273 Carb Size

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CaptRon

CaptRon

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I was looking at this video from the How to and found it pretty enlightening. I'm not a physics major but I understand enough (at least I think) to see that this guy makes a lot of since.
He explains a carb size calculation and one of the numbers is the RPMs where your engine hits peak power. Does anyone know what a 1965 Dart 270 273 engine peak power RPM range is?
My best guestimate will dictate me running about a 350CFM 4 BBL carb.
 
CaptRon said:
I was looking at this video from the How to and found it pretty enlightening. I'm not a physics major but I understand enough (at least I think) to see that this guy makes a lot of since.
He explains a carb size calculation and one of the numbers is the RPMs where your engine hits peak power. Does anyone know what a 1965 Dart 270 273 engine peak power RPM range is?
My best guestimate will dictate me running about a 350CFM 4 BBL carb.
Click to expand...


That chart he's showing ain't carb size but actual engine displacement at rpm, if you put a 390 cfm or an 850 cfm both carbs will only flow what's needed, different carb size are just different amount of restriction think if it like headers 1 3/4 is less restrive than 1 5/8. A 600/650 cfm would easily work on your engine but if your overly worried 500/550 cfm , 390 cfm would work but give up a few hp the only thing I'd put a 390 cfm is a stock /6. To me we already know what generally works no need for so called formula's. It more to match hp to cfm and matters what use a light weight deep gear high stall drag only car can get away with way larger carb than heavy high gear low stall street only car.
 
I always go with a larger carb a 750 because with vacuum sec.it will only use what it needs and if I go with a larger engine down the road you won't need a new carb
 
Original HP273 carb was approx. 500 cfm. (535 sounds familiar but I can't say that for sure) a 600 or 650 works fine.
 
It depends on your build. I've run anywhere from the stock AFB to a 750 cfm TQ. Many carbs are more forgiving and cfm does not really matter. Why limit yourself?
 
Keep in mind that the AFB's had an air door which made them operate as a quasi vacuum secondary. This is different than the upper door on the newer AVS but it has some of the same effects. This means they could be somewhat oversize and still work fine.

Also realize that different carb manufacturers (Holley vs Carter) measure carb airflow DIFFERENTLY and in fact, Holley measures 2bbl carbs different than 4bbl carbs. As I recall, 4bbl Holley were measured at 1.5"Hg "drop" and 2bbls were at 3"Hg. This means that 2bbls "seem" higher flow than they actually are, AKA if they were measured at the lower drop as the 4bbls, and thus compared directly, they would come up lower flow numbers

Also, some brands flow "wet" (I think Carter) and Holley flowed dry, which makes a tremendous difference in readings
 
273 said:
That chart he's showing ain't carb size but actual engine displacement at rpm, if you put a 390 cfm or an 850 cfm both carbs will only flow what's needed, different carb size are just different amount of restriction think if it like headers 1 3/4 is less restrictive than 1 5/8. A 600/650 cfm would easily work on your engine but if your overly worried 500/550 cfm , 390 cfm would work but give up a few hp the only thing I'd put a 390 cfm is a stock /6. To me we already know what generally works no need for so called formula's. It more to match hp to cfm and matters what use a light weight deep gear high stall drag only car can get away with way larger carb than heavy high gear low stall street only car.
Click to expand...
Unless I'm misunderstanding it, his formula is meant to be a way to calculate the right size carb for a specific engine based on how much air it pulls at the peak horsepower RPM. He may have it wrong but his theory of velocity through the venturi seems to make since.
As he says in the video, a bigger carb can and sometimes does make since if your only at the drag strip launching at 4000 RPMs but if your mostly using it as a street car being a bit smaller is better. Yes you can go too small but you can also go too big. I don't think anyone here would recommend putting a Holly 950 dominator on a stock 273 that was originally a 2bbl and in the other direction, no one would recommend getting a smaller 2bbl for more performance.
The point of my original post was to find out what the average peak HP RPM is for my stock engine.
1965 4 dr Dart 270
2bbl
That and to initiate discussion as to how much sense his formula makes to others here. I'm clearly not an expert.
Here are a couple of other carb charts that I found.
carb size chart.jpg
carbchart.jpg
 
CaptRon said:
Unless I'm misunderstanding it, his formula is meant to be a way to calculate the right size carb for a specific engine based on how much air it pulls at the peak horsepower RPM. He may have it wrong but his theory of velocity through the venturi seems to make since.
As he says in the video, a bigger carb can and sometimes does make since if your only at the drag strip launching at 4000 RPMs but if your mostly using it as a street car being a bit smaller is better. Yes you can go too small but you can also go too big. I don't think anyone here would recommend putting a Holly 950 dominator on a stock 273 that was originally a 2bbl and in the other direction, no one would recommend getting a smaller 2bbl for more performance.
The point of my original post was to find out what the average peak HP RPM is for my stock engine.
1965 4 dr Dart 270
2bbl
That and to initiate discussion as to how much sense his formula makes to others here. I'm clearly not an expert.
Here are a couple of other carb charts that I found.
View attachment 1715942852 View attachment 1715942853
Click to expand...

That formula Cid x rpm / 3456 = cfm x ve% gives you the dynamic volume of the engine. The problem carbs cfm is somewhat arbitrary, Take cylinder heads that formula should technically work for them too so say 400 cfm / 8 cyl = 50 cfm so you need a 50 cfm intake port, problem you can flow cylinder heads at different vacuum levels which is right ? Obviously the 28 hg doesn't match up. Now take carbs there measured at 1.5 hg but that's proven to be to conservative, like almost everyone will agree a 600/650 cfm will work perfect.

I feel this over carb thing came from people putting on 750 holleys on near stock engines and not tuning them right.
 
273 said:
I feel this over carb thing came from people putting on 750 holleys on near stock engines and not tuning them right.
Click to expand...

Just to be "picky" LOL I think the old days a 750 was a double pumper, 735 used to be the 'go to' dime store Holley. But the 735 was a vacuum secondary as well. No idea how that worked on say a good hot Commando with good exhaust
 
I can remember in 67-68 friend of mine had a hot 283 in a 48 Ford. It ran pretty good. No idea what cam, it was lopey and solid lifters, all built "back yard." Power Pack heads, big pistons, and a 'Vette aluminum WCFB inline 2x 4's manifold. But he didn't HAVE WCFBs on it--rather had port matched carb adapters and had the older, smaller AFBs on it. No idea what models those were.

Hurst "misery" shifter, Chev 3 speed (which got broken a lot) and 4.11's. Home made headers. I rode in it once when he raced a friend's brand new 68 Camaro 350. had dual exhaust added, and whatever the stock gears were. Beat that 'maro 2 out of 3. Had a Dixco? Rac? wide sweep 5K tach, just wind that ***** up until the needle is buried and then wind it until the points/ valves float.
 
My guesstimate is your stock hp 273 has peak power in the 4800-5000 range. While all these charts and formulas are fun and interesting, none of them take into consideration altitude/elevation or barometric pressure. In my opinion, I’m chosing the largest cfm carburetor with the smallest primary venturies that has vacuum/air door secondaries. Small primaries make for good throttle response, big size so I can use it on different size engines and air door so I can tune it to specific engines. The choices are vast, used rebuildable carbs, rebuilt warrantied carbs in all flavors, Avs, tq, or Holley. Or new carbs from edelbrock.
 
We ran the stock 273 AFB and a stock 340 750 cfm TQ on a basically stock 2 barrel 273 with good results. Daily driver. I'm sure a 340 AVS would work just fine also. I had stopped running Holleys by then, so if you want to run one of them, I can't help you.
 
66fs said:
We ran the stock 273 AFB and a stock 340 750 cfm TQ on a basically stock 273 with good results. Daily driver. I'm sure a 340 AVS would work just fine also. I had stopped running Holleys by then, so if you want to run one of them, I can't help you.
Click to expand...
Wow, no reason to turn this into brain surgery. Bolt the bigger carb on and adjust the idle mixture and idle speed. Take it out for a healthy flogging paying attention to bogging or backfire. Pull off the road and pull out a few spark plugs to read them. If the carb is too rich they will be black. The engine will only pull as much air/fuel as it can. Just because the carb is bigger than that number doesn't mean it's a bad thing or that it won't work. . Proper jetting is the key, all up to a point of course. (no Dominator on a stock 2 barrel 273) @318willrun ran dual quads on a stock 318 without issues.
 
[1] Nobody mentioned VE, unless I missed it. These charts assume 100% VE at the designated max rpm of the engine & production engines would be lucky to get to 80%. That means the carb size needs to be 80% of the chart size. So as an example the engine will only use 480 cfm of a 600 cfm carb at peak rpm.
[2] Another important principle has been lost. Somebody said use a 750 vac sec Holley & the engine will 'only use the airflow it needs'. Yep, true.....But 750 Hs have 1 11/16" pri & sec bores. A small engine like this could use a 500/625 AFB or AVS which have MUCH smaller primaries, 1 7/16" & have enough airflow. The smaller primaries will give better throttle response with no downside at all, & on a small-ish engine that needs to be considered...
 
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