360 carburetor choice?

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Cam1399

Cam1399

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Hey I am curious what everybody thinks of a new carburetor. I have a 74 duster with mild 360. Small cam. edelbrock intake and headers. 727 trans Current carb is a worn out 750 edelbrock. Im wanting to get a new carb and go smaller. I think I have narrowed it down to the edelbrock avs2 650 or the quick fuel brawler vacuum secondary 600 or 650. What do you guys think? Thanks
 
Either 650 should be fine. Pick whichever one you are the most familiar with for tuning purposes.
 
ch1ll said:
Prepped thermoquad.
Click to expand...
I'm interested to know where you buy a rebuilt and prepped 50 year old carb. Is somebody specializing in this? Is it really better than the latest and greatest AVS2?
 
IMO, before you start spending money, you need to do some tests;
1) do a compression test, so we can see what to expect, and
2) determine the flash-stall of your current convertor, and
3) determine what gear-ratio is in the back.
4) if the pressure is particularly low, I would do a LeakDown test as well.
5) check and plot on a graph, the ignition Power-Timing

According to Wiki, the elevation in Tennessee varies wildly, which, if true, would wreak havoc with your cylinder pressure, which was already pretty poor in the stock 360, never mind that it now has a "bit of a cam".
At a true 8.0 compression ratio, here are the predicted pressures at various elevations, in a 360 with an Ica of 62*, which is, "a bit of a cam";
121@800ft
117@1600
113@2400
109@3200
105@4000
as you can see, those are Not performance-type numbers, which would be closer to 160psi, and so your low/sub-3000rpm, is gonna be pretty soft.
To battle that handicap, you really need to completely eliminate that soft bottom-end, by installing a higher-stall convertor.
IMO,
If you currently have a stock convertor, and together with 3.23 or less rear gear;
You don't need a smaller carb per se; rather, you need either;
1) the aforementioned hi-stall convertor, so the engine rpm can quickly get up to rpm and begin inhaling some decent air. if you install a hi-stall, run whatever carb and intake you currently have, to get a baseline of the improvement. or,
failing that;
2) you need a carb with a smaller primary with maybe a triple booster, to keep the signal up, and a tuneable Secondary air door......"like" a Thermoquad design.
But if you do get a spreadbore of any kind, install it on a matching 180* spreadbore intake. And
if your cylinder pressure really is under 120psi, then, I recommend a small-port , small-runner, hi-velocity design ............... "like" something born at the factory, on a 273/318. This will get your torque up faster, to help increase the hit of the stock convertor.
3) at low cylinder pressure, your engine needs all the help it can get, so I would recommend that you fabricate a fresh-air intake, sealed to the airhorn. But, be advised, that with a lo-stall convertor and 3.23 gears, this is hardly worth it on account of it won't start working until something like 35mph. Whereas with something like a 3000 stall, it will be working as soon as it clears out the hot air, every time you mash the gas.

For your combo, if I got it pegged right, I can't overstress this hi-stall business enough. Handicapped by BOTH low pressure and a low stall; a smaller carb on your engine, will do nothing for it, unless maybe it gets the lo-rpm air speed thru the boosters up..... which will not change power hardly at all, but the throttle-response will be better, making it seem so..
I can't stress hi cylinder pressure enough. Not just for the compression factor, but also for the engine's increased ability to pull in lo-rpm air, because of faster-response time of the small combustion chamber.
Your 360 has a total chamber volume of ;
at 8/1 of 105.3cc ....But
at 9/1 this becomes 92.2cc, and
at 10/1 is 81.9cc, and
at 11/1 is down to 73.7
You can see the huge difference. So as the piston is falling on the Intake stroke, at low rpm, with the throttle cut way back, that chamber is gonna act a bit like an air spring, delaying the onset of airflow, and then the late closing intake valve, will pump some of it back into the intake-plenum, thus reducing your lo-rpm power. The smaller that chamber is, the sooner the falling piston can de-pressurize the cylinder, and the more eager atmosphere will be to muscle it's way past the nearly closed throttles.
Having said that, if you install a hi-stall convertor, this gets lost in the translation. It is still true, but not as critical, cuz your engine will rarely actually be required to do any real work at the rpms in which this matters.
What I mean is this;
If you had a 10/1 360 operating at 800 ft, with the aforementioned Ica of 62*, the cylinder pressure is predicted to jump to 163psi, and now you got yourself a powerhouse that doesn't much care about what stall or rear gear, that you might handicap it with, and the throttle response is guaranteed to put a smile on your face. Could it be better with a higher stall or gears? Sure, but now it is less of a big deal.
 
Thank you for everyones input. It’s already set up for edelbrock so I went with the avs2
 
1MeanA said:
I'm interested to know where you buy a rebuilt and prepped 50 year old carb. Is somebody specializing in this? Is it really better than the latest and greatest AVS2?
Click to expand...
Summit used to offer them, but no more.
 
Great choice! I know a a few people using the AVS2 carbs & have yet to hear a bad report.
 
AJ/FormS said:
IMO, before you start spending money, you need to do some tests;
1) do a compression test, so we can see what to expect, and
2) determine the flash-stall of your current convertor, and
3) determine what gear-ratio is in the back.
4) if the pressure is particularly low, I would do a LeakDown test as well.
5) check and plot on a graph, the ignition Power-Timing

According to Wiki, the elevation in Tennessee varies wildly, which, if true, would wreak havoc with your cylinder pressure, which was already pretty poor in the stock 360, never mind that it now has a "bit of a cam".
At a true 8.0 compression ratio, here are the predicted pressures at various elevations, in a 360 with an Ica of 62*, which is, "a bit of a cam";
121@800ft
117@1600
113@2400
109@3200
105@4000
as you can see, those are Not performance-type numbers, which would be closer to 160psi, and so your low/sub-3000rpm, is gonna be pretty soft.
To battle that handicap, you really need to completely eliminate that soft bottom-end, by installing a higher-stall convertor.
IMO,
If you currently have a stock convertor, and together with 3.23 or less rear gear;
You don't need a smaller carb per se; rather, you need either;
1) the aforementioned hi-stall convertor, so the engine rpm can quickly get up to rpm and begin inhaling some decent air. if you install a hi-stall, run whatever carb and intake you currently have, to get a baseline of the improvement. or,
failing that;
2) you need a carb with a smaller primary with maybe a triple booster, to keep the signal up, and a tuneable Secondary air door......"like" a Thermoquad design.
But if you do get a spreadbore of any kind, install it on a matching 180* spreadbore intake. And
if your cylinder pressure really is under 120psi, then, I recommend a small-port , small-runner, hi-velocity design ............... "like" something born at the factory, on a 273/318. This will get your torque up faster, to help increase the hit of the stock convertor.
3) at low cylinder pressure, your engine needs all the help it can get, so I would recommend that you fabricate a fresh-air intake, sealed to the airhorn. But, be advised, that with a lo-stall convertor and 3.23 gears, this is hardly worth it on account of it won't start working until something like 35mph. Whereas with something like a 3000 stall, it will be working as soon as it clears out the hot air, every time you mash the gas.

For your combo, if I got it pegged right, I can't overstress this hi-stall business enough. Handicapped by BOTH low pressure and a low stall; a smaller carb on your engine, will do nothing for it, unless maybe it gets the lo-rpm air speed thru the boosters up..... which will not change power hardly at all, but the throttle-response will be better, making it seem so..
I can't stress hi cylinder pressure enough. Not just for the compression factor, but also for the engine's increased ability to pull in lo-rpm air, because of faster-response time of the small combustion chamber.
Your 360 has a total chamber volume of ;
at 8/1 of 105.3cc ....But
at 9/1 this becomes 92.2cc, and
at 10/1 is 81.9cc, and
at 11/1 is down to 73.7
You can see the huge difference. So as the piston is falling on the Intake stroke, at low rpm, with the throttle cut way back, that chamber is gonna act a bit like an air spring, delaying the onset of airflow, and then the late closing intake valve, will pump some of it back into the intake-plenum, thus reducing your lo-rpm power. The smaller that chamber is, the sooner the falling piston can de-pressurize the cylinder, and the more eager atmosphere will be to muscle it's way past the nearly closed throttles.
Having said that, if you install a hi-stall convertor, this gets lost in the translation. It is still true, but not as critical, cuz your engine will rarely actually be required to do any real work at the rpms in which this matters.
What I mean is this;
If you had a 10/1 360 operating at 800 ft, with the aforementioned Ica of 62*, the cylinder pressure is predicted to jump to 163psi, and now you got yourself a powerhouse that doesn't much care about what stall or rear gear, that you might handicap it with, and the throttle response is guaranteed to put a smile on your face. Could it be better with a higher stall or gears? Sure, but now it is less of a big deal.
Click to expand...

The only thing in this reply that means anything pertaining to your question is the one and only partial sentence stating

“You don't need a smaller carb.”

And I agree.
 
650 700 750 dp
if keeping the vac sec rebuild what you have
dp rule your foot is better the vacuum signal 8 days a week twice on sunday
 
Brook [ post #20 ].

Yes, you can adjust the WOT position of the TQ sec blades to control CFM. But because the blades are so large, this will affect distribution & outright hp. The Mopar bulletins has the sec blades set at 81* at WOT for a dual plane & 88* for a single plane.
Better to stay with the factory setting unless experimentation on a dyno shows otherwise.

The air valve automatically controls overall air flow. If the engine needs less airflow, the AV will open at a higher rpm & vice versa.
 
Or just buy an AVS2 and have a nice drivable car that you don't have to mess with the carburetor all the time
 
1MeanA said:
I'm going to pick one of those shortly up for my 340 build.
Click to expand...
The local shop tried to talk be out of an AVS2 and into a Quick Fuel Slayer 600 :) I walked out of the store empty handed.
 
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