bore vs stroke 2

I've already read this post and have learned a lot
http://web.camaross.com/forums/showthread.php?t=159458&page=6&highlight=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.:D

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

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.

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.

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.

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.

Emissions!
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.

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.

Funny, but GM went the opposite way with their 8 liter stuff. They built some V10s for testing, but ended up with the Vortec 8100. The 8100 has a 4.25 bore and the 8.2 Viper has a 4.03 bore That's about 11% more piston surface area on the 8100.

Note that the Viper V10 was only available because the basic engine was going into trucks. Same with the LS1/6/etc series. It was the millions of truck engines that will be sold that paid for the development and tooling for the Vette, and F-body LS's.

That was a few years later wasnt it? Time and technology change a lot of things. My uncle has the 8.1 in a 3/4 ton for towing a 5th wheel and all I can say is DAMN. I didn't ride in it but I did follow him towing and I was impressed at how it handled hills.

Funny, but GM went the opposite way with their 8 liter stuff.

Which of course was in part because they could leverage the several decades of experience with the Mark IV.

My point being, with all due respect to everyone who posted; none of these decisions are based on single factors.

That was a few years later wasnt it? Time and technology change a lot of things. My uncle has the 8.1 in a 3/4 ton for towing a 5th wheel and all I can say is DAMN. I didn't ride in it but I did follow him towing and I was impressed at how it handled hills.


It's funny, I've had several friend's/aquaintances/family members over the years that have bought trucks to tow with; from one of the big three.

I always tell them that unless they plan on having at least 2/3 - 3/4 of their driving be steady state highway work for long distances to stay away from the diesels and go with the big gas engine option.

The ones that went diesel anyway did so because they couldn't believe that a gas engine could do the job.

501 cubic inches of splayed valve chevy not do the job?
It is to laugh... :D

I was following him with the LT1 95 Roady wagon and I was suprized at how much throttle I was using to follow him. He knows I am a gearhead so I am sure he was showing off a little but thios was in upstate NY, Watkins Glen area those familiar will attest to the grading there.

Going up the hill to the track? Anything coming out of the Glen going north or to the track is dam steep... the SS seems to still pull fine up that puppy at 110+ ;-)

Bret

We stayed at the KOA campground which is on top of the hill we came in a back way with the trailer still steep but it was more broken up than coming straight up out of town. Right out of town up to the campground(near the track) was fun in the wagon, those pesky German cars and such kept TRYING to pass guess they saw a Buick and didn't realize it was LT1 equipped. ANYONE who goes to the track should make time to hike the Gorge, much nicer than Niagra.

Dwayne,

Yeah I know the road you went up well... thats the way we go to the track.

I know the area well, we live 40 mins from the track and it was basically the half way point of a drive I took to get to college and back. Probably did that drive 100 times.

If anyone is going to the Glen for something give the old man and I a PM we will stop by or go grab a beer.

Bret

I have an aunt over in Vestal Center I try to go visit at least every other year and like to get to The Glen while in the area, that hike will wear you out but is so worth it.

Funny, but GM went the opposite way with their 8 liter stuff. They built some V10s for testing, but ended up with the Vortec 8100. The 8100 has a 4.25 bore and the 8.2 Viper has a 4.03 bore That's about 11% more piston surface area on the 8100.
I gotta disagree. They could've gone with the big bore BBC block and had even more cubic inches. Just imagine a factory 548cid (9.0L) BBC :eek: :D

The LS1 has a small bore/long stroke combo, but it looks like GM has learned how to get around that with the newer motors. THe LS2 has a 4.00" bore again and the 6.2L has a 4.065" bore. :D

I gotta disagree. They could've gone with the big bore BBC block and had even more cubic inches. Just imagine a factory 548cid (9.0L) BBC :eek: :D


I guess it depends on what you (they) were trying to accomplish at the time. If the 8.1 (496) gets the job done in 2500 trucks, an extra liter probably won't use less fuel on the highway. Obviously there were more inches available with the same stroke (see the 572). I wouldn't be surprised to see the 8100 become an 8700(530) or a 9000 (549) if the need arises. As you said, it worked for the LSX series. Rarely do they resdesign an engine and not allow for growth.

I've already read this post and have learned a lot
http://web.camaross.com/forums/showthread.php?t=159458&page=6&highlight=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?


The big bore piston would also move away from the cylinder head at a much slower rate is what you were forgetting.

Big bore engines simply allow bigger intake ports and thus more potential for power. When run with the same ports and valves they often will not make good power due to combustion ineffeciency and low compression or nasty chamber problems.

It's no accident that you don't see huge V4s with 5 inch bores and 1 inch strokes in production or in F1. No one here has been the first to think of this! Big bore engines are less efficient and also have worse emmissions because of these problems. It's a balance not a one sided argument.

If bore was all good a massive bore 1 inch stroke V engine could give great packaging and incredible power. Then some one would come along behind you and put a 3 inch stroker crank in that engine and destroy you and increase reliability ten fold.

I do realize that the piston moves slower. and I don't think that big bore engines are "all that". I was just trying to learn a little more in depth about the relationship that they share. I was just looking at it from the amount of force applied to the piston and what it translates to the crank through the leverage @ 90deg angle of the stroke. I saw a pattern form during my calculations that as the bore stroke ratio moved away from square that the amount of tq applied to the crank drops some. I now think that it was an error in my calculations though due to me rounding #'s, but havn't taken the time to redo the calculations to double check.

No **** Erik showed up in a thread about bore and stroke... must be a freaking becon or something when this happens. ;)

No **** Erik showed up in a thread about bore and stroke... must be a freaking becon or something when this happens. ;)

People actually send me to these threads Bret. I don't know why though?

I love bore AND stroke though. I try and maximize both if possible. Bore allows better heads and stroke then utilizes those heads.

If I take too much stroke out I then valvetrain limit my engine and/or kill my reliability big time. Taking stroke out a regular engine is plain stupid in general when looking at things from a performance perspective.

It's no accident that you don't see huge V4s with 5 inch bores and 1 inch strokes in production or in F1. No one here has been the first to think of this! Big bore engines are less efficient and also have worse emmissions because of these problems. It's a balance not a one sided argument.

If bore was all good a massive bore 1 inch stroke V engine could give great packaging and incredible power. Then some one would come along behind you and put a 3 inch stroker crank in that engine and destroy you and increase reliability ten fold.

A 5" x 1" V4 would have a major negative going for it in both F1 (where is wouldn't be legal, but forget that) and production type engines:

At 78.54 cubic inches it would be giving up about 104 cubes in the 3.0L format and about 68 cubes in the 2.4L format. Don't you always say go for the max cubes allowed in the class? Of course if you stroked that engine to 3 inches with the same bore you'd triple the displacement to about 236 cubes which probably would make (a lot) more power than the 78 cube engine.

Most folks reading this board don't have a tough time agreeing that 3 times the displacement will make more power.

Given a displacement limit, what bore/stroke ratio would you choose for a max performing engine? 5:1, 2.3:1, 1.67:1, 1.3:1 or ??? Why?

Given a displacement limit, what bore/stroke ratio would you choose for a max performing engine? 5:1, 2.3:1, 1.67:1, 1.3:1 or ??? Why?

I would choose a bore that works in the block I have and then add enough stroke to use it in an rpm range that makes sense.

If I could do my own block it would be a big stroke small block probably like 4.185 bore and 4.250 stroke with 375 cfm heads that turns around 7000 peak.

Bore/stroke ratio has no real effect on anything unless you hold cubic inches constant.

Bore sets how big your intake valves can be and how many you can have and stroke pretty much determines where in rpm you will make that power at.

Given a displacement limit, what bore/stroke ratio would you choose for a max performing engine? 5:1, 2.3:1, 1.67:1, 1.3:1 or ??? Why?


I would choose a bore that works in the block I have and then add enough stroke to use it in an rpm range that makes sense.

If I could do my own block it would be a big stroke small block probably like 4.185 bore and 4.250 stroke with 375 cfm heads that turns around 7000 peak.

Bore/stroke ratio has no real effect on anything unless you hold cubic inches constant.
Bore sets how big your intake valves can be and how many you can have and stroke pretty much determines where in rpm you will make that power at.


So you'd choose .985 bore/stroke ratio for a 468 cube engine because "Bore/stroke ratio has no real effect on anything unless you hold cubic inches constant?" That's not how most folks build 468s unless maybe detonation on poor 91 octane is a problem. Interesting out-of-the-box concept.

I'd choose a 4.500 x 3.678 large block for the same cubes with a 1.22 B/S ratio. I think B/S ratio has a real effect. Note that my choice gives about 8100 rpm at the same PS as your 7000.

"Different strokes for different folks." as they say. Thanks for your input.

The stroke has an effect on where you make power but the bore / stroke doesn't reallyl Oldstroker unless you are talking about the same inches.

A 2X2 will turn double the rpm of a 4x4 even though they are both square.

Oldsstroker,

I also said where I want my powerband (max 7000) at but for unlimted rpm I would go smaller stroke and larger bore. I would need much more info to know what actual dimensions I would use. At 468 I might even go higher in bore and lower instroke than you did especially if it was a 4 valve engine since I could use more rpm at that time but I might even go to a V 10 or 12.

I did not take "max performing" to be boundless because otherwise I wouldn't even stay wih 8 cylinders. Extremely large bores have poor thermo and burn characteristics and lower efficiencies when operated at extreme rpm. It's also really hard to make good pistons that are stiff and seal and last long at huge bore sizes.

Oldsstroker,

I also said where I want my powerband (max 7000) at but for unlimted rpm I would go smaller stroke and larger bore. I would need much more info to know what actual dimensions I would use. At 468 I might even go higher in bore and lower instroke than you did especially if it was a 4 valve engine since I could use more rpm at that time but I might even go to a V 10 or 12.

I did not take "max performing" to be boundless because otherwise I wouldn't even stay wih 8 cylinders. Extremely large bores have poor thermo and burn characteristics and lower efficiencies when operated at extreme rpm. It's also really hard to make good pistons that are stiff and seal and last long at huge bore sizes.


You are a moving target! :)

How about this one: A 2006 EMC rules engine that needs to be a "max performer" in the 2500-6500 range at 300-330 rpm/second. Scoring is average torque PLUS average hp in that range.

V8, pushrod 2 valve head, 434.0 cubes max, small or large block, 10.5:1 SCR max, one 4-bbl on a cast manifold, 91 octane pump gas. Those are the basics.

What bore and stroke would you use and why?

I would do a 4.300 stroke by 4.000 inch bore to make the smallest chamber possible. I could still run the airfow I needed which at 434 inches and 6500 rpm can easily be achieved on a 4.000 bore with commercial heads. I am still thinking of what I could do with what is really out there. i wouldn't put that in stone though but I have done some very detonation free stuff myself but at 10.5 to one it would be less important now.

I would do a 4.300 stroke by 4.000 inch bore to make the smallest chamber possible. I could still run the airfow I needed which at 434 inches and 6500 rpm can easily be achieved on a 4.000 bore with commercial heads. I am still thinking of what I could do with what is really out there. i wouldn't put that in stone though but I have done some very detonation free stuff myself but at 10.5 to one it would be less important now.

Interesting. I'd run oversquare with about 80% of your PS. Both of us would require special cranks. It will be interesting to see which way the EMC contestants go.

they seem to go a little under square. or like in John Case's bbf engine he found the bore the heads flowed the best at then had a crank made to reace the maximum ci.

OH and one more question now. how bad are the negitave effects on an engine when the chamber diameter is larger than the bore. how bad does the lip caused by the chamber hurt the engine. example I have a set of track 1's that the chamber is 4.185 from side to side. what if I put those on a 4.03 bore block. If I had a dyno or $ I would just test it, but I hope one of yall can help.

ps sorry for the side track.

OH and one more question now. how bad are the negitave effects on an engine when the chamber diameter is larger than the bore. how bad does the lip caused by the chamber hurt the engine. example I have a set of track 1's that the chamber is 4.185 from side to side. what if I put those on a 4.03 bore block. If I had a dyno or $ I would just test it, but I hope one of yall can help.

ps sorry for the side track.

It's not good especially if the gasket hangs in there too and can turn into a glow plug. Believe it or not a lot of head porters do that too and it's good in general.

OH and one more question now. how bad are the negitave effects on an engine when the chamber diameter is larger than the bore. how bad does the lip caused by the chamber hurt the engine. example I have a set of track 1's that the chamber is 4.185 from side to side. what if I put those on a 4.03 bore block. If I had a dyno or $ I would just test it, but I hope one of yall can help.

ps sorry for the side track.



If the bore is shrouding the intake at the LEDGE at the top of the block it's not good. Ya need to cut the bore back at this point to relieve the shrouding.

they seem to go a little under square. or like in John Case's bbf engine he found the bore the heads flowed the best at then had a crank made to reace the maximum ci.

I'm not sure that was the design approach.

The static compression limit in the 2006 EMC rules may have shifted the focus from anti-detonation. We'll see.

It's not good especially if the gasket hangs in there too and can turn into a glow plug. Believe it or not a lot of head porters do that too and it's not good in general.



DAMN! That's what I meant to say! :rolleyes: