If you're looking to squeeze every last drop of performance out of your V-twin, the predator 670 stage 4 kit is basically the holy grail of upgrades. It's not just a simple bolt-on project that you finish in an afternoon over a couple of beers. We're talking about a total transformation that turns a reliable, workhorse utility engine into a high-revving, fire-breathing monster. Most people start with a Stage 1 or 2, but once you get a taste for that torque, it's hard not to want more.
Going Stage 4 isn't for the faint of heart, though. It's the point where you stop worrying about "good enough" and start focusing on maximum output. Whether you're building a crazy fast go-kart, a mud boat that can cut through the thickest muck, or a custom garden tractor that sounds like a mini-dragster, this kit is how you get there.
What Actually Comes in a Stage 4 Setup?
When you step up to a predator 670 stage 4 kit, you're moving away from external mods and diving deep into the guts of the engine. A standard Stage 1 kit usually just handles the "breathing"—exhaust and air intake. Stage 4, however, focuses on high-strength internals and aggressive timing.
The heart of the kit is usually a high-lift performance camshaft. This is what gives the engine that aggressive, "lumpy" idle that gearheads love. But more importantly, it changes when and how long the valves stay open, allowing for a massive increase in airflow at high RPMs. To handle that extra lift, you're also looking at heavy-duty valve springs. If you kept the stock springs, you'd run into valve float almost immediately, which is a great way to ruin your weekend and your engine.
You also can't talk about Stage 4 without mentioning billet components. Stock connecting rods are fine for mowing the lawn at 3,600 RPM, but they're essentially ticking time bombs once you start pushing the engine toward 5,000 or 6,000 RPM. A Stage 4 kit typically includes billet aluminum rods that are much stronger and lighter than the factory cast pieces. Often, people pair this with a billet flywheel for safety. The last thing you want is a cast iron flywheel shattering right behind your seat because it wasn't rated for those speeds.
The Big Jump from Stage 3
You might be wondering why you'd skip the lower stages and go straight for the big guns. Let's be real: Stage 1 and 2 are great for a little extra pep, but they don't change the character of the engine. The predator 670 stage 4 kit changes the engine's DNA.
The biggest difference is usually found in the fueling and the headwork. While a Stage 2 might just have a larger jet in the stock carb, a true Stage 4 setup often moves toward a Mikuni or similar slide-style carburetor. This allows for much more precise tuning. When you combine that with some DIY porting and polishing on the cylinder heads—which many Stage 4 builders do—the engine stops feeling like a piece of industrial equipment and starts feeling like a racing engine.
The power delivery also shifts. Instead of having a flat power curve that falls on its face as soon as you get moving, a Stage 4 build keeps pulling. You'll feel a noticeable surge in the mid-range and top-end that just isn't there with the milder kits.
Installation Isn't Exactly a Walk in the Park
I won't sugarcoat it: installing a predator 670 stage 4 kit is a serious job. You aren't just swapping out a muffler; you're cracking the crankcase open. If you've never timed an engine or checked oil clearances with Plastigage, you're going to want to do some homework before you start.
Precision is everything here. Since you're installing billet rods, you have to ensure the clearances are spot on. You'll also need to be comfortable with a torque wrench and potentially some minor clearancing inside the block. Because the performance cam has much higher lobes than the stock one, sometimes the rods or the cam itself might need a tiny bit of "wiggle room" depending on the specific tolerances of your 670 block.
Then there's the valve train. Setting the lash correctly is vital. If it's too tight, you'll burn a valve; too loose, and you'll have a noisy, inefficient engine. It's the kind of project where you want to measure twice and bolt once. But honestly, that's half the fun for most of us. There's a real sense of pride in hearing that V-twin roar to life for the first time after you've personally overhauled the internals.
Supporting Mods You'll Probably Need
You can't just throw a predator 670 stage 4 kit into a bone-stock engine and call it a day. Well, you could, but it wouldn't run very well. To actually see the gains you're paying for, you need to support those internal changes with the right external parts.
Exhaust Upgrades
If you're sucking in more air and fuel thanks to a big cam and a better carb, you've got to get that air out. A high-flow header is a must. Most guys go with a "fatty" style header or something custom that allows the V-twin to exhale without restriction. Plus, it makes the engine sound absolutely mean.
Cooling and Oil
Higher RPMs and more power mean one thing: heat. The Predator 670 is air-cooled, so you have to be mindful of how hot it's getting. Using a high-quality synthetic oil is non-negotiable at this level. Some builders even look into ways to improve airflow around the cylinders if they're running the engine hard for long periods, like in a mud boat application.
Fuel Delivery
Don't expect the stock fuel pump to always keep up if you're wide open for a long time. Many Stage 4 users switch to a high-flow vacuum pump or even a small electric pulse pump to ensure the carburetor bowl stays full when the engine is screaming.
Where This Power Really Shines
So, why go through all this trouble? Because the result is a blast. A Predator 670 with a stage 4 kit is a torque monster.
- Go-Karts and Buggies: This is probably the most popular use. Putting a 22HP+ (after mods) V-twin on a light frame is a recipe for wheelies and dirt-slinging drifts.
- Mud Boats: If you're trying to push a heavy boat through thick vegetation, that extra Stage 4 torque is a lifesaver. It allows you to run a more aggressive prop without the engine bogging down.
- Garden Tractor Pulling: In the world of competitive pulling, the 670 is a popular platform. The Stage 4 kit gives you the high-RPM wheel speed needed to keep the sled moving at the end of the track.
Is the Investment Worth It?
Let's talk money for a second. A predator 670 stage 4 kit isn't cheap. Between the kit itself, the necessary tools, and the supporting mods like the exhaust and carb, you could easily spend more on the upgrades than you did on the engine itself.
But you have to look at the "bang for your buck" compared to other engines. To get this kind of horsepower out of a name-brand V-twin like a Vanguard or a Kohler, you'd be spending thousands more just to buy the base engine. The Predator 670 provides a budget-friendly foundation that allows you to dump your savings into the parts that actually make it fast.
If you're the kind of person who loves tinkering and wants an engine that stands out at the local meet or the boat ramp, then yes, it's worth every penny. There's nothing quite like the sound of a fully built 670 hitting its power band.
Final Thoughts on the Build
Building out a predator 670 stage 4 kit is a bit of a rite of passage for small-engine enthusiasts. It takes you from being a "bolt-on" guy to a "builder." It requires patience, a bit of greasy hair, and maybe a few frustrated nights in the garage, but the payoff is incredible.
Just remember to keep an eye on maintenance. These engines are fairly bulletproof in stock form, but once you start pushing 6,000 RPM, things wear out faster. Change your oil frequently, check your valve lash every few runs, and always keep an ear out for any new noises. Do that, and your Stage 4 Predator will be the terror of the neighborhood (in the best way possible) for a long time to come.