451mopar

[Morris Brown]

Mr 451 mopar, I appreciate your response to my question. In your reply, there were things that I did not think of before, such as pistons moving quicker with the less dwell time increasing cylinder volume this would answer some other questions that I have in mind. But several years ago, a friend of mine raced chevrolets and had a problem with the small block 400. It would not rev like the 350. It's power band was like 25 to 3500. The 350 power band was 35 to 4500. Both engines use the same cams, heads, everything. Then we noticed the rods were shorter on the 400. So we took the 350 and the 400, found absolute top dead center, back the engine up to the pistons .050, rotated the cranck shaft in one degree increments a full 360 degrees. We found the short rod had longer dwell time at tdc than the longer rod engine. We noticed that the piston moved faster at the center of the stroke than it did at the top and bottom of the stroke. Do you think that the 108 degree Lobe seperation angle would work better with the 6.123 rod than a 110 degree lobe seperation angle? Do you know of anyone that has any testing on the Dyno on this subject?

[Streetwize]

The longer dwell you recorded was due to the longer 3.75" stroke of the 400 vs the 3.48" of the 350. Stroke length can have an even larger effect on dwell than does rod length. You'll have longer dwell for longer rods vs. short relative to the same stroke.

The piston moves fastest approaching and just past the rod being 90 degrees to the crank throw. The shorter the rod the more combustion area is pushing down on the piston up to the point to where cylinder head flow or cam timing can no longer keep up. This is the principal reason short rods make more torque and less top end than a longer rod all else being equal.

If your motor only turns about 7000 RPM at redline and you say you go faster with long rods, I say (in general) you dont have enough head or cam in the car.

above 6500-7000, long rods win.


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Streetwize

[This message has been edited by Streetwize (edited December 10, 2000).]