Several of the recent threads, and my attempt to purchase a stroker 428, have got me pondering the physics of the old saying "no replacement for displacement".


What I have come up with:

It's not the displacement that directly contributes to the increased HP and torque values.

Independently, it is the bore and the stroke!

Off course these two variables define the displacement, but does this volume have any formulaic connection to the HP and Torque of a motor?

I think not:

The bore increase the force placed on the piston by simply multiplying the (lbs per square inch) pressure from the combustion by a larger surface area. So this increase in power is a product of the bore diameter.

force= PSI*area


As for the stroke- a longer stroke provides a longer moment arm about the crank. A kin to getting a bigger wrench. The moment or torque is defeined as force x distance, how many pounds force at what distance.


OK, so I just rambled on about nothing. But for a long time, I could not understand why displacement would increase power. So, now in my understanding, it's not realy the displacement but the bore and the stroke independent of each other.


I'm sorry, every so often the nerd with the B.S. in Physics comes out of the good old boy :-)

 

In a rambling reply to my rambling post:

I guess one of the reasons that I pondered this topic was the following contradiction, wouldn't a larger volume result in a lower pressure provided the same combustion paramters. I suspect it would, if the volume was a constant, but it isn't.


Just more ramblings..no I have not been drinking. Well, not yet :-)


Part of this thought process was started today as I talked to the folks at Flatlander racing about the 428-440 stroker I had ordered. They have had my money for 3 weeks but nobody can tell me what over-bore will be required on the 428 block or what pistons will be used to get the desired compression. I think I will cancel the order and just get some higher CR pistons for my 390. They obviously have too much work.

 

You are right Greg, in that cubic inches do not make horsepower. Never have, never will. Increasing the displacement of a given engine without changing anything else will increase torque. The HP will stay around the same and its peak will decrease in RPMS. Displacement offers a potential for more horsepower and that is all. For instance a 332 can make just as much horsepower as a 428 but will need many more RPMs to do so. It will never make the torque. Increasing cubes by increasing the size of the bore gives more potential than increasing the stroke as it allows for bigger valves and doesn't change the rod angle. You are right on about the bore vs. stroke thing. Simply put; the bigger the bore the more force on the lever and the bigger the stroke the longer the lever with the same amount of force applied to it. Of course most people choose to stroke their engines vs. bore them so they can keep that precious cylinder wall. On the downside, stroking decreases the maximum RPMs that the engine can handle.

Or you can just say screw it and bolt on a 250HP nitrous kit and make your 390 act like a 600 cubic inch monster!

 

This is just a thought: Wouldn't more cubes bring in more combustables ? Doesn't more combustables cause more pressure? More pressure would result in more power. Boy, you guys have really got my pee-wee brain smokin' now. I'd better stop now.

 

When you increase the cubes, you increase the demand for air. The engine will make more torque but will run out of breath sooner so Horsepower will stay about the same. HP is just a fancy mathematical way of measuring torque. THe smaller engine can perform the same amount of work but needs to turn a few more RPMs.

 

Let's look at this at a different angle before we make a decision.
What if Ford (or any other engine man.) made a mistake and put to large of an intake, heads, carb and exhaust system on an engine but had to detune it in order to run good. (Do you think that this could have ever happened). Well then here comes STROKER man with big ideas of boring this 352 out to a 428 and stroking this machine to the max. WaLa you now have a horse powered torque driven machine that creates twice the horse power and twice the torque and at a very low and conservative RPM with only a few minor carb jetting and a few tweeks.

Ok so that sounds a little far fetched but simply put " The only real horse POWER is POTENTIAL horse power. But the problem with potential horse power is that nobody understand what it is and how it effects an engine.

If you start with a 390 ci engine it will only have a specific potential horse power, and not one pony more. Even if you put bigger valves and port the heads it does not effect potential horse power it is ONLY ALLOWING THE ENGINE TO ACCHIVE IT'S POTENTIAL nad nothing more. This is why in engine building an performance engine it is so important to match every engind component with HP Torque rpm and what you are building the engine for to help you reach this potential horse power because it can NEVER BE EXCEEDED.

 

I've owned one example, the '70 351C 4V with 11:1 comp. They had to stick in a real dinky cam to make vacuum.

Barry