I've already read this post and have learned a lot
http://web.camaross.com/forums/showt...ighlight=sb2.2

there are some questions I have though

1. assuming that two engines are the same displacement one a big bore and the other a long stroke. wouldn't the big bore apply more instant (sooner) depression on the intake tract and the long stroke engine be more gradual causing the two engines to favor different valve trains and intake tracts?

2. no one brought up anthing about the amount of force in psi applied to the tops of the pistons then multiplied to the crank through the leverage of the crank stroke.

this may be way off but I'm here to learn. I'm assuming that both engines recieve 1000 psi of force in the chamber and taking out the rod to stroke relations and all the other stuff to just try and understand the relation ship between bore and stroke

area of 4 = 2x2x3.14=12.56 sqin
1000psi x 12.56psi = 12,560psi force
leverage arm of a 4in stroke = 2in

(2/12)12,560=tq

0.17x12560=2,135 lb ft

4.62bore and 3in stroke

area= 16.755
psi = 16755
levereage arm = 1.5
tq = 2,094

3in bore and 7.1 stroke

area = 7.065
psi = 7065
la = 3.55
tq = 2,090

if this is true it seems that a square engine is capable of appling more tq to the crank at any ginve point in the rpm band.

I do realize that the diminsions and the cost of parts (to handle the rpms) might stop a square engine from being built, but would it be a better choice if rules money and parts allow?

 

I'm not certain that you can pull out a part of the system (process) and make conclusions about the whole from a part.

Not that examining smaller parts isn't useful, but it can lead to incorrect conclusions based on your assumptions.

For instance, if I read your post correctly, you're assuming 1000lbs of pressure in the combustion chamber. If you happen to have 1000lbs in the chamber, the amount of pressure per cm^2 of surface area should be lower.

If by chance you assume 1000lbs/sq", where did the extra energy come from?


Yes, you can observe that the larger piston crown area unshrouded the valves and allowed more air/fuel to enter the combustion chamber, but now you are looking outside of the parameters that you first mentioned.


Perhaps, and this is only a perhaps; it would be better to look at the varying bore/stroke rations as a way to manipulate where in the rpm range the power will be made and the manner in which it's made.


Just a couple of thoughts.

 

the way I'm thinking is the relationship between just the bore and stroke have a dirrect impact on the amount of force aplied to the rest of the dirveline.

"If by chance you assume 1000lbs/sq", where did the extra energy come from?" lamerandomname

I'm just using psi. this is basicaly a hyd. force at the point of combustion correct? and the greater area of the piston the more force applied. is any of this correct? if both see the same psi only one has a larger bore it should apply more force to the crank.

 

I still don't get that logic.... maybe it's just me but the larger bore or longer lever arm theories never pan out.... this comes from a guy who is labled "Mr Theory" too lol

All the force of the piston is applied to the rest of the system the same way.

Maybe that larger bore with more piston area pressure counter acts the longer lever arm stroke so it comes out equal.

I really think there are more differences in the head flow and ring drag with the bore vs. stroke debate than anything.

OldSStroker and I have been looking at different combinations for the newest EMC... with the SCR rule of 10.5:1.... everything from a long stroke Windsor to a big bore BBC interesting differences. One has less friction, the other has more DCR given the same constraints.

Bret

 

It has been said for years that a big bore engine makes more power.(not talking 4.030 as big bore more like 4.125+)
The difference IMO is ya can run a larger intake valve and shroud it less.
Rod ratio diferences will not be seen below 7500RPM's and over that very little if not built for a long rod(6.300+ and that can't be done in a stock block with a stroker).That also depends on the head's and valve timing.
Ya will feel the stroke in the SOP meter from more torque.

 

ok using those calculations above an engine with a 4.165 bore and a 3.25 stroke will apply 12% less mechanical tq than one with a 4.03 bore and 3.48 stroke both bing similar ci, and one with a 4.03 bore and 3.75 stroke will apply 7% more than the 4.03 bore 3.48 stroke engine

I'm glad I've got yall talking a little just tell me if I'm wrong I'm not a get mad person I post here looking for answers and to gain knowledge so leter rip.

I'm a fan of the big bore engines also. my personal engine is a 354ci engine. it's a 4.165 bore and 3.25 stroke 10 to 1 256 @ 50 cam .580 lift has a set of l98 aluminum head that I worked.

so as of now I'm more toward a big bore but I don't realy want this thread to go to the big bore short stroke vs small bore short stroke route. I want it to stay more on the relationship the bore and stroke share with each other

 

The reason is the STROKE.
Figure a 3.5"stroke with a 4.030 then figure a 4.200 bore with the same stroke and both with the same rod.

 

ok first 4.03 bore and 3.5 stroke still using 1000psi

area=4.030/2= 2.015x2.015x3.14=12.749
psi=12749
leverage=1.75 = 15% of 1 ft
tq =12749x.15=1912 lb ft

1912lb ft of force on this crank

now 4.2 bore and 3.5 stroke
area = 13.847
psi=13847
leverage=1.75

lb tq = 2023


the 4.2 bore is 48ci per cyl
the 4.03 bore is onle 43ci per cyl and adding ci always will add force

here is another one with 48ci
4.1 bore and 3.65 stroke
area=13.195
psi=13195
leverage=1.825
tq=2006

another one but square

3.945 bore and stroke
area=12.217
psi=12217
arm=1.9725
tq=2008 lb ft

that's all 48ci I'de keep going the other way and the tq would start to fall off but I'm out of time bbl

 

So, what's the reason that most manufacturers are moving to longer stroke/smaller bore? It's a definite trend outside the 2-valve world. Take the 2.4l I4 from Honda- that thing has a 3.9" stroke! Lots of smaller engines out there are oversquare these days.

 

Oversquare means that the bore is bigger than the stroke.

As to the why, it is based on multiple factors, not the least of which is packaging. (size)

 

Emissions!

 

Seems to me that if you start with the same cylinder head volume, and burn the same amount of fuel, using the same amount of air, and assume the same thermal efficiency, the single cylinder (no matter what its bore/stroke relstionship) will have converted the same amount of energy from thermal to mechanical, by the time the piston reaches the bottom of its stroke. Doesn't matter what the bore/stroke relationship is. The only difference would be the instantaneous torque produced by the combination of cylinder pressure, piston face and crank moment arm at each degree of rotation of the crank. The instantaneous values might be different, but the area under the curve remains the same.

Its a gross oversimplification, but then most of what's been posted to this point is single-dimensional. Ignores the affect of each configuration on volumetric efficiency, air-fuel mixing, flame propagation, friction, and a few hundred other variables.

 

Longer stroke is also a result of better cylinder head technology. If you can run a longer stroke and have the cylinder heads to support it, you can produce quite a bit more torque without losing top end. I think the manufacturers have figured out that most Americans really do like big torque just as much as their horsepower, and that their choices over the past 15 years have been because of a lack of options more than wanting engines that have no "umph". The 4 valvers are not alone, either, all the LSx engines that are 5.7L and up all have strokes over the traditional SBC 350 3.48".

Packaging would also be another big issue, considering that changing the bore spacing requires a complete revamp of all the engine dimensions. From an engineers standpoint, lengthening the stroke is much easier, and less costly.

Emissions may have something to do with it as well, but it seems like the factory can make about anything emissions legal these days.

 

Well said!

Notice that when emissions, detonation resistance, packaging, cost, etc. are NOT a factor, large bore/short stroke engines rule. F1, Cup, ProStock, even Daytona Prototypes(LS1 derivatives), CTSV-R original killer engine (LS7 ~3.25 stroke derivative) are examples. The high-end sanctioning bodies are limiting maximum bore sizes (and consequently minimum strokes) rather than the other way around.

 

I was a few years ago reading on why Dodge did the V10 and this was the reason, they found that when bore sizes got too large it affected flame travel and emmisions even though it could still make good power, hence they went with more cylinders to use larger displacement without getting out of control on the emmions side.