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Engine Lugging, Drivetrain Lugging... Low RPM+WOT

('09-'13) 
5K views 14 replies 5 participants last post by  Sweetsixteam 
#1 · (Edited)
Hey guys, my main question here is; What is too low of rpm for my AWD drivetrain?

Here is my story... 2011 Subaru Forester 2.5X Premium with Manual 5 speed tranny. Purchased used @70,000kms, now has 100,000kms.

Last January (2017) I was cruising 5th gear, ~1,900-2,200rpm, ~50-75km/h. Because an uphill was coming I suddenly hit WOT without downshifting. As soon as I hit max engine Load/full TPS/Open Loop, engine stumble with loud sound (implosion like). Blown a bearing on shaft inside the transfer case. The transmission sounded differently, more winding sound. It got worst and worst, the bearing melted inside there, smell burnt oil for weeks after. During that time I didn't know anything about engine lugging btw.

2,000$ later I would like to take every precaution. I was told that AWD Subaru needed REVS for drivetrain to perform under high load / full throttle / high power output situations. ?
*I don't put the blame on 1 simple reason on that transmission fail. I think it is more a cocktail of; oversized/overweight tires/Engine lugging situation/low rpm/wot ***plus after the repair done, I came to conclusion that Bearings inside the transfer case was already damaged before failure!Before the repairs, my shifter stick was shaking a lot more! A lot more, but in a way acceptable for a fresh owner of subaru awd (back from Toyota Echo lol) Also there was a rattling annoying sound before when reaching the 4k rpm. Now all smooth up to the redline! Clutch was ridin hard by previous owner, had to change the clutch also.

I also tweaked the rims setup with my BFG Ko2 225/65r17. I was running the oem enkei 17 silver spokes rims, unfortunately those rims are too heavy. Swapped them with Enkei RPF1. Oem~25lbs, rpf1~15lbs, -10lbs, Ko2+10lbs, so that kinda net me close to flat.

Now I was wondering, what rpm is considered ''too low'' for a WOT in higher gear 5th ?
With my oversized tires, n/a engine, if I cruise 105 km/h~2,500 rpm I feel that rpm is not enough for full engine load. Full load on 5th gear 2,500 rpm has very low acceleration, feels like its struggling to increase speed but without any vibrations or bad sound. Its just smooth but very slow. So I think that 2.5k rpm may be to low putting stress on drivetrain/engine. Still dropping to 4th feels too much... Cruising closer to the 3,000 rpm (2,900 min) feels a lot better for acceleration and stress on hardware but I am going too fast.

http://www.subaruforester.org/vbulletin/attachment.php?attachmentid=388178&thumb=1




 
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#3 · (Edited)
Thanks for inputs, I do understand all this. What I am more concerned about is not the power outputs but the damage/stress givin to engine & drivetrain pushing out max power with so low rpm (2,500)...

For me, I prefer being close to the 3,000rpm when engine is pushing max load, but this is the whole questioning, maybe Im wrong. Being close to 3k rpm means I gotta drive too fast on the highway (5th gear), I would prefer 105km/h which is about 2,500rpm but then I don't like when max load at that low rpm.
 
#5 · (Edited)
As mentioned I run BFG TA Ko2 225/65r17 versus oem 225/55r17.

OEM ---- Ko2

Diameter
26.7" ----28.5" ---- +6.7%

Width
8.9" ---- 8.9" ---- 0%

Sidewall
4.9" ---- 5.8" ---- +18.4%

Circum.
84" ---- 89.5" ---- +6.7%

Revs/Mile
777 ---- 729 ---- -48

***I think my calculator is wrong. When I first checked it was 1'' taller, so 1/2'' lift from ground. Not almost 2''... I need to double-check that.
 
#6 ·
I can't imagine why there could be transmission damage in 5th gear no matter what. Engine RPMs, tire size/condition, road angles - nothing should do damage.

I suspect there was a defect in the transmission.

I've read that WOT in 1st gear is a bad idea but certainly not 5th.
 
#8 · (Edited)
I can't imagine why there could be transmission damage in 5th gear no matter what. Engine RPMs, tire size/condition, road angles - nothing should do damage.
I suspect there was a defect in the transmission.
I've read that WOT in 1st gear is a bad idea but certainly not 5th.
Yeah, like I said, we knew after all that there was a defect before the failure. BUT STILL, going WOT/Open Loop/Full Load @Low RPM is what caused the FINAL Break Point that blown one bearing completely into the transfer case.

Also going WOT on 5th low rpm is a bad idea, for engine crankshaft & combustion into cylinders, we call that Engine Lugging. The questioning here is more about Drive Train rotation/rpm versus Full Power Load. There should be more stress to a drivetrain components that is spinning 2,500 rpm versus 4,000 rpm i.e.
I just don't know at what degree? Enough to take that as special care or doesn't matter.
-------------------------------------
Wow didn't see the original tire size listed ? Anyways going almost 2 inches taller on your tires will throw your speedometer off and that will cause shift points to be off. but this would not cause damage to the trans only reduced fuel mileage and loss of power.
I think my calculator is wrong. When I first checked it was 1'' taller, so 1/2'' lift from ground. Not almost 2''... I need to double-check that.
 
#12 ·
The problem? I've explained it too much. I think that I will just let it go lol. I had a damaged transmission bearing from the car purchase, put overweight tires over it and what finished it off is low rpm full throttle situation. I am probably going too crazy with it.

The weight of tires is now corrected by a lot lighter wheels (enkei rpf1) roughly going from 25 pounds rims to 15 ish.

But still wheels higher diameter is there. So that require more rpm.

Living with it, doing my best. :p
 
#14 ·
Yea that's what I think too. Even if my car is equipped with cruise control I think its a bad idea because first it is manual (so it can't downshift by himself) and second because I ride over sized tires rpm needs to be higher than normal. Normally on a auto tranny with stock tires it wouldn't matter because is this legal / regular speed.
 
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