Reading through Titus's "my new favorite piece of speed equipment pics", I got to realizing what I've noticed before in multi-carb intakes that had divorced plenums or "logs". It seems that in almost every case, there's been some kind of passage between the seperated "halves", and I can only guess as to why. I think it's there to balance out vacuum between the plenums, but haven't bee able to figure out why there would be a need to balance it that way. Seems to me that the engine would run as if it were two engines connected by a common crankshaft. In my case, I'm thinking along the lines of a three 1-barrel setup for an inline six. I have yet to see one manufactered for a Mopar slant, so I'll have to fab my own. Is it that critical to have the balance passages between the divorced plenums, and why? Roger
Roger, Balancing the plenums for a street engine is kind of important. Not to say you can't make it work w/o balancing them, but getting it to run right at anything less than full throttle will be difficult, and a nice idle will be challenging to say the least. If you use something like Weber carbs, where they are designed to be tunned in that fashion, you can make it work well on the street, but the set up you are talking about really should be balanced. The bigger the balance tube the better, but remember....you can use something a little smaller if space is an issue. Just my 2 cents.... Matt P.S. I've built several log type manifolds over the years for V-8's. All but 1 were balanced....never do that again. LOL
OK.....Hmmmmm, it must have something to do with the firing order and the airflow "pulses" on the intake stroke. I'll follow the lead of the experienced, and engineer in fittings for balance tubes. Thanks.....Roger
I think it is done to keep the gas flowing through the carbs smoothly. If you have split manifolds there might be some "down time" between cylinders taking air.
A seperate log on each bank of a V8 is kind of a bad compromise. Because of the firing order the pulses are not spaced evenly ( in degrees of crank revolution, so that means the pulses are not spaced evenly in time with the engine running. ) That means that some Cylinders on one bank are helping each other, and others are fighting each other, with some Cylinders not getting all the mixture they could. A better way to build a Log Manifold would be to connect the Inner two Ports of one Bank with the Outer two of the other ( like a dual Plane 4BBl Manifold ), to split the V8 up in to two seperate 4Cyls. The balance tubes are there to bleed off the spikes of High pressure of one bank, at the time when the other side has a spike of Low pressure. The best is to seperate all the ports completely... Here is a pic of a Quad Weber setup I'm building.
The balance tubes are a band-aid of sorts, or insurance, you could say. In an engine where you have individual carbs for each cylinder, like a 4 cylinder bike, you have to be extremely careful to balance the carbs at idle so that the cylinders are all doing an even share of the work, as mentioned above. The easiest way to check them is with individual vacumme gauges on each cylinder. On bikes, they use mercury filled glass tubes that are mounted along side each other. You adjust the independent idle screw on each carb to get them as close as possible at the lowest rpm possible, then set the engine idle with a coordinated idle screw that moves all the carbs in unison. On multi carb set-ups on cars, the mechanics of setting up the gauges is a little more difficult and varys according to the system you use. Eight singles is done much like a bike, but 4 two bbls may be different. I have even seen a large tube with a floating ball that sits ove the carb throat to measure airflow on a carb. The dangers of failing to balance all cylinders varies, but is most important on the street. Race motors tend to idle higher, hence diminish the flow % that the imbalance makes up. On a street engine, a signifigant imbalance causes one or more cylinders to be drug along by the others. This puts enormous stress on the crankshaft and can cause harmonics through the valvetrain that will lead to sudden failure if left unresolved long enough. Metal fatigues with continuous vibrations like that. Even a forged crank can be damaged. The where and when of a problem is impossible to predict. By adding a balance tube between plenums or even using a common plenum at the top of a tunnel ram, you allow the carbs to appear as one to the cylinders as a group. That diminishes the effect of the imbalance, but the more carbs in play, the less it may help. If cylinder 8 in under breathing, it's hard for cylinder one to give away air to cover it. You still want to balance the carbs.
its all about pulses.you need the balance tubes if you want it to idle,its more important on 4 bangers and 6 bangers,but still needed on v8's randy
Thanks a bunch, guys. That's the info I was hoping to get. I figured I'd have to balance the carbs, but was looking for a decent explanation concerning the balance tubes. Panic asked how many plenums for a slant. Factory is one, in either a single barrel or 2-barrel configuration. I'm looking at fabbing up a "set" of manifolds to cover the front two, middle two, and back two cylinders, and mounting Holley 1-barrels on each. With this info, I will plan on adding the balance tubes to tie the plenums together, as I do plan on it being a street engine. On a related note; I've got Holley 1904 and Holley 1920 carbs available to me. The 1920's were factory stuff from '61 to about'73, while the 1904's are mid-fifties to very early 60's vintage Ford pieces. I'm really leaning toward the 1904's, as they all have the glass float bowls and brass floats. (Little bit of eye candy there.) The problem comes in at the throat, where the air filter mounts. The 1904's have a double tapered ring that looks like a thumb-screwed ring clamp would be tightened up to retain the cleaner. (Probably an old oil bath type.) I have nothing in the way of air cleaners to fit this arrangement.........any suggestions as to where I might get some air filter assy's, or at least the ring clamps/bases that I could weld into something more conventional? Thanks, guys.....Roger
That is not the way I would split them up. I think the Slant 6 has a firing order of 153624. If that is correct, then splitting up the intake into 3 two's side by side you'd have real uneven pulses in the outside two pleniums. I would pair up Cyls 1 & 6, 2 & 5, and 3 & 4. That way all the pulses are spaced out evenly. And you would have to make all the tubes equal length too... A lot of work, but I think the performance gain would be worth it...
********************************************** Right you are on the firing order......15; too young, 36;too old, 24; just right ****************************************************** Good God, man!! I can only imagine the nightmare of a can-o-worms THAT would look like. The main reason I wanted to split them up in the first place is the cylinder-to-cylinder fuel distribution imbalance that is inherent in that semicircle intake from the factory. With a "stock" intake, 1 & 6 run a bit lean, 2 & 5 run about right, while 3 & 4 run a bit rich. I figured if I could split them up into three and try to make something with equal length tubes, then at least I should have relatively equal fuel/air charges to all cylinders. You are now telling me it will be marginal due to #1 starting the intake stroke at about the same time #2 is somewhere in the power stroke (I think), right? Roger
Equal length is good, but even pulses are more important. Look at my High Tech illustration... I think a Custom Fabbed intake like that would look cool. If that would be too much work, you could go for a 1-2-3, and 4-5-6 split. That way they are evenly split too...
Im glad I got to catch this post, I was thinking of doing something like this on my comet. I am no intake/airflow/engineer but I do have common sense which has gotten through most. I guess from that I would have to agree that it seems that having the carbs split up in twos would still be better than the stock setup. Although I was thinking of just using a common plenum and mounting three carbs on that, then seperate runners to the intake ports. Like I said though I am no engineer, but I am always willing to learn so keep it coming.
Long runners like that will give you other problems though...With a long cool runner the fuel will drop out of the mixture and puddle on the floor of the manifold. Especially when the engine is cold. A Jag six with three SU's is set up exactly as he is first describing with 1-2, 3-4, 5-6 joined. Plenums have a little more to do also with simply the volume which the intake runner is drawing from. In the case of a log manifold, the balance tubes are for exactly the reason stated, to balance pulsations in the manifold. But plenum volume is increased also which effects the harmonics of the intake manifold. Intake runner length and size matter because of harmonics. Think about the pulsations in an individual runner. The intake valve is opening and closing creating pressure waves which travel up and down the intake runner. These pulsations occur at different frequencies depending on the camshaft and the speed at which the engine is operating. Just like a tuning fork, the runner is of a certain volume and contains a certain mass and at some frequency, it resonates. When this happens more air and fuel are pushed into the cylinder increasing power and the engines volumetric efficiency. Now if we complicate it a little more you can think about multiple runners where the runners are connected to a common plenum. Now the runner and the plenum will have their own resonance frequencies. Which will have the same effect at resonance as far as pushing more mixture into the cylinder. This is the reason for dual and single plane manifolds. A single plane manifold has short large diamter runners connected to a large plenum to resonate at a high frequency. A dual plane has longer runners connected to a smaller plenum to resonate at lower frequencies. Hopefully that helps.
Holy friggin crap! There are some smart people on here. Thats why I like this place. I never understood all the scientifics like harmonics in intake manifolds and stuff so its great to learn about this kind of stuff. I pretty much just understand: add 1 part air, 1 Part fuel, squeeze it real small, and light that shit on fire and it makes the wheel go round and round. So would it be better to make a log type manifold with a common plenum on a straight six engine or would it be better to group them in twos? It seems like either way would be better than stock but with a common plenum you could lose alot of low end torque correct? The info on the jag six is why people should not be hung for mentioning something other than a "traditional hot rod" on this site also. Cars share alot of basic things that can be useful in other cars. Some people need to open their brains. Thanks for the info.
Multi Weber setups are mostly used with big Cams, and often chosen too large to work well on the Street. When I put a twin DCOE setup on my 1.6 Kent powered Tow Car The Experts told me it couldnt be done. The Engine was completely stock ( including the Cam ) and I put the smallest venturi's in it that I could find ( 26 or 28mm if I remember correctly ), and a 4 into 2 into 1 Header with long primaries. Much more low RPM Torque than stock, and better milage too... Yeah that would work, that gives a even split with a 153624 Firing order. Long runners wouldnt worry me too much, although each sollution has its drawbacks. For the Slant 6 I'd try and stay as close to the length of the runners of the stock manifold as possible, because Mopar did a lot of experimenting with intake runner length through the years. ( lenght wise, what they came up with is probably not so bad... See pics ) Try and avoid sharp turns in the intake...
A good book on this topic is "Scientific Design of Exhaust & Intake Systems" by Morrison. It's on Amazon, and I've seen it at Borders before. It's based on testing done in the 60's, so it feels outdated (there's nothing there about VVT or active intake manifolds, but you weren't planning on using either anyway...). The fundamentals are there. That book, plus the theory of Helmholtz resonators, will get you 80% of the way to being an intake manifold designer. The other 20% is a flowbench, a computer modeling program, and experience.
I am no intake engineer by any means. Especially after reading this thread. I was considering "my hand" at building a custom intake. After reading this thread there is no telling how many hours you guys have saved me. But, if I was going to atempt to master the art of the intake I think I would try two different designs. The one that works would remain on the car, and the one that didn't would be a helluva conversation piece on my garage wall!
Check this out. http://victorylibrary.com/mopar/intake-tech-c.htm#ir http://www.phys.unsw.edu.au/~jw/Helmholtz.html
I am no means an engineer, but I built the intake on my slant six. It is for a 471 blower and it seems to work fine. This is not a race engine, it is on the street. I realize that a blower is different than a normally aspirated engine. I am also building the intake for my HA/GR. It is a log type manifold with two two bbl. Carters on it. I measured an old four bbl. manifold that they used on the slants and made my runners that long. This of course is for a race engine and it probably would not work on the street. I have yet to fire the motor off iwth this intake on it, so I do not have any more info to add.
The Jaguar 6 (XK120, XKE etc.) firing order is the same 1-5-3-6-2-4, and engines have been built quite successfully using both 2 carburetors (1-2-3, 4-5-6), and 3 carburetors (1-2, 3-4, 5-6). All had separate manifolds with a small vacuum balance passage. A direct passage from the bore to the port is much more important than the firing sequence. What appears to be a common plenum is a common casting with complex inner bits.
Ya metalshapes I didnt see the pic last night. It does appear to be a common plenum in the pic, unless its divided inside or something.
OK....so we're back to it "may" work with three 1-barrels with balance tubes, but if there's problems, I should rework it to two 1-barrels with balance tubes. I'd really like the three carbs, but two will work for what I've got in mind. Thanks for all the info, guys.....gives one some real food for thought. Roger.....The FNG with the tilted motor.
ok,i raced a 4 banger at b-ville and maxton,one thing i learned is that a 4-6 banger needs a small plenium,as small as you can make them.if you have a race cam its the same deal,small plenium. i built several intakes,ok way more than that! the ones that looked fast were slow. i had speed differences of 50mph with just an intake change.the longer the runner,the lower the rpm range that the engine makes power.keep em short,10'' from the valve centerline to the plenium. randy
ok,i raced a 4 banger at b-ville and maxton,one thing i learned is that a 4-6 banger needs a small plenium,as small as you can make them.if you have a race cam its the same deal,small plenium. i built several intakes,ok way more than that! the ones that looked fast were slow. i had speed differences of 50mph with just an intake change.the longer the runner,the lower the rpm range that the engine makes power.keep em short,10'' from the valve centerline to the plenium. randy
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