Choosing The Car – Part 2

So now that I've covered the basics of the options of the 350z, I have to pick which one to buy.

The standard factor in autocross is usually grab the lightest, followed by the most powerful. Simple equation. Not so much in this case..

As I mentioned earlier, the chassis among all 3 variants is identical, so I don't need to chasing a certain year with better geometry, or a faster steering rack. The same gear box  and rear end is also used, so no better gearing ratio to be had.

On the weight factor, I couldn't find anywhere specifically that there was 151 more lbs added to the HR variant cars. No mention of more supple powered airbag seats, or a steel vs aluminum hood.. nothing nice an clear cut. so I have to assume that weight is there to stay.

So, engine then? On two factors we can easily eliminate one candidate from the running, the 06 Up-Revs. They weigh as much as the 07-08 HRs, and have less horsepower and torque. On top of that big disadvantage they supposedly have oil consumption issues, possibly a byproduct of being spun harder (7000 RPM vs 6500) with (rumored) only a mild rework of the internals. No, an 06 is definitely off the table.

Continuing on the horsepower comparison, we're down to the orignal DE engine versus the new higher strung HR engine. There's defintely a noticeable output difference between these two engines. the wiki pages put them at 287 and 306 hp, 274 and 268 lb/ft respectively. Even at 151 lbs heavier, the HR has a power/weight ratio advantage over the DE, but is down on torque. Not good. But these are factory numbers.. separated by different SAE calculations. So I went digging, and came across a most helpful forum post: Stock DE vs Revup vs HR vs VHR numbers

[caption id="attachment_19" align="aligncenter" width="300"] DE Engine Dyno
234.8 HP - 222 Lb/Ft[/caption]

[caption id="attachment_20" align="aligncenter" width="300"] HR Engine Dyno
260.9 HP - 227.1 Lb/Ft[/caption]

Well, looks like there's a bit bigger gap there than originally perceived. Take note when looking at those graphs that they are on a different RPM scale. The DE engine revs to 6500 RPM, while the HR spins all the way out to 7500.  Adjusting for this, the HR has a very similar torque curve to the DE, and has a noticeably more aggressive HP curve.

This is all base horse power though.. what about post modification? maybe that air intake just frees up a ton of horsepower on the DE and is less effective on the HR. This too has been factored into my decision. It appears one of the biggest hold backs of the DE and UpRev engines were their intake manifold, as adding a spacer to the big pancake of a plenum frees up significant horsepower... horsepower that would be completely unavailable in STU class. Intake modifications are allowed up-to but not including the throttle body. The HR engines took care of this issue. So much so, that it required a different hood to clearance the engine. The replacement manifold no longer being a single plenum, but dual independent manifolds with a throttle body for each.

[caption id="attachment_23" align="aligncenter" width="300"] Notice the large Pancake plenum feeding the single throttle body?[/caption]

[caption id="attachment_21" align="aligncenter" width="300"] Unfortunately mostly covered by plastic, there are two independent manifolds, feeding all the way up into separate ram are intakes[/caption]

But the differences don't stop at the intake. The HR engine also received redesigned high-flow heads containing both intake and exhaust variable cam timing, as well as a compression ratio raised from 10.3 to 10.6:1. All of these differences seem to me like there's a lot more room for horsepower in the legal realm.

So, now that I've gushed all over about how much better the HR engine is, you'd think by this point that's really the deciding factor. and you'd be right, but maybe not for the reasons you'd think.

The real answer is... Tires.
In STU legal trim, the maximum width tire is 285mm initial digging has shown no shortage of people putting 275mm up front and 315mm tires in the back. Squeezing a squared set of 285mm tires all the way around shouldn't be a problem, and has already been done by Brian Peters on his STR winning 350z (DE).

If the chassis is the same, how do tires come into play in this decision? Well, it's slightly less about the tires, and more about the gearing. As I mentioned before, all 350Z use the same gearbox and rear end.

Gear Ratios (that I care about)

• First: 3.794


• Second: 2.324


• Third: 1.624


• Final: 3.538

use some maths (or an internet calculator), pair this up with the stock tire size (245/45-18) on the rear end, and a stock DE will be going 64 Mph when it hits the redline in second gear. Now, our younger HR brother gets 1000 extra RPM to play with.. meaning it's second gear tops out at 72 MPH. Now 64 is a nice sweet spot for autocross speeds, and topping out would be pretty rare.. but this is on the stock tire size. The whole goal here was to shove 285s under this thing and build a rubber monster. There's only two sizes of 285 that will fit the 350z.. a 285/35-18 (Hankook RS3) or a 285/30-18 (Dunlop ZII, Bridgestone RE-11, and Yokohama AD08). On a DE car, the 35 series tire will top out at 62 MPH.. the 30 series, a paltry 59 MPH, where as the HR engine will top out at 70 and 67 respectively. 59 is pretty much unacceptable and 62 is definitely lower than I'd like on car that will have this much horsepower and grip. It is legal to bump up the rev limiter in Street Touring classes, but with the up-rev engines being so similar to the original DE, but still being reinforced to handle only 500 more RPM, I don't know how comfortable I would be doing that.. especially on something that could be over a decade old by this point.

Then we factor in the tires themselves.. the Hankooks having a history of being squishy and demanding of heat may not work well In the northern climate, where as the options from Dunlop, Bridgestone, and Yokohama are all considerably more favorable to our mixed temperatures as well as being a generation newer tire.

So, after clearly some real thought into this.. it's settled.I'm seeking a 2007-2008 Nissan 350z Enthusiast.