Suggested Mods for TPI engines

By Tom Keliher (tkeliher@pacbell.net)

The Basics

The very first order of business should be a proper tune-up. It is amazing how many people expect mod’s to do something for the car, when in fact the state of tune prevents any modification from being beneficial.

In order for any modification to be worthwhile, the engine has to be running at its peak, as designed. In order to accomplish this, here are the items that should be looked at:

1. Spark Plugs
2. Ignition Wires
3. Distributor Cap
4. Rotor
5. Air Filter
6. Fuel Filter
7. TPS Voltage
8. IAC Valve
9. Engine Oil
10. Oil Filter

I recommend replacing all "standard grade" ignition parts with "premium grade" parts. ACCEL; MSD; MALLORY; etcetera are all better than the factory stuff you have on there now. It only costs a few dollars more than standard grade parts for the premium parts. And if you have your car serviced at a dealer, it is probably less expensive than dealer installed factory parts. So, learn to do it yourself, and put in the high grade equipment. You will be rewarded with better performance, and money in your pocket.

Since changing spark plugs on the L98 is a real pain in the butt, it would behoove you to install one of the highest quality spark plugs available. That way, if you decide to "put it off" again for a while, your plugs will still be working properly. I recommend BOSCH PLATINUM highly. They are one of the reasons car mfg.’s have gone to the extended time between tune-ups, such as 100,000 miles.

The ignition wires should be upgraded as well. The more voltage you can carry to the plugs, the better the performance. Install a new set of the "spiral core" type wires in 8MM or larger size.

The distributor cap also is responsible for directing the high voltage to the plugs, so compromise here is out of the question. Get an ACCEL or similar distributor cap and rotor. The brass contacts and high quality carbon brush allow much higher voltage to pass on to the plugs without damaging the cap and rotor.

The air filter is a restriction. A necessary one, yes. So, why not install the best air filter, that flows the most air, and filters the best as well? There is only one answer: K & N FILTERS.

Fuel filters– How often do you think about them? Well, in our cars they are extremely important if you want maximum performance. Don’t forget, we are running fuel injection with a pressure in the 50 PSI range and a demand for all that it can give. If the fuel filter is only slightly inferior in its ability to flow, we are going to be down on power. Not to mention potential idle problems and general drivability problems. I recommend replacing the fuel filter every 10,000 miles.

TPS Voltage– It "must" be right on, or your engine will not be running at its peak potential. It must be read with a digital VOM and set accordingly. This is outlined here.

IAC Valve– This determines your engines idle speed, and it also must be set correctly or you will be experiencing idling and low speed drivability problems. This is outlined here.

Engine Oil– Well, I just know that if you are an enthusiast and are reading this board, than you MUST be changing your oil regularly. Religiously. Without fail. At least every 3000 miles. Right? Nuff’ said. Ditto for the oil filter. Now. Want some "free horsepower"? Switch to a full synthetic motor oil, such as Mobil 1. It will give you more power. It will run cleaner. It will give you better mileage.

First Level Mod’s

You already did the first go around of suggested mod’s. Surprised? The upgraded ignition component is a real world; seat of the pant’s difference you will genuinely feel; modification you can make to your car. So is the K&N air filter. You have performed a full tune-up. You have upgraded your ignition and engines breathing. You should be burning rubber like never before by now! Now comes the 2nd level.

Second Level Mod’s

Here is where the "little things" start to add up. The following mod’s, by themselves, are not necessarily going to reap huge gains in power. However, as a total package, they do make "measurable horsepower."

1. Throttle Body Airfoil
2. Modified Mass Airflow Sensor
3. Modified Air Cleaner Housing
4. Exhaust System Upgrade

I recommend everyone with a TPI engine gets a copy of "TPIS Insider Hints" book. It will go into detail about how to modify your air cleaner housing and mass air sensor. It will also show graphically how much horsepower can be gained by these mod’s.

Throttle Body Airfoil– This is an easy to install mod that anyone can do. It is supposed to be good for 7 HP and more torque. It is bolted into the front of the throttle body and straightens and smoothes the incoming air.

Modifying the Mass Airflow Sensor– This is a tricky one, and if done wrong, can ruin the part and cost a ‘lot’ of money for a new one. Only for the seriously hard-core horsepower seeker, and one who is familiar with working with handtools. The modification is in two steps. The first step is to remove the screens in the front and rear of the MAF. This allows for much greater airflow. The screens are not really needed for protection of the hot wire inside the MAF, that is what the air filter upstream is doing. The second step is to grind away the cooling fins inside the MAF. This is the critical operation that can ruin the part if done wrong. It is best for the novice to just remove the screens and leave it at that.

Modified Air Cleaner Housing– This entails cutting rectangular "windows" in the sides of the air cleaner to facilitate better breathing. It made a noticeable difference in the "butt-dyno" after doing it, especially getting on the uphill freeway (poor mans’ dyno) on-ramp.

Exhaust System Upgrade– Here is where you can gain more power, especially after making the improvements to the intake side. And, you get a nice exhaust note to listen to, and to tell the "other guys" you mean business. Simply installing any of the popular aftermarket mufflers available will make a big boost in performance. It really depends on what type of sound you like. Some are more loud than others. It is a highly subjective item here. Just be aware that all of the aftermarket mufflers will give you a boost in power. The sound you have to take your chances on that you will like it.

A full "cat-back" system is better yet. It has fully mandrel bent tubing, instead of the factory crush bent tubing, for an increase in flow area of 33%. However, just adding the muffler alone will be beneficial.

Third Level Mod’s

This is where it starts to get a little more involved, and more mechanical skills are needed.

1. Adjustable Fuel Pressure Regulator
2. Underdrive Pulleys
3. Electronic Coil
4. MSD Ignition Amplifier Box

AFPR– This is one of those great mod’s that does not cost much money, but can make "serious" horsepower improvements. And with the extra breathing capability you have created for your engine, you now definitely need higher fuel pressure than the factory setting. Again, TPIS’s book, "Insider Hints" has a graphic chart that shows the gain in horsepower and torque for all the various pressures and at various rpm’s. But it seems to be accepted now that 50 PSI is just about right for everyone with the mod’s up to this point.

Installing the AFPR requires removing the throttle body and plenum from the runners, and having a fuel pressure gauge and hose to determine what your base setting is and then to slowly up it to the best pressure for your particular requirement. It can be tough getting the plenum off, because it likes to stick to the gaskets. And you have to bleed the pressure off of the fuel system before you hook up the fuel gauge, or else you get a nice flammable douche all over the top of your engine. But it’s one of those "must do" mod’s.

Underdrive Pulleys– "March Performance" claims as much as 15 horsepower can be gained by reducing the parasitic loss from the stock crank and alternator pulleys. It requires getting under the car, and removing the crank pulley in a rather confined area. And the alternator needs to be removed from the engine so that it’s pulley can be changed. And then a new, shorter serpentine belt must be installed.

Electronic Coil– This is another ignition system upgrade that helps with high rpm performance, and quicker starts. It is basically a plug in deal, but it means modifying the stock coil bracket to accept an aftermarket coil; or else mounting it in another location somewhere.

MSD Ignition Amplifier– This mod is a real good boost for getting that last 500-1000 rpm out of the TPI system a lot quicker than it can do stock. It also is a plug in deal if you buy the harness adapter. But it is a fairly large box, and requires finding an appropriate remote location somewhere inside the engine compartment. It’s well worth the effort, however.