reefedjib Posted March 5, 2012 Share Posted March 5, 2012 (edited) I'm not sure you're getting why they're called constant velocity, read the wiki article. http://en.wikipedia.org/wiki/SU_carburetter That's not the correct etymology of the "constant velocity" term. Also, there is no "great feat of Japanese engineering" here. The natural firing order for any inline-6 is 1-5-3-6-2-4 and there were straight-sixes with twin SUs way before the 240Z, e.g. the Jaguar XK140, predating the first Z by 16 years. FYI... Ok, so that's my incorrect use of "constant velocity", which seems to have more to do with the jets. I am not sure what you would call my observation... "manifold velocity"? It is the velocity of the air-fuel in the manifold. After the valve closes, the momentum of the air-fuel means its velocity does not change much and the air-fuel builds up for another intake opening. It is very interesting that this occurs with these carbs. It does not occur with triples. Edited March 5, 2012 by reefedjib Quote Link to comment Share on other sites More sharing options...
Leon Posted March 5, 2012 Share Posted March 5, 2012 Ok, so that's my incorrect use of "constant velocity", which seems to have more to do with the jets. I am not sure what you would call my observation... "manifold velocity"? It is the velocity of the air-fuel in the manifold. After the valve closes, the momentum of the air-fuel means its velocity does not change much and the air-fuel builds up for another intake opening. It is very interesting that this occurs with these carbs. It does not occur with triples. In no way is manifold velocity constant, it's a complex, pulsed flow. Besides that, I'm not even sure what you mean by "the air-fuel builds up". I think Steve's got his answer. This SU carb psuedo-theory is just clouding up the thread. Quote Link to comment Share on other sites More sharing options...
steve260z Posted March 5, 2012 Author Share Posted March 5, 2012 Clouding up??? If I didn't know better, I'd be velocitized into a constant state of compressed manifold confusion. Quote Link to comment Share on other sites More sharing options...
Leon Posted March 5, 2012 Share Posted March 5, 2012 Clouding up??? If I didn't know better, I'd be velocitized into a constant state of compressed manifold confusion. Quote Link to comment Share on other sites More sharing options...
BlueStag Posted March 5, 2012 Share Posted March 5, 2012 That's not the correct etymology of the "constant velocity" term. Also, there is no "great feat of Japanese engineering" here. The natural firing order for any inline-6 is 1-5-3-6-2-4 and there were straight-sixes with twin SUs way before the 240Z, e.g. the Jaguar XK140, predating the first Z by 16 years. FYI... Aston Martin, Triumph, Austin Healy, and several others, I am sure. The Brits built lots of pretty good straight sixes after the war. And ran them with SUs. Quote Link to comment Share on other sites More sharing options...
Tony D Posted March 6, 2012 Share Posted March 6, 2012 Once again, John C distills the discussion eloquently... The question was NOT "best setup" -- the question was "will these work admirably"? That answer is yes. Anybody arguing that three SU's or larger units are required needs to hop a plane to Charlotte and make an appointment to go for a 12 second ride in Norm Siempers STROKER 240Z. You want to talk BUDGET BUILD that car personifies it. And yeah you could say Norm's car "pulls to 6500"...scratch that, 7000...+!!! Quote Link to comment Share on other sites More sharing options...
Tony D Posted March 6, 2012 Share Posted March 6, 2012 I'm not getting into the clouding of the SU CV term other than to agree the discussion about manifold waves has nothing to do with why it's called CV. Ford and Honda both used VV carbs (Variable Venturi) which operate on identical principles. They did their own design for various reasons, but mostly so they didn't have to license and pay money to Skinners Union... Quote Link to comment Share on other sites More sharing options...
HowlerMonkey Posted March 7, 2012 Share Posted March 7, 2012 1925 norton motorcycle used a variable venturi carburetor with a flap to size the venturi as well as the rochester varajet. I knew carbs were doomed when honda stopped producing the carburetor with the pinky sized venturi. The hardest variable venturi carburetor to work with is easily the feedback carb used by toyota in which the piston was coated with teflon. Spray the wrong thing down the throat, and it's done........and expensive to replace. Quote Link to comment Share on other sites More sharing options...
Tony D Posted March 9, 2012 Share Posted March 9, 2012 Yes, that is a VERY good point Howler---I was going to mention Honda used VV (CV) carbs WELL into the 80's. When you pull them off, they look very odd, nothing like an SU, but indeed look down the barrel and you see the same rising suction piston and jet needle. They do what they do very well, but like anything with emissions requirements, the tighter they got, the more complex they became. Most guys toss Flat Tops as being 'junk' mainly because of a misunderstanding of how they function. But really, Honda really doesn't put junk on their cars, and their 80's SU Style (VV/CV) carbs are a good example of just how nice a sows ear can be made if you give enough money to the engineering department! Swap to EFI, and you're done with all that complexity! You made me remember the Rochester Vara-Jet you BASTARD! I'd put that one out of my memory and you BROUGHT IT BACK! Quote Link to comment Share on other sites More sharing options...
PMC raceengines Posted March 9, 2012 Share Posted March 9, 2012 hehhe i had to work on them every day in the 80s dam they were crap ,,, holden vk what a shiver box Quote Link to comment Share on other sites More sharing options...
Tony D Posted March 9, 2012 Share Posted March 9, 2012 shared misery I suppose! Quote Link to comment Share on other sites More sharing options...
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