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Our quest for big power has led us down the road of complete engine disassembly and back again with the construction of a 131-cubic-inch beast that still has some build details that need to be addressed. In our final installment of this build, we put the finishing touches on the quest for 150 horsepower.
We started out to make serious horsepower with our Milwaukee-Eight and built a really stout motor where we left no stone unturned. We paired an S&S Cycle stroker crankshaft with some big-bore pistons and cylinders, breathing through ported heads and being fed by a monster cam which adds up to a whopping 131 cubic inches of V-twin fury.
But before we can wrap up this quest for power we still need to address a few things. How are we going to feed this monster? Meaning what are we going to use to get air in and out and how are we going to control fuel delivery? And once we have fed the beast we have to figure out how to control it. We can’t turn our monster motor loose on a stock drivetrain unless we want a future full of smoked clutches and broken drive belts.
To get here, we’ve torn down our engine, primary and everything in between, and we’ve reassembled it with some of the best parts we could lay our hands on to build not only big horsepower but also a streetable engine that runs on pump gas. But now it's time to finish the job, and the next thing we need to tackle is our stock fuel system. You can have the biggest engine with the fanciest cylinder heads and most exotic valve train, but unless you can feed it air and fuel efficiently you’ll be leaving horsepower on the table. So we need to address the weak links in the system and those links are the throttle body and injectors.
The stock M8 throttle body is a paltry 55mm unit that bolts to a plastic manifold to feed the engine the air it needs to make power and it's really not much good for anything past a bolt-on modification in the horsepower-making department. With our increased internal engine capacity, high flowing heads, and big cam, using this setup would be the equivalent of putting a kink in a garden hose. The throttle body and manifold are known weak points when squeezing power out of an M8 and there are some exotic options available, like ported manifolds and throttle bodies bigger than 70mm. Even though we’re looking to build power, we’re also building a street motor, so I opted for a Screamin’ Eagle 64mm throttle body and upgraded aluminum manifold. It will give us the airflow we need without breaking the bank.
Once we had the air flow taken care of, we had to address fuel delivery and how we were going to control it. The stock injectors will duty cycle (meaning run out of the ability to flow fuel) long before our thirsty engine is done making power. To solve this problem, we swapped our injectors for a set of Feuling 6.2 injectors that will provide plenty of fuel flowing at a rate of 6.2 grams per second. To control our fuel flow and timing, we opted to replace our ECM with a Thundermax tuner, which gives us complete control of our fuel and ignition system. It also allows us to run a wideband closed-loop tuning system. Meaning it replaces the stock narrow-band oxygen sensors with wideband units that “sniff” our exhaust gasses for air fuel mixture, report back to the ECM, and make changes in the tune so it runs at our targeted air-fuel ratio and does it all in milliseconds. That not only lets us tune different parts of our RPM and throttle position ranges with specific targets, but it also constantly makes changes to keep the tune perfect.
To put the wraps on our air and fuel system, we capped it off with a S&S Stealth Intake to handle outside airflow and a Bassani Road Rage stainless exhaust with stepped headers to give us good power all the way through the RPM range.
But if we can't get all that power through our drivetrain and ultimately to the ground, we really haven’t accomplished much, so we started addressing weak points in the primary and final drive. The compensator sprockets in Milwaukee-Eights don’t respond well to big power and will fail if pushed too hard. So we opted for a solid comp sprocket from Evolution Industries to make sure we can transfer power through the primary without a catastrophic failure.
Next in line was our clutch and I was under no illusion that our stock clutch would even make it around the block with our new big engine. So we installed a Rekluse Torque drive clutch that increased the friction area in the clutch by adding plates and gave us some adjustability with our clutch springs. Last, but not least, we had to make a call on whether or not to stick with our stock final belt drive or swap it to a chain. Ultimately, I chose to go with a chain not only for added strength, but also because it gives us the option to change our gearing with different sprockets, something that we couldn’t do with a belt drive.
The Thundermax gave us the ability to tune quickly and without a dyno during our initial break in so we could get some miles before heading to the dyno to see what our power gains were. When we finally made it to Speeds Performance for the big dyno day, I was a very happy builder. Our 131 turned out 148 rear wheel horsepower and 147 foot-pounds of torque on a mid-80-degree day at 3,500 feet of elevation running on pump gas. Would I like to have seen 150 flash on the dyno screen? Yes, but I am in no way disappointed. In fact, I’m quite pleased with our build. It annihilates tires at any RPM in the first three gears and puts a smile on my face every time I twist the throttle.
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