Some comparisons

Hi guys,

First, I'd like to draw your attention to this article:

http://purplesagetradingpost.com/sumner/techinfo/350%20chevy%20engine.html

The article does a pretty good job stating it's case that destroking a motor doesn't necessarily mean you're building something with limited torque potential. (Dyno Charts.) But I'm wondering if there's more here than meets the eye. I've heard and read lots of bad press about production 400 Chevy blocks. Is it a possibility that this particular buildup is merely a happy compromise to make up for the shortcomings of the 400 block, or is there really some potential to build a better overall performing motor without the "risk?"

This leads me to the following questions:

1. If you have a crank with a shorter stroke, (327), in order to make whatever motor that crank is installed in produce the same amount of torque as a motor with a longer stroke crank, (400), what do you need to do to that motor?

2. If I understand physics correctly, if you have two cranks that both have different strokes and rev both of them to the same RPM, the shorter stroke crank will produce less centrifugal force. From what I've heard, the main webs on a 400 seem to be it's achilles' heel. Not enough "meat" to hold the main caps under high loads. If you installed a shorter stroke crank, a cam that would make up for the loss of HP from losing the cubic inches, and revved the motor no higher than you did when the 3.75 crank was there, would it be safe to assume that the main webs would have less stress on them due to the decreased centrifugal force of the shorter stroke crank? Or is this over-simplified?

3. Does the size of a valve in a head contribute to the level of port velocity the head sees, or is port velocity dictated by the size of the intake ports alone? Reason I ask this is I'm wondering how the tq/hp curve of this engine would be affected if they had used 2.02/1.60 size valves instead of the 1.990/1.550's? Would port velocity be decreased? Would the torque curve be moved up in the RPM range?

Thanks in advance for your time and any feedback you can provide.

Rob

Hi guys,

First, I'd like to draw your attention to this article:

http://purplesagetradingpost.com/sumner/techinfo/350%20chevy%20engine.html

The article does a pretty good job stating it's case that destroking a motor doesn't necessarily mean you're building something with limited torque potential. (Dyno Charts.) But I'm wondering if there's more here than meets the eye. I've heard and read lots of bad press about production 400 Chevy blocks. Is it a possibility that this particular buildup is merely a happy compromise to make up for the shortcomings of the 400 block, or is there really some potential to build a better overall performing motor without the "risk?"

This leads me to the following questions:

1. If you have a crank with a shorter stroke, (327), in order to make whatever motor that crank is installed in produce the same amount of torque as a motor with a longer stroke crank, (400), what do you need to do to that motor?

Keep the displacement and the flow equal and torque will be pretty much the same.


2. If I understand physics correctly, if you have two cranks that both have different strokes and rev both of them to the same RPM, the shorter stroke crank will produce less centrifugal force. From what I've heard, the main webs on a 400 seem to be it's achilles' heel. Not enough "meat" to hold the main caps under high loads. If you installed a shorter stroke crank, a cam that would make up for the loss of HP from losing the cubic inches, and revved the motor no higher than you did when the 3.75 crank was there, would it be safe to assume that the main webs would have less stress on them due to the decreased centrifugal force of the shorter stroke crank? Or is this over-simplified?

In your example, you are not going to make up for the loss of 50 or so cubic inches with a cam change and without reving higher. The advantage of the short stroke vs the long stroke is less piston speed (less friction hp loss) and fewer piston gs (load on the rods, crank, block) for a given rpm. You are on the right track with forces, but you are going to experience much of the same forces when you rev higher to get the power. There's no free lunch.

Additionally the larger bore (to get the same cubes like in the article) generally allows more flow thru the same heads. It's a shame the article didn't compare a 4.03 x 3.48 to the 4.155 x 3.25 engine with the same equipment. The short stroke engine would have probably been the stronger one by a little.

3. Does the size of a valve in a head contribute to the level of port velocity the head sees, or is port velocity dictated by the size of the intake ports alone? Reason I ask this is I'm wondering how the tq/hp curve of this engine would be affected if they had used 2.02/1.60 size valves instead of the 1.990/1.550's? Would port velocity be decreased? Would the torque curve be moved up in the RPM range?

Velocity is flow divided by flow area. If larger valves resulted in more flow at a given depression, which isn't always the case, the average velocity would go up, of course, and probably the torque and power. With the same cam, a better flowing head (not just larger valves) should boost the entire torque curve across the range. In the old (1997) magazine example the torque curve was very flat, with 90% of peak from below 2500 to 5400. More flow might move the peak torque rpm a few hundred revs, but the main gain would be more torque everywhere.

My $.02


Thanks in advance for your time and any feedback you can provide.

Rob