Bump 'N' Grind Porting
So you have put on a cold-air intake, headers, flowed exhaust, and done the standard SS fueling modifications (see "Modifying your SS for Street Performance" article) in an effort to gain some more power out of that stock engine while not bankrupting your savings account. Then you add a set of heads and a good cam to increase the flow a little, but alas, you are hungering for more power again after a few months of driving and stomping 'Stangs. What do you do now that won't break the bank?
Well, if you can massage metal with a die grinder, you can do a basic port job on your throttle body to increase its airflow. This is not a very tough task or one that requires tons of experience. This is a project that most members can undertake. You're not race-porting a set of heads or widening the venturis of the throttle body (TB), so a little basic practice is all you really need. It is recommended, however, that you get an old/damaged TB, heads, or other product on which to practice a little, just to get the feel of it, and realize what portions of the units and areas in the airflow path you should be concentrating your efforts on.
Also, the 454SS TB is aluminum and so is much easier to cut than iron, so if you choose to practice on iron parts, be forewarned that aluminum will cut and "work" much easier than iron will. Keep this in mind so you do not use excessive force on the grinder and grind away too much material, or use too high a speed on your grinding tool. Also, as with any precision work, it is imperative that you have patience. It is much harder to add material than it is to remove it, so take your time and go slowly. This should take you about 4 hours to grind and blend alone.
A Dremel tool is an indispensable power tool that is useful for many things (in particular, porting and polishing), especially when combined with the 2 foot flex bit adapter that you can buy for it. It is worth the price of the tool and bits alone for this job, and will come in handy for many other projects in the future. If you can't justify the cost of it, you can borrow one or use a die grinder, provided it is small enough to get into tight spaces with relatively small bits. This article will involve the working of a 1992 TB, so some specifics for a 90 model will differ. However, the practices and tips herein remain similar.
Remove the air cleaner assembly and take off your TB unit. There will be 3 mounting bolts (all the same length), 4 hoses on the front, one on the back, two fuel lines (feed and return), and two throttle cables to remove from it. Now drain it of fuel (completely), allow it to dry, and start taping up the openings in it. I chose to remove the two sensors on my TB and tape up all the orifices leading to them to prevent shavings from getting in there and screwing something up. This includes the hose grottings and sensor grottings. Don't forget to put a little piece of tape on the tiny openings in the venturis themselves, located parallel to the butterflies.
Next, get yourself a marking pen and plot out what parts you will grind out. To start with, look at the venturis. See how the smoothed radius abruptly ends in a neatly finished bore, leaving a sharp edge around the mouth of the TB? This is not optimal for flow. Air does not like to change direction swiftly like that. Your task is to blend the funneled mouth into the bore, in essence making a true funnel-shaped design so that air is drawn into the bore smoothly, with as little resistance to compressing and bending as possible. Start with a fine grit roll cartridge and the tool on its lowest power setting and begin grinding away the high point on the edge. A good technique is to hold the tool at an angle and try to continue along that angle as you work around the venturi. This works best if you can place the TB on a stand of some sort. Don't let the tool sit on one area for any length of time. This may cause gouging, and gouging is tough to blend out or repair. Sweep the roll along the edge line, avoiding one section for more than one or two seconds. You want to just sweep the grit over the metal to shave off a little at a time. Make it as homologous as possible, as the funnel slope must be as uniform as possible all around. Once the high point is ground off fairly level and looks clean, grind down the two edges (created by your grinding) on the top and bottom of the fresh aluminum mark. This should be a rough funnel that now needs minor blending to be completely finished. Move on to the other venturi, and make it look identical.
Once that is done, now you can start working on the sharp edges in the centermost edge of the venturis and the sharp unfinished edge by the mounting screw cutouts. Shape the edges in the same fashion as the funnel-to-bore transition you just did. This will take some time, as there is more material there. Keep the center post thick enough to support the air cleaner screw, and remember to blend the funnels to the post walls. You will need to switch out from the sanding roll to a more precise stone bit or if possible, thinner sanding roll, in order to properly shape this area before further blending with the large diameter rolls.
Now that the heavy work is done, you can final blend the ground areas. You will need hardly any more pressure than the weight of the tool for this. Using an extra fine grit, blend the transition from the funnel into the bore. It helps to rock the tool back and forth, rounding out the transition. This should be a super smooth transition when done; you should not feel any lumps or bumps when you are finished with the blending, and the transition should be smooth all around the venturis. A larger-diameter sanding roll comes in handy here but is not necessary. What is most important though is that you properly radius the cuts and begin to polish out the grit marks in the aluminum surface. Some minute marks are acceptable, like the pattern in the bores left by the factory machine, but anything larger is not. Don't rush the program either; you are almost done.
With the bores final-honed, you are now about done. It is recommended that you use a very fine wire brush to polish out irregularities that might have been left by the grit, and to dislodge any grit that might have become embedded in the walls of the funnel. Use a steel bristle at high speed, and "wash" the bores with it (press down so as to just barely flatten the bristles and work the radius quickly). This should leave a finish as smooth or smoother than the OEM bores. Now you should have a modified TB that looks like the factory did it.
But wait. There is more! If you get creative, you can shave off as much of the butterfly mounting screws as possible and shave down the throttle shaft. These parts are steel, and should be removed before grinding on them so as to gain better access to them. Thinning the throttle shaft will make it weaker, and so is not something I recommend for a daily driver.
Lastly, check out how the TB mounting bolts obstruct the airflow into the bores. On 92 units, there are two bolts that can be shaved down as much as ¾" and still remain useful. Also, try to shrink tube all the wires going to the injectors. One semi-thin wad of wire is going to be much less detrimental to airflow than 4 separate wires hanging out especially if that batch of wire is routed up the air cleaner mounting screw. Finally, install the TB unit and re-tune your fuel delivery via the adjustable regulator.
With a little technical knowledge and some tools, you can have a TB that will flow much better than stock and at nearly no cost. So do the bump 'n' grind routine. You might find that you really enjoy it.