Can someone clear up this "myth" for me.
11-13-2006, 10:51 PM
#1
TECH Regular
Thread Starter
Join Date: Mar 2005
Location: DFW, Texas
Posts: 401
Likes: 0
Received 0 Likes
on
0 Posts
Can someone clear up this "myth" for me. Correct me, please, if I am wrong.
I thought that more stroke generally gave more low end torque. It always seemed that way when I saw these motors with less stroke end up with less torque and all the motors with more stroke end up with more torque.
Say you take two identical motors with the exception that motor B has more stroke. Would motor B be making more torque because of the stroke or simply because it has a higher displacement?
I thought that more stroke generally gave more low end torque. It always seemed that way when I saw these motors with less stroke end up with less torque and all the motors with more stroke end up with more torque.
Say you take two identical motors with the exception that motor B has more stroke. Would motor B be making more torque because of the stroke or simply because it has a higher displacement?
11-13-2006, 11:03 PM
#2
TECH Fanatic
iTrader: (5)
Join Date: Apr 2005
Location: Phoenix AZ
Posts: 1,005
Likes: 0
Received 0 Likes
on
0 Posts
longer strokes deliver power for a longer duration as they are simply moving downward farther. That makes for more TQ. Shorter strokes make for higher revs and more top end. The strokes in mst diesels are retardedly long. Hence the extremeloy low rpm band. GL man
John
John
11-13-2006, 11:32 PM
#4
TECH Resident
Join Date: Jan 2003
Location: AUSTIN TX
Posts: 855
Likes: 0
Received 0 Likes
on
0 Posts
Originally Posted by adillhoff
Say you take two identical motors with the exception that motor B has more stroke. Would motor B be making more torque because of the stroke or simply because it has a higher displacement?
Trending Topics
11-14-2006, 07:45 AM
#8
Staging Lane
Join Date: Jul 2006
Posts: 53
Likes: 0
Received 0 Likes
on
0 Posts
CR is not a dominant factor in determining the torque of either a spark or a compression ignition engine.
Here, let C.J. Baker explain diesel torque in the Banks Engineering tech talks:
Here, let C.J. Baker explain diesel torque in the Banks Engineering tech talks:
... Now let’s discuss effective cylinder pressure on the power stroke of a diesel as compared to that in a gasoline engine. We’ve already mentioned that higher turbocharger boost raises the effective cylinder pressure, but let’s look at what else comes into play. In “Understanding Today’s Diesel” elsewhere on this site, the way fuel is introduced into the cylinder is thoroughly discussed for both gasoline and diesel engines. Gasoline engines mix the fuel with the air before it enters the cylinder, so when the intake valve closes, the power potential of that air and fuel charge is set. The timed spark ignites the mixture and cylinder pressure rises to a peak at roughly 15º after TDC. Because the combustion process takes time, combustion may or may not be complete by 15º after TDC depending on engine RPM, but for all practical purposes, we can say that the process of combustion is concluded early in the power stroke and that no more heating of the working fluid (the gases in the cylinder) occurs. This means the force acting on the piston top is highest at a time when the connecting rod has very little leverage on the crankshaft pin. As the crankshaft continues to rotate past TDC, the leverage the piston can exert increases, but the pressure on the piston top is dropping quickly. This, too, is discussed in the aforementioned article.
Once you envision when combustion occurs and the relationship between cylinder pressure and leverage on the crankshaft, it becomes obvious that if we could continue the burning process longer into the power stroke, additional cylinder pressure could be generated to push on the piston top as connecting rod-to-crankshaft angle improves for more leverage, and hence more torque. This is exactly what happens in a diesel. Because the fuel is injected into the cylinder after the intake valve is closed and the air is compressed, the length of the fuel injection pulse, called pulse width, can be extended well into the power stroke. This means the average effective cylinder pressure acting on the piston is higher in a diesel than in a comparably sized gasoline engine. The higher turbo boost pressure, high compression ratio, and greater heat content of the fuel all add to the generation of cylinder pressure that is substantially higher than in gasoline engines too, but it is this continued injection of fuel that really makes the big torque numbers for diesels. And all of this taken together makes it apparent why diesel engines have to be built with such robust parts to withstand this high cylinder pressure and torque.
Once you envision when combustion occurs and the relationship between cylinder pressure and leverage on the crankshaft, it becomes obvious that if we could continue the burning process longer into the power stroke, additional cylinder pressure could be generated to push on the piston top as connecting rod-to-crankshaft angle improves for more leverage, and hence more torque. This is exactly what happens in a diesel. Because the fuel is injected into the cylinder after the intake valve is closed and the air is compressed, the length of the fuel injection pulse, called pulse width, can be extended well into the power stroke. This means the average effective cylinder pressure acting on the piston is higher in a diesel than in a comparably sized gasoline engine. The higher turbo boost pressure, high compression ratio, and greater heat content of the fuel all add to the generation of cylinder pressure that is substantially higher than in gasoline engines too, but it is this continued injection of fuel that really makes the big torque numbers for diesels. And all of this taken together makes it apparent why diesel engines have to be built with such robust parts to withstand this high cylinder pressure and torque.
Last edited by LILS; 11-14-2006 at 08:01 AM.
11-14-2006, 08:17 AM
#9
TECH Resident
Join Date: Dec 2004
Location: K-W, Ontario
Posts: 845
Likes: 0
Received 0 Likes
on
0 Posts
Say you take two identical motors with the exception that motor B has more stroke. Would motor B be making more torque because of the stroke or simply because it has a higher displacement?
engine had a larger bore as opposed to the stroke, the results would be very
close.
The larger bore in this example would also breathe a little better because the
valves would be unshrouded.
Where the torque peak occurs has to do with the depression created by the
piston at a certain RPM. The longer stroke would normally place the peak
lower in the RPM range as the shorter stroke requires more RPM to achieve
the same piston speed.
Diesel engines make more torque because of fuel energy, and compression.
Comparing the two motors is a far stretch. Diesels are normally boosted as welll.
11-14-2006, 08:21 AM
#10
Teching In
Join Date: Oct 2006
Location: N. Chicago
Posts: 4
Likes: 0
Received 0 Likes
on
0 Posts
If the peak volumetric efficiencies of two identical engines occur at a mean piston speed of 2000 feet per second,
the longer stroke one will reach this speed at lower rpm.
http://www.wallaceracing.com/enginetheory.htm
Paul
the longer stroke one will reach this speed at lower rpm.
http://www.wallaceracing.com/enginetheory.htm
Paul
11-14-2006, 08:24 AM
#11
Originally Posted by adillhoff
Say you take two identical motors with the exception that motor B has more stroke. Would motor B be making more torque because of the stroke or simply because it has a higher displacement?
11-14-2006, 10:12 AM
#12
TECH Addict
I'm not sure where I read it (maybe MM&FF or something) but they built two motors of identical displacement, one small bore long stroke and the other big bore short stroke.
The Long stroke motor did make more low end but not by as much as was thought, however due to the long stroke it didn't like the high rpms so made less PEAK hp.
Also another thing to remember is the rest of the configuration of the motor. Most DOHC setups are big bore short stroke motors, this is beacuse it suits the valvetrain for higher rpms.
Most OHV motors don't favour high rpms due to limitations in the valve train, so to maximise potential a long stroke motor is often the better answer.
A victim of getting it WRONG is the Ford Modular SOHC 4.6 2v unit as found in the sn95 Mustang. OHC valvetrain ideal for high revs but they give the motor a long stroke, this of course gives it the American style low end grunt but totally voids the advantage of running OHC. Combine this with only a small bore and the breathing abilty and that's why in stock trim they aren't very powerful and feel breathless as you close in on the red line.
The Long stroke motor did make more low end but not by as much as was thought, however due to the long stroke it didn't like the high rpms so made less PEAK hp.
Also another thing to remember is the rest of the configuration of the motor. Most DOHC setups are big bore short stroke motors, this is beacuse it suits the valvetrain for higher rpms.
Most OHV motors don't favour high rpms due to limitations in the valve train, so to maximise potential a long stroke motor is often the better answer.
A victim of getting it WRONG is the Ford Modular SOHC 4.6 2v unit as found in the sn95 Mustang. OHC valvetrain ideal for high revs but they give the motor a long stroke, this of course gives it the American style low end grunt but totally voids the advantage of running OHC. Combine this with only a small bore and the breathing abilty and that's why in stock trim they aren't very powerful and feel breathless as you close in on the red line.
11-14-2006, 09:13 PM
#14
The simplest way to think of it is with leverage. The same force of the piston on a longer moment arm from the stroke means more torque. Of course you can't just change stroke without either changing displacement or bore.
Same thing applies to bore, since the force on the top of the piston equals pressure times piston area, increasing bore increases the force of the piston on the crankshaft. More force times the same distance equals more torque. But also more displacement, just like stroke.
Maybe a better comparison would be, how does stroke affect torque/displacement? But that's already been answered.
Same thing applies to bore, since the force on the top of the piston equals pressure times piston area, increasing bore increases the force of the piston on the crankshaft. More force times the same distance equals more torque. But also more displacement, just like stroke.
Maybe a better comparison would be, how does stroke affect torque/displacement? But that's already been answered.
11-14-2006, 09:17 PM
#15
Originally Posted by Theox
If the peak volumetric efficiencies of two identical engines occur at a mean piston speed of 2000 feet per second,
the longer stroke one will reach this speed at lower rpm.
http://www.wallaceracing.com/enginetheory.htm
Paul
the longer stroke one will reach this speed at lower rpm.
http://www.wallaceracing.com/enginetheory.htm
Paul
Bingo!!
This has to do with piston speed and thermal effiecency. Higher piston speeds can typically induce a higher VE.
11-14-2006, 11:24 PM
#17
Wow.... this is a hell of a thread full of BS. Longer strokes do not make more HP, given equal displacement and cylinder filling it's the same.
There was a good article in Hot Rod that dyno tested and verified this exact issue at hand and they found that the larger bore, shorter stroke motor made a few more ft lbs of TQ at the low RPM area of the test range.... either way it was within testing differences.
I think it was June 2005 pg 98-106
I suggest some of the posters on this thread read that before they go laying opinions on the matter down.
Bret
There was a good article in Hot Rod that dyno tested and verified this exact issue at hand and they found that the larger bore, shorter stroke motor made a few more ft lbs of TQ at the low RPM area of the test range.... either way it was within testing differences.
I think it was June 2005 pg 98-106
I suggest some of the posters on this thread read that before they go laying opinions on the matter down.
Bret
11-14-2006, 11:52 PM
#18
Originally Posted by SStrokerAce
I think it was [Hotrod Magazine] June 2005 pg 98-106
I suggest some of the posters on this thread read that before they go laying opinions on the matter down.
Bret
Bret
Waaaayyy too much of that stuff going on here at LS1tech. People, if you're not CERTAIN of something, don't post up. This is not a frickin' democracy. Too much misinformation here!
11-27-2006, 06:18 PM
#20
TECH Veteran
Join Date: Dec 2001
Location: Ft. Lauderdale, FL, USA
Posts: 4,117
Likes: 0
Received 0 Likes
on
0 Posts
Originally Posted by Nate_Taufer
This is a completely incorrect blanket statement.
I'm talking about daily driver type engine builds.
.