Guest Rolling Parts Posted July 13, 2009 Share Posted July 13, 2009 All things being equal, there is a performance advantage to the "straight shot" design of sidedrafts! With a 4 barrel, there are several severe corners to flow around that will cut down on air flow into the motor, hence, performance. ......EFI and direct ignition on the new motors can't be beat. Just my 2 cents. I have to disagree. I have 2 different 4BBL intake manifolds on the workbench at home. Both have massive air passages up to the point where they bolt onto to the engine or adapt to the N33 intakes. With large diameters and smooth transitions I just don't see anything that's restrictive at all at the air velocities were talking about. As far as airflow is concerned, The SU is called a "variable choke" carburetor for good reason. The simplicity of the SU means that when you throttle-up fast that the only way to get more gas for acceleration is that the carbs CHOKE for a short time (hesitates rising the piston) so that more fuel than normal can be added. More choke is more restrictive. The newer design Holley has an accelerator pump that squirts extra fuel while the throttle is opening wider. There is a designed-in flow restriction on an SU when throttling up quickly, that's eliminated on more modern carbs (like the Holley). The new EFI and ECU driven Ignition system can't be beat (well, as long as you upgrade your alternator an have a great battery). Minor voltage variations can make an EFI car run badly and low voltage can actually kill some turbo ECU's. Good old simple carbs and dizzy will start and run even down to something like 5.5 volts. Just TRY to push start an EFI car with a low battery! So yea, a single more-modern single Holley with a ZX dizzy are good bits of hardware for a simple daily driver. Since this is HybridZ I don't care what else that the carb was originally used on... Quote Link to comment Share on other sites More sharing options...
SHO-Z Posted July 13, 2009 Share Posted July 13, 2009 Any changes of direction will restrict air flow. Back in the day when these cars were new the guys racing them were not installing 4 barrels for racing, they would go with side draft webers or larger SUs. I really do not have anything against 4 barrels on Zs, but why are down draft webers considered junk? Heck all they are is a 2 piece 4 barrel. I have a set on my z and would much rather have a set of SUs. Quote Link to comment Share on other sites More sharing options...
Guest Rolling Parts Posted July 13, 2009 Share Posted July 13, 2009 Any changes of direction will restrict air flow. That is an incomplete statement. As long as the intake manifold is less restrictive than the head/valves then it does not really matter at all. As long as everything "scales up" in size in front of the valves then it won't be a limiting factor to the overall flow. Since the discussion here is on a daily driver then we are talking about unmodified heads and RPM's at or below 5,500. The single large Holley and large ID intake manifold won't be "the" restrictive item in the total airflow from air filter to tailpipe. Now if you change the parameters to include RACING with highly modified engines and want to rev to 7000 without a turbo, THEN the intake geometry begins to require some thought. Sure the old answer was to continue a side draft SU but today what you do is throw out the SU's all together and start looking at EFI throttle bodies! Nothing against SU's, but if you change the discussion to racing then that requires the best technology you can afford. Quote Link to comment Share on other sites More sharing options...
PanzerAce Posted July 13, 2009 Share Posted July 13, 2009 Leaving aside which is better (both suck, btw, get triple mikunis ), I have to laugh at the people that go on and on about how SUs are an old design. Last I checked, holly carbs were just as old (hence why any shop can work on them). Quote Link to comment Share on other sites More sharing options...
Guest Rolling Parts Posted July 13, 2009 Share Posted July 13, 2009 Leaving aside which is better (both suck, btw, get triple mikunis ), I have to laugh at the people that go on and on about how SUs are an old design. Last I checked, holly carbs were just as old (hence why any shop can work on them). I believe that people say that the SU is an old design is because that it is and old design. The design was the "Skinner Union" brothers back in the early 1900's and very little has changed. The Holley on the other hand is a staged multi-barrel with vacuum operated secondaries, automatic choke, and integral accelerator pump and is obviously a newer and more modern design. It's also available BRAND NEW TODAY for just $340. Much cheaper than a pair of professionally rebuilt SU's and way cheaper that tripple anything. Cheap, effective, zero maintenance, and a huge support network. That is "simplicity" you can live with. Quote Link to comment Share on other sites More sharing options...
cockerstar Posted July 14, 2009 Share Posted July 14, 2009 As for people chiming in on the price wagon, I paid $250 for my Z therapy carbs right here in the b/s/t. I wouldn't ever spend what they are new, considering that I could get a FI system running for that much. For the price, MS or the 300zx ecu on a stock intake with stock injectors is going to be the best. Quote Link to comment Share on other sites More sharing options...
Guest Rolling Parts Posted July 14, 2009 Share Posted July 14, 2009 For the price, MS or the 300zx ecu on a stock intake with stock injectors is going to be the best. Sorry to speak up again but... You forgot one very important fact for the daily driver scenario: Converting a car to EFI is a LOT of work compared to just swapping a carb! It's a lot of work to get it all the stuff installed, wired up, make a surge tank for the new EFI pump, replaced all the bad used bits, upgraded to high pressure fuel hoses, and get the thing running. The reward is not power, not reliability nor easier maintenance; the payoff is that you made your 1973 car run like a 1975 car. Quote Link to comment Share on other sites More sharing options...
donbyrd Posted July 14, 2009 Share Posted July 14, 2009 to Rolling Parts.. in one of your posts, you said you have 2 intakes on your workbench. Have you used either one? Would you mind posting who made them? I haven't had much luck trying to get my stock 240 running (let alone started). Installed is a new holley 390, the intake is from bob sharp racing, (looks very close to the arizonaZ one). I've also bought 4 new sets of jets, 51,53,54 &55's. Any suggestions for a starting point would be helpful, since Holley Tech themselves tell me it's more "trial & error". I've had a lot of the "error" part. Quote Link to comment Share on other sites More sharing options...
Guest Rolling Parts Posted July 14, 2009 Share Posted July 14, 2009 donbyrd... -One is from Car Tech and bolts to the stock N33 intakes in place of the carbs. -One is from Clifford and bolts on the same way as the Car Tech does. The one that is on a car right now is unmarked as far as origin (but it looks like some kind of Arizona Z knock-off or someone ground off all identifying marks) and is a complete one piece 4bbl manifold. As far as why you can't get the car stared or running with a 4bbl, you might want to read Arizona Z Car post. It's usually an old used carb or a vacuum leak that people have problems with... http://forums.hybridz.org/showthread.php?t=92930 Quote Link to comment Share on other sites More sharing options...
donbyrd Posted September 13, 2009 Share Posted September 13, 2009 since the last time I replied to this thread, I re-read (a few times) the post going back to '04 from Arizona Z. So I blocked off all the vacuum ports that were not needed. Only the choke and dist. advance are hooked up. Got the car to start, ( only about a minute). Then had checked the temp on the exhuast manifold. The front 3 were warm, but the back 3 were hot as h@ll. Would that sound like the back (right) jet of the carb is running way lean? Or the front is too rich, or both????? Thanks for any HELP! Quote Link to comment Share on other sites More sharing options...
Guest Rolling Parts Posted September 14, 2009 Share Posted September 14, 2009 ...The front 3 were warm, but the back 3 were hot as h@ll. Would that sound like the back (right) jet of the carb is running way lean? Or the front is too rich, or both????? That's particularly NOT how people usually experience a 4bbl since mixture variations are less extreme. New carb or an old used one? Quote Link to comment Share on other sites More sharing options...
donbyrd Posted September 14, 2009 Share Posted September 14, 2009 This is a new one. The numbers are still on the carb, and I cross referenced with the holley book to make sure it was the 390 cfm "universal" app. carb. Quote Link to comment Share on other sites More sharing options...
Guest Rolling Parts Posted September 14, 2009 Share Posted September 14, 2009 If it was hard to start, then chances are it's not the new carb (as many Arizona Z documents mention). If it was hard to start then you might check the usual suspects: ignition timing, compression, mechanical timing, valve adjustment, intake leaks, plug wires, points, etc. Even though a 390 is a tad much for a 150 cubic inch engine it should still start and run just fine on the standard jets. Quote Link to comment Share on other sites More sharing options...
zuum Posted September 14, 2009 Share Posted September 14, 2009 Being a new guy to SUs when I bought my 240z at the beginning of august, I must admit I HATED the carbs, I tuned them over and over and over, but never could get them right. I'm on a low budget right now so there was no other option for me to drive my car... So I reseached, spend several hours just reading, then reading some more, then watched some videos on youtube of a MG guy who teaches you how to tune them... no avail. So broke down and pulled them off, CAREFULLY, ripped them apart. DEEP cleaned them using gasoline and a toothbrush plus aerosol carb cleaner for all the little holes and crevices. Made new gaskets out of bulk gasket paper tedious and but cheap and works great. Reset the float height (they were not the same, one slightly off the other way off), and checked that the needles were set properly. Lastly, after reassmbling, I started experimenting with the oil... at one point the car ran WORSE than EVER, how?? The oil was wrong, so I went with a lighter weight oil and out of everthing I did I found out that THIS has the MOST effect on how SUs opperate! Download the FSM, it even states symptoms of low/high/wrong oil being able to cause practically every driveability/runnability/power/smell you may experience, an WAS the cause of every problem I was having. Now I love the carbs, they ROAR! Its great. Quote Link to comment Share on other sites More sharing options...
Guest Rolling Parts Posted September 14, 2009 Share Posted September 14, 2009 ... out of everthing I did I found out that THIS has the MOST effect on how SUs opperate! Download the FSM, it even states symptoms of low/high/wrong oil being able to cause practically every driveability/runnability/power/smell you may experience, an WAS the cause of every problem I was having.. Actually, old age and ham-fisted previous owners attempts at tuning tends to cause more issues than the dampening fluid. A very good read on theory and application of the old SU's is at: http://www.zparts.com/zptech/articles/mal_land/ml_sucarb2/SUcarb_111601b.htm He suggests a blend of oils to make an elixir that works well down under. The old worn parts combined with the almost black art of matching oil to the current weather is a one-two punch to simple driveability. The cost of a CHEAP factory NEW singe carb that manages without oils is the reason most people switch. I have a few pairs of SU's in the garage that I might rebuild one of these days; I should be able to make one running set from the 4 old ones that I have. Till then the single Holley seems to be running without any issues at all for the past 4 years... Quote Link to comment Share on other sites More sharing options...
Datsun#1 Posted December 26, 2014 Share Posted December 26, 2014 For a Daily, Id say go with the SU carbs.If you have any Decent size cam 560lift + then id get the holley and or triple Weber/Mikunie set up.the SU carbs (No matter what anyone tells you) Will NOT support a BIG cam and give you the best results. If you have a stock engine or a MILD build with a cam less than 500 lift, SU carbs are far better as far as tunability and MPG and will work fine for that set up.I dont know why no one has disscussed the use of a 4 barrel with a BIG cam,Ports etc...the SUs dont even come close when it comes to more air and fuel.there just EASIER to deal with. IMO and EXPERIENCE, there both great setups if you know how to tune them both right.It all comes down to what you want the car to do and what your SETUP is. Since you are looking at the Z being a DD, Get the SUs until you dont care about MPG Quote Link to comment Share on other sites More sharing options...
Datsun#1 Posted December 26, 2014 Share Posted December 26, 2014 (edited) "IF BUILT RIGHT AND NEW" SU carbs Pros and cons (from experience) PROs : - Easy tunability - Great MPG - retro/ Stock look - Great preformance In MOST setups - sidedraft carb for Inline engines CONs : - Hard to find parts - Not enough fuel and air for Larger camshafts (500+ lift) - Restrictive choke design carbs HOLLEY SetUp (Also from experience) PROs : - Easy to find parts - Great for bigger cams and port work - Accel Pump shot - More CFM - Cooler Look - Different - Great preformance - many options, Even EFI! CONs : - American parts on Jap car - Harder to tune and keep in tune - Bad MPG - Cable linkage instead of solid ------------------------------------------ This list is based on a Perfectly tuned set up. the SUs run great if tuned right, and so does the Holley. The 4 barrel just takes a little more tuning to get right but once its there, it works great over the SUs if you are running big ports,cam,exhaust etc.. hope this shed some light on everyone. I have used both. I still have my SUs but I'm still RUNNING the 4 barrel You Decide ! Edited December 26, 2014 by Datsun#1 Quote Link to comment Share on other sites More sharing options...
Tony D Posted December 27, 2014 Share Posted December 27, 2014 Well, if we're going to dredge up necroposted threads, remember well into the 80's SU's were still on emissions compliant Hondas in the USA. The four barrel had been abandoned for TBI at worst. CV downdraft carbs were used by Ford for a while due to the inherent ability to load-adjust mixture FAR better than dumping raw gas in an accel pump shot...which kills catalysts and belches HC's like crazy. I have a four barrel manifold on a Z, it's got a TBI body on it. LOADS better than ANY Holley Carburettor. For side draft....give me ITB's any day of the week, the power available from a set of 45 ITBs leaves a Holley carb sucking wind (losing 40+ hp at 8,200 rpms...) Personal Experience, verified by the record books of FIA & SCTA. Quote Link to comment Share on other sites More sharing options...
Tony D Posted December 27, 2014 Share Posted December 27, 2014 Also, THERE IS NO "BEST"! 1 Quote Link to comment Share on other sites More sharing options...
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