The Tuned Port Injection system first appeared on ´85 Corvettes. With fuel economy increases in the 11 percent range, a bump in horsepower to 230, and torque jumping from 290 to 330, the Tuned Port system seemed to outshine all other fuel-management systems. At least it did for a while. As technology moved ahead, so did the possibilities for improved fuel management. The TPI system was discontinued in 1992 with the release of the LT1, which turned out more horsepower in the first year of production than the TPI-equipped L98 did after many years of production and improvements.
Over the years, much has gone into the process of making an L98 faster than GM intended, and Myron Cottrell of TPI Specialties of Chaska, Minnesota, (612) 448-6021, knows a lot about how to do it. Long respected by those in the power realm, Myron has been working on the TPI systems almost as long as GM, and he's made vast improvements in the amount of horsepower you can squeeze out of a TPI system. Corvette Fever picked his brain to find out how he does it.
According to Myron, with these modifications you can have an L98 with about the same horsepower as a C5 for considerably less cash. Not bad, especially if you already own the L98. Any way you look at it, your Corvette should command respect at any red light and will drive like a dream on any highway. Isn't that what it's all about?
The first step is part performance, part tune-up. Possibly the best performance modification to start with is a set of good-quality spark plugs and plug wires. Remember, these engines are 10-15 years old and a large percentage of them are well overdue for a good tune-up. Also, opening up the breathing capabilities with a performance filter and Myron's Air Foil will head you in the right direction. At this point, a fuel-pressure regulator will help you tune in your car's diet of fuel for utmost power. These modifications are available as a kit from TPIS.
With the above-mentioned kit, you'd be wise to also purchase a fuel-pressure gauge. This tool will help you measure the adjustment changes and keep you from making changes that could damage some of the systems that keep your car running clean and strong. If you're going to make changes to the fuel system's pressure using an adjustable regulator, this is a must-have tool.
The following modifications will cost you only time, unless you want to buy the directions on how to do them from Myron. He suggests doing some porting of the plenum on your L98, modifying the mass airflow sensor and air-filter housing to get rid of some of the power-robbing turbulence in the TPI system. Myron states that you should own another 35 hp with this maneuver.
Once you get the engine breathing better than it ever has, you can go into the computer and make changes that will capitalize on the previous improvements. This "Level II" modification will improve the wide-open throttle performance and, according to Myron, it will also benefit the part-throttle response and should yield an increase of another 10 hp.
05. Throttle-Body Openings
If you feel there is more breathing capability yet to be found in the engine, you're right. You can increase the flow of the TPI by increasing the size of the openings in the throttle body. A good size for a mildly modified engine is a 52mm throttle body. There are larger sizes available, but they're not recommended unless you have increased the cubic inches of the engine to properly utilize the much larger opening. A slightly modified engine will make more power and have a much better throttle response with a stock throttle body than with a throttle body that's too big for the engine.
With all that air and added fuel going into the engine, it only makes sense that steps must be taken to ensure that it can get out of the engine with as little interference as possible. That can be done by using a set of headers and a performance exhaust. TPIS has a set designed for L98 Corvettes that are plumbed for use with an air pump to make them emissions friendly while still providing a free-flowing exhaust. Myron states that his headers and exhaust will give power increases of approximately 35 hp.
At this point you want to utilize as much power as possible, and that means burning all of the fuel that enters the combustion chamber. Myron suggests using MSD's 6 AL ignition. Throttle response should increase and you'll get every ounce of power your fuel can supply at the higher rpm.
If you still want more power from your L98 engine, you'll have to do internal modifications. Myron suggests his ZZ9 cam and, if you can swing the added cost, install his Big Mouth or Mini-Ram intake.
You'll have to remove the intake to install the cam, so why not take the time and make provisions for later improvements like larger-size runners? They can be installed at the same time or later, but you'll need to have the Big Mouth intake for them to work properly. If you have any intentions of using a larger-size runner, you can save yourself some time and hassle by installing the proper intake at the time of the cam install. Myron tells us the Big Mouth intake and larger runners are good for about 25 hp.
The final recommendation is the replacement of the cylinder heads. TPIS sells replacement heads that are checked, ported, and have 2.00 intake and 1.56 exhaust valves installed. He says they're good for 30 hp over stock L98 heads.