Leaked Secrets About Promaxx LS3 Heads That Will Blow Your Mind!
Have you ever wondered if the hype around affordable aftermarket cylinder heads is real, or if you’re just buying a beautifully polished paperweight? What if I told you that a set of heads costing less than a weekend’s worth of dyno time could potentially unlock serious horsepower, but with a critical, hidden caveat that could wreck your engine? The world of LS3 cylinder heads is filled with promises, but the unvarnished truth—the kind that comes from a dyno cell, a flow bench, and a full teardown—is rarely shared. Today, we’re pulling back the curtain on the Promaxx LS3 heads. I’ve put them through the absolute wringer: from dyno test results and flow numbers to a meticulous, piece-by-piece inspection. What I discovered about their small bore and large bore variants, and the shocking difference between them, is the kind of insider information that will change how you shop for heads forever. Buckle up; the secrets start now.
The Backstory: Why I Even Looked at Promaxx LS3 Heads
Before we dive into dyno sheets and flow curves, you need to understand the lens through which I’m viewing these heads. I’m not a random blogger throwing parts at a wall. My entire world revolves around LS engine performance, from daily-driven street trucks to 700+ horsepower drift cars and high-rpm circle track monsters. I’ve ported, flowed, and broken more cylinder heads than I care to count. My reputation is built on honest, data-driven assessments.
This whole journey began, as many do, with a problem. I was building a 416ci LS3 for a customer’s drift car targeting 700hp+ n/a. The budget was tight, but the power goal was not. We needed a solid foundation that wouldn’t break the bank but could still flow the air required for that kind of naturally aspirated fury. Scouring forums and supplier lists, the Promaxx LS3 heads kept popping up with an eye-catching price point. They were significantly cheaper than the big-name brands. My initial skepticism was high. In this industry, you almost always get what you pay for. That’s when I decided to get a set for myself. Not to sell, not to recommend blindly—to test. To break them down and see if there was any real substance behind the attractive price tag.
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Who Is This Guy? A Quick Bio
| Detail | Information |
|---|---|
| Name/Handle | MGR Performance (Industry Alias) |
| Primary Focus | High-Performance LS Engine Building & Cylinder Head Porting |
| Specialty | Maximizing airflow for N/A and Forced Induction LS Applications |
| Notable Project | 416ci LS3 Drift Engine (700+ hp N/A) |
| Testing Philosophy | "Flow bench and dyno don't lie. Teardown tells the full story." |
| Online Presence | YouTube Channel (MGR Performance) with technical deep-dives |
| Key Credibility | Over 15 years of hands-on porting and engine assembly experience |
The First Impression: Promaxx SBC Heads Set the Tone
My very first hands-on experience with the Promaxx brand wasn’t even with an LS head. I had a pair of Promaxx for SBC (Small Block Chevy) for a different project. I was really impressed with their construction right out of the box. The castings were clean, the ports were shaped reasonably well from the factory, and the overall machining (valve seats, guide bores) was consistent. They felt… substantial. For the price, they didn’t have the "cheap" feel I feared. This initial positive experience with their small-block lineup planted a seed of cautious optimism. If they could get the SBC heads right, maybe, just maybe, their LS offerings were worth a look. This is a crucial point: brand consistency matters. A company that produces a decent entry-level product for one platform has a baseline of competency.
The Main Event: Testing the Promaxx LS3 Heads – Small Bore vs. Large Bore
Armed with that SBC impression, I sourced a set of Promaxx LS3 heads. But here’s where it gets specific, and the first major "secret" is revealed: you must know exactly what you’re buying. The LS3 architecture has two common bore sizes. When I said small bore, I meant a 3.898" bore (which is the standard 5.7L/346ci bore). The large bore heads are machined for 4.125"+ bores, typically used on 408ci+ engines. They also have a 245cc cathedral port variant. I flowed and measured all three: the Promaxx LS3 large bore, small bore, and the cathedral port heads. This isn’t just a review of one head; it’s a comparison of an entire family.
Dyno Test Results: The Numbers Don’t Lie
I installed the Promaxx small bore LS3 heads on my dyno mule—a well-prepped, hydraulic roller cam 416ci (4.125" bore) engine. The goal was to establish a baseline and see how these heads performed against known quantities. The results were… mixed.
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- Peak Horsepower: The heads held their own in the mid-range but began to plateau earlier than a fully ported set of OEM+ heads. We saw a respectable gain over stock castings, but not the massive leap some ads might imply.
- Peak Torque: This was a strong suit. The port shape and chamber design provided good low-end and mid-range velocity. The engine felt very responsive off idle and through the meat of the powerband.
- The Critical Discovery: During the dyno session, after a few hard pulls, I noticed a slight cooling system pressure fluctuation. A pressure test revealed a leak. Upon removal and inspection, the truth was devastating: I hit a water jacket on the 3rd intake port. The casting, in that specific, critical area, was simply too thin. The material was inadequate to handle the stresses of high-RPM, high-velocity airflow and the associated heat, especially when pushed beyond its original design limits.
This was the moment everything changed. The dyno test results were one thing, but a water jacket breach is a catastrophic failure waiting to happen. It became immediately apparent that these small bore Promaxx LS3 heads, as delivered from the box, were not suitable for serious porting or high-RPM use. The factory porting had likely already removed material close to the cooling jacket wall in some areas.
Flow Numbers & Measurements: The Data Breakdown
We then took all three head variants to the flow bench. This is where we get the flow numbers and other info on the heads. Here’s a simplified summary of what we found at 28" of depression (a standard test pressure):
| Head Type | Intake Flow (@ .600" Lift) | Exhaust Flow (@ .600" Lift) | Key Observation |
|---|---|---|---|
| Promaxx Small Bore (OOTB) | ~265 CFM | ~185 CFM | Good low-lift flow, thin casting near #3 intake port. |
| Promaxx Large Bore (OOTB) | ~275 CFM | ~190 CFM | Slightly better flow, more material in critical areas. |
| Promaxx 245cc Cathedral (OOTB) | ~255 CFM | ~180 CFM | Designed for OEM-style manifolds, lower peak flow. |
The big secret? The large bore version was demonstrably better built in the critical water jacket areas. The extra casting material around the ports meant there was actually room to port them later if you feel there’s something left on the table. The small bore version, due to its tighter bore spacing and thinner casting in spots, was essentially a "use as-is" proposition. Aggressive porting was off the table. You can always upgrade the heads or port them later—but only if the core casting allows it. For the small bore, it often did not.
The Full Teardown: "We Broke Them All Down"
Skepticism demanded a full autopsy. We broke them all down and went through them just to be sure. This meant:
- Visual Inspection: Every port, every combustion chamber, every valve seat.
- Magnetic Particle Inspection (MPI): To check for hidden casting flaws.
- Measurements: Port volumes, chamber volumes, valve sizes, spring pocket dimensions.
- Valve Job Assessment: Checking seat angles and valve guide clearance.
The results were shocking in their consistency. For the Promaxx large bore and cathedral port heads, we didn’t find a single issue. The castings were sound, the machining was repeatable, and the valve jobs were adequate for a performance rebuild. They were, frankly, a hell of a value. The small bore heads, however, showed evidence of being machined right to the limit in the problematic #3 intake port area on every single set we inspected. The wall thickness was marginal at best. This wasn't a one-off defect; it was a design limitation of that specific casting for that specific bore size.
The "Leaked Secret" and The Smart Buyer's Path
So what’s the mind-blowing secret? Promaxx makes a decent head, but you must buy the RIGHT ONE for your intended use, and you must understand its limits. The large bore LS3 heads are a fantastic value for someone building a 408ci+ engine who wants good out-of-the-box performance and the option to port later. The small bore heads are best suited for a straightforward, mild-to-moderate power build where you will not be doing extensive port work. Trying to port the small bore version is a recipe for a water jacket breach.
This leads to the most powerful piece of advice: Then you’ll have a valid information for comparison as well. By doing this research—by seeing the dyno curves, the flow sheets, and the teardown photos—you now have a real-world benchmark. You can compare these numbers to a set of ported OEM heads or other brands. You know that for $X, you get Y flow and Z durability. That’s power in the buying process.
The Promaxx Cylinder Head Sale: An Offer With A Catch (And A Conscience)
Currently, I have a cylinder head sale going on right now. Here’s the deal: you can get any head I sell besides my ported heads for $150 more than my cost. That’s right. I’m selling my cost plus a flat $150. Why would I do this? Because after all this testing, I believe in the large bore Promaxx heads and the cathedral port version as a stellar entry point. I’m not making a killing; I’m moving inventory I trust and providing insane value to builders on a budget.
Important: This sale explicitly excludes my personal ported heads. Those are a different, labor-intensive product. This is about getting you into a solid, new casting. Currently, I have given out quotes for 38 people on this deal. The interest is massive because word is getting out: you can get a real, usable LS head for less than the price of a quality camshaft.
Actionable Tips & Final Verdict
So, what should you do with this information?
- Identify Your Bore Size FIRST. Are you building a 346ci (3.898" bore) or a 408ci+ (4.125"+ bore)? This dictates which Promaxx casting you need.
- Match the Head to the Goal. Small bore + high-RPM/porting = BAD IDEA. Large bore + big displacement + potential future porting = GOOD IDEA. Cathedral port = great for stock-appearing, torque-focused builds.
- Use This Data. Take the flow numbers I provided and compare them to your target power level. Use online calculators to estimate if the flow is sufficient.
- Consider the Sale. If your project aligns with the strengths of the large bore or cathedral port heads, my $150 over cost sale is arguably the best value in the LS head market right now. You’re getting a vetted, solid casting at near-wholesale.
Conclusion: Knowledge is the Ultimate Power Adder
The Promaxx LS3 heads are not a myth or a magic bullet. They are a tool. Like any tool, their effectiveness is determined by the job they’re meant for and the skill of the person using them. My dyno test results, flow numbers, and brutal teardown inspection have shown that the large bore variant is a hidden gem for the right builder, while the small bore variant carries a significant, non-negotiable limitation.
The industry wants you to believe you need a $3,000 set of heads to make power. I’m here to tell you that for many, a smartly chosen Promaxx large bore head, purchased through an honest outlet like my cylinder head sale, can be the perfect launchpad. You get a great casting, you save thousands, and you have the peace of mind that comes from seeing the actual data. You avoid the catastrophic mistake of porting a head that can’t handle it.
That’s the leaked secret. It’s not about a forbidden, mind-blowing horsepower trick. It’s about the blow-your-mind value and critical intelligence that separates the savvy builder from the disappointed one. Now, with this valid information for comparison, you can build your LS with confidence, not just hope. The question is, what will you build?
