I still see so many questions like, "I just got a 3.8, what should I do first?!". There is a ton of info out there on the forums, but for the most part, it's found in disparate locations and scattered throughout random threads in a sea of troll posts. I thought I'd chime in and give a guide for 3.8 owners to follow and not waste money.
First: If you have not read this thread The Ultimate Genesis Coupe Newbie Guide to Modding... I suggest you do it. While it's focused on the 2.0T, there is still great info in there in general.
Disclaimer: This thread is based on my own build and is mostly in my opinion. I believe my car speaks for itself. I don't care to argue any points with anyone, but if I do make a mistake, please point it out to me so I can correct it. Also, note that this is mostly related to daily drivers and performance. I don't know jack about suspension so I don't really touch on it here.
My Goals
My goals have always been to improve the performance of my car while spending as little as possible (don't confuse this with being cheap!) and just make the car a fun drive overall. IMO I have accomplished that goal.
Mods/Upgrades
Here is a list of modifications and upgrades. These are listed mostly in the order I recommend you do them. Some of these are free and some of them require spending money.
1. Start using premium quality 91/93 octane fuel
2. Check the spark plug gap (free)
3. Better ground mod (free)
4. Coolant bypass mod (free)
5. Easy intake mod (free)
6. Relocate the IAT sensor mod (free)
7. Remove the engine cover (free)
8. Catch can
9. Phenolic spacer
10. Brake pads
11. Heat wrap intake
12. Hood vents
13. Intake manifold balancing/porting
14. Throttle body bore
15. Test pipes
16. Lightweight pulleys
17. Cat-back exhaust
18 a. Shave IAT sensor (free)
18 b. Replace the IAT sensor with a fast-acting sensor
19. Tune
20. Rigid collars
21. Tires
22. Upgrade Brake fluid
23. Trans Mount
1. Start using premium quality 91/93 octane fuel
Honestly, there is no point asking your car to do better if you don't give it the tools it needs. If you plan on running 87 octanes from the cheapest gas station you can find, then just stop reading and move on. Quality fuel will benefit you in the long run and using an appropriate octane rating is key to getting the most from your car. Some of you can get away with 89 octanes but don't argue this point with me unless you are providing data logs to back up your statements.
2. Check the spark plug gap (free)
This might seem like a dumb one, but a lot of people have reported (not just GC owners) that their spark plugs from the factory were all different gaps. Ensuring they are the correct gap will help prevent any misfire or premature failure. Too low of a gap won't give you a clean burn so you'll end up with soot buildup on the plugs (eventually leading to fouling), valves, etc and you won't be getting the most power. Too wide a gap and you run the risk of misfires, knock and plug failure due to heat.
2a. Should I run a colder plug?
This is a question best answered by data logs. In most scenarios, no, you do not need to run a colder plug. Most of the time you won't be running hot enough to even need them and you won't burn all the crap from the plug and they will eventually foul. After you have done your entire build and you are tuned, data log and see what's going on. If you get a custom tune, talk to your tuner while he's doing it and discuss if they are needed or not.
Do not just go buy them, they are a waste of money and just cause a headache when you don't really need them. You can't just put them in and say, "my butt dyno" feels like an improvement. The improvement comes from preventing knocks when running advanced timing. Your stock tune will not benefit from them unless you're running the worse fuel possible, but even then probably not. I'm custom tuned and do not have any knock-on California 91 octane on stock plugs, so there you go.
3. Better ground mod (free)
When it comes from the factory, the ground strap going from the battery to the body is decently beefy. The problem is that Hyundai painted the body before bolting the ground to it so the only connection being made is from the bolts that hold it on.
The mod is simply to remove the battery, then remove the ground from the body, remove the paint where the ground plate mates to the body, then put everything back together.
After performing this mod I gained .5v on all test points with the car off. You might think this is insignificant, but it isn't. What does this do for you? First, it allows a better flow of current when your components need to draw power. Better flow means less amperage, meaning less heat and less stress on the components. But where you gain performance is from the fuel pump. When you're screaming along at WOT in the 6k-7k RPM range, you're asking your fuel pump to produce a level of fuel required to support the target AFR. The pump depends on proper voltage. If it can't get the voltage it needs, it can't deliver what you ask it to. There are a few platforms that benefit from running a larger gauge wire to supply the fuel pump because the factory wires are too small for the demand. I'm not sure if this is the case with the GC (yet, I'm looking into it), but you get the point.
Some people have reported smoother idles, better start-up, and better throttle response. YMMV.
How to: How-To: Get a better ground :video:
4. Coolant bypass mod (free)
If you live in an area where temperatures commonly get below freezing, skip this mod.
This mod reduces the intake air temperatures. It's common for cars from the factory to run coolant through/around the throttle body. This is done not to cool it down, but to heat it up. There are a few reasons like better emissions, preventing freezing, etc. But if you live in an area that is warm or hot most of the time, then what this setup does is increase air temps going into the motor. Hotter intake temps mean less timing and on our motors, timing is everything.
This is a very simple mod. I actually spent $5 to get a new length of hose, so it wasn't totally free for me, but you can do it without spending any money.
The results will vary power-wise, but you will notice the difference. Before doing the mod, drive around hard and then pull over and touch your TB. Very hot! Let the car cool down, perform the mod then do the same test again. Ice cold.
How to: DIY: 2013 3.8 Throttle Body Coolant Bypass (Pic Heavy)
5. Easy intake mod (free)
This is an odd one and some people may not like it, but it does help. Essentially you're removing the shroud that sits in front of the intake snorkel (by the hood latch). The stock air box flows really well, but this shroud negates that and helps add heat to the intake temps.
Removing this shroud will result in cooler intake temps (about 2-3* when traveling) and will improve throttle response up top.
How to: Easy stock intake mod
6. Relocate the IAT sensor mod (free)
One of the biggest problems with the stock 3.8 air box is the location of the IAT sensor (intake air temperature). It sits in a spot that does not get a lot of moving air and it's sitting in a spot that gets blasted with heat from the engine and coolant pipes.
The problem is that your sensor is seeing hotter temps than what is actually going into your engine. The issue with this is 1) higher temps will reduce timing 2) higher temps will reduce the fuel the ECU adds which will result in leaner AFR which could lead to timing reduction. The car will compensate fairly quickly with fuel trims, but why not just fix the issue?
This one is very simple. Just drill a hole in the top of the air box and stick the sensor there. Then block off the old hole with a piece of plastic and some gasket maker or RTV.
![Image]()
![Image]()
The result is that the temperature probe nipple is in the direct path of the incoming air stream which will give a much more accurate reading (it won't be 100% accurate, see 18a and 18b).
First: If you have not read this thread The Ultimate Genesis Coupe Newbie Guide to Modding... I suggest you do it. While it's focused on the 2.0T, there is still great info in there in general.
Disclaimer: This thread is based on my own build and is mostly in my opinion. I believe my car speaks for itself. I don't care to argue any points with anyone, but if I do make a mistake, please point it out to me so I can correct it. Also, note that this is mostly related to daily drivers and performance. I don't know jack about suspension so I don't really touch on it here.
My Goals
My goals have always been to improve the performance of my car while spending as little as possible (don't confuse this with being cheap!) and just make the car a fun drive overall. IMO I have accomplished that goal.
Mods/Upgrades
Here is a list of modifications and upgrades. These are listed mostly in the order I recommend you do them. Some of these are free and some of them require spending money.
1. Start using premium quality 91/93 octane fuel
2. Check the spark plug gap (free)
3. Better ground mod (free)
4. Coolant bypass mod (free)
5. Easy intake mod (free)
6. Relocate the IAT sensor mod (free)
7. Remove the engine cover (free)
8. Catch can
9. Phenolic spacer
10. Brake pads
11. Heat wrap intake
12. Hood vents
13. Intake manifold balancing/porting
14. Throttle body bore
15. Test pipes
16. Lightweight pulleys
17. Cat-back exhaust
18 a. Shave IAT sensor (free)
18 b. Replace the IAT sensor with a fast-acting sensor
19. Tune
20. Rigid collars
21. Tires
22. Upgrade Brake fluid
23. Trans Mount
1. Start using premium quality 91/93 octane fuel
Honestly, there is no point asking your car to do better if you don't give it the tools it needs. If you plan on running 87 octanes from the cheapest gas station you can find, then just stop reading and move on. Quality fuel will benefit you in the long run and using an appropriate octane rating is key to getting the most from your car. Some of you can get away with 89 octanes but don't argue this point with me unless you are providing data logs to back up your statements.
2. Check the spark plug gap (free)
This might seem like a dumb one, but a lot of people have reported (not just GC owners) that their spark plugs from the factory were all different gaps. Ensuring they are the correct gap will help prevent any misfire or premature failure. Too low of a gap won't give you a clean burn so you'll end up with soot buildup on the plugs (eventually leading to fouling), valves, etc and you won't be getting the most power. Too wide a gap and you run the risk of misfires, knock and plug failure due to heat.
2a. Should I run a colder plug?
This is a question best answered by data logs. In most scenarios, no, you do not need to run a colder plug. Most of the time you won't be running hot enough to even need them and you won't burn all the crap from the plug and they will eventually foul. After you have done your entire build and you are tuned, data log and see what's going on. If you get a custom tune, talk to your tuner while he's doing it and discuss if they are needed or not.
Do not just go buy them, they are a waste of money and just cause a headache when you don't really need them. You can't just put them in and say, "my butt dyno" feels like an improvement. The improvement comes from preventing knocks when running advanced timing. Your stock tune will not benefit from them unless you're running the worse fuel possible, but even then probably not. I'm custom tuned and do not have any knock-on California 91 octane on stock plugs, so there you go.
3. Better ground mod (free)
When it comes from the factory, the ground strap going from the battery to the body is decently beefy. The problem is that Hyundai painted the body before bolting the ground to it so the only connection being made is from the bolts that hold it on.
The mod is simply to remove the battery, then remove the ground from the body, remove the paint where the ground plate mates to the body, then put everything back together.
After performing this mod I gained .5v on all test points with the car off. You might think this is insignificant, but it isn't. What does this do for you? First, it allows a better flow of current when your components need to draw power. Better flow means less amperage, meaning less heat and less stress on the components. But where you gain performance is from the fuel pump. When you're screaming along at WOT in the 6k-7k RPM range, you're asking your fuel pump to produce a level of fuel required to support the target AFR. The pump depends on proper voltage. If it can't get the voltage it needs, it can't deliver what you ask it to. There are a few platforms that benefit from running a larger gauge wire to supply the fuel pump because the factory wires are too small for the demand. I'm not sure if this is the case with the GC (yet, I'm looking into it), but you get the point.
Some people have reported smoother idles, better start-up, and better throttle response. YMMV.
How to: How-To: Get a better ground :video:
4. Coolant bypass mod (free)
If you live in an area where temperatures commonly get below freezing, skip this mod.
This mod reduces the intake air temperatures. It's common for cars from the factory to run coolant through/around the throttle body. This is done not to cool it down, but to heat it up. There are a few reasons like better emissions, preventing freezing, etc. But if you live in an area that is warm or hot most of the time, then what this setup does is increase air temps going into the motor. Hotter intake temps mean less timing and on our motors, timing is everything.
This is a very simple mod. I actually spent $5 to get a new length of hose, so it wasn't totally free for me, but you can do it without spending any money.
The results will vary power-wise, but you will notice the difference. Before doing the mod, drive around hard and then pull over and touch your TB. Very hot! Let the car cool down, perform the mod then do the same test again. Ice cold.
How to: DIY: 2013 3.8 Throttle Body Coolant Bypass (Pic Heavy)
5. Easy intake mod (free)
This is an odd one and some people may not like it, but it does help. Essentially you're removing the shroud that sits in front of the intake snorkel (by the hood latch). The stock air box flows really well, but this shroud negates that and helps add heat to the intake temps.
Removing this shroud will result in cooler intake temps (about 2-3* when traveling) and will improve throttle response up top.
How to: Easy stock intake mod
6. Relocate the IAT sensor mod (free)
One of the biggest problems with the stock 3.8 air box is the location of the IAT sensor (intake air temperature). It sits in a spot that does not get a lot of moving air and it's sitting in a spot that gets blasted with heat from the engine and coolant pipes.
The problem is that your sensor is seeing hotter temps than what is actually going into your engine. The issue with this is 1) higher temps will reduce timing 2) higher temps will reduce the fuel the ECU adds which will result in leaner AFR which could lead to timing reduction. The car will compensate fairly quickly with fuel trims, but why not just fix the issue?
This one is very simple. Just drill a hole in the top of the air box and stick the sensor there. Then block off the old hole with a piece of plastic and some gasket maker or RTV.
The result is that the temperature probe nipple is in the direct path of the incoming air stream which will give a much more accurate reading (it won't be 100% accurate, see 18a and 18b).
