Installing a 4bbl on a 273 advice

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MassDart

MassDart

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I have an LD4B Edelbrock for my 67 Dart 273, and I'm planning on putting a cam in it most likely in the Spring.

Can I install the manifold and 500 cfm Edelbrock AVS2 carb in the mean time and expect the car to run reasonably or should I just wait.

Thanks.
 
500 avs2 will work fine even with 2bbl cam. The secondaries are driven by the demand of the motor. If the 2bbl cam wants the air, the avs2 will give it only as needed.
 
I may get to try the same modification out on a friends '66 273 with an old Edelbrock 318 street master. I've also advised him with a 500cfm AVS2. I like an LD4B better as long as the intake hasn't been modified.
 
A 500 is close to the ordinal carb size. I have a 600 Edelbrock AFB on my 273 and it works well and still get's me 18 mph/hwy.
 
MassDart said:
But you're cammed, no? You think a 600 would work on a stock 2bbl motor?
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I may be talked out of my dual quad intake set up.... a pair of 600's currently on my 318...... 1200 on top that 273 wouldn't hurt...
 
MassDart said:
But you're cammed, no? You think a 600 would work on a stock 2bbl motor?
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Isky E4. Not much over stock.



engine 296.jpg
 
318willrun said:
I may be talked out of my dual quad intake set up.... a pair of 600's currently on my 318...... 1200 on top that 273 wouldn't hurt...
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a pair of vacuum secondary on a 273/318 probably wont even open the secondaries. Not enough velocity across the orifice to operate the dashpot dragging it through 8bbls or the velocity driven air doors of an AFB. They tell you to remove the air doors on a 273/8 with carters. Maybe use a very light spring on a pair of 1850s......
 
pishta said:
a pair of vacuum secondary on a 273/318 probably wont even open the secondaries. Not enough velocity across the orifice to operate the dashpot dragging it through 8bbls or the velocity driven air doors of an AFB. They tell you to remove the air doors on a 273/8 with carters. Maybe use a very light spring on a pair of 1850s......
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I think mine open though....
 
Put a paper clip on the dashpot rod and take it for a drag. See how far the paperclips have slid on the dashpot rod. If its set up correctly for that motor, I guess they are gonna open. An 8bbl has way less vacuum across its openings so the CFM is not doubles, but its a factor of like .6 or something. Vizard has a graph that showed the cfm through a 6 pack (1350?) was closer to 990 with a 440 pulling on it and the line went down as the CFM got lower. I think its somewhere posted by me in teh massive FABO database.
 
That’s a great intake (LD4B) you’ve got for your project.
 
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Mom's 64 Barracuda, stock 273 two barrel, pushbutton 904, 2.94 gears. Early 1980s, put a small four barrel and dual exhaust on it, everything else stock including CR and cam, ran 16.70s. Thinking probably mid-17s stock. So yeah, just the four barrel and dual exhaust made a pretty big difference.
 
pishta said:
Put a paper clip on the dashpot rod and take it for a drag. See how far the paperclips have slid on the dashpot rod. If its set up correctly for that motor, I guess they are gonna open. An 8bbl has way less vacuum across its openings so the CFM is not doubles, but its a factor of like .6 or something. Vizard has a graph that showed the cfm through a 6 pack (1350?) was closer to 990 with a 440 pulling on it and the line went down as the CFM got lower. I think its somewhere posted by me in teh massive FABO database.
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I have a pair of eddies. It's on a 273 early bolt pattern dual quad intake. Of course the 318 isn't swallowing the cfm's available. It does perform better than the single 4 bbls did
 
MassDart said:
Can I install the manifold and 500 cfm Edelbrock AVS2 carb in the mean time and expect the car to run reasonably or should I just wait.
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Yes.
But, the engine will only pull as many cfms as it is physically capable of pulling. So if it can only pull 206cfm , it doesn't matter how big the carb is, 206 is all it will pull.
What I mean is this;
suppose you have 2.76 gears in the back, and a 904, and 25.5" tires (80"roll-out). At 3000 rpm this will be ~29mph, and by the math your 273 should pull 237cfm at 100% efficiency. But your stock 273 cannot hit 100%, with the small stock cam and heads. One might expect 60% so that would be 142cfm/29mph
By 4000rpm, the number at 65% now, might be 206cfm, now 39mph,
By 5000, your engine is way down the efficiency curve so maybe the number is 180cfm, now at 48 mph..... still in first gear.
No wonder it breaths thru that small single exhaust pipe (stock).

but go ahead and install a 500 . The 500 will pass the maximum required amount of air, without a hiccup.

Suppose your peak efficiency with the next bigger cam jumps to 70% at 5000rpm. Your new cfm requirement is 277cfm.
If you install a tunnel ram with a pair of 500s up there, and headers of course with a free-flowing exhaust, suppose your efficiency goes up 5% with those tiny heads, at a new higher rpm. Your new maximum cfm at 5000 is predicted to be, at 75%, is 297cfm . So I mean you got the potential up there to pass 1000cfm, but the engine will only pull what it can physically pull.

But suppose you spent a wheelbarrow of money on that 273 and got her up to 100% at 5000rpm, that would be 396cfm. So yeah, the 500 has lots of room to grow.

But I mean the stock 2bbl was rated at IIRC 180 cfm, in the 2bbl way, which I think, is 268 in the 4bbl way.
 
318willrun said:
I have a pair of eddies. It's on a 273 early bolt pattern dual quad intake. Of course the 318 isn't swallowing the cfm's available. It does perform better than the single 4 bbls did
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Remember, the optional V8 for 56 Corvettes was a dual quad 265. Had 225 hp vs the 4-barrel version's 210. They were small four barrels, though.

Here's a dual quad 273 on the cover of Hop Up Magazine, June 1965:

10221501 (4).JPG
 
AJ/FormS said:
But I mean the stock 2bbl was rated at IIRC 180 cfm, in the 2bbl way, which I think, is 268 in the 4bbl way.
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I believe you are backwards here. When four barrels were first introduced in the early fifties, carburetor flow benches (or whatever they used to measure a carb's cfm capacity) couldn't handle the added flow of a four barrel, so they were measured at less pressure. Result is that a 500 cfm two barrel flows about the same as a 350 cfm four barrel.

FWIW, I have twin Holley-Weber carburetors on the flathead six in my 49 Dodge. In other words, four barrels vs. the original one. I just put them on for looks, but I was pleased to find out that it made noticeably more power, too.

You can throw out all the formulas you like, but I know from experience that even an otherwise stock 273 will make more power with a small four barrel than with the stock two barrel.
 
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Just for fun.. I really don’t know what someonehad in mind with this one. But, a “full house” LD4B. Plenum divider milled, and ported to probably the largest set of intake gaskets available at the time. I know nothing of the history behind it, other than it looks professionally ported. I’ve seen plenty of LD340 intakes with the same treatment, but not one of these. This one is in my “someday” pile, as in someday, I may have a set of cylinder heads that merit using it…
 
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