon.com/Mr-Gasket-Street-Performance-Calculator/dp/B003DITJEY

Apps are less $$ so more $$ for HP! 

Android app: https://www.calculated.com/prd363/Hot-Rod-Calc-for-Android.html
iPhone app: https://www.calculated.com/64/prd360/Hot-Rod-Calc-for-iPhone.html

[RichFox]

If some is good - Mores better - And to much is just right

[hotrod]

Peak VE will be at the peak torque rpm of the combination (that's why it produces peak torque).

I would over size the carburetor maybe 10% over what you think it will actually need to help compensate for the lower air density.
Too much over size and you will have fuel atomization problems from low mixture velocities.

A hot summer day at Bonneville has about the same air density as a mid 50 degree day here in Colorado at (4200 ft altitude higher temp verses 5500 ft altitude and 50 degrees)

You might find this website useful for getting a handle on what your engine will do at Bonneville and how much air flow it will need.

http://wahiduddin.net/calc/cf.htm

He has several calculators and also explains how and why of the calculations.
Playing with the calculators is very educational.

[Polyhead]

Peak VE will be at the peak torque rpm of the combination (that's why it produces peak torque).

I would over size the carburetor maybe 10% over what you think it will actually need to help compensate for the lower air density.
Too much over size and you will have fuel atomization problems from low mixture velocities.

A hot summer day at Bonneville has about the same air density as a mid 50 degree day here in Colorado at (4200 ft altitude higher temp verses 5500 ft altitude and 50 degrees)

You might find this website useful for getting a handle on what your engine will do at Bonneville and how much air flow it will need.

http://wahiduddin.net/calc/cf.htm

He has several calculators and also explains how and why of the calculations.
Playing with the calculators is very educational.

That's what I was thinking, too little velocity means really poor atomization.  Bet... I don't even know what size I should use at this point.  The car IS going to be pumping 400 to 440cfm at 5500rpm.  But everyone I talk too seems to think a 500CFM carburator would be WAY too small.  And I see the reasoning, since carburtors flow rates are rated at 1.5" mercury of vacuum pulling on them.  Less restriction is of course more peak power. So since the flow through a carburator falls off with the square of reduction in manifold vacuum, the actual size of carburator I should buy gets much larger very quickly.

I think i'm going to go with Midget's idea and go with something over 600CFM that doesn't cost me an arm and a leg... A 650cfm AVS would be nice but... honestly, if someone throws me a bone on a holley anywhere between 600 and 800 cfm I'm going to run with it.  Something makes me think that getting help with a Holley out on the salt is going to be a damn site esier than asking for help with a Carter.

[4-barrel Mike]

Odds are, you can find exactly what you need at the swapmeet this Sunday.

Mike

[Polyhead]

Odds are, you can find exactly what you need at the swapmeet this Sunday.

Mike

what swap meet?  ummmmmmm. I mean, I'm gearing up for the big one at PIR in april...

[4-barrel Mike]

Northwest Drag Race Association's annual.  Clark County Fairgrounds.  Search NWDRA in Craigslist.

Mike

[Polyhead]

Northwest Drag Race Association's annual.  Clark County Fairgrounds.  Search NWDRA in Craigslist.

Mike

That's amusing, my super top secret storage facility for the Phoenix is about 1 mile from the fair grounds.

[fordboy628]

Maybe this will save some hair.........

CFM will be the same but density will change - each cubic foot will have less air molecules so less fuel and less HP.   

The new jet diameter is equal to the original jet diameter times (new density/original density)^0.5 [raised to the 0.5 power]
So if you have a Ø0.1000 jet at 100% air density and B'ville gets you 82% air density then your new jet is Ø0.0906 [Jet area is 82% of original.]

Remember that formulas are only a guide as reality can will be slightly more complicated since carburetors are a volume solution for a weight problem.

This calculator has a jet correction routine and other useful routines for plotting various schemes to raise HP by lowering the weight of your wallet  :
http://www.amazon.com/Mr-Gasket-Street-Performance-Calculator/dp/B003DITJEY

Apps are less $$ so more $$ for HP! 

Android app: https://www.calculated.com/prd363/Hot-Rod-Calc-for-Android.html
iPhone app: https://www.calculated.com/64/prd360/Hot-Rod-Calc-for-iPhone.html

x2.

Some suggestions:

A/   Spend $39.00 on Larry Meaux's "PipeMax" header design and calculator program.
2/   Take the time to "properly" model your engine in the program.   This means measuring accurately, NO guessing.    Don't forget that: GIGO
d/   Change the model to identify "trends" within the basic scope of your engine build.

WHY GUESS WHEN YOU COULD KNOW?   This method reduces the cost of parts that end up being "unsuitable" because of a guess about size/sizing.

However, if most of the pleasure you derive from your build is to try different things to see what works and what does not, by all means do it that way.    Lots of significant development has been done in that fashion previously.   I, for one, will not argue about the certainty of testing, specifically, testing to destruction.    I just don't want to be the one paying for it.

 
Fordboy