rotten egg smell? eyes iritated?


Staff member
while not something you may have thought about, regular periodic maintenance on the cars tuning and exhaust system and checking trouble codes and visually verifying the spark plug condition is a very good idea.
that smell is, fairly common in older (about 1984-2004) corvettes and usually a very good indication that either your catalytic converters on its last legs and needs to be replaced, or your O2 sensors may be defective,its usually caused by running a richer fuel/ air ratio mix or burning more oil than the cat can process, or rarely... due to use of too much fuel injection cleaner being used,
in any case you should be throwing trouble codes and looking into the source, a shop manual and a scan tool seem to be on your next trip to the auto tool store.
you can check for exhaust back pressure indicating a restriction, that can cost you a great deal of peak horse power.


having easy access to a scan tool, v.o.m. meter, timing light,vacuum gauge and a shop manual when you own a corvette is almost mandatory, and knowing how to accurately read spark plugs, and use an IR temp gun,and an air/fuel ratio gauge is a big help

CATALYTIC CONVERTERS ... anada.html ... erter.html ... CCwQrQQwAg ... erter.html ... stall.html

O2 sensors ... nsors.html ... rs&f=false






comparing catalytic converter flow rates ... 132001.htm ... 122001.htm

Data Converted to 28"
CFM @ 20.4" Inches of H2O

CFM @ 28" Inches of H2O

Passenger Side Catalytic Converter 2.50" in/out 299.5 350.9
Driver's Side Catalytic Converter 2.50" in/out 268.3 314.4
TTS Bullet Catalytic Converter 2.50" in/out 277.1 324.6
Carsound Catalytic Converter 2.50" in/out 372.7 436.7
FLP Catalytic Converter 2.50" in/out 376.0 440.5

CFM @ 28" Pressure Drop was calculated by using the following formula


FR0 = Flow rate at original pressure drop (ft3/minute)
FR1 = Flow rate (ft3/minute)
PD0 = Original pressure drop (inches of H2O)
PD1 = Desired pressure drop (inches of H2O)

Random Tech Cat-327CFM
Random Tech Metallic-535CFm

Catalytic Converters
Ready, set, flow!

Cats, not the furry four legged meow gremlins that that make you sneeze and place foot prints all over your freshly Zaino’d car. I am talking about the tree hugger’s solution to pollution and the horsepower-robbing enemy underneath your car. Every new car is shipped with catalytic converters in order to clean the exhaust emissions from your car to keep the air clean.

Unfortunately for us drag racer’s, the time is coming (and already arrived for some cities) for emission’s test for everyone in the United States. That means we have to find the highest flowing catalytic converters we can so we can pass emissions test and minimize horsepower loss.

Why use a flow bench

How do you go about finding out which catalytic converter will sacrifice the least amount of horsepower? You can track/dyno test each cat or flow them on a flow bench. Seeing how the later choice is the cheapest, and easiest, we ended up at White Performance in Kingsport, TN. Flowing pieces on a flow bench will allow you chose pieces that minimize flow restrictions and maximize horsepower.

First you have to find a flow bench to use. White Performance had a Super Flow 600 flow bench and was the site where our test took place. They manufactured an adapter to connect the catalytic converters to the flow bench prior to our visit.

All units were flowed at a 20.4†of H2O pressure drop and converted to 28†of H2O. Both results are given in Table 1. Why such an odd number? David Vizard’s exhaust flow research is the precise reason we flow exhaust pieces at 20.4â€. Pieces can be flowed at 28†or 1†and converted to 20.4†in just a few seconds with simple math. In this case we wanted the actual reading at 20.4†of H2O for accuracy since the research references a specific pressure drop.

David Vizard’s flow research shows the required air flow a muffler needs to attain, in order to keep horsepower loss due to back pressure to a minimum, is 2.2 cfm for every one horsepower. This general rule set by David Vizard keeps horsepower losses under one percent.1

The 2.2cfm per one horsepower represents a free flow condition and may be more or less depending on your car. We use that standard as a basic guideline with our test results. If the muffler has to follow this rule, then so should the rest of your exhaust system, including your catalytic converters.

Flow test and results

Five catalytic converters were chosen for the flow test. The passenger and driver’s side converters off of a 99 LS1 F-Body, TTS Bullet, FLP, and Carsound converters. The factory catalytic converters were modified from stock. Each was opened up to have a 2.5 inch entrance and exit to match those of the competitors. Of these, the passenger side converter has been regarded as the lowest flowing one for reasons expressed later in this article. Enough rambling, let’s get to the flow test.

Table 1

Inlet Diameter (in.)
Outlet Diameter (in.)
CFM @ 20.4†H2O
CFM @ 28†H2O

catalytic converters can be tested on car, with the engine running, at operating temperature.
its a good idea to test with a vacuum gauge as described earlier, but a quick check can be done with an I.R.TEMP GUN,
Using an infrared temperature tester, like I link to below, the "gun" style with the red laser pointer, measure the inlet and outlet temps of the converter.

The temperature difference from one end to the other, should be approximately 100 deg/F.

The INLET should be 100 deg/F or so COLDER than the OUTLET end.

If the outlet is significantly colder than the inlet, converter is usually plugged.

heres the one I use and recommend
T504-4254_product.jpg ... ermometer/

Passenger Side Catalytic Converter

Driver's Side Catalytic Converter

TTS Bullet Catalytic Converter

Carsound Catalytic Converter

FLP Catalytic Converter

Results discussion

On a stock LS1 F-Body, the driver’s side catalytic converter has been praised as the higher flow converter of the two that are on the car. A few reasons for this assumption have been: The driver’s side converter can be found on past Corvettes. The fact that the F-Body has a converter off of a Corvette surrounds the converter with an aura of performance history. Also, the converter is smaller and sleeker appearing than the passenger side converter and that gets the mindset that it has to be better.

The theory that the driver’s side catalytic converter is the “high flow†converter of the two found on LS1 F-Body, ranks up there with high performance muffler bearings. The data above shows that the passenger side catalytic converter flows more than 35 cubic feet per minute (cfm) more than the driver’s side cat. That is a significant difference.

The real shocker of our flow test involves the TTS Bullet catalytic converters. The total combined flow of two TTS catalytic converters would be 554.2 cfm. Compare that to the modified factory catalytic converters of 567.8 cfm. Something is definitely wrong with this comparison. The TTS cats are not even close to delivering the flow of either FLP or Carsound. Installing a set of TTS Bullet cats on your car would be taking steps backwards compared to the stock units found on LS1 F-Body’s. How TTS can sell these catalytic converters with their excellent long tube headers is beyond anything imaginable. TTS does have a new oval catalytic converter that should outperform their current catalytic converter. The new oval unit is currently being tested for OBD II compliance.

The FLP and Carsound converters strutted their stuff on the flow bench. Both sets flowed over 190 cfm per pair more than TTS converters. These test results were what we expected to find with every set of aftermarket high flow catalytic converters. No one should hesitate to place these to converters on their car.

Mathematical Models

Each pair of catalytic converters restrict about eight (8) rear wheel horsepower on stock LS1’s. David Vizard’s research shows that 2.2 cfm of flow at 20.4†of H2O will support 1 horsepower. 2.2 cfm is the optimum flow through the exhaust system and reflects open pipe flow. Removing the stock catalytic converters on the LS1 usually results in a gain of 8 rear wheel horsepower. Assuming that the average LS1 puts 300 horse power to the ground, that is a 2.67% increase in power.

Stock catalytic converters:

Final thoughts

Adding a cat back to your exhaust system helps, but by no means makes it more free flowing than replacing the stock exhaust manifolds on back to your muffler tips with headers, larger diameter pipe and high flow mufflers. Replacing just your catalytic converters with high flow pieces may not necessarily help flow if you have a factory muffler on your car. The same could be said if you have a high flow muffler or run an exhaust cutout, but have factory cats installed. So be smart and plan your exhaust system out and realize that replacing just one piece of the exhaust system isn’t the end of the job

1. Vizard, David. How to Build Horsepower. Volume 3. Page
Emissions Devices

The information contained within this article is intended for educational purposes only. It may be illegal to remove, modify, or tamper with any emissions device on your vehicle, depending on the state in which you live. Please check your local rules and regulations to ensure that you abide by them.

Before we discuss how these systems operate, I would like to comment on a general misconception associated with fuel injection in general. I have heard on numerous occassions that fuel injection in general is too complicated and restrictive due to emissions devices. In reality, the problem lies with people assuming that the emissions components are somehow required as part of the whole fuel injection platform, and that these devices pose a significant restriction in performance. Just because you are running a fuel injection system, does not mean you will have emissions devices. In addition, having emissions equipment does not prevent someone from running whatever engine modifications they want. The loss in power from these emission devices is quite small.

As many of you already know, Tuned Port Injection engines were factory installed on vehicles that were required to be emissions compliant. To ensure that emissions requirements were met, GM used several devices to lower emissions output. Basically, these can be broken down into three main systems : EGR, A.I.R., and EECS. None of these are absolutely necessary to run a Tuned Port Injection intake, but may be legally necessary if you want to be street legal in the state where you live.

Exhaust Gas Recirculation System (EGR)

The sole purpose of the EGR system is to reduce the formation of Oxides of Nitrogen (NOx). These are formed when the temperature in the combustion chamber reaches very high levels. To avoid this situation, the EGR system is used to lower combustion chamber temperatures by admitting small amounts of exhaust gas back into the combustion chamber. As you can imagine, exhaust gas does not burn, and does not help the combustion process. If the EGR system would admit exhaust gas into the combustion chamber at idle, it would cause a rough idle, or stalling. As a result, EGR does not allow exhaust gas into the chamber at idle (nor at wide open throttle, more on that later...). To accomplish this, there has to be some way of controlling EGR flow.

The way this is accomplished is actually quite simple. First of all, there is a round, flying saucer - looking part that mounts on the intake manifold. This is called the EGR valve. There is an opening in the intake manifold from the cylinder head that allows exhaust gas to move up to the valve. When vacuum is applied to the valve, it allows the exhaust gases to pass. To control when the valve receives vacuum, an EGR solenoid is used. This part mounts on the passenger side near the back of the intake manifold. It has a vacuum line which supples ported manifold vacuum, and another vacuum line running to the EGR valve. When the ECM requests EGR to be ON, it sends a signal the the EGR solenoid, which then allows vacuum to be applied to the EGR valve. When the ECM wishes EGR to be off, the solenoid will cease to apply vacuum to the EGR valve.

The ECM will turn on the EGR solenoid by grounding it. It does this via pulse width modulation (PWM). This means that the ECM will turn on and off the solenoid many times a second. Just how many times this occurs will affect the amount of vacuum applied to the EGR valve, and therefore the amount of exhaust gas admitted into the combustion chamber. During the time that EGR is requested by the ECM, fuel output and spark advance are also altered. More timing is added, and the fuel mixture is leaned a bit.

During WOT (wide open throttle), the ECM shuts down EGR. Since EGR will play no role whatsoever under WOT, it will not impede the engine's ability to generate horsepower.

How does the ECM know if there is a problem with the EGR system? Well, the answer depends on what year TPI setup you have. If you have an 85-89 setup, then there will be a single wire that goes to the base of the EGR valve. This is basically a temperature switch. When the ECM requests EGR, it will check this wire to see if there is a change in temperature. If no change is detected, the ECM will think there is a problem with the system, and will throw a code 32. Most of the time, the problem is the temperature switch that is bad. However, you should first check for vacuum leaks, check that all vacuum lines are properly routed, check the harness connector at the EGR solenoid with a voltmeter, and make sure that the EGR passages are not clogged with deposits.

The 90-92 TPI setups did not use a temperature switch on the EGR base. Instead, the ECM monitors the MAP voltage to determine if an EGR request was successful or not. When EGR is turned on, engine vacuum will lower a little. The ECM will throw a code 32 if it suspects that the EGR request was not successful.

So what would someone gain by disabling EGR? Just about the only thing would be less parts under the hood. Basically, removing EGR gives you more space (although not much). To correctly disable EGR from your vehicle, you MUST at the very least, disable it in the prom. This is done by setting the minimum temperature to enable EGR to 151 degrees Celsius (maximum allowable temperature), and setting the minimum vehicle speed to enable EGR to 255 mph (maximum allowable speed). Since the engine will never reach either of these conditions, EGR will never be requested by the ECM. Since EGR will never be requested, it does not matter if you leave the EGR system all installed in its original place, or if you remove it from the car. If you decide to remove it, you will need an EGR block off plate to cover the hole in the intake manifold.

If you simply remove the EGR system from the car, but do not disable it in the ECM, you will run into significant problems. You will likely run into detonation, the engine will run very poorly, lack power, and will probably run on the hot side as well. In addition, you will get a code 32 before running very long. If you recall from before, the ECM alters fuel and spark advance when it thinks EGR should be on. If no EGR flow is possible because you removed it, you will have a lean condition which will be further aggravated by advanced timing.

I have not had a chance to discuss A.I.R or EECS (canister purge). I will finish the article as soon as I have some spare time, but I wanted to post the EGR section for the moment atleast.
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