383 240z Posted January 18, 2003 Share Posted January 18, 2003 I was wondering if swapping my 750 vac holley for a 600 vac holley would affect my mpg? Im running a 4.11 rear gears with an auto trans. My thinking is the carb only dumps enough fuel for the air the motor ingests, right? so if the 750's 2ndry dont open because the motor is a lame stocker the amount of fuel used should be about the same or does cfm's figure into this? Keith Quote Link to comment Share on other sites More sharing options...
Mike C Posted January 18, 2003 Share Posted January 18, 2003 The myth that a vac secondary only uses as much fuel as the engine needs is mostly still a myth. The flow of fuel through the primary venturi reaches a certain point, it creates enough vacuum signal to the secondaries that they open. Fuel metering is still controlled by jetting. What kind of motor do you have? IS it a 383 as your name suggests or a lame stocker? Assuming lame stocker 350, a 750 vac is WAY overjetted in the primary and a 350 ci motor will still be able to injest plenty of fuel through the primaries to open the secondaries. If you have a 4160 with a plate in the rear, the secondary jetting is fixed. Either you are running way rich on the primaries or your vac secondary spring is too light and they are opening quite a bit and a low throttle settings is another scenario. You can improve your economy to some degree by working with ignition timing and carb jetting. In that sense the 600 is much more appropriate for your combination. Honestly, 4.11 gears is just too much for a stock engine. Your best bet IMO would be switching to a 3.54 geared rear axle. Now, if you have a built 383 as your name suggests, you have 10% more displacement and are turning a ridiculous RPM at cruising speeds if you have a 3 speed auto and the 4.10. A better solution would be a TH700 and the 3.54 gears along with a correctly tuned and adjusted 750. Quote Link to comment Share on other sites More sharing options...
383 240z Posted January 19, 2003 Author Share Posted January 19, 2003 The car that I am talking about is my 87 rx-7 with a 350/700 combo. Ive jetted the primarys down to 70's and have the ing timing pretty close. The car has pretty good throttle response but the mpg sucks. The 383 in my screen name is sitting on the build bench for my datsun. I would change the gears but the only other option is 3.90 I have a set of better heads that will bump the compressin a bit but that will entail a cam change, that means a new torque converter, and intake. that means the motor has to come back out. oh well I guess I can drive the camero a bit longer ( I really hate that car). I'm gonna toss my 600 on the motor just to see the difference. Keith Quote Link to comment Share on other sites More sharing options...
Mike C Posted January 19, 2003 Share Posted January 19, 2003 With my built 327 I think I had 67 in the primaries. With a stock 350 I think you can jet it down even further, say 65, and add the stiffest secondary spring you can get. 4.10, 24" tires, and 5.7liter engine just isn't going to get very good mileage... Actually, if you keep engine speed down (cruise the slow roads) and keep your foot out of it, no reason you shouldn't get 14-16 mpg. One thing about 4.10 is the engine doesn't work very hard to get up to speed! Quote Link to comment Share on other sites More sharing options...
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