You know that feeling when you've finally got your Small Block Chevy (SBC) on the stand, but you're staring at a catalog of camshafts that looks more like a phone book from 1994? It's overwhelming. Honestly, most guys just pick the biggest lift and hope for the best, which is exactly how you end up with a car that sounds like a popcorn machine and stalls at every stoplight. But then there's the Comp Cams 12 600 4.
It isn't some experimental, "flavor of the week" profile. It's basically a Thumpr. Specifically, the Big Thumpr. If you’ve spent any time in the muscle car scene, you know that name carries a lot of baggage—some people love the aggressive chop, while others think it’s "all bark and no bite." But there’s a nuance here that most forum experts miss.
What Actually Makes the Comp Cams 12 600 4 Different?
Most cams are designed with a specific functional goal: more torque for towing, more top-end for the drag strip, or smooth vacuum for a daily driver. The Comp Cams 12 600 4 is part of the Thumpr series, which means Comp’s engineers actually prioritized the exhaust note. That sounds like heresy to a pure racer. However, they did it by playing with the lobe separation angle (LSA) and the overlap.
By narrowing the LSA and increasing the duration on the exhaust side, they created a camshaft that creates a heavy, rhythmic lope. It sounds like a pro-street monster.
You’ve got a 291/311 advertised duration here. That is huge. But look at the lift: .500 on the intake and .486 on the exhaust (with a standard 1.5 rocker ratio). It’s a flat tappet hydraulic cam. That means it’s relatively affordable, but it also means you absolutely cannot skip the break-in process. If you don't use high-zinc oil (ZDP) during those first twenty minutes, you’re basically just turning your expensive new cam into a smooth metal stick. It happens faster than you'd think.
The Reality of Vacuum and Power Brakes
Let’s talk about the "no-go" zone.
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If you're running a stock power brake booster with this cam, you might be in for a surprise when you try to stop at the end of your driveway. Because of that tight 107-degree lobe separation, the Comp Cams 12 600 4 doesn't pull much manifold vacuum at idle. It’s "dirty."
Usually, you’re looking at maybe 8 to 10 inches of vacuum if you’re lucky. Most power brake boosters want 12 to 14 inches to feel "right." If you're dead set on this cam—and let's be real, that sound is addictive—you should probably budget for a vacuum reservoir or an electric vacuum pump. Or just get really good at standing on the brake pedal with both feet.
Stall Converters: Don't Cheap Out
You cannot run this cam with a stock torque converter. Period.
If you try to keep that factory 1,600 RPM stall, the engine is going to fight the brakes every time you put it in gear. It’ll chug, stumble, and eventually die. To make the Comp Cams 12 600 4 livable, you need a stall converter in the 2,500 to 2,800 range. This lets the engine rev up into its "happy place" before the transmission starts grabbing.
It makes the car feel snappy. Suddenly, that "all show" cam starts to show some "go" because you're actually using the power band, which really kicks in around 2,500 RPM and pulls hard up to 6,000.
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Real World Installation: The Little Things People Forget
When you're installing the 12 600 4, you aren't just swapping a stick of metal. You're changing the entire harmonics of the valvetrain.
- Valve Springs are mandatory. You cannot use stock springs. They will bind, or worse, they won’t have the seat pressure to keep the lifter on the lobe at 5,500 RPM. Comp usually recommends the 981-16 springs for this setup.
- Check your gear ratio. If you have 2.73 gears in the rear end, this cam will feel like a dog. You want at least 3.55s, though 3.73s are the sweet spot for a Small Block Chevy running this much duration.
- The Carburetor Struggle. Expect to spend a Saturday afternoon messing with your idle circuit. Because the vacuum signal is weak, your carb’s primary boosters might not want to behave. You’ll likely need to crack the secondary throttle blades a hair or drill small holes in the primary blades to get it to idle at 850-950 RPM without loading up on fuel.
It’s a bit of a diva. But when it's dialed in? Man.
Why the "Thumpr" Reputation is Only Half True
There's this persistent myth that the Comp Cams 12 600 4 loses power compared to a "normal" cam.
Is it the most efficient way to make 400 horsepower? No. A modern Xtreme Energy profile will probably give you a broader torque curve. But the Thumpr isn't "weak." It just shifts the power. You get a massive hit in the mid-range. For a street car that spends its life between stoplights, that "hit" feels fast. It's visceral.
The 107-degree LSA makes the torque come on very suddenly. It’s not linear. It’s more like a light switch. For some drivers, that’s exactly what they want out of a weekend cruiser. It’s about the experience, not just the dyno sheet.
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Compatibility and Blocks
This cam is designed for the 262 through 400 cubic inch Small Block Chevys from 1955 to 1998 (non-roller blocks). If you have a later model engine with a factory roller cam, you could technically use this, but you'd be downgrading your tech. Stick to the flat tappet blocks for this one.
Also, keep an eye on your compression ratio. If you’re running a smog-era 350 with 8:1 compression, this cam is going to be a disaster. It has too much duration; it’ll bleed off all your cylinder pressure and feel gutless. You really want to be in the 9.0:1 to 10.5:1 range to make the 12 600 4 work properly.
Actionable Steps for Your Build
If you are ready to pull the trigger on this cam, don't just buy the box. Do it right so you don't flatten a lobe in the first 500 miles.
- Verify your compression ratio first. If you’re under 9:1, look at a smaller cam like the 12-600-8 (the Mutha' Thumpr) or even a standard High Energy grind.
- Buy the complete kit. Get the 12-600-4 kit that includes the matched lifters and springs. Mixing and matching brands here is a recipe for premature wear.
- Order break-in oil. Brands like Driven Racing Oil or Lucas Oil's high-zinc break-in formula are non-negotiable.
- Set your timing. This cam loves initial timing. You’ll likely end up somewhere around 14 to 18 degrees of initial advance, with a total of 34-36 degrees by 3,000 RPM.
- Check pushrod length. Don't assume your stock pushrods are the right length. Use a $20 checking tool to ensure your rocker arms are centered on the valve stems.
The Comp Cams 12 600 4 is a specific tool for a specific job. It’s for the guy who wants his Chevy to sound like it’s about to eat the car in the next lane, and who is willing to do the extra tuning work to make that happen. It's not a "set it and forget it" part, but the first time you crack the throttle in a parking lot, you'll understand why people keep buying them.