updraft vs. downdraft carbs

wingski

wingski

Senior Member
OK, I know I’m opening a can of worms here, but isn’t a downdraft carb superior to an updraft carb? Since the Vortex intake manifold that I have for my twin Marvel Shebler carbs can be inverted so the fuel flows down instead of up, why not mount twin 97’s? I don’t think I’d have that much trouble adapting a linkage system that would be higher on the firewall to control the 97’s.

Fire away,

Mike
 
I have seen a lot of problems when running down draft carbs.

The old Stromberg downdraft carbs are notorious for sticking floats where the float valve will not close. fuel from the tank can run thru the carb and fill up a cylinder and leak down onto the crankcase. Not a good thing.

For me and My Model A's, I like to stick with up-draft carbs. They seem to run fine and if a float valve does stick, fuel will not finds its way into the engine.

Try 2 up-drafts and you may like that set-up.

Now, this is JUST MY OPINION. I am sure others will have different ideas on the subject.

Chris W.
 
I am running twin Strombergs with adjustable main jets and so far after around 10,000 miles, no problems. I did have both overhauled first.
Keith
 
If I could do it over I would go with a Holley 94 which I think is now a Edelbrock 94, I heard no problems on that Carb.
 
Besides a fuel pump you would probably need a fuel pressure regulator . If you wanted to run dual 97s you would be far better off with a manifold that was made to run dual downdraft carbs . a dual updraft manifold turned upside down would be a rig that I don't think would give optimum results !!! A properly setup dual downdraft setup would outperform a dual updraft setup . The downdraft carbs would put out more CFM and would have gravity working on their side but would be a better choice for racing than street use . Downdrafts have got to be setup JUST right are they won't work at all . Besides all of that , down drafts that are setup with an electric fuel pump are subject to completely flood the engine with raw gas , from the oil pan to the tops of the pistons if a person doesn't know what they are doing .

Updrafts are very simple to setup . I build swap meet carburetors with original jets and have never had a problem . I can disconnect the linkage and run with one carb if I wanted to. Though I wouldn't do it I could run two different brand carbs and it would still work . As far as performance goes, without higher compression nether down draft or up draft is going to show much if any seat of the pants improvement that a person can feel . more or larger carburetors will add some top speed but you won't feel much if any torque . As Rainmaker Ron once said it was torque that we raced with. Dual updrafts is a good street setup and dyno tests have shown a 10 horsepower increase The more mods that are used , the better results with either setup will be. Higher compression , hotter spark and free flowing exhaust are all important and all work together .
 
its all about airflow. flip that vortex and run a pair of weber ICT's (about 160 cfm,the engine handles about the same) the weber ICT's bolt right to the vortex,I was going to run that lash up till I got a Holley 94,I can get an Ansen intake cheaper than the vortex.The ICT's need 3 psi,so a rotary electronic fuel pump in in order,they are fool proof and set for pressure,no regulator needed.'Hell with a common runner balancding can be done by ear,they are a sweet little carb,IMO better than the 32/36 setup the vendors sell.

https://vwparts.aircooled.net/Rotary-Electric-Fuel-Pump-3-25-psi-7mm-Fittings-p/rotary.htm
 
Railcarmover said:
its all about airflow. flip that vortex and run a pair of weber ICT's (about 160 cfm,the engine handles about the same) the weber ICT's bolt right to the vortex,I was going to run that lash up till I got a Holley 94,I can get an Ansen intake cheaper than the vortex.The ICT's need 3 psi,so a rotary electronic fuel pump in in order,they are fool proof and set for pressure,no regulator needed.'Hell with a common runner balancding can be done by ear,they are a sweet little carb,IMO better than the 32/36 setup the vendors sell.

https://vwparts.aircooled.net/Rotary-Electric-Fuel-Pump-3-25-psi-7mm-Fittings-p/rotary.htm
Click to expand...



What Railcarmover said!!!
 
In the past 15 years, I’ve run the Mooneyes EFI, two old 97’s, a single new 97, a pair of new 97’s a 84 and a pair of 84’s. My A and my B are both equipped with Weber 32/36 set ups and my dyno sheets are proof positive that for my applications the Weber’s are superior.
 
No doubt Webers are superior. I've yet to hear anyone complain about the 32/36 as used on the model a/b.Its not the case in other applications of this carb. The ICT has a 34 mm venturi,its almost a perfect match for two of the A's cylinders at 82cfm.Use a common runner intake like the vortex between the two balances the two singles naturally,negating the tuning difficulty running two singles on a four cylinder engine independent of each other. ****,I got that holley 94 for twenty bucks and three gallons of 15/40..and here I am, talking myself into running the ICT's again..
 
Thank you all. The great thing about the forum is that I can get other people’s input before I do something stupid. I hope all of you know how much I appreciate the time and effort you are sharing with me.

Happy Holidays,

Mike
 
i have an 81 on my PU runs good but need fuel pump as gravity won't work unless ya got a full tank..looks better too..sc
 
I have been wondering if two SU side draft carbs would work well in an A. I have run them on my MGA for years and find them to be a great carb. I have a couple that I would like to install if there is room. Just wondering if anyone has tried that or is this a pipe dream
 
updraught said:
"... but isn’t a downdraft carb superior to an updraft carb?"

Hello?

Am I in the right forum?
Click to expand...

I've heard it is, but by how much? Probably not a lot, and I think a side draft would be an even straighter air flow, so that should be tops.

Oh heck, just install a turbocharger.:D
 
updraught said:
"... but isn’t a downdraft carb superior to an updraft carb?"

Hello?

Am I in the right forum?
Click to expand...

There's no technical reason why an updraft carb would perform differently than a downdraft. All experiences are based on the fact that the 97, the Weber, and probably some others are just pretty good carbs. I'm sure Weber could make a great updraft if they tried. Stock carbs tend to be sized for fuel economy and good fuel signal strength. This means they are undersized for most sporty work. Picking a size that is big enough to flow and small enough to tune on your engine is the trick. The fact that a side draft has fewer bends is a real benefit.
 
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They all have mitchells with 'touring' motors..6 to 1 heads,B cams,insert bearings.12v systems with those ridiculous turn signals..old men putting turn signals on so they can forget to turn them off..FSI zipper elctronic ignitions,hell the list goes on and on..but once you mention a downdraft carb youre a juvenile delinquent,a dirty hotrodder..
 
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