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.