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Old 11-27-2008, 08:16 AM
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Mechanical v.s. vacuum carbs

Hey guys I'm tryin to decide on a carb for my 406 street/strip. 10.2:1 CR, cam is 241/249@.050" duration with 525/546 lift 110 LSA. I'm figuring it'll have around 8"HG vacuum, so I know if I go with VS I'll hafta use the light diaphram spring, I just don't know which will be better.
It will have a TH400 3300-3500 stall. I plan on being able to run around town occasionally, and hit the strip on the weekends.

Everyone raves about the DP carbs for performance, but some say your stupid to use them on the street. How does one meet in the middle for a TRUE STREET/STRIP MACHINE??? I can get a 750 VS HP holley 3310 or 80508 for about 100 bucks cheaper then a DP 750, but if a DP is gonna give me better performance and throttle responce through the rpm range I'd rather pay the extra 100........ Any advice??

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Old 11-27-2008, 09:10 AM
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Btw

by the way HAPPY TURKEY DAY TO ALL
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Old 11-27-2008, 12:09 PM
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I'd say you could go either way depending on how competitive you are and how much you actually plan to drive on the street.

The DP will provide better WOT performance, but the VS will have better street manners. Properly tuned, they should both give you full WOT and the same ET at the strip.

Put it this way; for a street build I would say VS and for a race build or a manual tranny in a hot build I would say DP. Since you have elements of both in a street/strip build, take your pick.

I would personally do the VS, but my idea of racing is for tuning and fun. I would gladly give up a 1/2 a tenth to get better street manners. I'm not about bragging rights or winning races, I just want to go fast on the street
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Old 11-27-2008, 01:58 PM
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I agree with Curtis.
98% of the time I use either VS or Air Valve secondary Edelbrock for snappy response and decent mileage.
Wide open throttle, if properly tuned, carbs are pretty much identical... but that is not the real world we drive in, is it?
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Old 11-27-2008, 03:51 PM
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A vacuum secondary will not open until the engine needs it. A mechanical secondary will open whenever you put your foot all the way down. Floored, at a high enough rpm, both primary and secondary are wide open. If you force the mechanical secondary open without the engine rpm to support it, it might be difficult to tune the engine at that particular condition. With a vacuum secondary, that is not a problem. With a mechanical secondary, You have to control the secondary opening.
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Old 11-28-2008, 07:17 AM
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The engine knows when it needs the secondaries to open...better than the Driver. Vacuum secondaries are the way to go...
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Old 11-28-2008, 02:12 PM
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thanks guys

I appreciate the responces, although they all make logical sence, It's still confusing...what confuses me is why if "the engine knows when to open the secondaries" why to all the racers use the dominators, proforms, quik fuel's, and so on which are all mechanical d/p carbs???
Is this just caues thier cams are so radical that they don't produce enough vacuum to use a VS carb or is there more to it??
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Old 11-28-2008, 02:43 PM
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Quote:
Originally Posted by eatonde
I appreciate the responces, although they all make logical sence, It's still confusing...what confuses me is why if "the engine knows when to open the secondaries" why to all the racers use the dominators, proforms, quik fuel's, and so on which are all mechanical d/p carbs???
Is this just caues thier cams are so radical that they don't produce enough vacuum to use a VS carb or is there more to it??
Vacuum secondary carbs sense the amount of airflow going through the venturi and open the secondary valve in proportion to equalize the flow at the chosen velocity. (adjusted by springs in the diaphram can)

Air valve secondary carbs (Edelbrock) do essentially the same thing but use the air pressure pushing against the secondary air door to do the opening. (adjusted by changing the weight, or adjusting the spring tension, depending on which version it is)

Mechanical secondary carbs are opened by your foot. Whether the engine can stand the instantaneous big gulp of air (bog) or not...... open is open.
The double pumper aspect is to squirt raw gas into each carb venturi (double means front and rear) when the throttle is opened to crutch the sudden gulp of air which would cause the bog or stall.
Other types of carbs don't have "squirters" on the secondaries because they don't open instantaneously, but progressively as the engine demands more flow.

When all carbs are wide open there is no effective difference between them, assuming of course, they are all tuned equally.
(bear in mind that there are differences in the carb boosters, etc. that may or may not be delivering the proper fuel atomization for the engine combination.
BUT BUT, if they are equal, wide open power will be equal.)
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Old 11-28-2008, 02:45 PM
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They use DPs for one big reason; they tune their engines to run at ONE specific point - wide open throttle in a very narrow range of RPM. They need repeatable, reliable, WOT performance. As long as it gets there without bogging, idles at 2000 rpms, and always opens fully every time they mash the pedal, its best to have a mechanical carb. They don't care about part throttle mixture, cruise manners, and they don't need the compensation for altitude and other atmospheric conditions. They tune the carb for every track and often times even in between races, so they don't need the adaptability of a VS carb.

A VS carb will let you run it to the track, maybe tweak the secondary vacuum opening based on track conditions, and then run it home with much more versatility.

A DP can be made to run fine on the street. Most of your driving will be in the primaries anyway, and that is the same basic operation for both carbs, but the VS secondaries are much more forgiving of everything from 2/3 throttle up to WOT.
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Old 11-28-2008, 06:08 PM
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Let's talk VACUUM

Quote:
Originally Posted by eatonde
if "the engine knows when to open the secondaries" why do all the racers use the dominators, proforms, quik fuel's, and so on which are all mechanical d/p carbs???
Is this just caues thier cams are so radical that they don't produce enough vacuum to use a VS carb or is there more to it??
Idle manifold vacuum and the vacuum that pulls the secondaries open are quite different.

Manifold vacuum at idle is low due to the timing of the valve events of the hot cam diluting the intake charge with exhaust etc. Essentially the engine is tuned to perform well at higher than stock speeds so it isn't a very efficient pump at idle.

The vacuum that opens the secondary throttles in a Holley 3310 is vacuum that exists right inside the primary venturi. It drops to essentially nothing at idle, and rises drastically as you step into the throttle and get the engine revs up.

So, when someone says that the engine "knows when to open the secondaries" you can imagine this orifice at the edge of the primary venturi sees a strong vacuum if there's a healthy 406 inches sitting underneath it running at 4000 RPM and the throttle open. Enough vacuum will be present in the primary venturi to overcome the spring working against the diaphragm that opens the secondaries. It's a measurement of how much air the engine is pulling in.

Put the same carb on a stock 305 and it may not even pull the secondaries fully open, 'cause the engine doesn't pull enough air through the primary venturis to "deserve" the secondaries. The fact that the secondaries don't pop wide open just 'cause you floored it ensures a decent vacuum signal in the primary venturis or 'boosters'. That's where the good street manners come from.

Put a double-pumper on this same 305, pull out into traffic and floor the throttle. Blahhhhh... You've now got a rather dismal volume of air flowing through all 4 venturis. The vacuum signal is so weak that it doesn't pull any fuel through into the boosters and it just lays there, misfiring 'cause it's so lean.

The vacuum secondaries ensure that the minimum required amount of vacuum is maintained inside the venturis to keep them working properly.

Of course with 406 inches and a loose stall you are ensured lots of air (and fuel!) will be moving through the carb so you can go either way.
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Old 11-28-2008, 07:31 PM
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Normally you would use a mechanical secondary with a standard trans and vacuum with an automatic...but I see your running a 3500 rpm stall so I would run a mechanical.


Spend the $100, it will be worth it.
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