In our 5 Best Duramax L5P Mods guide, we covered downpipes and mentioned them as our favorite exhaust upgrade. While a larger DPF-back exhaust system will provide solid sound gains, the biggest power gains within the exhaust actually come from upgrading the downpipe. Additionally, there are a number of ancillary benefits like lower EGTs and less exhaust backpressure which also make this a great reliability mod.
This guide is going to discuss what a downpipe is, the benefits of upgrading it on the L5P, and cover a few of our favorite aftermarket downpipes.
*Picture of a stock L5P downpipe next to a WCFab 3.5″
What is an L5P Downpipe?
On the L5P, the downpipe is the first component of the exhaust system. It bolts-up directly to the turbocharger and connects with the diesel oxidation cat, or catalytic converter. Since it sits before the cat it actually doesn’t have any emissions equipment within it and therefore upgrading it doesn’t remove any factory equipment.
Since it connects directly to the turbo, it has a lot of impact on backpressure. Backpressure is the build-up of exhaust gases in the downpipe that then creates a negative pressure against the turbine wheel, making it more difficult for the turbine wheel to spin quickly and efficiently. Less backpressure in the exhaust allows the turbine wheel to spin more quickly and freely, increasing efficiency and power. Backpressure also leads to increased EGTs since it requires the turbo to work harder, therefore producing more heat.
The stock downpipe on the L5P is skinny and crushed in multiple spots which restricts airflow. While it is okay for un-tuned and stock trucks, it becomes especially restrictive when a tuner and intake is added and boost levels are increased.
Duramax L5P Downpipe Upgrade Benefits
The biggest benefit of an upgraded downpipe on the L5P is the reduction of backpressure. Less backpressure means more power and torque. Additionally, it lowers EGTs, creates a quicker turbo spool, and reduces stress placed on the turbo itself.
On a completely stock L5P you probably won’t notice a ton of power gains. The gains are the biggest and most noticeable when you are using a tuner and running above stock boost levels. On a stock truck you will notice some EGT improvements, especially when towing, but the power gains will be minimal as the stock downpipe can handle factory power levels just fine. No tuning is required to run an aftermarket downpipe, but it is recommended to maximize the performance benefits.
- Approx. 10whp gains
- Approx. 15-20wtq gains
- Lower exhaust backpressure
- Quicker turbo spool
- Lower EGTs
- More efficient turbo
Again, the power numbers quoted above are going to be when running the downpipe with a tune. We generally always recommend an intake with a tune as well as the extra airflow helps hit the higher boost targets.
Downpipe Sound Gains
While an upgraded downpipe is larger than the stock one, it doesn’t provide much in the way of exhaust sound gains. It will give a slightly (barely noticeable) deeper exhaust note and also provide more turbo spool noises, but not a very noticeable amount. Since the downpipe doesn’t have any emissions equipment in it, it doesn’t add a lot sound wise.
If you want more exhaust sound, your best bet is upgrading to a 5″ DPF-back exhaust system, or deleting the DPF and getting a turbo-back exhaust. Outside of the gains from the downpipe, the best exhaust gains are going to come from deleting the L5P. However, if you don’t want to go this route, getting a 5″ DPF-back exhaust will also reduce some backpressure and provide some minimal power gains.
Best Duramax L5P Downpipe Upgrades
There are a few different options for L5P downpipe upgrades. The first thing you’ll notice is that some companies offer 3″ downpipes and others offer 3.5″ downpipes. A 3.5″ downpipe will have greater flow and therefore more backpressure reduction. Secondly, they come in various different materials. Stainless steel downpipes are the best as they provide corrosion resistance unlike other materials.
So, our recommendation is to stick to 3.5″ stainless steel downpipes. Stainless steel options tend to be slightly more expensive, but they are worth the extra money for the additional durability provided. Additionally, you’ll see a number of the downpipes come with heat wraps. Heat wraps help reduce the amount of heat radiating from the exhaust and lowers engine temps. We also recommend downpipes with heat wraps over ones that don’t have it.
1) PPE L5P Downpipe
PPE offers a stainless steel, heat wrapped downpipe for a solid price. This is our current favorite offer on the market due to the value, price, and quality combined. PPE also offers an exhaust manifold and up-pipe kit for an extra $1,500. If you are looking for extra performance then upgrading these at the same time as the downpipe makes sense as you will have to remove a lot of these parts to get the downpipe installed anyways.
The only downside to the PPE downpipe is that it is a 3″ downpipe and not a 3.5″, so it won’t have as much of a backpressure decrease compared to the larger downpipes. However, it will still be a big improvement over stock.
Overall, solid product at a great value which is why it tops our list.
2) WCFab 3.5″ L5P Downpipe
WCFab offers a 3.5″ downpipe that is also heat wrapped. The one caveat with the WCFab product is that it is not made of stainless steel. It is black ceramic coated which will help prevent corrosion, but it isn’t quite as durable as stainless steel options. It is a larger downpipe though so it will have maximum performance benefits. WCFab manufactures all their products in-house in the USA so it is also a more expensive downpipe.
If you want the bigger downpipe and want to support American businesses, at the sacrifice of price, then this is a great option as well.
Duramax L5P Downpipe Upgrade Summary
An upgraded downpipe is one of the best L5P mods and the best overall exhaust upgrade. The factory downpipe is small and crushed in multiple areas which restricts airflow and results in increased backpressure. High amounts of backpressure in the downpipe work against the turbo, reducing turbo spool and decreasing efficiency. Upgrading to a larger downpipe will lower backpressure which there increases power and torque. Additionally, it has numerous additional benefits such lower EGTs, faster turbo spool, and better efficiency.
Downpipe upgrades are most valuable for L5P’s that are either tuned or running a piggyback like the Derringer. There will be minimal power gains on a stock car, but it will still result in lower EGTs. The power and spool benefits are most noticeable when you are running above stock boost levels.
Our favorite downpipes are 3.5″, with heat wraps, and stainless steel piping. Unfortunately, there aren’t any downpipes on the market right now that meet all three of these. However, there is a company currently developing a downpipe that will hit all of these marks. In the meantime, our favorite options come from PPE and WCFab.
Ultimately, this is a great mod for those looking for more power and a healthier engine.