Bore Vs Stroke - how it shapes up in the real world --

[statsystems]

EMC engines as examples of long stroke advantage.

Many posters have used the fact that a lot of the top producing EMC engines were under square configurations and therefor justified the belief the the long stroke was inherently superior - at least when it came to producing torque. I have to make one important point here is that most of the guys who compete in this challenge could, from the results they get, generate a strong argument as to why we should all build flat head Fords. These guys are top notch so please factor that into your thoughts on the subject. At the end of the day you would be surprised how many work very much with what they have got not necessarily what they want.

One aspect we have not brought up here is that the geometry of a short stroke/big bore actually beats out a long stroke with a small bore given the same cylinder pressure profile.

DV

I have called the over square stuff geometrically retarded for years. Superior markets trumps facts everyday of the week.

[Rick360]

One aspect we have not brought up here is that the geometry of a short stroke/big bore actually beats out a long stroke with a small bore given the same cylinder pressure profile.

DV

With the same R/S ratio between 2 identical displacement engines of different strokes exactly how does the geometry favor the short stroke?

The ratio of piston area between a given long and short stroke engines would be inversely proportional to the stroke ratio between the same 2 engines. Therefore, with the same R/S ratio and pressure profile in the cylinder, how could turning force on the crank differ?

Rick

[Leftcoaster]

Variables, variables, variables

For performance, a lower rod/stroke ratio provides less time for induction requiring larger diameter ports and valves, until a practical limit is reached

Now factor in the rod to crank leverage ration during combustion, the combustion cycle, and the increased piston to wall thrust resistance therefore power losses

And whether it's a two valve or four valve head

Working with stock blocks and heads introduces so many compromises its like trying to create a silk purse from a sow's ear

Obviously, the architecture of one stock design will favour one set of extremes over another

Engines designed for race use do not answer our dilemma - - as they're street/strip performance is uncompetitive

Simulations aside, deciding which bore/stroke ratio is "best" requires custom everything including block heights, and even then remains open to interpretations as the likes of 4 barrel manifold runner lengths change also

And on we go chasing our tails until giddy

[Belgian1979]

I run a short stroke big bore 350. Basically what DV once described in one of his books (4.125X3.250X6.250)

Favors high rpm. So combined with a short rear end it still has good bottom end and pulls like a freight train on top. Doesn't seem to run out of air.
Cam needs to be made for the combo.
Run ITB's on it and EFI. Cam has too much overlap imo, but that's what you get when you want to run higher rpm.

Only downside so far is the piston pin protruding in the ring land. Personally don't like it, but that's how it is.

[Stan Weiss]

One aspect we have not brought up here is that the geometry of a short stroke/big bore actually beats out a long stroke with a small bore given the same cylinder pressure profile.

DV

With the same R/S ratio between 2 identical displacement engines of different strokes exactly how does the geometry favor the short stroke?

The ratio of piston area between a given long and short stroke engines would be inversely proportional to the stroke ratio between the same 2 engines. Therefore, with the same R/S ratio and pressure profile in the cylinder, how could turning force on the crank differ?

Rick

Rick,
No sure about my math, but I do come up with them being the same.

Stan

[Stan Weiss]

From Max Race Software / Larry Meaux Forum

Bore -VS- Stroke at equal Cubic Inches and Rod Ratios

http://maxracesoftware.com/bulletinboar ... c50f28b743

Bore -VS- Stroke at equal Cubic Inches and Rod Ratios

from Formula One ( even with old Rules= Pneumatic ValveTrains ) -to- NASCAR
then from NASCAR -to- ProStock
then from ProStock -to- largest Engine offered by Sonny Leonard 's = 1006 CID

Sonny Leonard 's 1006 CID small-Bore large-Stroke bad RodRatio "rules the Planet"
in terms of HP per Cylinder Head Flow CFM , Brake Specific Fuel Consumption, BSAC by far ahead of anyone else .-on Earth

in fact Formula One 4-Valve is not much better than old NASCAR 2-Valve
and NASCAR is still way behind current NHRA ProStock 2-Valve .... all in terms of BMEP or Torque per Cubic Inch

Big Bore + Short Stroke rules HP/CID wars
but Small-Bore + large Stroke rules HP per CFM output

these are a Physics FACTs => you want the most HP per CFM ?? , then its Small-Bore + Large Stroke
you want the most HP per CID ?? , then its Large-Bore + Short-Stroke

Question :
since equal Cubic Inch displacement + equal Rod Ratios will move the very same AirFlow CFM at all Crank angles and at same VE%
whats going on ??

Why ... is Sonny's 2-Valve restricted 1006cid so much superior to ProStock in making the most HP per CFM ???
and comparing Formula One 4-Valve .. even with Pneumatic ValveTrains , is a joke against Sonny's HP per CFM

then again , likewise vice/versa .... Sonny's 1006 is a Joke in the HP/CID department -vs- Formula One

but what's going on to get so much more HP out of so little CFM ( Sonny's 1006cid ) ????

in the last week.. i took a hard look at Sonny's 1006cid , using every equation i had , i ran a bunch of simulations,
they all show Sonny's 1006 relatively Small-Bore + Large-Stroke engine to make superior HP per CFM

BTW ... all calculations show or prove Sonny's Dyno is almost exactly correctly calibrated or correlated to my Dyno,
in other words, he is not just trying to sell engines with some inflated HP and TQ numbers, they are the Real-Deal ,
at least in that one article on his 1006cid 2150 HP engine at this Link :
http://www.maxracesoftware.com/bulletin ... =19&t=1234

so still the Question lingers => " What's going on ?? "

it appears its the Piston Top in the way ... the distance the Piston is from the Valves
at max piston velocity ... larger stroke 1005.84cid has Piston more than 1/2 inch away downward from Valves ( Bore= 5.220 Stroke= 5.875 Rod C-C = 7.913 )
than the equal Big Bore Short Stroke same Rod Ratio version ( Bore= 5.875 Stroke= 4.638 Rod C-C= 6.247 )
...... you could go extreme opposite and somehow create a 1006 cid engine with a 0.500" or 1/2 stroke, Piston would always be in the way of Flow )
...... remember, Flow has velocity and inertia and there's acoustical wave pressure , and the Piston top is blocking and bouncing velocity back up into Valves/Port )
and during Overlap ... theres no place for Flow to go, except to escape more out the exhaust )

in Sonny's version , the Piston is moving downward ... more keeping in step with acoustical wave FPS and Valve discharge velocity FPS and getting out the way of Valve Lift quicker/sooner



and if you somehow were able to make Ring Friction or all Friction disappear ( + better Ring Seal, less Bore distortion from larger Strokes )
..... then Sonny's version would be even more superior at turning CFM into HP
( this was with all Aluminum Engine ... even much better CFM to HP if it were all cast-iron engine )

a Large-Bore Short-Stroke engine of equal CID, Rod Ratio , Compression Ratio
will ALWAYS need to make more VE% to turn the CFM into HP,
and it will make a higher BSAC and BSFC, in the process

its a good thing the large Bore allows use of larger Valves , to make it easy to get higher VE%,
which then allows greater HP per CID , but worse HP per CFM

Larry,
I believe this is like if I have an engine that has to small a carb on it and I put the right size carb on it. I make more HP but the HP per rated cfm is down. Because I had a higher than rated depression on the smaller carb and it was really pulling more cfm than its rating.

Stan

Anyone else have any other ideas?

Stan

[David Vizard]

In a frictionless world there is no advantage to the big bore short stroke if just the bore stroke is manipulated. As the excell spread sheet below shows that with two differing configs and a fixed cylinder pressure (taken at 72 degrees ATDC from a TFX generated pressure graph) we find that the instantanious torque is identical and will be so through out the power stroke. for a given CID this should show the long stroke theorists the fallacy of a 'long arm'' being better for torque. All other things being equal it should be identical.

Bore ----------------4.33 ----------4.56
Stroke---------------4.25----------3.8321
Cylinder CID -------62.583-------62.583
Total CID--------- --500.664 -----500.664
Piston area --------14.7254------16.331
Pressure ------------145-----------145
Force ----------------2135.183----2367.995
Instantaneous Tq - 378.1-------378.1

If we now throw in all the possible advantages of a bigger bore and the ability to have a longer rod in a given block height then the short stroke starts to display it's advantages. First the rod angle is reduced and the side thrust into the piston wall is reduced as is friction. A bigger set of valves can be used and for any given piston speed the rpm the engine can turn up to is proportionally increased. The bigger valves require less duration to make a given HP and the shorter duration is a means toward increased torque. Net result here is a wider power band.

The longer rod ratio delays peak piston air demand on the intake valve to a later point in the cycle. This means that when peak demand is reached the intake valve is slightly more open. The bore friction is reduced almost in proportion to the stroke reduction.

The only down side is that if the advantages of a big bore/short stroke are used the bearing and valve train loads are going to be higher.

DV

[David Vizard]

Larry,
Just read the post that Stan showed a page or so back. This all sounds good but did you take into account the difference in CR used by Sonny's engine and those of an atmo F1 engin?
DV

[MadBill]

Larry,
I believe this is like if I have an engine that has to small a carb on it and I put the right size carb on it. I make more HP but the HP per rated cfm is down. Because I had a higher than rated depression on the smaller carb and it was really pulling more cfm than its rating.

Stan

Anyone else have any other ideas?

Stan

First we need to define what CFM is the yardstick. From the context I believe it is 28" H2O depression flow bench data, in which case we need to realize that a 500 c.i. engine at peak power will pull more air through the meter than an otherwise-identical 400" one at its peak. Therefore unless someone has actual B.S.A.C. data from the engines under review, the comparison is flawed. On the bench data basis though, I agree: When airflow is limited by bore and thus valve size, the builder will do everything he can to utilize all the available air, leading to very good HP/bench CFM*. BUT since there are no prizes for that, If he could get 20% more flow and make 'only' 10% more power, he'd take it in a heartbeat.
*Which somewhat debunks the common equations like "HP = 0.257 x CFM x # of cylinders." "So, if I take these heads off my 302 and put them on my 440" SBC, it will make exactly the same power as the 302?"

[digger]

If I have a given bore and increase it without changing the head in any way it's possible the pressure recovery goes down slighty as the bore is an extension of the chamber to some extent

[Belgian1979]

In a frictionless world there is no advantage to the big bore short stroke if just the bore stroke is manipulated. As the excell spread sheet below shows that with two differing configs and a fixed cylinder pressure (taken at 72 degrees ATDC from a TFX generated pressure graph) we find that the instantanious torque is identical and will be so through out the power stroke. for a given CID this should show the long stroke theorists the fallacy of a 'long arm'' being better for torque. All other things being equal it should be identical.

Bore ----------------4.33 ----------4.56
Stroke---------------4.25----------3.8321
Cylinder CID -------62.583-------62.583
Total CID--------- --500.664 -----500.664
Piston area --------14.7254------16.331
Pressure ------------145-----------145
Force ----------------2135.183----2367.995
Instantaneous Tq - 378.1-------378.1

If we now throw in all the possible advantages of a bigger bore and the ability to have a longer rod in a given block height then the short stroke starts to display it's advantages. First the rod angle is reduced and the side thrust into the piston wall is reduced as is friction. A bigger set of valves can be used and for any given piston speed the rpm the engine can turn up to is proportionally increased. The bigger valves require less duration to make a given HP and the shorter duration is a means toward increased torque. Net result here is a wider power band.

The longer rod ratio delays peak piston air demand on the intake valve to a later point in the cycle. This means that when peak demand is reached the intake valve is slightly more open. The bore friction is reduced almost in proportion to the stroke reduction.

The only down side is that if the advantages of a big bore/short stroke are used the bearing and valve train loads are going to be higher.

DV

Is that taking into account the piston dwells more ATDC and thus more pressure is generated ?

[Stan Weiss]

In a frictionless world there is no advantage to the big bore short stroke if just the bore stroke is manipulated. As the excell spread sheet below shows that with two differing configs and a fixed cylinder pressure (taken at 72 degrees ATDC from a TFX generated pressure graph) we find that the instantanious torque is identical and will be so through out the power stroke. for a given CID this should show the long stroke theorists the fallacy of a 'long arm'' being better for torque. All other things being equal it should be identical.

Bore ----------------4.33 ----------4.56
Stroke---------------4.25----------3.8321
Cylinder CID -------62.583-------62.583
Total CID--------- --500.664 -----500.664
Piston area --------14.7254------16.331
Pressure ------------145-----------145
Force ----------------2135.183----2367.995
Instantaneous Tq - 378.1-------378.1

If we now throw in all the possible advantages of a bigger bore and the ability to have a longer rod in a given block height then the short stroke starts to display it's advantages. First the rod angle is reduced and the side thrust into the piston wall is reduced as is friction. A bigger set of valves can be used and for any given piston speed the rpm the engine can turn up to is proportionally increased. The bigger valves require less duration to make a given HP and the shorter duration is a means toward increased torque. Net result here is a wider power band.

The longer rod ratio delays peak piston air demand on the intake valve to a later point in the cycle. This means that when peak demand is reached the intake valve is slightly more open. The bore friction is reduced almost in proportion to the stroke reduction.

The only down side is that if the advantages of a big bore/short stroke are used the bearing and valve train loads are going to be higher.

DV

Is that taking into account the piston dwells more ATDC and thus more pressure is generated ?

Turning force on the crank journal is what counts. ATDC as long as there is modifications to the standard geometry like offset wrist pins or crank center line, etc. there is zero turning force no matter how much pressure is pushing on the piston because ATDC the rod crank angle is 180 degrees. Unless I misunderstand it the longer rod will produce less turning force earlier in the crank rotation and more turning force later in the crank rotation. As the rod stroke ratio increase the point in crank degrees where the rod and crank are at 90 degrees becomes farther from TDC.

Stan

[pdq67]

I run a short stroke big bore 350. Basically what DV once described in one of his books (4.125X3.250X6.250)

Favors high rpm. So combined with a short rear end it still has good bottom end and pulls like a freight train on top. Doesn't seem to run out of air.
Cam needs to be made for the combo.
Run ITB's on it and EFI. Cam has too much overlap imo, but that's what you get when you want to run higher rpm.

Only downside so far is the piston pin protruding in the ring land. Personally don't like it, but that's how it is.

There is an old article in a magazine that was titled, "The 350 engine Chevy should have made", only I think that it's rods weren't quite as long as yours.

They did make one, but it was an old, 348-"W" engine and I am here to say that onna them ran damned good!

The article stressed small combustion chambers, long rods, and flat-top pistons so that a good CR could be ran on our 87 octane gasoline back then.

pdq67

[Belgian1979]

In a frictionless world there is no advantage to the big bore short stroke if just the bore stroke is manipulated. As the excell spread sheet below shows that with two differing configs and a fixed cylinder pressure (taken at 72 degrees ATDC from a TFX generated pressure graph) we find that the instantanious torque is identical and will be so through out the power stroke. for a given CID this should show the long stroke theorists the fallacy of a 'long arm'' being better for torque. All other things being equal it should be identical.

Bore ----------------4.33 ----------4.56
Stroke---------------4.25----------3.8321
Cylinder CID -------62.583-------62.583
Total CID--------- --500.664 -----500.664
Piston area --------14.7254------16.331
Pressure ------------145-----------145
Force ----------------2135.183----2367.995
Instantaneous Tq - 378.1-------378.1

If we now throw in all the possible advantages of a bigger bore and the ability to have a longer rod in a given block height then the short stroke starts to display it's advantages. First the rod angle is reduced and the side thrust into the piston wall is reduced as is friction. A bigger set of valves can be used and for any given piston speed the rpm the engine can turn up to is proportionally increased. The bigger valves require less duration to make a given HP and the shorter duration is a means toward increased torque. Net result here is a wider power band.

The longer rod ratio delays peak piston air demand on the intake valve to a later point in the cycle. This means that when peak demand is reached the intake valve is slightly more open. The bore friction is reduced almost in proportion to the stroke reduction.

The only down side is that if the advantages of a big bore/short stroke are used the bearing and valve train loads are going to be higher.

DV

Is that taking into account the piston dwells more ATDC and thus more pressure is generated ?

Turning force on the crank journal is what counts. ATDC as long as there is modifications to the standard geometry like offset wrist pins or crank center line, etc. there is zero turning force no matter how much pressure is pushing on the piston because ATDC the rod crank angle is 180 degrees. Unless I misunderstand it the longer rod will produce less turning force earlier in the crank rotation and more turning force later in the crank rotation. As the rod stroke ratio increase the point in crank degrees where the rod and crank are at 90 degrees becomes farther from TDC.

Stan

I do not agree. Higher pressures at around 15° ATDC because the piston moves slower, will translate to higher pressures acting on the piston anywhere ATDC untill the exhaust valve opens.

[user-612937456]

I do not agree. Higher pressures at around 15° ATDC because the piston moves slower, will translate to higher pressures acting on the piston anywhere ATDC untill the exhaust valve opens.

I don't have an engineering background and I am slow to comprehend sometimes. But is I understand what you say it doesn't take into account that the combustion reaction is at the same speed in a particular environment no mater what speed the piston travels. So If the piston speed slows there is more time for the combustion to introduce more heat expansion during the whole cycle resulting in less left over combustion expansion after the exhaust opens. My apologies if I missed the point.