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Doing my homework on 306's for my 95 GT. But nowhere seems to talk about how much power you actually get out of the overbore itself. That's 4 more ci, doesn't sound like much. Then again some people will tell you that the overbore is worth 15-20 hp. Just a minor curiosity I guess, since of course that's not where your power is going to come from in a 306 engine build.

I just always wondered if the bore itself actually nets you any more power.
 

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I checked out a few threads and most said about 5 hp.

Thats if every thing stays the same, and the only difference is the bore.
 

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It would be minimal, & not worth doing if you have round cylinder bores that are in spec.
 

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15-20 hp for 4cid???? Lol so somebody has figured out how to make 5hp/CID with these things huh? Lol. Maybe 1Hp/CID, so maybe 4 HP.
 

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Yep, I'd say 4 to 5 hp.
 

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Taking a .030" cut with the specific goal of gaining horsepower isn't worth it, once you've factored in time for dis and re-assembly, new parts, and labor to machine. It's typically only done when the standard bore size is unusable.

We sometimes overbore VW VR6 blocks 2mm, which is .080". At .080", on a 302, you're almost in the water.
 

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A stock 302 bored to 306 still uses the same crank and rods so there isnt much of a power increase. The stroke does that.
 

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Stroke can also lose Hp in some cases.
 

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The place where you'll see gain in going to a 306 is bumping up compression at the same time.

If you go from stock 9:1 and your 4 inch bore to 11:1 and a 4.030 bore, you'll probably gain a solid 20-25hp depending on the combo.

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Some engines are going to see more benefit from increased displacement via bore than stroke and vice-versa. It's my understanding that with the pre-Coyote mod motors, a big-bore 5.0 makes more power than a similarly set up 4.6->5.0 stroker. I believe Toyota R motors are the same way, and see more of a gain from increasing bore than from increasing stroke.

I wish I better knew the physics of it, I'm sure it's a combination of the geometry of the engine internals, the weights of moving parts, the nature of how the flame propagation is different (fire wants to go laterally more than it wants to go down), and the like.
 

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Some engines are going to see more benefit from increased displacement via bore than stroke and vice-versa. It's my understanding that with the pre-Coyote mod motors, a big-bore 5.0 makes more power than a similarly set up 4.6->5.0 stroker. I believe Toyota R motors are the same way, and see more of a gain from increasing bore than from increasing stroke.

I wish I better knew the physics of it, I'm sure it's a combination of the geometry of the engine internals, the weights of moving parts, the nature of how the flame propagation is different (fire wants to go laterally more than it wants to go down), and the like.
So in your opinion, do you think a big bore 347 will make more power then a standard 347 ?

What i mean by that is..... a 331 crank, rods, and then boring the rest to make the extra cubic inches ( obviously an after market block would be needed to bore that big)

Vs. The standard 347 crank, rods, and .030 over pistons.
 

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So in your opinion, do you think a big bore 347 will make more power then a standard 347 ?

What i mean by that is..... a 331 crank, rods, and then boring the rest to make the extra cubic inches ( obviously an after market block would be needed to bore that big)

Vs. The standard 347 crank, rods, and .030 over pistons.
I honestly don't know. I'm guessing there's some relationship between bore and stroke that's ideal, and if you're outside of that you see more gain increasing the side that brings you closer to that ratio.

I think that there's also going to be a ton of other contributing factors, so isolating just the effects of bore vs stroke would be very difficult.
 

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After thinking about this for a while, I want to ammend my comments with this: if its a traditional bore and hone, then my original statement stands. But, and this is a big ole but, if the cylinder is bored and then torqueplate honed, then race style pistons put in, even if comp stays the same, then considerable power can be gained. But the gain isnt from just the overbore, rather over all the displacement. Just one of those engine builder things.
 

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After thinking about this for a while, I want to ammend my comments with this: if its a traditional bore and hone, then my original statement stands. But, and this is a big ole but, if the cylinder is bored and then torqueplate honed, then race style pistons put in, even if comp stays the same, then considerable power can be gained. But the gain isnt from just the overbore, rather over all the displacement. Just one of those engine builder things.
Efficiency!
 

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Efficiency!
Nah bro the added weight from the bigger piston adds momentum to your rotator. Therefore your engine doesn't work as hard to go up in the rpm range! :lmao

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Discussion Starter #19
I've just always noticed that 306's make fast mustangs. Don't know why. Even combos that are quite mild have decent power if its a 306. Could be that displacement is a sort of sweet spot that even Ford didn't know about.
 

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Compression, lightweight rotating assemblys, total seal rings. When all this gets put in compared to stock stuff it will run much better. Effiency

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