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Reply to "New CMC Speedster project"

And don't either be like Ed.

For reference, Ed's gears, in both his clown cars, are 3.80 / 2.06 / 1.26 / .93 with a 3.44 R&P. Car 1 is a 1700-lb MG TD replica with a 2.2l Subaru engine and stock sized 165/80 tires. The engine is rated at 137 HP @ 5200(?) RPM and makes about 140 ft-lbs torque pretty much from 2000 to redline, which is rev-limited to 6000.

Car two is very well known on this site. It weighs 1550 wet and the engine is rated at 120 HP @ 5500 RPM and makes about 125 ft-lbs peak torque at 4000. It also redlines at 6000 RPM. Both of these vehicles, weighing substantially less than a Beetle but with two to three times the stock Beetle's power, possessing but two seats (instead of 4) and with limited trunk space, have no need for the Bug's stump-pulling 4.12 R&P + first gear, designed as that was to haul four passengers and luggage up an alpine pass with 40 horsepower.

It's that first gear, which in a stock Bug tops out around 20 mph and necessitates shifting into second halfway through the intersection, that limits the utility of the gearbox for our more sporty pursuits. In my applications, with the much taller 3.44 ring and pinion and higher-than-stock shift point, top speed in first is increased to about 35 mph, which is very near what the original 550 Spyder did.

Top speed in second (6000 RPM) is a bit over 60. This is equal to a 1955 Spyder geared for LeMans (although the Spyder achieved that at a rather show-offy 7500 RPM).

Third tops at 100 mph, again, same as a Spyder. And 4th is theoretically above 140 mph, which is a little more than the original Spyders could reportedly manage.

Now, as every schoolboy knows, successful as they were, the early Porsche racing gearboxes were a compromise, which is why the company rushed to get a 5-speed box in place. But remember too the engines they served: the four-cams were moon-revving beasts with no torque down low. They needed closer gear ratios in a way our street-rod engines—with their broad torque bands and 6000-ish redlines—do not.

In fact, the lower the redline on the engine, the wider-spaced the gears can actually be—which is why all the ultra close ratio boxes first came out on the high performance and race vehicles. It's not that close gear spacing isn't nice to have behind a pedestrian street engine. It definitely is! It's just that, when you look at the dyno sheet on a race motor you tend to see a very small gap between peak torque and peak horsepower. They're both way, way up there in the rev range. So if you're trying to keep the engine revving between those two figures (which you absolutely are!) then you need very close gear spacing. To win races.

Case in point: Boxster v. Spyder. 2.5 liter Boxster 6: 201 hp @ 6000; 181 Ft-lbs @ 4500. A 1500 RPM gap between peak torque and peak HP. 1.5 liter Typ 547 flat 4: 120 HP @ 6200 RPM; 96 ft-lbs @ 5300. A 900 RPM gap.

Despite their ancient pedigrees, our hot-rodded Type 1 engines behave—if they're built right—much more like an early Boxster mill* than a four cam. We're getting broader, flatter torque curves that make it easier to keep them "on cam," as the old chaps used to say. They don't blow up if you lug them to the donut shop when the oil's still cold.

Anyway, don't be like ED, and spend your life thinking about it. Just get a good stock-geared Bug transaxle, preferably one with a 3.88 r&p, put it in the car and be happy.



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*Excepting the dreaded IMS bearing issues, of course.

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