Increasing carb cfm
#2685053
08/06/19 05:31 PM
08/06/19 05:31 PM
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roadrunner2
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I’m limited in my class to a single Edelbrock 800 cfm carb. What are my options to get more flow?
Thanks.
Bill
1968 Road Runner 383 727 3.91 8 3/4 3520 with driver 11.04 @ 124.26 with 1.67 60’ Done on G70-14 redlines thru factory manifolds New quickest ever 383 build in FAST. 10:1 451” replaced with 14:1 499”.
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Re: Increasing carb cfm
[Re: roadrunner2]
#2685058
08/06/19 05:43 PM
08/06/19 05:43 PM
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Joined: Nov 2003
Posts: 36,041 Lincoln Nebraska
RapidRobert
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Bill I read an article once on reducing the OD of the (2) horizontle booster legs & reducing the dia of the choke shaft & as far as removeing it, John the carb guru at "the carburetor shop" says its better to keep it installed as it straightens the air flow & is very beneficial. Another tidbit was to use another gasket under the booster cluster to raise it for more airflow & shortening the vertical boosters. Can you enlarge the venturis bore(s) & booster nozzles, their ID? EDIT I heard that that round black plastic airhorn piece that I think Holley sells helps straighten the airflow & is good. I would give John a call & he only takes calls on Mon/Tues, I dont have his # handy but you can Google it. He's the man on this stuff
Last edited by RapidRobert; 08/06/19 05:50 PM.
live every 24 hour block of time like it's your last day on earth
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Re: Increasing carb cfm
[Re: RapidRobert]
#2685063
08/06/19 05:48 PM
08/06/19 05:48 PM
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roadrunner2
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Bill I read an article once on reducing the OD of the (2) horizontle booster legs & reducing the dia of the choke shaft & as far as removeing it, John the carb guru at "the carburetor shop" says its better to keep it installed as it straightens the air flow & is very beneficial. Another tidbit was to use another gasket under the booster cluster to raise it for more airflow & shortening the vertical boosters. Can you enlarge the venturis bore(s) & booster nozzles, their ID? All internal mods are good. Carb must look stock on the outside. Thanks for your input.
1968 Road Runner 383 727 3.91 8 3/4 3520 with driver 11.04 @ 124.26 with 1.67 60’ Done on G70-14 redlines thru factory manifolds New quickest ever 383 build in FAST. 10:1 451” replaced with 14:1 499”.
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Re: Increasing carb cfm
[Re: roadrunner2]
#2685105
08/06/19 07:57 PM
08/06/19 07:57 PM
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Joined: Oct 2007
Posts: 4,212 New York
polyspheric
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All internal mods are good?
How much money do you want to spend? Boring the body oversize and installing larger throttle discs is 90 years old TIKO.
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Re: Increasing carb cfm
[Re: Cab_Burge]
#2685209
08/07/19 05:31 AM
08/07/19 05:31 AM
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roadrunner2
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I have to use the factory cast iron intake. Internal mods are allowed and have been done. No spacers allowed. Using a custom solid flat tappet now, switching to a roller this off season. 👍🏽 Bill
1968 Road Runner 383 727 3.91 8 3/4 3520 with driver 11.04 @ 124.26 with 1.67 60’ Done on G70-14 redlines thru factory manifolds New quickest ever 383 build in FAST. 10:1 451” replaced with 14:1 499”.
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Re: Increasing carb cfm
[Re: dvw]
#2685210
08/07/19 05:34 AM
08/07/19 05:34 AM
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roadrunner2
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I've seen ore than a few modified ones running in N/SS. To be honest they haven't shown much improvement. The AFB style can use a stub stack. But we've tried one modified to fit and AVS and went slower. You can do the usual stuff. Thin the boosters (seen them break off). Streamline the throttle shafts. Doug That’s kinda where I’m at now. Maybe Edelbrock will make a 1000 cfm carb someday. 🤔
1968 Road Runner 383 727 3.91 8 3/4 3520 with driver 11.04 @ 124.26 with 1.67 60’ Done on G70-14 redlines thru factory manifolds New quickest ever 383 build in FAST. 10:1 451” replaced with 14:1 499”.
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Re: Increasing carb cfm
[Re: roadrunner2]
#2685295
08/07/19 11:13 AM
08/07/19 11:13 AM
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Joined: Oct 2007
Posts: 4,212 New York
polyspheric
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John Kaase: "that's not a spacer, it's a base gasket". The difference? He's important. I think anything thick enough to matter will be detected.
Just based on laws of physics: we radius the bottom of the manifold's plunge into the plenum (I did very "directional" work inside my 383/343 HP 2 X 4 factory manifold based on the direction and proximity of the ports being supplied). I'm tempted to radius the carburetor exit below the base where it's not accurately placed on the manifold, or hanging above a machine cut. Taken many apart to feel a 1/16" ledge in places. In general, any surface interruption is a trip wire, and air stumbles and slows down as it goes past. Care to share any info/pictures of your manifold interior??
Haven't inspected your carb, the main venturis are the intersection of 2 angles. Is their transition sharp or radiused? A small radius will improve flow, but try to avoid moving the "vena contracta" (smallest point) which is located about 1/32" above the booster discharge. Both ends can be enlarged at the same angle (steep on top, shallower below) making a smaller triangle with the same angles to increase venturi ID but it's Impossible to get in there. Easier to bore them out (yes, take the whole thing part) and make an insert. Much, much time for small gain.
Any re-shaping of the air horn, even intuitive blending the sharp edges, makes me very nervous: the factory calibration of air bleeds, blah is based on what happens there, and more CFM may upset the transition. Vizard has warned about this (but not specifically the Edelbrock IIRC).
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Re: Increasing carb cfm
[Re: polyspheric]
#2685313
08/07/19 11:59 AM
08/07/19 11:59 AM
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Joined: Jan 2003
Posts: 14,514 So. Burlington, Vt.
fast68plymouth
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As a point of reference, a bigger cube motor than the OP’s, that ran in the same series built to a much higher level(no disrespect, just is it what it is), that made what I’d estimate is roughly 100hp more........ only used 730cfm@6100.
If you want to know “how much” ET is left on the table with a carb, from either a calibration or capacity standpoint, borrow something like a Holley 850 or 950 from someone and see what kind of improvement there is.
On another build for that same series, also bigger cubes....... there was basically zero difference on the dyno between a Holley 750dp with the choke removed vs an Eddy 800. That motor used 720cfm @6500
Motors of this type, in legal trim.......are going to be down about 100hp+, compared to what you’d see out of a comparable bracket build would make.
That being said, we tried a correct 383 avs on one of the motors compared to the Eddy 800, and it was down about 30hp.
68 Satellite, 383 with stock 906’s, 3550lbs, 11.18@123 Dealer for Comp Cams/Indy Heads
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Re: Increasing carb cfm
[Re: fast68plymouth]
#2685329
08/07/19 12:58 PM
08/07/19 12:58 PM
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roadrunner2
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As a point of reference, a bigger cube motor than the OP’s, that ran in the same series built to a much higher level(no disrespect, just is it what it is), that made what I’d estimate is roughly 100hp more........ only used 730cfm@6100.
If you want to know “how much” ET is left on the table with a carb, from either a calibration or capacity standpoint, borrow something like a Holley 850 or 950 from someone and see what kind of improvement there is.
On another build for that same series, also bigger cubes....... there was basically zero difference on the dyno between a Holley 750dp with the choke removed vs an Eddy 800. That motor used 720cfm @6500
Motors of this type, in legal trim.......are going to be down about 100hp+, compared to what you’d see out of a comparable bracket build would make.
That being said, we tried a correct 383 avs on one of the motors compared to the Eddy 800, and it was down about 30hp. I should’ve just called you. Also I’m going for that 383 record next year. 👍🏽
1968 Road Runner 383 727 3.91 8 3/4 3520 with driver 11.04 @ 124.26 with 1.67 60’ Done on G70-14 redlines thru factory manifolds New quickest ever 383 build in FAST. 10:1 451” replaced with 14:1 499”.
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Re: Increasing carb cfm
[Re: roadrunner2]
#2685363
08/07/19 02:14 PM
08/07/19 02:14 PM
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Joined: Jan 2003
Posts: 14,514 So. Burlington, Vt.
fast68plymouth
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Also I’m going for that 383 record next year. 383........ or “383”? I think the “383” record is around 11.50....... and there was def more left in that combo if he’d have stuck with it.
68 Satellite, 383 with stock 906’s, 3550lbs, 11.18@123 Dealer for Comp Cams/Indy Heads
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Re: Increasing carb cfm
[Re: fast68plymouth]
#2685369
08/07/19 02:55 PM
08/07/19 02:55 PM
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roadrunner2
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Also I’m going for that 383 record next year. 383........ or “383”? I think the “383” record is around 11.50....... and there was def more left in that combo if he’d have stuck with it. “383”. That’s the one.
1968 Road Runner 383 727 3.91 8 3/4 3520 with driver 11.04 @ 124.26 with 1.67 60’ Done on G70-14 redlines thru factory manifolds New quickest ever 383 build in FAST. 10:1 451” replaced with 14:1 499”.
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Re: Increasing carb cfm
[Re: fast68plymouth]
#2685373
08/07/19 03:16 PM
08/07/19 03:16 PM
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Joined: Oct 2007
Posts: 4,212 New York
polyspheric
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a correct 383 avs
Rated 480 CFM, same for the 340, 580 for the 440, IIRC
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Re: Increasing carb cfm
[Re: fast68plymouth]
#2685605
08/08/19 08:58 AM
08/08/19 08:58 AM
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Joined: Nov 2017
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69ACME
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Also I’m going for that 383 record next year. 383........ or “383”? I think the “383” record is around 11.50....... and there was def more left in that combo if he’d have stuck with it. 120mph is 6-pack territory in a B-body...How much did the dart weigh with Jim in it?
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Re: Increasing carb cfm
[Re: 69ACME]
#2685621
08/08/19 09:53 AM
08/08/19 09:53 AM
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Joined: Jan 2003
Posts: 14,514 So. Burlington, Vt.
fast68plymouth
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I don’t know what one of those weighs, but I don’t think his had any major lightening done to it, plus it had the added weight of the roll bar. It’s still an all iron big block with ex manifolds and full ex system, steel wheels, etc. His is a bucket seat car...... originally a 4 speed. I actually think it went 122-ish with the stick.
The fastest of the 6bbls went either 129 or 130, and that was at 3900lbs. Number two was 127 iirc.
68 Satellite, 383 with stock 906’s, 3550lbs, 11.18@123 Dealer for Comp Cams/Indy Heads
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