Dual carbs and Ram effect.

[theKbStockpiler]

I know that duals on a acvw help keep fuel from condensing like in single carb manifold but isn't the ram effect moved way up in the rpm range as a consequence? A short runner is only good for high rpms ,correct?

[Steve Arndt]

Answer: It depends. Generally longer is tuned lower RPM. Volume and length of the runners are both part of the engines response characteristics during transients.

With 90 Kpa barometric pressure here I can easily see 95 Kpa map as I pass through the torque peak while data logging a hard and quick pull.
edit that is with IR ITBs but it is the same with dual carbs on the same manifolds.

[theKbStockpiler]

Let's say we use a FI induction for the example. Wouldn't a single throttle body with multiport work better for a street car that needed torque?

[Marc]

Lots more to consider than just runner length....runner/plenum volumes are big factors too. Are we talking about dual 1-bbls or dual 2-bbls? The stock 1-bbl upside-down-bicycle-handlebar arrangement was chosen strictly for simplicity and low production cost, performance was the last thing considered. Reasonably-sized dual carbs often yield better low-end grunt than a single - my daily driver is a stock 1600DP with dual 34ICT 1-bbls and it pulls better at low RPM than any single-carb engine I've ever driven, to the extent that I have to remember to downshift in order to avoid lugging/overheating.

Let's say we use a FI induction for the example. Wouldn't a single throttle body with multiport work better for a street car that needed torque?

Better than WHAT? Are you aware that there's a forum more appropriate for this sort of discussion?

[Steve Arndt]

I hate quad/dual linkage. It is never perfect on a flat four. I have ran lots of ITBs and five kinds of linkage over the last 10 years. So ITBs are best for max power and easy to tune in that case for a torque boost compared to a plenum, but harder to get dialed for drivability in the 1 to 3 % load areas where the engine spends 99 percent of it's time on the street.

[Marc]

Sounds like your life would be much simpler if you were to find some decent linkage. Ever tried Berg's? I've used it successfully on everything from 32mm 1-bbls to IDAs....street, off-road, circle-track, drag-race... all it requires is a stable shroud to be mounted to, and once set up works perfectly "forever".

[theKbStockpiler]

Better than WHAT? Are you aware that there's a forum more appropriate for this sort of discussion?

I'll do a better job choosing a forum next time,


I guess I'm trying to reason with a short intake having a ram effect at RPMs that you may never reach, but will support better mixing because of their lack of length. The torque increase from better mixing is greater than the torque increase from the longer runners.

How much increased torque is produced from using velocity stacks on duals? It has to reduce the rpm that peak Hp is reached. I would think that using a multiport FI system with dual throttle bodies with longer runners would be best for everything except a drag car but I don't see anyone doing it.

[FJCamper]

Hi theKbStockpiler,

Yes, all things being equal the taller carb manifolds produce more low end torque. And your asking if shorter manifolds with better mixing might compensate for the shorter length is a valid question. In my opinion, I think it might all even out ... but the answer you really need is short or long manifolds, in road course racing, you are up and down through the gears so much at full throttle, you never have time for manifold length to have too much influence on your laps times.

You referenced "ram air," but you are actually talking about intake velocity. Ram air is air forced via a scoop into a carb at high speed, which has a small supercharging effect. You probably get more benefits from the cold air than actual "ram air."

Under steady state conditions, manifold length is important. But there is only about a 500 RPM loss or gain between short to long on a VW engine! All manifold lengths are compromises.

What are you running now?

FJC

[theKbStockpiler]

I'm thinking out my induction system before I start it. My car is a big and long project.

I'm using 'Ram Effect' as a synonym for the cylinder filling force of the intake charge that is stored after the intake valve is closed and then released as the valve re-opens.

My goal is to fill in the gaps I have with acvw carb/manifold combinations. If the stock type I/II manifold is replaced with IR manifolds the power band should move way up.

Can anyone tell me if the stock manifold actually limits peak RPMS or If the short IR manifolds move the power band of the manifold up higher than the engine can benefit from? I'm thinking that the IR manifolds are making the greatest filling pressure at 10k when the engine may not even hit 7k. Hence the common IR manifolds are actually too short for a street car. The stock vw single carb manifold most likely is only good for a high manifold vacuum for drive-ability issues ,especially cold. Domestic V8's ; with big induction systems ,tend to run like hell when cold.

[FJCamper]

Hi theKbStockpiler,

Yes, I understood how you were using the "ram air" term.

Now, how to fill the cylinders as much as possible with fuel/air mix is your question. That's "volumetric efficiency." Luckily, we don't have to guess much about how to go about it, as the math and principals are well founded. It works like this ... a piston is on the down stroke, creating suction. How much suction depends on the size of the piston and length of the stroke.

The modifier is how large are the intake valves. Too big and intake velocity suffers. Too small and not enough fuel/air gets through.

The next modifier is how long the intake valve stays open.

Then, things get complex as the engine runs. And this is where manifolds matter. You don't want the manifold to be a problem. It should be as direct a link between the carb and the intake valve as possible. And it has to be the right diameter to maintain intake velocity. Again, too big and velocity suffers. Too small and not enough fuel/air gets through.

To begin to optimize flow, if there are two intake runners in the manifold, each one feeding an intake valve, they should be the same length.

A real world detail is the longer the runner(s) the more time the fuel/air charge has to separate. So manifold length is related to carb design. A Weber can create a better mixture than a Solex, but other physical laws dictate at which point fuel/air separation begins.

OK -- we've just opened the door here on design. I'm presuming you've read all this before somewhere in theory.

If you're with me so far, or have questions relating just to what we've covered, let me know.

FJC

[theKbStockpiler]

One explanation I read on ram effect is that the runner length has to be such that the inertia of the fuel charge has to still be compressing at the speed you want the ram effect to be maximized. A short runner only has a short distance for the fuel charge to travel and hence compress so the runner length effectively "times or synchronizes" the ram effect. A long runner has more time for the fuel charge to compress which is needed at lower RPMs. If the timing (length) is off,the fuel charge will actually be moving away from the valve when it opens.

Acvw IR runners are pretty short so I guess the ram effect would be maximized over 7k rpms.

Then there is the plenum ideology where there is a chamber that collects the inertia of all of the runners and improves the ram effect at a lower RPM of around 5k.

There's a lot to consider.

[Piledriver]

Any runner "optimized" at 7K also has a harmonic peak at 3500.

Not just runner length but just as vital is cross section area , as well as plenum volume and inlet area and length of plenum inlet.
If second harmonic lengths don't work, 3rd harmonic probably will, even though lower % still beats nothing.

Note: The plenum "inlet area" does not have to equal the throttle area.
It can be MUCH larger, or smaller....or variable size/length.
(assuming ITBs feeding off an airbox AKA plenum)

The tuning also tends to work over a broad range with a peak at center.
Some setups can manage multiple peaks in the power band.

A really long 1st or second harmonic runner sounds great on paper, but has to be ~huge cross section to offset surface friction losses at high RPM...
...so it doesn't really work that much better in meatspace, if at all.

Hoping for the best possible compromise, initially based on (and in the end constrained by) what actually fits is the norm.

[FJCamper]

Above: For best intake runner length on a VW or Porsche 4 cylinder pushrod engine, 15 inches from intake to valve is max optimization at 6000 RPM.

Hi theKbStockpiler,

Like Piledriver says, we more often have to use what fits rather than what is optimal. When it comes to carburetion, the Porsche factory figured out the best applications for the four-cylinder 356's before we VW guys even got the 40-horse.

Fuel injection beats carbs in almost every way, the big one being that the injectors can be right down on the intake valves and you get a beautifully mixed fuel/air charge that has little to do with intake runner length.

I went to Porsche, VW, and BMW fuel injection schools (all Bosch systems back in the day) and have a good understanding of them. But I'm a carb guy at heart.

I believe had fuel injection existed first, and someone later invented the simple carburator, he would have won the Nobel prize.

FJC

[Piledriver]

I believe had fuel injection existed first, and someone later invented the simple carburator, he would have won the Nobel prize.

FJC

It was a very close call.
Early carbs were very primitive beasts, literally relying on evaporation of fuel as air passed over a puddle of gas.
A carb with a wick was a huge tech advancement at the time.
Fuel injection was relatively sophisticated almost from day one.
Things moved pretty fast... On high spec gasoline engines, FI was the norm from the mid 1890s.
FI was invented to fix the issues with carbs almost from day one.

From Wikipedia:
The carburetor was invented by an Italian, Luigi De Cristoforis, in 1876.[citation needed] A carburetor was developed by Enrico Bernardi at the University of Padua in 1882, for his Motrice Pia, the first petrol combustion engine (one cylinder, 121.6 cc) prototyped on 5 August 1882.[citation needed]

A carburetor was among the early[when?] patents by Karl Benz as he developed internal combustion engines and their components.[3]

Early carburetors were the surface carburetor type, in which air is charged with fuel by being passed over the surface of gasoline.[4]

In 1885, Wilhelm Maybach and Gottlieb Daimler developed a float carburetor for their engine based on the atomizer nozzle.[5] The Daimler-Maybach carburetor was copied extensively, leading to patent lawsuits, but British courts rejected the Daimler company's claim of priority in favor of Edward Butler's 1884 spray carburetor used on his Petrol Cycle.[6]

**************
1891: Herbert Akroyd Stuart invents the first internal combustion engine to use a pressurised fuel injection system.
1892: February 23, Rudolf Diesel obtained a patent (RP 67207) titled "Arbeitsverfahren und Ausführungsart für Verbrennungsmaschinen".
1892: Akroyd Stuart builds his first working Diesel engine.

So it appears gasoline injection predates Diesel, maybe. Diesel may have used injection prior to recieving the patent for obvious reasons, but it is possible to feed diesel into the intake like a carb, just not terribly efficient vs high pressure injection.

I'm now curious if they were using port or direct injection... and how Diesel metered his fuel in the first Diesel.

[Ol'fogasaurus]

Don't forget the placement (location) of the carb/manifolds on the head, the shape of the valve pockets, manifold design/shape (for flow patterns) and a myriad of other tiny little things that also affect air-fuel flow and atomization. I think a good flow bench (for manifold design and head work) would be of some initial advantage to the working the problem(s) also.

What about a balance tube or does it seem to be not necessary here; something I have also heard debated but never heard if it was resolved or not. The big engine boys I think are still playing with this as well as cam design.

And don't forget about exhaust tuning for RPMs. Most equal length headers are a compromise at best.

Not trying to be a deadhead on this but everything is tied together and can't be separated. About the time I was a kid making single changes were changing as the whole was affected by all the components was discovered (not really the right term but that is what it seemed like) which drove rapid changes/discoveries' started to come out of it. The eco problem and the limits put on drove changes/upgrades at a rapid rate such as electronics to measure and control things like FI and precise engine timing and advance. They are still fighting the good battle as they learn more about things and how to control them.

You just can't focus only on a small aspect: it was tried and found that they had to deal with the whole.