360 carb selection

[Chapter2]

hey guys, i posted a thread titled 'cam degree help' not too long ago. i was able to settle that with yalls help. now, im at the point to decide on a carburetor for it. for those who arent familiar with my set up, here is a basic list of what i have going on: 360 LA block, 0.030, stock stroke, balanced, comp cam .498/.527 with 1.6 roller tip rockers, magnum heads, headers, dual exhaust. im building this engine for a the street to drop in the truck in the picture here. half ton, short bed, etc. with auto trans and 3.55. i have a mechanical advance dist too. im wondering what cfm carb would be good for the street.. that formula "max rpm x total ci / 3456" gave me 625cfm but that seems small considering some 360s came stock with the 800?..... any help is appreciated !

[72Challenger]

A lot depends on how you plan on driving it. A smaller carb will (generally, all other things being equal) give you better driveability, throttle response, and gas mileage; while the larger carb will give you better acceleration and top-end horsepower. The reason some factory engines get away with 800 cfm carbs is because they use vaccuum secondaries. They only open up the secondaries when the engine is ready for them, and then only as much as is needed.

Another thing to consider is that a carb rated at 625 will flow that much at 1.5 inches of vaccuum. If you're racing, that's too restrictive. You may only be ingesting 625 cfm into your engine, but at, say, 0.5 inches of vaccuum, the intake charge will be denser, and therefore make more power. Plus, don't overlook the fact that a well-matched engine combo can exceed 100% volumetric efficiency at its peak, and will need more than the basic formula allows for. Of course, for everyday driving, you may only be getting 80-85% efficiency, so an even smaller carb can work, too. That's why 2-barrels are still popular among the fuel economy minded folk.

If you want the best of both worlds, go with a spread-bore carb like a Thermoquad, or spend the big bucks and go fuel injection.

[rumblefish360]

For a spreadbore carb, I vote T-Q.
For a sqaurebore carb, I vote Holley street avenger, 670 cfm.

[451Mopar]

670 cfm Holley Street Avenger. Do not get the truck avenger, it is not calibrated for performance use. I have one, and I had to drill the metering block restrictions to get it to run right with my 360.

[dwc43]

With that cam, get a big block thermoquad.

[dgc333]

Don't go comparing the cfm rating of a TQ to a square bore carb. The reason a large TQ works on a small block is because the primaries are smaller than even a 600 cfm square bore and in a sb application the large secondaries never fully opened on a stock motor.

If you are going to buy a square bore carb like a Holley then something in the 650 range is more than enough. I have a Street Avenger 670 on my 360 and it works extremely well. I picked up 5 mpg over the Holley 3310 750 I was using and didn't loose anything on the top end by the butt dyno and it has it all over the 750 in low rpm driveability and throttle reponse.

[aarracer]

Chapter 2

For pulling that barge around (no offense intended) you would be much more efficient with the 72 and newer smalll block(800 cfm) thermoquad!

[Chapter2]

Awesome, so it seems like the Holley 670 street avenger and the big block TQ (<800cfm?) whats the difference between a spreadbore, and squarebore ?

[dgc333]

A spread bore carb has two small primary venturis and two much larger secondary venturies. A square bore all four are the same size. If you flip them over the difference is real evident.

[dodger1]

You will also want to use a matching intake for best results. Weiand and Edelbrock make spreadbore intakes as well as squarebore. The stock iron intake is also not bad at all - some say it flows as well as the Edel. Performer, but you might want more air for your combo. The Weiand is a bit taller and may flow more. With square-bore intakes, the sky is the limit.

[Chapter2]

wow that should work out then.. i have an intake by Professional Products, which i heard is = to the airgap by Edel, the intake i have is a spreadbore.
so spreadbore intake... with a holley 670 street avenger carb.. or 800 TQ ?

[Chapter2]

woops.. i meant to say squarebore* .. not spread bore.

[65BELVEDEUCE]

650 holley double pumper all they way! great response without being to big, and easy to mod if you ever go wilder with the engine.

[mrb]

look up biggs performance they are the best on the planet for carbs

[451Mopar]

The problem with the Thermoquad is they have been out of production for 30 years, and tuneing parts are hard to find. I think there is one place that specializes in that carb and may sell rods and jets for it?

The Holley and Edelbrock carbs are easy to get parts for. Even the local Checker Auto here sells jets, metering rods, power valves, etc for them.

[dwc43]

That holley is too small. Just pick up the adapter plate for your intake and get the thermoquad. Tuning is not a major issue with them either. You don't need a box full of parts like you do with a holley either. The Tq has better throttle response by design and that will help with the weight of the truck.

[aarracer]

See

http://thermoquads.com

Demon Sizzler does some rockin stuff.

[rumblefish360]

Originally Posted by Chapter2

wow that should work out then.. i have an intake by Professional Products, which i heard is = to the airgap by Edel, the intake i have is a spreadbore.
so spreadbore intake... with a holley 670 street avenger carb.. or 800 TQ ?

I noted the next entry you made, the correction to the intake description.

The Pro-Products with a 670 Holley Avenger is what I would do. IMO, the 625 is a bit small. The extra CFM will be well used.

Accroding to DWC, a adapter can be used, but hood clearance is now an issue. He also made mention to a big block T-Q. It should be noted that small blocks came with both sized T-Q's. Big blocks as well. But it is far and few that get the small T-Q.

T-Q's are rated by Carter @ 800 & 850 cfm. A small block is hard pressed to use all 800 cfm.

[dodger1]

There are pros and cons regarding adapter plates to match spreadbore carbs (TQ, etc.) to squarebore manifolds, (or vice-versa) increased height being one as mentioned, and I believe airflow can be affected as well. Personally if I already owned an aftermarket squarebore, I'd stick with a matching square-bore - 650-700 cfm range-carb. Even though dwc and I both love our TQs!

[Tarrbabe]

I have never been a fan of adapters on carbs. It appears to me that the flow is interrupted with the change of venturi size. Going from a small venturi carb to a large slows the flow and does less or atomization of the fuel/air. Going from a large venturi carb increases the flow and can force the flow into the floor of the intake and interrupt the flow thus. On the opposite of what just said, I have seen it work wonderfully. But I'm sure it could have been much better.
The NASCAR boys spend millions on matching their parts exactly and they constantly get more than 2 hp per cubic inch. Most of us are on a budget and can't afford to throw money at the problem. I would try to match my parts for maximum flow interference. If you use a square bore carb, you should also use a square bore intake and visa versa. As mentioned before, the reason the TQ works so well is because it won't feed more than the engine can use, plus it does it on a spread bore intake that isn't impeded by the miss match intake. Even if it is, the lower speed of the carb is easier to re-direct the flow into the intake. This will allow the atopmmation of the fuel to be maintained rather than being blasted into the floor of the intake or the other extreme of coming from a small carb opening into a much larger intake volume. This can be used correctly if you can figure out how ram technology works.
For the average person I would think that the matching parts idea is a most productive idea. I had a 650 Holley spread bore double pumper on a stock 72 intake that ran great at all speeds except for over 115 mph because it was pretty well matched on my 340. I think I ran out of cfm then. The beauty was I got over 19 mpg on the highway and had all that in reserve.

Just thinking out loud.

[dwc43]

Originally Posted by dodger1

There are pros and cons regarding adapter plates to match spreadbore carbs (TQ, etc.) to squarebore manifolds, (or vice-versa) increased height

It's only a 1/2" thick. Should not be too much trouble on most cars.

[Tarrbabe]

I disagree. If you really want it to WORK! You should match all the parts to match what the others are designed to do.

Sorry, just thinking out loud.

Yes, I have seen setups work very good but if you want it to work near 100 or even 200% you have to get a lot closer.

[rumblefish360]

DW is right that space should not be an issue, but measure before ordering or installing. Clay is good to use.

The extra open plenum area isn't so great on the feul droplets as they leave the carb on some intakes. The reason being is that the divider that makes the dual plane what it is acts as a partail damn. The fuel crashs into it and ends up puddling. The air flow around this wall isn't the best.

MoPar recomends cutting down the divider wall to get around this problem. This isn't the best idea with dual purpose rides and small cams. Your cam is small and your build is mello.

[rampage_82]

Well this could be debated forever, however the way I size a carb for a motor, is to take it's displacement, and multiply it by 2 and find the closest size carb I can to that number..It's worked well for me. I.E. 318 = 636 (600, 650) 360 = 712 (700, 725, 750) 440 = 880 (850, 900...dare I say it 930 predator...lol)

[Chapter2]

Okay .. hmm.. i was just looking at my intake, its definitely square bore.. and looking down the openings.. it seems one side goes down further than the other... its kind of hard to explain. i suppose thats how most are ? i dont remember my 8007 being like that. maybe i was wrong. would that make any difference which carburetor i choose? i feel like 50% of the repsonses are leaning towards something in the 650-700 range. and the other 50% are recommending the TQ 800. where would i be able to order something like the TQ from? summit, jegs? mancini ?

[Tarrbabe]

Sometimes the problem seems mute, but one day you take your carb off and look inside the intake. I have found the floor shiney from the charge was blasting the floor. This isn't good for atomazation. The cure is to increase the volume of the intake to allow for a smoother transferance of the flow to the intake runners. This can be done by a spacer or a different intake manifold or even a larger carb. If your charge is shining the floor, the flow is being intrupted big time and your charge is not remainging as atomizied flow, but into liquid which is not good.

Just thinking out loud.

[dwc43]

Originally Posted by Chapter2

the 650-700 range. and the other 50% are recommending the TQ 800. where would i be able to order something like the TQ from? summit, jegs? mancini ?

The 650 700 is too small for a 360 so I would not go smaller than a 750 double pumper in a holley or it will suffer for it. And it wont have the throttle response of the TQ anyways. And you can get a TQ for about $200 or so from the local Advance Auto. That's where I get some of mine before I mod them for use on race cars. You can get them cheap from e bay and just put a kit in them.

[dgc333]

DWC43, you are wrong there!

A 650 to 700 cfm is about the ideal size carb for anything less than a max effort 360. A 750 double pumper can be made to work but you will be giving up throttle response and driveablity in the rpm range that a street or street strip car will be spending most of its time. A vacuum secondary carb is more forgiving of being over sized but you are better off using all of the capability of a smaller carb than part of the capability of a larger carb.

A 360 with a 6000 rpm red line that operates at 100% volumetric efficiency (which nothing short of a NASCAR small block does) only consumes 625 cfm of air (729 cfm at 7000 rpm). A more realistic efficiency for a well sorted out street strip motor might be 90% which would require 562 cfm. You need to select a carb that can deliver the needs of the engine without it being a restriction so something larger than 562 is in order but say anything smaller than a 750 and a double pumper to boot is to small is an irresponsible recommendation.

I know you like to praise the virtues of a Thermo Quad and remind everyone that the factory used 800 or 850 cfm rated units on small blocks BUT. As its already been stated in this thread you can't compare that carb to a square bore Holley or Edelbrock style carb with respect to CFM ratings. The primaries are extremely small even smaller than a 600 square bore, this gives you great throttle response and driveability. The very large secondaries on a TQ would never fully open on a small block so in essence you still only had a 650-700 cfm carb.

[dwc43]

You can't use that stupid little cfm formula cause it does not take into effect temps, rpms, compression, cams or anything else. That's why it always comes up too small. And with the smaller holley you suggest, you don't gain any low end throttle response over the Tq and you just killed it on the top end. And the reason for the Tq's great throttle response is due to the double ringed booster design on the primaries. The quadrajunk has small primaries too, but it has nowhere near the throttle response and it's too small as well. And it's quite easy to get the TQ fully open on any small block. We've run them wide open on several of our cars over the years.

[Slingshot383]

Having one side of the intake higher than the other when looking down through the carb mounting spot means that it's a dual plane intake. The easy way to get more mid-range and top end out of one is to grind down the center divider to 5/8" above the upper floor. This allows each cylinder to pull from all 4 barrels of the carb instead of just the 2 over that half of the intake opening.

As far as carb sizing, a 650 or 700 double pumper would be a better choice on a 360 over a 750, and you won't be sacrificing any top end, unless you are using W-2 or better heads. If you are running a stick shift or a tranny brake car, and talking strictly track or highly spirited street driving then MAYBE look at a 750 or 800 double pumper. If it's a max effort 360 with good heads, big cam, lot's of compression, a tunnel ram and pair of 650 double pumpers is the best way to go.