Much debate is brought about over throttle body sizing and the effects of each.
I decided to bring out a little fact and a little common practice, so pardon some of it being unorganized, but I put this together quickly and wanted some feedback and thought. If there is anything you see that you feel is inaccurate or bias please post up and we will go from there
I notice that there are a few stangers out there with the notion that a bigger throttle body has no effect on engine performance or drivability and is needed to get the correct amount of air in the engine. I attend to show this by thought that this is not the case all the time. These numbers are accurate ballpark figures. Not exact.
At 6,000 rpm a 347 can flow 600cfm. That is with NO restriction. Any 347 or similar has a ton of restriction. That cfm rating does not include a heads, cam package on it. The cam only is open for little amounts of time, commonly referred to as the duration of a cam. So the cfm rating of the engine is further cut.
Stangers are routinely sticking on throttle bodies that flow two to two and half times greater than their engine sees at its peak.
I have to ask...why?
If you've got a throttle body that delivers 100% of the peak air requirements of your engine when the throttle plate is fully open, you have control of the air throughout 100% of the throttle position range. If you go to an oversized TB that delivers 100% of the air that your engine can consume while the throttle plate is only 60% open, you have given up usable throttle-control range for no advantage.
Guys that are constantly defending the oversized throttle bodies are only talking about wide open performance (WOT). What about the 99% of the time that we cruise around at part throttle?
With a bigger blade like on a 75mm throttle body, the throttle modulation becomes more difficult. The gas pedal modulation becomes touchier. The reason why is do to the fact with the larger surface area of a 75mm blade compared to a 65mm blade, you get more air that passes thru with less pedal effort. This causes the gas to be less manageable and many post of bucking problems do to the very problem described above.
From Tom Moss, do to the nature of velocity the air charge moves faster (for each degree of TB blade movement) when using a smaller TB and that fills the cylinder better at lower rpms. Velocity is effected for part throttle performance as well.
Again, look somewhere else for a restriction, not an oversized throttle body.
An Accufab 65mm throttle body flows more than enough for a 347 stroker with 664cfm continuously, while the 347 would see 600cfm if it had no heads, cam, or intake on it, which of course is impossible.
Enthusiasts need to quit ignoring the facts.
Read an article from the top throttle body maker them self and let them tell the tale:
THE AIR PUMP UNDER YOUR HOOD
By George Klass – Accufab Designer/Tech
What size throttle body or carburetor do I need?
Good question but the real question should be “how many cubic feet per minute (CFM) of air will my engine require?”
An engine is an air pump. Based on the size of the engine (displacement) and how fast it will be spinning (RPM), it will pump out a certain amount of air. Works just like an air compressor. Of course, there are many variables to CFM requirements, such as cylinder head flow capabilities, intake and exhaust manifold flow capabilities, etc., but the basic engine block will pump a certain amount of air over a specified period of time, measured in cubic feet of air per minute.
If the engine is to be carbureted, it should be a relatively easy decision to pick out the correct sized carburetor. Carburetors are defined by CFM. If your engine calls for (or pumps out) about 664 CFM, the correct choice is probably a 650 CFM carburetor. Unfortunately, throttle bodies are rarely defined by CFM ratings. Instead, most throttle body manufacturers define their throttle bodies by the inside diameter of the unit, measured at or around the throttle blade, and usually in Millimeters (MM). Unless you have a direct correlation between the measurement in MM and the related CFM of that particular unit, the selection is going to be based on “best guess”.
And to further complicate the “guessing” process, all throttle bodies of the same dimension, do not have the same CFM rating. You might think that Brand A’s 75 MM throttle body would flow the same as Brand B’s 75 MM throttle body. Such is not the case, because all throttle bodies have a “major obstruction” in the middle of the air path, namely a throttle blade and shaft. How well the air flows over and under this obstruction will define the CFM a specific throttle body will flow. A big fat shaft with the attachment screws for the blade sticking up into the air stream will impact the CFM of that throttle body.
While most enthusiasts with EFI engines continue to purchase throttle bodies based on Millimeter size, caring little about the actual CFM ratings, the carb guys purchase carburetors based on CFM ratings, caring little about the size of the throttle plates. Score one for the carb guys.
But, back to the original question, “how much CFM do I need”?
Below is a chart with the CFM requirements, based on displacement (in cubic inches) and RPM. This chart will work for any piston engine with any number of cylinders. After you have determined the CFM for your specific engine combination, you can then choose the corresponding throttle body or carburetor size to best fit that combination.
DISPLACEMENT………….6000 RPM……….6500 RPM……….7000 RPM
280………..…………………486………………..527………………..567
290……..……………………503………………..545………………..587
300………..…………………521………………..564………………..608
310……..……………………538………………..583………………..628
320……..……………………556………………..602………………..648
330………..…………………573………………..621………………..668
340……………..……………590………………..639………………..689
350……..……………………608………….…….658………………..709
360………………..…………625………………..677………………..729
370………………..…………642………………..696………………..749
380………………..…………660………………..715………………..770
390………………..…………677………………..734………………..790
400………………..…………694………………..752………………..810
410………………..…………712………………..771………………..830
420………………..…………729………………..771………………..830
430………………..…………747………………..809………………..871
This chart should give you a general idea of the amount of air your combination will pump. Engines will pump less air because of the restrictions in the cylinder head or intake manifold design or valve lift, or all three. But, the chart still gives you a ball park starting point.
One other thing to know. A carburetor requires air speeding over the venturi to draw the gasoline into the mixture. Using too large a carburetor (high CFM rating) will usually cause derogatory performance in the lower or midrange. This is because the lower air velocity is inefficient in mixing the gasoline with the air. In general, and particularly for street use, a slightly smaller carb (less CFM) will give better overall performance.
With an EFI system, this is usually not a problem. The throttle body only controls air flow. A computer monitors the gasoline supply and the mixing of gasoline and air takes place inside the intake port, and not inside the carburetor. Using an oversize throttle body is not nearly as detrimental to low and midrange performance as is using an oversize carburetor.
So, to find the CFM ratings of a carburetor, all you need to do is to look in any catalog from Holley, Edelbrock, Barry Grant, etc. That’s how the carburetors are listed. To find the CFM ratings for a throttle body is going to be more difficult, unless you happen to choose an Accufab throttle body.
Because the Accufab throttle bodies are designed to “race engine specs”, the flow ratings are going to be greater than most of the other aftermarket throttle body designs, so don’t automatically expect a “75 MM Brand B” throttle body to flow as much as an Accufab 75 MM unit.”
Top reasons why mythical ‘gains’ are seen by larger throttle bodies:
1. The owner did not port match the intake or have an intake that had a same or larger diameter intake opening. The owner then port matched the inlet or switched with a different intake.
2. The throttle body was swapped after a new or rebuilt engine had broken-in. It is common knowledge that a engine will loosen up and gain clearance tolerances which reduces friction. For example, if a stanger had a 70mm throttle body on a freshly rebuilt 331 and dynoed 350rwhp with 500 miles and at 7,000 miles swapped to a 75mm throttle body and gained 8hp ‘across the board’. You may want to think about engine break-in and tolerances of an engine to be the culprit not the larger throttle body.
Car and Driver do long term testing on cars/trucks as well as others. They do baseline runs when new and do more performance testing at 50,000 miles. Every single time the 50,000 mile period shows to much noticeable quicker to 60mph and ¼ mile times. Be careful on a new engine thinking the throttle body gave you the gain when it was an extended period between a dyno or track session.
3. Another thing I have seen is when there are dyno graphs that show gains ‘across the board.’ This simply can not happen at wide open throttle if you take this example: Take a 302 engine with a 65mm throttle body and you add a 75mm throttle body. Let’s say it shows a gain from 2,000 rpm – 6,000 rpm. That would imply that the 65mm throttle body could not flow enough at 2,000 rpm which is guaranteed not to be true. So what is the culprit? Check when the dyno was done compared to the new dyno? What was the mileage? Temperatures? Different dynos? Were there any other changes along this timeline or tuning? Then you get your answer. You will not see gains ‘across the board’ when you are dealing with N/A 302’s or even strokers under 350 cubic inch. The 65mm throttle bodies flow enough. You may see gains in the top half of the rpm range if you have some healthy parts on a stroked engine.
4. It is done inefficiently by using our own seat of the pants. Many that make the unnecessary swap think since the gas is touchier assume they have gained power by ‘feel.’
5. Much ‘testing’ is done at the track and many post how X stanger gained X mph with just a throttle body change. So again ask this stanger was the track the same? Headwind/Tailwind the same? Same shifting speed? Same 60ft time? Same rpm shift? Same day? Same or similar temperatures? This may seem tedious but many factors remain. A dyno will help take care of some of the ‘questions’ by taking out some other parameters that change.
In short, if there is information given about a gain look for the simple things like: Were there any other changes to the combo? Was the intake port-matched? Was the engine new or freshly rebuilt recently? Were the dynos the same? Was the temperatures/humidity the same?
The most accurate way to see if a larger throttle body gives you a gain is to do a back-to-back dyno test from one throttle body to the next. That way you have same mileage, same temperature, same dyno, same supporting parts, and same parameters in general. But be wary of promotional dyno test for a company or brand.
There can be instances were gains can be seen, on track cars that see HIGH rpm and have larger cubic inches that us 5.0L stangers usually see. And the gains will be up top in the higher rpm.
Throttle Body CFM Flow Ratings:
Stock 5.0L 60 MM - 526 CFM
SVO 65 MM - 540 CFM
Accufab:
65 MM - 664 CFM
70 MM - 787 CFM
70 MM - 896 CFM (Race version)
75 MM - 924 CFM
75 MM - 1045 CFM (Race version)
80 MM - 1142 CFM
85 MM - 1322 CFM
90 MM - 1369 CFM
105 MM - 1550 CFM
Holley:
65 MM - 750 CFM*
70 MM - 790 CFM*
75 MM - 840 CFM*
80 MM - 892 CFM*
*Information given by Tech Rep.
BBK:
70 MM - 726 CFM
Edelbrock, Ford Racing and Professional Products have no cfm information after calls/emails.
Information to keep in mind when picking out a throttle body for your application:
A 300 cubic inch engine (302 c.i.) flows 521 cfm (ballpark) at 6,000 rpm.
A 330 cubic inch engine (331 c.i.) flows 573 cfm (ballpark) at 6,000 rpm.
A 350 cubic inch engine (347 c.i.) flows 600 cfm (ballpark) at 6,000 rpm.
With the above information from Accufab's website, you can see that the aftermarket throttle bodies offered flow much more than your engine can breath (302-347). Those cfm ratings are even given with NO restrictions and of course our engines all have restrictions, via our heads, cam, intake packaging. An actual running engine flows somewhere in the neighborhood of 200-400 cfm with some rough math. The aftermarket throttle bodies flow two times this amount.
In short, be careful on picking 'too big' of a throttle body for your car. An application with boost (forcing air) allows for you to efficiently select a bigger throttle body, although N/A engines do not.
Throttle Body 101 from Accufab article: http://www.accufabracing.com/article 2.htm
Feedback welcome
If you dispute this information feel free to post up
I decided to bring out a little fact and a little common practice, so pardon some of it being unorganized, but I put this together quickly and wanted some feedback and thought. If there is anything you see that you feel is inaccurate or bias please post up and we will go from there
I notice that there are a few stangers out there with the notion that a bigger throttle body has no effect on engine performance or drivability and is needed to get the correct amount of air in the engine. I attend to show this by thought that this is not the case all the time. These numbers are accurate ballpark figures. Not exact.
At 6,000 rpm a 347 can flow 600cfm. That is with NO restriction. Any 347 or similar has a ton of restriction. That cfm rating does not include a heads, cam package on it. The cam only is open for little amounts of time, commonly referred to as the duration of a cam. So the cfm rating of the engine is further cut.
Stangers are routinely sticking on throttle bodies that flow two to two and half times greater than their engine sees at its peak.
I have to ask...why?
If you've got a throttle body that delivers 100% of the peak air requirements of your engine when the throttle plate is fully open, you have control of the air throughout 100% of the throttle position range. If you go to an oversized TB that delivers 100% of the air that your engine can consume while the throttle plate is only 60% open, you have given up usable throttle-control range for no advantage.
Guys that are constantly defending the oversized throttle bodies are only talking about wide open performance (WOT). What about the 99% of the time that we cruise around at part throttle?
With a bigger blade like on a 75mm throttle body, the throttle modulation becomes more difficult. The gas pedal modulation becomes touchier. The reason why is do to the fact with the larger surface area of a 75mm blade compared to a 65mm blade, you get more air that passes thru with less pedal effort. This causes the gas to be less manageable and many post of bucking problems do to the very problem described above.
From Tom Moss, do to the nature of velocity the air charge moves faster (for each degree of TB blade movement) when using a smaller TB and that fills the cylinder better at lower rpms. Velocity is effected for part throttle performance as well.
Again, look somewhere else for a restriction, not an oversized throttle body.
An Accufab 65mm throttle body flows more than enough for a 347 stroker with 664cfm continuously, while the 347 would see 600cfm if it had no heads, cam, or intake on it, which of course is impossible.
Enthusiasts need to quit ignoring the facts.
Read an article from the top throttle body maker them self and let them tell the tale:
THE AIR PUMP UNDER YOUR HOOD
By George Klass – Accufab Designer/Tech
What size throttle body or carburetor do I need?
Good question but the real question should be “how many cubic feet per minute (CFM) of air will my engine require?”
An engine is an air pump. Based on the size of the engine (displacement) and how fast it will be spinning (RPM), it will pump out a certain amount of air. Works just like an air compressor. Of course, there are many variables to CFM requirements, such as cylinder head flow capabilities, intake and exhaust manifold flow capabilities, etc., but the basic engine block will pump a certain amount of air over a specified period of time, measured in cubic feet of air per minute.
If the engine is to be carbureted, it should be a relatively easy decision to pick out the correct sized carburetor. Carburetors are defined by CFM. If your engine calls for (or pumps out) about 664 CFM, the correct choice is probably a 650 CFM carburetor. Unfortunately, throttle bodies are rarely defined by CFM ratings. Instead, most throttle body manufacturers define their throttle bodies by the inside diameter of the unit, measured at or around the throttle blade, and usually in Millimeters (MM). Unless you have a direct correlation between the measurement in MM and the related CFM of that particular unit, the selection is going to be based on “best guess”.
And to further complicate the “guessing” process, all throttle bodies of the same dimension, do not have the same CFM rating. You might think that Brand A’s 75 MM throttle body would flow the same as Brand B’s 75 MM throttle body. Such is not the case, because all throttle bodies have a “major obstruction” in the middle of the air path, namely a throttle blade and shaft. How well the air flows over and under this obstruction will define the CFM a specific throttle body will flow. A big fat shaft with the attachment screws for the blade sticking up into the air stream will impact the CFM of that throttle body.
While most enthusiasts with EFI engines continue to purchase throttle bodies based on Millimeter size, caring little about the actual CFM ratings, the carb guys purchase carburetors based on CFM ratings, caring little about the size of the throttle plates. Score one for the carb guys.
But, back to the original question, “how much CFM do I need”?
Below is a chart with the CFM requirements, based on displacement (in cubic inches) and RPM. This chart will work for any piston engine with any number of cylinders. After you have determined the CFM for your specific engine combination, you can then choose the corresponding throttle body or carburetor size to best fit that combination.
DISPLACEMENT………….6000 RPM……….6500 RPM……….7000 RPM
280………..…………………486………………..527………………..567
290……..……………………503………………..545………………..587
300………..…………………521………………..564………………..608
310……..……………………538………………..583………………..628
320……..……………………556………………..602………………..648
330………..…………………573………………..621………………..668
340……………..……………590………………..639………………..689
350……..……………………608………….…….658………………..709
360………………..…………625………………..677………………..729
370………………..…………642………………..696………………..749
380………………..…………660………………..715………………..770
390………………..…………677………………..734………………..790
400………………..…………694………………..752………………..810
410………………..…………712………………..771………………..830
420………………..…………729………………..771………………..830
430………………..…………747………………..809………………..871
This chart should give you a general idea of the amount of air your combination will pump. Engines will pump less air because of the restrictions in the cylinder head or intake manifold design or valve lift, or all three. But, the chart still gives you a ball park starting point.
One other thing to know. A carburetor requires air speeding over the venturi to draw the gasoline into the mixture. Using too large a carburetor (high CFM rating) will usually cause derogatory performance in the lower or midrange. This is because the lower air velocity is inefficient in mixing the gasoline with the air. In general, and particularly for street use, a slightly smaller carb (less CFM) will give better overall performance.
With an EFI system, this is usually not a problem. The throttle body only controls air flow. A computer monitors the gasoline supply and the mixing of gasoline and air takes place inside the intake port, and not inside the carburetor. Using an oversize throttle body is not nearly as detrimental to low and midrange performance as is using an oversize carburetor.
So, to find the CFM ratings of a carburetor, all you need to do is to look in any catalog from Holley, Edelbrock, Barry Grant, etc. That’s how the carburetors are listed. To find the CFM ratings for a throttle body is going to be more difficult, unless you happen to choose an Accufab throttle body.
Because the Accufab throttle bodies are designed to “race engine specs”, the flow ratings are going to be greater than most of the other aftermarket throttle body designs, so don’t automatically expect a “75 MM Brand B” throttle body to flow as much as an Accufab 75 MM unit.”
Top reasons why mythical ‘gains’ are seen by larger throttle bodies:
1. The owner did not port match the intake or have an intake that had a same or larger diameter intake opening. The owner then port matched the inlet or switched with a different intake.
2. The throttle body was swapped after a new or rebuilt engine had broken-in. It is common knowledge that a engine will loosen up and gain clearance tolerances which reduces friction. For example, if a stanger had a 70mm throttle body on a freshly rebuilt 331 and dynoed 350rwhp with 500 miles and at 7,000 miles swapped to a 75mm throttle body and gained 8hp ‘across the board’. You may want to think about engine break-in and tolerances of an engine to be the culprit not the larger throttle body.
Car and Driver do long term testing on cars/trucks as well as others. They do baseline runs when new and do more performance testing at 50,000 miles. Every single time the 50,000 mile period shows to much noticeable quicker to 60mph and ¼ mile times. Be careful on a new engine thinking the throttle body gave you the gain when it was an extended period between a dyno or track session.
3. Another thing I have seen is when there are dyno graphs that show gains ‘across the board.’ This simply can not happen at wide open throttle if you take this example: Take a 302 engine with a 65mm throttle body and you add a 75mm throttle body. Let’s say it shows a gain from 2,000 rpm – 6,000 rpm. That would imply that the 65mm throttle body could not flow enough at 2,000 rpm which is guaranteed not to be true. So what is the culprit? Check when the dyno was done compared to the new dyno? What was the mileage? Temperatures? Different dynos? Were there any other changes along this timeline or tuning? Then you get your answer. You will not see gains ‘across the board’ when you are dealing with N/A 302’s or even strokers under 350 cubic inch. The 65mm throttle bodies flow enough. You may see gains in the top half of the rpm range if you have some healthy parts on a stroked engine.
4. It is done inefficiently by using our own seat of the pants. Many that make the unnecessary swap think since the gas is touchier assume they have gained power by ‘feel.’
5. Much ‘testing’ is done at the track and many post how X stanger gained X mph with just a throttle body change. So again ask this stanger was the track the same? Headwind/Tailwind the same? Same shifting speed? Same 60ft time? Same rpm shift? Same day? Same or similar temperatures? This may seem tedious but many factors remain. A dyno will help take care of some of the ‘questions’ by taking out some other parameters that change.
In short, if there is information given about a gain look for the simple things like: Were there any other changes to the combo? Was the intake port-matched? Was the engine new or freshly rebuilt recently? Were the dynos the same? Was the temperatures/humidity the same?
The most accurate way to see if a larger throttle body gives you a gain is to do a back-to-back dyno test from one throttle body to the next. That way you have same mileage, same temperature, same dyno, same supporting parts, and same parameters in general. But be wary of promotional dyno test for a company or brand.
There can be instances were gains can be seen, on track cars that see HIGH rpm and have larger cubic inches that us 5.0L stangers usually see. And the gains will be up top in the higher rpm.
Throttle Body CFM Flow Ratings:
Stock 5.0L 60 MM - 526 CFM
SVO 65 MM - 540 CFM
Accufab:
65 MM - 664 CFM
70 MM - 787 CFM
70 MM - 896 CFM (Race version)
75 MM - 924 CFM
75 MM - 1045 CFM (Race version)
80 MM - 1142 CFM
85 MM - 1322 CFM
90 MM - 1369 CFM
105 MM - 1550 CFM
Holley:
65 MM - 750 CFM*
70 MM - 790 CFM*
75 MM - 840 CFM*
80 MM - 892 CFM*
*Information given by Tech Rep.
BBK:
70 MM - 726 CFM
Edelbrock, Ford Racing and Professional Products have no cfm information after calls/emails.
Information to keep in mind when picking out a throttle body for your application:
A 300 cubic inch engine (302 c.i.) flows 521 cfm (ballpark) at 6,000 rpm.
A 330 cubic inch engine (331 c.i.) flows 573 cfm (ballpark) at 6,000 rpm.
A 350 cubic inch engine (347 c.i.) flows 600 cfm (ballpark) at 6,000 rpm.
With the above information from Accufab's website, you can see that the aftermarket throttle bodies offered flow much more than your engine can breath (302-347). Those cfm ratings are even given with NO restrictions and of course our engines all have restrictions, via our heads, cam, intake packaging. An actual running engine flows somewhere in the neighborhood of 200-400 cfm with some rough math. The aftermarket throttle bodies flow two times this amount.
In short, be careful on picking 'too big' of a throttle body for your car. An application with boost (forcing air) allows for you to efficiently select a bigger throttle body, although N/A engines do not.
Throttle Body 101 from Accufab article: http://www.accufabracing.com/article 2.htm
Feedback welcome
If you dispute this information feel free to post up
:
