is 600cfm really enough?

[Max Power]

when using a formula to determine carb sizing, you need to use the rpm at which you develope maximum torque, as that is where your volumetric efficiency peaks. for most healthy street engines that is around 4500rpm. a simpler formula i use, and it does very well by me, is to double the engine displacement and multply by .85 for the street, and .95 for a race motor, and that will get you very close to what you need. as far as your problem goes, you just need to tune your combination better. start by setting your initial timing at 10 degrees, assuming the 13 degree slot on your dist advance mechanism that should set you right at 36 degrees total timing(without the vacuum advance). once you have that set, then hook up the vacuum advance. next you need to tune the holley to your engine, lighter secondary springs, a jet change, go down about 4 sizes and that should put you close, and perhaps even a 1" thick 4 hole spacer.

By that formula, a 393 stroker with AFR 205s and a single plane intake would be just fine with a 675cfm carb. I know that no formula works in all situations, but I think most here agree that a 675 cfm carb on that motor is not near enough.

CFM stands for cubic feet per minute. But who goes WOT for a minute? How about in 10-15 second blasts? While 675 might be the ideal size carb for the motor listed above, a 750 will give you a stronger and faster 10-15 second blast than a 675 will. That not only works in theory, but it has been proven at the strip time and time again.

Holley and Edelbrock err on the side of caution because there has been a history of backyard rodders throwing the biggest carb they can at a motor, thinking that cfm means performance. In this day and age, we know better. Holley doesn't want to sell carbs that are too big for the application and process returns on carbs that don't work for the application.

Quarter mile times don't lie.

Also, remember that Holley are flow tested dry and Edelbrocks are flow rated wet. Wey means there is fuel in the mix, and fuel displaces air. Therefore, a Holley 750 will outflow an Edelbrock 750, for the most part.

Good discussion, by the way....

 

[D.Hearne]

Also, remember that Holley are flow tested dry and Edelbrocks are flow rated wet. Wey means there is fuel in the mix, and fuel displaces air. Therefore, a Holley 750 will outflow an Edelbrock 750, for the most part.

Good discussion, by the way....

Also one more thing, all those carbs are flow rated using a set vacuum level, and no two engines will pull that level of vacuum in real life, that's the reason why that formula is useless other than to recommend the Minimum size carb that's needed.

 

[302 coupe]

Go with a Holley 4777, 650 double pump. You will not regret it. My build is similar to yours...I was using a tuned 600 w/ vacuum sec's, then switched to an out of the box 650, picked up 3 tenths and 2 mph in the 1/4. Got it tuned and picked up a little more.

 

[gp001]

To assume that formula is correct, you also have to assume that ALL engines that displace that cubic inch displacement, also pull the same amount of vacuum, which in the real world, that ain't gonna happen. There are too many variables in real life to assume that formula is correct 100% of the time. You can take two engines that have the same cubic inch displacement, but have different bore & stoke sizes ( with all other components being the same) and and they will not pull the same vaccum at the same rpm. I've run a Holley 600 on a milder 306 than jon350 has, and it performed flawlessly with the 600. And in case your thinkin the secondaries weren't fully open, you'd be wrong there too, cause I tune all my vac secondary Holleys to open the secondary's almost immediately after you nail the gas. A carb rated at 600 cfm doesn't necessarly flow 600 cfm with any motor it's bolted on. it may flow more , or it may flow less depending on the motor. According to that formula, my 331 stroker motor needs no more than a 575 cfm carb. Well tell that to it's face when you nail the gas and open up ALL THREE 250 cfm 2 bbl carbs and it takes off like a scalded dog.

Like I said, Maybe some of you know more than he does.

Also, I don't really consider anything vacuum operated a performance item. More of a street item. Maybe that is why they don't discuss vacuum in relation to any of those calculations.

Here's another quote for the Derriere Dynos to consider.

Mike Urich from the book Holley Carbureators and Manifolds

An ordinary low-performance engine has a VE of about 75% at maximum speed; about 80% at maximum torque. A high-performance engine has a VE of about 80% at maximum speed; about 85% at maximum torque. An all out racing engine has a VE of about 90% at maximum speed; about 95% at maximum torque. A highly tuned intake and exhaust system with efficient cylinder-head porting and a camshaft ground to take full advantage of the engine's other equipment can provide such complete cylinder filling that a VE of 100%-or slightly higher-is obtained at the speed for which the system is tuned

My out of the box (adjusted, but no mods) 650cfm on my 347 performed well on the dyno all the way to 6000 rpm, and my car pulls all the way to 7000, and it is still a little too much.

 

['69Stang]

Real world reply.....

You have a very nice set-up for a 302 and I can guarantee you that you will make more power across the board with a bigger (well tuned) carburetor. I think a 650 double pumper would be great, even a 700 if you are adventurous.

As a point of reference, I had a 302 with 9.7:1 compression, Gt-40 heads, Performer RPM intake, a 230/235 duration @ .050 .510/.515 lift hydraulic cam, and used a 600 Holley double pumper cam. Guess what? At the dyno the engine ran lean at middle to high RPM's, like high 14's to 15:1 air/fuel ratio. It needed a 650 or 700 CFM carb but it was only a 302. So much for the formulas. Nothing against the formulas, they are a good baseline for general performance, but for high performance you need to analyze your own particular combination.

Yous combination is stronger then the one I had due to the great cylinder heads you have and the roller cam. it should be able to process air flow even better than the engine I had. I have to assume that you need at least a 650 CFM carburetor. I'd choose a VS or DP based on what trans is being use. Automatic = VS, manula = DP.

 

['69Stang]

Real world reply.....

You have a very nice set-up for a 302 and I can guarantee you that you will make more power across the board with a bigger (well tuned) carburetor. I think a 650 double pumper would be great, even a 700 if you are adventurous.

As a point of reference, I had a 302 with 9.7:1 compression, GY-40 heads, Performer RPM intake, a 230/235 duration @ .050 .510/.515 lift hydraulic cam, and used a 600 Holley double pumper cam. Guess what? At the dyno the engine ran lean at middle to high RPM's, like high 14's to 15:1 air/fuel ratio. It needed a 650 or 700 CFM carb but it was only a 302. So much for the formulas. Nothing against the formulas, they are a good baseline for general performance, but for high performance you need to analyze your own particular combination.

Your combination is stronger then the one I had due to the great cylinder heads you have and the roller cam. It should be able to process air flow even better than the engine I had. I have to assume that you need at least a 650 CFM carburetor, maybe a 700. I'd choose a VS or DP based on what trans is being use. Automatic = VS, manual = DP.

 

[ratio411]

Again...
Those formulas don't take into account cam variables, header tuning, overlap, effieciency of intake tract... and too many more to list.
When they say street, they are assuming stock ports, low compression, cast manifolds, etc...
When they say race, they are thinking of aftermarket heads, headers, good intake, dual pattern cams, and all sorts of goodies that suck air in and evacuate exhaust quickly. These formulas have been around and worded the same for DECADES. From back when the best heads we could get were 351Ws and the best intakes for Ford were Streetmaster and F4B. Cams only came single pattern and "3/4 race" back then too.

I don't have Desktop Dyno, so I don't know if you can do it, but my modeling program gives a chart of VE% through the rpm range. All it takes is a good matching of parts, not anything wild, just that the parts work together, and you can get 115% VE at peak, with 2000 rpms showing 100% or better before and after peak.

About the comment of someone "...vacuum operated carbs are not performance..."
Smokey Yunick, a man that has won more races than your engineer has every seen, said when asked about DP or Vacuum Holley, "The 3150 (vac 750) is the runninest carb on the planet".
I'll take Smokey's word over anyone on the planet. It served me well, I got my best performance with a 750 Holley on a single plane intake. Against all recommendations of magazine geeks. It killed the RPM and any carb or spacer combo I tried.
Dave

 

[6Stang7]

Again...
Those formulas don't take into account cam variables, header tuning, overlap, effieciency of intake tract... and too many more to list.

When you use them with the VE that they give you, yes that is true. If you figured out your true VE though, then it would work, because all of those play a roll in your VE.

 

[gp001]

Again...
Those formulas don't take into account cam variables, header tuning, overlap, effieciency of intake tract... and too many more to list.
When they say street, they are assuming stock ports, low compression, cast manifolds, etc...

Yes, there are more complex formulas that can be used, but then asking on a Classic Mustang forum generally indicates that the person does not have access to all the pertinent information, access to a computer program to crunch those numbers into their formulas, or both.

When they say race, they are thinking of aftermarket heads, headers, good intake, dual pattern cams, and all sorts of goodies that suck air in and evacuate exhaust quickly. These formulas have been around and worded the same for DECADES. From back when the best heads we could get were 351Ws and the best intakes for Ford were Streetmaster and F4B. Cams only came single pattern and "3/4 race" back then too.

I don't have Desktop Dyno, so I don't know if you can do it, but my modeling program gives a chart of VE% through the rpm range. All it takes is a good matching of parts, not anything wild, just that the parts work together, and you can get 115% VE at peak, with 2000 rpms showing 100% or better before and after peak.

Well, they as you call them have been around nothing but carbs, intakes, etc their whole career. They have tested combinatiions on the engine and chassis dyno. They have written several books on the subject. Where do you think they come up with these formulas?

Like I said, some of you know more than them

About the comment of someone "...vacuum operated carbs are not performance..."
Smokey Yunick, a man that has won more races than your engineer has every seen, said when asked about DP or Vacuum Holley, "The 3150 (vac 750) is the runninest carb on the planet".
I'll take Smokey's word over anyone on the planet. It served me well, I got my best performance with a 750 Holley on a single plane intake. Against all recommendations of magazine geeks. It killed the RPM and any carb or spacer combo I tried.
Dave

I wonder how many vacuum operated accessories ol Smoky's cars had to contend with at low rpm operation? I wonder how modified thos vac carbs were? Most high performace street cars (that is what we talk about here isn't it?) have issues with vacuum and vacuum operated accessories.

So, if we can't use formulas derived by people in the field for this exact situation what should people do? Buy a carb in every range and schedule some dyno time? Buy a modelling program and some precision measuring devices and do a real world model of the exact engine in question? If someone comes into a forum asking about "which carb" they usually don't have the time, money, resources, or experience to do a real world model of their car. I think the evil formulas do a good job of getting them into the approximate right sized carb without all the BS.

 

[Max Power]

Yep, you could say that, but I ain't spending $4000 on a 393 to choke it at 675cfm because some white lab coats tell me to. After years of working on Fords of all stripes and sizes, I know better.

 

[blue66tang]

The formula is for minimum carb needed...
NOT for maximum.

Also, the VE of 85% is very low for a performance build.
A carbed engine built to a good combo can reach 115%...

The whole over-carb and dual plane issues are way overblown.
I put different carbs on my 302, and swapped them between a mild single plane and an RPM intake.
I got better throttle and top end, with no noticable bottom end loss with the large carb and single plane.

I guess the point is that you have to build to your combo, not what the geeks in Hot Rod preach to you.
Dave

No kidding, i hate when people start bringing out the math. Every freaking engine is different, everybody has different, head, cam , intake, piston, setups. Im sorry but if i put a 600 on my car like it calls for id be lucky to get out of the 14s with it. Lets not forget ford put what was it 670s or something on there hipo 289s. Way more than what that math calls for. Its all about trial and error i think.

 

[ratio411]

When you use them with the VE that they give you, yes that is true. If you figured out your true VE though, then it would work, because all of those play a roll in your VE.

That is what I am getting at...

The engineers that have been around all this stuff all their lives, as suggested in another post, seem to be stuck on the same formulas that their grandfathers put out years ago.

My point is that even today, they are still using the same formula that I can dig up from my books and mags from the 60s. All these engine brains that know more than everybody have put no thought into the situation, they just keep plagerizing (sp) the same old formula.

Parts we commonly use today on the street would have been considered exotic race parts up to the late 70s!

Dave

 

[10secgoal]

D.Hearne is right. All motors are different...even if you build 2 motors identical to each other, they will be be different. The formulas are theories, which means a good guess with no proof.
I have personally seen a 347 dynoed that started with a 650 that increased all the way around with a 950 !!! And it was more than 6hp,too. 48.6 to the rear to be exact. And even on mountain motors... You think a 1100 cfm would be enough. Of course not. They set 3, count them three dominators, on one motor.
Most four barrels are rated at 3 in of vac. on a single carbed situation. If you had a vaccum guage to look at while you put your foot it in and drew more than 3 in, that carb is flowing more than 600 cfm.
I think you would be better of with a 650, but a 600 wouldn't KILL performance.

 

[charlies]

so if some of you are so opposed to using a more scientific method, what do you propose?

maybe an engine psychic?

or maybe just a good feeling you have when you look at a part?

or maybe just ask someone and hope that someone isn't b.s.ing?

 

[brianj5600]

A formula is only as accurate as the variables you insert. I don't know if you even need one hand to count the number of people on this thread that actually know the true VE of their motor. If you guess at any number in the formula it is a waste of time. You might as well just guess at the carb size. I would think a vacuum gauge would be a good tool to judge carb size at wot. I don't know what would be ideal vacuum at wot, but I would not think it should not rise as rpms rise. I loaned out my holley book or I would check it out. My personal experience is all seat dyno but it told me the more under carbed a motor is the easier it is to get throttle response. W/o well monitored dyno time it is educated guess followed by trial and error. With a dyno it is still trial and error, it just takes less time. If it is a driver, I'd err on the small side for ease of tuning. If all out performance is the most important thing, I'd err on the large side, butthere are drivability concerns.

 

[D.Hearne]

A formula is only as accurate as the variables you insert. I don't know if you even need one hand to count the number of people on this thread that actually know the true VE of their motor. If you guess at any number in the formula it is a waste of time. You might as well just guess at the carb size. I would think a vacuum gauge would be a good tool to judge carb size at wot. I don't know what would be ideal vacuum at wot

I totally agree here. I don't know either what vacuum reaeding at WOT should be, but there has to be a slight vacuum there for the carb to function properly.

 

[rbohm]

By that formula, a 393 stroker with AFR 205s and a single plane intake would be just fine with a 675cfm carb. I know that no formula works in all situations, but I think most here agree that a 675 cfm carb on that motor is not near enough.

CFM stands for cubic feet per minute. But who goes WOT for a minute? How about in 10-15 second blasts? While 675 might be the ideal size carb for the motor listed above, a 750 will give you a stronger and faster 10-15 second blast than a 675 will. That not only works in theory, but it has been proven at the strip time and time again.
Good discussion, by the way....

obviously you have something more than what would normaly be considered a street engine, thus you should use the .95 factor which gives 746.7(rounded up to 750) cfm. also remember that any formula is merely a place to start. testing is required to sort out ANY engine when trying to get best performance or fuel economy, or the best compromise between both. as for your 393, is 750 even enough? only testing will tell for sure, and since i dont know what is in your motor, other than the heads and intake, i cannot say if 750 is enough or not, but it is a good starting place.