#68343 - 01/29/12 06:55 PM
600 CFM ON A 250
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Registered: 06/12/11
Posts: 7
Loc: dfw
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I have a 250 with an offy intake would a summit 600cfm carb be to much? What carb would you fellas recomend besides a holley 390 or edelbrock 500?
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#68344 - 01/29/12 07:22 PM
Re: 600 CFM ON A 250
[Re: oliver65]
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Active BB Member
Registered: 01/23/12
Posts: 11
Loc: Victoria, Australia
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I would go 465 Holley 4 barrel with vac secondaries..but overall I would still go with EFI, although it seems to be a bit of a bugger to set-up!
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#68371 - 01/31/12 11:31 AM
Re: 600 CFM ON A 250
[Re: Tony P]
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Registered: 01/11/05
Posts: 1614
Loc: Hempstead, Long Island, NY
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Some guys think a bog is good, the engine stumbles,then picks up and they think it's accelerating harder,but it's actually slower. X2 - this is very common, and very important. The human body is not too sensitive to absolute forces (you can't tell how fast a plane is going), but far more sensitive to changes in force. Anyone can feel + or - 10% change, better drivers can tell down below 5% change. Recovery from a bog feels like "coming up on the cam", but is instead going from very bad to only slightly bad. Other things that feel like an improvement, but need more exploration and diagnosis: Late spark curve Late secondary opening Too much overlap for the exhaust system
If Tony will permit? "Supports" in this context means that the carburetor will be within the manufacturer's designed range of vacuum, and the air bleeds and venturis will function predictably without major tweaking. If the engine is much larger, of course peak power will be down (compared to a larger carb), but also the metering circuits will be operating outside their intended range and may need expert advice.
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#68373 - 01/31/12 02:10 PM
Re: 600 CFM ON A 250
[Re: panic]
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Active BB Member
   
Registered: 04/30/00
Posts: 540
Loc: Western New Yak state
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Yes,that's correct...a carb is designed for a certain engine situation. The Holley 600 ,out of the box,will be jetted just a touch rich ,for safety,at full throttle on typical 350 cube V8.Same for similar sized Carter and Edelbrock. In theory the Holley 390 is pretty close out of the box for less powerful engines,like 180-250 hp.But not always. The simple carburator size test;rig a vacuum to the intake,run the engine from let's say 2000 rpm to redline in second gear.When you first go wide open the vacuum gauge needle will zero then rise up a bit as the engine nears redline.If the vacuum rises to about 3-4 inches ,a larger carb may help. My observations found that GMC 302 inline 6 with a single central 4 barrel carburetor needs a richer fuel mixture at maximum torque peak than the carb will supply without some fussing with power valve restriction channels on the Holley.The Edelbrock or Carter wouldn't respond to metering rod or air flap adjustments.Finally I bumped into some tech info about fuel bleed holes in the secondary venturi clusters on some Carters.I drilled holes on a Carter 400 CFM 4 barrel on the GMC and it cleared up 95 percent of the bog and detonation.Supposedly all engines need a richer mixture at peak torque,then a less rich at it winds up to peak HP. I believe many guys run too rich a fuel mixture on modified carburated inline engines and thus the complaints about poor fuel mileage,especially with GM 235-261's and GMC.I run street engines on the verge of detonation with lean mixtures ,aggressive spark timing,180-190 degree thermostats heated intakes and relatively high compression with as tight a quench as practical.Downside is any problems with fuel mixture curves show up instantly as heavy throttle detonation,it's a slippery dlope and you have to be careful.But the return is super sharp throttle response and good fuel mileage.And I don't drive with the throttle wide open for more than 10 seconds on the street.
_________________________
70 Triumph 650 cc ECTA current record holder
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#68398 - 02/02/12 11:41 AM
Re: 600 CFM ON A 250
[Re: panic]
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Active BB Member
Registered: 09/23/10
Posts: 8
Loc: Sweden
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Thanks! that enriched the knowledge library.
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#68665 - 02/19/12 07:11 AM
Re: 600 CFM ON A 250
[Re: THE FRENCHTOWN FLYER]
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Registered: 01/26/10
Posts: 485
Loc: left out
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I ran two diff holleys on my '67 half ton short wide bed 250 with a cliff intake and hedmans/flowmasters. The first was a 450 cfm holley "economaster", a small butterflied carb like a wcfb/4GC, but with MECH. secondaries. With a certain amount of "copying" from a holley book, I was able to duplicate the mods holley said was necessary to run it on a 238 buick V6 (close enough in size-my carb was a replacement on a 350 chevy). After mods,(one of which was to add a set of idle holes and transfer slots under the SECONDARY butterflies to handle the slight bogs) this was a nice carb.
Later I put on a stock 600 cfm "belly-button" holley with vacuum secondaries (NO DOUBLE PUMPER). With virtually no mods, other than idle screw and pump adjustments, it ran real nice too. It seemed that since the back barrels would not open 'til engine needed them, that the old 3.70 geared truck had more pep (from bigger front bbls than other carb) at bottom end making it the more fun of the two carbs.
Now aint those front bbls on the 600 the same setup and situation as a 500 cfm holley TWO BBLs have?. Therefore why cant the 500 two bbls work well on 250s? Most overcarbed motors are going to exhibit bogs, and stumbles. The 600 cfm was really nice, and so was the smaller 450 cfm mech, no-vac secondary carb(after modin'). Vacuum controlled secondaries is the saving virtue. Q-jets, factory mopar thermo-quads ('cept they got a lotta smog stuff on 'em) an' I guess a pair of Pintos too, and most controlled (mech or vacuum-vacuum the better) secondaries carbs should work fine. There are some good carb books on the market-ya ges gotta find 'em and read'em snowman.
Edited by preacher-no choir (02/19/12 07:30 AM) Edit Reason: missin' letters an' dots
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#68672 - 02/19/12 10:31 AM
Re: 600 CFM ON A 250
[Re: oliver65]
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Active BB Member
Registered: 04/17/06
Posts: 530
Loc: Northern CA
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Oliver, Deja Vu, I thought I replied already cuz I looked up my test data but maybe Exploder blew up while responding. I had Gtech tested a stock 250-Powerglide-3.08 using 4bbl carbs ranging from 390cfm to 625cfm. There was no clear trend with CFM, it mattered a lot more that the carb was "happy" - right jets, good shooters, launch technique - you could "stomp" on the 390cfm but not the bigger ones. But if launched correctly, the 390cfm cut a 2.574 60ft and the 600cfm Carterbrock was close behind at 2.580 sec. That is too close to call by far. In 0-60mph te 600cfm Carterbrock actually won, 10.13 sec vs 2nd place 10.21 sec for the 450cfm Holley. Again very close. At 1/8 mile et: 11.238 450cfm Holley R4548  11.266 600cfm Carterbrock 1406 11.284 390cfm Holley R6390 11.324 500cfm Autolite 4100 c3af-t To show how it depends on carb, some "similar" carbs ran 11.406 600cfm Carterbrock 1405 11.538 450cfm Holley R3492 before tuning 12.130 500cfm Autolite c5af-f before tuning, what a dog Most of the 1bbl Rochester B carbs ran from 11.50 to 12.50 so those last couple 4bbls were real dogs before tuning.  A lot of this was really testing the primaries. The secondaries on the 390 opened about 1/2 way, on the 450 a little bit, on the 500 a little bit, and on the 600s hardly at all tho the secondary transition circuits did help. Bottom line, if U have a 4bbl in any of these size ranges, TRY IT and play around, if it runs good then just chill and shop around til U find a good deal on just the right CFM. It makes some diff but not that much. 
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#68701 - 02/20/12 08:27 PM
Re: 600 CFM ON A 250
[Re: Tony P]
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Active BB Member
Registered: 04/17/06
Posts: 530
Loc: Northern CA
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Yup and I knew that. Partly I was just rebuilding some carbs and this was the easiest engine to work on and test em.
My formula for cfm is roughly "double your horsepower" so about a 300-350cfm 4bbl would be perfect, but there is no such animal unless you go back to the early 1950s. A 500cfm (at 3" Hg) 2bbl is about the same size but the whole point was to go progressive.
But the surprising point was it didnt matter. I really thought the big 600cfm Carterbrocks would be soggy but they weren't.
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