Worried about Knock

[bigdaddy]

So lately I have been reading up on the topic of engine knock, and I'm starting to worry. I go WOT on the highway fairly often when Im in 6th at like 2800 rpms, and sometimes less rpms. I sometimes feel a little stutter right before it builds boost and I take off. I think this might be the "hesitation" that is spoken of with the stock plugs, but I just wanted some opinions. I will stop doing so, but is it bad to have the car in 6th at like 1500 rpms, just cruising, no WOT?

 

[Darth Vader]

It's certainly bad to get on it at 1500 rpm at any time, that's seriously lugging the engine and stressing majorly the rotating assembly. 2800 rpm? Well, I don't hammer it under 3000 myself. Low-load cruising is fine, though I wouldn't just jump on the car after a particularly protracted spell of it as the cylinder head temps will be really high at that point.

 

[RuZec]

yes. I heard that flooring it while you in low rpms and in 6th gear is bad.. like the OP mentioned you putting a lot of stress on the engine.

 

[bigdaddy]

yeah. I will stop doing it as of now. I just didn't know that it was hard on my engine

 

[Darth Vader]

Nothing harder, save completely uncontrolled combustion and, even that is usually momentary. Lugging is long, protracted crushing force on the pistons, rods, piston pins, and crank. It's the most damaging thing you can do in the course of normal driving. Engines like ours let you do it because we have a pinner turbo and alot of static compression = massive low end torque.

 

[Saskatchewan17]

But if you are cruising at a low RPM, but not putting barely any load on the engine, is that still bad? Say barely tapping the gas when cruising in 6th at 45 MPH.

I mean I do this often, I just ALWAYS ALWAYS ALWAYS downshift before passing, going uphill, downshifting to a light/for a turn. I never-ever go WOT below 3k RPM.

And how is it a good thing for an engine to always have it cruise at 3500 rpm or even more? I would think that is more taxing on an engine to keep the pistons/crank and whatnot rotating at such a high rate all the time.

 

[Darth Vader]

No, that's not bad. It's putting high load on the engine at low rpm that's bad.

...and keeping an engine at higher revs IS NOT bad for it. You have to understand the physics involved. It's kind of like cheating in the gym by lifting fast. You can pump out alot of reps before fatigue but, if you take it slow, you have to strain alot harder and will fatigue faster. Your thoughts are a common misconception of how engines work.

Like I said before though, you shouldn't jump on the gas on any engine after PROLONGED cruise, as the engine timing has been retarded for a long time, mixtures are around stoich (~14.5:1 air/fuel) and combustion chamber temps are sky-high.

I believe this engine's tendency is to run pretty high combustion chamber temps normally, as this reduces emissions and boosts fuel economy. The back blade of this strategy is outlined above.

 

[tunersteve]

I have a few questions for the OP/anyone who seems to be experiencing 'knock'

-when you are seeing knock, is the car hesitating/bucking?
-what are you using to see this?
-have you tried letting the knock cure itself?

If you're seeing spikes randomly, let the car run itself and adjust accordingly. You should see a decrease in the random spikes by doing so. I was getting spikes of 7-8 on my DH, on light cruise during warm up on the freeway, now I rarely see anything. As long as the knock isn't audible or pulling fuel/hesitating, you don't need to be freaking out.

 

[bigdaddy]

i did do WOT a decent amount below 3K, and now im freaking out. I have only had my car for 6K miles, and I have stopped doing this, but do you think it has done permanent damage to my motor?

 

[3speedy]

You should be fine don't worry about it.

 

[Darth Vader]

i did do WOT a decent amount below 3K, and now im freaking out. I have only had my car for 6K miles, and I have stopped doing this, but do you think it has done permanent damage to my motor?

First question is, why? The motor makes a whack more torque at 3k than 2K. As long as you weren't going WOT in higher gears at 2k I wouldn't be too concerned. The mechanical advantage of the lower gears eases the load on the motor.

Anyway, anything is possible but, there ain't much you can do about it if you have hurt it. You can find out, if you're really concerned, by running a leakdown test on all four cylinders. If you've hurt one, it'll likely be the rod that's hurt and the cylinder will leak like an SOB past the rings if the rod's tweaked.

 

[Saskatchewan17]

I think you would notice if the engine was badly damaged.

 

[bykeryder4life]

OP if you are worried you are gonna blow your motor and you have knock, you need to change your driving style. Driving around you should stay in vacuum, drive like a granny. if you need to accelerate downshift(you got six gears for a reason) and ease on the gas after to avoid part throttle boosting. If you are gonna boost, it's all or nothing- none of this half pedal low ass rpm s***. you need to have it around 3k rpm and give it full. You cant be boosting and still have the car in closed loop operation its certain fail...

 

[Saskatchewan17]

?
You can't give the car 5psi when at 3,000 RPM?

 

[Darth Vader]

C'mon, Bykeryder, stop spreading panic and dissent. You can most certainly boost this car and run it like any other car. You can most certainly use lower than max boost and lower than max load without blowing up. It's just common sense not to lug the engine, like I described earlier but, other than that, you can drive it like you want.

I think you would notice if the engine was badly damaged.

On a less cheery note: You might not. a tweaked rod, for example, can go unnoticed until it decides it's had enough non-linear motion and fails. I've dismantled running engines that had 7 of 8 rods tweaked, had cracks in every rocker arm, cams ground flat, etc. that were reported to have been running well and "just broke".

 

[bigdaddy]

C'mon, Bykeryder, stop spreading panic and dissent. You can most certainly boost this car and run it like any other car. You can most certainly use lower than max boost and lower than max load without blowing up. It's just common sense not to lug the engine, like I described earlier but, other than that, you can drive it like you want.




On a less cheery note: You might not. a tweaked rod, for example, can go unnoticed until it decides it's had enough non-linear motion and fails. I've dismantled running engines that had 7 of 8 rods tweaked, had cracks in every rocker arm, cams ground flat, etc. that were reported to have been running well and "just broke".

what can I do to run this test? Take it to Mazda?

 

[Darth Vader]

No, any regular car shop can do it for you, with no risk or perception issues if modded.

 

[bykeryder4life]

.

 

[bykeryder4life]

?
You can't give the car 5psi when at 3,000 RPM?

Yes you can, but dont do it part throttle. you gotta get on it enough to make the open loop transition or else your car will be at 5psi running ****** 14.7 afr's with a s*** ton of timing (thats closed loop operation basically). go ahead and do it, im not saying you will blow but its certainly putting stress on your motor thats not needed.

C'mon, Bykeryder, stop spreading panic and dissent. You can most certainly boost this car and run it like any other car. You can most certainly use lower than max boost and lower than max load without blowing up. It's just common sense not to lug the engine, like I described earlier but, other than that, you can drive it like you want.

Yeah you can use low boost and low load, but again, its all about the open/closed loop thing. If you dont give it ENOUGH load or boost or whatever triggers the car to make the switch to open loop, you will be running in boost with a lean afr and high timing advance designed for fuel economy and emissions driving around town. If you own a DH, watch the lower right corner of it. driving around it should have a "C" in the corner, which is closed loop. when it goes to "O" you are in open loop. An "L" means open loop due to engine load. theres even one more i forgot that means open loop with engine failure-limp mode basically.

 

[Saskatchewan17]

Ok, I sort of understand... but don't at the same time (as in I understand the closed/open system concept). I understand what you're getting at byker... it's just sort of hard for me to grasp that you have to use your gas petal and turbo as an on off switch. Surely the car can be boosted saftley at less than max psi and under lower revs (say 3,000 rpm...)? I don't think the car should be driven like an on/off switch. The car has to be tuned from the factory to be able to handle low boost at lowish rpm or low boost at high rpm.