KAAMA

iTrader: (1)

Can anyone that has experience or a good clear working knowledge with carburetors tell me what the bennefits, pro's & con's are between a mechanical secondary carb vs a vaccum secondary carb for a hi-perf marine application? It seems to me that a vaccum secondary would only shoot in the fuel when the engine requires/demands it vs the mechanical secondary where if you touch the throttles it shoots in fuel whether you really need it or not.

I guess my thinking is that if it is unwise to stab the throttles forward while running along so as not to be abusive to the Bravo One drives, etc, as compared to slowly pushing the throttles forward, then perhaps a vaccum secondary would be the wiser choice.

Am I right, wrong or just splitting hairs? Thanks

robyw1

A vacuum sec as you said opens the secondaries as engine load increases. The rate can be adjusted by spring weight and it takes a lot of trail & error to get it to your liking. The Vacuum secondaries will be a little more efficient UNLESS you use the wrong spring. Personally I prefer a mechanical sec (double pumper) because setup is easier and you know that the blades will fully open. You can also get to use jets in the secondaries as opposed to the metering plate. Yes you get a shot of fuel when the sec are moved on a double pumper. If you're on and off the throttle constantly then a double pumper will use more fuel but I would use it anyway.

Roby

MESABALANCING

Kaama
I just last week finished freshining up a 454 for a friends P&D here in Miami the engine made originally 465 hp and 525 ft lbs of torque with a modified vac secondary carb by Bo Laws out of Orlando and I tested a mechanical secondary I tailored for another 454 and made 496 hp and 535 ft lbs of torque we went and tested it Friday before the regata and the boat picked up 2.5 mph and it's flawless the guy is tickled and the acceleration is even quicker than before. In my opinion the double pump is the way to go. Sincerely Laz Mesa

KAAMA

iTrader: (1)

Thanks for the input guys. Sounds like I should stick with the mech secondaries.

jhnrckr

If you idle a lot in no wake zones you may end up dumping too much fuel with the double pumper and you can wear out rings and dilute your oil with fuel.

mcollinstn

In general terms, the vacuum secondary carb improves full throttle metering from low engine speeds. Stab it wide open from idle, and you still stay in the primaries until you build some engine revs. Then the secondaries start opening to give you proper flow.

Mechanicals can bog badly in the same circumstances because you stab the throttle and all the plates fly open.

A properly tuned vacuum carb can run real nice.
But like has been mentioned, mechanicals are easier to tune (and they have pumps on the secondaries, too which is a tuning aid).

articfriends

iTrader: (1)

At wot or steady cruising a properly tuned and jetted vacuum secondary will run exactly the same as a double pumper,its still just 4 booster venturis atomizing fuel into 4 throttle bores. I would probably use a double pumper on a boat for the benifit of the extra fuel shot when you are cruising on the primarys and want to crack the whip(more instant response). I ran a holley 9381 830 cfm annular discharge carb on my last boat,at one point i substituted a holley 650 vacuum sec on it,it ran virtually the same but lagged temporarily when you nailed it. The biggest thing you should be looking for in a carb is don't get too crazy with size(I can't understand a big dominator carb on a med size bigblock that turns 5400 rpms) and the best running holleys have annular discharge booster venturis(they atomize fuel MUCH better). There was a article years ago in a car magazine where they showed that jetted correctly for conditions a carb with those big fuel boosters made as much power as a equally sized fuel injection ,Smitty

robyw1

Well keep in mind here that part throttle movement will not actuate the mechanical secondaries. (unless he is using a progressive linkage) But even still if you must use a "carbonrator" then use the double pumper Holley.

Roby

robyw1

I want a recount, I bet there were some hanging chads on those dyno sheets


Roby

RumRunner

Either carburetor tuned, and set up properly should work well on the combination. A vacuum secondary carburetor can give you slightly better economy (if you can use that word with regards to a boat), while the mechanical secondary carburetor will give you more positive throttle control.

It really depends on how you use your boat. If you spend a lot of time at low speed cruse the vacuum is going to be a better choice. If you spend a lot of time in and out of the throttle (and water) the mechanical secondary is the way to go. At idle, and WOT they should both be the same.

As far as HP fuel injected vs. Carbureted it's common for a good carburetor system to make more HP. This is due to the fact that the air/fuel will have further distance to travel giving better atomization of the fuel, and there are generally better intake manifolds available for carbureted applications. Just put a carburetor & intake on an LS1 and made a bunch more power than the injection did.