throttle body theory??

jetuomi · Nov 8, 2004

hey how about we talk some throttle body sizes here so we can set the record straight about what size is needed when on an NA car?

on a stock car, MAF flows 800 kg/hr or so @ WOT

so 800 kg/hr after conversion = 375 cfm... that shows that a 55mm t-body would be okay..

so, another sample is that a 300rwhp 302/306 will usually run a pro-m MAF, and will likely have max maf readings of around 1100 cfm, = 525 cfm.. so on a heads/cam/intake car a 60mm t-body would be JUST joked a bit.. 65mm would be optimal

in my case, a 393, w/ 450rwhp or so (hopefully?) will use a 83mm pro-m and hit max flow of around 1400-1500 kg/hr = 675 cfm, that means a bbk 70mm t-body should work just fine..

but, these are @ WOT, if you want more air to go through at part throttle you get a bigger t-body, hence why most equations add about 5 mm to the bore to gain that extra bit of "seat of the pants" feel...

So, use these #'s and add 5mm to get your optimal t-body..

sound like a science?? comments??

nmcgrawj · Nov 8, 2004

Jay Allen and many others still say to just get a 75mm on a h/c/i 302. But i dont know the science behind it, just what different people recommend.

Joes95GT · Nov 8, 2004

You looked way too far into it. People get the carburetor and throttle body theories confused, big time.

A carburetor relies heavily on velocity to pull the fuel into the motor. A fuel injection car doesn't. The long runner intake manifold keeps the airspeed relatively high. You want to stuff the manifold with air. A large throttle body will let more air in. It's really that simple.

Don't fall into flow numbers either.

I ask this question not knowing the answer either: How can a larger diameter circular piece of steel (i.e. the mass air meter) flow less air than a smaller diameter circular piece of steel with a butterfly in the middle of it (i.e. the throttle body). It just doesn't make sense.

I'd bet money that on a completely stock 5.0, you could put a 75mm TB on it and see no great gains or losses in PEAK horsepower. You just might lose a tad under 3000 RPM, but I don't think it would be too terribly noticeable. Again, my contentions are very subjective, but it shows my theory.

I'm sure I'll have more to add when others post more, I just can't think of more examples or reasoning to back myself up.

Joe

nmcgrawj · Nov 8, 2004

Joes95GT said:
You looked way too far into it. People get the carburetor and throttle body theories confused, big time.

A carburetor relies heavily on velocity to pull the fuel into the motor. A fuel injection car doesn't. The long runner intake manifold keeps the airspeed relatively high. You want to stuff the manifold with air. A large throttle body will let more air in. It's really that simple.

Joe

Thats exactly how Jay Allen described it to me when i asked him. People will proobably say you need the smaller TB to get velocity...but where does the velocity of the intake begin? thats the real question and most guys probably dont know it. I didn't.

final5-0 · Nov 8, 2004

Joe brings up some good points about the carb & fi compare thing.

I'm such an old fart that I have lots of carb experience on small block fords.

I can say for sure that for a carb combo on a sreeet car from mild to pretty wild a 600 to 650 cfm carb will do the job quite nicely. Again we are talking srteet car and not race car.

Me and my gearhead buds saw time and time again kids who would show up at the strip with 780 cfm and larger carbs and would ask why they had no low end and did not understand why they would always bog off the line.

A larger carb would screem at the top one third of the rpm range on a pertty stout combo but the low to mid range was soft due to the carb being too big.

Not sure if any of the above would apply to our topic of discussion. However air flow is air flow.

Later
Grady

jetuomi · Nov 8, 2004

good answer.. the driver for the velocity comes from the vacuum created by the stroke of the piston... My thoughts are that as long as you're all port-matched along the intake route, you should be fine no matter what the opening size is (to a point, you'll never get enough air into a big block through a 50mm t-body)..

I know a lot of people like to put 75mm t-body's on a 302 but I don't see the point, you don't gain, you don't loose, but you spend more..

Joes95GT said:
You looked way too far into it. People get the carburetor and throttle body theories confused, big time.

A carburetor relies heavily on velocity to pull the fuel into the motor. A fuel injection car doesn't. The long runner intake manifold keeps the airspeed relatively high. You want to stuff the manifold with air. A large throttle body will let more air in. It's really that simple.

Don't fall into flow numbers either.

I ask this question not knowing the answer either: How can a larger diameter circular piece of steel (i.e. the mass air meter) flow less air than a smaller diameter circular piece of steel with a butterfly in the middle of it (i.e. the throttle body). It just doesn't make sense.

I'd bet money that on a completely stock 5.0, you could put a 75mm TB on it and see no great gains or losses in PEAK horsepower. You just might lose a tad under 3000 RPM, but I don't think it would be too terribly noticeable. Again, my contentions are very subjective, but it shows my theory.

I'm sure I'll have more to add when others post more, I just can't think of more examples or reasoning to back myself up.

Joe

jetuomi · Nov 8, 2004

a mass-air meter does outflow the throttle-body (from what I can gather anyways) ???

my 80mm pro-m flows more than a 80mm accufab t-body..

or am I missing the point?

Joes95GT said:
I ask this question not knowing the answer either: How can a larger diameter circular piece of steel (i.e. the mass air meter) flow less air than a smaller diameter circular piece of steel with a butterfly in the middle of it (i.e. the throttle body). It just doesn't make sense.
Joe

GTJake · Nov 8, 2004

jetuomi said:
I know a lot of people like to put 75mm t-body's on a 302 but I don't see the point, you don't gain, you don't loose, but you spend more..

Accufab 65mm, 70mm, and 75mm TB's are all the same price.

Jake

OrangeMustangGt · Nov 8, 2004

but when you say 75mm, you must consider your intake opening, i dont know what a cobra is or an edlebrook, but there is no need for a 75mm tb if you cant flow that, actually i think if your tb is larger then your intake opening you could create air flow problems with air hitting the upper intake face.

Joes95GT · Nov 8, 2004

jetuomi said:
a mass-air meter does outflow the throttle-body (from what I can gather anyways) ???

my 80mm pro-m flows more than a 80mm accufab t-body..

or am I missing the point?

Yes you did.

A mass air meter is measured by diameter of the opening. So is a throttle body. How can a LARGER 80mm mass air flow sensor flow less CFM than a SMALLER 75mm TB. Lets pretend they're 2-D

Area of a 75mm diameter circle = pi (r^2) = pi ((75/2)^2) = 4417.86 square mm.

Area of a 80mm diameter cirlce = pi (r^2) = pi ((80/2)^2) = 5026.55 square mm.

Now, the area of the 75mm circle (i.e. throttle body) also has a big contraption in the middle of it (i.e. throttle blade), which also takes up some of the ~4400 square millimeters.

How does a smaller opening flow more air? I don't get it.

Point being - don't pay attention to flow numbers, especially comparing TB and MAF's. It's useless.

Joe

Joes95GT · Nov 8, 2004

jetuomi said:
I know a lot of people like to put 75mm t-body's on a 302 but I don't see the point, you don't gain, you don't loose, but you spend more..

I wouldn't say that.

Aftermarket cylinder heads have bigger intake and exhaust ports than stock heads. More air in = more air out. To get more air in, you need a larger throttle body.

Look at it this way - Since airspeed isn't (too) important here, you want to eliminate all bottle necks in the intake tract. (Relatively speaking) a throttle body can't be too big. If it's "too big", you can let your foot off the gas pedal and close the butterfly up, restricting the amount of air getting into the motor. Making it act "smaller".

This same effect is MUCH harder to achieve with a carburetor. This, again, is where the throttle body and carburetor theory get mixed up.

Grady is one of them old guys D) who knows about carbs. I'll leave the detailed explainations of carburetors to him.

Joe

TrueBlue95GT · Nov 8, 2004

Wouldn't a bigger tbody mess with the computer though? If you have a 75mm tbody on a stock engine, you're getting full air flow at part throttle, right? So wouldn't the TPS send wrong readings to the computer at part throttle, decreasing driveability? This is just a question. I'm not sure if it's true.

jetuomi · Nov 8, 2004

hey Joe, okay, good points.. I admit I learned a lot on this thread.. (good thing)..

So, how about this question, I have a 393 ready to go in next spring, and I'm going with the 75mm t-body and victor EFI 5.8 manifold.. I guess I'm wondering why I don't port the victor to 80mm and go with an 80mm t-body, or, will it get all the air it can need (AFR 205 headed) through the 75 mm t-body..

I figured analyzing the #'s would show that the 75mm t-body on this combo was sufficient? I appreciate your thoughts.. tx guys..

jetuomi · Nov 8, 2004

ACTUALLY, changing your t-body probably won't mess your idle up as much as changing your Mass-Air Meter... Using my Tweecer there are as many as 6 functions and 1 scalar to change as you change MAM's.. the t-body can be manually adjusted to flow the same MAF voltage as the previous t-body, but with a different MAM it sends a new MAF voltage indicating the airflow at idle. the processor needs to know this base voltage (idle with IAC unplugged) plus it needs a linear curve that tells it MAM flow at specific points to help it idle, and control coasting idle as well as idle w/ AC on, etc..

hope that helps..

TrueBlue95GT said:
Wouldn't a bigger tbody mess with the computer though? If you have a 75mm tbody on a stock engine, you're getting full air flow at part throttle, right? So wouldn't the TPS send wrong readings to the computer at part throttle, decreasing driveability? This is just a question. I'm not sure if it's true.

TrueBlue95GT · Nov 8, 2004

I wasn't talking about idle. I was talking about part throttle voltages. As the TPS sensor notes that the butterfly is open to 3/4 throttle, the MAF actually senses full throttle.

final5-0 · Nov 8, 2004

TrueBlue95GT said:
Wouldn't a bigger tbody mess with the computer though? If you have a 75mm tbody on a stock engine, you're getting full air flow at part throttle, right? So wouldn't the TPS send wrong readings to the computer at part throttle, decreasing driveability? This is just a question. I'm not sure if it's true.

No, a tb upgrade would not cause a prob for the pcm.

Unless:

The larger tb caused a maf air flow over flow (that sounds weird)
or
The larger tb caused a fuel starvation from the inj's being too small

Later
Grady

nmcgrawj · Nov 8, 2004

jetuomi said:
good answer.. the driver for the velocity comes from the vacuum created by the stroke of the piston...

Thats what i thought to!

Jay Allen told me when we first met to talk about my cam that it in fact starts in the exhaust at the header collector. The vacuum is caused by the exhaust leaving the combustion chamber thus pulling the intake charge into the combustion chamber.

jetuomi · Nov 8, 2004

yeah during normal driving it wouldn't affect anything UNLESS your t-body was a restriction and upgrading it caused your MAM to peg out, otherwise, you'll just be upping the MAFV reading, which throws more fuel, ie: normal operation..

TrueBlue95GT said:
I wasn't talking about idle. I was talking about part throttle voltages. As the TPS sensor notes that the butterfly is open to 3/4 throttle, the MAF actually senses full throttle.

94Blue302GT · Nov 9, 2004

not to mess with the thread, but what about a blown app? people usually say go as big as you want, but could the stock be ok then too? i'm lookin for 450 horse peak, which should be around a 70mm, but it's forced in, so i really don't need that, right?
shoot, the turbo itself has an opening about 2.5 inches anyways.

Killercanary · Nov 9, 2004

nmcgrawj said:
Thats what i thought to!

Jay Allen told me when we first met to talk about my cam that it in fact starts in the exhaust at the header collector. The vacuum is caused by the exhaust leaving the combustion chamber thus pulling the intake charge into the combustion chamber.

Now you know why the inatke valve starts to open before the exhaust is closed, even though the piston is still on the way UP in the cylinder... that is overlap. In a boosted car, you want to minimize this as the incoming air has more pressure than the exhaust so you will actually push your incoming air/fuel mixture right out the tail pipe if the overlap is too much.

As for the whole TB thing... I have to think about this some more...

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