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Old 07-03-2016, 05:10 AM   #1
Gentlemanjim
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Question Rasing the compression?

I am putting a 1974 DT175 top end on my AT2. While its apart I thought it would be a good time mill the head a bit.

Question is how much to remove to improve power and still keep it Reliable?
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Old 07-03-2016, 09:24 AM   #2
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Question

What is your compression right now?
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Old 07-03-2016, 09:43 AM   #3
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What ever is stock DT175 .25 mm
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Old 07-03-2016, 10:55 AM   #4
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Default Greg French means...

What he means is, when you put a compression gauge in the spark plug hole and measure the actual compression, what does it register on the gauge? If you are talking about milling the head and decreasing the volume in the cylinder, you had better be very careful about your measurements. Ballpark won't do.
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Old 07-03-2016, 10:56 AM   #5
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Default Also...

You might want to read up on the "squish band" threads here on the site. Lots of info there on measuring that as related to compression as well.
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Old 07-03-2016, 12:55 PM   #6
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Default

All good advice.
Remember.... it's a two stroke. Not a small block Chevy.

Some "experts" will throw out numbers because they sound reasonable, but you never do anything without knowing where you are starting. What if the last owner took off 0.20" without your knowledge. You decide to follow this advice and now that brings the amount removed to 0.40"

And you can't trust a guess of 0.01" = 1 psi.

Remember: raise the compression=raise the temperature. So you will have to figure out a way to keep the engine cool (richer F/A ratio) without burning holes in pistons.



Although it may seem simple, there is a TON of science built in... that no one understands.

(Well, almost no one.... Dave Sigle is probably the only one.)



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Old 07-03-2016, 05:10 PM   #7
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Smile

Gentlemanjim, instead of milling the head I would recommend that you have the top of the cylinder milled down to around .035" (to achieve zero deck height).
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Old 07-03-2016, 11:44 PM   #8
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Just to be safe I'm going stock thanks for the input.
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Old 07-03-2016, 11:51 PM   #9
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Default

You did not mention that you are doing any of the much simpler things.
First, Are you using the reed from the DT? It flows far more than the ATs.
Second, Use the DTs carb joint as it allows you to use a spigot type 28mm mikuni.
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