Let's talk about carbs

[69stang_250]

How many of you guys have used the carb calculators online? What's the rule of thumb for picking a carb based off of it?

I just had a talk with my machinest about carbs and he thinks I am picking a good size carb with the 570 Holley, however, he told me with the head, intake, and all of the other goodies I have going into this engine to contact Holley directly and see if what their suggestion is. I have sent them an email and going to see what they have to say. While I was on the site I saw the carb suggestion area and filled it out. Once I clicked on the box it suggest 670 carbs for my engine build. This has me wondering if there is more to carb picking than just the size of the engine, VE%, and max RPM.

Each time I have ran my numbers on the CFM calculators the y say 403-530 CFM. My thought with the 570 is that the primaries will supply 285 of that required CFM and then pull what is needed from the secondaries and still shouldn't use all of the secondaries CFM. Is this how is should be looking at it? Or am I missing something?

After seeing that page on the Holley website and also a few others, it has me wondering if there is a factor I'm missing or if they have a % more CFM over what is required that they suggest?

What say you guys?

 

[xctasy]

Any thing works on a Ford 250 if the intake is good enough. The Classic Inlines intake is one of the best around.

I'd use the 650 double pumper. Plenty of 350 to 400 hp Aussie crossflows do.

The Ford 4180 series so called 600 cfm vac sec 4-bbl is very good too.

So is any four barrel from Carter, Edlebrock, Quadrajets. Some Autolite and some Motorcraft carbs are excellent.

Vac sec operation is designed for economy. Mechanical secondary Holley's aren't economy carbs like some Edlebrocks, and Qjets are.

Off the shelf Holleys and some of the Sportsman 4011/4010 versions are built down to a price carbs, yet plenty of people get a great run out of them.


The sizing relates to a well designed intake manifold with good volumetric efficency. You've got one!. So you can go up on the minimum suggested size without any concerns.


Or stay small and recalibrate the primary, secondary, PVCR's and secondary springs to suit.

 

[69stang_250]

I think I should have just asked what you guys down under run on the 250s. After talking to the Holley guy this morning and discussing the engine build and all the specs I will be going with the 670 street avenger. I know the double pumper would be fun, but for me the vacuum secondaries make a lot more sense for how I will be driving the car.

After talking with all of these guys about carbs it's just interesting to me how people seem to stay or suggest the 400 CFM range, but when you get way down into the details of the engines the recommendations tend to be more than you thought.

 

[xctasy]

Continue on!


I learned this from members from two Aussie Ford six forums.


The US tunners have found that when your halfway between port on port 1 carb per cylinder, and your 4bbl carb on 6 cyl or 2bbl carb on 4cyl venturi area is close to the idealised Weber ones...your car behaves like its independent runner.

The point is when venturi area is nearly the same as the "idealised Weber chart".


You loose 50 cfm when going 4-bbl at about 650 cfm anyway. For various reasons, two separate 500 cfm carbs flow 354 cfm at 1.5" Hg, or 707 cfm. When sucking from a 4-bbl, that drops to 650 cfm.

On something with three duces, like on Ak Miller's 250 Mavi GT or FalconSedanDeliveries 66 Mustang 250 3 x 1-bbl which was an appropriation of all Ak Millers best ideas, the 450 cfm and about perhaps 3.9 sq inches of venturi doesn't get close to Edurdo Webers areo engine derived venturi to displacement formulae. About 64 cubic inches per 1 sq in of venturi isn't close enough to the 21 to 23 cubes per square inch that makes real power.


A 650 4-bbl has about 5.938 sq in of venturi area. Its is effectively two 500 cfm carbs joined together. 250/5.938 is 42 cubes of engine per inch of venturi.

Best three examples.


Ist is the Esslingers 2.3 and 2.5 liter SOHC Lima fours running just one 500 cfm 2-bbl Holley. 268 at 7500rpm with no power valves and 79 jets! 152 cubic inches likes just 2.969" of venturi area. 152/2.969 is 51 cubes of engine per inch of venturi.

With a 3.80 bore by 3.35 stroke, the 152 cubic inch 2.5 Liter version of the 2300 Lima easily makes 268.1 hp at 7500 rpm and 211.2 lb-ft at 5750 rpm with just two 79 jets, the 6-32 PVCR block off, a trick as Essinger custom bias plate which corrects cylinder to cylinder EGT temperatures to 5 deg F variation. No power valve indicates the engine is behaving like and independent runner engine. IR engines don't need power valves.

all for a V8 style Esslinger intake and 269 degree at 50 thou cam installed heads up.

Same 35 mm venturis that make 135 hp on a near stock 2.0 EAO engine.

I'm quite sure a routed out 2300 series carb with 650 cfm like the 1.4375" 37 mm #6245 sieries 2-BBL,

or the brilliant, unlimited # 4412 2300 series based aftermarket 890 cfm C&S 1.56" (40 mm ) 2-bbl carbs would make even more, but they are illegal in circle track.

Second is the formerly triple weber 45 DCOE carb junker 240Z that was supposed to run six 32 mm ventris runs better on a 390 cfm 4-bbl. Just four tiny 1.0625" ventris with only 3.545 sq in of venturi area. 146/3.545 is 41 cubic inches per sq in of venturi.

146/7.478 or 20 cubes per sq in of venturi was the 240 Z Webers vital stats.

Third is the benchmark triple carb sixes from 1971.
Both an Aussie E49 Valiant Charger with a 265 cubic inch Hemi head, triple 45 DCOE Weber with over 300 hp and 3000 pounds does a 14.4 second quarter. So does a triple 45 DCOE DBS Aston Martin Vantage with 244 cubic inches of twin cam six, 325 hp, 3600 pounds. Well, 14.8, anyway.

Each has 40 mm venturis and about 11.684 sq in of ventri area. 21 to 23 cubic inches per sq in of carb.


American cars are so reliable because of non idependent runner single carb fuel delivery system simplicity. Plenty of 1978 to 1982 BMW's, Benz 280's and Opel and Vauxhall 2.8 and 3.0's got a US style French made Solex 4-bbls. They were a good start, but weren't a patch on the normal US 4-bbl carbs.


Now, back to non idealized veturi area again.


Just one 38 mm venturi 1435 cc A series can poke out 145 hp with one 45 DCOE. Thats 87.6/ 1.757, or just 50 cubic inches per sq in of venturi.


A good 2.4 240z just needs nice little 390 CFM 4-BBL like all killer 240Z's and BMW sixes end up with.

http://bringatrailer.com/2012/09/29/bat-exclusive-1971-datsun-240z-race-car/

The engine was recently freshened by west coast Z engine-guru Paul Ruschman at Rusch Racing. Induction is an unconventional but highly effective 390 cfm Holley 4150HP carb on a Bob Sharp style custom intake manifold. These manifolds were sold by Bob Sharp back in the early 70’s, and this is an improved reproduction. The Holley carb outperformed the 45 DCOE Webers on the car originally, and the seller says he was as astonished as anyone by this.

And BMW'S...http://www.seniorsix.org/faq/2_4bblconv.html

The rest of the world needed to take some stern 4 bbl simplicity lessons from the US of A...