ldamelio

What do folks think about fuel for the track for the 718S? I'm a noob and just getting started in HPDE with my first track event in a couple of weeks. By definition, I won't be running the car at 10/10 (at least for now). Track (NJMP) has 93 and 100 available. Aside from cost, any pros/cons to using either of these for beginner HPDE? How about later if I do get close to running 10/10 as I progress?

iliveoncaffiene

If you're not running 10/10, 100 octane won't make too much of a difference. It'd probably be the equivalent of getting a GTS instead of an S (93 vs. 100 in terms of power)
Also, I'm not sure if the ECU is programmed to take any advantage of 100 octane. If so, you won't even get any benefit to running 100 octane in the first place

manifold danger

Here- http://www.badasscars.com/index.cfm/...rod/prd427.htm

I actually took the time to do a little internet sleuthing and discovered I had a bit to learn in this department.

Short answer- race gas belongs in race cars. If you don't already know that your car needs race gas, then it's probably best to avoid it. Because race gas is designed to make the detonation COOLER, you will probably actually lose power in a factory car.

edit- A couple interesting things from more poking around the internet... the 718s have a compression ratio of 9.5... which is lower than the 981 because it's turbocharged. My 981 has a compression ratio of 12.5... which honestly is the highest I've ever heard of for a factory car. 9.5:1 seems pretty high for a turbo car too- I remember how everyone considered the 2000 Civic Si's 10.2:1 really high back in my youth, and that car is obviously naturally aspirated. Fun.

Anyway, you could probably be fine with ~100 octane unleaded, but I wouldn't expect to notice anything significant one way or the other (but you will have a cool smell that will remind you of all the fun you had for a few days after). If it's leaded, I'd stay away because it will likely be pretty harmful for your catalytic converters.

ldamelio

Great link, thanks!. I learned a lot. 93 it is.

spdracerut

Yup, don't run much more than what the engine is calibrated for. Every car tuned to run on Premium in the US can run on 91oct. For track use, especially in hot weather, a little extra octane can help prevent detonation. So 93 octane would do the trick. The only reason to use 100oct is if the car were actually tuned for it.

Vista6019

No need for Race fuel in these vehicles. A good grade 91 or 93 should do the trick..

85Gold

Wild Weasel

Firstly, to answer the OP... use 93. The car is built and tuned to run on 93 so that's where you'll get optimum performance. It'll run without damage on 91 but it'll adjust for it. It will not take advantage of anything higher. If someone had an aftermarket tune for 100 hp that cranked up the boost or something then there could be an argument for it but without changing anything, you won't see an improvement.

Next... if your track is anything like mine is, anything you buy at the track is probably hugely expensive. Fill up before you get there. I leave the track at lunch and go fill up at the nearest gas station that has 93. The pump at the track is convenient, but good God is it ever expensive!!

Now a little explainer on this. If you're thinking that a 981 needs higher octane fuel because of the higher compression ratio and that the 718 compression ratio is relatively low in comparison then you're missing the full story. Almost all boosted (turbo/supercharged) engines have relatively low compression ratios compared to high performance NA engines. This is because the air is already being compressed before it gets in there in the first place, so even with the lower ratio the actual amount of pressure it's put under is much higher. Higher octane fuels are designed to resist exploding under pressure so you can control the ignition with the spark plugs. It's not ultimately the compression ratio of the cylinder that matters. It's how much the air/fuel mixture gets compressed. Think of it as being squished twice. First before it gets into the cylinder, and then even more so when the piston comes up.

manifold danger

I get that, I was just musing about how surprising it was to see how high the compression ratio was for the 718 being a turbo car. I used to think 10:1 was high even for naturally aspirated; I remember back in the day this was a regular discussion when me and my friends were running around turbocharging stock engines. Motors like the ka24de in the US market 240sx were super boost friendly because they had a 8.6:1 compression ratio, whereas cars like the aforementioned civic si were finicky because of the relatively high 10.2:1, and usually required being built to get any real power out of (didn't stop people from trying, but anything past 5-6 psi and they'd blow **** up). Then along came the Celica GT-S and Honda S2000 with 11:1 and blew everyone's minds. Seeing the 12.5:1 for the 981 was pretty shocking (and virtually all NA Porsche flat 6s as far as I can tell are at least this high). I guess I never thought about it but it was a pleasant reminder that these cars don't share the same degree of compromise that most other marques are subject to.

Just interesting throwback conversation I found interesting, that's all.

Wild Weasel

Mazda makes an engine with 14:1 compression that runs on 87 octane. That blows my mind!!

manifold danger

Mazda has always been one to push the envelope on engine innovation. Nobody else was ballsy enough to build a rotary except Mazda. Now they're like, no spark plugs? NO PROBLEM*!

But they won't bring back the RX-7, so they're dead to me.

wriggly

wriggly

85Gold

GrantG

In places where only 91 is available, adding a couple/few gallons of race gas to the 91 to bump the octane to 93 or a bit more is helpful. Just make sure to avoid the Leaded race fuel.

BTW, my new GT3 has 13.3 compression but my modified Evo IX requires much more octane (8.8 compression with 24 psi boost) - luckily a switchable E85 ECU map does the trick (106 Octane for $1.85 per gallon)