Throttle Body size question
 

Car referance (if it matters) 2003 Mustang GT Automatic

Does anyone know if it hurts the car to have the 75mm T-Body over the 70mm?

I read that the 75mm TB doesn't help unless you have a very heavily modified car.

Anyone have any details on that they can share?

It won't hurt the motor or the car any but having to large of a T-body will take away from hp...not much...chances are you won't notice. I'd rather go big so that someday when I need the 75mm's...I'll have it.

Right on. Going too big will hurt you down low and from what I've seen, screw up you AF ratio. Stick w/ stock or 70mm. Remember, the engine is only 4.6L, it doesn't need that much air unless you force it in there.

You can even check out BBK's 78mm i love it.

sounds like I go with the 70mm then when it comes time since I'm not going to put a turbo or super charger or anything on it.

I havent seen any proof on this anywhere yet between the 70 and 75s. Also, many people argue scientifically that you cannot go too big on throttlebody size as far as power loss at peak because the engine can only take in what can be sucked in by vacuum and piston pumping.

I do agree that chances are you will suffer in driveability with going too large. Considering that the stock tb is supposed to be 65mm (right?) seems to me the 70 is barely any larger and 75-78 would work fine. I personally think I would benefit more from 75 to increase throttle response with 5 speed. Honestly I think the better flowing plenum designs are what makes the power and not the throttlebody.

The argument isn't at peak power, since the larger size is a benefit at that point and since it's WOT, there's not much of vacuum. It's at the lower rpms where the larger size kills velocity. And as for the size difference, a 65mm throttle body has an area of .0357 cu. ft. 70mm has an area of .0414 cu. ft. If the engine is sucking in say 200 cfm at the moment, the 65mm has a air velocity of 93.37 ft/s and the 70mm has a velocity of 80.52 ft/s. That 5mm of diameter dropped the velocity by 13 ft/s. You can see how much 75 or 78 would decrease the velocity.

bassman I see you are on top of your math, and I suck at math. I get what you are saying about velocity in your numbers but what you just said applies to having the full throttle bore wide open at low rpms. Are we talking driveability or dyno numbers at low rpms? With the bigger throttlebody in normal driving you are only letting in more air for how far you push the pedal and not really affectin velocity until you go wide open, or am I not taking something into account? You give less throttle position per theoretical amount of air but end up using the same amount of air for those purposes, you just step on pedal less.

Also the size of the intake tube has not increased with any of these throttlebodies bore sizes increasing so you are still at the same velocity entering the throttlebody in that case?

Well what about using a CAI or K&N filter in the example?
I'm not going to be on a drag strip with this car, just freeway and streets it's ment as a fun street car slash project car for me to have fun with.

From what bassman said it sounds like the larger T-Body actually hurts your power. Does it still hurt your power with a high flow filter rated at 500 cfm on it's worst day?

so u think that a bbk 78mm t/b & plenum , bbk cold air intake and typhoon intake manifold that my power will be affeted because of too much flow ?

Yes, the air going into the throttle body remains the same. However, as I mentioned, though you are allowing the same amount of air to enter the engine, because of the difference in area, the velocities are different. I probably should have used areas for part throttle but I was in a rush and that would require even more math. However, the point remains the same. At lower rpms, where air velocity is important, it is crucial to maintain a high velocity to promote power down there. At WOT, velocity means nothing as making sure as much air as possible enters the engine is what's important. This is also why running exhaust pipes that are too big kills your low end but boosts your peak power.

i'm going to be getting the 75mm tb/plenum combo from accufab. i'd stay away from BBK, from my research its not the best out there.

What does WOT stand for? I haven't seen that one before.

noob, WOT (Wide Open Throttle)

We're here for you.

Wide Open Throttle

I had the accufab 75mm TB and plenum combo on my 03 GT and if I had to do it all over again i would have went with the 70mm. I'm pretty sure the 75mm TB messed with the A/F ratio, especially with exhaust and CAI. If you are planning on supercharging or using nitrous go with the 75mm, but if not go with the 70mm. That's just my opinion.