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Old 11-09-2008, 11:26 PM   #1
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Default Stock bore Vs 20 over

Im looking at the Weisco and Im pretty sure I can get them in stock bore size. I cant imagine there being a price difference so the only difference would be having the block bored Vs honed. What I would like to know is if there is good reason to go with the 20 over Vs stock size?
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Old 11-10-2008, 12:01 AM   #2
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the slightly extra displacement?
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Old 11-10-2008, 12:20 AM   #3
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I'd bore it out and get the bigger pistons, like .040. A spec more displacement and it will allow you to unshroud the head a bit more for more HP.
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Old 11-10-2008, 12:40 AM   #4
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now how much more hp are we talkin here? like 5-7hp vs stock and 10-15hp vs turbo?
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Old 11-10-2008, 12:42 AM   #5
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Meh, it's not a ton. But it's a little. I did the math before. It's somewhere in the engine conversions forum. It's like 1.2% gain in displacement going with a .040 bore IIRC.
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Old 11-10-2008, 12:49 AM   #6
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Quote:
Originally Posted by patsmx5 View Post
Meh, it's not a ton. But it's a little. I did the math before. It's somewhere in the engine conversions forum. It's like 1.2% gain in displacement going with a .040 bore IIRC.
So I guess its just a numbers game? Do I want to spend the extra $100 for a couple of HP?
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Old 11-10-2008, 12:52 AM   #7
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Quote:
Originally Posted by levnubhin View Post
So I guess its just a numbers game? Do I want to spend the extra $100 for a couple of HP?
I searched and can't find my own post on the matter. I ran all the number for a 1.8 miata before. I think a .060 bore gets you like 116 cubic inches or something like that.

Anyways, whether it's worth it or not is debatable. More displacement helps power everywhere. Helps low end, helps top end, helps spool, etc etc. One of the few mods where there really aren't any compromises. So to me, it's worth it (if I were building an engine). And as I mentioned, it will allow the head to be unshrouded more, which will also help HP. To me, it would damn sure be worth it.
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Old 11-10-2008, 01:03 AM   #8
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Found it.

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Originally Posted by patsmx5 View Post
Guys .020 or .060 from the cylinder walls is NOT Gonna hurt the cylinder walls. It's a thick *** cast iron clylinder, you will never hurt it. It's the strongest thing in your motor. It's more displacement, which our undersquare engines can use. By increasing the bore .060, you could unshroud the combustion chambers that .060 as well, which helps flow. More flow and more displacement is...... desirable. Do the math but that's surely a few % more displacement. Boring it the minimum is ok so it can be rebuilt again. They do this for commercial equipment, where they would like to get several rebuilds out of the engine for maximum service life.

Sorry Rotornut but reread my post. You were asking stuff I outright already answered.

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Originally Posted by patsmx5 View Post
Ok, using the factory specs of 3.2678" bore and 3.346" stroke, I get 112.25 cubic inches of displacement.

Let's punch this motor .020 over.
3.2878" bore and 3.346" stroke= 113.63 cubic inches (.000 to .020 displacement changed 1.378 inches)
1.23% more displacement then stock.
1.43 cubic inches larger than stock.


Now let's punch it .040 over
3.3078" bore and 3.346" stroke=115.01 cubic inches (.020 to .040 displacement changed 1.385 inches)
2.46% more displacement then stock.
2.76 cubic inches larger than stock


.060 over
3.3278" bore and 3.346" stroke= 116.35 cubic inches (.040 to .060 displacement changed 1.400)
3.65% more displacement then stock.
4.1 cubic inches larger than stock

Though insignificant, as bore size increases, area increases exponentially. This can be seen as the change in displacement is not the same for each .020 iteration. For example, look at the change in displacement for .020 and .040. The difference between them is 1.385-1.378=.007. However, this is not the case for .040 to .060. Here, the change is .015. Hence it's exponential by a factor of 2. This is because the formula for area is pie times radius squared times height.

An extra 4.1 cubic inches is 3.65% more displacement, or 3.65% more power. However, it will also allow you to unshroud the combustion chambers .060, which will increase flow. I would say the benefits of unshrouding will be greater than the benefit of the larger bore, but at the minimum, equal. If only equal, that's 7.3% increase in power.

That's my inexperience showing. Night night.
Original thread: https://www.miataturbo.net/forum/t17594/
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Old 11-10-2008, 01:06 AM   #9
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Great info, thanks man.
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Old 11-15-2008, 11:20 AM   #10
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My MazMax engine is a .060 overbore, feels a lot more spunkier than a stock bore motor.

EDIT: btw, I also made 168whp off my old sebring at 6 psi, considering that most make 145-150whp, thats a decent jump.

Can't wait to see what it does turbo'd

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Old 11-25-2008, 04:53 PM   #11
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It really depends on your budget and goal for the car. Of course any little extra displacement will help. You just need to decide if the money could be better spent elsewhere. For a street car I don't know that it would be worth it if you're on a limited budget.

But if you have the extra hundred or so lying around, then why not? Then you will know for sure that the bores will be round with a fresh hone.
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