I've heard a lot about destroked engines revving up faster and how stroked engines have more low-end torque.... but can someone try and explain WHY the destroked rev's better (quicker) is it just the lesser stroke?? making the pistons and rods have to travel less ? or is there a more interesting theory ??

 

I know you rev a destroked engine higher, but I never heard of it reving faster, just wondering if that part is true.

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The main reason for reving faster is less mass. Most 283 cranks weigh several pounds ligher than a 350. If you are carefull you can get some pretty light (and strong) parts for just about any stroke now. My 400 crank is only a couple of lbs heavier than a 350.

 

It isn't the rotating mass, it's the reciprocating mass. At least with rotating mass, once you get it spinning, it doesn't cost any power to keep it spinning; but with the reciprocating mass of the rod and piston, on each stroke you have to accelerate it upwards (consumes power); stop it at the top (consumes power); accelerate it downwards (consumes power); and stop it at the bottom (consumes power). So every single stroke, i.e. 4 times per crank revolution or 8 times per cylinder firing, the engine throws away all that energy involved in the up-down motion of the rod, pin, and piston.

There are 2 ways to reduce this parasitic loss: use lighter parts, or move the parts a shorter distance. You can only go so far with lighter parts before you get into reliability problems. You accomplish the other thing by shortening the stroke. That is why a 302 or a 327 revs faster than a 350, and they continue to produce useful power at a higher RPM; it's less power since they're smaller, but it peaks at a higher RPM. Stroking a motor typically increases the power output but lowers the RPM at which it occurs, because of the parasistic losses that increase with RPM.

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"At least with rotating mass, once you get it spinning, it doesn't cost any power to keep it spinning."

Thats obviously not true.

 

its like this basically, the longer the stroke, the longer the piston has to travel, to say stick you arm out and move it side to side real fast for mabey 3 inches, then do it at about 6 inches, you'll noticed you can move your arm as fast the longer the distance, and this is the same way the pistons works, also the longer the stoke the more power you can put into each movement back and forth (IE more torque) also a lighter mass (forged pistions) will make the stokes faster, a big myth is that the more R's the more power, thats just not true in most cases unless its a constant high r' engine like in circle track, other wise allways for for the extra torque over the HP, HP looks nice on papers, but low end torque wins races.

 

According to Mr. Newton, and object in motion will stay in motion in the same direction and at the same speed unless acted upon by an outside force. That goes for rotational motion as well as linear motion.

The pistons having to be stopped and started twice per revolution would be an outside force on the crank rotation. So, basically, it isn't really true (although the longer stroke would produce slightly more rotational momentum).

I'd have to agree that it is the piston having to go a longer distance, and therefore having to be accelerated at a greater rate for a single revolution that makes a longer stroke engine less capable of higher RPM's. Since acceleration is force divided by mass (remember, F=MA), it will take more force and therefore more power (P=work x time = force x distance x time) to move a longer stroke rotating assembly to the same RPM's, even if the reciprocating mass is the same. Therefore, there is more parasitic loss in a long stroke as RPMs increase, so the net effect is: keep the RPMs up on a short stroke to produce more power, and down on a long stroke engine to maximize torque (HP=torque x RPMs / 5252).

Does "There's no replacement for displacement" still apply? Probably. The mag articles on 383 stroker 350 vs. 377 destroked 400 tend to prove that out.

Oh, never addressed the part about longer stroke engines producing more torque. Torque is force times length of the lever arm; the force in this case is the pressure in the cylinder times the area of the piston, and the lever arm is the distance from the centerline of the crank main bearings to the centerline of the crank throw (i.e., 1/2 the stroke). A longer stroke engine will have a longer lever arm, so at the same cylinder pressure and piston area, will produce more torque than a shorter stroke engine.

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mr RB's description is correct.

 

Sounds like if the stopping and starting were the biggest factors that an engine with a very long stroke would be less prone to that problem (it would be spending a larger precentage of it's time with the piston traveling at a average speed.) Maybe I slept through that lesson.

I guess i better do some of the work they pay me for.

[This message has been edited by jcb999 (edited October 30, 2001).]

 

ahhh but you see, that law of motion takes place in where there is no frition any where...do this, if you take off the con rods and piston and anything eles connected to the crankshaft so it is just sitting in the block and spin it what happens, well it will spin the stop, all the kinetic energy it had was used up. but once it is spinning it takes alot less force to keep it going b/c that extra force is the force required to overcome the frition of the cranks in its mounts. So don't be so quick to respond that someone is incorrrect unless you are sure what you are saying, and have the whole story correct...this was not a flame. and spelling does not count

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short stroke engines only rev up faster with no load on it. under load a longer stroke engine built the same will rev quicker just not as high. i most cases, short stroke engines are smaller and are given deeper gear ratio's from the factory so they also feel to rev quicker, when in fact they are mearly taking advantage of less load. piston speed is much lower on a short stroke engine which helps for higher rev's and as well as better breathing from a larger bore. this is about as simple as i can explain it. going into the whole physics of it is obviously worthless or you'd have thought of it yourself.

 

The other day I asked why there aren't any counter weight cranks for SBC's.

Some engines use them and that helps with the starting and stopping of motion.

Clearance isn't an issue I don't think......

But they are effective.


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