depends on which one you get, and how well you set it up. I personally don't like thm, but I've only ever worked with one kind. The owner thought it ran great and was telling everyone that a 4 barrel was the way to go. It actually ran horrible, sending different mixtures to different cylinders.....the reason he thought it ran great was that he didn't know how to tune anything, and didn't even know when something was tuned...He figured it was tuned because it didn't buck off the line like his old carbs did. Apparently some of them work better than others, though, but you are going to have trouble figuring out which is which, since you are gonna be getting lots of advice from people like the guy I was just talking about!
For a stock 260 like you have, I would highly recommend getting a set of non-smog SU's from an earlier 240. That will be the easiest, and probably the cheapest, and most authentic setup. On a stock motor, they flow plenty; you won't go faster with the 4 barrel.
> No idea where this bizzare idea that a 4bbl setup gives a bad
> mixture distribution is coming from since it should actually be
> BETTER with a single source carb that with 2 carbs on their own
How about because of the extra turns the mixture has to make and the great disparity in runner length? This concept, like the downdraft Weber swap, was initiated to overcome the really bad smog carbs of '73. It was a crude and inefficient conversion then, and still is today. Bad idea.
Never had a 4B intake on a datsun but I have tuned a lot of holleys. Holleys are great! And very easy to tune. However, I would recommend a jet tune kit, accelerator pump tune kit, and a vacuum secondary spring tune kit.
Holleys don't work to well out of the box. I just installed a new 750 on my truck and it needed leaner front jets, weaker secondary spring, higher number power valve, and a much bigger accelerator pump squirter. If you don't want to buy the whole kit then get jets from 68 to 76, power valves from 6.5 to 8.5, get all the secondary springs, and squirters from 31 to 41.
Read the plugs and change jets
Measure idle vacuum and install a power valve that has 1/2 that value.
Use a squirter that gives the best "take off" responce
Use the smallest secondary spring you can before it bogs when the 4bl opens.
Adjust the idle mixture screws to get highest vacuum at idle (smoothest idle)
Do the jets, power valve, and squirters first then mess with the secondary spring.
As for the intake......I would think the intake designers would design the intake so that the mixture would be as similar as possible. I'm sure they considered the problems with the end cylinders running leaner and tried to make compensations in the intake. I think most of the problems occur when no tuning is done on the carb.
I like the 4brl set up and I am a hard core EFI guy. If you get the right cam for the intake you should see good torque numbers. Maybe even build a motor with a little more compression for the extra "pop". I would rather a 4brl over a tripple carb set up in most motors, specially drivers.
Unless you have flow matched SU's with perfectly matched jetting sitting on flow matched manifolds with perfect linkages, then dual carbs do not have perfect "mixture distribution for all cyl's. Saying that a single carb is fundamentally worse for street applications because of "mixture distribution" is crazy.
I am planning on going single carb on the 260. I have never had the twin 240Z SU's working perfectly together in all conditions at all rpms. At least with a new Holley 390 it will be **** close on all cyls all the time.
The problem is you have a nearly staight shot with the #3 and #4 cylinder, but not the others, thats why it has distrubuion problems. For #1 to get fuel it has to get past the #3, then it has to travel a little ways then take a corner while passing #2. How is that going to distribute fuel evenly?
TonyD has proof, he used heat probes in each exhuast port. The particular 4-bbl manifold he tested had way differnt temps from the end ports than it did the center ports.
So, the mixture travles a bit further on some cylinders. SO WHAT? The fuel mixture is introduced at one distribution point. ****, some even say that the longer run aids in mixing of the fuel/air and actually HELPS mileage!
The same argument hapends for headers Vs stock exhaust. Truth is, in a stock Z engine, it does not make a real difference in the exhaust travels longer in some cyl on a stock manifold OR all cyl are made to flow the same distance in a header. It all comes in and all goes out about the same. The distances are so short that measuring the any effect is **** hard to see.
i enjoy all the "talk" about the 4 barrel manifolds. what people need is everyday use and driving results. i run the arizonazcar manifold with a new holley 390.
i drive it everyday as a work car. and many out of state runs. as well as quarter mile runs.
it performs very well even with a/c
29 miles to the gallon on the freeway and it ran perfect right out of the box with no changes to anything.
no regrets on my end...
all my other Z cars run SU's and i enjoy them as well, this was just a different set-up i wanted to try first hand and it has worked out better than i had hoped
Like I said...different ones work differently. The one I worked with distributed so unevenly that a brief inspection of the spark plugs made it entirely obvious.Interestingly enough, the most lean running cylinder by far was one of the center ones....I have DGV's on my car right now, and the long flow path with a sharp bend gives me trouble, especially in cold weather, with a false lean caused by fuel puddling or condensation on the runners. I run manifold heat to solve the problem.when I look at the Arizona manifold, I would be inclined to guess that you might have this problem with # 1 and #6, but with it only affecting two cylinders, mostly at transition, it would be a real pain to diagnose and fix.
Otherwise, I definitely agree that Holleys are great carbs. I got my first car when i was 13 ( I lived in Iowa, where 14 year olds drive legally) and trashed the factory six pack in favor of dual quads within a year.....I would never run anything else with a big american motor. I just don't like to see them on a z....seems wrong, somehow.
Man, that's NOT the manifold if you have fuel "puddling". Sounds like someone had a trash carb on the car if it was dribbling in fuel like that... Did that car have a new Holley or was it a castoff carb from another car?
Been there, done that. Now I open my laptop, and say "richen that rpm range 10%, at this MAP, and lean it 5% at that MAP...
Regardless of what anyone believes, EFI is lightyears easier to precision tune than a carb is... I hate those tune kits. I welcomed the Megasquirt that made prices for Standalone EFI comparable to a good new four bbl...
No idea where this bizzare idea that a 4bbl setup gives a bad mixture distribution is coming from since it should actually be BETTER with a single source carb that with 2 carbs on their own manifold.
Have you ever worked on a car with one personally? I have and when you pull the plugs and they all are different colors, that's a mixture distribution problem.
Also no idea where "tuning" is an issue on a 4bbl since that is a classic problem on dual SU's and not on any 4bbl that I know of.
Su's you turn a nut, Holley's you have to take them apart, many come with metering -plates- that don't have jets you can change. etc etc.
So, the mixture travles a bit further on some cylinders. SO WHAT?
Man, that's NOT the manifold if you have fuel "puddling".
Different color in the plugs could be due to oil issues, bad compression rings in some cyls, or bad ignition in a cyl, or simply a bad plug. Did you diagnose it or just conclude that the single carb was somehow responsible?
There is no "tuning" since there is only one carb. No need to try to balance carbs when there is only one.
I've seen lot's of straight 6's with single carbs over the years. Not a **** one has had problems with the mysterious "mixture distribution" that would cause such wildly different plugs in the same engine. If they did have such wild differences, it was not the single carb!
Plugs that are so different in a single engine is probably either a sealing problem or an ignition problem. The basic design of a straight 6 with a single carb has been around forever.
Oil looks different that fuel, at least to someone with experience reading plugs, so again have you personally -worked on- a z car with a 4bbl intake setup? Some of the intake designs do have problems with fuel puddling, especially with a cold engine and most require a HUGE accell pump squirt to overcome the problems from fuel wetting the long runners causing a misfire under sharp accelleration.
"There is no "tuning" since there is only one carb."
You never gave any history of the engine. You did not give mileage. You are not giving manifold type. You are not giving carb age or history. You have not said that you checked for vacuum leaks in the manifold. You have not given the experience level of the owner that installed the manifold or the completeness of the swap, or how the car was driven, what kind of carb it was. Nothing.
So I do not trust your information since there IS NO information, just your yapping. So if a plug was fuel fowled, did you check ignition? The plug wires, the cap for cracks near that lead, do a compression test? A vacuum leak test? Test the carb for a stuck float? Test the carb for a binding linkage or a stuck choke? What?
You did no diagnosing so you can not make a conclusion.