Stu,
The short answer to your question *might* be: Tr250's made 1968
emissions of 275ppm HC and 1.5% CO with 8.5CR heads, so it seems reasonable
your 1974 could too, if it's being *completely* rebuilt. Take a hard look
at your carbs though. Are the jets worn where the "biased" needle rubs on
it? Your CO and HC will go through the roof, and the jet is not replacable,
at least not easily. I believe that CR's were dropped to meet NOx standards
starting in 1972. That's also one of the purposes of the EGR valve as well.
(lower temps in combustion chamber lead to lower NOx).
The long answer is: I don't know, I'm making a guess, and I'm not
an expert. I'm reading a book called "Emissions From Combustion Engines and
Their Control" that gives a lot of theory, and in theory higher
compression, all things being equal, should not significantly hurt CO or
HC, and may even help HC due to more complete combustion.
All things are seldom equal, though. If you wind up adjusting the
carb throttle plates toward the closed side to get a reasonable idle speed,
the carb may enter a flow regime where it's unable to meter fuel
accurately. If you have to run rich to avoid pinging with pump gas, your HC
and CO will both climb significantly. I have no idea what CR does to the
effective operation of the EGR system, I know reducing exhaust back
pressure hurts it (keep that header on the shelf unless it has a CARB #).
Also, removing metal from the head may result in a less efficient
combustion chamber shape, and the resulting unburned fuel will cause HC to
climb. These are a few examples of how changing one thing may change
something else, there are probably a hundred, and I don't know the answer.
There's a book at most Barnes and Nobles stores called "Understanding
Automotive Emission Control" that's very good for guys like me with no
automotive engineering background, but it won't give you the answer either
(it's sitting here as I write this). Most of the wild conjecture expressed
above comes from this book, there are probably much better sources on this
list.
If you need a spare '74 TR6 head to experiment on, I'm selling one
in the N. Virginia area. But that's not why I wrote you!
Good Luck
Tom Marincic
CD3574L
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