Powerband?

Just thought I would post an odd question. On my 89 XJ 4.0 AW4, I notice that when I push it and it gets above 2500 rpm, there is noticeable imporvement in acceleration. It kind of reminds me - on a much lesser scale - of the powerband on dirt bikes. Is this a feature of increased torque or more HP at that RPM range? Does anybody have a graph showing teh horsepower at various rpm's on this engine?
Thanks
BSD

Increased torque. "Torque makes you quick, horsepower makes you fast."

I do have a batch of dyno-sim charts around here somewhere, I used them in my book. Maybe I'll have to put them up in the "Tech Archive" section of my website...

As I recall, tho, the torque peak is just shy of 3K RPM, and it's nice and flat, so I'd say that "powerband" runs from off-idle to about 3800-3900rpm (going wide,) or from 1800-2000 RPM to 3500 or so RPM.

Is it any wonder we cruise around 3K RPM? Torque peak is where an engine is at its most efficient...

While we're on the subject, I'd be grateful if anyone with a Dyno report scanned could send it to me - I'd be able to verify the DesktopDyno reports. I know they're "optomistic," but just how far I'm not sure. If you don't mind, I'd post them alongside the DD2K reports just for "correction."

5-90

You have a booK? I want to see torque curves!

5-90,
Please explain "so I'd say that "powerband" runs from off-idle to about 3800-3900rpm (going wide,) or from 1800-2000 RPM to 3500 or so RPM." I am a little dense on this.
As far as 3000 being optimum efficiency, that means for me, I cruise at about 2400 at 70mph, man I would be flying at 3000 rpm!
Thanks
BSD

Doesnt work like that, the peak torque is at 3K, so thats when the motor is working at peak efficiency. Its "easy" for the motor to be there, think of the motor running 5th gear at 15mph, youd be floored to keep that speed, not very good.

OK...

"Torque peak" is when the engine is making peak power (a dyno run measures torque, then calculated horsepower based upon torque production. At low RPM - below 5252 - the engine is making high torque. At high RPM - above 5252 - horsepower takes over. Horsepower and torque are ALWAYS equal at 5252rpm - it just works out mathematically that way. I'd have to find the formula to convince you.)

"Power band" or "torque band" is the area around the torque peak. In come cases (such as ours,) the phrase "torque peak" is a misnomer - since it's just a high spot in a high area. Some engines make power curves that look like a side view of a mountain, ours makes one that looks like a plateau. Therefore, there is a wide range in which "peak" power (say, 90% or so of peak) is made, which amounts to a wide "cruise band" and more room for gear selection.

"Off-idle" is also a generalisation - in my experience, "off-idle" encompasses the area from 1000-1500rpm. If you have good torque production "off-idle," that means you are making torque low in the RPM range, and you don't have to turn the engine quickly to make useful power (which really saves on fuel, since you're burning less per unit time.)

Again, I'll have to find the dyno/sim curves to point out the areas where it happens - since I'm on the mend and can't get out to the shop much, I'm working on updating the site and I'll post here when it happens. That way, you can get a more detailed explanation than I'm able to give here (limited formatting.)

5-90

BSD said:

5-90,
Please explain "so I'd say that "powerband" runs from off-idle to about 3800-3900rpm (going wide,) or from 1800-2000 RPM to 3500 or so RPM." I am a little dense on this.
As far as 3000 being optimum efficiency, that means for me, I cruise at about 2400 at 70mph, man I would be flying at 3000 rpm!
Thanks
BSD

Click to expand...

thanks, makes much more sense.
GAB

BSD said:

Just thought I would post an odd question. On my 89 XJ 4.0 AW4, I notice that when I push it and it gets above 2500 rpm, there is noticeable imporvement in acceleration. It kind of reminds me - on a much lesser scale - of the powerband on dirt bikes. Is this a feature of increased torque or more HP at that RPM range? Does anybody have a graph showing teh horsepower at various rpm's on this engine?
Thanks
BSD

Click to expand...

Don't have one for the Renix but I do have one for various other 4.0's:

'96-'99 4.0 XJ

'99-'05 4.0 WJ

'96-'98 4.0 ZJ

Figure on the Renix 4.0 HP/TQ curves being closest to those of the 4.0 ZJ, but the torque curves are pretty flat from 1200-4600rpm on all of them.