throttle body/intake question

Paul (killercanary) ran a 65mm TB and a 75mm TB on his AFR 165/FTI cam/Unported Performer intake 302 combination.

Gained ~50 rwhp by adding 29 cubic inches and a Holley Systemax. Everything else was the same, including the throttle body and MAF. Now, he may want to look into a change as he is fully aware of.

He ran 112.51 MPH at 3650 lbs.

I would imagine the restriction was very minimal, if at all.

The throttle body is only going to be a benefit if the throttle body runner is larger than the effective throttle body area.

Port matching the intake only helps keep down turbulence. It does not take into account the dimensions after the throttle body plate.

Some intakes react great with a 70-75mm TB, and some do not.

I have a 65mm and a 75mm t-body. My intake and heads both flow 250cfm. 75mm is going on with the combo as is. i'm using an anderson cam and 30# injectors. All going through a AOD. The only advantage I see to using the 65mm is the fact that it would blend in to the rest of my "stock" engine compartment.

Ttop - Do a back to back dyno test run for both and entertain us all

My bet would be no change in HP but gain 5lb ft with the 65mm.

No need to cut an explorer upper.Not much to gain inside of it. The casting itself is suprisingly smooth. I opened it to 77mm and smoothed out transition. I cut mine for deception. I'm altering the appearance a bit. The lower flows 205cfm stock. With a good port job you can see 250cfm. 250 may not sound that impressive, but that is all the more wedge heads flow.

for the name droppers.........

THE AIR PUMP UNDER YOUR HOOD

By George Klass

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.

Below are the various throttle body sizes and corresponding CFM ratings for the Accufab throttle bodies. 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.

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

my comment - once you have met the max needs of the engine you have, TB size then affects throttle repsonse more than anything else.

So a 347 with wedge heads that has a shift point of 6100rpm would only need a 65mm t-body? Please correct me if I am looking at this wrong or missing something. Thanks for the info Tom.

You'd want a 75mm or more depending on the combo.

Remember, I was recommended a 75mm for my 6,200 rpm 302 by Ed Curtis @ FTI. I'll take his suggestions over ANY internet chart

The "internet chart" was put together by George Klass.

He works on many more projects. Look him up sometime when you get some time...

A larger throttle body can help with pressure loss, but it has some very sharp diminishing returns as the diameter of the blade increases.

A 1000 CFM throttle body on a 600 CFM engine.

That is the equivalent of a 160% VE engine.

Even NASCAR rides range in the 120% VE with every trick in the book for a 500 lap race.

I have never seen any good solid evidence that a 75mm TB was better than a 70mm TB.

Just get a good brand TB and be done with it.

I have no issues running a 75mm or similar, but you have to realize where there is no return.

I'm not looking for huge numbers. I'm looking for stealth. If a 65mm will feed enough air for a 331 with tfs heads and a stockish cam I will be tickled ****less. I am anticipating 320hp and 350tq at the rear. If it's more it's a bonus. As long as you walk past my "stock" engine compartment and shake your head I have done my job.

Hey,

What impact would bigger TB/MAF have on the computer??

does any tuning need to be done or just a power reset ?

Thanks

Al

Grn92LX said:

You'd want a 75mm or more depending on the combo.

Remember, I was recommended a 75mm for my 6,200 rpm 302 by Ed Curtis @ FTI. I'll take his suggestions over ANY internet chart

Click to expand...

I can`t believe he would have recomended something as stupid as that.
And its not just an internet chart.:Zip2:

Well what's done to the OP's motor. We are all taking engines that are far different from a stock 5.0. Im guessing that if he just picked up an explorer intake and TB that he doesnt have a crazy HCI package. If he doesn't need the extra breathing room that a 75mm tb provides than he shouldn't use one.

As for the tuning aspect. The tb size also matters here. There's a function in the computer called throttle body air flow, measured in pounds per minute (air). This value changes if you increase the size of the TB. While this scalar value may not be extremely important in WOT operation. I'm sure as hell it would effect part throttle performance. It may not cause large drivability problems but it certainly wont run "just like stock" The difference going from the stock 58mm TB to a 75mm TB on a stockish motor (stock heads) IS OVERKILL. THink of it this way. The "intake volume" area of the stock 58mm tb is 841 mm^2 the same area for a 75mm tb is 1406 mm^2. Nothing larger than a 65 or 70mm TB is needed for an NA 302.

Charts are just an organized way of explaining the real world, by those that do this sort of thing for a living. Aka, George Klass @ Accufab.

ninety15.0 said:

Nothing larger than a 65 or 70mm TB is needed for an NA 302.

Click to expand...

Wrong. Talk to Ed Curtis, among a few people.

Okay, lets do some very simple math here.

I will take my 6,000 RPM 302, that would have dynoed in the 295 rwhp range. A 75mm TB was suggested to me.

Then, lets take a 6,800 RPM 347, that may dyno in the 385 rwhp range. A 75mm TB was suggested for it.

Now, piston speed for the 6000 RPM 302 is 3000 mean feet per second.

The piston speed for the 6800 RPM 347 is 3,853.33 mean feet per second.

That is a 28.4% difference in piston speed between the two engines.

Take that same 28.4% number and apply it to 295 rwhp. You get 378.78 rwhp.

So why is that a 75mm TB is suggested for an engine that has 28.4% less output?

Simple, because a bigger throttle body does not hurt straightline performance at WOT.

If you use that same line of thinking, slap on a 96mm TB (28% larger), and get a touchy throttle with no straightline performance gain.

It is not due to the fact that, it lacks airflow. The larger throttle body is meant to cover up any inconsistencies.

There is a 14.9% difference in engine size between a 302 and a 347, but only a difference of 7.1% between a 70 and a 75mm TB.

A typical throttle body blade sits at 15 degrees (est.), and there reaches a point where there is diminishing returns.

A larger throttle body only becomes beneficial when the upper intake throttle body runner is opened up to the area of the actual throttle body elliptical (minus blade thickness (1-2 mm), and shaft diameter).

Why have a barn door, when you only have a hen house?

Now, if you have an Sn-95, converting to a Fox set-up could get you a few HP, but not the size.