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GT Bob -> RE: Does (Insert Random Part here) need a tune? (2/6/2008 8:38:51 AM)
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Mod: One Piece Driveshaft
Tune: No
Why: Same idea as UDP’s. The extra power is gained by reducing rotational mass and reducing parasitic drag on the power being put into the rear tires. Less things eating up power before it gets to the rear wheels, means more power gets there,
Popular Brands: Spydershaft, shaftmasters
Mod: Axleback exhaust, Catback Exhaust, Mid Pipes (X, H or Prochamber)
Tune: See below
Why: The stock exhaust flows pretty well on the S197. Well enough in fact that with the exception of moving to an Offroad style midpipe or headers, there are no real power gains to be had by changing out the exhaust. Headers I have already talked about so I will not go there again here, but with the exception of midpipes with no Cats (aka offroad pipes), none of these will require a tune. This is basically because the parts they are replacing are not measured volumes. Once the exhaust gases pass the rear O2 sensor, as far as the Computer is concerned its gone and the computer could care less about it. Now, if you replace your mid pipe with an “Offroad” type pipe (which is illegal in most states, and hence the name), you will need to disable the rear O2 sensors. It is the only exhaust Mod behind the headers that will require a retune
Popular Brands: Almost too many to list… FRPP, Borla, Pypes, JBA, Flowmaster, Dynomax… these are only a few of the many.
Mod: Cams
Tune: Yes
Why: If you think of your car as a human body, the Cam is the brain. It tells everything when to happen. As the cam turns, it pushes open the valves, which then tells the PCM to pump blood…errr… spray fuel into the cylinder. It then closes the valve and a sensor on the camshaft then tells the coilpack when to send a spark into the chamber to light the fuel on fire. It then turns more, and opens the exhaust valve, so the piston can push all the spent fuel charge out. Now, while the Cam controls when these things happen, the computer has an exact model of what the cam is doing and knows what to expect. By changing the cams, but not changing the model that the computer is working with, it’s going to be out of synch with whats going on.
Popular Brands: BBR and Comp
Mod: Throttle Bodies
Tune: By model.
Why: The why on this is very similar to what I talked about in CAI’s. The computer has a pretty specific model of how much air the throttle body can flow based on how open the butterfly’s are. This measurement however, is not as important as the MAF’s, but it still plays a part in everything. These values can be updated in a tune, but as anyone who has run a BBK or even a JDM kit that uses a GT500 TB, it’s not 100% needed. I’ve seen a few articles that have tested TB’s and they all follow one general theme. Without an aftermarket tune, the gains for the TB were in the 8 to 15 HP range at the rear tires, depending on other mods done to the car. Once an aftermarket tune updated for the TB is added to the mix however, things change considerably. The difference between having the TB and not drop to as little as 1 hp and 3 ft/lbs when the tune is updated for the Throttle Body. Now, while no one ever made any hints as to why, I have an idea on this. Please keep in mind that this is purely speculation on my part and I haven’t done any testing to prove this, it’s just me following what seems like logic. It strikes me that that without updating the tune for the TB, the airflow model is going to be expecting less air than the larger TB will flow. By having more air and the same amount of fuel, it’s going to lean the AFR’s out a bit… which is exactly one of the things that tuners do since the stock commanded AFR’s at WOT are in the 12.0 to 12.4 range. However, once you update the airflow model to accurately reflect the amount of air that is flowing past the butterflies, the AFR’s go right back to what is commanded. As a side note, most say (and I tend to agree) that the stock TB’s are good enough to support airflow to horsepower levels that are well past what the stock bottom end can handle and the only time that a TB is really worth it is in radical NA builds (i.e. “Cammer 5.0” engine, 298ci Strokers and the like) or in forced induction setups in the 450+ RWHP range… tho there are examples of people using the stock TB’s to the 600 horsepower range.
Popular Brands: BBK, Grantelli, and JDM’s GT500 adaptor kit.
Mod: Electric Water Pump
Tune: No
Why: Same idea as the Underdrive pulleys, just taken to the next step. Instead of increasing the mechanical advantage that the crank has over the pulley, you eliminate the drag on the pulley altogether. No drag on the pulley means that there is no power used up to drive it.
Popular Brands: Meziere
Mod: Tires other than factory size
Tune: Yes
Why: The computer uses the height of the tires to help calculate the speed and distance traveled. If you change this height, then your speedometer and odometer will be off.
Popular Brands: You name it…
Mod: Coil Pack (a.k.a. COP’s)
Tune: No
Why: The computer just tells the coils when to discharge. How much they discharge, how fast they do it, and how fast they recover are all a function of how the coil is made.
Popular Brands: Granatelli, Accel, MSD, and Summit Racing.
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