Increasing bore VS raising compression -- Discuss/debate

[jrm81bu]

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. True, but you just can't hardly get enough of a displacement increase in most stock blocks form boring to make much difference. Take a sbc 350 for example, .030 over will get you a whopping 5 extra cubes that would result in zero measureable increase in power. While i guaruntee going from 8:1 to 10:1 compression will get gains you don't even need to measure to notice.

And why won't anyone mention the drawbacks of increasing compression, such has added heat, and pinging, etc.. It all depends in how much of an increase you go for. Like it or not the power an engine makes starts out as heat energy. More heat more power.

Or that the increased bore will greatly increase torque. I think you are confusing bore with stroke. Which is also much easier to use to increase you actual displacement than what the bore is.

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. The biggest being that most blocks can't be bored enough to make a large increase in displacement.

Not to mention, it all depends on other factors (ie. cam lift/duration, flow rate, etc..) True, but take that 350 I mentioned above. Assuming no other changes than a max overbore vs. say two points of compression increase, only the compression will change the power on any significant level.

It's been stated before but a the better argument would be stroke vs. compression. And even then you reall need to have an intended goal or use decided before you can really choose one over the other.

 

[454muscle]

Hehe thanks Jrm81bu for that excellent answer and explanation.

Yes about the stroke being the significant way to increase cubes as opposed to just bore. Perhaps the op intended that assumption?

 

[MadMax]

I have an sbc 350 bored .030 over. Stock stroke and all, just rebuilt and bored. I have 4 valve relief flat tops and some stock non ported 58cc heads. It has a very mild almost stock RV cam in it, and a 2 barrel intake and carb. The car will absolutely MOVE for a 2 barrel (as long as i have some 93 octane in it) :wink:

So i would say that compression definitely helps! I really need to put a small 4 barrel on the thing so i can open it up some

 

[othtim]

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

Just want to mention that increasing stroke vs bore doesn't affect engine tq output - displacement is displacement. I'd say that increasing stroke is a "worse" method of increasing displacement than adding bore. You've got more drag on the rings, harder to rev, smaller valves, etc. More stroke doesn't equal more torque - more displacement = more torque. The whole stroke "level-arm" makes tq thing isn't neccessarily true. One thing that a longer stroke gets you is more piston speed which can help carb signal, but that isn't really due to the "lever-arm" effect of stroke as much as it is a side effect of stroke. It may be easy to add stroke, but it's not the most "efficient" way. But its cheaper, lol! Better do both, just to be safe XD

Don't agree? read this article by david reher
http://www.rehermorrison.com/blog/?p=220

 

[jrm81bu]

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

Just want to mention that increasing stroke vs bore doesn't affect engine tq output - displacement is displacement. I'd say that increasing stroke is a "worse" method of increasing displacement than adding bore. You've got more drag on the rings, harder to revthere's plenty of stroker motors out there that would disagree, smaller valvesAnd this would only be for an engine that can't fit a bigger valve, using my 350 sbc example, more bore doesn't really allow for any bigger valves, etc. More stroke doesn't equal more torque - more displacement = more torqueAnd in most cases you can get a lot more displacement with stroke than bore. The whole stroke "level-arm" makes tq thing isn't neccessarily true. One thing that a longer stroke gets you is more piston speed which can help carb signal, but that isn't really due to the "lever-arm" effect of stroke as much as it is a side effect of stroke. It may be easy to add stroke, but it's not the most "efficient" way. But its cheaper, lol!I wouldn't really say cheaper but you can get a lot more for it than with boring. Better do both, just to be safe XD

Don't agree? read this article by david reher

http://www.rehermorrison.com/blog/?p=220

Again like i said before, using a 350 sbc for example, a max bore with no room for future rebuilds will get you a whopping 10 cubic inches. Whereas stroking can get you about 41. So if displacement makes torque and you want as much power as you can easily get, what would you do?
The real answer would be go big with as much compression as you can afford to keep from detonating. I don't really see the bore as the first place i'd look to gain cubic inches.

 

[GhettoRacingKid]

I really cant believe this is actually a debate or for that matter still going on.

No one ever says im going to pull compression out to increase power.

Compression gives more power.

bore and stroke come into play to make some serious cubes. going up 5 ci is nothing. your better off taking out a gallon of gas to lighten the car to make the car faster.

Look up all the SBCs from the mid 60s till now. look at power ratings and the compression ratio. youll see a 400ci making just about the same power as a 350 with the same compression. maybe a minor increase.


when you increase compression you need to have the motor match everything else. Cam, heads, intake, carb all need to work together.

 

[Dr. Rudy]

all i meant when saying that you generally increase stroke to increase torque is that thats the best way to get more displacement out of the engine as opposed to boring it.

 

[-83MONTESS-]

when you increase compression you need to have the motor match everything else. Cam, heads, intake, carb all need to work together.

Well stated.