GRM Exclusive Reveal: Exocet Spec-A

Y'all are NOT helping my automotive ADD. I have to agree that, while a V8 version is the shizznit, a turbo 1.8 will be nearly as fast without the weight penalty and will be much easier on consumeables like tires and brake pads. For the average schmuckazoid (like me ), 15's are much more affordable than 17's.

Then again some of you shoestring types make me envious because you have room to work on cars and keep 3-4 half finished projects around :)

I understand the appeal of a super-easy kit, but the more power that you throw in there, the more that I'm bothered by the weight distribution. Good luck getting V8 power successfully to the rear wheels with that lump up front. My Stalker with the old, underpowered V6 (200 ft lbs, 170 HP) was difficult to hook up coming out of slow corners. You put 1.5x the power into a similar rig while moving the weight distribution towards the nose, and it's less than ideal.

I take it that the only easy way to improve weight distribution is to keep the drivetrane location and extend the chassis and front subframe forward?

In reply to nderwater and kreb:

First off, the weight distribution change of an LSx and T56 isn't as bad as you think. You're replacing a heavy iron-block engine with one of the lightest all-aluminum V-8s out there. The T56 and rear diff add considerable weight, but they're not at the front of the car. The rearward-mounted position of a V8 Miata puts the engine's midpoint right at the front contact patches. We're not talking Audi engine-placement, here.

The driver sits a bit further back in an Exocet, but yeah, the only way to balance it out would be to extend the motor mounts back a little. As far as "improving" the weight balance, you really need to consider what you want out of it. In a car with this power/weight ratio, I WANT a bit of front weight and more polar moment to make it more forgiving and to get some heat in the front tires. 430hp gives you a power-to-weight ratio of 3.2 lb/hp, which is better than a McLaren F1. That is the equivalent to putting 850hp in a C6 Corvette. Any way you look at it, you're going to have wheelspin on demand with those numbers.

Ultimately, you have to decide what kind of car you want out of this. The Exocet is not a car designed for the numbers, it's a car designed to be fun and predictable. Yes, you can be faster with more rear weight bias, but the car will not be as "chuckable" or fun. We're not making a formula car, we're making a giggle machine. Cheap, effortless, natrually-aspirated, controllable acceleration is my definition of fun. I've driven rear-biased cars with high hp/weight ratios, and it's a balancing act more akin to work than fun. If you want to tear apart everyone at the track, you'd be better off with buying an older formula racecar or an Atom 500.

I did a little bit of math to figure out where we're at with a 53% front-weight LS3-powered Exocet. (That's my math for a worst-case scenario, I suspect it will be a bit more rear-biased than that). With F-Body T-56 ratios and a 3.23 rear end, you're traction-limited all the way through third gear. That said, the acceleration is 0.77g!. You can launch in third with no problems. 0-60 is 3.5s, 0-100 is 6 seconds, and that's on regular summer rubber. Hoosier R6s at a longitudinal traction ratio of 1.4 would get you 0.90g of acceleration. 0-60 would be 3.0s, and 0-100 would be 5.0s.

Here's my math: https://docs.google.com/spreadsheet/pub?key=0AvUlKfewYcCNdEZXdGFYUnZWT1NhQlhuOTg2SHhnbFE&single=true&gid=4&output=html

Do you really need more traction than that? Personally, 0.77g is more than enough grip for me. If you want to accelerate harder than that, you can add weight to the rear end. 100 extra lbs on the rear gets you to 50% front, and a 200lb passenger gets you to 51% front. With that extra 100 lbs on the rear, you'll be right below the limit of traction in third gear and be able to accelerate up to 0.82g. The difference between 0.77g and 0.82g is very tiny, and handling of the car would be worse with that extra rear weight. Acceleration times would not change.

53% is about what you can expect from a Miata with an LS3. What's a normal Miata-powered Exocet look like for balance?

Personally, I'd like a bit of rear weight bias. My Seven is like that, and it's a very happy little machine. But that's pretty hard to pull off with the engine up front.

With a 180lb driver and 5 gallons in the tank, the 1.8-powered Exocet is right on the nose at 50/50. The tank is 13" further back and over the rear wheels, so more gas = more go.

In reply to Keith Tanner:

Just for fun, I tossed an LS3-powered NB in that little calculator. Do those numbers look reasonably right for warm summer rubber? https://docs.google.com/spreadsheet/pub?key=0AvUlKfewYcCNdEZXdGFYUnZWT1NhQlhuOTg2SHhnbFE&single=true&gid=3&output=html

I can change the rear end ratio to something you've driven, but basically you're playing with wheelspin in second and pretty stable in third.

Interesting on the weight distribution. I wouldn't have thought that you'd be so close to 50/50 since there's so little over the rear wheels and so much over the front. I'd like to see the real-world corner weights sometime. Until then, I'm not so sure of your math.

I have no doubt that your car will be plenty fast, and no end of giggles, but I disagree that front bias is preferable. You can get better braking and the ability to get on the throttle earlier coming out of a corner with rear bias. But I also get your point about ease of driving. I've been to Lotus track days and it seems that the majority of the owners of mid-engined models are a bit scared of their cars, considering how easily they'll spin if not managed correctly.

In reply to kreb:

The 50/50 measurements are from the original designer, Stuart Mills, who was very meticulous about weighing components. That number was with MEV's very light (too light in my opinion) fiberglass seats, too. Hopefully we'll get a US-built car on corner weights soon.

The Miata's rear subframe and diff weigh quite a bit. The Miata "backbone" (subframes, engine, trans, diff, suspension, wheels, tires) balances about 5" in front of the shifter, which is 38.5" behind the front contact patch. As such, the backbone's F/R distribution is 57% front, and those mechanicals weigh ~880lbs. The driver's center of mass is located 30/70. The Sport Exocet chassis is 43/57, the Race is around 42/58. The fuel tank weighs a substantial amount and is centered on the rear tires. The battery is mounted in the rear.

kreb wrote: I understand the appeal of a super-easy kit, but the more power that you throw in there, the more that I'm bothered by the weight distribution. Good luck getting V8 power successfully to the rear wheels with that lump up front. My Stalker with the old, underpowered V6 (200 ft lbs, 170 HP) was difficult to hook up coming out of slow corners. You put 1.5x the power into a similar rig while moving the weight distribution towards the nose, and it's less than ideal.

not a problem with appropriate tires also by putting a fuel cell and battery over the rear subframe, i added a bit of weight backt here as well as compared to stock configuration. plus with the addition of a wing mount made off the rear roll cage bars and a proven airfoil, it will only be a problem from a standstill.

In reply to v8exocet:

Hey, the wing was supposed to be a surprise. Last time I tell you anything. :)

it was my idea berkeleyer. its just not there cause im not caged!

Sounding better all the time. Thanks for continuing the discussion, guys.

How good/terrible is aero on a car like this? Do a front and rear wing actually work?

Would the wiring/lighting kits you sell for the other models work OK for the Exocet? I ask because I'd like to put in a later 1.8 engine and they have a separate engine harness (like hte BMW's I'm used to) if I understand correctly.

Aero would work. My plan is to sell the v8 version when it gets to saudi (ok, well a little after, gotta show it off) then build a turbo 1.8 version and research different aero options. I think rear aero is going to important for corner exit traction with 225+ hp and non super wide section tires (like 225 series instead of 275/35 or whatever people are running) front aero should be rather simple using another wing airfoil mounting it to the front with the added ground clearance of the shortened nose and angle of attack. i think it would greatly help for sure. but im not an engineer, so i cant speak to facts, just theories.

I would also like to see a stationary fender that is incorporated into the front splitter to reduce turbulent air over the wheel, however, you have to have a pretty stiff suspension so it doesnt look stupid with fender to tire gap

z31maniac wrote: How good/terrible is aero on a car like this? Do a front and rear wing actually work? Would the wiring/lighting kits you sell for the other models work OK for the Exocet? I ask because I'd like to put in a later 1.8 engine and they have a separate engine harness (like hte BMW's I'm used to) if I understand correctly.

you reuse the wiring from your car. it doesnt come with a harness splice in things as needed

i used a custom harness in mine with circuit breakers and switches

I'm working on a 3D scan of the actual bodywork so I can run some CFD and get some ballpark FA and Cd numbers. Thankfully I already have a rolling road setup in ANSYS Fluent from my senior design project, so all I need to do is mesh the sucker.

Drag

One thing that people mistakenly latch onto with aero is Cd. Cd is important, but it's multiplied by the frontal area to get your drag force. Cd numbers aren't quite interchangable between cars due to differences in frontal area, but hey, it's easy for the marketing guys to turn into a metric. Why am I bringing this up? The frontal area of the Exocet is quite tiny. I would say it's about half the area of a Mustang, if that. Even if the Cd is ridiculously high, I am certain that the overall drag is less than a new Mustang.

Wing Effectiveness

Modern airfoils are stupidly effective. With a multi-element wing, you can generate a hundred pounds of downforce at 50mph. There is a bunch of virgin air where a front wing would go. The rear wing would be placed based off CFD results. The tubes from the roll hoop will reduce effectiveness slightly, but it will still be well worth it. A diffuser and center tunnel to take advantage of the flat floor would be fun to experiment with, but that's in the future. There is tons of room for a diffuser.

Wiring Later 1.8? Do you mean the Miata 1.8 or something else? Generally you'll want an ECU to match the year, but there is some wiggle room between the 1.8s. Did you find a 1.8 without a harness or something? I think the 1.6's harness won't work due to the barn-door AFM.

In reply to v8exocet:

Hah, thanks for the free customer service!

As for drag - exposed tubes are terrible for it. There's a lot that could be done to clean the car up by putting on some simple panels. I've seen this with my Locost, it hits a brick wall at about 80-90 mph - that car has a fairly small frontal area, but exposed control arms and the flat windshield don't help it at all.

Wings would work well if you get them clean air, but I'd have to have a look under the car to see if there's anything that could be done down there.

v8exocet wrote:

z31maniac wrote: How good/terrible is aero on a car like this? Do a front and rear wing actually work? Would the wiring/lighting kits you sell for the other models work OK for the Exocet? I ask because I'd like to put in a later 1.8 engine and they have a separate engine harness (like hte BMW's I'm used to) if I understand correctly.

you reuse the wiring from your car. it doesnt come with a harness splice in things as needed i used a custom harness in mine with circuit breakers and switches

I know, but the early model cars the engine/chassis harness is integrated. I like the later NB cars where they are separate.

ok? nothing a little wire rerouting cant fix?

Use a later engine. In the end, it doesnt matter what setup you use, it goes back together the same way it came apart unless you choose to do some custom stuff

All the talk of the perfect weight balance being 50/50,so why are damn near all the highpowered/lightweight race only/super cars in the 60+ percent rear heavy territory?.

Its like the horse pucky(IMO)that the GTR designer says about why the car is so heavy....because thats about what an F1 car weighs WITH downforce he says.Hmm if he was so smart why are all race series rules setting min weights and not max weights??.

Btw I do love the build,keep at it.

kevlarcorolla wrote: All the talk of the perfect weight balance being 50/50,so why are damn near all the highpowered/lightweight race only/super cars in the 60+ percent rear heavy territory?.

I want an Exocet, and I don't think the weight distribution is a big deal, but... he's right, sort of:

Pagani Zonda: 44% front 56% rear

Ferrari 458: 42%/58%

Lamborghini Aventador: 43 % - 57 %

Audi R8: f/r (%), 39/61

But those were all mid engined cars

Viper (old one): 48/52

Viper (new one): F/R, 49.6/50.4

Corvette ZR1: 51 / 49

Ferrari California: 47:53

Ferrari FF: 47/53

Ferrari 599: 47%-53%

Aston martin DB9: 50/50

So it's not totally cut and dry, and I believe there is a lot that can be done via suspension tuning to make up for those differences.

v8exocet wrote: ok? nothing a little wire rerouting cant fix? Use a later engine. In the end, it doesnt matter what setup you use, it goes back together the same way it came apart unless you choose to do some custom stuff

From what I understand, the NB Cars have a separate harness for the engine, a separate harness for the body.

That's what I'd like vs the integrated one on the NA cars. I'd like the headlights/brakelights/etc to all be separate. Ideally, their wiring kit (like on the MEVBusa) would make it even more sparse........less weight.