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 Post Posted: Sat Nov 19, 2011 10:29 am 
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Does it really make sense to put annular boosters in the primaries and something else in the secondaries?

The reason I'm asking is, as a few of you know, I've been messing with a 4150 (3310) that was so modified.
I think both QF and BG have offered this in some of their off the shelf carbs. But I've come to question whether the concept has merit.


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 Post Posted: Sat Nov 19, 2011 10:31 am 
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Probably on an emission or fuel economy deal you could get away with it, annulars on the primaries. I would just opt to do all 4, you don't lose that much in CFM from doing one set.


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 Post Posted: Sat Nov 19, 2011 12:07 pm 
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I guess that's the heart of my question. If the secondaries are going to be used for any extended period of time, does it make any sense to mix two different atomization characteristics?

Now you got me thinking Mark. If the setup really prefers the annular, then go all four barrels. But if it doesn't, then maybe the better way to improve the low and part throttle is smaller primary venturies, even a spread bore.
Correct me if I'm wrong, in writing this I'm assuming:
a spreadbore's ability to provide even distribution depends on the manifold design?
On a heated intake, too fine of atomization could actually be a negative characteristic - creating a dry manifold?

- Matt


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 Post Posted: Sat Nov 19, 2011 1:32 pm 
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It all depends on what you are after. An annular does provide a better metering signal and fuel distribution, but at the expense of some airflow. On a 4150 if you size the carb right it usually isn't needed, A bigger one on a small engine or one with strictly economy in mind an annular is the way to go. If you end up with downlegs and are installing them or having them installed Braswell has a nice stepped downleg booster that provides a little better signal and is cast with a better shape than the standard Holley stuff.


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 Post Posted: Mon Nov 21, 2011 10:16 am 
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Mine already has downlegs in the secondaries. I guess QF was also thinking that would offset the cfm loss in the primaries. It did have at least one Braswell float in it, not sure whose booster this is - build sheet says the downleg p/n is 45206. Was the extra cfm needed? 340 cid with hp peak in the mid 5k range.

let's see if we can get away with a hotlink from innvte forum
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But in general what happens when the secondaries are open? Aren't we then feeding big droplets to some cylinders and little ones to others or does it all balance out when there is enough flow? Can this even be generalized or is so dependent on the shape, temperature and so forth in the intake tract?

To answer your question about what I'm trying to do. I have two uses where the secondaries are in extended use. The first is autocross, where the engine spends 50-70 seconds being run up and down from 1000 to 6500 rpm. The other is that I'm planning to take the car to some track days. Basically 20 minutes with the engine running up and down from 2500 - 6000 rpm or so.

The carb currently sits on a heated dual plane with 1/2" phenolic 4 hole spacer. I've got a 3310-1.5 ;) that someone has lent me to try out. I've also got a 650 vac secondary carb that had a dead spot in part throttle that needs to be worked out. Crisp, controllable throttle response is critical for autocross. You could say that's my primary goal. The forward acceleration forces are not that great even for events with drag race style starts. Side to side and braking forces are generally higher. In fact jet extensions in the primary side has helped with stalling on braking.


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 Post Posted: Sun Jan 01, 2012 4:01 pm 
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Interesting question by the OP. In fact, I was thinking about doing this exact thing. I did testing with both 4781 and 9380 this past year. In fact, I won at least one race with each carb! I like the idle and part throttle of the 9380 much better. It is clean and crisp. It makes the engine act like it has a smaller cam. Given the choice though I prefer the 4781 on track. Both carbs like the same jetting! That really surprised me! Both carbs are very close in on track performance on my car.

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 Post Posted: Sun Jan 01, 2012 4:31 pm 
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Put adjustable bleeds and idle feeds in the 4781 and you can tune it a little cleaner for idle and transition.


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 Post Posted: Sun Jan 01, 2012 8:38 pm 
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Thanks for the tips, Mark! I have tried just what you recommend. It is a little better but I'm not finished yet. I've learned quite a bit playing with the IAB and IFR on this carb. I'd say I know enough to be dangerous at this point. The 4781 is better but I got it a little too lean in transition. I tried to duplicate the idle circuit of the 9380 and, not surprisingly, the 4781 is too lean in transition. I mapped both carbs and was surprised to find the 9380 does not have PIAB's! I don't remember for sure but the hole is about .115"! I called Holley and they confirmed this!

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 Post Posted: Sun Jan 01, 2012 9:01 pm 
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Is the 4781 a 4 corner idle?


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 Post Posted: Wed Jan 04, 2012 1:44 pm 
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The one I have was not but I converted it. It is probably a 4781 or 4781-1. The choke was cut off when I got it. It is so old it had the flathead bowl screws and the linkage like the 9380 comes with. I replaced the throttle plate with a newer one that is 4 corner and drilled the passages in the main body.

The changes I made really cleaned up the idle but it's now a little too lean on the transition. I think it just needs a little smaller PIAB. Right now I don't have any PIAB in it. Just like the 9380.

So, Mark, what do you use for screw in air bleeds in a stock main body? It is really thin in the air bleed area. I thought about using set screws but I would need a big enough hole that the allen portion would be gone. I guess I need to use the 10-32 screws and install in the main body at the metering block flange. I'm reluctant to do that because it's hard to change and I like being able to see them to keep them clean. It seems like the dye in my gas seems to bridge them over easily. I sure wish they didn't put that stupid dye in there!

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 Post Posted: Wed Jan 04, 2012 2:48 pm 
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Here's a pic of my Holley 4777 DP with 10x32 set screw and .060 something IABs. So you can put a 10x32 in there but I wouldn't try it with a hand drill. I did these on my milling machine so I could control the tools (drill and tap) better.

On my old style 4150 methanol carbs I put 8x32s in the MABs and it worked well but those holes in the set screws were only .033. I "might" would try this one with a hand drill and tap but I'd try to find some way to fasten that carb body down so it couldn't move around while you're drilling.

Image

OH, FYI, you can put those screw in set screw bleeds in the face of the main body where the metering block bolts up if you're afraid to try it up on top. Just a PITA to change'em is all.

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 Post Posted: Wed Jan 04, 2012 8:17 pm 
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sorry to be slow on the response.

I still think that mixing the boosters is not a great idea. One instance that might be an exception is the dual carb tunnel ram. Look for the screen name vanishpt or something like on Innovate and Moparts for a guy who has that arrangement. He's sort of jammed for space and with the big plenum and two carbs maybe all the fuel gets dstributed nicely.

Instead of tapping the airbleeds you can use press in bleeds. Not many places stock them, but I'm pretty sure both Braswell and BLP do 'cause I was lookng at getting some myself. I have one pair we had made in the machine shop (long ago). We took the old ones out very gently with a drill bit. However the guy helping me had mentioned the wet tissue method. I'm going to try that and see how it goes.

In the mean time, if you haven't done so already, try some wire in the holes (although that's a pretty big hole now).

That's very interesting that the jetting is the same for both carbs. On my 3310, I've got the primaries down to 65 main jets and its still not lean enough at WOT. I too have to richen up the transition - lean spot on low speed part throttle when the vacuum gets around 10". So far the annular conversion hasn't impressed me as being superior to the small 650 carb that was on there before. But its been a great learnng experience :)


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 Post Posted: Wed Jan 04, 2012 9:01 pm 
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I dont see any point in mixed boosters. running different boosters with different vacuum amplification causes unknown problems when trying to do the emulsion as well. You dont know what main jet to run and what emulsion pattern to run, nothing is easy to fiddle, at least if you have all the same you can start by jetting all the same. Much easier. And then there's the droplet size changes so how do you know what your getting, are all cylinders receiving the same treatment? How do you know which of them are right? KISS works. If your very accomplished at the trade then fair enough but for most situations the best is to square everything and then observe whats different about the cylinders. Fix the real cause of the problems causing variation, not tune it with a carby stuff up.


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 Post Posted: Thu Jan 05, 2012 11:23 am 
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If memory serves, I "think" the press in MAB holes are very close to the tap drill size of an 8x32 screw. :-k I don't think I had to do any drilling for those, just ran a tap in'em for the 8x32. When tapping this stuff, make sure you use plenty of WD-40 on the tap and back the tap up often to keep it from locking up. You also want to limit the depth of the thread so that the top of the set screw is somewhere near the top of the carb body and tight when it's installed. I've heard of guys "horking" up the top thread of the set screw to keep it near flush but this also "horks" up the thread in the carb body. Best to try to control the depth of the thread.

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