Why do some engines have a long flat torque curve in the upper register ?

What engine characteristics produce this ?

Long rods, good intake/exhaust balance, not too much overlap, high compression, maybe these, I don't know what else ?

Sufficient valve spring, fuel, and ignition too, but those are not design parameters.

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Big valves for the displacement ?

Boiling it down to the basics.........

The required air flow(and trapped VE) is keeping up with the demand as the rpm goes up.

That can be done by a variety of methods, and/or by mixing and matching different methods.

Different engine platforms need help to accomplish that in different areas.

mainly its the cam. everything else sets up around that.

It's the combination of parts working together. Wouldn't say it's mostly -cam -heads -valvetrain stability, induction/exhaust flow, or anything else because just one thing off in the entire combination can cause the high rpm VE to drop like a rock

And here's a good example of exactly that with VE on our mild 10:1CR 589 wedge motor, 572-13's 365cnc heads, 260/270@.050 s/roller, 440-2, 1.55 Jesels, bit of a mismatch of parts as it came all built/dyno'd but it runs real well with that small carb, the small venturi probably shows up on the ET rather than on the dyno hp. Torque holds on reasonable as well through 1100rpm, it did what we wanted it to do ET wise in the car and then some.

This one has a torque curve that’s within 20ft/lbs for over 1500rpm........

It has all the usual stuff to achieve that.
Good heads, nice cam, high CR, stable valvetrain, big intake and carb, good oil pan.

Almost all the pump gas stroker motor BB Mopar V8 I've built, and dyno tested that had less than 10.5 to 1 compression made more torque than HP, especially with iron heads below 10.0 1 to 1 compression ratio
My first 400 stroker motor I built had a 4.25 stroke crank with 6.8 long Chinese H beam steel rods with 9.25 to 1 comp ratio 23.0 CC dished pistons with a set of iron 906 heads with big valves, low deck six pack intake and carbs, solid roller cam that motor made 612 HP at 55000 RPM with 644 Ft.. Lbs. of tire frying torque at 4500 RPM
I don't remember the exact torque numbers now but we started the first pulls at 2500 RPM and it made a hoop of torque from down their to 4500 RPM where it started to taper off slowly

Here area few I have online. Not sure what you consider broad and flat but see if these fit the bill. I believe they qualify. We also have one in our dragster that is not really flat but the thing is a torque monster for sure as it peaked at 960 ft/lbs.

This engine was within 56 ft/lbs from beginning of pull up til peak HP, 2300RPM Predator headed race gas 604" sheet metal intake



This one is 25ft/lbs from start til peak HP, 1200rpm. This one is a pump gas 536" deal.



Here is another. 59 ft/lbs from start to peak power, 2100 rpm. 610" Predator with a single 4 cast intake

446" 14# 112 LSA .631 lift Edl Vic heads std port,2.19" int/1.81"exh 4.35 bore

Using Al’s criteria of tq variation from the start of the pull up to peak HP.......... the one I posted still fares pretty well.......

41lbs variation over 23 lines on the sheet.

Nice results guys ^ Mine is mild but carries some rpm. This is at the wheels through full exhaust to tips with accessories (and steel wheels). I've made some changes since and will update with results but think I can get it in the 615 wheel range.

The 440 ShortRam motor I built, ( very mild motor) makes 500+ ftlbs from 2500 to 4300, peaks at 543 ftlbs@3800. Motor doesnt make a boatload of hp (429), but it's a great little torque monster.

Gents,
Thank you all, the charts don't lie. "Extended torque" is relative I know.
I have an idea about valve sizing. Essentially, bigger engines don't have valve sizes that are proportionately as big as
the valves in smaller engines. The dyno charts are good, could I further ask what heads and valve sizes are in those engines ?

IMHO biggest single source of big dip/spike in torque: strong exhaust wave return at the wrong/right time.

Poly,
Duration / timing, or exhaust valve size / port flow ?

Indy 572-13 365CNC heads have 2.30/1.88 valves.......

I agree. That dip on my graph 4200-4500 range I think could be fixed with tri-y's.

During OL, slightly off TDC either way depending on symmetrical IO/EC or not

Two of the ones I posted were Predators as I listed on the post. 2.45/1.84. The middle engine was a 2.30/1.88 B1

OK Thanks All,
Interesting topic, hard to see a pattern but there must be something these engines have in common.

The opposite effect, where the torque curve just drops, seems to be related to too much duration and a smallish intake port.

Where breathing hits it's limit, the torque curve will slope down in a line, and the HP will plateau for a while with increasing RPM.

Maybe a key player in the flat torque curve is an intake port on the small side but that flows really well, a highly raised port.

Anyway thank you.