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Old 31-07-2020, 11:13 PM   #1
Olderisbetter
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Default Compression ratio

I have been involved in the rebuild of a 1600 TP engine from a camper and the heads on it were 040 but the ones supplied to rebuild it were 113 heads so with the current deck height being 2.15mm i think the compression will be quite low as the 113 heads have a bigger chamber volume, the engine has been refitted and started and ran fine but we are taking it back out to do some other work, should i just fit thinner shims under the barrels to get the CR back up to 8.5?
I used http://cbperformance.com/v/enginecalc.html to do the maths and this is just a 1585cc 69mm crank nothing fancy.

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Old 01-08-2020, 08:56 AM   #2
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VW made the compression ratio seven point something for good reason. It's best not to tinker with an otherwise stock engine!
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Old 01-08-2020, 02:54 PM   #3
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The current CP is 7.1 if i am correct in assuming the 113 heads have 52cc chambers, will that not run a bit flat and i think the stock should be 7.7? the old heads were 040 so with smaller chambers maybe the CR was more like 8.6 if the deck height was the same as it is now, that might explain the cracked heads.
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Old 01-08-2020, 06:41 PM   #4
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The current CP is 7.1 if i am correct in assuming the 113 heads have 52cc chambers, will that not run a bit flat and i think the stock should be 7.7? the old heads were 040 so with smaller chambers maybe the CR was more like 8.6 if the deck height was the same as it is now, that might explain the cracked heads.
There are things more important than worrying about getting CR absolutely correct to any real degree, but aiming for 7.7 keeps it correct and I would work towards good consistency between cylinders rather than 7.7000:1 accuracy!
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Old 01-08-2020, 08:40 PM   #5
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The current CP is 7.1 if i am correct in assuming the 113 heads have 52cc chambers, will that not run a bit flat and i think the stock should be 7.7? the old heads were 040 so with smaller chambers maybe the CR was more like 8.6 if the deck height was the same as it is now, that might explain the cracked heads.
Deck height needs to be 1.0mm to 1.5mm for an effective squish and good efficiency. If that gives you a CR of around 8.0:1 it will be fine with a standard cam. 7.0:1 is very low and performance will be poor. 7.7:1 isnít necessarily correct; itís just one CR VW may have used.
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Old 01-08-2020, 08:51 PM   #6
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Thanks i think for now i will see how it runs local and we can always come back to the CR another time, it starts and runs very well and i have electronic ignition to fit yet so at least it should be reliable but a bit flat, notes taken on all info thanks both for your inputs.
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