Carbs - why will triple produce more power?

[Azon]

Ok abit of a stupid question, but... I keep hearing about people using tripple carbs - but never quite understood the theory behind using 3 cabs instead of 2 and why would 3 produce more power? From what I undestand there is a perfect air/fuel ratio that will produce the maximum amount of power and I presume the stock say 240 carbs will be able to supply even L28 with plenty of fuel and air - so whats the point of switching to tripples - is it just a matter of flow (as in tripples flow slightly more air/fuel in a period of time)? Or is it because you can tune triples more accurately (each for 2 cylinders instaed of 3) than twins - but then still dont see if that will make any significant difference on its own without any accompanying mods such as cam, porting etc.?

 

Also another question - why are 240 carbs considered to be so much better than 260 carbs?

[gav240z]

Ok from my understanding it's like this.

 

Questions 1.

First of all Twin SU's are great carbs and can perform great on any L-series. The problem begins to become evident when you are getting to higher revs and the engine needs to be able to breathe as much as possible.

 

SU's can be bored out so that the throats are larger and port matched to the intake to allow good flow.

 

However if you have ever looked at high performance machinery you will often see throttle bodies along the side of the motor nicely lined up and often on an almost 45 degree angle.

 

Just like the exhaust system on your car the less bends in the system the less restriction there is. Of course in theory this is great just make everything as straight as possible and no problems.

 

However you also have to take into account air turbulence when it hits items such that stick out like an unmatched manifold this is why port matching is so important.

 

Triple carbs come in a range of sizes depending on the size of your engine and usable rev range. The flexibility that you are given with Triple's is alot greater than with the more simplistic SU's.

 

Triple carbs will give a good power increase but to really benefit from them you need to have a nice lumpy cam and some port / valve work to match.

 

Keep in mind with an engine there is an ideal amount of vacuum. Just like your garden hose when you turn the tap on you get a nice dribble of water provide a certain amount of blockage and it will scream out the end. In an engine it's about finding the right flow balance and back pressure.

 

I'm sure someone will correct me here but this is how I understand it.

 

Questions 2

 

Why are 240z carbs better than 260z carbs? To put it quite simply the 240z carbs were far more simplified because of no emission controls.

 

The 260z carbs in most cases are in need of desperate rebuild and are almost useless after 30+ years of life. I'm sure they can perform well but the amount of modification and work required to them just to get them to a standard that the 240z ones are already at is simply not worthwhile economically.

 

Many 260z owners have had issues with vapour lock and in general they are considered a less than ideal design.

 

I guess Nissan realised the design flaw and went with EFI on the L28.

[Toecutter]

Hi Gav, I've been spending a lot of time reading basics on how an engine works. I know in general terms how cranks, cams, carbies etc work but I've been trying to look into specifics to iprove my basic knowledge and understand why I would get performance gains from mods liken specific head work. I don't mean to hijack this thread but this question sort of moves from the air flow issue discussed above re through carbs through to inlet manifold and then inlet port. I've just read some info on Wikapedia thats got me confused. Some of the info below sounds contradictory to what I've heard from many people, especially the stuff about reducing port size and rough texture for better performance???

 

[edit] The "Porting and Polishing" Myth

It is popularly held that enlarging the ports to the maximum possible size and applying a mirror finish is what porting is. However that is not so. Some ports may be enlarged to their maximum possible size (in keeping with the highest level of aerodynamic efficiency) but those engines are highly developed very high speed units where the actual size of the ports has become a restriction. Often the size of the port is reduced to increase power. A mirror finish of the port does not provide the increase that intuition would suggest. In fact, within intake systems, the surface is usually deliberately textured to a degree of uniform roughness to encourage fuel deposited on the port walls to evaporate quickly. A rough surface on selected areas of the port may also alter flow by energizing the boundary layer, which can alter the flow path noticeably, possibly increasing flow. This is similar to what the dimples on a golf ball do. Flow bench testing shows that the difference between a mirror finished port and a rough textured port is typically less than 1%. The difference between a smooth to the touch port and an optically mirrored surface is not measurable by ordinary means. Exhaust ports may be smooth finished because of the dry gas flow but an optical finish is wasted effort and money.

 

The reason that polished ports are not advantageous from a flow standpoint is that at the interface between the metal wall and the air, the air speed is ZERO (see boundary layer and laminar flow). This is due to the wetting action of the air and indeed all fluids. The first layer of molecules adheres to the wall and does not move significantly. The rest of the flow field must shear past which develops a velocity profile (or gradient) across the duct. In order for surface roughness to impact flow appreciably, the high spots must be high enough to protrude into the faster moving air toward the center. Only a very rough surface does this.

 

http://en.wikipedia.org/wiki/Cylinder_head_porting

 

*EDIT: after reading this again it might be a good idea to move it or not drag it out at the least :oops:

[zedevan]

i think what the wikipedia bits saying is that since the fluid (air) is stationary on the walls no matter how smooth it is, it doesn't matter if the walls are slightly textured as all it means is there is stationary air trapped in the indents?

 

However you also have to take into account air turbulence when it hits items such that stick out like an unmatched manifold this is why port matching is so important

 

that makes complete sense from the fluid dynamics crap i've done and just common sense in general, however i'm guessing the porting the wikipedia bits saying isn't just talking about this?

 

re 240 and 260 carbs, my car had 240 carbs put on it at some stage as the orginal owner kept having issues with the 260 carbs, there are semi detail notes as to what was going wrong with them, so if someone wants i can try to find them again

[thehelix112]

I don't have time to respond to all of this, or to write an essay on head flow (which would be entirely theoretical and supposition anyway), but I believe the reason why you have rough walls, is to prevent boundary layer separation.

 

If the wall is completely smooth the boundary layer just gets bigger and bigger and bigger and bigger until it separates, and you end up with slow turbulent boundary layer flow near the walls. So you make it rough so you continually break up the boundary flow, creating indents of dead-flow as zedevan says, and it never gets the chance to become separated and turbulent.

 

Dave

[zedevan]

so dave why will tripple webers produce more power? i'm not looking to ever get them, however it would be interesting to know!

 

i would of thought that if you were happy throwing to much petrol into the middle cylinder of the three carbs a dual carb would operate, that there woudln't be much difference? or is it just a matter of how much air they let in? therefore if you have 3 carbs the same size each as the 2 carbs you had it would let in more...but what if they equalled the same area? woudlnt it be the same?

[thehelix112]

Lets clarify, when you talk about triples, you are really talking about three dual-barrel carbs/throttle-bodies/inlet tracts. Thus you have an individual throttle body for each intake port.

 

This HAS to flow better than a standard SU-based manifold where one port feeds three cylinders and the manifold splits off. Imagine the air on its way in. With an ITB setup (triple webbers for example), the air goes into the trumpet, past the butterfly, then it sees the valve, and voila in the chamber.

 

With a factory setup, air goes in the trumpet, past the butterfly, then sees a triple fork in the road. So the air slams into this fork, and spills off to the sides, now all disrupted and slowed down.

 

The path through an ITB system is much cleaner, and will provide much less of a restriction to flow. This is of the utmost importance to N/A engines as the amount of air you get in the chamber is determined by the pressure difference between atmosphere, and the vacuum generated by the piston's descent during the intake stroke (and valve timing/lift of course).

 

For forced induction cars, we have control over this pressure differential, and can literally force more air in almost at will. The downside is that the more boost, the hotter the air, so we get to a point where the pressure differential is outweighed by the reducution in charge density from increased temperature.

 

I don't want to proof read that because it'll be dribble, but I hope that makes atleast <some> sense.

 

Dave

[Rod F]

Everything mentioned matches what i've read: higher air flow at higher RPM's, better airflow as well. Also better tunability for each cylinder. I've heard tripples are incredibly thirsty as well (not surprising).

[Toecutter]

Oh ok yes I understood the theory behind the rough walls. I imagine the fuel building up in waves like condensation in an steam pipe on the smooth surface but what about the comment:

 

Often the size of the port is reduced to increase power

 

is this standard practice?

[thehelix112]

Ah.. now this is where things start to get complimacated. You have to match the port size with the cam in this respect.

 

The larger the port the more revs will be required to produce intake flow at an ideal velocity. By ideal I mean within the range that will make appropriate use of the air's inertia. I am still learning as much as I can about cams and cylinder head flow, so the rest of this will be conjecture.

 

Obviously, assuming that a port is so big that it isn't being made best use of until a certain RPM range, it is also necessary to have valve timing that facilitates maximum flow in the same range.

 

Atleast, this is how I understand it at the moment. I'm sure its a simplistic view, and disregards a lot of important factors. Hence the signature.

 

Dave

[nat0_240_chevZ]

yea dave is on the money there.

 

Its all about port velocity, this is the speed of the air through the port, smaller ports = higher speeds of intake charge aswell as the natural pressure equalisation that tries to occur high to low, the speed is also affected by steps (large to small and vice versa) which affects the final filling pressure in the cyl, bends and things such as our throttle butterflies, gasket protrusion.

 

reduing all of these restrictions ie port match (not boring out the ports to make them larger) slimmer butterfly shafts, gasket trimming helps flow in a sense of reducing restrictions, now if we look at flow, its about HOW much air we can get through a pipe, which if my fluid mechanics and dynamics is recalled correctly flow = area x speed, so you can see where i am going, only opening up the ports to match which reduces restriction but mantaining as small a port cross sect area as possible increases the speed....

i have seen (not on the net or a forum but in person also while being done) a twin SU setup which has had nearly a year alone put into it and the extent at which porting and welding, yes welding, was done to ensure that the cross sect area does not change and if it does to within only 2% (around where the valve insert in the port). Results were only within around 5% of that of tripple 45's.

 

Beauty of the tripples is that they are really 6 off single carbs. utilising a common float chamber and other ancillaries. Having the throttle butterfly so close to the valve head also increases the feel we call 'pick up' or 'response', as what you get with all motorbikes efi or not, still have individual throttles.

 

Pick up is not an easy thing to measure (barely measurable at all, not even on the dyno graphs) it is just a sudden velocity change for the fuel metering device (jets, emulsifier tube, pilot air, bleed etc etc or EFI) this is where the multi-throttle side draught or similar carbs come into their own, because they respond so rapidly to throttle actuation, where as an SU has a large piston which movements are dampened by fluid above the piston (here is an area of improvement, aswell as the inertia of this piston, ie lightening etc), this gives that slight response delay, aswell as the restriction untill they fully open and even then its like a square top to a round port.

As you can probly tell, the tripple or beit 3x2 = 6 throttles have a wicked sense of right foot movement, and the also sound beautifil at full noise.

 

ill stop rambling, any specific q's just post or i could go for hours, hope it stayed inline with the thread... :oops:

 

nato

[Toecutter]

thanks guys that certainly helps. Good to get a basic understanding of the theory howvere I suppose as far as head work goes it really comes down to the engineering business thats doing the job!? Any recommendations in Melbourne area for the L series?

I take it they do the best job they can and flow test it after which one would try and find the most suitable carbs!? I suppose what I'm trying to say is do they take into account the carbs and manifold that would be fitted to the head ie 40mm triples or 45mm triples?

[nat0_240_chevZ]

size of the tripples more depends on engine application, (revs/displacement/cam) for a really hot road going car the 40's are more than adequete. 45's, 48's and 50's are for all out race engine regularly seing the redline and not falling below 4.5k

 

carburettion type and size would/should be kept in mind whilst performing any 'work' as such but not entirely essential. Port work however, ie manifold matching etc should be done to suit your manifold you are going to use as their ports on their mating flange are almost certainly going to differ.

 

for a road going car, i wouldnt stress too much on it, but also depends on how you drive your car i suppose.

 

nato

[thehelix112]

Tis an excellent point Sulio, and highlights the fact that the engine (and the car) is a system. You can't change one thing without it having an impact on something else. And as such, everything should (needs!) to be designed to work together.

 

Dave