600 CFM, all I need. Check my math!

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7milesout

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Guys - Long story short. My 72 Plymouth Scamp (that was given to me) came with a brand new fuel pump and 750 cfm edelbrock (1407). As I got it roadworthy (Feb / Mar), and begin to spend money on it (exhaust), and test it at the strip (before improvements data), I find it is just as I suspected. I think it is running rich ... way rich. I'm going to *try* to tune the 1407 because within about a month I should have a whole new exhaust, including headers (versus oem manifolds). When I do the exhaust, I will put a clamp on bung on the exhaust and a wideband. I'll use the wideband to tune the 1407. I mean, I would just dump the 1407, but I'm going to try to tune it first. The 1407 was purchased new in November of 2016, and the only miles on it were from me, since February. Maybe 500 miles. So if I pull it, it will need a new home.

Anyway, from my math I figure several things. I want the car to run a 13.99999999 second quarter mile. After that, assuming the carb is tuned well, I will just drive the car and enjoy.

At 60 mph, it is turning ~3,000 rpm (727 / 8.75). The Scamp and I weigh in at about 3,210 pounds. Based on that, and using some "hp from ET" formulas to run a 13.99, I would hit the line at 107.8 mph.

Knowing the car is turning ~3,000 rpm at 60 mph (cruising, top gear), doing the math it should be turning 5,390 rpm at 107.8 mph.

Also, since the car is a 360, at 5,390 rpm, the engine would be consuming 561.4 CFM. I'll explain that below. But only requiring 561.4 CFM, a 600 CFM carb would be about perfect in size. No need for a big ole 750 CFM. I'm thinking the car would be faster and more efficient with a 600 CFM. Faster because it should produce better torque off the line with a better mixture from the 600 CFM.

Explanation (for those who care, and haven't done this themselves)

A V8 pulls in 4 intakes in 1 revolution. Which means it intakes 360 / 2 = 180 cubic inches of air per revolution.

Divide 180 by (12*12*12) to convert from cubic inches to cubic feet = .1041 cubic feet.

That is .1041 cubic feet per revolution.

Multiply that times 5,390 rpm = 561.4 CFM.



QUESTIONS

  • Is my math correct?
  • Should I just dump the 1407 now and go for a 1406?
    • I think I want an electric choke, I read most guys prefer it, and will tune the carb regardless of the initial settings using the wideband.

7milesout
 
I ran a 750 Edelbrock on mild 340 with a dual plane intake in a '66 Valiant. I had problems with it being EXTREMELY rich and decided to try new jets/rods for it...that's when I discovered the factory had put the wrong jets/rods in it to begin with (HUGE...bigger than what they even list "factory" in the 800)! Ordered the set that should have been it in...plus a few above and below. What a difference...even got 18-19mpg at 80mph...with 2.94 gears of course. The guy I sold it too didn't believe me until he drove it from CO to NC. He called me and said 18+ every tank except one when he played with it....better than his 318 Duster!

Bought another new Edelbrock a few years later for another project and checked the jets/rods BEFORE I used it...wrong again! My advice...open it up, check what's in it, and play with the jetting. It should work just fine with a little tuning.
 
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Electric choke is nice for daily driving, when adjusted properly they are trouble free...

Here's a nice 600 vacuum secondary Holley with electric choke that I have run for years on daily drivers with success...

Holley 4160 Aluminum Street Carburetor
 
7milesout said:
QUESTIONS

  • Is my math correct?
  • Should I just dump the 1407 now and go for a 1406?
    • I think I want an electric choke, I read most guys prefer it, and will tune the carb regardless of the initial settings using the wideband.

7milesout
Click to expand...
 
Your math is correct figuring at 100% volumetric efficiency. Most engines don't run at that level, though adding headers will help.
I ran my 340 for years with a 600. Many people said it was too small, but it ran fantastic. Check this
Carb CFM Calculator
 
Nice link, thanks. Do you think the default 83% VE is about right? If so, makes me wonder if a 500 CFM would be even a smidge better...

Naaaaah. The P.O. said it had upgraded cams, he said it was 268's both intake and exhaust. No telling what else was done, it's not exactly stock. So I upgraded the VE to 88% (halfway between 83% and 92%) and considered that most V8's redline about 6,500 rpm. That put it at 595 cfm.

So a 600 should be the best application.

Next would be, dumping the 1407. No doubt, I'm going to dump it. Should I dump it before messing with it? Or should I save myself ~$350 and tune it as best it can be and see if I'm happy?
 
7milesout said:
Nice link, thanks. Do you think the default 83% VE is about right? If so, makes me wonder if a 500 CFM would be even a smidge better...
Click to expand...

A 600 vacuum secondary will work fine...

The vacuum secondaries will only pull what you need, if you don't need all of them they won't open all the way..

I used one for 300,000 miles on a 318 daily driver in Michigan winters...
 
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Wait for some more knowledgeable folks here to answer that one, I know just enough to be dangerous, & don't like spreading mis- information.
 
Per your question..........no, your math is not correct, it does not take into account of pumping efficiencies/inefficiencies. You are assuming 100% efficiency and that doesn't happen in an internal combustion engine with out alot of help......
There is more to carburetor selection than simple cfm numbers......air velocity thru the carb and intake are far more important.....for instance.....lets say you pick what would be the ideal carb for your engine based on your cfm calculation, you drive it and it seems ok, you race it and the time slip says the car ain't doing what you thought it should, friends and others at the track say put a bigger carb on it, you resist but eventually break down and try a larger carb, the car goes faster.......WHY??

A strong possibility is that the velocity of the air passing thru the ideal cfm calculated carb is too great and as the air-fuel mixture turns as it comes to the floor of the intake fuel drops out of suspension due to centripetal force and puddles on the intake plenum floor..........with the larger cfm rated carb, the velocity of the air slows down, fuel stays in suspension and the engine runs better. If you have several carbs, try them both, jet up/down as needed, watch your time slip, MPH means HP, jet up till MPH falls off, jet back to the faster combination.


Good luck
 
That carb is not too big. The math you are using has been proven wrong for decades. Yet people keep using it.

Your carb is a self adjusting unit. If it's rich, it's because it's not correctly tuned, not because it's a 750.

Second, I don't know how you figure 107 MPH is only good for a 13.9x pass. At that MPH you need to be running 12.6x-12.8x or something is wrong.
 
yellow rose said:
That carb is not too big. The math you are using has been proven wrong for decades. Yet people keep using it.

Your carb is a self adjusting unit. If it's rich, it's because it's not correctly tuned, not because it's a 750.

Second, I don't know how you figure 107 MPH is only good for a 13.9x pass. At that MPH you need to be running 12.6x-12.8x or something is wrong.
Click to expand...
I agree with this post. And to the OP, yes, you are using way too much math, and I pretty much disagree with it all.
 
I like the throttle response of the smaller carb 600/650.

Jake
 
Have you checked your fuel pressure? It should be best at about 5#. Most mechanical fuel pumps will go as high as 10# and will force fuel past the needle and seat causing a rich condition that can't be tuned out. A friend found that out on his Super Bee with 2 different engines, 2 different carbs, and 2 different fuel pumps. A simple pressure regulator fixed the problem.
 
I have a moderately modified 340 (375 HP) in a 69 Barracuda with a 4 speed and 3.55 gears. I had a Edelbrock 750 CFM on it, and it ran a After about ten years, the Carb went south, and I replaced it with a Edelbrock 650 CFM Thunder series carb. It is a great carb, and my car runs great. I do not think you will be happy with the 600.
 
I'm listening / learning. Seems like the website I used yesterday for the trap speed is snarffled up. I put the same info in a couple other ET / hp / trap speed calculators and it's saying more like 96 or 97 mph trap speed with a 13.99 sec ET. And at 96-97 mph, the engine would be turning 4,800 - 4,850 rpm.

Either way, based on what I'm reading, I'm going to get the exhaust straightened out, and then tune the 750 edelbrock based on what I see on the wideband, which sounds like fun. Once I get it dialed in, I'll take it back to the strip and see what it does.

Thanks for the info.


7milesout
 
You can pick up a used 1850 holley for cheap. It'd be interesting to run both at the track to see which one runs better.

As a side note, 17 years ago when I was in high school, I ran an 1850 on my 408 Duster. It kicked *** around town and ran 12.70s with a lot of mismatched parts.
 
7milesout said:
I'm listening / learning. Seems like the website I used yesterday for the trap speed is snarffled up. I put the same info in a couple other ET / hp / trap speed calculators and it's saying more like 96 or 97 mph trap speed with a 13.99 sec ET. And at 96-97 mph, the engine would be turning 4,800 - 4,850 rpm.

Either way, based on what I'm reading, I'm going to get the exhaust straightened out, and then tune the 750 edelbrock based on what I see on the wideband, which sounds like fun. Once I get it dialed in, I'll take it back to the strip and see what it does.

Thanks for the info.


7milesout
Click to expand...


That's the way to do it. You have to learn for yourself. I will tell you what I tell everyone. That is this: do not waste your time with ANY vacuum secondary Holley. I can't tell you how many hors I have wasted first testing them and second, replacing them. Most people don't realize that most of the time the secondaries NEVER even open. I've see 480 CID Chevys on the dyno with 3310 hollers on them and the secondaries didn't open until 6000.

Work with what you have and make it right. It will run well. If you decide down the road to go to a Holley, skip all the nonsense and buy a mechanical secondary carb and don't waste your time.

I lovingly call those vacuum secondary Holley carbs boat anchors.
 
yellow rose said:
-- Second, I don't know how you figure 107 MPH is only good for a 13.9x pass. At that MPH you need to be running 12.6x-12.8x or something is wrong.--
Click to expand...
-- I had a Mild 440, in a 3500 lb. Dodge, - at a 12.52 ET., the MPH was 108.xx.
 
yellow rose said:
Work with what you have and make it right. It will run well. If you decide down the road to go to a Holley...
Click to expand...

My thinking is this. I'm new to carbs. The Edelbrock from my understanding, is easier to adjust / tune than the Holley. I have no desire to go to a Holley just based on that alone. The 1407 does not need to be removed from the intake. At my skill level (low for carbs), I need to stick with what is simplest. To learn. Aside from what I believe to be flooding the car somewhat when cranking cold, the 1407 seems to run smooth around town. It also gets crap for fuel mileage, and is low on power. I don't fully blame those last 2 aspects on the 1407. The exhaust is in bad shape. If the exhaust were done (should be complete in a month's time I hope), it would likely help lean it out. But the wideband will spell it all out. And I should be able to tune the pi$$ out of the 1407 to make it run as good as it can.

Don't get me wrong, I'm not expecting excellent mpg in the Scamp. But I use mpg as an indication of the state of tune of the engine. I expect that a 360 cubic inch old school V8 in a 3,100 pound car to yield 20 mpg cruising the interstate. However, with the 3 speed, and turning 3,000 rpm, I don't think it will achieve that. But I think 16 - 18 should be feasible. When I get on the interstate, I cruise (this thing) slow and steady. 55 - 60 mph, and I don't care who or what tries to read the serial number on the bumper. That's why other lanes exist. I'm in the slow lane, they can get left.
 
7milesout said:
My thinking is this. I'm new to carbs. The Edelbrock from my understanding, is easier to adjust / tune than the Holley. I have no desire to go to a Holley just based on that alone. The 1407 does not need to be removed from the intake. At my skill level (low for carbs), I need to stick with what is simplest. To learn. Aside from what I believe to be flooding the car somewhat when cranking cold, the 1407 seems to run smooth around town. It also gets crap for fuel mileage, and is low on power. I don't fully blame those last 2 aspects on the 1407. The exhaust is in bad shape. If the exhaust were done (should be complete in a month's time I hope), it would likely help lean it out. But the wideband will spell it all out. And I should be able to tune the pi$$ out of the 1407 to make it run as good as it can.

Don't get me wrong, I'm not expecting excellent mpg in the Scamp. But I use mpg as an indication of the state of tune of the engine. I expect that a 360 cubic inch old school V8 in a 3,100 pound car to yield 20 mpg cruising the interstate. However, with the 3 speed, and turning 3,000 rpm, I don't think it will achieve that. But I think 16 - 18 should be feasible. When I get on the interstate, I cruise (this thing) slow and steady. 55 - 60 mph, and I don't care who or what tries to read the serial number on the bumper. That's why other lanes exist. I'm in the slow lane, they can get left.
Click to expand...


They are easy. All of them are to work on once you look at them.

If you really want to get the hang of it, start reading all you can about emulsion and how the carb actually functions. Understand they only know pressure differential. Once you get that concept downy out can understand why you need emulsion. Why air bleeds are needed and how the trim the fuel curve. Carbs are simple in operation, but complex in function.

Work with what you have. There isn't a thing wrong with it. Just tune it. Once you get comfortable, you can start watching the plugs. I've been able to move the fuel ring up and down the plug nose with simple jet changes and also changing emulsion.
 
I'm running a warmed up 318 with mild cam and milled 2.02 heads to increase compression. I have 600 Holley on a dual plane intake on my car and it runs great. Excellent throttle response and plenty of power for my needs. Mileage tends to be variable based on how I drive it.
 
My 273 did 12.72 ET at 108 MPH with a 600 CFM Holley. Any street small block could get by with the same size carb in my opinion.
 
once you understand the carter-brock, try an AVS or TQ with that wideband. It was the most advanced non feedback carb designed...and it had tiny primaries for huge velocity at low RPM. You can alter Carter rods to fit a TQ.
 
Your math is correct on approximately how many cfm's your engine needs. But what that formula don't tell you is which carb you need. No matter what carb you run is gonna with in reason will pull 561 cfm's thru.

Just cause you run a smaller or larger carb your engine isn't gonna all of a sudden use more or less fuel and air.

What you got to decide is how much vacuum you want to pull at full throttle (restriction). So 1.5" vacuum should be close the fourmlas out but be on the restrictive side probably want to be around 0.8-1" probably the carb you have now.
 
You guys bring up an excellent point. I understand the principle behind vacuum. But I have no vacuum gauge, and no relevant idea of what I should be running (as far as vacuum). I can tell you I'm reading the Edelbrock carb manual, and it is very informative / educational, and well written. Easy to understand. I believe it also makes mention of expected vacuum, so I'll go back on that. I've dabbled a little bit here and there doing what the manual suggests. For instance the car was idling low and almost stalling when in gear (like stopped at a light). Even when fully warmed. So I read up on where the LA 360 should idle. It was idling (as I recall) 550 - 600 rpm. And near stalling. I read that it should be 750 - 850 rpm (as I recall). So I targeted 800 rpm.

I read in the Edelbrock manual how to set (I think it was) "Optimal Lean Idle." And it covered how to set the rpm, then make adjustments for optimum lean. I'd have to go back and look but I think it mentioned reading vacuum too, probably as the best way to set Optimal Lean Idle. But it also covered how to do it with no vacuum gauge. And I followed that. Like I said, a well written document to include pointers how to do it with no vacuum gauge. In the end, the Scamp now idles spot on 800 rpm (in Neutral), and maybe 775 or so in gear. It does not feel at all like it's going to stall at traffic lights, and comes right up off of idle, at small to medium inputs. If I just instantaneously whomp the pedal to the floor from idle, it will more or less just discontinue running, no rpm increase. I'm not concerned about that ... until AFTER the exhaust is complete.

QUESTION: Can you guys recommend a good beginner's set of vacuum gauges?
 
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