Why should I go with a double pumper? I currently have a 750 vac sec holley. It's on a 30 over 454 10:1 with stock rebuild 049 heads and a air gap manifold. Marine cam. Mercuiser exhaust manifolds (center rise aluminum). MSD ignition (dizzy and 6a). 20.5 Caribbean with Jacuzzi WJ. MPH guess is between 45-50. I've heard I should go to 850. It's a pricey part. Unless I will gain something very noticeable I'm hesitant to purchase a new carb. The carb on there is brand new. Straight from the box. No issues. SCHOOL ME.

I would not change what you have now a mechanical sec you will notice a little but the 750 vac sec will work fine for what you are running.  Who ever is telling you that borrow Thiers for a weekend then make you choice. lol

What will help get to 55 MPH?

  I would think you would need some exhaust mods and a shot of NOS.  But really the boat should be close to that I'm not sure what you have that is limiting you at this point.  If I were to take a guess I would start with the pump and exhaust.

I'm waiting for GT.......

Put a vacuum gauge on a manifold vacuum source and monitor it while you are making a full-throttle pass for a minimum of 12 seconds at WOT.  If the vacuum is high, you could gain from the increase in CFM.  If the vacuum is low, you are not taking full advantage of the CFM that you have now.

Post your findings and then a better educated guess can be made regarding CFM.

So what are the bench marks for high and low?


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I don't remember.  I have to look it up.  If I have the energy when I get home, I will.

Point is, the only way to know what you need is to test and figure out where you are.  Just throwing carbs at it is not going tune 5 MPH out of it.  And unless there is something wrong with his tune, no carb change is going to net a noticeable difference.

I maintain, the OP needs to go out and power tune his current combination, THEN evaluate and make changes.  Any carb that he puts on will need to be tuned and the process repeats.

A couple lessons that I learned and have stuck out in my mind:
1. I dynoed a 355 that made 418 HP @ 5400 RPM and 442 TQ @ 3700 RPM.   This engine had a 3310 vacuum secondary carb with the Proform body conversion.  Actual measured CFM peaked around 400 CFM.
2. A friend's 302 making low 300 horsepower, made the most horsepower with a 900 CFM.

Respectfully submitted,

Joe

Put a vacuum gauge on a manifold vacuum source and monitor it while you are making a full-throttle pass for a minimum of 12 seconds at WOT.  If the vacuum is high, you could gain from the increase in CFM.  If the vacuum is low, you are not taking full advantage of the CFM that you have now.

Post your findings and then a better educated guess can be made regarding CFM.

Spot on.

I want to make an  attempt to explain the train of thought of why the larger, mechanical secondary carburetor.

Lets use the 760 vacuum secondary vs an 850 mechanical secondary carburetors  because that has been established as the benchmarks.

Keep in mind, these numbers are likely not realistic and are for explanation purposes only.

First, a vacuum secondary carburetor (speaking specifically a Holley) needs two things to work velocity and manifold vacuum. Unfortunately, a properly tuned jet boat has neither.

A mechanical secondary carburetor needs very little vacuum and the velocity can be crazy low and still maintain a stoichiometric fuel mixture.

So here is the theoretical part.

A vacuum secondary carburetor first, lets say you have a wide open throttle manifold vacuum of 2.5"hg. It wont be a lot lower than that because it takes vacuum to open the butterflies. That means the engine needs to pull the air (and fuel) in under somewhat a restriction.

Looking at the mechanical secondary (again, theoretical numbers) the secondaries open mechanically regardless of vacuum, this allows the manifold vacuum to be much higher, I have personally witnessed numbers in the .5"hg range.... As long as the velocities and fuel ratios are spot on, she will continue makng horsepower.

Now here is the bad ass part (IMO). With the vacuum deal, with the 2.5"hg, the manifold  pressure is -1.25PSI (or 13.45PSIA), same engine, potentially more RPM on the mechanical deal, can run .5"hg, this converts to -.25 PSI (or 14.45PSIA). Now I'm no genius, but I am seeing a 1 pound difference in the two.

The reason this works is because the atmospheric pressure at sea level is 14.7PSIA... Less negative is more positive.


Now this does not apply to a car, simply because the torque requirement on a jet pump is totally linear..... Meaning the torque goes up as the rpm does.


In your application, I would feel that the 750 VS may be a good choice, can it work better? My best answer is, maybe.

GT