Best overall SBF combo for a street/strip Cobra

[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.

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 thats 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...not how much peak HP or Peak torque.

[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.

[eschaider]

Not true.

Back at the basic engine configuration, I still think the 427 cylinders should be in the middle of the engine, and the short rod 347 holes should be closest to the flywheel. Of course, if the rods are made of a depleted uranium enhanced alloy, you would want them at the front of the engine to protect the driver's feet ...

[My427stang]

The majority of my builds peak well under 6000, run 91 or less octane, have power brakes and absolutely pure street, in traffic, drive anywhere FEs, so I generally build where you are talking, and yes I am pre-internet too. However a ton has changed since I started, both parts and process

We don't have to agree, but cam timing, not just centerline but all events and effects, as well as proper matching induction and exhaust parts, make much more difference than stroke or rod length, by a wide margin. Not just on a dyno, off idle as well, and why you'll see such a focus on overlap by guys like Brent and I nowadays.

I used to have a chart I built on the rate of volume change in a cylinder, based on rod length, stroke and bore. My idea was to find enlightenment on combinations based on the most rapid changes in cylinder fill for torque.

It worked so well, I don't even know where I saved it

That being said, I am more happy when guys think about where they want their power, regardless how they get there. For me, I spend more time finding out what the customer wants, which they don't always know at first, so kudos on you for not being a power only guy, although I would say, that sometimes being a little softer at shift recovefry can be a little helpful in a handful of hot rod.

[Thor maine]

Best small block engine in a Cobra kit car , GM LS but that's just me?

[PDUB]

...where's the popcorn? This one's getting interesting!

[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

[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 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.

[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.

[blykins]

Here's another example of how there is no correlation between the stroke and the torque:



That's the dyno sheet of a 311ci SBF with a 3" stroke! It bested the 351C by ~25 lb-ft and was only within ~10 lb-ft of the 363 that I posted.

That little engine is an excellent example of how to manipulate hp/tq peaks and hp/tq curves. It's only 10.25:1 compression....

Actually, that little 311 would trigger a lot of internet forum keyboardists. The cam timing was very advanced, 109 LSA with a 101 ICL, but yet it pulled to 7400-7600 with only a 239° @ .050" solid roller camshaft and made more torque than a lot of other larger engines.

[Streetwize]

My definition of Tractible is I guess Nostalgic...I had a 340 69 Dart in the early 80's that weighed about 2850 Pounds that I built a 11.5:1 W2 headed motor with a SBC grind oval track solid roller 4 speed combo that was just (again to me) "PERFECT" .

I could hook on skinny early 80's street tires while most big blocks were either bogging or blowing off the tires. 11.60's at 115mph in a car I drove to work in New Jersey traffic all week and racing on weekends was pretty damn respectable. I put a 250 shot on it but never sprayed it before somebody made me an offer on it I couldn't pass up.



Tractible means (easy to drive) on street tires. And in a cobra I mean balanced in the corners with roll on power and lots of RPM capability. I picked a power level I like...say 525-ish....Like my 427 now. Only this time i'd rather have that power spread over 7500 RPM with a close ratio than over ~5500 with a wider ratio.

It's really that simple. Again , that's just me, your mileage may vary.

Everybody's different. for me Cobras are better for brief bursts of acceleration and enjoyment than for any kind of sustained high-speed driving....

but if a cobra had true active suspension with a flick of a switch, a 750 ish HP motor might really be fun. And yes, you can drive that powerful of a motor on the street in a light car, I just don't want or need that much but more power to those who do.

To set a Cobra up to Handle in a balanced (optimum Sports car handling) way it's nearly impossible to launch it (in terms of the lowest possible 60' ET) straight (lift both front wheels more or less equally) at full power at anywhere near it's optimum potential, and to set it up where it would go down the quarter mile the fastest (Stiffen the Right rear and loosening the front sway bar) upsets the optimum handling.

[eschaider]

Engine power is directly related to air mass processed by the engine per unit time. In round numbers, it takes 10 lbs of air per minute or 600 lbs of air per hour at STP (standard temperature and pressure) to produce 100 HP.

Once you have settled on the engine displacement, power production is directly related to how completely you fill those cylinders and what RPM you test the engine at, i.e. how much air you process — period!

If you have inadequate heads that choke off the airflow at some rpm, then that is where the torque begins to fade. If you have adequate heads but the wrong cam, wherever that cam runs out of steam, so too does your engine.

Until you go to a boosted engine, you are pretty much stuck with the above metrics and power model.

You can play with some engine metrics, such as squish, compression, timing, etc., for optimization, but the big enchilada is the displacement and the pounds of air per minute that the mechanism can process.

When you start chasing specialty length rods and other interesting but ineffective BS power enhancements, you are just wasting time and money. If you want to waste money, at least do it creatively. Use the small block with the four 427 bores at the center and the depleted uranium superalloy rods at the front of the engine. They not only protect the driver's feet up there, but they also produce a nice soft blue glow, making it easier to find your car in the pits at night races.

If you think the 427/347 engine is stupid (with or without the depleted uranium alloy rods), then you should run as fast as you can from all the trick rod/stroke/crank BS the internet regularly dumps in our heads to either sound smart or sell parts you don't need.

Brent is telling the straight story; it is worth paying attention to! You will have a better-performing engine, spend less money, and have a happier experience!

[blykins]

I think I probably come across more hateful than what I sound like in my own head but there are so many internet fallacies that are continually passed around from forum to forum. When it gets passed around a certain amount of times, it somehow becomes gospel and then guys will defend it to the death.

[eschaider]

Telling it like it is, is not hateful. It is simply setting the record straight and sweeping out the internet-spawned BS.

[Buzz]

Question from a non-expert in the field who is genuinely interested in the topic. Shouldn't the argument (I mean discussion) focus on the impact of the bore to stroke ratio for a given displacement instead either the stroke or the displacement on their own?

To me - and I'm absolutely open to correction - basic physics should dictate that the characteristics and nature of the power delivery of an engine with a big bore and a short stroke would differ from that of an engine of ~ identical other specs that uses a smaller bore and a longer stroke to achieve that same displacement. Naturally, tuning and adjustment of other parameters can compensate, but would the two base combinations not perform or at least feel different to the seat of the pants?

Admittedly this has been my belief based on the above ASSumptions for a long time and I'd be surprised - though happy to be educated - to learn differently.

[blykins]

Buzz, if the valves are not shrouded by a small bore, then you generally won't see much difference.

The differences that you will see are affected by piston weight, rod weight, etc. A short stroke engine will have either a longer rod or a taller piston in comparison to the same displacement with a long stroke. Weight does make a difference in how an engine accelerates or makes power.

[Buzz]

Interesting, thanks. That does make sense. I started thinking about this in depth (obviously not enough depth, lol!) when someone was trying to convince me to replace the 350ci based 383 stroker engine that I had with a 377" short-stroked 400ci based engine that supposedly would rev faster and higher and ultimately produce more power in the upper rpm range.

[Streetwize]

But another part of drivability, it's not only how the motor applies power to the ground, but in a light car like a Cobra with a manual transmission, it's also how the engine decelerates (compression braking) in gear and how much that effects driving on either a road course or something like nice twisty turns up in the mountains. That's a driving parameter that really comes into play in a Cobra really moreso than most other cars, because the power to weight ratio (hp per pound) is more like a sport bike.
Compression braking isn't as abrupt with an engine that spreads it's power over a wider RPM range, and I think part of why the FIA racers preferred the 289's is they could let off the gas and roll back into the power more gradually with less chassis "upset" , keeping the car more neutral. I know that bigger inch motors with narrower RPM powerband ranges can almost feel like pulling a parachute when you close the throttle.

It's a parameter of the engine that is seldom considered but in a good handling lightweight car like a Cobra it's 'a thing', particuarly with a manual trans.