[TR] TR4a ignition advance

[Darrell Walker]

This table has been floated around the list before:

Vacuum Advance dist. 40795

Inches Hg           Degree Crank advance
2.0                     0
3.0                     1
4.0                     1.75
5.0                     2.5
6.0                     2.75

Centrifugal Advance

RPM                    Degrees Crankshaft advance
500                     0
700                     0-2
1200                   5-7
1600                   9-11

So if you believe this is indeed crank degrees, you get a total max  
advance of:

4 + 2.75 + 11 = 17.75 degrees

But I don't believe the above figures are in crank degrees, but  
rather distributor degrees (half of crank).  But the initial advance  
maybe crank.  So that gives you:

4 + 5.5 + 22 = 31.5 degrees

Which seems like a reasonable number.  I recently read a book on  
powertuning ignition systems, and it claims the combustion chamber  
shape is the biggest factor in how much advance the engine can  
tolerate.  The one that most closely matches the TR head was called a  
"bathtub" chamber, and they said 33-34 degrees.  So that would be in  
line with second calculation.

-Darrell


On Sep 21, 2007, at 8:36 AM, spamiam at comcast.net wrote:

> Does anybody know the ignition advance information for the TR4A?
>
> I know that stock static advance is 4 degrees BTDC, but what is the  
> full vacuum advance?  What is the full mechanical advance?
>
> Someone said to me that these engines can not tolerate all that  
> much total advance (I.E. less than 30 degrees).  I would have  
> expected that a total advance in the mid-30's would have been about  
> right!
>
> -Tony
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