Building for 400whp (Engine Build Series Part 1)

One of the most popular blogs on our website is “How to make 300 hp on a 10th generation Civic L15”. It has successfully guided many of you to this milestone in performance. With full bolt-ons and some ethanol-blended fuel, we find the majority pull in the 310-340 whp range which is solid numbers for a fairly small 1.5L 4-banger. 

But like all things in life we look forward to the next progression. So what comes after making the 300hp mark? Well making 400hp of course. 

However, going from 300whp to 400whp requires more effort and time than it did going from 200whp to 300whp. The parts needed and the skills required are greater. Today we are going to dive into that in some detail.

I want to preface this by saying there is no specific magic # of horsepower that the stock bottom end can hold before bad things happen. And by bad things I mean - “my engine blew up”. We have seen engines let go as little as 280 and have seen a few get pushed over the 400 mark with correct tuning and fuel upgrades. That being said, when customers call us and tell us how much power they get on the stock engine my answer is always the same. Experience tells us that around 375 whp is when one should consider a built engine. Again this is not to say that it will run all day at 374 or blow up at 376. This is to say that as you get closer to that power range you should realize that you are on borrowed time. Don’t be surprised if she happens to let go when you get up in this range. Let’s talk about how to avoid a blown motor.

Our 10th gen Si has lived happily and healthy in the ~350 WHP range for a while but it’s time for 400+. Our goal is a solid ~435 daily on setup capable of doing 500whp. To get to that goal an engine build is in order. While still working on and daily driving our 2018 Civic Si; we sourced a spare L15B7.

Our goals for the build were simple; make an engine that could easily support 500hp and do it with off-the-shelf parts. We didn’t want a build that would require custom this or really exotic that. It was to be a build for the people. A build that most any well-equipped machine shop could do and with parts you can easily get. 

The recipe was quite easy in terms of beefier internals: forged connecting rods, forged pistons, head studs, and valve springs. Nearly everything else used was OEM parts and of course a full suite of 27WON goodies for all the power adders.

We know that on these cars the biggest hurdle to 400+ HP after the engine is fuel. The OEM fuel system simply can’t support 350+ HP. An upgrade is required. We will share details about that with you in a later blog. 

Before you can start building a new engine you need to teardown the old one, so check out this sweet video and see ours done in 2 minutes flat!

In part 2 of our Engine Build Series, we will dive into the specific aftermarket parts that we used (with parts numbers) so you can easily source them yourself and share how we resolved the fuel issue.

Stay tuned because at the end of this blog series, there is a BIG surprise :)

REDEFINE the Aftermarket 

-Vincent