Mr. Sanderson and V8pwr are correct, you get more torque from the added leverage.

To increase stroke, you move the rod journal out farther from the centerline of the crank. In doing so, you increase the amount of torque placed on the crank. (Think: what exerts more force? 50lbs on a 1-foot breaker bar or 50lbs on a 5-foot breaker bar.)

But, it isn't the only reason you get such a nice increase in torque.

With the longer stroke, you also get a longer distance in which the combustion gasses are exerting force on the power stroke. This means each cylinder has more time to exert force on the crank during each power stroke.

Combine these two effects and enjoy a very nice torque monster.

 

It should be noted:

With a longer stroke you add more torque, but you also exert more force on the rod bolts on the upstroke due to the increased speed at which the piston is travelling at any given rpm. This limits your max rpm a little more unless you upgrade rod bolts.

 

Yep, if you are tightening a bolt, you are correct.

However to relate this to engines, with a given displacement a longer stroke engine will necessarily have a smaller bore and less area for the expanding combustion gasses to push on. That means less force pushing on the longer lever arm. It equals out. *

OK, I'm assuming both engines get the same combustion pressure because they breathe in the same amount of air. Usually the larger bore engine breathes better with the same heads, but let's say they are the same. We also don't need to go into more friction from a longer stroke, but I'm not telling you anything you don't already know as a professional engine buiilder.

I just don't like to see less knowledgeable folks misled.

*Do the math for a 4.000 bore x 4.000 stroke 402 then figure the bore for 4.250 stroke 402, and then compare areas of the pistons and the stroke ratio. Or you could just take my word for it.

 

Theres no school like old school!
Right stroker?

 

HOW ABOUT THIS ONE, STROKER!
Biggest stroke the block will take.
Biggest bore the block will take w/out compramising wall strength.
Maximum compression of desired fuel{street most likley}
1.63-1to 1.7-1 r/s ratio
w/good heads w/highvelocity/hi-volume ports
the right cam for the combo!

 

Old School and Old Fool...I resemble both of those.

 

The only bubble needing popped is yours. Old SStroker said it best.

 

totally wrong... while the statement about 50lbf on a 2 foot lever arm making more torque than 50lbf on a 1 foot lever arm is CORRECT, you completely missed the fact that you dont have a constant force because for a given displacement, your bore is changing as well (i.e. decreasing the force in a porportional rate to the increase in lever arm)

What makes torque? Increasing the amount of chemical energy that can be released and utilized efficiently during a combustion event. So..

A. you can work on adding more air and fuel per event
B. you can work on increasing the efficiency of the event

 

WOW , this keeps going and going . An example some of you might respect is the C5R program which was a 427 and a rev limit under 7k if I remember . They choose a larger bore shorter stroke combo than the usual 4.125x4.0 for some odd reason , and obviously it worked .

 

larger bore allows larger valves..

but yes, there are tradeoffs going extreme in either direction

good engine design usually takes full advantage of the regulations... thats why limited displacement class engines usually have large bore and spin fast.

Displacement = torque (in a very general sense)
Power = torque x rpm

selection of bore, stroke, valvetrian design, materials, etc.. are all to optimize the engine for its operating environment..

pretty simple really.. but it seems we have an unlimited supply of people who only made it through the first week of statics class.. and only know force x distance = torque... and assume large stroke means more torque.

 

O.K. - the math tells all people.

Engine Torque = (hm ) (hc ) (ht) (hv) (ra) (Vd) (N/2) (F/A) (Qhv) (1 / 4pi)


Where hm = Engine mechanical efficiency (power in / power out)

hc = Engine combustion efficiency

ht = Engine thermal efficiency

hv = Engine volumetric efficiency

ra = Air density

Vd = Engine displacement volume

N = Engine speed in radians per second

F/A = Mass of fuel divided by mass of air

Qhv = Heating value of engine fuel

pi = 3.14

If you are comparing engines with the same efficiencies, F/A ratios, fuel, and air density, then there are only two variables left: RPM and Displacement.

Raise and lower RPM and/or displaced volume in the equation and you can see the impact on torque. Stroke is a direct impact on displaced volume, so the advantages are clear.

I believe that stroker engines may also have an impact on the hv (volumetric efficiency) term in the above equation. Stroker engines often have longer piston dwell times at the top and bottom of the cylinder, which allows more time for air and fuel to enter/exit the cylinder. This provides a better A/F charge and contributes to the power gains.

 

That made me ROFL, DanO. Thanks!

Unfortunately, the "one-weekers" are preaching to those who never had the opportunity to take those kind of classes. I'm not trying to be a snob about having a physics or engineering background, but it surely helps one understand how the world works.

I hate to see bad info passed on as gospel. That just keeps dumbing down the understanding level of many car enthusiasts. The torque wrench idea is easy for most folks to grasp, while the force resulting from combustion pressure/piston area is not as intuitive. It can be taught however. IMO, that's what "Advanced Engineering Tech" forums should be about.

Jon

 

Wow, still chatting about this topic.

All I know is a 422 and a 428 built by the same guy, using the identical top end, the cams were almost exactly the same and they both have the same compression 11:1 and are M6's in F-Bodies.

Only difference is the 422 has a longer stroke and is an iron block. (we know iron/aluminum have no bearing on power output). It makes more power and torque everywhere. Don't have both dyno sheets, but I do know that the 422makes 535 RWHP and the 428 makes 515 RWHP, both at 6,400 rpm, same chassis dyno. The torque was more on the 422 everywhere.

Increased Cubic Inches obviously will keep making more power than the smaller engine, doesn't take a class to tell a person that. But its pretty clear that a longer stroke, all else being equal, and in this case all else is pretty much identical right down to the man who built them both using the same exact brand parts......makes more Torque, period. Even more than an engine with a little more cubes than another, if again, all else is held close to equal.

So the argument is over I guess, I can tell these few guys that were arguing about it, stroke is what causes more torque to be made, compared to a shorted stroke being used in a given cube engine.

 

i always heard that stroke makes the torque while the advantages of a small stroke but big bore is for high rpms and hp, thats why small engine cars like Ferraris DONT have big long strokes, because with a long stroke it would have to move too deep, but if you have a small stroke you can even it out with a really big bore, so that now you dont move so deep so you can go much faster, hence low torque but higher peak hp due to being able to rev much faster.

Ok i tottally over complicated it the way i explained it but hopefully you get my point. Im so done talkin about deep long strokes lol.

 

Yeah, F1 and CART type engines that have tiny strokes and spin to the moon make huge HP, but not much TQ, comparitively.

 

ugh.. yet again..


"i know this guy who built two engines"

"there is this engine builder who"


please, i dont mean to be a jerk but this is an advanced engineering forum.

 

How can one dispute such well documented facts especially when run in two different cars? You asked the original question, but you really already had your mind made up. Too bad.

The chugging smilie seems apt for you. You do that a lot? Reread your own post.

 

One probably had a 10 bolt and the other a 12 bolt.

 

Its the closest thing I see for this comparison.

Unless someone wants to build two identical engines, one with a longer stroke, and put them in one car and do tests for us. Doubt anyone will waste their money and time.

Everything is identical except the stroke in these two engines, as already I said. When one makes considerably more torque.....what would you say is the reason????

Do you think the 428 car has cement in the drive shaft or something and its a trick.

Its plain to see why the 422 makes more torque. Whats the big deal. I asked here to settle an agrument between some people. Everyone is wondering why the 422 has the 428 beat pretty good.

STROKE!!!!!

 

Only by its title, DanO. Too many zero-weekers with closed minds...a deadly combination if ever there was one.