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Hi Guys,
I’m wondering if anyone can tell me the benefits of overboring the m20b25 engine to 0.5mm. Im in the middle of refreshing my head however am thinking of doing this also but am not sure if its worth the extra work for the potential gain in the end?
If i do go down that path, will anything differant be required for the head?
Overboring the block means more compression. If your block’s cylinders are no longer round you either overbore or get a new block. You do not need to do anything to the head. Overboring adds cost tho because pistons aren’t cheap. Now that we have a dyno rule it might not be worth doing. As a datapoint, neither my engine nor it’s predecessor was overbored. I didn’t want to spring for new pistons.
You add both compression and displacement - albeit a fairly small amount.
With the head off it’s a great idea to measure the cylinders if you have a bore gauge. Not cheap or easy to repair/improve however.
We are in the same boat with 2 engines.
-Ken
Sleeving an engine would likely cost more than an overbore and new pistons.
A max legal overbore and max decking of the head will push the CR to about 9.1 (8.8:1 is stock). If we could legally advance the ignition timing this would help with power.