Quote:
Originally Posted by Streetwize
Well, I still respect you 
But I understood basic engine theory and was building motors .....long before the internet.
Here's where I think we're 'Apples to Bowling Balls'
I think where we are "not synching-up" is I believe you are looking more at the WOT and HP/torque output....but what I am speaking more about part throttle/roll-on power and torque....and this is particularly where a longer stroke motor tends to produce better torque for drivability. How it feels just driving around. A similar thing would happen at 2000-2500 RPM with a smaller cross section port on the same motor on a Larger cross section port as long as the head and the cam flow adequately to make the HP goals.
But I agree 100% that if PEAK POWER is the primary objective, you can build a combo that can produce equal or near equal results. but that's not really what I'm referring to.
Please think about this:
If you built 2 motors on your dyno with the same cubes, heads, compression and cam timing but one with a long stroke and one with a short stroke, they might make around the same power and torque....as long as you start the pull at ~3000 or 3500 rpm.
Now, try the same test starting to load up the motor at only 2000 or 2500 RPM....I can about guarantee you the longer stroke motor will accelerate harder. Because the longer (bicycle pedal, lol) generates intake runner VELOCITY sooner, which helps get it up on the cam sooner.
^^^^ THIS is my (only) point....3000 or 3500 for a dyno pull is great, but we all know that less than 2500 is really much, much more realistic for a true street driven car, regardless of the weight.
Again, I'm talking more part throttle and roll-on drivability torque...tractability is "just the right amount" to optimize acceleration...thats all I've been talking about.... not how much peak HP or Peak torque. You can design for that target parameter with your heads compression and cam.
On street tires, I'm not always chasing the most on the dyno, I'm chasing the most I can hook up. |
Regardless of what you're trying to explain now, and whatever fruit or sports-related item you're comparing sizes with, your initial premise was that you were basing everything around a 3.250" stroke, because it was the best of all worlds.
My response, which I still back 120%, is that a stroke number is not a magical number. It's just a part of an equation. All of those old factory engines didn't have special powers just because all of them had a 3.250" stroke. Again, do you think that a 4.250" x 3.250" combination would behave the same as a 4.000" x 3.250" combination just because the stroke is the same????? *Displacement* is what you should be focused on, not the stroke number.
You also can't make the point that a 3.250" stroke is just going to make a vehicle more tractable, with less chance of someone planting their foot down and the car getting away from them. You can put a clapped out engine out of a 1991 Mustang GT into a FFR Cobra, turn the wheel and hit the gas and the tires are instantly going to go up in smoke and you're going to get a very significant donut.
You're so far zoomed in on stroke that you're forgetting all of the other very, very important variables that go into building an engine: port flow, port volume, camshaft specs, compression ratio, intake manifold, etc. Just going from a low compression ratio to a high compression ratio by itself will make an engine very zippy and very responsive to the throttle.
You also stated in the beginning that there was a correlation between stroke and torque and that's obviously not a correct statement. I've got 20 years of dyno sheets that say otherwise.
Sorry, you asked for opinions, and even named me specifically, but I'm not buying what you're selling.
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