Carb CFM recommendations

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thecatsfan said:
Not sure that writeup is factually accurate. I called Edelbrock. 1405 and 1406 have extremely minor differences. Primary and secondary venturies are identical as well as boosters. Primary jets are 0.98 and 1.00 - a 2% difference in flow area. Secondary jets are identical. Metering rod is the same diameter but slightly shorter on the 1405, which makes the carb run leaner. So slightly leaner primary jet on 1406 and slightly leaner metering rod on 1405, which makes both carbs essentially equal out of the box. And for $5, I can change the primary jet on the 1406 to make it identical to the 1405. Or for $75, I can put an electric choke on the 1405. Don't forget at altitude, engines run rich unless jetted down. And just for information, higher compression does not flow more air. Bore and stroke do not change. It still pulls in 318 inches of air per combustion cycle. You still need a 13.8:1 air to fuel ratio - same amount of air and gas.
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Well, "whichever" you choose, I would stay in the 600 CFM range. I don't think you will be disappointed. Even if you choose the 1406 and it's a little lean you can always make changes. The one I had was on the stock 351M in my 75 F250. It's a pretty heavy truck with a larger engine and it did pop badly when cold and even sometimes when warm. I went up 3 jet and metering rod sizes and it ran great. Now I have a 625 Street Demon.....one of the little three barrel carburetors. It runs very good with that.
 
RustyRatRod said:
Well, "whichever" you choose, I would stay in the 600 CFM range. I don't think you will be disappointed. Even if you choose the 1406 and it's a little lean you can always make changes. The one I had was on the stock 351M in my 75 F250. It's a pretty heavy truck with a larger engine and it did pop badly when cold and even sometimes when warm. I went up 3 jet and metering rod sizes and it ran great. Now I have a 625 Street Demon.....one of the little three barrel carburetors. It runs very good with that.
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Thanks! I will definitely keep that in mind.
 
thecatsfan said:
Thanks. Great advice. I am going to stay small. I am starting to lean towards 500 cfm.
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IMHO the smaller carb argument is just for best driveability and ease of tuning. If you want max power, then you'll still need the larger carb, regardless of altitude but I doubt your cam will pull in the extra air to need it, and big is certainly not needed for your intended use. And IIRC, at your altitudes, carbs are starting to go rich vs standard atmospheric settings, so a lean carb at sea level may be just about right up at you elevation.
thecatsfan said:
It's actually about 86%, but close enough. Your point is very valid and I am leaning towards a smaller carb.
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And the carb size versus elevation situation goes both ways so I am not sure my thinking of going smaller is good advice. If you want the same power output, you gotta open the throttle to pull in more of that thinner air so a smaller carb will get into the secondaries, etc., sooner.
 
Correction on the Weiand stealth, the port size is 1.96 by 1.00 inch. It's a direct match for the 318 heads with a rated power range of idle to 6800 rpm. It's the Weiand answer to the Edelbrock performer 318-360 but is more closely related to the LD4B.
 
Garrett Ellison said:
Correction on the Weiand stealth, the port size is 1.96 by 1.00 inch. It's a direct match for the 318 heads with a rated power range of idle to 6800 rpm. It's the Weiand answer to the Edelbrock performer 318-360 but is more closely related to the LD4B.
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Weiand Action plus is Weiand's answer to the eddy performer. The Stealth is their answer to the Eddy RPM. The Action Plus has the same width port as a 340/360, but not quite as tall. I like the Action Plus better than the 318/360 performer by edelbrock. I have two Action Plus's and 1 Stealth. Another video I made.....
 
I'd go with the 650 AVS2, like I recently did on my 340. Nothing but a positive experience.
My second choice for mild street use was the 625 Street Demon.

I'll pm you regarding an LD4B
 
nm9stheham said:
IMHO the smaller carb argument is just for best driveability and ease of tuning. If you want max power, then you'll still need the larger carb, regardless of altitude but I doubt your cam will pull in the extra air to need it, and big is certainly not needed for your intended use. And IIRC, at your altitudes, carbs are starting to go rich vs standard atmospheric settings, so a lean carb at sea level may be just about right up at you elevation.
And the carb size versus elevation situation goes both ways so I am not sure my thinking of going smaller is good advice. If you want the same power output, you gotta open the throttle to pull in more of that thinner air so a smaller carb will get into the secondaries, etc., sooner.
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I understand what you are saying about needing more air to produce the same power. Problem is, the only way to pull in more air is more displacement. There is nothing I can do to make a 318 engine pull in more than 318 cubic inches of air per combustion cycle. Obviously changes to cams and valves impact the exact airflow in and out of the engine, but the maximum amount of air still cannot exceed displacement on a naturally aspirated engine.

We actually pull in the same volume of air in Colorado as they would in Florida - the air just has less density. That is the reality we have to live with at altitude. So we make intakes as free flowing as possible, but we have to live with the fact that our air has less energy in it. And if we oversize a carburetor, the lower density only compounds the issue of poor signal at the boosters. Thinner air passing through the venturis has less vacuum than denser air. This is even higher than the standard air energy loss. We lose approximately 16.8% vacuum pressure at 5K feet. This means the problem of an oversized carb will only be compounded at our altitude. I think your original advice is pretty solid.
 
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thecatsfan said:
I understand what you are saying about needing more air to produce the same power. Problem is, the only way to pull in more air is more displacement. There is nothing I can do to make a 318 engine pull in more than 318 cubic inches of air per combustion cycle. Obviously changes to cams and valves impact the exact airflow in and out of the engine, but the maximum amount of air still cannot exceed displacement on a naturally aspirated engine.

We actually pull in the same volume of air in Colorado as they would in Florida - the air just has less density. That is the reality we have to live with at altitude. So we make intakes as free flowing as possible, but we have to live with the fact that our air has less energy in it. And if we oversize a carburetor, the lower density only compounds the issue of poor signal at the boosters. Thinner air passing through the venturis has less vacuum than denser air. This is even higher than the standard air energy loss. We lose approximately 16.8% vacuum pressure at 5K feet. This means the problem of an oversized carb will only be compounded at our altitude. I think your original advice is pretty solid.
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Way too much science. Gets in the way of a good build every time. I tune by letting the car decide and not a calculator, but that's just me. In your case it doesn't matter, because you just want a good driver and performance isn't your concern. I'm very familiar with your altitude. I used to live in Wellington and for years and years I have/had family in Loveland.
 
I grew up in Colorado (mountains west of Denver, ~9000' elevation) and raced out there for quite a few years...what was interesting is on a normal hot summer day, the density altitude at Bandimere Speedway (5800' elevation) would be well over 9000'....cars were slow, unless you bring your own atmosphere...I preferred to keep mine stored in a blue bottle. My old 360 motor, would run 13.8 NA, but would drop to 11.0 on a 180 shot. That same motor when I moved to Indiana ran 12.4 NA (850' elevation, DA in the probably in the 3000' range (typical summer air guesstimate)).

My point is, air density makes a huge difference. Car ran over a second faster at lower altitude.
 
flyfish said:
I grew up in Colorado (mountains west of Denver, ~9000' elevation) and raced out there for quite a few years...what was interesting is on a normal hot summer day, the density altitude at Bandimere Speedway (5800' elevation) would be well over 9000'....cars were slow, unless you bring your own atmosphere...I preferred to keep mine stored in a blue bottle. My old 360 motor, would run 13.8 NA, but would drop to 11.0 on a 180 shot. That same motor when I moved to Indiana ran 12.4 NA (850' elevation, DA in the probably in the 3000' range (typical summer air guesstimate)).

My point is, air density makes a huge difference. Car ran over a second faster at lower altitude.
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correct. Every time I go to visit out there, feels like your pulling a trailer... LOL. Just got back from Colo Springs, 6200 ft. Yep, you can feel it. I always say a rule of thumb is you loose 2 tenths every 1000 ft. Probably 2.1 would be more accurate
 
flyfish said:
Being a scientist, this statement makes me want to pull the wheels off your stomper....:lol:
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LOL..... science is only "fact" until a new way of measuring is discovered, then everything we once believe becomes "false" because of "new evidence". LOL
P.S. - I ate eggs when they were healthy, I ate them when they were unhealthy, and I'm still eating them after we learned they are healthy again :D
 
318willrun said:
Way too much science. Gets in the way of a good build every time. I tune by letting the car decide and not a calculator, but that's just me. In your case it doesn't matter, because you just want a good driver and performance isn't your concern. I'm very familiar with your altitude. I used to live in Wellington and for years and years I have/had family in Loveland. My brother had a 850 DP on a mild 340 and never lost a race out there (Big block, small block, didn't matter).
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I get it. But at the end of the day you cannot defy the laws of physics, because physics does not care if you agree. If you simply ignore science, you will get to learn the hard way. I am not a fan of wasting money because I guessed.

I am not saying that cfm calculators are perfect. If they were, I never would have bothered asking. But they are still written by automotive engineers based on sound science and decades of engineering experience. I am not going to live and die by them, but I also am not going to ignore them.

I absolutely am concerned about performance. In no way does the fact that I am not drag racing mean I do not care about performance. There are many different measures of performance. I do not care about boiling tires or redlining at 6,500 rpms. But in between, I want this engine to run as well as it possibly can. Because if I do step on the gas, I want it to respond instantly and strongly. Just because the majority of driving will be mild, does not mean the engine does not need to perform when asked. Oversized carbs kill low end throttle response. It is unavoidable. There is no jet combination you can install to make the wrong carb perform properly on an engine. I may not be an expert, but I know enough to acknowledge that fact.

I appreciate everyone's input. I am not saying anyone is wrong - not at all. I got some amazing advice on this thread, and an amazing amount of disagreement among that advice. Bottom line is this definitely confirmed which way I will go. And I am more than confident it will work GREAT!
 
thecatsfan said:
I get it. But at the end of the day you cannot defy the laws of physics, because physics does not care if you agree. If you simply ignore science, you will get to learn the hard way. I am not a fan of wasting money because I guessed.

Oversized carbs kill low end throttle response. It is unavoidable. There is no jet combination you can install to make the wrong carb perform properly on an engine. I may not be an expert, but I know enough to acknowledge that fact.
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Like science, physics, and facts???? FACT: my stock 360 with stock converter and 2.45 gears cut a 2.06 60 ft with a great running 600 eddy, the "perfect" carb. I put the "wrong" carb on this car, a Holley 770. 1.94 60 ft. Then 750 DP (with 2.76 gears), 1.92. No wheel spin on any of the passes. We ALL know that you DON'T put a Double Pumper on a stock engine with stock converter and 2.76 gears.... Funny how it made the fastest pass. Those that don't know will never know..... :)
 
318willrun said:
LOL..... science is only "fact" until a new way of measuring is discovered, then everything we once believe becomes "false" because of "new evidence".
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Gravity has never been disproven. None of the primary laws of physics ever have been. There is an entire profession of engineers who would disagree adamantly as to whether the laws of physics can simply be ignored while we wait for new evidence. Seriously. But if I need some advice on eggs, I know who to call. :)
 
thecatsfan said:
Gravity has never been disproven. None of the primary laws of physics ever have been. There is an entire profession of engineers who would disagree adamantly as to whether the laws of physics can simply be ignored while we wait for new evidence. Seriously. But if I need some advice on eggs, I know who to call. :)
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Law of physics said the bee cannot fly, it's wings are too small to support the body weight. I just sit and wonder every time one of those little buggers just up and takes off in flight.... I just wonder... :D
 
thecatsfan said:
When? Who? Show me a source for a physicist that said bees cannot fly. I call BS!
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"But there was once a certain breed of scientist that continued to ask the question, even when faced with the obvious visual “proof”. Antoine Magnan studied bees He concluded that it is physically impossible for bees to fly!"

copied the quote for ya...….
 
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