Increasing bore VS raising compression -- Discuss/debate

[jrm81bu]

Sorry if I made it unclear. I paraphrased what they said there. Yes, 3.5% per point increase in power and the 10% was an increase in displacement and (he implied) power. I don't know where he got the 3.5% figure from and I highly doubt that the increase in displacement will be equate to the same power increase. Again, I have a great amount of respect for the knowledge these guys have. That's why it's weird that they seem so far off on this one.

Not unclear, just pointing out where i found fault in the logic. I agree with what your saying. Some people claim "x" gain in power with "y" increase in compression but it's not guarunteed. And I really don't see an increase in displacemnet having an equal increase in power.

 

[DRIVEN]

Honestly, I was really hoping someone would chime in on the pro bore side. I'd love to hear some theory on that side.

 

[Clone TIE Pilot]

One of the problems with a big bore is that it is harder to fill than a small bore. Big bores are also prone to having a less evenly mix air fuel charge (more lean spots) which increases spark knocking. Smaller bores are easier to fill (at lower rpms) and evenly mixes their charge up better so they are less prone to spark knock. Ever wonder why 1980s 305s had 9.5 cr while 350s only had 8.5 cr? That is why. While 305s do suffer from some valve shrouding but it isn't a problem until 6000 rpms. This is why modern engines are more square, it's easier to run higher cr that way.

 

[bill]

I think the real debate is whether more horsepower is better than more torque......

and desired end result has more to do with the overall design of the engine....

 

[othtim]

One of the problems with a big bore is that it is harder to fill than a small bore. Big bores are also prone to having a less evenly mix air fuel charge (more lean spots) which increases spark knocking. Smaller bores are easier to fill (at lower rpms) and evenly mixes their charge up better so they are less prone to spark knock. Ever wonder why 1980s 305s had 9.5 cr while 350s only had 8.5 cr? That is why. While 305s do suffer from some valve shrouding but it isn't a problem until 6000 rpms. This is why modern engines are more square, it's easier to run higher cr that way.

Wait what? How is a smaller bore easier to fill than a larger bore? Is that some engineering thing? Are we talking identical everything else? Or are we talking stuffing in the largest valves that you can fit in the bore? Do you have some sort of evidence to back up this statement? Not saying I disagree, cuz I dont know one way or the other, I'd just like to see some reason behind the statemnt.

 

[khan0165]

If you were to take a mid-70's smog SBC and throw a set of 64cc double hump heads on it, dyno the engine before and after the swap ... take the block to the machine shop and have it bored, we'll say .030" over, and replace with the same -Xcc pistons that the engine had from the factory. Place the 76cc smog heads back on the block and dyno again.

I guarantee you would have had more of a power gain using the double hump heads on the old shortblock than boring it and replacing it with factory pistons (.030" over of course) with the 76cc smog heads

I don't know if this is a good way to compare the two scenarios.
for one, the 64cc double hump heads have larger valves and flow better than the 76cc smog heads (assuming both are in factory condition). The flow characteristics alone would greatly improve power and performance of the engine. And obviously only 0.030" over bore would not equate to the same power gain as increasing 2 cr points. Honestly, I don't even think you'd match the performance of 2 cr points with even a 0.090" over bore.

maybe then the question is...
how much increase in bore size would it take to match each point of CR?
(given that all other components and flow characteristics of the engine remain equivalent)

 

[Clone TIE Pilot]

One of the problems with a big bore is that it is harder to fill than a small bore. Big bores are also prone to having a less evenly mix air fuel charge (more lean spots) which increases spark knocking. Smaller bores are easier to fill (at lower rpms) and evenly mixes their charge up better so they are less prone to spark knock. Ever wonder why 1980s 305s had 9.5 cr while 350s only had 8.5 cr? That is why. While 305s do suffer from some valve shrouding but it isn't a problem until 6000 rpms. This is why modern engines are more square, it's easier to run higher cr that way.

Wait what? How is a smaller bore easier to fill than a larger bore? Is that some engineering thing? Are we talking identical everything else? Or are we talking stuffing in the largest valves that you can fit in the bore? Do you have some sort of evidence to back up this statement? Not saying I disagree, cuz I dont know one way or the other, I'd just like to see some reason behind the statemnt.

Best way I can explain it is that the larger a volume is, the longer it takes to fill, which means you have to use more energy to fill it at a faster rate to fill it in the same amount of time as filling a smaller volume. For example, one guy using a footpump fills a 1 gallon bucket in a minute, another guy trying to fill a 2 gallon bucket would either have to pump twice as fast, or use a footpump that pumps twice the amount but needs twice the effort. Either way he has to work alot harder to move the extra amount water at a faster rate so he can fill his larger bucket in the same amount of time even with a larger pump.

So while a larger bore can flow more with larger valves, it has to since it has more volume that needs to be filled. Since oversquare engines rev faster, it compounds the problem more with air drag and other factors. Just because a engine "flows" better doesn't mean it is "filling" better. But this is just one of many factors in designing an engine. However the bigger problem with a oversquare bore is that it doesn't mix the air/fuel charge as well as an squared or undersquare motor, resulting in more limited CR. But that doesn't mean square or undersquare are always better either.

 

[bill]

I would agree with the above statement, even though Im not sure I understand it....lol

 

[454muscle]

What??? Just one vote for increasing bore (me)? WTF

Whatever happenned to "there's no replacement for displacement"? I know it's relative, but still.

And why won't anyone mention the drawbacks of increasing compression, such has added heat, and pinging, etc..

Or that the increased bore will greatly increase torque.

Assuming that the "increasing compression" means doing it mechanically speaking (as opposed to adding boost via supercharging or turbo or even nitrous -- all of which increase compression).

And yes, I see there are drawbacks to increasing bore size too.

Not to mention, it all depends on other factors (ie. cam lift/duration, flow rate, etc..)

 

[Dr. Rudy]

how does an increase in bore greatly increase torque? thats generally a characteristic of stroke.