The BIG THROAT Throttle Body

A few months ago, after going through all of the effort and expense of installing all of the new goodies on my Black Beast, the Feuling heads, the hydraulic roller cam, and the MSD ignition module, I began thinking of ways to increase the airflow into the engine, while still keeping it smog legal.

One way to increase the airflow into an engine without going the supercharger route is to make the intake system air passages less restrictive by making them smoother and/or larger. Turbo City wants $229 +shipping for modifying the throttle body (TB) by shaving the throttle shaft from 8.9mm down to 5mm and boring the throat diameter from the stock 51mm to 53mm. These modifications produce a 25% increase in throat area. (Just shaving the shaft alone is a 14% increase!) The limiting factor is the wall thickness near the rear mounting holes. For whatever inane reason, GM counterbored the bottom of these holes, which severely limits how large you can bore out the TB without breaking through the walls. Because I already had the Edelbrock MPFI system installed on my truck, my TB is only an air door so modifying it for greater airflow didn't mean having to mess with the injectors at the same time.

But how to enlarge the throat area even more, when the TB walls are so thin? The answer for me was to bore the body out as large as possible, breaking through the walls everywhere, and then sleeve the bores to reseal them. By going this route I was able to increase the throat diameter to 56mm. This by itself is a 22% improvement over stock. In addition, I reduced the throttle shaft thickness from 8.9mm down to 4.8mm in the throat area. When combined, these modifications increase the throat area of the TB from 31 cm2 to 43 cm2 At WOT, this is a total increase of 40%. Then, as an afterthought I added a cool looking set of air horns.

After doing this once, I now know it is possible to go to a 57mm bore, which is the absolute maximum. Any larger and you will cut the throttle shaft bearing off the drivers side of the TB, and this would not be a good thing. While going to 57mm adds another 6% in throat area, I wouldn't recommend it because the wall thickness of the sleeves becomes very thin.

All things considered, putting sleeves into the bores was not a trivial exercise, due to the fact that the engine management system is very sensitive to the location of the vacuum bleed holes in the throttle blade region. Get these locations wrong and none of the emissions stuff works any more. Minor little things like the MAP sensor, the exhaust air system, the EGR valve, the vapor canister, etc. Oh, and the IAC can't control the idle anymore either. A miss of only 0.3mm on two of the bleed holes messed things up the first try, but everything works now. All in all, it was a fun project though, figuring out solutions to the all of the problems I ran into as I went along.

The process goes something like this:

1) Totally disassemble the TB. This involves removing the TPS, the IAC, the throttle blades and the throttle shaft. You will have to machine it later anyway. (The screws holding the blades in place are staked on the backside and usually break off. But not to worry, you won't use those holes again anyway.)

2) Machine out the throttle bores in the TB to a diameter of 58.2mm. This will break through the walls just about everywhere, and scare the crap out of the uninitiated, but trust me it's okay.

3) Next you need to machine the new sleeves to fit the larger bores. They should be a nice slip fit. If you are still going to use the stock injectors you will have to make the sleeves taller than I did.

4) While you're at it you might as well machine the air horns to fit the new throttle bores. They will fit down on top of the sleeves.

5) Here comes the precision part of the job. You must locate and drill the 3 (one of which is square!) vacuum bleed holes in the sleeves.

6) Now machine the throttle shaft down to a thickness of 4.8mm in the bore region. Remember that it must still be round in the bearing areas. Next, drill and tap it for the new throttle blade mounting screws.

7) Using a good quality, medium viscosity epoxy, you now glue the sleeves into place making sure all of the vacuum bleed and throttle shaft holes line up properly. You have now re-sealed all of the holes you cut through the body in step 2.

8) Now comes another fun part. Making the new throttle blades. Ever hear of the old machinist's adage "feel, file, and form to fit?" Well, it applies here. A tedious job, but the only way to get it right.

9) Now is a good time to "gasket match" the throttle bores to your intake manifold. Take your time and do it right, the clearances don't allow much leeway here as you need to make sure all of the holes on the bottom are still sealed to the manifold

10) For purely cosmetic reasons, I used DEVCON Aluminum Putty and filled in the holes that were used with the stock injectors, but this is not really necessary. But if you are still using the stock injectors....

11) Reassemble your new "Big Throat" throttle body. With those air horns, it really looks cool doesn't it?

12) Insert the special studs you had made (to accommodate the smaller holes after the sleeves are installed) and install the modified TB on the manifold. You will have to grind off the side of two nuts so the air horns will fit.

13) Now go out and feel that Beast really start to breathe above 2500 rpm. Hear that baby suck! Fun isn't it?

If you were to have all this work done by someone else, (I have access to a full machine shop and am capable of performing just about any machining job) the costs would look something like this:

Replacement throttle body: I paid $150 for mine on e-bay, including the injectors. (The truck is DOA while all of this is being done if you don't have a spare.)

Machining the throttle body to accept the sleeves: $100

2 - sleeves machined to my drawings: $125

Modifying the throttle shaft: $50

2 - Custom throttle blades: $50

3 - Custom studs for mounting: $50 (You didn't think part this was going to easy did you?)