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Tofu
 
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Joined: May 2006
Posts: 1
 
The fact that the VQ35DE makes the same 300bhp as the 4.6 3 valve is a simple reflection of basic engineering, marketing, and financial and philosophical differences - not a question of "why is one good and the other weak".

Take a look at some of the differences between the two engines, and some of the things that influenced the engineers:

The VQ35DE makes the power it does simply because it breathes extremely well, has very precise and expensively machined parts, and had a better cost amortization than the 4.6L 3 valve.

For instance, the heads have very large, direct intake runners that are oriented very straight, with no shrouds and few obstructions. The finish on the ports is also very fine, close to the finish on the extrude honed SVT Contour ports. Both the intake and exhaust valves are large, and there's a total of four per cylinder, not three - there's more relative air flowing through those four valves given the cylinder displacement than the 4.6L. Remember that the original 4.6L engine was not really a "heavy lung" engine at all - opening up and reshaping the intake ports on the PI heads gave great gains in power.

The reciprocating mass of the VQ35DE is also very precise. The crank has microfinished journals, the rods are matched to each piston and cylinder by both size and weight - which is very rare for mainstream production engines - and the pistons have a moly coating and shortened skirts for lighter weight and lower friction. The rods also have tapered small ends for less mass around the piston pin. The big ends of the rods are mated to the rod caps with small dowels, instead of the cracked cap design on 4.6L rods. There are also piston cooling oil squirters in the block to help cool down the pistons at high engine speed. Basically, there's a lot of aftermarket tricks already present in the production version of the VQ35DE. There's less frictional loss in the head as well - the cams act directly on bucket tappets instead of on rockers with hydraulic lifters like on a 4.6L.

Better breathing, higher revving equals more efficiency, which equals 280bhp from a 3.5L engine. The 300bhp version adds slightly lighter pistons and rods, a little more aggressive cams, stiffer valve springs, and stronger retainers; basically a little additional strengthening to allow a more aggressive fuel and air strategy to get the extra twenty horses.

But, let's look at some other realities here. First, those 300bhp come accompanied by less torque generated at a higher rpm, so you'll have to work the VQ35DE a bit more than a 4.6L - think Honduh VTEC. Second, some of the engineering choices present in the 4.6L are there for reasons other than ultimate power - hydraulic lifters don't require valve adjustment, producing a two cam engine is cheaper than producing a four cam engine, producting a cracked cap rod is much easier and cheaper than producing a doweled cap rod. The 4.6L is also much simpler than the VQ35DE - for example, changing the timing cover on a VQ35DE is an eleven hour job (by factory techs!) that requires hoisting the engine out of the car.

Economies of scale also come into play here - the a version VQ35DE goes into nearly every car Nissan sells in the US (except the bottom and top of the ladder), while the 4.6L 3 valve is actually pretty unique when compared to its 2 valve cousins. That Ford could have made 4.6L 4 valve V8 that produced 400+ bhp is certain - of course they could have, but they damn sure weren't going to be able to do it at the same price point. What they did do was make a very nice, tractable, and powerful engine that fulfills 99% of most anyone's needs that doesn't require huge amounts of maintenance, special care, or arcane trickery to keep going, and stuck it in a car that mere mortals could aspire to own. In its own way, that's a bigger achievement than simply seeing who can squeeze what number out of what sized engine. (Besides, then we'd end up talking about the "weak" VQ35DE with its 300bhp when 1.4L turbo Renault R5s were making 350bhp back in '80s)

Horsepower numbers are pretty meaningless anyhow, and the only more pointless number is horsepower per liter. All that matters is how you use what you got in the real world.

Oh yeah, I have an '06 Pony.
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