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Old 05-13-2012, 10:42 PM
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Dual motorcraft 2 barrels vs holley 4 barrel ???

ok, let's start this way, no, I can not just order a 750 from holley, it's expensive and hard from here.

so I was thinking of a way to feed the hunger of my supercharged 358 SBC, it's a 144 blower so a 750-800 CFM is perfect, all I have is a couple of 600 CFM 1850's

I have two Motorcraft 2 barrels, came out of stock 351W engines, what if I make an adaptor and run this two babies?

what are they... like 500 CFM each ? that'll be 1.000 CFM for the pair..!!!

anyone ever done this before?

thanks.

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Old 05-14-2012, 12:02 AM
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They are 350cfm each for the large venturi versions, the small venturi are 287 cfm. Don't forget two barrels are rated at a different level of vacuum drop so its not comparable.

My first suggestion is where are you going to get jets to make it work? I used to have a selection of MC jets that ranged from 47-62 that took a long time to collect from the boneyards, the largest (62) only came in the MC four barrels off a big block AMC.

Then of course there is the issue of dual accel pumps and nozzles that aren't replaceable, how are you going to tune that? Can't just block one off either because the original pump cavity is pretty small compared to a Holley 50cc Rio pump.

Anything is order-able via the internet these days, it will just cost you more...my guess is either your time is worthless to you and you have tons of it to burn so this project which might save you a few hundred dollars makes it a good idea, I don't know Augusto, anything is possible but have fun reinventing the wheel. My opinion is though, its a fire waiting to happen at the first cough when you whack open those twin carbs without dyno/flow testing it first.

If it were me, and I had unlimited time to burn I would do a triple setup, stage the outboards carbs to half throttle and custom cast/fabricate an AL manifold to make it all fit. At least this way you can run on the primary center carb to get it running and fool with the outboard carbs to find a tune that will work...you will likely have to custom make or silver solder/braze up some old jets and redrill to a custom size to make it run right...at least not impossible and it will run until half throttle at least.

Where are you going to get a manifold to fit all this? Weld it up out of pipe and some sheet-metal? Just so you don't too excited, every time I have seen some hair-brained scheme like this put into action the motor ended up making a lot less or only a few HP more than a stock setup and it ran like crap and was impossible to tune.

Other than those caveats, good luck on your project!

P.S. This is a SBC right? Why not just find a modern fuel injection and forget this idea, a lot less work and your at least guaranteed a performance/economy gain...it will be a lot easier.
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Old 05-14-2012, 03:17 AM
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Quote:
Originally Posted by Augusto
ok, let's start this way, no, I can not just order a 750 from holley, it's expensive and hard from here.

so I was thinking of a way to feed the hunger of my supercharged 358 SBC, it's a 144 blower so a 750-800 CFM is perfect, all I have is a couple of 600 CFM 1850's

I have two Motorcraft 2 barrels, came out of stock 351W engines, what if I make an adaptor and run this two babies?

what are they... like 500 CFM each ? that'll be 1.000 CFM for the pair..!!!

anyone ever done this before?

thanks.
I believe you can make the Holley 600's work, providing you can mount them on that small of a blower case. I know they're usually used w/a single 4 barrel, but because the 600 carbs are vacuum secondary, you can still not be over carbureted using them- if you can mount them, that is.
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Old 05-14-2012, 05:39 PM
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Quote:
Originally Posted by cobalt327
I believe you can make the Holley 600's work, providing you can mount them on that small of a blower case. I know they're usually used w/a single 4 barrel, but because the 600 carbs are vacuum secondary, you can still not be over carbureted using them- if you can mount them, that is.
you mean mouting two 600CFM atop the blower?

since they are vaccum secondaries... makes a lot of sense to me.. I guess I should use the hardest springs in the vaccum pots to prevent over-carburation.? or maybe not?

fabricating a funnel adaptor would be pretty simple and would look even meaner than the two motorcraft 2 barrels..
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Old 05-14-2012, 05:47 PM
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Quote:
Originally Posted by 4 Jaw Chuck
They are 350cfm each for the large venturi versions, the small venturi are 287 cfm. Don't forget two barrels are rated at a different level of vacuum drop so its not comparable.
are this figures the equivalent for the 4bbl rating at 1.5" ?

Quote:
Originally Posted by 4 Jaw Chuck
Where are you going to get a manifold to fit all this? Weld it up out of pipe and some sheet-metal? Just so you don't too excited, every time I have seen some hair-brained scheme like this put into action the motor ended up making a lot less or only a few HP more than a stock setup and it ran like crap and was impossible to tune.
atop the supercharger, only a sheetmetal funnel would be needed


Quote:
Originally Posted by 4 Jaw Chuck
P.S. This is a SBC right? Why not just find a modern fuel injection and forget this idea, a lot less work and your at least guaranteed a performance/economy gain...it will be a lot easier.
this is for a racing engine, a racing EFI would be quite expensive.

seems like it's not a very good idea, maybe the two 600 holleys seems a better idea, what do you think?
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Old 05-14-2012, 05:56 PM
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Quote:
Originally Posted by Augusto
you mean mouting two 600CFM atop the blower?

since they are vaccum secondaries... makes a lot of sense to me.. I guess I should use the hardest springs in the vaccum pots to prevent over-carburation.? or maybe not?

fabricating a funnel adaptor would be pretty simple and would look even meaner than the two motorcraft 2 barrels..
Yeah, two 600s on top of the blower.

The secondary springs would need to be experimented with until you found the right tension for them. This could mean cutting or shimming the stock springs as needed to get them to behave like they need to. You might want to use a secondary throttle stop to keep the secondaries from opening 100%- that might let you use a lighter secondary spring w/o over carb'ing it.

Same thing for the accelerator pump shooter size and cam; gonna take a fair amount of trial and error to dial it all in. But I don't see a reason you couldn't get satisfactory performance out of it.

Something else you can look into is boost referencing the power valves.
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Old 05-14-2012, 09:42 PM
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you make me wish I had bought a 6-71 instead...

the two 4 bbl would look just awsome, instead of limiting the secondaries travel, how about plugging the secondaries of the second carb? (nobody would know about that)

that would be a 6 barrel setup, good for maybe 900 CFM ?

what do you think?
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Old 05-15-2012, 06:37 AM
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Quote:
Originally Posted by Augusto
you make me wish I had bought a 6-71 instead...

the two 4 bbl would look just awsome, instead of limiting the secondaries travel, how about plugging the secondaries of the second carb? (nobody would know about that)

that would be a 6 barrel setup, good for maybe 900 CFM ?

what do you think?
The only possible downside I could think of is air/fuel distribution possibly being a bit upset by the difference in the distance between the "primary" carb's (the one using just the front bores) throttle bores to the secondary carb's primary throttle bores. But I'd bet that won't amount to enough to matter one way or the other, especially if the primary carb sits behind the secondary carb so the primary carb's throttle bores are more centered. This will matter even less if the engine will be running wide open most of the time.

If it wasn't for the throttle linkage, putting the carbs facing primary-to-primary on top of the blower would center both of the primaries. In other words, the rear (primary) carb mounted normally, the front (secondary) carb turned around "backwards".

BTW, what kind of competition is this for?
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Old 05-15-2012, 08:48 AM
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is for drag racing, so most of the time it will be at WOT

I'm thinking of making a funnel type adaptor, not very flat, this will help with the airflow, but I'm not worried much because above the blower this does not matter too much, even fuel distribution takes place in the manifold under the blower and this won't be disturbed.

maybe the large plenum under the carbs may induce some little lag and may requiere a large fuel shot, but since there will be two squirters maybe it won't happen, only way to really know is making it.

I just saw that weiand, for their tunnel ram manifold reccomends for a hot natural aspirated street engine up to two 600 cfm carbs, and for racing up to two 750's so I guess for a supercharged 358 two 600's won't be overkill.

the visual impact alone I think is enough reason to make it
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Old 05-15-2012, 11:58 AM
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sure I was thinking of using boost referenced power valves, I know how important they are,

the pulley ratio is 2.4 6in drive, 2.5in driven.

I have a q-jet from a 454 that's supposed to be 850 cfm

but I kinda felt in love with the idea of twin carbs atop the blower
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Old 05-15-2012, 06:54 PM
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finding parts to tune a q-jet here is near mission impossible, there are plenty for the holleys, I better use them.

I like the idea of removing the primary piston and rods, but maybe I'll try that in other engine.

what do you think of this cam?:

http://www.lunatipower.com/Product.aspx?id=1998
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Old 05-16-2012, 07:38 PM
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Quote:
Originally Posted by Augusto
I like the idea of removing the primary piston and rods, but maybe I'll try that in other engine.
Only consider this if you are drag racing and if the Q-jet was originally a computer controlled model. Without a PP and metering rods, a Q-jet will be miserable on the street. A non computer controlled Q-jet has better uses than to be gutted this way when feedback carbs are cheaper than non feedback Q-jets, IMO. Something to remember next time someone is throwing away a feedback Q-jet.

Last edited by cobalt327; 05-16-2012 at 07:44 PM.
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