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DCOE's and fuel condensation at the mouth of the carb, input


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I am looking at finally making an airbox that fits the sidedrafts WITH the regular booster.

One thing I was thinking about on the way to work was when I had RAMFLO filters on, they would just get wet with fuel, and without the filters on there always seems to be some fuel sitting at the mouth of the carb, like condensed fuel vabors sitting there.

So I am wondering, for the people with velocity stacks, if you get fuel at the opening of the bellmouth?

What I am getting at here is if I have an airbox, I am wondering if eventually a buddle of fuel would condense outside the bellmouths.......possibly leading to a fiery combination in the wrong circumstances.




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as the air gets sucked, in it generates a change in pressure which cools the air, causing condensation. you'll notice this more in very humid environments.

its might smell like fuel, but it is likely just smelly water. if you do have fuel there then you may have a timing issue cause some back pressure from the intake. i've heard of filters catching fire due to backfiring, might be something to resolve before selecting a new filter setup.


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The above is true but there is also another thing coming into play.

When an intake valve opens and a piston sucks in a fuel/air "charge" it does so with a lot of velocity. When the valve closes, the remaining charge smacks the back of the valve and sorta bounces back till the valve opens again and the whole thing starts over. This is one of the many ideas Metric Mechanic uses in their designs. If you take off your air filters on your Webbers and rev the motor you should be able to see the actual suspended fuel up to an inch out of the carb. This is also another good reason why velocity stacks are a good idea.


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Huh... here's a third factor- it's called 'reversion standoff'

There are yootoob videos of s14's with big alpina stacks being run through

the revs, and you can watch the reversion cloud move in and out of the stack,

and even sit a foot out... and this with injectors that shoot at the base of the

valve stem.

What happens is that as the exhaust valve is closing, the intake is opening.

As it opens, residual pressure inside the cylinder blows back out the carb.

It builds a fuel cloud inside the intake- often a long way back up- and then the

exhaust valve closes and the cylinder sucks air back into itself. Except that

it's now air with some fuel in it...

This effect is very dependant on revs, and to a lesser extent, on throttle

opening. As you wind a motor out, the reversion cloud may build

and fall several times as you go through resonance points in

both exhaust and intake systems.

This is part of the reason why Webers have all these trick metering features,

and also part of the reason why engines with significant cam overlap

(intake and exhaust are open at the same time) are so sensitive to

intake and exhaust length changes. There's a phenominal amount

of shit going on to determine exactly how much air and fuel

get mixed in a high- performance engine running at the

peak of its tune.

That all said, you can stuff a weber 34 ICH on a single downdraft

manifold and still have a reasonable approximation of a car...

So yes, anyone who runs DCOEs will get fuel reversion. I get rings

on th inside of my intake plenum (I race with avgas, which is dyed blue)

and they MOVE depending on what my average rev range is for a weekend.

...it's kind of fun...

( I couldn't find the video, here

is a less exciting version)


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I have a car (not BMW) where the Webers stick up out of the hood. If I run the car at speed with no air cleaners, the standing charge gets ripped off by the wind and the car runs lean (and my head gets covered with gas.) If you have found the right length horns for your rev range, it's important to protect the air source so you don't do this. Notice how the early Ferraris that have a hood bump don't let the air in the front unless there's a baffle there. Standing charge is important.

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