Best overall SBF combo for a street/strip Cobra

[blykins]

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.

[sunman]

Ain’t no replacement for displacement
Once you go big block it’s hard to go back

[blykins]

Originally Posted by sunman

Ain’t no replacement for displacement
Once you go big block it’s hard to go back

In pretty much every situation except for a situation where some sort of race rules dictate the displacement (or a replica is desired, i.e. 289 FIA, etc.), that is correct. A larger engine will always have the potential to make more horsepower while still being streetable.

The whole premise of building an engine just so that it's tractable makes me scratch my head. It's like every Cobra out there is either at idle or WOT with no way of controlling anything in between. I've driven/ridden in some VERY raunchy Cobras (780 hp FE in one of them) and you can always just give them enough throttle to keep traction until a higher speed or higher gear gives you a hand. That's applicable to not just Cobras but to any vehicle out there.

In racing applications, such as sprint cars or dirt track, they do not change the displacement in order to gain traction, they make as much horsepower as they can and then adjust the cam timing to make them keep traction with larger inputs of throttle.

[Streetwize]

When you say "Again, do you think that a 4.250" x 3.250" combination would behave the same as a 4.000" x 3.250" combination".

Like me (or anyone with any sense) would think an extra 40 cubic inches wouldn't make any difference at all?

That sounds a bit condescending, but maybe you missed the "all else being equal" part...Like displacement, static compression ratio, etc

I only spoke in generalities about stroke. It's physics and trigonometry, not much to argue about. But Can you tune each one to Optimize each combo? Of course.

"Again"...I never said any of the other elements of engine design were not important for power production...but it seems like you want to harp on one thing I did say and maybe making it seem (to others) like that's the entire point of my thread?

I'll leave it at that.

But what about the question about loading up the 2 same displacement (1 long stroke one short stroke, all else the same) motors at 2000 rpm instead of 3000?

And again, I listed type of motor and powerband I (PERSONALLY) would like for my next Cobra....and back in the 60's there were certain factory bore stoke combinations (like the ones I mentioned, 327 Chevy, 340 Mopar), and with close ratio manual transmissions, they all have similar powerband characteristics as to how they put down power....and coincidently(?) they all just happened to all had a ~3.25"-ish stroke....and that's the kind of "linear" power curve I'd like to emulate in my next Cobra, only I just want to turn up the wick a bit.

Anybody who builds a motor either wants to (or settles for) a particular short block size / combo, this is the one I want to build.

The point of the thread was to ask what everybody else's Best Cobra combo is....and why they like it.

[blykins]

Originally Posted by Streetwize

When you say "Again, do you think that a 4.250" x 3.250" combination would behave the same as a 4.000" x 3.250" combination".

Like me (or anyone with any sense) would think an extra 40 cubic inches would make any difference at all?

That sounds a bit condescending, but maybe you missed the "all else being equal" part...Like displacement, static compression ratio, etc

I only spoke in generalities about stroke. It's physics and trigonometry, not much to argue about. But Can you tune each one to Optimize each combo? Of course.

"Again"...I never said any of the other elements of engine design were not important for power production...but it seems like you want to harp on one thing I did say and maybe making it seem (to others) like that's the entire point of my thread?

I'll leave it at that.

But what about the question about loading up the 2 same displacement (1 long stroke one short stroke, all else the same) motors at 2000 rpm instead of 3000?

And again, I listed type of motor and powerband I (PERSONALLY) would like for my next Cobra....and back in the 60's there were certain factory bore stoke combinations (like the ones I mentioned, 327 Chevy, 340 Mopar), and with close ratio manual transmissions, they all have similar powerband characteristics as to how they put down power....and coincidently(?) they all just happened to all had a ~3.25"-ish stroke....and that's the kind of "linear" power curve I'd like to emulate in my next Cobra, only I just want to turn up the wick a bit.

Anybody who builds a motor either wants to (or settles for) a particular short block size / combo, this is the one I want to build.

The point of the thread was to ask what everybody else's Best Cobra combo is....and why they like it.

If you want to change your argument now after thinking about it more, that's fine. But in your first several posts, you were very adamant about using a 3.250" stroke because (in your own words) "it's a great balance of 'just right' torque and a predictable power curve", along with saying that there is a correlation between torque and stroke.

Those arguments, along with a few more, are what I zoned in on because I see those incorrect applications being made almost on every engine/car forum.

There is no correlation between stroke and torque.

A particular stroke number does not (by itself) change any balance between torque and power curves.

We do not pull engines down to 2000 rpm on the dyno with a full load. It's very hard on an engine to do that and exposes them to detonation. It's also not necessary to do because higher rpm ranges give you a great view of torque and horsepower curves.

But no, I do not agree with your argument that the longer stroke engine "will accelerate harder because it generates intake velocity sooner". Getting the intake charge started sooner is not the job of the crankshaft stroke! It's the job of everything above it!

Again, you are so hyper-focused on a stroke number, and the stroke number isn't as important as what you're wanting it to be.