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Thread: Secondaries: Vacuum or Mechanical?

  1. #1
    SeaSlut
    Olds Berk combo. I'm running a modified 4150, 750,vacuum secondaries. Short yellow spring to open fast, 6.5 power valve. Runs good, But maybe I could get more ooomph out of a mechanical secondary carb.
    What are you guy running and why?
    [This message has been edited by SeaSlut (edited September 06, 2001).]

  2. #2
    phillyray
    I like Vac. tried them both.The motor takes it when it needs it.Unless you'r always drag raceing.Better fuel mileage to boot!

  3. #3
    Cas42
    Philly,
    Boy I hate to disagree with you but I am anyway http://free.***boat.net/ubb/tongue.gif I changed from a Holley 750 DF/VS to an Edelbrock 750 with MS, the gas consumption was almost cut in half without the loss of performance. The main area I can attribute that to is just in acceleration, mostly from a dead stop. VS's usually will open the secondaries before mechanicals will, thus dumping more fuel into the motor.
    Granted, this is from experience on my ole heavy tub that takes quite a bit to get it on plane. HP to weight has a lot to do with fuel consumption, IMHO.

  4. #4
    Cas42
    oh yea, hey SeaSlut, you're going to hear a bunch of pros and cons with both. What it's going to boil down to is whatever works for what you're runnng.

  5. #5
    77charger
    I run the vs too did remove the metering block and replaced with the adjustable jet block.Have also tried the dp both the same results at wot.

  6. #6
    fryzll
    I just put my boat together, its a 18.5 Challenger w/ a 460 (est. 425 to 450 hp) w/ a Jacuzzi. I bought a new Holley 4150 DP/MS, tore it apart before I used it. I had a buddy mill the choke horn off and form the throtle body (looks like a baby Dominator now) jetted up to 76 pri and 88 sec (I think those are the right #, been awhile) put in a 5.5 power valve and used the blue gaskets out of the Holley Trick Kit. The carb works great, 35 mph at 3000 rpm and barely on the throttle, atleast 62 mph (gps was slow due to coverge) at 4800 rpm. Ive allways had beter luck with man sec carbs but that just my 2 cents.
    Chris

  7. #7
    bajaruner
    SEASLUT it all boils down to what size engine your running, how it's built, and how heavy your boat is?

  8. #8
    beached1
    I agree with bajarunner. You should figure out your CFM's to choose your carb size. Then I was told by my enginer builder that Vac sec's will give you better gas milege, mec's will giver you better throttle response. I haven't checked this out but I was told that if you look up in holley's web sit it has the chart in which to figure out your CFM's. I think it goes something like, your CU's X peak RPM's = then take that number and divide it by something like 3457. anyway, hope this helps.

  9. #9
    phillyray
    Originally posted by Cas42:
    Philly,
    Boy I hate to disagree with you but I am anyway http://free.***boat.net/ubb/tongue.gif I changed from a Holley 750 DF/VS to an Edelbrock 750 with MS, the gas consumption was almost cut in half without the loss of performance. The main area I can attribute that to is just in acceleration, mostly from a dead stop. VS's usually will open the secondaries before mechanicals will, thus dumping more fuel into the motor.
    IMHO. Cas,I must correct myself or did not clarify...I went from a 715 vac. sec. and tryed a 850 dp.Used more fuel,motor was flat in the midrange,did not gain any rpm up top.I failed to note that in the above post.I agree, with equal comparison in( cfm)the DP may give better response and or performance.No need to over carb the motor.

  10. #10
    flat broke
    Rememnber that there are two factors that determine when a VS will open the secondaries, vacum pod resistance, and linkage actuation. If you have too light of a spring in the VS, it can open too soon, and without the fuel enrichment of the MS squirter. Cas hit it on the head when he stated that ultimately it will come down to what works best for your weight/hp/driving style. While MS might work well in a light boat where throttle response is accentuated by quicker acceleration due to less drag, it might overenrichen a heavier slower planing boat that is gonna take a little longer break the hull loose and remove some of the load from the engine. Then again in Cas'case it worked the opposite way. Regardless of the secondary actuation method one thing is easy to figure out and that is the CFM requirements for your application. Time spent there will only aid you in finding the better setup regardless of the secondary type.
    Chris

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