1997 Mazda MX-5 Miata: Valvetrain Tech
June 21, 2012
Ported head, check. But port work, while a critical ingredient in a proper high-performance cylinder head, is best served with hardware that can fully exploit it. Running a ported head with stock valves and cams would be suboptimal, like training for a marathon wearing Florsheims.
And when it comes to high-performance VVT Miata heads, upgrading the valvetrain also creates an opportunity to enhance its durability.
Underpinning this bootstrap effect in Project Miata's Keegan Engineering-ported head is a thoroughly overhauled valvetrain. Fortunately, Supertech makes this easy -- they manufacture a whole range of high-performance valvetrain hardware for many auto makes, and it's engineered to work together.
Project Miata's new head has Supertech +1mm valves, their single valve spring kit, titanium retainers, shim-under-bucket conversion and Viton valve seals. It's all in support of our goal of maximizing this little rat's area under the curve on 91 octane. Here's the nitty gritty.
Larger valves are a straightforward way of increasing flow - the increased valve curtain area they provide is easy to envision. As a general rule, valves that are 1mm larger than the stock Miata valves provide a meaningful increase in flow potential with no real downside. And with the Miata head, Keegan has showed us that properly unshrouding the valves will make any valve perform to its potential.
Supertech offers a wide variety of valves in stainless, Inconel and titanium. We went with their entry-level MAIVN-1206 and MAEVN-1206 stainless "+1mm" valves for the intake and exhaust sides, bringing them to 34mm and 29mm, respectively.
Ever wonder why an engine's exhaust valves are always smaller than its intake valves? It's because the speed of sound rises with temperature. Hot gases don't "choke" a valve as readily as do cooler gases, so the hotter the gas, the more of it you can cram through a valve of a given size... provided that gas has enough pressure to offset the density lost because it was hot. Good thing the exhaust side (unlike the gas on the intake side) has cylinder pressure to help bust that gas out. The more you know...
Supertech's stainless valves are made of SAE EV8 stainless steel, a grade of austenitic stainless with high chromium and manganese content that has been developed specifically for high-temperature valve use. The slight necking-down of the valve stem means there's less blockage of incoming airflow, and the black nitrided finish provides a hard, smooth exterior while the underlying steel structure retains ductility.
One neat touch - Supertech's valves stems are purposely sized 0.6mm too long. This extra stock can be sized specifically for the depth of your valve seats and selection of shims. This is one example of why it's important have your high-performance engine assembled by somebody who knows what he or she is doing, or, if DIY-ing, to ensure you're paying attention, know what to measure, have the right equipment and know how to use it.
Higher-Rate Single Valve Springs
Up through model year 1997, Miatas all had a hydraulic lifter valvetrain. Hydraulic lifters use engine oil to take up valve clearance, or lash, providing a maintenance-free valvetrain for the life of the engine.
Unfortunately, hydraulic lifters are heavy. Heavy things in a valvetrain that are accelerated and decelerated many thousands of times per minute induce higher forces and are harder to control than lighter ones. Ultimately the total valvetrain mass and spring rate limits how aggressively the cam can open the valve at high engine speeds. In simple terms, more valvetrain mass makes your cams wimpier or your redline smaller, or both.
This reality is partly why Mazda switched to a solid lifter setup in 1999. The solid lifters are much lighter, reducing valvetrain forces, which in turn allowed a more aggressive cam to be fit. These changes had a lot to do with the '99 Miata's bump in power and torque.
Solid lifters use shims to set valve lash or clearance. Unlike hydraulic lifters, the valve lash of solid lifters requires periodic inspection and adjustment if necessary. It's a fair tradeoff for a better valvetrain.
In 2001, Mazda again tweaked the head, this time with variable intake valve timing. The solid lifter setup was retained, and a yet-slightly more aggressive cam profile was added. With this setup, little margin was retained. Racers that have raised their rev limits on stock VVT Miata heads learned this the hard way -- destructive valve float occurs just 300 rpm after the stock fuel cut of 7200 rpm.
With our deep-breathing head and strong bottom end, that's an arbitrary rev limitation we just can't abide. The solution is less reciprocating mass and higher-rate valve springs to raise the rev threshold at which valve float occurs and once again, Supertech has this covered with their single valve spring kit and titanium retainers.
Supertech's entry-level single valve springs (more aggressive dual spring setups are also available for the Miata) and lightweight titanium retainers are integral components of a valvetrain that won't hem us into a buzzkill redline or a weaksauce cam.
Spring surge is a phenomenon that can occur when the natural frequency of the spring itself is excited by the speed at which the cam is driving it. Higher rate raises a spring's natural frequency, and so does lower weight. However, a stiffer spring typically has more mass, and this can create a surge situation.
Ideally you want a higher spring rate to avoid float and a low spring mass to avoid surge, a combination that requires specialized spring steel. Supertech's SPR-MM18S single valve spring kit neatly addresses this conundrum, with a 24% higher rate on the intake side while weighing 23% less than the stock spring. Adding additional lightness are the company's titanium retainers, which replace the stock steel ones.
Shim Under Bucket Conversion
It turns out that the factory solid lifter setup still isn't as light as it could be. The stock buckets are mass-produced items that haven't had every gram pared away since, hey, the stock valvetrain works fine as long as everything stays stock. And because the stock lifter setup places the shims between the cam lobes and the lifters, each shim is necessarily as large as the diameter of the lifter. There is opportunity for improvement.
Supertech's CF-30/16LC shim-under-bucket conversion is that improvement. It replaces the factory buckets with new ones that relocate the shim to beneath the lifter (or bucket). In this location, the shim only needs to be as big as the top of the valve stem (6 mm) instead of the entire surface of the lifter like in the stock setup. The buckets in Supertech's shim-under-bucket conversion are some 30% lighter than the factory ones, and the shims are a fraction of the stockers' mass.
Shim-under-bucket is a standard practice in high-performance valvetrains and can be found on many stock sportbike engines.
Moving the shim entirely away from the cam lobe in this manner also removes any possibility of the shim being dislodged by a big cam lobe and shot out at high rpm. Usually when the shim is tiddlywinked in this way, it wreaks havoc elsewhere in the head, not to mention the shimless lifter that's now running with more clearance than Agent Kay and the cam smacking said lifter into oblivion.
Here's how the component weights shake out:
|Supertech||Stock (VVT head)|
So on the intake side, we've reduced reciprocating valvetrain mass on the intake side by 14% (17 grams) and by 18% (21 grams) on the exhaust (I counted half of the spring's mass as reciprocating). Not listed here are the shims, which represent an additional mass savings.
It's all ready for a pair of cams to make the whole thing come together. We're working with Miata specialists Trackspeed Engineering on cams that play nicely with the above hardware, and will have more geeky tidbits for you soon.
--Jason Kavanagh, Engineering Editor
Supertech Performance - http://www.supertechperformance.com/
Trackspeed Engineering - http://trackspeedengineering.com/