Engine What Can I Do For Extra Hp And Keep It Smog Legal?


New Member
Jan 20, 2014
Las Vegas, NV
Hello all;

New to the site, looks AWESOME!!! I live in one of the only 2 main cities in Arizona that require emissions testing. I have the opportunity of picking up a 93 5.0 that I'd like to start building for replacement. Am I stuck with a cam, CAI, intake/plenum and chip (JET Stage 2)?

I want more ponies yet keep it legal for testing....

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Wow. Where do we start? You can free up power by adding things such as:
-Underdrive pulleys, Electric fan, aluminum driveshaft.
-Gears! Put a set of 3.73's in it if its a 5 speed. 4.10's if its an automatic. It will feel like its got a hundred more horses.
You can get more power by adding things like:
- Upgrade the stock cattpipe with a better flowing cattpipe. I used Bassani for years. Really picked up some power.
- Larger tube headers and a catback exhaust help alot.
- As far as heads go ,there are street/emissions legal heads.
GT40 style "Ford Explorer" heads are a very popular upgrade because they are cheap, huge bang for the buck, and are emissions legal.
There are plenty of aftermarket aluminum heads that are emissions legal . Do some research to find out which heads in your state.

Most of all add a supercharger. Won't affect emissions (I think) and will make it a ball to drive.
I don't live in your state so correct me if I'm wrong on emissions rules there.

Glance at my list of mods in my SIG below. My car passed emissions fine until I removed the cats last month. Luckily I don't need to worry about that anymore.
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First thing to do BEFORE changing the cat H pipe is to check and see if your smog inspector looks for the original 4 cat H pipe to be in place. I understand that the California smog inspectors look for the original 4 cat H pipe. If all they do is an emissions check, you may slip by with a 2 cat high flow H pipe.

Remember that the old H pipe contains a valuable platinum or other rare metals catalyst. That scrap platinum or rare metals catalyst is valuable on the scrap metal market and can range from $50-$200 in value depending on the amount and current market prices. It could be worth your time and effort to check into this possibility.

Most of the aftermarket parts will tell you if they are 50 state emissions legal or not. Stick with the parts that are 50 state emissions legal and save the manufacturer's paperwork that proves that they are 50 state emissions legal. If you do that, there are all kinds of things you can do depending on your budget. Heads, cam, intake are just a few of the options along with a supercharger.

How to pass emissions testing:

High NO - high combustion temps - retard timing, check EGR for operation.
High CO – Rich condition - fuel pressure too high, check O2 sensors, replace air filter, Clean MAF element.
High HC – Lean misfire, vacuum leak, common misfire due to worn or weak ignition system components. On rare occasions, an overly rich mixture may be the cause. Do the ethanol/E10 fill up as suggested.
High CO & HC - Cat converters, smog pump, and smog pump controls. Make sure the smog pump has good air output at 1200-1700 RPM

How to pass emissions testing:

1.) Make sure all the emissions gear the car was made with is present and connected up properly. That includes a working smog pump and cats. The smog tech will do a visual check to make sure that all the original equipment is present and connected up.

2.) Make sure that you have fresh tune up with spark plugs, plug wires, cap, rotor, fuel & air filters. An oil & filter change is a good idea while you are at it.

3.) [Dumping the computer diagnostic codes on 86-95 Mustangs

Revised 26-July-2011. Added need to make sure the clutch is pressed when dumping codes.

Codes may be present even if the check engine light hasn’t come on, so be sure to check for them.

Here's the way to dump the computer codes with only a jumper wire or paper clip and the check engine light, or test light or voltmeter. I’ve used it for years, and it works great. You watch the flashing test lamp or Check Engine Light and count the flashes.

Post the codes you get and I will post 86-93 model 5.0 Mustang specific code definitions and fixes. I do not have a complete listing for 94-95 model 5.0 Mustangs at this time.

Be sure to turn off the A/C, and put the transmission in neutral when dumping the codes. On a manual transmission car, be sure to press the clutch to the floor.
Fail to do this and you will generate a code 67 and not be able to dump the Engine Running codes.



If your car is an 86-88 stang, you'll have to use the test lamp or voltmeter method. There is no functional check engine light on the 86-88's except possibly the Cali Mass Air cars.


The STI has a gray connector shell and a white/red wire. It comes from the same bundle of wires as the self test connector.

89 through 95 cars have a working Check Engine light. Watch it instead of using a test lamp.


The STI has a gray connector shell and a white/red wire. It comes from the same bundle of wires as the self test connector.

WARNING!!! There is a single dark brown connector with a black/orange wire. It is the 12 volt power to the under the hood light. Do not jumper it to the computer test connector. If you do, you will damage the computer.

What to expect:
You should get a code 11 (two single flashes in succession). This says that the computer's internal workings are OK, and that the wiring to put the computer into diagnostic mode is good. No code 11 and you have some wiring problems. This is crucial: the same wire that provides the ground to dump the codes provides signal ground for the TPS, EGR, ACT and Map/Baro sensors. If it fails, you will have poor performance, economy and driveablity problems

Some codes have different answers if the engine is running from the answers that it has when the engine isn't running. It helps a lot to know if you had the engine running when you ran the test.

Dumping the Engine Running codes: The procedure is the same, you start the engine with the test jumper in place. Be sure the A/C is off, and clutch (if present) is pressed to the floor, and the transmission is in neutral. You'll get an 11, then a 4 and the engine will speed up to do the EGR test. After the engine speed decreases back to idle, it will dump the engine running codes.

Trouble codes are either 2 digit or 3 digit, there are no cars that use both 2 digit codes and 3 digit codes.

Your 86-88 5.0 won't have a working Check Engine Light, so you'll need a test light.
See AutoZone Part Number: 25886 , $10

Alternate methods:
For those who are intimidated by all the wires & connections, see Actron® for what a typical hand scanner looks like. Normal retail price is about $30 or so at AutoZone or Wal-Mart.

Or for a nicer scanner see Equus - Digital Ford Code Reader (3145) – It has a 3 digit LCD display so that you don’t have to count flashes or beeps.. Cost is $30.

Dumping the Engine Running codes: The procedure is the same, you start the engine with the test jumper in place. Be sure the A/C is off, clutch depressed to the floor and the transmission is in neutral. You'll get an 11, then a 4 and the engine will speed up to do the EGR test. After the engine speed decreases back to idle, it will dump the engine running codes.
Cylinder balance test
Warm the car's engine up to normal operating temperature. Use a jumper wire or paper clip to put the computer into test mode. Start the engine and let it go through the normal diagnostic tests, then quickly press the throttle to the floor. The engine RPM should exceed 2500 RPM's for a brief second. The engine RPM's will increase to about 1450-1600 RPM and hold steady. The engine will shut off power to each injector, one at a time. When it has sequenced through all 8 injectors, it will flash 9 for everything OK, or the number of the failing cylinder such as 2 for cylinder #2. Quickly pressing the throttle again up to 2500 RPM’s will cause the test to re-run with smaller qualifying figures.
Do it a third time, and if the same cylinder shows up, the cylinder is weak and isn’t putting out power like it should. See the Chilton’s Shop manual for the complete test procedure

4.) Post the codes and get help to fix them. Don’t try to pass with codes not fixed. Clearing the computer just temporarily removes them from memory, it doesn’t fix the problem that caused the code to be set.

5.) Be sure to do the testing on a hot engine. Drive for 15-20 minutes prior to taking the test to get operating temps up into the normal range. Do not shut off the engine while waiting for your turn on the test machine. An engine up to full operating temperature puts out fewer emissions.
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I have found emissions testing very easy on my set up, although I do not have a lot of power.

I have a stock 4 cat mid pipe that I cut to fit my long tubes. Only 2 of the original cats remain. I put my smog pump back in, run the line to the H pipe, and pass emissions. No dumping codes, no fuel mixture adjustments, no changes other than throwing these parts back on.

With that said other areas might be different. My local guy climbs under my car to hook the sniffer up to my dumped mufflers. They have never cared about missing 2 cats, and although I do have a smog pump, I always thought there was a whole mess of vacuum lines on the passenger side that was emissions related. I have all of that removed and never put anything back in.

Great advice above. Best I can tell you is if your ok using 2 cats, you can make some good power while still being emissions compliant.
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