Vacuum Secondaries

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moper said:
The "right" size carb will vary depending on the use of the engine. That's why 600, 750, 850, and 950+ may be the "right" carb. One needs to know how to tune them. Then match the combo and use with the size. The typical math basing cfm on displacement and rpm required really has very little to no bearing on sizing a carb. Especially when some carbs are dry flow rated, some are wet, and 2bbls are tested differently.
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Exactly. Those are approximate numbers used the marketing department.
Some general trends related to sizing
Smaller primary venturis and throttles naturally produce higher signal both for the idle circuit (vacuum dependent) and at the main (velocity dependent).
Higher manifold vacuum tends to produce better fuel distribution amongst the cylinders. The better the manifold distribution, the less important this is.
Big venturis and throttles produce the least restriction and have potential to produce the most horsepower during high air flow demand. The better the cylinder to cylinder fuel distribution is, the more this can be taken advantage of. Droplet atomization size, etc can also play role.
 
furrystump said:
For sgat ever reason 340’s like BIG carbs.
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I can't really speak for 340's but my machinist has always told me big block mopars like big carbs because the restriction on the intake side is at the head, so it doesn't want one before that. I can say that on my dyno testing I've yet to find one of my 440's or other peoples that didn't like a bigger cfm carb and a bigger plenum. Currently I run a 950 proform on a performer RPm with a HVH super sucker reverse ball mill spacer. It is like cracking a whip as far as throttle response and it works well under load and WOT as well.


krazykuda said:
Slanging match???

You were the one that called the 340 Dart with a 600 vac secondaries a slug...

We built one that will spin the tires as long as you can hold it steady...

Not everybody does quarter mile... Many of us drive our cars on the street and want throttle response and a little fuel economy to go with it...
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Could it spin those tires like that with a real set of tires under it? Spinning tires doesn't equate to big power.

You don't tune throttle response by selecting a smaller carburetor. By no means do I think a 600 is too small for a small displacement V8. I make over 450hp with a 500cfm carb on my dirt track engine. Could it make more? Yes. Vacum secondaries don't give you better throttle response or less gas milage. If anything you get better milage as you can control if and when they open. YOu can literally feel the gas pedal when you get to the point they are going to open.

I am a huge proponent of building a car for the street, not he strip. People love to say their car is a street strip car. Even if you raced every weekend that car lives 86% of it's life on the street. This is why I hate overcamming an engine or wanting that radical idle so many think they want.
 
Lots of good stuff here!
I'd take the whole assembly off and check the cork gasket first.
I think if you push the diaphragm rod into the closed position and hold your finger over the hole, it should stay in if there are no holes in the diaphragm.
and by applying vacuum to this port you should see it move in and out.
Only if it passes these tests should you look at the spring.
Sometimes the color on the springs has faded, so compare the old spring with the ones in the kit by stacking them and seeing which one compresses more.
That will be the softer spring. Many of these carbs came with the "plain" spring, which is kinda stiff for a light car. There is also a check ball that can be removed which will also affect the opening, as will the size of the vacuum port, but drilling this should probably be left to experts!
 
My street 367 has had 5 different carbs on the AirGap; from the lowly AVS, to the mightyTQ, to 3 different Holleys. It liked the750DP the best, the biggest I had.
I have come to believe that the more cylinder pressure an engine has,and the better the LD results, the better the throttle-response will be.... with any carb.
With my cylinder pressure up at 185psi, I bet I could run a lot bigger carb yet,and not suffer any response issues. Of course I'm basing this guess on the awesome response of the current 750DP. I would under no circumstance, run less.
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And of course the fact that with the 750DP, and with 3.55s,the 295s ;scream mercy,bloody mercy, to past the legal speed limit, and smoke in a straight line for over six seconds, and very often howl while drifting around a corner in second gear,and occasionally have been busted doing a near full-lock slide in a 150ft or so circle;
well,
I ain't ever going back to a wiener 600vs.
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That published formula worked bang on for my combo; (367x7000)/3450=744cfm, heehee. Let her buck,Freddie.
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The very first trip around the block I ever took with my combo I knew I was in trouble; those 245/50-14s were a joke. And so were the 225s up front. I could hardly burn them off fast enough. Same for the 275/50-15s. And the 275/60-15s. And yes even the 295s. But at least I can control the power-slides with the 295s.
It was sorta ok :( with the skinny tires and the 600vs, sorta like that Dart, but with more tire-smoke;
But the 750DP made it an animal, that no street-tire on the stock rear end, could harness.
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If it sounds like I am talking down my nose at wiener carbs, it's only because I don't have a wiener engine. Get rid of that 150 psi cylinder pressure!. Step up to the plate that starts at 180psi. My engine is nothing special. It's a slammed together item that anybody can build. There is only one thing different about it from most builds, and that is the cylinder pressure. Well that and, maybe the leakdown results have something to do with it too,lol.
Build it once.
I get that aluminum heads cost money. But so does the disappointment of an underpowered slug at 155psi. And .......
you just can't put a price on the ear-to-ear grin of a 180psi ,any size, SBM.
 
Good grief. All Karl was trying to do was show how well the vacuum secondary carburetor can work and all you bunch of morons can do is run the car in the ground. No wonder people think Mopar guys are assholes. They are.
 
RustyRatRod said:
Good grief. All Karl was trying to do was show how well the vacuum secondary carburetor can work and all you bunch of morons can do is run the car in the ground. No wonder people think Mopar guys are assholes. They are.
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and the same guys put on a carb that is more than the engine requires.
 
I put a Thermoquad on a bone stock low-C 318 with a small-port cast-iron spread-bore intake, TTIs, and 3" cannons,
And it ran beautifully,too.
Course I have no idea if it was any quicker, than with the 2bbl........ but it sure sounded quicker; I love TQs.
 
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