5spd GT

Jason, you are 100% right about the above.

However, you have to ask, is it a restriction?

I am with a few of Bob's thoughts. Those 105mm units flow over 1500 CFM, and are running on 2500 HP vehicles.

75 vs. a 105mm is a 40% difference.

350 HP vs. a 2500 HP engine is a 714% difference.

A larger throttle body will not hurt you, however.

Mustankiller - Keep it cool...

Joel5.0

WOW...... MAF and TB now depend on velocity to work.....

Would a MAF be a restriction if it can "flow" twice or three times what the engine capacity is limited to?.....

In that case, and using the 2500 HP engine with 105mm/1500 CFM TB example given above, what size engine is that one?...... lets say it's a 347 turning at 9000 rpms with a VE of 105%...... wouldn't that make the engine CFM "capability with no restrictions" 949 CFM? ....... if you add the "normal" restrictions, wouldn't that CFM rating be lower?........ then why go with such a big/high-flowing TB?

The answer is not contained in a formula, nor should it be attempted to draw conclusions based solely on simple correlations. I wonder why I had such good "luck" using a 850 CFM carb on a 289, non-ported Hi-Po heads, 210° 'sh duration HFT cam, a single plane and a DD '65 Mustang? ..... and yes....I also had and tested it against a 600 CFM carb.

There are other factors that are affected by "matching" components outside the "flow capability window" of an engine..... and I'm not talking about claims, I mean ET performance. HP is just a tool to use, but it does not mean that the higher HP producing setup (in the same car) will be faster and out-accelerate a lower HP config 100% of the times.

5spd GT

My response in bold...

Joel5.0

Exactly!...... yet everybody forgets about the "fine scaling" effect a bigger MAF causes, which provides better electronic control through its operating range and prevents MAF "pegging". 100 g/sec through a 65mm MAF is not equal to 100 g/sec through a 90mm MAF....... why is the OEM going with "higher" flowing units over the engine's capacity then?

Yet it requires a higher flow capacity over the engine "limitation" to provide the power.... which again, is the same thing the OEM continues to do in all "intake related areas".

The reason why I brought the carbs, is to follow the same trend in the comparison areas used between both. If you recheck the article at http://www.accufabracing.com/article%203.htm , you will notice the following statements:

See my point?

... my post was not "directed" at you Dave, it's more directed at the information that is being generated wrongly.

1. A MAF does not require velocity, it's a measuring device.
2. Matching the CFM capacity of a specific intake component, only using the engine flow capability, does not take into consideration other effects. As already proven above with the 2500 HP setup.
3. If a TB is simply a valve, how could it affect engine torque? ....wouldn't it affect "velocity" in a positive way, where the real restriction is at?
4. The "validity" of the continuance of changing a single component on a setup, and derive how effective that component is, w/out resetting the rest of the combo for that component.

..... just like the 650HP+ turbo euro-imports I see (one of my son's is working on his VW Turbo Beetle), lighter than a Mustang, and yet only performs high 11's at the 1320'..... VS an over-carbureted, less HP setup, at roughly 405 HP, ~ the same weight, open headers (no backpressure.... but that's "another 20 bucks") dips into the 10.6's..... so why not bring it up.

Point is simple, why limit the setup based on CFM tables that have been proven to not be accurate, based on many experiences with DD's, S&S and race-only setups?

5spd GT

Sorry, but I dislike the "quote" set-ups on this website, so I am just going to simplify it for myself.

Exactly!...... yet everybody forgets about the "fine scaling" effect a bigger MAF causes, which provides better electronic control through its operating range and prevents MAF "pegging". 100 g/sec through a 65mm MAF is not equal to 100 g/sec through a 90mm MAF....... why is the OEM going with "higher" flowing units over the engine's capacity then?

I am not "up" on MAFs, so I cannot provide any of the 2 cents here. My comments above, both times, were not related to MAF scaling.

I can say that a MAF is not a valve, and does not effect the driver, so I imagine you would be talking to someone else here, because a "big MAF" is only going to provide accuracy, not driveability issues that a large throttle body may give.


Yet it requires a higher flow capacity over the engine "limitation" to provide the power.... which again, is the same thing the OEM continues to do in all "intake related areas".

Which factory set-ups are you speaking of? Take the well known LS1, with the 71.5 mm (back side measurement) throttle body. A couple measurements I have taken are 74.20 mm for the initial opening, and the minimum section was 73.14 mm.

Now even with a 71.5mm (~75mm opening) TB from the factory, they include a throttle cam. Take the '98-'99 pulley system for opening the throttle, which has very close to the same cable pull to the angle of the blade. Now, what did GM do for the later years following '99? They created a nautilus shell style throttle cam, where the part throttle area of the cam requires more pull on the cable, with less degree angle actually being pulled. As the blade gets closer to WOT, the radius of the cam, shrinks and allows the blade to open up wide open. So why would GM, do this?

Easy, to help with part throttle touchiness.


The reason why I brought the carbs, is to follow the same trend in the comparison areas used between both. If you recheck the article at http://www.accufabracing.com/article%203.htm , you will notice the following statements:

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.

See my point?

... my post was not "directed" at you Dave, it's more directed at the information that is being generated wrongly.

1. A MAF does not require velocity, it's a measuring device.
2. Matching the CFM capacity of a specific intake component, only using the engine flow capability, does not take into consideration other effects. As already proven above with the 2500 HP setup.
3. If a TB is simply a valve, how could it affect engine torque? ....wouldn't it affect "velocity" in a positive way, where the real restriction is at?
4. The "validity" of the continuance of changing a single component on a setup, and derive how effective that component is, w/out resetting the rest of the combo for that component.

I am not here to tear apart an article. George Klass was strickly talking about CFM requirements, not which Carb is right for every combination. Even George Klass has said this himself over on HC50. Go check it...

1. Yep, I agree.

2. I like to go slightly above the requirements for daily driven or true street cars. Most of these cars are not above 100% VE, especially since NASCAR is only in the 125% range, with the most highly researched engines in the world. I don't think either one here could touch that with our current resources. By the way, NASCAR runs 750-830 "CFM" carbs on 800 HP rides. Now that is cool...

3. I never said it would effect torque. Just getting that straight

4. I do not believe that one needs to recharge a combination by increasing the throttle body size. I have never seen the need, and what I do know is as the larger throttle body diameter increases, so does the throttle response at part throttle, or in other words, a touchy or goosey engine.


..... just like the 650HP+ turbo euro-imports I see (one of my son's is working on his VW Turbo Beetle), lighter than a Mustang, and yet only performs high 11's at the 1320'..... VS an over-carbureted, less HP setup, at roughly 405 HP, ~ the same weight, open headers (no backpressure.... but that's "another 20 bucks") dips into the 10.6's..... so why not bring it up.

Point is simple, why limit the setup based on CFM tables that have been proven to not be accurate, based on many experiences with DD's, S&S and race-only setups?

I have not seen them to be unproven, and I have looked and looked. Usually all I find is "gains" done with other changes as well, or some inconsistent track/dyno sessions to "prove" something, that in my eyes, is not proven.

[b][color=#000000]I never said your vehicle was over-carbed. I also do not want to compare just E.T. (no MPH), rear wheel dr