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Mein 12

Mein 12 by Jon Darlington

Jon Darlington – Leamington Spa (I guess West Midlands)

Car model & year
1968 LHD SWB 912

Length of ownership
10 years (purchased when I was 21)

Reason for choosing a 912
I wanted a Porsche instead of VW which myself and my group of friends where interested in at the time (late 1990s), I couldn’t stretch to a 911 but loved the shape. So I investigated 912s, found a solid car from California imported it and restored it. Over the years I have wanted to use the 912 to its full potential, which has resulted in a full body restoration and diet (car now weights 880Kg) and a performance enhancement with an Andy Prill engine re-build to fast road spec (120-130 bhp although not been on a rolling road yet). Plan now is to prep the car for FIA road rallies and have some fun, I think the 912 will be a competitive car in the under 2000cc category, really looking forward to finding out.

Best time you’ve had in your 912
I would have to say the 912 register run to Reims, this was a great weekend organised by Gavin Joule , we covered around 1100 miles in the weekend taking in various parts of France with a spirited drive around the part of the old Reims track that is still open.

Interesting changes to my car:

Full rebuild by Maxted-Page & Prill to the below spec:

912 crank
337s Norris cam
Shasta pistons (1720 cc)
Shasta billet barrels
Stock valves
Stock pushrods
Balance connecting rods (912)
Flywheel lighten and balanced
Solex carbs
K& N filters
Power pulley
Full flow oil system

Around 120 BHP (yet to be confirmed on a rolling road).

356 sports exhaust fitted, gives a great deep sound and nice high rev edgy snarl, also sound great on the overrun with lots of audible engine draw. No changes are required to the body work to fit the exhaust just bolts straight on.

All gauges standard expect the addition of a calibrated combination pressure and temp gauge. This is a great addition which gives added piece of mind and some key info on what the engine is up to, a would say a necessity if you plan to use your car for track days etc.

Full body bare metal restoration to original beige grey (special order colour) by Curves Body shop (Terry Allen)
Lightweight vent rear fixed qtr glazing
FIA towing hooks front and rear
Safety devices roll cage (rear 1/2 cage)
Lightweight RS carpet
Lightweight RS door panels
Replica lightweight 911 R fixed back seats

Wind tunnel:
I am currently preparing my 912 for road rally competition as part of this I had a chance to take some basic wind tunnel data on my car (SWB 912) in the MIRA Wind tunnel. The main aim was to baseline the car for lift and drag and also to get some understanding of small things that could be done to which may affect aerodynamic performance. I only had a short amount of time in the facility so the focus was on small changes that could be done to the car while still being within the MSA/FIA regulations which state that the silhouette of the vehicle must remain as production.

MIRA had on file a test of a SWB 911 measured in 1969, this was a nice piece of information to refer back to as the wind tunnel is still running the same inlet/outlet and balance today as it did on 1968. This makes the drag and lift data directly comparable.

1968 911 2.0l SWB measured in the same tunnel (on the same balance)

CD = 0.329 (CDA = 0.576)

Combined front and rear lift = 0.275 (96.3 kg @ 130 mph)

Summary of test:
1.Baseline (external form changes to vehicle from production 912: ride height lower in the region of 80mm, Fuchs wheels fitted with 185/205 tyres front and rear, 356 sports exhaust fitted, front and rear closure straps fitted).

NAV190F Baseline:
CD 0.323 (CDA 0.565)
Front lift = 0.028 (10.7 kg @ 130 mph)
Rear lift = 0.246 (85.6 kg @ 130 mph)
Combined front and rear lift = 0.275 (96.3 kg @ 130 mph)

2. Lowest drag: this was focus on reducing frontal openings and changing the leading edge flow condition – fog lamp covers in place, horn grilles covered, bumper plate angle (vertical to ground line)/height (5mm reduction in Z).

NAV 190F best drag (Le Mans set-up!): CD 0.320 – 3 count drag improvement over baseline

Front lift = 0.005 (2.5 kg @ 130 mph) – 82% reduction in front lift (change due to number plate angle/height change)

Rear lift = 0.252 (87.7 kg @ 130 mph) – 2.4% increase in rear lift.

3. Lowest (overall lift): this condition gave the lowest overall lift although the highest drag coefficient – fog lamp covers open, horn grilles open, bumper plate angle (vertical to ground line)/height (5mm reduction in Z).

NAV190F best lift:

CD 0.329 (CDA = 0.576)

Front lift = 0.005 (2.5 kg @ 130 mph) – 82% reduction in front lift (change due to number plate angle/height change)

Rear lift = 0.238 (82.7 kg @ 130 mph) – 3.3% reduction in rear lift (change due to removing fog lamp covers)

In summary opening holes on the front of the car (fog lamp covers or horn grilles) increases drag and front lift, although reduces rear lift. With the fog lamp covers open there is an increase in flow to the brakes so this also needs to be considered when selecting the final setup (I would like to run a brake temp test at some point in the future). Changing the front number plate angle to vertical (it is currently at the same angle as the bumper skin on my car) gives a significant reduction in front lift without increasing drag but does increase rear lift slightly (inspiration for this test came from looking at the development of the 911 front bumper, take a look at the RS and RSR).

The other thing we tested was yaw stability which doesn’t seem to be to bad the lift figures do change but at fairly progressive rate so early SWB cars shouldn’t have a Audi TT issue in a cross wind at high speed!

What does it all mean?
To put these figures into context a drag coefficient (CD) of 0.323 as a baseline is quite good, the latest turbo (997) is 0.295 (although the lift is much better, lower and more balance front to rear). The main reason for being CD being that low (in relative terms considering its age) is due to next to zero cooling drag as all the opening for cooling on the early cars is at the rear. Also when you consider the real world figure of CDA (CD corrected for frontal area) the aero dynamic performance of the early cars is very good indeed.

The effect on Performance
So I am no expert but I have put my noise in a few text books and calculated theoretical (no gearing effects taken in to account) maximum speeds the various set-ups tested:

Calculation for V.max
v max (kph) = 100 * 3√P/(k*CD*A)
p = engine power at the flywheel (kw)
k = vehicle losses (rolling resistance, drive train loss, electrical system load)
CD = drag coefficient
A = Frontal area (squ M)

Note: mass is not considered as it has been assume there is not gradient.

Best drag set-up (CD = 0.320)
V max (kph) = 100 * 3√97.2/(14*0.320*1.75)
V max = 231.3 kph (142.8 mph)
Reduced lift set-up (CD 0.329)
V max (kph) = 100 * 3√97.2/(14*0.329*1.75)
V max = 229.2 kph (141.5 mph)

Can you tell the difference on the road?
I have also had a chance to try the three main configuration tested in the wind tunnel on a track. This is the fun part can you actually tell the difference between the set-ups? To make this as consistent a test as possible, all set-up where tested on the same day on the same tack to reduced the effect of surface and wind conditions as much as possible. The track I was using had a number of open high speed corners (95-110mph exit speed) and a long 1.8 mile straight (max speed recorded here from a GPS unit) perfect for subjective assessment of aerodynamic set-ups.

I started with the baseline (a setup I know well), this gives a fairly neutral feel to the car with a tendency to under steer in high speed corners when pushed. The max speed achieved on the GPS unit fitted in the vehicle was 120.6 mph. We then moved to the “best drag” condition, this resulted in a light feel to the front end in the high speed corners, I was finding it much harder to stay within the mark lanes at the same cornering speeds as the baseline condition, the max speed achieved was 121.2 mph, although the corner exit speed seem slower and defiantly felt less secure. The final condition to try was “best lift” as the data suggests this gave a much more planted feel to the front end in the high speed corners with subjectively less under steer, the corner exit speed felt higher with the max speed on the straight being recorded at 120.9 mph.

So which setup for my plan to road rally the car (what I am going to run?), I think best lift as the extra brake cooling is a good thing for road rallies and track work, also long runs at v.max are unlikely, so trading in corner stability and better brake cooling for a few additional mph on the top speed of the car doesn’t feel like the right direction to take! So my 912 is now set-up in the “best lift” condition.