Quote:
Originally Posted by DavidBoren
@ dave,
I very much appreciate your response, and data. I was just thinking about the rod length earlier today, actually. I found myself wondering why wiseco made their pistons for use with a rod length other than what GM produced. And I was thinking that I would probably get pistons made so I could use the LS7 titanium rods.
Regardless, I am trying to understand the data you provided. Why does the small bore fall off? What causes it to not at least be equal? If head flow is equal, then shouldn't the power be equal? It definitely shouldn't decrease, should it? Why would the 4" stroke reduce the power, if the heads flow the same?
Is it the piston speed leaving top dead center? What causes the decrease? The 3.78" bore cylinder heads can be made to flow AT least as much as bone stock LS2 heads... Even in the 3.78" bore. So what is causing the decrease? Heat through friction? If the heads flow the same, why is the 4" stroke showing a decrease?
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Stock rods are metric. Most aftermarket rods are standard - and pistons are made to match these rods.
Heads
WILL flow less on smaller bore - valves are more shrouded by cylinder. So regardless of what you get heads to flow, they will flow more in a larger bore engine - but as I said, I did
NOT change flow numbers for this comparison (which will make difference even greater than these numbers show).
So let's look at some additional differences:
While you are correct in assuming a longer lever arm will increase torque, this is only true if the cylinder pressure times surface area is the same. Say each LS2 head combination results in the same cylinder pressure. One is working on a total surface area of 12.566 in^2 (4" bore) and the other is working on a total surface area of 11.222 in^2 (3.78" bore) - or ~89% the surface area of larger bore. So to make same FORCE, will need 112% the cylinder pressure. Peak cylinder pressure occurs ~12 deg ATDC. The difference in lever arm at this crank angle for a stroke change of 0.4" is pretty damn small - certainly much less that the required 112% change in force.
Data below shows frictional losses for longer stroke are greater (no surprise). Pumping losses are slightly better for longer stroke (no surprise again). Mechanical efficiency favors the larger bore slightly. Both LS2 headed combinations flow nearly same airflow (again - I did not alter flowbench data for smaller bore). All of these are relatively small differences - up until you get to the aforementioned piston force. Now you see the primary difference between the combinations.
BTW: I do
NOT believe for one second any of these combinations will make the high RPM power numbers that these simulations show. I would fully expect any of these to make peak power ~6500 rpm, and start falling rapidly after this point. If I took more time to detail the model, it would certainly reflect this.
Dave