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Speedy Squirrel Speedy Squirrel is offline
Rocket Scientist
Join Date: Jun 2008
Location: Detroit
Posts: 898
Originally Posted by mark houghton View Post
Which still begs the question, why not US also? Surely not emissions related. I won't be comfortable with this until the answer is clear.
I don't like to speak generally for all cars, as there are 4 different distributors out there, but let's take the case of a 1978 to 81 USA (California) car.

USA (California) - It is timed at 31 nominal at 4000 rpm with the vacuum hose disconnected. It has 16 degrees of centrifugal advance at 4000. That means it has a static timing of 31 - 16 = 15 degrees.

EURO - For the Euro distributor the timing is 0 with the vacuum hose connected. At idle the vacuum retard is about 10 degrees, and the centrifugal advance at idle is 0. That means it has a static timing of 0 + 10 + 0 = 10 degrees.

So the USA has 5 more degrees static timing than the euro car. In order to maintain their safety margin Porsche decided that 5 degrees had to be pulled out under boost. That is why the USA car has boost retard. The boost retard is actually more than 5 degrees because the USA car also has more centrifugal advance. The boost retard is around 8 degrees.

Under full load both cars have the same timing:

EURO: 10 static + 12 centrifugal = 22
USA(California): 15 static + 16 centrifugal - 8 boost retard = 23

While I'm at it, let's explode a few more internet myths.

Euro cars have more timing under full boost - Wrong, as we have just seen.

Euro cars have a "more aggressive" timing curve - Wrong, the USA cars have more centrifugal advance, with about the same vacuum retard.

Euro cars had more timing because they had better fuel - Wrong in the late '70's US fuel was, on average, better than european fuel.

If you want more performance, get a euro distirbutor and pull off all the hoses - Wrong, the USA distributor can run more static timing safely with the boost retard feature, and it has more centrifugal advance. Just disconnect the vacuum retard hose.
Old 04-02-2010, 06:32 AM
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