Why dual, twin-barrel carbs when one, single-barrel would do?

Dumb question from carb amateur.

1) With a four-cylinder, four-stroke VW Type 4 engine I assume only one cylinder can be sucking at any one time.

2) Assuming a 2lit engine, each cylinder is 500cc.

3) Thus, the carb would need to be sized to feed a 500cc cylinder at max rpm.

4) Again, assuming four cylinder/four stroke, a single carb would be flowing continuously.

5) Now lets assume one carb is replaced by two, single-barrel carbs. They would also need to be sized for a 500cc cylinder but would only flow once every two strokes.

6) Now lets assume two, single-barrel carbs are replaced by two, twin-barrel carbs (like the Webers I'm looking at). They would also need to be sized for a 500cc cylinder but would only flow once every four strokes.

Here is my question. Two, twin-barrel carbs won't flow any more air than one, single barrel carb but they will be more expensive and difficult to balance. So why are Type four engines configured this way.

 

With a single centrally mounted carb the fuel falls out of suspension as it passes down the long manifolds

 

good question! VW put 2 single barrel carbs on type 3 and 4 engines.

 

Factory T4 engines had two single barrel carbs, but I see what you're asking.

Two twin barrels (ie one carb per cylinder) does seem overkill, but I assume folk fit Webers and Dells because they're readily available and that they're a much better design of carb than, say, ICTs. Not all carbs are created equal.

If the opportunity's there to fit something that (in the case of Dells, IMHO) was the very peak of carb design, fit 'em!

 

Why do modern cars have 1 injector per cylinder?

 

I appreciate that the manifold vacuum will reduce the temperature and cause the fuel to condense so one solution is to mount two carbs on short manifolds. However, that sets up two more questions.

1) Would it not be cheaper/easier to heat the manifolds?

2) Why two, twin barrel carbs rather than twin, single barrel?

 

Regarding injectors, I think one reason for one per cylinder was to reduce the operating frequency. On a four cylinder engine a single injector would have to operate four times faster than one per cylinder. I recall a project to inject a high revving two stroke engine which was abandoned because the fastest injector was not fast enough! That was 20-years ago so things might have changed.

 

You're looking at it too linearly... engines are dynamic, and therefore each cylinder is slightly different...

before we get into that, here's another thing to consider... if the engine is running at 1000rpm, that means each cylinder fires 1000 times per minute... so a single carb would draw through 2000 litres of air per minute (based on it being a 2l engine) so 2 2 barrel carbs would only see 500 litres of air a minute, so the cooling effect on the carb is less, and the engine will also run more efficiently as it's not pulling all from one source!!

 

but petrol injectors work on a frequency, they don't speed up or slow down, they all fire at the same time (valves opening control fuel flow into the engine) the only thing that varies is the fuel pressure!!

And the air source is generally a plenum chamber!!

 

Fiiiiiiight !

 

1 inj per cylinder is more to do with fuel efficiency and leaner emmissions than anything else
you will get a better performance from 1 carb per cyl than 1 carb supplying 4 cylinders, think bike engine

 

I'm know very little about fuel injection. On my injected engine there is a fuel pressure regulator at the end of the fuel rail to keep the fuel pressure static. I assumed the engine controlled the flow through the injectors by controlling the injector opening frequency and duration?

 

nope... fuel flow is controlled by fuel pressure and injector size

if you look at the regulator it'll have a vac hose on it connected to the inlet manifold... the vacuum controls the pressure... normally the lower the vac the higher the fuel pressure!!

I agree... vw did a single point injection on the 1.8 mk2 golf driver... they soon dropped that

 

Weeel, not strictly true, I believe. With correctly dimensioned injectors and stable pressure, it does all come down to duration of opening. It's the only tool in the engine ECU's box.

 

well more modern stuff should do... but I generally don't touch that

 

Whether 400 or 500cc per cylinder on the induction stroke, the more fuel getting to it the more power the engine will produce per minute. The greater the "barrel width" the greater the power throughput.

Why anyone wants more power from a 50bhp camper is beyond me......... 67mph flat out is plenty

 

Back to the original question, I think what I've learned is.

1) Shorter manifolds are better
2) The twin-barrel Webers are better quality products
3) By splitting the work between four carb barrels they each operate more efficiently.

Is that fair to say?

 

I'd agree with that

 

Sorry, I forgot you were stuck in the FI stone age . I also believe injectors work individually now there's proper brains behind it...

 

1's always true, I'd say, but the biggest reason for fitting is likely no 2.

The heftier carbs were designed for higher-performance cars, whereas the simpler ones just had to do an adequate job in a Bedford van or Land Rover (and be cheap).

You'll have one carb per cylinder - how cool is that? Be happy !