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Old 11-14-2002, 07:32 PM
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Post mechanical or vacuum secondaries

I am looking to buy a carby and am unsure whether to get a mechanical or vacuum secondary carb. Is there much difference in fuel economy (when driven sensibily) between the two? I was thinking a mechanical secondary spreadbore would be good as I already have a 4 barrel manifold. Are stock fuel pumps adequate for double pumpers?

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Old 11-14-2002, 08:10 PM
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depends on what you are doing with this setup. If it's a street setup in a heavy car I would recommend vacuum secondaries.
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Old 11-14-2002, 10:42 PM
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Vaccum secondaries open up only as fast as the engine needs them to- they are "sucked" upen. Mechanical secondaries open up when you tell it to. Usually this means with vaccum secondaries your engine will tend to not bog as much. Mechanical secondaries are pretty much for race engines where the engine revs quicker.
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Old 11-15-2002, 12:23 AM
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I have always prefered double pumpers mainly because they have slightly larger fuel circuits designed into them because they are mainly used on high performance motors...plus I have always felt I was smart enough to know how much throttle an engine needs much more than a little spring does. Am I right? Maybe, but one thing is for sure you can never go wrong with a good square bore 750 DP on any engine you care to name including a couple of small sixes I have tried them on, I like to think it reduces the inventory of spare parts I need to have on hand.

Fuel economy is in the right foot, engine combination and tuning...not the carb size or secondary actuation.

Fact: An engine develops maximum fuel economy when it is at wide open throttle (least pumping losses). If you want economy put in a tall rear gear, design your engine combo to run at WOT at highway speed and watch how well that "race carb can function as a "economy" carb. Smokey Yunick taught me that in his book "Power Secrets", a must read if you are at all interested in good old common sense engine development.

May he rest in peace.
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Old 11-15-2002, 07:09 AM
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Another opinion to cloud the picture; mechanical secondaries were designed for radical engines in race applications. If you do the calculations (available in any carb tech book), 99% of aftermarket carbs are WAY oversized for efficient performance. For example, all but the most radical 350 engine should have no more that a 400-500cfm carb. The Holley 390 4bbl is probably the best carb for this engine. Of course, rodders like to go racing occasionally so a 600cfm would probably be justified for warm 350s. IF it had big valves, big aftermarket heads, good flowing high speed cam, the engine would spin to higher revs and move enough air to justify a 750cfm carb, however this engie would not be very pleasant as a grocery getter (you wife wouldn't like it - I LOVE babying a cold-blooded monster engine to life!). Vacuum secondary 4bbl carbs are a wonderful compromise that gives you a relatively manageable low speed engine because it runs as a 2bbl carb 99% of the time but you have a big top end in reserve when you stomp on it and start moving a lot of air through the engine. The secondaries come on as demand requires mor air flow. With a 750 double pumper, when you stomp on the throttle from idle, all 4 bowls open up and there is so little air flow past the fuel flow ventuires that no fuel is sucked into the carb. The thing is too big to start with and opening all 4 basically kills air velocity. Thus the engine bogs for lack of fuel. Racers make up for this bog by adding two fuel pumps instead of the normal single to flood gas into the carb when the air flow is too weak to suck it in through the metering system. Conversely, a vacuum secondary carb can be tuned with diaphragm springs and fuel pump cams and nozzle sizes to maintain optimal air velocity and air/fuel ratio in all barrels at all times, with optimal fuel pump flow rate to totally avoid bog. For the Saturday racer, a vacuum secondary carb can be tuned to out perform a mechanical secondary double pumper by a big margin.
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Old 11-15-2002, 07:52 AM
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Mechanical secondaries are my choice. Like Jaw says, the (knowledgable) driver is the best element to control when (and if) the secondaries open. IF the secondaries open "prematurely" and flood the engine, then the carb is WAY too big for the engine. With the correct combination the engine should be revving sufficiently to handle the secondaries when they do come in. Being an old timer, I recall the days when the "only" high performance carburetor was the Carter AFB. The AFB has mechanical secondaries ane at one time was the basic performance carb used in virtually all GM and Chrysler performance engines until 1964.
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Old 11-15-2002, 12:35 PM
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Okay then what size carb would you recamend for a warmed over 351c with 4v closed chamber heads with 2.19 intake and 1.71 exh valves, i am still trying to figure out what cam to use, i have a overbore of 0.030" and i curently have a 4 speed toploader with a 3.25 9inch rear. i am trying to find a performer rpm intake for itand i have about 10.5:1 compression? <img src="confused.gif" border="0">
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Old 11-15-2002, 01:17 PM
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750 Holley DP, a match made in heaven.
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Old 11-15-2002, 02:10 PM
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[quote]Originally posted by willys36@aol.com:
<strong> If you do the calculations (available in any carb tech book), 99% of aftermarket carbs are WAY oversized for efficient performance. </strong><hr></blockquote>


You said it. I had a 450 holley on my old '69 Rambler's 232 L6. It was a little too big, would bog if I got on it hard below 2 grand. But, once I got going fast enough, I'd open her up all the way. I always looved the look of guys in Mustangs and Cameros when they would see me come screaming past them from behind in the other lane after beating me off the line, where that little car should have spanked anything.

[ November 15, 2002: Message edited by: 78novaman ]</p>
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Old 11-15-2002, 02:21 PM
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I've also read that formula for carbs in my engine building book. Personally I think its a bit bogus. Basically, they're telling you that with (for example) a stock 350 that redlines at 5500rpm you need a 417cfm carb. But with a warmed up 350 with say 100%Volumetruc effeciency you only need a 557cfm carb???? I've read pleny of articles in magazines where they put on a bigger carb and get way better performance. In muscle mustangs and fast ford they have a project 85 mustang with a 392 stroker (redlines at 6500) and a 750dp (i think) that runs consistent mid 10s. Just for the hell of it they put on a demon 1050 and CUT 2 TENTHS off their quarter mile times. Another example is edelbrocks performer RPM package for a 302 ford. They recommend their 750 carb. According to that formula you would only need a 567cfm carb. I think everybody knows that to a certain extent bigger is better, this doenst mean ill put a 1050 on my 302. But definately not a 550cfm carburetor either. I dunno, it just doesn't really make sense.
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Old 11-15-2002, 02:33 PM
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You are right Mustang, for high performance, the bigger carb will give you better top end. However, in drag racing, your are at WOT the full time and at high RPM to get into the power band so the carb will always be seeing great velocity signals at the boost venturies. However, that setup in the real world will be challenged when accelerating from idle at a stop sign with a CHP right next to you. For an all out racer, get the 750 double pumper, by all means. For a dual purpose car, get a vacuum secondary 600. Incidentally, the most efficient engines out there don't get close to 100% VE. They are more like 80% efficient. Our wannabe hotties are even worse, so conservative is better unless you are an all out racer.
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Old 11-15-2002, 04:28 PM
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I once ran a 289 Ford with a 1150 Dominator carb and I redlined it at 7500. It ran perfectly fine (tuning is the key) and even got 12 MPG in the city. The trick of course is the Dominator has an extra fuel circuit that is tunable compared to the 750 DP I took off and since I could dial it in that much more closely the engine ran better. The biggest problem with big carbs is the loss of signal at low rpm, you may have to play with idle bleeds to get it to work with your combo. Don't forget the extra fuel shot you get with a DP so if you have a radical cam and an open plane manifold you will never have the fuel shot capacity to fill that volume no matter what you do to your single accelerator pump vacuum secondary. Of course a carb is not a drop on and away you go modification, a vacuum secondary is much more forgiving in this regard.

Sometimes some guy comes to see me with a carb related problem and I start tuning, invariably the carb still has the same components in it as it came out of the box. Plain spring in the vacuum secondary etc. etc. etc. after some tuning I manage to drop a second and a half off the 0-60 times and the customer is amazed, big deal it was there all the time waiting to be discovered with some good tuning skills. Usually the vacuum secondaries were not even opening...at all, and the engine is just responding to the extra air and fuel.

The Gods live in the details.

Oops almost forgot, that 289 with a 305 duration cam developed a VE of 102% on the dyno. Not bad for an old hunk of cast iron. Don't believe everything you read, usually it's just to keep people out of trouble.

Me I look for it. :p

[ November 15, 2002: Message edited by: 4 Jaw Chuck ]</p>
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