Throttle Body Size

[final5-0]

Some say bigger is better
and
others say smaller is the way to go.

I got a good friend who is in the business of air flow, in pipes, ducts
and the like. He also works a lot with automotive ac stuff.

anyway .............

We were talking cars the other night and the subject of tb size came up.

He reminded me about how when we were kids, we saw many times, peeps
believed bigger carbs were better cause they made more power.

I told him you might not wanna compare carbs to tb's
as
tb's don't deal with a liquid like carbs.

He asked me how that made a difference when you just look at the
air flow side of both applications. Both have a butterfly to pass air!

He then asked me to tell him why, in the past, we used different size
carbs for different cars even though all were sbf powered and
to be specific with my explanation.

I said it was simple and it was backed up by what did and did not work.

600 to 650 Holleys for Street Cars
750 to 780 Holleys for 1/4 mile only Cars

We found the larger carb was great on Street Combos for WOT
but
It was lazy down low and at mid range

on the other hand ............

The smaller carb was great on Street Combos for low and mid range
but
It would not produce as much power at higher WOT rpm

He looked at me and said
You're saying ... different size for different application

His next question to me was .......

Why should a Throttle Body be any different
and
I really could not come up with an answer other than .......

Seems like it should be the same principle at work as the carb

btw ... It was Tom Moss talking about this the other day that
got me thinking along these lines and the talk with my friend
was just more info to motivate me to say something.

Just thought I'd share the info for who ever cares
and
I can see how both beliefs have merit here

I very rarely start threads
and when I do .......
I usually look for some good discussion

so

As Always ....... Feel free to agree or disagree

Grady

 

[95opal]

I will have to agree with this type thinking myself. 65-70mm for street and 75mm for track or blown cars. A friend of mine came across a TFS street w/ 75mm t-body, so we bolted it up and low and behold it was a dog , we switched to the stock tb and the car ripped through out the entire rpm. This was with a mild bolt on car. I still remember my 85 306 w/ mild cam and intake sportin a holley 650, would have never even thought about throwing a 750 on it, and that was back in the day where t-bodys didnt exsist.

 

[Grn92LX]

Jay Allen has posted many times on corral the real deal tech as to why you can not compare a tb to a carb. I would suggest you post this on www.sbftech.com you'll get a good answer there.

Rule of thumb is to match the tb to the inlet of the manifold. If you need bigger, break out the dremel!

 

[BlackVert]

I also contributed to the thread you are referring to.

The velocity of the air going through the intake and the pressure waves that bounce back and forth in the intake (created by the air hitting the valves when they close) and how those waves match up with the valve events, has alot to do with maximizing the power the engine makes.

In our cars, the pressure wave bounces back and forth between the valves and the large space in the intake three times between the time the intake valve closes and re-opens. The longer the runners, the longer it takes the waves to cycle, so the waves match the valve events better at low rpms.

When it is timed correctly, the wave rushing towards the valves gets there right when the valve opens. That affects how much air gets into the cylinder, and that of course affects how much power you make.

 

[gmkillr]

according to Jay, in a 1/4 mile racing application the biggest that you can possibly fit will produce the most peak hp & best 1/4 mile times. Think about it, when your racing the car never see's anything less than 3800-4000rpm. The car spends most of the 1/4 mile in the 4500-6000 range or whatever your redline is.
Now if your talking cruising on the street, then is say go w/ a 65 or 70mm so that you will have quick throttle response.

 

[MustangGT95]

I have a 75 ready to go on mine. I got it cheap so I am going to try it. Really hoping I can get blackverts kenne bell off him. That would make good use of the 75mm

I am no expert and do not claim to be but if you intake has a 75mm outlet I can't see a 75mm throttle body hurting it.

 

[Grn92LX]

I am no expert and do not claim to be but if you intake has a 75mm outlet I can't see a 75mm throttle body hurting it.

Yup!!

When I had my little 302 I lost ZERO throttle response switching from a 65mm tb to a 75mm tb as per Ed Curtis's recommendation

 

[Car Nut]

Very interesting discussion. I totally agree and to me it makes total sense.

Over the years, I've heard of people putting on all different sizes of TBs. As mentioned before, the best advice I keep running into is to match the TB with the intake opening for street use.

Now I need to do some research on MAF size, so I can choose the right one the first go round on my future engine combo.

 

[final5-0]

Now I need to do some research on MAF size, so I can choose the right one the first go round on my future engine combo.

I've never seen much to think about with this subject myself

I am Pro ProM for typical NA street combos.

In their line of meters for that application ..........
You can go with the old tried and true 77mm
or
You can go with the newer 80 mm

Both of these use the same electronics

There is also the 75 mm bullet meter.

When they were still ProM, their tech guy told me the bullet
was not as accurate as the 77 & 80. I asked why and he
basically just said ...........

It was one of those ... You get what you pay for kinda things

btw ... I like ProM cause you get a flow sheet with it and
sometimes, that can be helpful with tuning.

Our 94-95 cars pcm uses load to determine how to administer many
different things such as fuel, spark, and others.

It is the maf that the pcm gets data from to determine that load.

An accurate meter is critical for our cars
The mass air meter is one place you don't wanna cheap out on

C&L meters are less costly
but
I don't think they got the same quality

Size really doesn't make a difference here IMHO
but
Accuracy really does!

Grady

 

[Black95GTS]

I will have to agree with this type thinking myself. 65-70mm for street and 75mm for track or blown cars.

Thats nonsense. With forced induction, throttle body size becomes less important. Since you're forcing the air in, size is less relevant. Go to turbomustangs.com and look at all the 10 second cars running FMS 65mm throttle bodies. How many NA cars run 10s with 65mm TBs?

I want someone to explain to me how throttle response gets worse with a bigger throttle body. With the bigger blade, more air gets let in then with the smaller blade given the same pedal depression. If more air gets in with less pedal depression, that means more fuel gets put into the cylinder, wouldn't that mean MORE power with less throttle depression, giving you BETTER throttle response?

I can understand the argument that one may need to get used to having more power with less pedal effort, but I'll never believe that throttle response is worse if you are getting more power with less pedal work. At the least, it should be the same.

Adam

 

[Car Nut]

Thanks, Grady.

I'm glad you said what you did about accuracy, because that seems to be something overlooked in some cases.

 

[WhiteDevil]

Yeah i remember before doing self tuning that you dont really think about MAF accuracy. But since i have been tweecing i can stress it enough. Also i run a 75mm tbody and i did on my 302 as well. I didnt lose any throttle response that i remember.

 

[parchisi]

Thats nonsense. With forced induction, throttle body size becomes less important. Since you're forcing the air in, size is less relevant. Go to turbomustangs.com and look at all the 10 second cars running FMS 65mm throttle bodies. How many NA cars run 10s with 65mm TBs?

I want someone to explain to me how throttle response gets worse with a bigger throttle body. With the bigger blade, more air gets let in then with the smaller blade given the same pedal depression. If more air gets in with less pedal depression, that means more fuel gets put into the cylinder, wouldn't that mean MORE power with less throttle depression, giving you BETTER throttle response?

I can understand the argument that one may need to get used to having more power with less pedal effort, but I'll never believe that throttle response is worse if you are getting more power with less pedal work. At the least, it should be the same.

Adam

I think the theory is that with a smaller TB, the air comes in with a higher velocity and gets to the ports faster or something.......I dunno, lol.

 

[blksn955.o]

On the maf end IMHO if you dont want to deal with a tune for the most part but run a larger inj. Pro-M/PMAS is the way to go.

Now if a tuner or tuning device is had with the amount of larger (80-90mm) factory mafs out there now from the 4.6 line of motors I dont see why people have not started running those as a meter to fit a combos needs of air flow. Now at a certain point the stockers start to lag even the 03 cobra/Lmaf has its limits but then (again) IMHO the SCT line of mafs takes over with a sim. style of factory maf.

The maf and amount of airflow is why I started that maf list topic in the tuning section a year or so ago. That thread is why I went with a 98gt meter as I felt it would be the best bet and they are going to be even more accurate even over the Pro-M/PMAS units. That and I picked up mine for like 45$ and a pigtail for 45$ or so...not a bad alt. for money spent.

 

[BlackVert]

I want someone to explain to me how throttle response gets worse with a bigger throttle body. With the bigger blade, more air gets let in then with the smaller blade given the same pedal depression. If more air gets in with less pedal depression, that means more fuel gets put into the cylinder, wouldn't that mean MORE power with less throttle depression, giving you BETTER throttle response?

ok, i'll try. try to think of it this way. with a smaller opening, the air is coming in faster than it is with a larger opening. we assume that enough air is able to come in, so the throttle is not a restriction. so the issue is not how much air CAN come in, it is how fast the air that ACTUALLY DOES COME IN is going. when the intake valve opens, if the air that is going through the intake is moving fast, you will get more air into the cylinder if it is moving faster than you can if it is moving slower.

there is more to it than that, because there is the dynamic of what happens to the air when the valve closes and the pressure waves that bounce off the back of the valve and so on, but hopefully i have shed a lttle light on why air velocity is an important factor in part throttle response.

 

[wytstang]

The smart way to chose a TB is based on CFM and how much CFM can the entire induction system handle. I have yet to see a 302-347 N/A needed/used 800+ CFM...

 

[Black95GTS]

ok, i'll try. try to think of it this way. with a smaller opening, the air is coming in faster than it is with a larger opening. we assume that enough air is able to come in, so the throttle is not a restriction. so the issue is not how much air CAN come in, it is how fast the air that ACTUALLY DOES COME IN is going. when the intake valve opens, if the air that is going through the intake is moving fast, you will get more air into the cylinder if it is moving faster than you can if it is moving slower.

there is more to it than that, because there is the dynamic of what happens to the air when the valve closes and the pressure waves that bounce off the back of the valve and so on, but hopefully i have shed a lttle light on why air velocity is an important factor in part throttle response.

As an enginner I can appreciate the theory of waves and all that www.howstuffworks.com stuff, but the motto of my college is Lehr and Kunst, which means Theory and Practice.

On the combos that this site is familiar with, larger TBs make for smaller ETs.

People who upgrade to larger TBs do not lose throttle response. I've seen people's friends brothers cousin's cars do it, but I've yet to see someone ditch their 75mm TB for a 55mm TB off a supercoupe for velocity reasons.

Air is pulled into the engine by the vacuum created by the exhaust system and the induction stroke of the engine. Not the TB size. The more air that gets in there, the denser that air will be, and the denser the intake air is the more fuel is mixed with it and the more energy is produced. The throttle body chokes off this air flow on purpose to control power. Sometimes it chokes it off too much and we have to turn the idle adjustment screw with a hex wrench or a Fox guy drills a hole in the blade with a 1/4 inch drill. (I've seen it work )

There's been talk of Nascar engines and how they work with volumetric efficiency, etc. Well I say, you can't compare the two and that argument is complete bunk.

Don't you think there is a slight difference between a carbed 354 making 800rwhp at 9500 RPMS and a fuel injected 302 making 310rw horsepower at 6000 rpms? Apples to oranges. Carbuerated engines mix fuel and air via Venturi vacuum. Venturi vacuum increases as total mass flow through the engine increases. Venturi vacuum is completely different from manifold vacuum.

My point is twofold:

1 - Results and first hand reports speak for themselves.
2 - Using carb theory to dictate throttle body practice doesn't make sense.

Furthermore, say brand X TB flows 1000 CFM. Does anyone here really think that after the air goes through an air filter, twisted inlet tubing that often varies in diameter and texture, and through an MAF before it hits the TB that you're going to have 1000 CFM to flow? I don't.

Adam

 

[BlackVert]

well, i tried. i leave you with one other analogy ...

when cleaning my driveway, i put a nozzle on the end of the hose to cause the water to come out faster. even though less water is coming out, it is going much faster and has much more power than it would be if the opening were larger (no nozzle).

i can already see that people are going to say that is more like a blower case, not a n/a case, but the concept is similar. the faster an object is moving, the more energy it has, and the more pressure it can exert on what ever it happens to be pushing against.

if you draw the same amount of air through 2 different sized openings, the air going through the smaller opening has to move faster. therefore, that air will have more energy and it will be able to fill the cylinder more effectively.

 

[Car Nut]

The smart way to chose a TB is based on CFM and how much CFM can the entire induction system handle. I have yet to see a 302-347 N/A needed/used 800+ CFM...

Interesting. And a really good discussion so far by everyone..........

I'm probably going the 347 N/A route in the next 12 months and I've wondered what the needs would be for the TB and MAF. This is a critical topic, because now we're dealing with stuff that affects how much tuning will be needed.


Here's a setup that is fairly basic for an example:

CHP 347
Typical mostly forged internals (all new)
AFR 185 or Edel Performer heads
Mildish cam (emissions friendly or rather, good for a daily driver)
Matching TB (70mm most likely)
Larger injectors
Exhaust would be typical aftermarket stuff for street (shortys, X, catback)


Just how good or bad would the stock TB and MAF be on the setup above? What about a matching TB and stock MAF?

blksn955.o mentioned a possibility of using a stock MAF from a newer stang to get very accurate results. Really? Almost seems too easy, but maybe there's more to it than what I see on the surface.


If someone has a thought on which TB and MAF option to use on this "street only" engine combo, maybe someone can take it a step further and compare that to an engine combo that is much more agressive with wilder intake, heads, injectors, and cam. Just how much difference is the flow rate requirements realistically in both combos compared to stock?

What would flow enough, but not too much or too little? More cubes make a difference, right? Sounds simple, but maybe others have different opinions.

 

[Grn92LX]

Car nut, you need a 75mm tb for your 347.

Black95GTS, you da man!!

Im too lazy to do it but someone should look up Ernie302's stock tb vs 75mm tb dyno results on his little 302. The results may suprise you, but im NOT at all suprised. He GAINED low end, GAINED mid range and GAINED up top Thats odd because according to uneducated internet racers, he shoulda lost a lot of low end