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Dirt Trackin'

Carb on an EFI Manifold

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Might sound like a weird question, but has anyone tried mounting a carb on to the 318i intake manifold?  I'm just wondering if it will be worth trying.  Would there be a benefit over the stock down draft manifold?  The intake runners are pretty long, which I read is good for torque.  The plenum doesn't look to overly big to cause problems.  But I haven't been able to find any info on anyone that has done anything like it.

Thanks

Edited by Dirt Trackin'

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How exactly would you go about that? The 318i manifold is somewhat opposite the carbureted manifold in that the throttle body and air intake is underneath, on the bottom (down between the starter and the oil filter). 

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It surely would be one of a kind.  Suppose you would need an updraft carb.  Farm machinery used them, but sorta small for this engine.

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The other issue is that your air/fuel mixture will be happening a long way from your valves.  If your intake velocity was high enough you might be fine at higher rpms, but you'd have a heck of a time getting it to idle.

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I'd block off the bottom, and mount a carb flange on to the top of the plenum. Similar is done on the VW 2.0L when racers want to carb the engines.

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Edited by Dirt Trackin'

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drill a hole in your hood

This, I was thinking, too.

 

But the Zenith updraft carb on the trencher would almost bolt on, and it's very nearly as adjustable as a  Weber, if you can find jets for it.

 

Downside?  The venturi's probably 16mm.  It's sized for a 1750cc VH4D that redlines at 2800 rpm...

 

t

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I have no intention of using an up draft carb.  If I do it, it will be a weber 38/38 fitted on to the top of the EFI Plenum.  The car I'm building is a dirt oval track car.  I'm not worried about a hole in the hood. It's expected whether I am using the stock 2BBL manifold or this. 

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Then you have to do it and let us know how it does!

 

t

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The other issue is that your air/fuel mixture will be happening a long way from your valves.  If your intake velocity was high enough you might be fine at higher rpms, but you'd have a heck of a time getting it to idle.

 

There's a lot of science  that goes into modern manifold design, but there's nothing wrong with pure dumb luck - if it is on your side.  When I hear updraft carburetors I also think of handmade copper manifolds that worked  .  .  .  but probably not as well as a properly designed cast aluminum intake manifold with tapered plenum chambers.

 

If I remember correctly, the 318 manifold is designed to flow only air and not an air/fuel mixture.  This may or may not be an impediment to acceptable torque production versus an engine that merely runs on its own.  I would carefully consider whether the manifold has any low lying areas that might promote unwanted fuel pooling.

 

Other things to consider:  Runner sizes versus stock 2bbl manifold and weight comparisons of the two manifolds.

 

 

 

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With air cooled VW engines that have a centrally located single carburetor, icing can be a problem.  The fuel can pool because it gets too cool.  They try and combat this by plumbing hot exhaust gasses up through a tube that is cast in an aluminum 'casing' which includes the intake tubes, as they split under the carb.  You can see ice/frost on the tube under the carb in this photo.

 

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What Percy said about the 318 manifold flowing only air has me wondering whether it is heated with a jacket full of coolant (ironic as that sounds), like the stock 2002 manifold.  That heat is added to help keep the fuel in suspension, but if there was no fuel mixed, maybe they did not bother to heat it.  

 

I know it is advantageous to take cool outside air into the carb, yet also important to heat the passages to keep them from icing.

 

Can anyone explain how to achieve an ideal balance of cool air/heated manifold?

This sort of stuff is what makes me not want to shy away from original equipment.

 

I want to fit that stock air cleaner/snorkel onto my 32-36, that sits on a stock, heated, ported manifold and call it good.

I do admire creative endeavors, but I really respect what the engineers put into the original equipment.

Although a little desmogging is a good thing, imho.

Edited by '76Mintgrun'02

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Generally when we do manifolds on race cars, we want the fuel a cold as possible. This means going as far as removing the water jacket from manifolds.

Keep in mind this is a race car. A circle track car, that will see one gear and high rpm, and nothing but high rpm. There is never a risk of freezing the fuel.

The problems we have come across in past attempts at manifolds have been to smooth on the inside. Which does not allow the fuel to stay suspended in the air. For EFI, you want as smooth as possible of a finish on the inside of the manifold. It's only moving air, and you can achieve greater air speeds.

That, and having to large of a plenum. I've though I had it figured out. Strong vacuum, nice idle, crisp throttle repsonse, then I got on the track, and it would barely run once over 30 mph. All the fuel was falling to the back of the manifold and flooding the rear cylinder, while starving all the rest.

We found, that a ported stock manifold, with a slight increase in plenum size and spinning the carb 90° gave us best results.

Of course this was on a chevette, which did not have different factory manifolds to choose from.

In this case, my main concern might be runner length on the 318 manifold being to long to be beneficial. But with the sweep, velocity should increase which may even things out

But at this point it's all speculation. I will most likely attempt it down the road. But this engine is still a year plus down the road.

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