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Performance Tuning the Turbo 4.9

Like many performance cars, Trans Ams can be victims of modifications over the years in the quest for more performance.  

It is a good idea to verify what you have before making any modifications.  

Check the following: 
-Correct carburetor.  #17080274 (1980)   #17081273 (1981 US)

-Correct secondary metering rods in the carb?  Stamped “DX”.

-Does it have a PEVR?  (1980 and Export 1981 only)

-Any cracks in passenger side exhaust manifold just below the bolt heads of the center exhaust port?  

With everything checked out, we can now move on to making sure it is all working properly.  These steps should be performed before moving on to other performance modifications.  After doing this, you should have a well performing, stock, Turbo 4.9.  

1. Oil change

Use a good quality oil and filter (I like Wix filters).  Rotella T oil is great for Turbo engines.  Synthetic oils are also good.  (I like Amsoil synthetics).  Be aware that if converting to synthetic oil, you may develop engine oil leaks over time.  I recommend using thicker oil viscosities like 10w-40 or 15w-50. 

2. Tune up

Distributor cap, rotor, wires and plugs.  Use good quality components like Standard Motor Products for the cap rotor and wires.  A cap with brass terminals is good.  Replacing the coil and ignition module is not necessary unless they are bad.  For the spark plugs, I like to use NGK UR5’s.  They have a cooler heat range to help fight detonation.  Spark plug gap should be around .040” for stock boost pressures.  Do not gap them to the factory .060."  That is too large. 

3. Ignition timing 

1980 and export 1981: Verify that the distributor rotor mechanical advance moves freely and returns correctly.  Over time the mechanical weights can get stuck or gummed up.  I do not recommend changing the weights and springs on the distributor.  The Turbo 4.9 has a specific mechanical advance rate of 16° by 2000 RPM.  Combined with a base timing advance of 8° gives a total of 24° at full throttle above 2000 RPM.  You will also want to verify the vacuum advance moves freely and holds vacuum properly.  Using a hand held vacuum pump, attach the hose to the vacuum advance can and pump vacuum into it.  The arm inside the distributor should move and it should hold vacuum. 

To properly set the ignition timing, the engine needs to be up to operating temperature and the vacuum advance hose disconnected.  You can also verify that the ESC module is not retarding the ignition timing by disconnecting the knock sensor wire with the engine running and checking the timing. 

1981 US spec with CCC: The US spec 1981 Turbo 4.9 with Computer Command Control distributor is different from the 1980 version. The 1981 CCC distributor does not have mechanical advance weights or a vacuum advance can.  All ignition timing advance is controlled by the ECM.  Base timing is still adjustable.  When setting the timing, there is a blue, single wire connector (the wire color is tan) near the driver’s side valve cover that needs to be grounded with a jumper wire.  A simpler method is to unplug the flat, 4 wire connector on the passenger side of the distributor.  This will put the ECM in by pass mode for the ignition timing so the base timing can be set.  Base timing is 6° advance on the 1981 computer controlled cars.

4. Vacuum leak check  

A simple check of spraying brake cleaner or carb cleaner around the base of the carburetor, turbocharger, intake manifold etc.  We are looking for a change in engine RPM.  No change is a good sign everything is sealed up properly.  Inspection of all the vacuum hoses is also a good idea.  Look for broken, disconnected or plugged vacuum hoses and ports.  Also check the condition of the hoses.  Age and heat will deteriorate the hoses causing leaks.  Any leaks after the carburetor is a vacuum leak.  Any leaks after the turbo is a vacuum/boost leak. 

5. PEVR check

See the PEVR page on operation and how to diagnose a bad one.    

6. Cold air intake

You do not want to draw in hot under hood air into the carburetor.  The turbo is going to be heating up the incoming air enough.  And hot air gets you closer to detonation issues.  The Turbo 4.9 had a special flexible intake hose connecting the oval air cleaner arm to the round fender duct.  A replacement hose is no longer available.  On the majority of turbo cars, this hose is missing.  You can either get creative with your own set up or purchase a reproduction plastic adapter and flexible hose from the restoration places.  This system was used on non-turbo Firebirds.  The plastic adapter is not correct for the Turbo 4.9, but it is better than drawing in hot, under hood air.    

7. Exhaust leaks

You will want to check for any exhaust leaks before the turbo.  These can cause sluggish or delayed performance from the turbo.  Since the turbo is relying on exhaust heat and flow to spin, any small leak prior to it will cause a loss in performance.  Make sure all gaskets and seals are tight.  Key areas to check are manifold to cylinder head, pipes to manifolds and pipe to turbo.  Another area to check for exhaust leaks is the front of the cylinder heads where the plate for the AIR pump tube bolts on.  The gasket there can leak exhaust pressure.  It is common to find cracks under the center exhaust port bolts on the passenger side manifold.  The only way to fix this is to find a good replacement manifold or send us your manifold to have the crack repaired.  An easy way to find exhaust leaks is to take a piece of garden hose and hold one end to your ear and point the other end around the exhaust areas.  With the engine running you will hear any exhaust leaks.

Exhaust leaks after the turbo will not affect performance but should still be addressed due to the possibility of exhaust fumes getting into the interior of the car. 

8. Got Boost?

With the car running good and everything checked out, now is the time to take it for a ride and verify you are making boost.  Make sure to use the highest octane fuel available.  Since Pontiac didn’t give us a vacuum/boost gauge an aftermarket one needs to be installed.  It is best to “tee” into the line between the turbo and the waste gate to measure boost.  You should see boost starting to build around 2500 RPM while driving under heavy acceleration or wide open throttle.  YOU WILL NOT BUILD BOOST REVVING THE ENGINE IN PARK.  Turbos are load sensitive.  The car needs to be accelerating down the road to build boost.  Maximum boost pressure should be no more than 9 psi on a stock Turbo 4.9.

To help prolong the life of your turbo, you should always let the engine idle for 1 minute before shutting it down after “spirited” driving.  This lets the turbo cool down so that it doesn’t “coke” the oil in the bearings. 

Modifications for more performance...

(Note: The following information is based on theory and real world testing. TTA Performance, LLC is not responsible for engine damage that may occur from these modifications.) 

Stage 1: These modifications are just to make the car more efficient and prepare you for heavier modifications later.  The performance increase may be small but it will be more enjoyable to drive.

  • 180° thermostat
  • Mufflers eliminated
  • 2.5” down pipe and a test pipe to eliminate the catalytic converter or install a high flow catalytic converter. 
  • Manual Boost Controller - 9 psi of boost.
  • 10° base timing (8° for 1981 US spec) if no detonation is heard.
  • Replacement high flow air filter (K&N or equivalent).
  • Shift improver kit for transmission. 

Stage 2: These modifications are a little more serious and will get you more of a performance increase.  It is wise at this point to start installing some other gauges to monitor air fuel ratio and exhaust gas temperature (EGT/Pyrometer).  If installing an EGT gauge, I would recommend installing the probe in the rear exhaust port of the driver's side exhaust manifold (cylinder #7). 

Stage 3: These modifications are more involved and are for more of a street/strip application. 

Beyond Stage 3:  These are internal engine modifications that can be performed for better performance.  Some of the modifications listed have not been tested yet and are based on theory. 

  • Cylinder head modifications. Polished combustion chambers, unshrouded intake valves, 1977-79 non AIR pump cylinder heads, larger valves, porting and polishing of the exhaust ports, back cutting of valves.
  • Polished piston tops to eliminate sharp edges and roughness that can cause detonation.
  • Notched cylinder walls in the block to unshroud exhaust valves. 
  • Modified head gaskets for more coolant flow. 
  • External coolant by pass hoses.
  • AIR pump elimination.
  • Electric cooling fans.
  • 160° thermostat.
  • Cylinder head studs.
  • Fuel injection conversion.
  • Smaller spark plug gap? 



These are some things I suggest not to do or don’t waste your money on.  These turbo engines do not respond well to some conventional hot rodding tricks. 

1.  Do not change the carburetor trying to gain performance.  Trying to replace the factory Rochester Quadrajet with another Quadrajet or an aftermarket carb might be asking for trouble.  As described on the PEVR page, other non-turbo carbs won’t work correctly.  Especially on PEVR equipped cars.  The stock Quadrajet is rated at 800 cfm. It is plenty big.  Some have had success modifying for a Holley carb for draw through blower applications when a stock carb could not be located, but if you have the stock carb, use it.  1981 computer controlled cars can possibly get away with using a different computer controlled Quadrajet if an original can not be located. 

2.  Do not change the secondary metering rods in the Quadrajet.  The Turbo 4.9 used the richest secondary metering rods available for a Quadrajet.  They are very skinny and are stamped “DX”.  Sometimes they get damaged and replaced with leaner rods from another carb.  If yours are damaged or are incorrect, new ones can be made by Cliff Ruggles.   

3.  Do not install a large camshaft.  There are several reasons for this, one is the Turbo 4.9 does not turn the high RPM required to get in the power range of a large camshaft.  Plus, turbos have a different dynamic compared to a non-turbo engine.  A really “lumpy” camshaft is most likely going to hurt performance. Bigger is not always better.  The Comp Cam 252H is a good cam for this engine.  Some have successfully used the next larger 256H cam with other modifications.   

4.  Do not alter the timing advance.  Experimenting with higher base timing is ok, but be careful and listen for detonation.  10° base (8° for 1981 US cars) should be ok with 93 octane fuel and cool intake temps.  Some engines may be able to tolerate even higher timing.  Experiment with what works for you.  Some cars may like more timing and less boost vs. more boost and less timing.  I personally would not waste money on an aftermarket boost/retard ignition box, although some have found it to work well in their application.  I don’t see a need to alter the mechanical advance.  You really can’t get the advance to come in any sooner than with the factory weights and springs.  Avoid trying to get the total advance beyond the mid to high 20’s.  Non-turbo Pontiacs can run in the 30’s for total advance, but not the turbo cars.  Also be aware of aftermarket performance distributors.  They are most likely set up for a non-turbo advance curve and will deliver more mechanical advance at a later RPM. 

5.  Low rear end gears.  Installing 4.10’s or 4.56 ratio gears may help get the car off the starting line, but being that the engine doesn’t like to turn high RPMs and that the turbo likes to have a load on it, low gears may not help performance.  And if the car is able to make boost with the low gears, you will run out of RPM very quickly in a street car.  

6.  Intake ports and porting.  The 301 heads are a Siamese port design.  With one intake port feeding two adjacent cylinders.  When looking at the cylinder heads, you will notice that cylinders 5/7 (drivers side rear) and cylinder 2/4 (passenger side front) have a larger opening than the others.  When looking at the intake manifold you will see that the 5/7 intake runner is larger than the others.  This is due to the firing order of the engine.  Cylinders 5 and 7 fire sequentially, one after the other.  Since they share the intake port, cylinder 5 was taking the air/fuel charge and starving cylinder 7.  So the intake port and runner was made larger.  Since the same cylinder head is used on both sides of the engine, this made cylinders 2 and 4 have the larger intake port.  But the intake manifold runner is not larger on 2/4.  Some people see this larger intake port on the head and want to make the smaller ports match thinking they will gain more flow.  This is not true.  The intake manifold runner is still small.  In addition, enlarging the cylinder head port may hurt performance in that it could potentially slow down the speed of the air coming in due to the larger volume. 

7.  Spark plugs.  There is no need for fancy, expensive spark plugs.  All they do is lighten your wallet.  I always use regular spark plugs.  It is also possible that platinum tip plugs could cause problems in a turbo application.  Same goes for ignition components.  No need to buy some expensive big coil or ignition box etc.  Stock replacement components will work just fine.  If you are experiencing ignition problems with higher boost pressures, try closing up the spark plug gap to .030”-.035”.  And it doesn’t hurt to pull a spark plug after a “spirited” drive to visually check for detonation.  It will show up as tiny silver “balls” collected on the spark plug electrode.  Those silver balls are molten aluminum from the piston.  Some call it “salt and pepper” on the plugs.  In that case make the appropriate changes to stop it from happening. 


IN SUMMATION, I use a few simple rules when modifying a Turbo 4.9:

  • Use the highest octane fuel.
  • Make sure the engine can get the coolest air possible.
  • Listen and look for detonation.
  • Makes sure everything is working correctly before modifying.
  • Make small modifications one at a time and note improvements.