Idle torque for 4.0 HO?

[vetteboy]

Does anyone have a dyno run or torque figures for a 4.0 that show the maximum torque available at idle? As in, without any throttle input, how much torque I can expect before the engine stalls.

The engine in question is a '94 4.0 HO, pretty much stock except for a K&N filter (done out of necessity for engine compartment space).

 

[j99xj]

I read somewhere that the later H.O.s (96+) develop 85% of maximum torque at idle. So if max torque is 225 lb-ft, you are looking at about 191 lb-ft at idle. But the earlier H.O.s had a slightly higher torque curve so that idle figure might be a little high in your case.

I'm not sure if this is with, or without the throttle being open or closed. If I had to guess, closed throttle. You are not at idle if the throttle is open.

Anyway this 85% figure is outstanding compared to nearly all engines except diesels. Go drive a modern gas V6 or a modern small displacement gas V8 and you will know what I mean. (pretty much gutless below 3000 rpm, and they require more throttle for the same "jolt" in your seat when taking off from a stop)

 

[JJacobs]

Interesting topic you've got here. I suppose it'd be no big deal to put one on the rollers and start a run at idle, but why? This is something you can't really measure (and have no need to measure.)
191 lb/ft at idle? Um, no, don't think so, not by a long shot. Maybe 100 lb/ft at 1000 rpm with the throttle open, not at idle with the throttle blades closed.

 

[j99xj]

Maybe I'm not interpreting what I read right. Perhaps the Jeep ad I read meant "off idle" with the throttle all the way open.

That would make more sense. But I always thought "idle" was no throttle input at all.

 

[JJacobs]

Makes you wonder.. somewhat related, I've always been very wary of lugging engines down with high throttle, low rpm. Supposedly the high combustion forces combined with low crank speed and oil pressure can squish the oil right out of the rod bearings and do some damage.

 

[j99xj]

Yea no kidding. Low rpm with high combustion pressure is probably just as bad as revving to redline.

This is why diesels are beefed up big time. Low rpm and high pressures are the names of the game for these types of engines.

 

[vetteboy]

Interesting topic you've got here. I suppose it'd be no big deal to put one on the rollers and start a run at idle, but why? This is something you can't really measure (and have no need to measure.)

Well, from experience I can say there's definitely not 190 ft-lbs available at idle with no throttle.

My reasoning behind the question is this: I'm in a situation where I've got 182:1 crawl ratio available behind a 4.0, and I have frequently overstepped the boundaries of my Dana 60 front axle. To date:

- Broken front 35-spline inner shaft (stock)
- Broken front 30-spline outer shaft (stock)
- Broken front D60 Detroit locker
- Broken front 35-spline drive flange
- Twisted splines on 35-spline Yukon 4340 outer stub

The last three all happened at the same time.

ALL breakage happened at crawling speeds. Meaning, just creeping along, a little *blip* on the throttle, and carnage ensued.

Now, I know what a D60 Detroit should be capable of. I know what 35-spline alloy Yukon shafts should be capable of.

What I'm more wondering here: I've been very impressed with the gearing I'm running. The control and stability I have when crawling over stuff at that low a ratio is awesome. BUT - it has apparently come at the cost of many expensive parts, and I'd rather not be replacing them all the time.

So the bottom line is this - I know what kind of torque a 4340 35-spline stub should be able to handle. At 182:1, this opens up a new world of torque capacity to the axle shafts. I'm fairly certain that I can stall the engine at idle without breaking anything, however I'd like to know if I have any safety factor above this - I.E., if I stall it at idle, I'd better back up and try something else. So far there have been a few situations where a quick throttle blip has gotten me over something, and a few situations where I've broken some expensive components because of it.

As an interesting counterpoint, the rest of the drivetrain consists of Napa 1310 u-joints in all driveshafts, and regular Spicer Life-series U-joints in the axle shafts, and stock front and rear outputs in the D300 case. None of which have suffered at all compared to the front axle.

 

[privatejoker]

You need to realize what the objective of idle is. The engine sucks in a little air as it can while maintaining a smooth rpm. When a small load, such as AC kicks in, the idle raises to compensate. There is very little torque available at idle, if you open the throttle and apply a load, that 100 ft lb. figure may be close. I don't know anybody that has ever dyno tested one at that low of an rpm.

 

[vetteboy]

You need to realize what the objective of idle is. The engine sucks in a little air as it can while maintaining a smooth rpm. When a small load, such as AC kicks in, the idle raises to compensate. There is very little torque available at idle, if you open the throttle and apply a load, that 100 ft lb. figure may be close. I don't know anybody that has ever dyno tested one at that low of an rpm.

I know exactly what the 'objective' of idle is. That's not what I was asking.

What I am wondering about is essentially what it takes to stall a 4.0 without any throttle input.

 

[j99xj]

I test drove a 98 manual 4.0 and it took a lot of brake pressure to stall it. But this was a stock Jeep. It almost felt like the engine was getting stronger the harder I pushed on the brake. Perhaps as the rpms dropped, the computer opened the IAC more allowing it to produce more torque. Anyway, I would have to say that the best manual transmissioned vehicles to learn to drive on are behind Jeep 4.0s. I learned to drive a stick on a 22r Toyota and it sucked because there was practically no low end even if the engine was revved a bit.

My current 99 is an auto and it still makes a lot of torque at idle. For example when I'm stuck in stop and go traffic, I'm riding my brakes almost the entire time. If I let off the brake I actually accelerate to 10-15 miles per hour. Most of the traffic around me can clearly be seen "goosing" there throttle a little bit to get rolling again.

Different Jeep but still on topic is my friends work vehicle, an 06 3.7 Liberty. That thing won't move unless throttle is applied. But I guess that's what you get when you put an engine suited for a lighter car into a heavy SUV like the Liberty.

 

[Dr. Dyno]

A stock '91-'95 XJ 4.0 puts out about 85% of maximum torque at 1000rpm.

 

[vetteboy]

A stock '91-'95 XJ 4.0 puts out about 85% of maximum torque at 1000rpm.

That's the most though, right? Assuming you're giving it the most fuel/air it can handle at that RPM?

Looks like I'll have to do some testing or something to get the numbers I'm after. Oh well.