Does a smaller bladed TB give a better throttle response? Like 75 mm inst of 80mm?

[BlackVert]

...

Now, what did GM do for the later years following 1999? They created a nautilus shell style throttle cam, where the part throttle portion of the cam requires more pull on the cable, with less degree angle actually being applied to the throttle body. As the blade gets closer to WOT, the radius of the cam shrinks and allows the blade to open up completely to WOT. So why would GM, do this?

Easy answer; it is to help with part throttle touchiness.

...

My car was easier to drive over a stock TB as well. The FRPP throttle body I had was much smoother. It had nothing to do with airflow, just the design and age.

that is what i was referring to in my post earlier about the SN95 style throttle bodies. ford did it too, starting with our cars

Thanks 5Spd! What abut going from a 4" cai pipe to a 70mm tb for example, to a 80mm intake opening. Will that screwup air flow enough to make a problem or decrease in hp? Thanks.

even though a 70mm might flow enough, IMO it makes more sense to match the 80mm intake opening

 

[Pokageek]

Ok. Thanks BV. I called accufab, my air leak needs to be addressed. This may well be why I do not like the funky throttle response. I will post up results.

 

[Black95GTS]

Why does the Accufab throttle body for a 96-98 Cobra, at 281 cubic inches, flow 1284 CFM, which is more then an 80mm throttle body? By the article on the site and your posting, that would be a huge waste of resources because a 281 cubic engine could in no way make use of all that flow. Yet there they are selling a throttle body for a 281 that flows more then a throttle body he's looking at for a 410. Seems to me that its more then just engine size dictating the throttle body.

Adam

 

[5spd GT]

Why does the Accufab throttle body for a 96-98 Cobra, at 281 cubic inches, flow 1284 CFM, which is more then an 80mm throttle body? By the article on the site and your posting, that would be a huge waste of resources because a 281 cubic engine could in no way make use of all that flow. Yet there they are selling a throttle body for a 281 that flows more then a throttle body he's looking at for a 410. Seems to me that its more then just engine size dictating the throttle body.

Adam

The twin throttle bodies are a little different. You need to add the areas together of each individual blade, so the total areas are:

57 mm - 7.90”
62 mm - 9.36”
65 mm – 10.28”

Now compare that to a 80 mm TB at 7.79".

A couple reasons for the twin TB's are that they it is easier to control two small blades, than one big blade. Also, look under the hood of a DOHC engine and look at there engine design. There engine is wide and is has some long runners ( ~ 12.70") and fairly decently sized plenum. Packaging is the concern here.

Are you familiar with the IRMC valves that came on the '96-'98 Cobras and there function. You may be surprised what there purpose is. I have replaced a couple, with the latest being on a '96 3.4L SHO.

Also, one needs to remember to not compare a 75 mm stock TB to a 75 mm aftermarket TB. They are not the same and do not flow near as much.

Most rides now-a-days have Nautilus style "cams" attached to the TB linkage so that the "large" TB's are manageable.

This is why I dislike the argument, "well look at the "new" car that is running a 87 mm TB (Ls7 zo6)." It is not the same as the 5.0L engine and TB set-up, so it is not a fair comparison.

In this case, it is best to read between the lines and not just the lines.

About, whether adding a 4" pipe (12.56" of area) and then a 70 mm TB (5.96"), I would not advise it. You can do one of two things to avoid it:

- Don't get a 12.56" of square area pipe
- Don't get a 5.96" of square area TB

With EFI, a large TB will not hurt WOT performance. However, why have a barn door on a hen house?

 

[Black95GTS]

You still didn't answer my question regarding flow rate vs. cubic inches. How can a 281 cubic inch motor with a rev limit of, IIRC, 7000 RPMS, possibly use 1284 CFM?

Using your formula above (which there is an error in, it should read 1728 x 2, not 1728 / 2) I get the following for a 281:

281 * 7000 * 1 = 1967000 divided by (1728 x 2) = ~569.16

I know what the IRMC does; getting rid of them and gearing accordingly is a very popular mod for those DOHC folks. All that engine needs is more moving parts!!

Adam

 

[5spd GT]

I do not use gears to band-aid lost HP/TQ at lower RPM's. I also have a buddy with a '96 Mystic Cobra. He deleted them and noticed nothing, EXCEPT a slight decreased feel at lower RPM's.

As for the CFM per cube question. I did answer it.

Packaging. If you can only fit one ~ 57 mm TB, throw on two, or else you will have two very small TB's, in which a large intake tube and the air in it will have to minimize down significantly. For anything further, contact Accufab.

However, your question defeats the purpose of what you are trying to prove. How can many of the rides mentioned and the TB's that come on them use all the CFM? This is where design and minimum restriction comes into play. That is why TB A at X size may not flow near as much as TB B at the same, X size.

Also the Cobra's have oval TB's in there aftermarket arsenal. That is less restrictive as well, then an "air foil" between two small TB blades.

Thanks for typo correction.

As for the IRMC purpose:

The air induction system improves engine performance by using the intake manifold runner control (IMRC) assemblies as follows:

The intake manifold has two runners per cylinder, feeding each of the intake parts in the cylinder heads. The IMRC assemblies are located between the intake manifold and cylinder heads, providing two air passages for each cylinder.

One air passage is always open and the other passage switches from closed to open by means of a valve plate. Below 3000 rpm, this valve plate is closed to improve low-speed and cold engine performance. Above 3000 rpm, this valve plate opens to improve high-speed engine performance. This is what you hear as your car hits 3400 rpm's; the "secondaries" open up. If you can't hear this on your car, and your SHO/Cobra seems a bit slower or sluggish, your IMRC deactivation motor may be damaged. It will need to be replaced to bring the SHO/Cobra back up to speed again.

The valve plates are opened and closed by the IMRC deactivation motor, which is controlled by the powertrain control module (PCM).

 

[Black95GTS]

My question proves my point. You post that Accufab article all the time, yet that same article contradicts itself by having an aftermarket throttle body that, according to you and that article, flows over 200% of what the engine could use.

My point from the beginning is that there is a lot more to sizing the throttle body then that calculation. Otherwise people with 7000RPM 410 cubic inch engines would be out buying 70MM throttle bodies and intakes for 351s would be sized with a 70mm opening.

If I hired you to build me an engine and you told me that you were going to put a 70mm throttle body (896 CFM for race version) on a 7000 rpm 410 engine (830 CFM by the article), I would throw up the red flag.

Adam

 

[5spd GT]

VE can fluctuate from 70-130%.

The Cobra TB set-up is for packaging.

A 6000 RPM GT vs. a 7000 RPM Cobra, goes from a 60 mm TB to twin 57 mm TB's. Why, such a huge increase with minimal power difference and RPM?

It is all about packaging. Look at the Cobra intake tube, and then ask why would the OEM go with twin ~ 25 mm TB's. It would "neck down" way to much, so they install two larger TB's to keep airflow consistent through the intake tract.

Also, bolting on a throttle body is looked at as a waste of money on a DOHC Cobra, or similar.

Do I have to remind you of the many aftermarket intakes that "neck down" after the initial TB opening size?

Throttle bodies can be very helpful, IF the intake can support it.

 

[GRGT1994]

Interesting stuff guys, although I understand very little of it. The early posts in this thread struggled to define throttle response. I think that is a fairly easy and well understood concept that got blown out of proportion for the original question at hand.

Throttle response, as commonly understood, is the time lag from when you put your foot in the throttle and the engine responds - pushing you forward.

Some cars have notoriously bad throttle response, especially slush boxes for some reason. I remember reading an early 90's Car & Driver article that complained about a 4 second delay from the time you push the go pedal on an F body (with slush box) to when you felt any sort of acceleration.

So can I ask the clearly well informed posters here to respond to that original question. How is throttle response impacted by a one step change in throttle body size (75 mm to 80 mm)? I got the detail, now I'm curious to read the short answer.

 

[BlackVert]

Interesting stuff guys, although I understand very little of it. The early posts in this thread struggled to define throttle response. I think that is a fairly easy and well understood concept that got blown out of proportion for the original question at hand.

Throttle response, as commonly understood, is the time lag from when you put your foot in the throttle and the engine responds - pushing you forward.

Some cars have notoriously bad throttle response, especially slush boxes for some reason. I remember reading an early 90's Car & Driver article that complained about a 4 second delay from the time you push the go pedal on an F body (with slush box) to when you felt any sort of acceleration.

So can I ask the clearly well informed posters here to respond to that original question. How is throttle response impacted by a one step change in throttle body size (75 mm to 80 mm)? I got the detail, now I'm curious to read the short answer.

actually, from the title, i think the original question was more about going from 80mm to 75mm, not the other way around

 

[Black95GTS]

Interesting stuff guys, although I understand very little of it. The early posts in this thread struggled to define throttle response. I think that is a fairly easy and well understood concept that got blown out of proportion for the original question at hand.

Throttle response, as commonly understood, is the time lag from when you put your foot in the throttle and the engine responds - pushing you forward.

Some cars have notoriously bad throttle response, especially slush boxes for some reason. I remember reading an early 90's Car & Driver article that complained about a 4 second delay from the time you push the go pedal on an F body (with slush box) to when you felt any sort of acceleration.

So can I ask the clearly well informed posters here to respond to that original question. How is throttle response impacted by a one step change in throttle body size (75 mm to 80 mm)? I got the detail, now I'm curious to read the short answer.

A larger throttle body will give you better throttle response as you define it. With a larger throttle body, less pedal travel will result in more air and fuel entering the engine, which results in more power being put out by the engine.

A smaller throttle body will give you worse throttle response because you will have to push the pedal in further to get the same amount of air and fuel into the engine. You will "feel" as if you have to "give it more gas" to get moving with a smaller throttle body.

Just through of this... the old expression give it more gas no longer applies to fuel injected engines.... really you're giving it more air; the computer gives it the gas.

Adam

 

[Pokageek]

This makes sense to me. I just remember the FRPP 65mm TB having the best throttle "feel" so far. Perhaps it was a combination of throttle "response" AND feel both. It would jump off the line AND I could graduate it.

 

[5spd GT]

A larger throttle body will give you better throttle response as you define it. With a larger throttle body, less pedal travel will result in more air and fuel entering the engine, which results in more power being put out by the engine.

A smaller throttle body will give you worse throttle response because you will have to push the pedal in further to get the same amount of air and fuel into the engine. You will "feel" as if you have to "give it more gas" to get moving with a smaller throttle body.

I have a couple issues with this post Adam.

First, the larger throttle body may NOT give you more power. An EFI engine can only use so much air. It is about finding that happy medium between restriction and overkill.

Second, a smaller TB may GIVE you better throttle response. Do you know why?

Because it all depends on the sizes discussed and being changed and it is also based on the OWNER'S perspective, not some NHRA guy.

Anyways, the difference in diameter between a 75 mm and 80 mm is 6.67%, but the difference in area is 13.89%.

The difference in AREA is where the touchiness comes from, not the diameter difference.

As I have stated before, more throttle response can be described in two ways:

1. Touchier throttle response
2. More throttle response

Take Accufab with a 15* closed throttle angle, right?

Let's compare a 75 mm and an 80 mm throat area quickly.

From 15*, let us open up the TB 20 more degrees (total of 35*).

The 75 mm TB has a open throat area of 1.04" squared.

The 80 mm TB has a open throat area of 1.18" squared.

Yes, part throttle will be effected, but you have to ask the question.

Will WOT be effected?

Other than that...

 

[GRGT1994]

. . . as you define it.

Thanks Adam, but I don't think I getting my point across quite right. Throttle response, again as I understand it - maybe I'm the only one who uses the term this way - is the time lag from putting your foot down to observing ANY engine response. It's not a matter of how much go at various degrees of throttle application. I'd suspect there is some different terminology for that concept, like throttle sensitivity or something like that.

To remove all ambiguity, consider stepping the pedal to the floor (WOT) and the time lag from when you do that until the engine responds. The engine does not immediately react. It takes a few moments. And even as it begins to rev up, it's still just a bit longer before you feel anything. The faster the lag time, the faster the throttle response.

I haven't heard anyone yet write directly about this understanding of throttle response, and the impact of a change between 75mm and 80mm (whichever direction you'd care to consider). Rather, there have been some very articulate and well developed descriptions of related, but different, considerations.

I'm not completely alone in this understanding of the definition of throttle response. Take a look at Wikipedia: Throttle response - Wikipedia, the free encyclopedia Of course, they're not the authority, but it does show that at least some other people think of the concept in precisely the same terms as I do.

Anyway, with all the technical knowledge here, we should be able to answer this question, in addition to all the other helpful insights in this thread.

 

[Black95GTS]

I give up with the throttle body thing. Everyone buy smaller throttle bodies.

Put 70MM throttle bodies on 400+ cubic engines because an article on the internet says so. Whatever you do, don't think for yourself, or do any research. Because sizing a carb is the same as a throttle body right??

Everyone completely ignore dyno results and track results. Real world information is useless. Apply fluid dynamics concepts and rationalize smaller parts even though nobody has yet to build an actual mathematical model of a mustang fuel injected engine that I've seen. I can dig out countless threads or articles where people switch to larger throttle bodies and gain horse power and torque, but pay no attention to that.

Its more interesting to apply theories then to observe the real world.

I will be at the track with "too much compression" "too short of a screw ratio," "too big heads," "too big throttle body," "too big camshaft," "too many RPMS," "too big MAF," "too much flow," going "too fast" while the paralysis by analysis crowd is wallowing in "too much BS."

Adam

 

[5spd GT]

I give up with the throttle body thing. Everyone buy smaller throttle bodies.

Not everyone needs to buy smaller TB's. Some need to buy bigger ones.

Put 70MM throttle bodies on 400+ cubic engines because an article on the internet says so. Whatever you do, don't think for yourself, or do any research. Because sizing a carb is the same as a throttle body right??

That "article" happens to be from the leading TB manufacturer. George Klass has did many test on the very thing the article is based upon. Just ask him.

That is exactly the point Adam. No one is thinking for theirself and going with a larger TB than needed.

Who said sizing a TB was the same as a Carb? A TB is just a valve and uses no vacuum signal through the boosters like a Carb. Just talking Cubic Feet Per Minute.

Was there anywhere I said that a large TB will hurt you?

Everyone completely ignore dyno results and track results. Real world information is useless. Apply fluid dynamics concepts and rationalize smaller parts even though nobody has yet to build an actual mathematical model of a mustang fuel injected engine that I've seen. I can dig out countless threads or articles where people switch to larger throttle bodies and gain horse power and torque, but pay no attention to that.

I have never ignored either, especially dyno results (less variables).

Those countless articles of gains all have to many variable changes (time between dynos, different dynos, different ambient conditions, tracks, altitudes, other changes, etc.) or go from "stock to 75 mm" TB's. Does that mean that a nice 65 or 70 mm TB would not do the same?

Nobody pays attention when someone does not gain anything either.

The tree sways both ways here Adam.

Its more interesting to apply theories then to observe the real world.

I apply real world, as mentioned above. Observations in the real world, lead people to Q/A theories.

I will be at the track with "too much compression" "too short of a screw ratio," "too big heads," "too big throttle body," "too big camshaft," "too many RPMS," "too big MAF," "too much flow," going "too fast" while the paralysis by analysis crowd is wallowing in "too much BS."

Adam

Who said too much compression? Who said too short of a screw ratio? Who said "too big heads," or "too big throttle body" (in your definition), or too big camshaft, or too many RPM's, or too big MAF, or too much flow, or going too fast?

Just answer me those questions.

Otherwise, good post.

GRGT1994 - I suspect most of the "lag" is from an ECU to TB response. Otherwise an 80 mm TB would react quicker than a 75 mm at part throttle conditions.

 

[Pokageek]

"Otherwise an 80 mm TB would react quicker than a 75 mm at part throttle conditions."

this is not totally true according to george. Because the 80mm takes in a HUGE GULP of air on part throttle..it can actual cause the car to stumble a little on part throttle if it is too big for the motor.

Good discussion. But lets not get feathers ruffled guys by talking track vs street. I prefer an ideal TB feel in all respects. If I can get that FEEL (however it is defined and maybe all the above) by going 75mm for example rather than 80mm and NOT lose HP then that's what I want. I think 5spd and all of you are addressing that aspect very well. So if bigger gets to the point where it ain't better, then why would I put an 80mm on my car when it don't give me more HP and the throttle feel on street is worse?

 

[5spd GT]

"Otherwise an 80 mm TB would react quicker than a 75 mm at part throttle conditions."

this is not totally true according to george. Because the 80mm takes in a HUGE GULP of air on part throttle..it can actual cause the car to stumble a little on part throttle if it is too big for the motor.

Good discussion. But lets not get feathers ruffled guys by talking track vs street. I prefer an ideal TB feel in all respects. If I can get that FEEL (however it is defined and maybe all the above) by going 75mm for example rather than 80mm and NOT lose HP then that's what I want. I think 5spd and all of you are addressing that aspect very well. So if bigger gets to the point where it ain't better, then why would I put an 80mm on my car when it don't give me more HP and the throttle feel on street is worse?

I do believe that George was talking about wide open throttle "gulp", not part throttle "gulp." I do not believe an 80 mm TB would take in a "HUGE" gulp of air. Let us just say that a 65 mm TB at 50% depression of the throttle, takes in the same amount of air as an 80 mm TB at 30% depression, your 65 mm TB is not going to "gulp", so why would the 80 mm TB "gulp" at 30%?

I want to make it clear that I think a larger TB is great, IF the engine can use the power.

If you have a big intake tube, small TB in comparison, and a large intake opening and upper plenum throttle body area, I believe the small TB could pose a problem in extracting every last HP.

I agree with you Pokageek, let us not get our boxers in a bunch. It is just the internet.

I wish someone, like yourself Pokageek, could borrow a few TB sizes and have them back to back dyno'ed on your 414^3.

 

[Rick 91GT]

5spdGT-If you have a big intake tube, small TB in comparison, and a large intake opening and upper plenum throttle body area, I believe the small TB could pose a problem in extracting every last HP.

 

[Pokageek]

Ok..GOOD news! Accufab sent me a new race 80mm TB AND a new egr spacer. This time the egr spacer was just that, WITHOUT the egr stuff . In other words its called an EGR "blank." Guess what. NO MORE DAM SQUEAL!! And prolly no more hesitation..Let's see how it drives tomorrow. I will prob send the race TB back. Looks like the main diff is that it fits my 4" intake tubing better without a shim. Why change it out if it works.