Carbs, mechanical or vaccum secondaries...
 

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

Re: Carbs, mechanical or vaccum secondaries...
 

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

Re: Carbs, mechanical or vaccum secondaries...
 

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

Re: Carbs, mechanical or vaccum secondaries...
 

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

Re: Carbs, mechanical or vaccum secondaries...
 

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.

Re: Carbs, mechanical or vaccum secondaries...
 

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).

Re: Carbs, mechanical or vaccum secondaries...
 

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

Re: Carbs, mechanical or vaccum secondaries...
 

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" :D then use the double pumper Holley.

Roby

Re: Carbs, mechanical or vaccum secondaries...
 

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

:D:D
Roby

Re: Carbs, mechanical or vaccum secondaries...
 

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.

Re: Carbs, mechanical or vaccum secondaries...
 

Thanks again for the great input guys. I was told by a guy named Marty at Quick Fuel that the HP950 actually flows 860cfm, but will flow 950cfm if the engine demands it----not sure if I said that correctly, but he said something to that effect. He told me his shop can modify my existing HP950's to a vaccum secondary carb for about $100 each.

Here's something that I think you guys will find kind of interesting. When I originally had my 540's built I had the Holley HP830cfm annular discharge carbs. The following year I had JimV FULLY port the heads (Dart Pro-1's 310cc), so I thought a larger Holley HP950cfm carb might do the trick---but they did NOT offer one with the annular discharge, so I bought a HP950 the only way it came---down leg boosters (did I say that right?). Anyway, I took one of my 540's to Tom Earhart's dyno with the two different carbs and dyno'd them back to back on the same 540. Here are the results...

RPM TQ HP (HP830cfm w/annular discharge)
3000 592 338
3500 598 398
4000 662 504
4500 694* 594
5000 688 655
5250* 664 664
5400 646 664
5500 636 666*

Same 540" engine only with the HP950cfm NO annular discharge
RPM TQ HP
3000 614 351
3500 598 399
4000 662 504
4500 692* 593
5000 684 651
5250* 658 658
5400 655 673
5500 651 682
5700 639 699*

As you can see the two carbs are pretty much neck and neck with the HP830 making a hair more torque than the HP950, but after 5250rpm the HP830 is all done and the HP950 takes a pretty commanding lead. The HP950 may have even made more power at a higher RPM level, but we never ran it past 5700rpm.

Please continue to post any thoughts or input.

Re: Carbs, mechanical or vaccum secondaries...
 

Interesting,i wonder if the higher cfm gave you the power or the dropleg boosters,i have used the 830 w/annular discharge on several motors(although they were 455 ci) and they always seemed crisper than normal 800 or 850 double pumpers,Smitty

Re: Carbs, mechanical or vaccum secondaries...
 

Hey, Since you guys are all talking carbs here, lets see what you think....
I have an 88' formula 311 w/ twin 454's 330 hp each. When I open up the engine hatch I get a sick feeling in my stomach! (They are bone stock and pretty ugly). HERES THE PLAN.... I want to scrap the carbs and intakes, Ill call this "stage 1" of my plan. Im thinking of installing edelbrock performer RPM high rise intakes on both, AND I'm thinking maybe a set of spacers for additional height?? (like someone said longer distance better atomization). And I want to trash the quadra-toilets and slap on a pair of double pumper holleys... don't know what size though. 750 or 850??? Im also wondering if I do this will the stock fuel pumps be sufficient? Or should I put in electric fuel pumps (one for each motor, or one for both.... holley blue or black).........Also
what kind of hp gains do you think would result????????????

Re: Carbs, mechanical or vaccum secondaries...
 

toxik,

Some of the other guys can offer up some good info, but I would suggest Edlebrock's "Air Gap" intake manifold as it would work well. The intakes will really help dress up the engines much better than those stock Merc heavy cast iron pancake lookin' anchor weight intakes that Merc uses!

A 750cfm Holley is adaquate for a 454. An 800cfm would probably be a little better if you happen to eventually to go a 509 cubic incher in the future, but I have seen them work well on 454's too. An 850cfm Holley is kinda large for a 454 in my opinion.

Not real sure about the fuel pump. With only an intake and carb change you MIGHT get away with the stock fuel pump, but as you add more hi-perf components you may have to add a more sufficient fuel pump.

You probably won't see much of a gain with an intake and a carb, but it's a good start. What kind of exhaust system--stock Merc?

Re: Carbs, mechanical or vaccum secondaries...
 

I believe it best to use vacuum secondary carbs with dual plane intakes and double pumpers with single plane. Dual plane gives stronger signal to carb which benefits secondary actuation. Dual planes typically start to fall off over 5k rpm or so. Single planes are just coming in at that point. Therefore, if your goal is WOT running above 5k rpm, single plane with double pumpers sized at approx. 125% theoretical engine cfm at desired max rpm would be best. If that doesn't work, try bore notching :rolleyes:

BT

Re: Carbs, mechanical or vaccum secondaries...
 

Yes, both carbs were tuned/jetted so that the engine came out to have the same the same bsfc. The smaller carb (830) required larger jets (86's) vs the jets we used in the 950's which were 78's.

blue thunder, thanks for the carb input, but not real sure how that correlates with bore notching. Good to see you---how did your engines run for you this year?

Re: Carbs, mechanical or vaccum secondaries...
 

Engines stong Kaama. Happy as a pig in poo :cool: Now, to make them even better....

BT

Re: Carbs, mechanical or vaccum secondaries...
 

I agree with Roby. A year or so ago there was a thread on this, I believe involving Dennis Moore. It got rather emotional. If you're dumping fuel from the secondaries while idling, you've got a problem. Just my .02

Re: Carbs, mechanical or vaccum secondaries...
 

Well, I am going to do some searching and try and locate that thread and see what was said. I don't see how the secondaries can dump fuel while in the idle mode.---??? I do know that if you touch those throttles that the carb gives a shot of fuel that may be unwanted or not necessary---and whether it's on the primary side or secondary side I don't know, but I would like to find out.

Re: Carbs, mechanical or vaccum secondaries...
 

toxikdump,

On your combination 750's would be your best option.

As far as the 830 vs. 950 carburetors go...

An 830 is a bastard carburetor that was built for NASCAR, and Trans-Am racing. They are essentially 850 Carburetors with 750 Baseplates. Going from the larger venturi to a smaller butterfly slows down the air velocity, and won't produce as much HP. These carburetors are designed to restrict air flow. The 950HP on the other hand is a 750 Venturi with an 850 baseplate installed. This will give better air velocity. Combine that with the fact that the 830 Carburetor had annular boosters which will decrease the air flow, and your 950 is going to flow a lot more air which why it made more top end power. You may have been able to help the low end numbers on it with a 4-hole spacer.

Re: Carbs, mechanical or vaccum secondaries...
 

I would like to thank everyone for the advice... I guess Im going to go with the performer rpm air gap, and a 750 carb. There are only a few minor details I am not shure about. With the carb, should I get a real holley 'double pumper' or a regular 750 w/ dual feed. The double pumper has 2 accelerator pumps and costs alot more $$$ than a regular one. The regular holley cost about $250 and the DP holley is around $400. Also should I use an open spacer or the 4 hole w/ this setup. I would like to make most of the power beween 4-5k rpm. Thanks again !!!!!!!!!! I'll get this formula to run 65 yet!

Re: Carbs, mechanical or vaccum secondaries...
 

Please don't be offended by this but you couldn't be more wrong. The reason why a carburetor can't compete with port fuel injection is because of the reason you said it was better. The further the air/fuel mixture has to travel the worse the atomization will be by the time it's gets to the combustion chamber. There is no way a carburetor can atomize fuel as fine and consistent as an injector. Not only that; the injector (port) atomizes the fuel right at the intake port of the head. It doesn’t have to change directions and be pulled apart by other cylinders drawing fuel. Ever wonder why you always have some cylinders that tend to run leaner than the others? It is all because of the carburetors inability to properly atomize distribute fuel in an even pattern throughout the RPM range. With a port injection system you can have limitless dry intake shapes finely tuned to specific engine characteristics without have to consider wet flow. It makes a tremendous difference and significantly widens your options when building an engine for all out horsepower, efficiently, and reliability. EFI is so much greater that you can’t honestly compare it to carburetors. It is like comparing a Sony plasma screen to an old Curtis Mathis TV. Again I mean no disrespect to anyone. Once you take the plunge and starting building good EFI systems you will never go back.

Roby

Re: Carbs, mechanical or vaccum secondaries...
 

Roby,

I take no offense to your comments. Not sure if you’ve actually ever tested this or not, but you’re not looking at right. It is simple physics. If you look at a high-end race engine the injector is mounted, as far away from the combustion chamber as possible so there is more time for the air, and fuel to atomize properly. An F-1 engine will have the injectors mounted in the air breather assemblies about 42 or 44 cm (18”) away from the chamber. It’s not practical to do this on most applications. It is true on carbureted engines you will have rich/lean cylinders, but still on the average engine due to better designed intakes available a carbureted engine will make more power than the injection. Injecting the fuel through the booster/venturi combination of a carburetor (using electronics to monitor, and maintain your fuel curve) will make more HP than direct port injection. I’ve done the testing on this.

Since we’re dealing with marine engines here, talk to the majority of engine builders, and you’re not going to find any HP gain on a marine engine going fuel injected vs. carbureted. Especially when you take the difference in price into consideration.