Nissan VQ35DE Build - The All-Motor VQ35DE

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Nissan VQ35DE Build - The All-Motor VQ35DE

Ever since we drove Endless Racing's Z Car Challenge machine in Japan (naturally aspirated, 400bhp), building the same engine here has topped our list of geek obsessions. The only reason we haven't gone ahead with it is because Project Z has already been committed to turbocharged power. Plus, the cost of building such a fully prepped engine gets pretty outrageous-and it would only drink 100-octane. But automotive karma works in strange ways.
Castrol has invited us to face off against our sister magazines (Super Street, Modified, eurotuner, Import Tuner, Turbo, and Lowrider) by teaming up with a shop of our choice to build a no-compromise engine in its Castrol Syntec Top Shop challenge. Naturally, everyone else is going for the one-pull-wonder, ranging from a GM crate motor small-block with twin turbos to the usual big-power 2JZ or 4G63. The competition is based on peak horsepower and torque per displacement (displacement is multiplied by two for anything with forced induction), total horsepower under the curve, a 30-minute endurance death match (all tests administered on an engine dyno), and an engineering challenge to impress a panel of engine-guru judges. All of which sounds suspiciously like our USCC rules. We're looking to build something real-world that might one day end up in some lucky raffle-winning schmuck's engine bay (the victorious engine will be given away to a member of the public).
Picking a naturally aspirated (NA) engine to go into a horsepower battle with turbo engines might not sound like the smartest thing to do, until you look at the rules and consider real-world performance. For example, if we chose to build a 3.0-liter Supra motor targeted at 1000bhp, the numbers would break down to around 166bhp/liter, since a turbo motor has its displacement multiplied by two. A 5.7- liter small-block would really suffer, even at a wishful 1500bhp. That would land it somewhere around 131bhp/liter, just a little more than Honda's out-of-the-box S2000 engine.
Even if we can't make the most power per displacement with a naturally aspirated VQ, there's another side of the equation: power delivery, or area under the curve. If you've ever seen the power curve of a big-turbo, small-displacement 1000bhp car, it's essentially useless until the last 1000rpm and undriveable, regardless of engine speed. For most of the powerband, the engine will be struggling to make a fraction of its peak power until the monster turbo needed to flow this amount of air finally spools and sky-rockets the power at an uncontrollable rate. Do the math and the area under the power spike won't compare to the steady power coming from a naturally aspirated engine.
The Engine And The Shop
We decided on Nissan's VQ35DE as our platform. It has the ideal combination of displacement, rpm, fundamental design and flow capabilities, plus a respectable amount of low-end torque. Add in the 100-octane gas everyone will be using and we'll be able to raise the stock compression ratio to a respectable race engine standard. Our aim is to build an easily replicated, street-usable, 400bhp naturally aspirated VQ. Several respectable tuners like Tomei, Nismo, Cosworth and Jun Auto already have extensive research and racing programs based around this V6.
When it came to picking a shop among the notable VQ tuners, we decided on Cosworth Engineering for several reasons: they spoke English (though with a strange accent at times), its US headquarters are right in our backyard here in Southern California, most of the parts we would be using are off-the-shelf, and, mostly, because few organizations have the engineering capabilities, expertise and experience to rival Cosworth.
Geek theory
By picking a manufacturer like Cosworth, we suddenly have a wealth of engineering resources available: engine simulation software, flow bench data and computational fluid dynamics (CFD) head flow analysis. These guys possess the same tools OEMs use to design an engine from scratch. In theory, We could run engine simulations of how different bore, stroke, rod length, piston, compression ratio and displacement combinations would offer optimum power, torque, flow and engine speeds.
Our original hope was to reverse-engineer the VQ and analyze the engine to see why Nissan made certain choices in its design and what could be optimized without sacrificing durability or its innate character. But while Cosworth has the technology, the labor involved is far beyond the scope of this project-this isn't an IRL or an F1 engine we're talking about here.
As powerful as these analysis and modeling tools are, they're still only guides to be used in conjunction with practical and realworld knowledge. So we've gone for a more conventional method of design and tuning, working around the VQ's basic architecture.
Real-world tuning
Our high-output VQ will be loosely based on Cosworth's concept for a drop-in crate motor, featuring an assortment of parts already released or in testing. The Top Shop motor will take it up another notch.
Since we're using a stock block, our engine design is constrained by some basic physical limitations. The Castrol Syntec Top Shop Challenge makes no restrictions on displacement or flow, so we've started from the bottom end to aximize our total displacement and take advantage of the lack of a displacement modifier for NA engines.
A simple approach to increasing displacement is to punch out a motor as far as its bore spacing will allow and add as much stroke as possible. This works for old rev-limited V8s with cast iron blocks (where piston speeds aren't such a concern, since reciprocation mass ultimately limits revs). But in a VQ with the potential to spin up to 9000rpm, that's not the case. From a basic displacement perspective, the available room allows Cosworth to increase stock stroke by 6mm from 81.4 to 87.4mm and bore (limited by the factory bore spacing) from 95.5 to 96.0mm, increasing total displacement from 3498cc to 3796cc, or 3.8 liters.
Increasing the stroke by 6mm has a noticeable effect on piston speeds-a significant concern for an NA engine designed for highrev power. The original dimensions are over-square (larger bore than stroke) with a bore/stroke ratio of 1.17, while the 3.8-liter dimensions would bring the engine closer to square (equal bore and stroke) with a ratio of 1.10. Although the engine is still over-square, which is typically good for higher revs, the mean piston speed at 8500rpm has increased from 23.1m/s to 24.8m/s; 25 m/s is roughly F1 engine territory.
However, mean piston speed (or the average speed of a piston through one revolution) only helps classify the type of engine. Higher speeds usually mean higher performance. When changing an engine's internal geometry, such as adding stroke and changing rod lengths, what matters more is the piston velocity profile and the amount of sideways thrust added to the piston.
When an engine is stroked, the rod journals get moved further outboard from the crank centerline. The original VQ35DE has a stroke of 81.4mm, meaning the rod journal rotates 40.7mm (or half the stroke) from the crank center axis. This is because an engine's stroke is the total vertical distance the piston travels as the crank rotates 180 degrees between one and six o'clock, which translates exactly to the distance the rod journal has to travel. By adding 6mm of stroke, Cosworth had to increase the journal offset radius by 3mm. The added radius has a side effect, though, since it will shove the piston harder against the cylinder wall as the crank sweeps from one o'clock to five o'clock on the combustion stroke. This can increase wear, drag and may introduce ring flutter and piston wobble at high speeds.
Increasing the journal offset radius by 3mm also brings up another issue: now the piston will pop 3mm above the top of the block into the head at TDC. Since the VQ's deck height can't be changed, there are only two options to compensate for this. Either shorten the rod by 3mm, or modify the piston design and push the piston wrist pin position up by 3mm so the piston sits lower than stock.
Here's where rod ratios come into play. Rod ratio is basically rod length divided by stroke length. Long rods or short strokes will have a large rod ratio (roughly 2:1), like high-revving engines such as the B16B or SR16DE. Short rods or long stroke will have a low rod ratio (approaching 1.5:1) like Nissan's 2.5-liter QR25. These can't spin fast, but have massive low-end torque. While rod ratio has implications on torque and engine speed, it's more an indication of how much side load the pistons are subjected to.
In our case, if rod length was decreased by 3mm to match the stock deck height, we would end up with a lower rod ratio of 1.61:1. Imagine the right triangle formed by rod, crank and piston position. A larger journal radius (which makes up the horizontal leg of the triangle) combined with a shorter rod (the diagonal leg) will increase the angle the rod forms to the cylinder wall. The larger the angle, the harder the piston shoves sideways into the cylinder wall. The increased rod angle would also require shorter piston skirts that might compound the issue of piston wobble.
In the VQ35DE, using a shorter rod to compensate for the stroke increase would be a double whammy. Instead, Cosworth chose to reduce the wrist pin depth and retain the stock rod length.
This gives a slightly higher rod ratio of 1.65, compared to the stock VQ35DE's 1.77, resulting in a peak piston speed increase of 9m/s (or, for ease of comprehension, 22mph). The only compromise here is that the ring packs also have to be moved up, reducing the space available for each ring land. Loss of ring land is bad, since it doesn't hold up as well under knock. But as we're running 100-octane gas under NA power, the trade-off is minimal. Small ring lands are also cleaner on emissions and could provide a fractional increase in power.
Compression ratio is only limited by the fuel used. And since our engine will deploy off-the-shelf, CNC-ported Cosworth cylinder heads-designed for both turbocharged or NA applications-our 12.1:1 compression ratio will come strictly from the high-compression Cosworth slugs thrown in. This ratio is based on previous experience, since combustion efficiency, chamber design, tuning and octane all interact to restrict how much compression an engine can run.
That takes care of our bottom end. Next time, as we continue work on our Castrol Syntec Top Shop Challenge engine, we'll address all the less analytical and more black-art stuff requiring more real-world testing, like heads, ports, cams and tuning.
The Rules
Engine Setup
* Only production engines from production based cars are allowed
* Only one forced induction system permitted (turbocharger or supercharger systems are allowed but nitrous oxide is not)
* Factory turbo or supercharger systems count as one forced induction system
* No methanol, auxiliary fuel or water injection is allowed
* Engine management systems are open. The competition's tests will be performed on an engine dyno
* Custom parts are allowed. Aftermarket parts may be modified
* Heat-treating, cryo-treating and thermal coating is permitted
* No welding of the cylinder head to the engine block
* Testing will be done with no muffler, but the header or exhaust manifold(s) must fit a production chassis
Fuel And Oil
* Engines are required to run on spec 100-octane gas. Additives or oxidizers for the gasoline are not allowed. Gasoline will be distributed at the test location with fuel testing done before and after the dyno session
* Engines are required to run with Castrol Syntec oil only. Oil weight is open, as long as it's an off-the-shelf Castrol Syntec oil weight. No oil additives are allowed
The Tests
* Peak horsepower and torque per liter of displacement. Turbo and supercharged engines are given a displacement multiplier of two. If a 3.0-liter turbo motor makes 600hp, then 600hp/(3 liter x 2 multiplier) = 100hp per liter. Naturally aspirated motors receive no multiplier. Exception: rotaries (13B, 20B, etc. will have a 2x displacement multiplier)
* Horsepower under the curve
* Build quality and craftsmanship (judged by a panel of three experts)
* Survive a 30-minute drive cycle

Photo Gallery: Nissan VQ35DE Build - Sport Compact Car Magazine

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