That formula is good at telling us how much air your engine will displace at a given rpm, on a dyno when they measure how much are your flowing it's probably close, problem is it's got nothing to do with carb size, Lets use cylinder heads to make the difference easier say for your application that formula says you need 600 cfm that's how much air you will displace at peak hp. So divide that by 8 to see what each cylinder will displace which is 75 cfm. We know what most head will flow how come that formula is of no help to figure what cylinder head we should be using or at least narrow the choices down ?? Cause we flow cylinder heads like carbs at a chosen vacuum level not the level it will be on each individual application, since it will vary greatly from cases to case. What an engine displaces in air in cfm and the cfm which carbs and cylinder heads are tested at are not related.
Carburetor cfm calculator
- Thread starter Dan the man
- Start date
-
That formula is good at telling us how much air your engine will displace at a given rpm, on a dyno when they measure how much are your flowing it's probably close, problem is it's got nothing to do with carb size, Lets use cylinder heads to make the difference easier say for your application that formula says you need 600 cfm that's how much air you will displace at peak hp. So divide that by 8 to see what each cylinder will displace which is 75 cfm. We know what most head will flow how come that formula is of no help to figure what cylinder head we should be using or at least narrow the choices down ?? Cause we flow cylinder heads like carbs at a chosen vacuum level not the level it will be on each individual application, since it will vary greatly from cases to case. What an engine displaces in air in cfm and the cfm which carbs and cylinder heads are tested at are not related.
Yes, IIRC, the 350 hp 440 had a 575 CFM vac sec Holley
I always wonder if anyone actually ever convinces anyone else in these threads..... 
The folks that run 600's will continue to do so, and those that run 750's will continue to do so... and I think every OP kind of/sort of has something in mind but not really so sure but in the end takes the view of the folks that sort of support what he/she was originally thinking of anyways ... Like I said, I wonder...
P.S. - this post isn't this thread specific
The folks that run 600's will continue to do so, and those that run 750's will continue to do so... and I think every OP kind of/sort of has something in mind but not really so sure but in the end takes the view of the folks that sort of support what he/she was originally thinking of anyways ... Like I said, I wonder...P.S. - this post isn't this thread specific
@273 To answer the only question you asked, it is because your choosing from stock or ported head and his is a stock head.
What the engine consumes in air is completely different to what carb will perform the best on it which is much larger than what a driver would like in most cases, person depending.
You are one wacky dude.
What the engine consumes in air is completely different to what carb will perform the best on it which is much larger than what a driver would like in most cases, person depending.
You are one wacky dude.
Didn't realize I come off as wacky
lolI'm just trying say that formula seems to do more harm than good, by that formula a 650 would be over carbing a lot of 440's.
I think most here would agree that a stock to mild (350 ish hp) 318-360 would do good with a 600-650 carb especially where driveability is of most concern, no need to go smaller and or to use that formula. The idea of velocity is making people too cautious a lot of times about carb head valve etc... choices.
AJ/FormS
68 Formua-S fastback clone 367/A833/GVod/3.55s
- Joined
- Jan 19, 2014
- Messages
- 25,875
- Reaction score
- 13,184
I have to agree, sorta, lol. My 750DP idles just fine at 550 on my Airgap 367 with a 230cam..... lol. But you just gotta know, that wasn't always the case ...............
I agree with a lotta stuff you present, even if it sometimes seems hard to understand. It's sorta like your thoughts sometimes struggle to go from the thinking stage to the written form.
But if I read the hard for me to understand parts, most of the time I get it. But other times it seems "whacky" to me too, lol. But as I read more and more of your comments, or read them over a few times, I think I'm starting to get the hang of it.
Don't stop posting, I am listening, and hopefully I'll get more of it.
Learning for me is like I'm struggling to find my way thru a forest. I can see this Big Tree at the end of the trail, but the trail is so overgrown that I have to keep taking rabbit trails around the brush. Off-trail I can't see the Tree, and it can be hard to find the trail again. I do a lot of thrashing around.
Stock or mildly built ?
I have dual quads on my "almost stock" 318. Pair of 600 eddies hooked up 1 to 1
I take it You have a 2bbl on the engine now? If You gotta buy an intake, the RPM will be a better choice IMO, leaves room for growth. A performer ain't no better than a stk. TQ intake, just lighter. An AVS/TQ or Vacuum Secondary carb like a Street Avenger will let You go larger, again with room to grow, without destroying fuel mileage like a Double Pumper.
As is, a 650 AVS or 670 St. Av. will do You fine, a 750/770 gives room for more cam etc.
Don't get too "bent up" about headers, You can get into the low 12's on a set of 1&5/8" heddmans, so big tubes ain't needed, Quality & Fit are more important to You....
Whoops! Got your message backwards then.Didn't realize I come off as wacky
I'm just trying say that formula seems to do more harm than good, by that formula a 650 would be over carbing a lot of 440's.
I think most here would agree that a stock to mild (350 ish hp) 318-360 would do good with a 600-650 carb especially where driveability is of most concern, no need to go smaller and or to use that formula. The idea of velocity is making people too cautious a lot of times about carb head valve etc... choices.
I agree….
Quality and fit would be great if the expense wasn’t crushing in cost. I dislike the standard header design a lot! When I mentioned a 1-3/4 header that I use, I forgot to mention there on a ‘79 B body (Magnum) and there Hooker Super. Know with excellent fit and ground clearance. I got them way way back for $330. Now there a good bit more than that! But IMO, it was worth every dime spent.
A little bit of an Ouch! Not horrible, $581 black paint, $870 coated, vs TTI?
1979 DODGE MAGNUM Hooker Headers 5115HKR Hooker Super Competition Headers | Summit Racing
Here they are for a Duster… $666.16
Hooker Headers 5204HKR Hooker Super Competition Headers | Summit Racing
These have always fit and sealed great!
$172!
1972 PLYMOUTH DUSTER Summit Racing SUM-G9040 Summit Racing™ Headers | Summit Racing
$172!
1972 PLYMOUTH DUSTER Summit Racing SUM-G9040 Summit Racing™ Headers | Summit Racing
Unfortunately, some of the anecdotes [ went 2/10ths quicker with a bigger carb ] are misleading people into thinking that more cfm must be better. Using that logic, a tunnel ram with twin carbs has got to beat a single 4 bbl every time....which it doesn't it.
None of the examples given in this thread of 'bigger is better' have been conducted in a controlled test environment. Other factors to consider other than just the cfm rating.
None of the examples given in this thread of 'bigger is better' have been conducted in a controlled test environment. Other factors to consider other than just the cfm rating.
Dan the man
Well-Known Member
I think that for my application, I don't need a lot of carburetor, cam,compression. I'm sure that a compression ratio of around 9 - 9.5:1 is all that I'll need. Selecting the parts so that they work together is the biggest thing, then dialing in the ignition system and the fuel system. Also using the proper size exhaust system is important. A lot of people think that it takes a big carburetor, cam, and a high compression ratio to make power, but it doesn't.
I think as for the math and the formulas, the closer an engine is to stock, the closer the math is. 440s came right from Chrysler with carburetors in the 600CFM range and did just fine.
True but a 440 runs fine with manifolds but headers are better for power, bet it would run fine with a with a 2 bbl at a cost to power, put a 750/850 on a stock 440 picks up a decent amount of power. Bet if you put a 390 4 bbl on a 440 would work better than people would think, My cousin Late Model 372 pulls strong to near 7000 rpm with a 500 cfm 2 bbl, carbs are pretty flexible. People could use the formula but they would be short changing themselves in power especially in a performance situation but they would definitely not worry about over carbing
I think carbs should come with 3 cfm ratings the conservative 1.5 hg for stock and maybe add something like 1 hg for performance and 0.5 hg for race.
Like I said. STOCK engines will respond fine to the math.True but a 440 runs fine with manifolds but headers are better for power, bet it would run fine with a with a 2 bbl at a cost to power, put a 750/850 on a stock 440 picks up a decent amount of power. Bet if you put a 390 4 bbl on a 440 would work better than people would think, My cousin Late Model 372 pulls strong to near 7000 rpm with a 500 cfm 2 bbl, carbs are pretty flexible. People could use the formula but they would be short changing themselves in power especially in a performance situation but they would definitely not worry about over carbing
I think carbs should come with 3 cfm ratings the conservative 1.5 hg for stock and maybe add something like 1 hg for performance and 0.5 hg for race.
Rat Bastid
Dunamis Metron
True but a 440 runs fine with manifolds but headers are better for power, bet it would run fine with a with a 2 bbl at a cost to power, put a 750/850 on a stock 440 picks up a decent amount of power. Bet if you put a 390 4 bbl on a 440 would work better than people would think, My cousin Late Model 372 pulls strong to near 7000 rpm with a 500 cfm 2 bbl, carbs are pretty flexible. People could use the formula but they would be short changing themselves in power especially in a performance situation but they would definitely not worry about over carbing
I think carbs should come with 3 cfm ratings the conservative 1.5 hg for stock and maybe add something like 1 hg for performance and 0.5 hg for race.
Carbs shouldn’t have a cfm rating. You should base your decision on venturi and throttle blade diameter and ignore the CFM ratings.
"run good". Put a 318 2bbl carb and intake on a 340 and it will "run good". Put a stock 318 cam in a 340 and it will "run good". Put 318 manifolds and single exhaust on a 340 and it will "run good". Will it run better than a 340 with a 3310 Holley sitting on a LD340 with a 340 cam and 340 manifolds with 2.5" dual exhaust. NOPE. The 2 bbl set up 340 won't run any better, won't idle better, won't drive better, just be a ton slower.
Calculator... LOL. Ok, 600 cfm for a 360 at 5500 rpms. What 360 ? The factory stock one in a Maxi Van with single exhaust ?? They reference no heads, cam, etc.Like I said. STOCK engines will respond fine to the math.
Not an easy thing to figure out for new guys or even some seasoned wrenchers.
IMO, this is also why carb threads drag on with a million replies and mini arguments.
Rat Bastid
Dunamis Metron
Yeah, the carb manufacturers should post the venturi and throttle blade sizes right next to the CFM rating. That would clear up a bunch of issues. If you can run a smaller venturi and throttle blade and get the same CFM at the same test pressure the smaller venturi and throttle blade sized carb will out perform the bigger one quite easily.
That’s what most guys do not understand. It’s a bit weird as well. The bigger is better thing gets in the way. Even though they get the “Combo is everything!” Mantra.
It also gets worse with a thousand opinions and suggestions thrown in. In most carb threads, I’ll just make a basic recommendation and that’s it really. I never really get into the race stuff recommendation. I’m not super versed in it and there is normally something more and deeper to it.
It also gets worse with a thousand opinions and suggestions thrown in. In most carb threads, I’ll just make a basic recommendation and that’s it really. I never really get into the race stuff recommendation. I’m not super versed in it and there is normally something more and deeper to it.
I don’t get this idea where someone wants to mod their engine but thinks their gonna get it at the 1st stab, instead that it might take a couple of cam swaps, carb swaps, gear changes etc… to get tailored to suit you.
I’ve yet to hear anyone state how much vacuum their engine pulls on a full throttle run when asking if they should go with a different size carb.
I’ve yet to hear anyone state how much vacuum their engine pulls on a full throttle run when asking if they should go with a different size carb.
Now you're splittin hairs. But that's ok.
-
