I'm looking for a bigger bore TB for my car. I had the stock one taper-bored by A&J (like the Spoon 70mm) but want something bigger now.

Holley makes one (68mm) but I'm not sure if this is at the butterfly.

Is there anyone who can help me here?

ALSO: Just wondering what people have done in the past to get the Spec B's to run a bit more on the lean side. (yeah I did a search and got squat)

Phat R - who someday will get ITB's but wants a b.b.TB for now....


[Modified by Big Phat R, 12:53 PM 6/17/2001]

 

Bigger isn't always better.

 

Yeah I know, but my Spec B's are running really rich (and yet making good power) with Yoshio's fuel map. I'm upgrading the ignition system now to the Nology Powercoil with the MSD dist. cap and figured that more air into the motor would be a good thing at this point.....

Phat R- who is willing to try anything at this point.

 

sheesh jonathan, why don't u ask frank at dea? ronnie at illustrix/moto-r does all it's tb's in-house.
http://www.illustrix.net

still need to talk to you dammit!

 

i did already....dammit! I will talk to ronnie - sound like they replace the butterfly?

 

Geez, its on the website. Don't you guys surf for info?

"Throttle Body Detailed

Each throttle body is precision bored, true to the centerlines of the throttle body and throttle plate shaft and bored completely through the air passage. Machined into the approaching side of the throttle body is a 5° - 10° taper causing a venturi effect, increasing throttle response and top-end power. A new throttle plate is bored .001" under the inside diameter of the new throttle body dimensions. This plate is made from aluminum to match the expansion and contraction rate of the throttle body so that the new throttle plate can shift naturally with temperature change. For nitrous oxide applications, a brass throttle plate is used.


What is a throttle body?
A throttle body is the unit found on electronic fuel injected cars that enables the driver to control the amount of air flow and fuel delivered into the engine (via the throttle plate and throttle position sensor) using the gas pedal. Before electronic fuel injection, the carburetor handled the tasks of the modern day throttle body, fuel injectors and the ecu (electronic control unit).

How does a bored throttle body increase horsepower and effeciency?
Assuming that you understand how an internal combustion gasoline engine works, this will shed light on the concept behind boring a throttle body and porting of engine components in general. If you have no idea of the combustion process, then you will want to refer to our 'tech talk' section.

Almost all production cars come from the factory with an air/fuel ratio less than a stoichiometric mixture. Boring a throttle body introduces more air into the engine while the same amount of fuel is injected. This leans out the mixture, which creates more heat. Since heat is energy, this results in more power. Under normal driving conditions (not racing) with a bored throttle body, gas milage is slightly improved. This is due to each combustion cycle using the same amount of fuel but producing more power... efficiency.

The process.
The illustrix throttle body process is a precision operation done with the correct tools. To bore the center throttle passage a lathe is used with a positive angle precision cutting tool. The size of the bore depends on a two things:

1. The application. If nitrous oxide is going to be used, then the bore will be slightly smaller than if it were going to be for a turbo/super charger or naturally aspirated engine. This is because of the characteristics of nitrous backfires. A nitrous backfire typically occurs on the intake side of the engine as opposed to the exhaust side. To ensure that the throttle body does not get damage in this event, less material is taken off the original bore.

2. The physical limitations of the throttle body itself.

Once the desired size is determined, the throttle body is then disassembled and mounted on the lathe. Centering the throttle body is important so that the centerlines before and after match. The throttle body is then machined to the predetermined inside diameter by making several passes with the cutting tool. A finishing pass is then made to give the inside surface a smooth finish. The entrance angle is now machined into the front of the throttle body. Using a 5° - 10° taper angle, the funnel is created. Having tested many techniques, illustrix found that this angle is most effective.

Making the throttle plate is the most important aspect of boring a throttle body. It will make or break the whole throttle body. If you look closely at a stock throttle plate, you'll notice that it rests at a slight angle. This is because a round plate cannot occupy the same inside diameter of a cylindrical bore if it is perpendicular to the walls. If the throttle plate were to rest like this, then you would feel resistance and a click everytime you stepped on the gas pedal and eventually metal would wear away from the plate and the walls creating a gap causing the engine to idle at an obscenly high RPM. (click here for a better understanding). illustrix developed a special jig to cut throttle plates correctly so this is never an issue.

The Results.
A smooth, stock idle with noticable increases in power throughout the entire RPM range and peaks within the engine's natural powerband. Dyno sheets will soon be available for common applications."

 

ttt

 

Loss of intake velocity

 

You'll probably lose performance if you go too big with your throttlebody. As KJ pointed out, if your throttlebody is too big, you'll lose intake velocity. This makes your power extremely peaky, losing a lot of low-end and mid-range power, and a little more peak power at extreme RPM's. I personally think a overbored ITR TB would be more than enough.