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98 BMW M Roadster (II)
98 BMW M Roadster (I)
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79 Datsun,(I)
The Beginnings

June 2, 2007

2007, V8-like Performance, I6 Fuel Economy: 1998 BMW M Roadster (Post-DINAN) Part 1

At the end of my previous story I described the upcoming update for the car to be performed on the fall of 2006: Suspension system replacement. It was done on schedule and it included re-valved Konis, stiffer springs, thicker anti-roll bars and adjustable front/aft camber plates.

Roundel, BMW Car Club Associationís magazine, reviewed the DINAN M Roadster prototype when it was first manufactured (dubbed ISR3 for Inter-cooled Supercharged Roadster); and concluded that the suspension and chassis modifications (which my car has), placed the car in a totally different league from that of the stock car. In a bid to experience for myself said difference, I decided to test the car in a twitchy road, before and after the suspension upgrade. I chose CA-152 between Gilroy and Watsonville, California. It is a 2-lane road which, on its mountain segment, has a speed limit of 40 mph. Most of the curves have a suggested speed of 25 mph, a few are marked 20 and one is so closed that it is marked at 15 mph. Although I had driven in twisty roads before, I had never tried to test the carís limits and therefore I did not push it to the point for the tires to chirp. The best I could manage to do at the 15 mph curve was 30; and I felt the carís body roll. I was able to keep the car at the roadís top limit of 40 mph on the other curves and tried to imprint in my memory the firmness (or lack thereof) of the car going into and out of the curves in order to compare them with the handling of the soon to be modified car. A few weeks after the upgrade, I took the car on the same course for another run. Even though I was determined to evaluate all the curves, I knew that the litmus test for the upgrade would be the 15 mph curve. Without tire chirp, I was able to get 35 mph; and the car felt more secure with less amount of roll. This time I had to carefully watch the instruments in order to keep the car within the roadís speed limit as I found myself exiting from some curves at 45 mph.

The new limiting factor is the rubberís footprint. At the time of this writing, the first set of tires due for replacement, are the rear ones. Without changing the stock wheels, I can increase tire width by 10 mm front and aft. Since the car is also my daily driver, I wonít put R compound tires as I already spend a lot on them as they are. Overall, the suspension upgrade was exactly as described by Roundel and other magazines that tested the DINAN-modified car: Exhilarating fun on dry windy roads.

Dinan Supercharger

While the stock car was never a slouch, one can always welcome a few more ponies under the hood. At the beginning of 2007, DINAN fitted my roadster with their supercharger system; designed to greatly increase the performance envelope of the car while keeping the original around-town drivability. The upgrade included a higher capacity radiator fan and fan clutch, and I added a bigger throttle body. The end result is an increase of 100 hp and 70 lb/ft of torque; although, with my engine, being as old as it is, I see lesser increases. Nevertheless, the jump in performance was instantly noticeable. Equally noticeable was the stock-like drivability: On my way home the day I picked the car from the DINAN shop at Mountain View, CA, I was impressed with the docility of the car on the streets. Aside from a minor increase in pressure on my back while accelerating the car from 1st to 2nd gear and the unavoidable noise, I would not have noticed the existence of the blower.

I have not been able to perform comprehensive tests yet, but I already have experienced substantially faster launches and, provided I keep the engine at a minimum speed of 3500-4000 rpm, I always have acceleration power on reserve, even on 5th gear. The icing on the cake comes when I go to the gas pump: The lofty performance comes with only a 2 mpg penalty. I have not yet ran down a full gas tank while demanding the most from the supercharger as I have done several times with the unmodified engine; but I have routinely confirmed that with the cruise control engaged at 65 mph, the supercharged car makes 23 mpg. I never got above 25 mpg before on the same speed conditions with the stock engine. When in ďracingĒ mode, the stock engine gave me 22 mpg. For comparison, a new M Roadster, with a 3.2 litter, I6 engine tuned to output 330 hp has been rated by the EPA to yield 24 mpg on highway. My supercharged roadster makes 23 mpg. Many V8 powered cars with comparable performance have more taxing fuel consumption.

To me, the supercharger is all about acceleration, rather than top speed. What I love the most is the improved ability to increase distance from cars following me. I almost dig to find an impertinent driver pressing me to go faster. I would quietly shift down one or two gears and, pressing the pedal to the metal, watch the personís car getting smaller by the second. More often than not, after a couple of minutes I would revert to cruising speed and move one lane to the right; waiting for the poor devil to catch up with me and letting him/her get ahead. By that time they know that I am letting them go because I want to, just because I canÖ I know, I know, I am showing off. But, hey, I love it!

Could it get any better? Aside from wider wheels and tires, I still have more upgrades in store: A limited-slip differential which would multiply torque at the wheels by about 8% with only a minor top speed penalty (I donít drive my car at the drag-limited top speed anyways). This upgrade will also increase the ability of the car to accelerate during turns. An oil cooler would be a welcome addition for those summer months; and finally, a larger diameter air flow meter will be a nice and cheap complement to the increased throttle body.

The DINAN ISR3 featured an air-to-air intercooler and a larger diameter intake manifold valve which resulted in another 50 hp and some extra torque. However, they donít do those anymore and probably the performance increase vs. its cost is not justifiable. Besides, I also need to consider that, in order to keep the car as a daily driver for the next 5 years (at least), I will have to put a brand new engine; and those M machines are not cheap.

Because of the way I have structured my business, it is more advantageous for me to maintain my current car (however expensive it might seem) than to purchase a new vehicle. With all the enhancements I already have and the few I still want to add, I can afford to be patient and aim for the big guns: A Porsche 911 GT3 or the ultimate Porsche: The 911 Turbo! But letís not get ahead of ourselves; I still have plenty of experiences to enjoy in my now Supercharged M Roadster.

This was just Part 1Ö

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