Do I go for a 2 barrel or a 4 barrel manifold for my straight six?
 
I get asked this a lot, “What’s the best, a 2 barrel manifold or a 4 barrel manifold?”
 
When I get questions like these I have to assume you are probably a novice at tuning or you pay somebody else to do it for you, nothing wrong with that but with this in mind this article is not aimed at the hard core racers because they know as well as I do that there are many variations you can experiment with to get the optimum out of your engine, gearing and chassis set up. So hopefully this will shed some light on it for you and hopefully you will be able to answer these questions for yourself by reading further.
 
First up a carburettor should be selected and sized based on engine power and maximum rpm.  Simply put the engines power is governed by the camshaft selection, compression ratio and the volumetric efficiency of the engine.  The carby does not make the power it just delivers the correct air/fuel ratio to meet what the engine demands at any given rpm. Engine demand is measured as cfm [cubic feet per minute] and there is a formulae that you can calculate a theoretical engine demand, if you are interested download our Kustom Bitz Dyno spreadsheet [click here] and on the “engine calcs” sheet you can see the relationship and calculate this and more.
 
Basic rule of thumb for straight six engines up to 250 cubic inch
Holley 350 2 barrel – Stock or near stock engines that never sees higher than 4000rpm
Holley 500 2 barrel – Hot street engine that gets revved on occasion to 5500rpm
Holley 465 – 4150 or a 570 Avenger 4 barrel – Hot street engine and many race applications that run up to 6500rpm often
Holley 650 double pumper 4 barrel – all out high rpm race engine or supercharger application
 
Why not triples, side drafts – Triple SUs webers or dellortos.
Classic race car induction for the straight six and very nostalgic looking. Great power makers and great for stick shift cars where you are either on the throttle or off it.  Some people swear by them and are great at tuning triples. You can make just as much power with a single carb as you can with triples, it all depends on how good a tuner you are with either of the set ups. I will say however that typically a 2 barrel or 4 barrel is more streetable for engines that have to work at part throttle and off idle in heavy traffic, that being the car will be more comfortable to drive on the street, even on real hottie engines.  We have had feed back from circuit racers going from triples back to a 4 barrel AussieSpeed inlet manifold, especially on the Ford Crossflow and OHC engines, and when dyno tuning they have reported a fatter torque curve and better peak horsepower with the AussieSpeed 4brl intake. So what I am trying to say is that one is not better than the other, but one may suit your application [the car set up as a whole] better than the other.
 
Swap Meet and EBay second hand specials - some history on the older intakes around.
The only manifolds for the Ford, Holden and Valiant straight sixes that have been available in the past before the AussieSpeed manifolds came out were not developed for even flow, they had poor runner design and huge, uneven plenums which caused low air speed, fuel drop out and uneven cylinder to cylinder fuel charge. This may sound like a sales pitch but it is not, it is a well known fact amongst engine builders who have had to overcome problems with these older manifolds on race engines in the past. 
 
We have measured up to 40cfm difference from port to port on old swap meet manifolds and some current brand manifolds are still using these same old manifold patterns. Basically in the past these manifolds were just a bracket to hang a big carbie off a straight six. This is why some old mechanics say you cannot run a 4 barrel carby on a Holden six, or I have heard them say a 500cfm 2 barrel is too big for a six. This is because the old manifolds that they are experienced in, were that bad a design that the bigger the carburettor the worse the driveability problems became.  The little 350 two barrel Holleys on these old manifolds seem to perform best because of their small throttle body helped air speed due to the restriction in the carbie itself and hence less fuel drop out and masked the manifolds real deficiencies.
 
2 barrel AussieSpeed manifolds - Carby selection
For example we have found the best performing 2 barrel set up for an AussieSpeed manifold is the 500cfm Holley, it responds well to jet changes, has crisp off idle throttle response and has a strong mid range torque. We have found that in allot of cases the 350cfm 2 barrel Holley seems to be overwhelmed by the manifold vacuum and appears to be too small to work properly.  However some baby engine applications the 350cfm has been perfect, so there is some trial and error on your part.  Please make sure you read through our Holley tuning guide for the AussieSpeed manifolds [click here] for the right way to dial a carb in.
 
4 barrel AussieSpeed manifolds - Carby selection
A small 4 barrel can be the perfect street set up for a hottie street machine or hot rod with high compression, mild cam or lumpy cam, good breathing head, extractors, free flowing exhaust and stiff gears. You think about it, at normal slower off idle street speeds you are running around on the primaries [small, good air speed for crisp throttle response] then when you give it a squirt the secondaries open up to allow your engine to breath and unleash its potential. If you have a 465cfm vac sec Holley then use that but if you need to buy a new one then the Holley 570 avenger is the best set up because the secondary jets can be changed and they are more tuneable to dial them in for the smaller capacity six cylinder engines. Edelbrock 4 barrels are a good choice too.
 
Offerings of tuning wisdom - take it or leave it
However a 4 barrel on a six is harder to dial in the tune, you need to work at it a little to get good results. Remember that all out of the box smaller capacity Holley’s are tuned for 300-350ci V8s so it will be too rich for the straight six. The other thing with a 4 barrel is that you also need to throttle car more like a race car, you cannot just stand on the throttle from idle and expect it to go, it will most likely flood and cut out.  So if you just like to stand on the throttle flat to the floor from idle and have no right foot restraint for throttle control, then a 500cfm 2 brl is much more forgiving or at the very least a vacuum secondaries 4 barrel that is properly set up on the dyno.
 
For the novice tuner a 500cfm 2 brl is a much more forgiving set up on a straight six up to 250 cubic inch and you can get good consistent results.

For the tech guy who has the tenacity to chase things down and problem solve and enjoys dicking around with engines then go for the 4 barrel because on a real hottie it is the ducks guts.

For those of you who are prepared to spend money on a good tuner and your engine is a real hottie then the 4 barrel is the go and you will need a tuner like the tech guy described above and make sure they follow our tuning guide for the Holley carb and the AussieSpeed manifolds click here otherwise find a different tuner.
 
So what is the best for you, a 2 barrel or a 4 barrel? You tell me.