The Definitive Guide To Building a Honda L15B/C (1.5T) Engine
So you've got a 2016+ Honda Civic with one of those 1.5T engines; the car is fun to drive, the engine is torquey and engaging while also getting great fuel economy, but you find yourself lusting for more power. You know you should probably beef the engine up to handle the extra power, but you don't know where to start, what upgrades are worthwhile and which ones may not be worth your money or what's a good fit for your goals. Well keep reading because we're going to answer all of that here!
Before we go into mods for these engines, we're going to cover the variations because there are some slight differences to be aware of in case you find yourself sourcing a new engine to build to minimize downtime of your car. Fortunately these engines have much less variation than the older K series platform so it's fairly easy to keep things straight and understand what you've got and what you want/need!
L15B vs L15C
Honda didn't keep things very clear with the notation of these engines as there are some with the same name yet are different in form (eg the L15BE was used in both CRVs and Accords but the cylinder heads are different), so we're not going into the details of each variant as it can cause confusion, rather we're going to explain the important part differences and where they are usually found.
Engine Block
One of the best things about building these engines is that 98% of the blocks you'll see are the same block. As of this writing (2025) all L15B/C blocks are exactly the same, with one small exception: Honda changed the block where the alternator mounts in 2017. The width where the alternator bolts up was changed for some reason so if you get a 2016 or 2017 block there is a chance you may need to get a new alternator too if your car has a later engine. The early style block is only found in 2016 and 2017 Civics (not including the Si). ALL L15s made after 2017 (plus 2017 Civic Si and CRV) use the later style block and are the same part number. The older style block is part number 11000-59B-305 and the new style block is 11000-59B-010.
Cylinder Head
There are two cylinder head options for the L15B/C:
- Single exhaust port. No VTEC. Came in 10th gen Civic (2016-2021) and 5th gen CRV (2017-2022)
- Twinscroll exhaust port. VTEC (exhaust side only). Came in 11th gen Civic (2022+), 6th gen CRV (2023+) and both 10th and 11th gen Accord (2018+)
The single port head is preferred if you're going for peak power, the lone port on this head has a larger cross sectional area than the twinscroll head, this results in lower preturbine exhaust back pressure which allows for higher peak power numbers in exchange for a tad slower turbo spool. There are more camshaft options for the single port head.
The twinscroll head is preferred for faster turbo spooling which can make it more engaging to drive, but gets more choked at high RPM when trying to make much more power. The exhaust side has VTEC, which adds some extra complexity over the VTEC-less single port head.
Since the exhaust flange on the head is different, the turbos are different as well, so single port turbos can only be used on single port heads and the same goes for twinscroll turbos on twinscroll heads. The intake ports are the same on both heads so intake manifolds are interchangeable.
Crankshaft
The factory crankshaft in the L15B/C is a fairly robust piece, Honda has always used very well made crankshafts and this platform is no exception. The only differences to be aware of here is the L15B uses a smaller connecting rod journal than the L15C. Both are plenty strong so there's no downside to the L15B's smaller journal. There are more aftermarket forged rod options for the L15B than the L15C because the B predated the C so that's something to be mindful of. The main journals are all the same size and they all have a stroke of 89.4mm.
Pistons + Oil Squirters
From the factory there are two different pistons in the L15B/C, and on paper the main difference is the compression ratio: 10.3:1 and 10.6:1. The 10.3 pistons are found in both 10th and 11th gen Civic Si, 11th gen non-Si Civic, CRV and Accord. The 10.6 pistons are only found in the 10th gen non-Si Civic. Off paper, they are constructed very differently and this difference also changes the oil squirter. The 10.3 pistons have an internal oil cooling channel that has oil sprayed directly inside of it by a single nozzle oil squirter. The 10.6 pistons have a traditional construction with no cooling channel so they use an oil squirter with two nozzles for extra oil spray cooling. If you are using forged pistons you will want the dual squirters, they do the best job of keeping pistons cooler which reduces the chances of having bore scuffing from the piston overheating.
Connecting Rods
The rods are the main weak point of this engine, which is pretty normal for a turbo 4 cylinder engine. There's really nothing special about the stock rods, they're a basic powdered metal rod and if you're building your engine they should absolutely be replaced with an aftermarket forged rod. The L15BA found in the 10th gen Civic hatchbacks (and only the hatchbacks) has a different part number and are marked 59B (all other small journal rods are marked 5PA) and seems to have a different heat treat process because they are proven to fail at lower power levels than the other rods used in all other L15B/C engines. Because the crankshaft centerline is offset from the bore centerline on these engines the block is notched from the factory for the rods to clear it on the intake side of the block. All aftermarket rods are wider and require these notches to be enlarged for the beefier rod to clear the block.
Oil Pump
All L15B/C engines use the same exact oil pump. It is a gerotor style pump driven directly by the snout of the crankshaft. The gerotor gears are made of powdered metal and are relatively weak, if you are going to rev the engine much higher than stock it's absolutely a good idea to upgrade this.
Valvetrain
Due to the twinscroll head having VTEC and the single port head not having VTEC there are a fair number of differences in the valvetrain, but still a fair amount of similarities. Since the VTEC is only on the exhaust side, the exhaust cam and exhaust rocker arms are different between the two heads, but the intake cam and rocker arms are the same. The intake cam is different between 10th gen Si and 10th gen non-Si but the only difference is the lobe that drives the high pressure fuel pump has more lift on the Si cam. This larger fuel lobe intake cam is used in the Accord, CRV and 11th gen Civic as well.
Critical Fasteners
The critical fasteners in these engines are the head bolts, main bolts and rod bolts. The head bolts should 120% be replaced with aftermarket head studs. These are usually the first thing to become compromised in these engines and when the head lifts the head gasket gets taken out. This is usually addressed before people resort to building their engine. The main bolts are fairly strong and we've yet to see anyone have an issue with the factory main bolts or main caps. The rod bolts get replaced and upsized when moving to aftermarket forged rods so there isn't much to say about those.
UPGRADES
And now we're finally at the part you came here for, UPGRADES! What do you need to achieve your goals? Well everyone has different goals so we're going to break down some of the when and whys as we get through these. Note: All of the parts we talk about are what's on the market at the time of this writing (2025) so if you're from the future there may be more stuff not mentioned but we'll try to add to this post as new stuff becomes available.
Head Studs
We're starting here because this is the easiest thing to address on the L15B/C and even if you're not building your engine it's a great idea to put some head studs in to ensure you'll never have to worry about compromising your head gasket. On other engine platforms there are different grades of alloy for different options but for the L15B/C pretty much all of the current options are 8740 steel or some tweaked form of 8740. Two Step Performance has a direct fit kit made for them by ARP. Optitorque also has a head stud kit that is very on par with the TSP kit. Lastly, if you're on a budget you can use Suzuki Hayabusa head studs, ARP has a kit that doesn't break the bank but the catch is they have to be installed upside down, this can be hard to do if you're installing them in the car without removing the head since the broached end of the stud isn't facing up when installed like this.
Note: If you have had issues with the head lifting or suspect it's happened we strongly recommend taking the head off and replacing the head gasket as well as having the head and block checked for warpage. Head studs are not a problem solving solution, they are a problem avoiding solution, if you already have an issue with head lift it's too late to just toss studs at it.
Forged Connecting Rods
This should be the first thing on your shopping list for your engine build, it's the weakest point in the engine (besides the head bolts). There are only a handful of options and none of them are bad choices, Brian Crower, Carrillo and Saenz all have solid offerings. Avoid any cheapo names like the plague, connecting rods are a fairly basic component but poor quality machine work can result in bearing issues and worst case turn your shiny new built engine into a paperweight and tears. There isn't a whole ton to say here other than to avoid the questionably cheap stuff, all of the offerings we listed will easily handle 500whp and potentially more.
Forged Pistons
Forged pistons go hand in hand with forged rods. All of the options on the market are made of 2618 aluminum and are very strong. The reputable companies for pistons on this platform are JE, Wiseco, and Traum. There are some different options for compression ratio, our recommendation is the stay around stock in the 10-10.6 range. Lower compression isn't going to give much discernable advantages and will make for a slightly slower turbo spool. As for bore size, it will depend on the block you're using. We're big believers in keeping as much thickness in the cylinder walls as possible for max strength, so if you're using a new or low mileage block then get 73mm pistons. If your block is higher mileage or shows signs of scuffing, scoring or any other damage in the cylinder wall then get 73.5mm pistons and have the machine shop bore the block. Any of the pistons mentioned should be good for 600whp.
Oil Pump and Pan
Boundary Engineering offers their billet steel oil pump gears to replace the factory powdered metal gears. Extended periods of time at high RPM or revving much over the factory redline (6500RPM) greatly increases the chances of the factory oil pump gears breaking. If this happens there is a high likelihood that your engine will become scrap. Not only are the Boundary gears strong and able to handle far more abuse, but they also have a smoother pumping action which creates more steady and stable oil pressure in the engine, a win-win! The other side of oiling is making sure the pump can receive good air free oil, the best way to ensure this is to install a pan baffle. We've had our 10th gen pan baffle and 11th gen pan baffle on the market for a while now with great results and it's the perfect solution for keeping oil around the pickup tube, it also allows for some extra oil capacity to bring oil capacity over 4qts (in the 10th gen pan only)!
Head Gasket
The factory head gasket works great and we recommend that for basically all builds, but if you want to slightly tweak your compression ratio up or down Cometic offers quality head gaskets of different thicknesses to help dial in compression. We don't recommend any other brand head gaskets than Honda OEM or Cometic.
Camshafts and Valve Springs
The factory camshafts and valve springs are mediocre and not designed with performance in mind, they leave lots of room for improvement. Aftermarket cams aren't a huge mod for this platform yet but Kelford Cams offers a few profiles and with Kelford's vast experience in making hotter cams for direct injected engines these are the cams to go with. The only other cam option is Skunk2, but for the same price we'd take the Kelford cams every day. Kelford also has a spring kit that pairs perfectly with their cams (springs are required for all of their grinds except the EZ grind) but even if you're not doing hotter cams upgraded valve springs are a good idea to avoid valve float at higher boost and RPM levels. Brian Crower also makes a nice valve spring kit and Skunk2 has a spring kit, but again, we'd recommend the Kelford or BC springs as first choices. All cams currently on the market are only for the single port head, there are no twinscroll head aftermarket cams.
Cylinder Head Porting and Valves
We're lumping head porting and valves together because just like cams and springs they are closely tied to each other (all 4 are fairly well intertwined) in that they have a big impact on airflow. Porting is a largely unexplored frontier for these engines (yes, people have done it but it's not be validated), but there's absolutely gains to be had in it. The factory head ports aren't terrible flowing, but with how small the valves are, having a really good flowing port can go a long ways for making power. Head porting is a very job dependent result, AKA not everyone will give you the same port, so be sure to find someone who has experience porting a DI cylinder head. As for valves Ferrea is the only option for aftermarket valves, their stuff is generally considered to be the cream of the crop so there is no concern about them being poor quality.
Deck Insert
There are a few companies that offer deck inserts or "block guards" that are basically machined pieces of aluminum that are pressed into the open gap of the deck in the cooling jacket of the engine block. This strengthens the top of the block and can allow for better head gasket sealing. It's a good idea for uber high power, but also requires extra machine work on the block and isn't needed for moderate power level goals.
Figuring Out What's Right for YOU!
To keep this simple we're going to outline what we recommend for power levels as well as additional parameters:
- 300-330hp = stock engine w/head studs
- 350-390hp = rods + pistons, head studs
- 400-430hp = rods + pistons, head studs, valve springs, camshafts
- 440-480hp = rods + pistons, head studs, valve springs, camshafts, head porting
- 500hp+ = rods + pistons, uprated head studs, valve springs, camshafts, head porting, valves, deck insert
- High revving = Add Boundary oil pump and Wunderladen oil pan baffle
- Faster turbo spool, less top end power = Twinscroll head
- More top end power, slower turbo spool = Single port head
Thanks for reading along, hopefully you learned something here and let us know what you think about building an L15B/C! Are we missing any details? What combo is right for your goals?