So why are 4-stroke dirt bikes badass? Two-strokes ruled...

BilletMan

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It's probably been 20 years that the 4 strokes have been finding their way into the motocross scene...grab a beer, pull up a stool. How and why?

The two-stroke engine is rather power-dense (hp/lb of engine wt) because they fire twice as often as a four-stroke....the 2 stroke fires every revolution, the four stroke was to spin 2 turns to get a power pulse. There's a lot of friction in a running engine...piston skirt friction is big due to the shearing of the lubricant. Piston ring friction is huge. In a 4 stroke, you're moving the parts a long way in non-power producing to get a nice power event. Two stroke has half the friction per power event. Further, the 4 stroke has the friction of camshaft and valve actuation, significant.

Making a 4 stroke competitive in a 2 stroke MX world is not an easy task! The 2 stroke can spin fast with a cast aluminum piston. If you think about it, the 2 stroke connecting rod is always in compression...if they had "rod caps" like a four stroke, you could run it without the cap...the piston is compressing the nitrogen on the way up and that same super fucking hot nitrogen is pushing it down on the power stroke. The 4 stroke has major yanking on the connecting rod (and rod bolts if so equiped) when the piston goes up to expel combustion products, then reverses to draw in intake air. The connecting rod has major compression for 2 cycles, and a pretty good tension for the other 2.

So the 2 stroke isn't all that concerned with reciprocating mass as it's all compressive and trying to stay together, not EXIT like the 4 stroke that's pulling hard to stretch it's connecting rod half of the time.

More to follow....
 

Stites

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Evan - I have had this new 4 stroke for a little over a week. I love the dam thing, love the amount of power it has, love the newer technology and I love how responsive the throttle is and how she sounds.

With all that I honestly might sell the fucking thing. I just love the sound of the two stroke, love the smell and man do I love the fucking hit! The hit reminds me of a turbo busa.......with all that all the technology in the new 4 strokes is in the new 2 strokes.

I might sell all three of my fucking dirt bikes and go buy a new/used YZ250X.

In all honesty I feel like I am riding a ticking time bomb when on the 4 stroke and it scares the hell out of me.
 
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BilletMan

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So the 4 stroke has to cover a LOT of ground to be able to hang with the 2 strokes. The 4 stroke has to be careful of engine speed due to connecting rod tensile stresses and all sorts of valve dynamics that are entirely absent in the 2 stroke. But the 4 stroke weighs a lot more and produces power half-as-often as the 2 stroke! Fuck! That's a stacked deck! You gotta flow more air, do more with the air that goes through the engine, and do it much more often than a conventional 4 stroke. The key is in the bore-to-stroke ratio!! There, I said it.

Here's what happened. Build a way over-square engine (ala Formula 1) so that you can now have a combustion event maybe twice as often as you could before. If you cut the stroke in half (for example), you can now run twice as fast. Now your combustion frequency is equal to the 2 storke, but you're spinning it at twice the speed. It's all about piston speed....this is related to reciprocating stresses. So now, you zing that motherfucker twice as fast, but the connecting rod is cool with it, as the accel/decel forces are reduced due to the reduced stroke.

Here is where the fun starts! The huge bore/short stroke now gives you big poppet valve area...4 valve cylinder heads DO rock! If you look at "instanteous curtain area" of 2 valve versus 4 valve, it's a quick study....the curtain area is the circumference of the flow diameter times the lift. Four valve has much more impressive valve flow areas thoughout the cycle.....further, two small intake ports can have sweet velocitiy profiles that are absent in big-ass 2 valve ports.....Maybe even more to follow....:fu::fu::biggrin1a::biggrin1a::biggrin1a:
 
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Intake stroke: connecting rod is in tension
Compression stroke: connecting rod is in compression
Power stroke: connecting rod is in compression
Exhaust stroke: connecting rod is in compression
 
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BilletMan

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nitrogen?
You likely missed my discussion about nitrogen...it's THE SHIT!! If you mention nitrogen in drag racing or engine folks, people won't like you. It ain't fuel, it ain't turbo, it ain't nitrous, it ain't supercharger.....yeah, but there is ZERO game without nitrogen!! Nitrogen is what is doing THE work inside of every internal combustion engine on our planet! So you've just burned badass C-16 fuel and loaded it up with SLEEPERBUSAS' 1000hp fogger kit, eh? What are you gonna do with all this fantastic heat you just released? You gonna heat metal (SLEEPERBUSA DOES), or are you gonna make power??

The coincidence that our atmosphere is 20% oxygen and 80% nitrogen happens to bode VERY WELL for internal combustion engines! So you burn the fuel, it heats the nitrogen. That is THE WORKING FLUID of the engine. It converts heat to mechanical work. Same with jet engines. Nitrogen is fucking badass and entirely needed for your engine to run. Raise your hand if you've leaned something today...OK, I gotta get back on track with the 4 stroke MX engine...
 
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BilletMan

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Intake stroke: connecting rod is in tension
Compression stroke: connecting rod is in compression
Power stroke: connecting rod is in compression
Exhaust stroke: connecting rod is in compression
Nope, if you look at the pressure/area term of the exhaust event, the rod is in tension as soon as it starts slowing down at about mid stroke...exhaust pressure is nothing compared with compression or firing pressure. Inertia forces of the parts are well in excess of the force created by exhaust pressure acting on the piston's area....

And if you think about the inlet event, it's only tensile while accelerating the piston down from TDC...once you get to mid-stroke, you slow it down with compressive force...
 
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BilletMan

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The Yamaha YZ-250f that I owned shows this trend...IF you have a huge bore, you can put lots of big-ass valves in the head...lots of "instananeous area" as the valves open......the intake ports feeding the 3 intake valves have higher velocity at lower lifts than one big intake valve....so the engine flows well over a broad range of speeds...makes great torque down low because you can have modest cam overlap, yet flow well at high speeds, too! Big bore, short stroke: Tons of valve area....spin it fast with short stroke...This combination pulls damn well from pretty low, and just KEEPS pulling...this thing redlined at 13,500, and pulled all the way there....

Easy bike to ride at the track...no pronounced hit...if you're in the wrong gear in a turn (too high or too low), no sweat, just twist the throttle....

Fuck me, this is OEM shit! 5 titanium valves in a 13,500 rpm stock dirtbike!?
38809

38810

38811
 

DavidDole

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"The 4 stroke has major yanking on the connecting rod "

That's prob why some of us were jerkin wrist pins out of the underside of the pistolas leaving a scattered mess behind.
 

whipit1K

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Back in the 2000-2006 era, motocross magazines routinely did comparisons ("shootouts") that included both 450 four strokes and 250 two strokes, since they were expected to compete with each other.

Even if you read or understood nothing else, all you needed to see in these shootouts was the dyno chart with all bikes shown. The 250 two strokes made nearly the same horsepower as the 400-426-450's of the day , but it was delivered in a short, steeply climbing curve that had a sharp peak and abrupt dropoff. Looked like a profile shot of the matterhorn.

The four strokes had a smooth buildup of power that pulled hard long before the peaky two stroke did much, and continued pulling for thousands of additional rpm after the two stroke dropped off like a rock. an ideal, smooth rideable powerband.

This is rideability. This is the type of powerband that works. two stroke powerbands can smoothed out a bit with heavier crankshaft mass - but it is just a bandaid solution- making it rev slower

Honestly, I do not know why anyone is worried about four stroke motocrossers and longevity these days.

They are built with all the same tech that current sportbikes have , but with tougher crankshafts and greater durability, in my opinion. When properly maintained, they are mosty rock solid.

Trouble is, dirtbikes are very frequently sold to young dumbasses who know nothing about proper maintenance and just run them into the ground for lack of proper filter service and oil changes. Then said dumbass tells everyone who will listen that four stroke dirtbikes are unreliable....

I would not dream of going back to a two stroke. I can hardly stand them, they are borderline unrideable in my opinion....at least if you want to be fast and smooth
 

pimpslayer

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This is just my opinion...

I like light bikes, especially in the dirt.

I would rather have a nice, simple 300 or 400cc 2T with a little extra flywheel to it as a woods bike, than a heavy 450cc 4t.

They would both live just as long with maintenance, but the 2T would be simpler/cheaper to freshen up when needed.
.
 

r1racer

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I would not dream of going back to a two stroke. I can hardly stand them, they are borderline unrideable in my opinion....at least if you want to be fast and smooth
If you're referring to motocross 2 strokes (YZ,KX,CR, RM etc) then sure however Beta, KTM, Sherco smokers are designed for hard enduro. They're plenty fast, smooth and lighter than comparable thumpers.
 

whipit1K

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If you're referring to motocross 2 strokes (YZ,KX,CR, RM etc) then sure however Beta, KTM, Sherco smokers are designed for hard enduro. They're plenty fast, smooth and lighter than comparable thumpers.
I have also ridden the YZ250X two stroke, which is eye opening because it looks exactly like the standard YZ. It is much more tractable and happy to lug if needed but still pulls well in the mid ....for a two stroke. It has a different head, different pipe, different internal gear ratios, etc etc. I still would not want one. The two stroke powerband sucks , big time, compared to a modern 4 stroke. They are dinosaurs ....hard enduro is their last real domain
 

agri

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I have ridden over the years several 2 strokes and 4 strokes.
I am faster on a 4 stroke but love the simplicity of the 2 strokes. Almost all of the 2 strokes especially the 300's run like electric motors, the FI injected ones are magic.
Most people would be surprised how light some of the 4 strokes are now. That being said 20 lbs difference over a long ride does make a huge difference.
For me, its way easier to do a top end on a 2 stroke and its way cheaper. Another thing, although the 4 strokes I have had were reliable my 2 strokes have been bullet proof.
With age I have gone in the last 40 years from a CR500, XR600...to a KTM 150xcw.
I find the light weight and decent power to be better for me on long runs. Most of the time I am in front of the bigger bikes and never do I say I need more power.
The power I do have goes to the ground and I don't have the wheel spinning wasting energy.
This weekend a friend borrowed my 125 and on some fast twisting trails we could barely keep up.
Any how, what ever makes you happy, and boy are we lucky how fucking great these new bikes are.
 

pimpslayer

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I have also ridden the YZ250X two stroke, which is eye opening because it looks exactly like the standard YZ. It is much more tractable and happy to lug if needed but still pulls well in the mid ....for a two stroke. It has a different head, different pipe, different internal gear ratios, etc etc. I still would not want one. The two stroke powerband sucks , big time, compared to a modern 4 stroke. They are dinosaurs ....hard enduro is their last real domain
Have you ever ridden a KTM 300? MX or EXC?

(It doesn't really matter which one I guess, they all come with a 12 alternator on them (read as flywheel))
.
 

whipit1K

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This is just my opinion...

I like light bikes, especially in the dirt.

I would rather have a nice, simple 300 or 400cc 2T with a little extra flywheel to it as a woods bike, than a heavy 450cc 4t.

They would both live just as long with maintenance, but the 2T would be simpler/cheaper to freshen up when needed.
.
According to KTM's own web site, the 2021 Transfer Port Injected 300 two stroke weighs 228 pounds dry, the 2021 450XC is 223 pounds
There are heavy four strokes still, such as my 2018 (old design) Yamaha YZ450FX, but KTM leads the way on all things dirtbike these days and their four strokes are lighter than most two strokes.

Kawasaki's last KDX200 was also said to be 223 pounds dry.

We are long past the old days of 285 pound, 30 horsepower Honda XR's. New four strokes blow two strokes out of the water in every way
 

agri

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All the new 300s and even the 150FI two strokes are so far advanced from the old fire breathing 2 strokes its crazy.
Most of them have power bands like trials bikes.
I have ridden both the Husky and KTM 300 FI 300 and this weekend a new 250 KTM 6 days.
Smooth tractable power and you don't have any jetting issues.
When I did Costa Rica on a KTM 450exc, for me often I wish I was on a 300. Its all personal preference.
 

agri

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Whipit,
Up here in Northern Canada, say 5 years back 90% of the rides were 4 strokes.
Now, 90% of the guys are on 2 strokes.
You can say this, what ever your choice is there are few bad bikes now, which is good.
My worst 2 stroke..... a Canam 400 qualifier . It would try to kill you with a vile power band, shitty handling and bone jarring suspension.haha
 

Fever500

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In all honesty I feel like I am riding a ticking time bomb when on the 4 stroke and it scares the hell out of me.
Honestly, I do not know why anyone is worried about four stroke motocrossers and longevity these days.

They are built with all the same tech that current sportbikes have , but with tougher crankshafts and greater durability, in my opinion. When properly maintained, they are mosty rock solid.

Trouble is, dirtbikes are very frequently sold to young dumbasses who know nothing about proper maintenance and just run them into the ground for lack of proper filter service and oil changes. Then said dumbass tells everyone who will listen that four stroke dirtbikes are unreliable....
Stites ... read and believe... so much misinformation about modern 4 strokes these days.
 

Stites

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Stites ... read and believe... so much misinformation about modern 4 strokes these days.

Thanks, many of his posts about 4 strokes is what has given me a boost of confidence. I am just new to four strokes on dirt bikes and unfortunately have heard the stories.
 
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BilletMan

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Four strokes are fine, just make sure you break it down and accurately measure the piston and bore every 50 hours...this is critical with the huge bore and very short piston....it doesn't take much clearance to cock the piston in the bore...BOOM!
 

maui

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My last trials bike was a gasgas, 2 stroke, unbelievably smooth on power. It was simply put designed for a low RPM operating range and it was fine. My issue with 2 strokes here is noise. Thats basically my only complaint.

as for the connecting rod only being in compression, what about when you're at 13million rpms and you cut the throttle?
 

Master-Cylinder

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My last trials bike was a gasgas, 2 stroke, unbelievably smooth on power. It was simply put designed for a low RPM operating range and it was fine. My issue with 2 strokes here is noise. Thats basically my only complaint.

as for the connecting rod only being in compression, what about when you're at 13million rpms and you cut the throttle?
First thing the asshole do around here is take the muffler off their junk four stroke dirt bike and make a shit load of noise that travels a good distance. Same with the ATV's. Stock new mufflers on either two stroke or four stroke work well, until they fuck with them.
 
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BilletMan

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Maui, of course there could be conditions where compression of the rod is at a minimum, and given a heavy enough piston and high enough speed, you could impart some tension. But unlike a 4 stroke that has the highest piston acceleration at TDC with zero cylinder pressure during the exhaust-to-intake event, the 2 stroke will still have it's compression pressure of 150psi+ when you chop the throttle . For an 80mm piston at 150psi, that's 1169 pounds of compressive force.

While it seems strange, it makes sense if you think about this. A Detroit Diesel 2-stroke (and EMD locomotive engines) experience the most vexing of lubrication of the wrist pin-to-small-end of the connecting rod. It is difficult to lubricate this one area because it never comes unoaded. A 4 stroke unloads during the exh/intake tension reversal and can replenish the oil film. The 2 stroke can't. Further, it never gets the velocity to utilize hydrodynamic lubrication like the crank pin and big end of the rod enjoy....DD wrist pin bearings have a tough life!
 
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BilletMan

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And they often run worse than stock....imagine that the OEM engineers actually DO know what the hell they're doing. Yep, I have an asshole neighbor who tears the muffler off of everything he owns... :fu: :fu: :fu:
First thing the asshole do around here is take the muffler off their junk four stroke dirt bike and make a shit load of noise that travels a good distance. Same with the ATV's. Stock new mufflers on either two stroke or four stroke work well, until they fuck with them.
 

maui

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I used to roll around here on a YZ400. They could hear me a looooong ways off. I did like the power that bike had but the lack of stealth was a disadvantage. At the same time I had a 125 4 stroke and while it was an absolute dog on power, it was a really fun trail bike. I probably had 4x the time on that bike then the big YZ
 

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BilletMan BilletMan

Thread on the FB about jetski pistons with holes in them ... here is one guys explanation ... (though we know they were for the Goki reed conversions)


its because when the piston is heading to bdc, the negative pressure created by the combustion exhausts sound wave, can draw through it and down into the cases, so it adds to the overall breathability of the engine.
Then goes on to explain how this also helps "supercharge" 2 strokes ... like the expansion chamber ...

The sound wave creates a negative pressure in the chamber, which draws through the entire engine from intake , cases, transfers, combustion chamber, all the way through into the expansion chamber. The "reflex" wave, or "return" wave caused by that same sound wave hitting the convergent conical area of the end of the pipe, is what stuffs the excess a/f (air/fuel) mix thats drawn out into the expansion chamber, back into the combustion chamber just as the port(s) closes. This "over stuffing" of the cylinders apon closing, compression, and then combustion is what is called "supercharging" the cylinder(s), and its how a 2-stroke engine is able to make more power with a tuned exhaust, than what it could make on its own without one.
Interested in your take on this ...
 
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I never cared for starting them compared to 2 strokes. Of course they all have a button now.
If only my budget was that big.

I'd probably get a YZ250FX
 
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BilletMan

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Fever 500, yep, that's right on. When the engine is "on the pipe" it's a pipe organ. That's why I wrote that crankcase compression ratio isn't too important (your 550 "T" cranks has low CC comp ratio) when the engine is on the pipe....the pipe on a loop-scavenged 2-stroke is a great thing! Also, BSFC goes down a lot when you're on the pipe because it's putting the fuel back, not wasting it...
 

Master-Cylinder

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Now do reed valves, they are more than just a flap. You could melt a slide rule just trying to state how they work mathematically. We're talking deflection, resonance and junk like that.
 

Master-Cylinder

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Fever 500, yep, that's right on. When the engine is "on the pipe" it's a pipe organ. That's why I wrote that crankcase compression ratio isn't too important (your 550 "T" cranks has low CC comp ratio) when the engine is on the pipe....the pipe on a loop-scavenged 2-stroke is a great thing! Also, BSFC goes down a lot when you're on the pipe because it's putting the fuel back, not wasting it...
I "four stroke" the shit out of my DT250 riding it around town.
 

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First thing the asshole do around here is take the muffler off their junk four stroke dirt bike and make a shit load of noise that travels a good distance. Same with the ATV's. Stock new mufflers on either two stroke or four stroke work well, until they fuck with them.
I have been asked more than once how I made my Harley so quiet. I have stock mufflers, and I just tell them "this is how they come"

When I picked up my 2 stroke snowmobile a few years ago, it had an aftermarket "race" pipe on it already. I put the stock system back on before I even got it registered.
 

Master-Cylinder

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All my stuff has stock exhaust on it, even the Olde Ratt Maxx. It had a four into one when I got it, sounded like a Pro Stock drag car. But way too loud.
 
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BilletMan

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Now do reed valves, they are more than just a flap. You could melt a slide rule just trying to state how they work mathematically. We're talking deflection, resonance and junk like that.
Indeed....as you're well aware, the 2-stroke is a gas dynamics nightmare! Accurately computer modelling a loop-scavenged two-stroke is WAY more complex than a four-stroke engine! The 4 stroke has it good, it has pure scavenging events...you pump almost all of the exhaust out, cleanly close the exhaust valve and open the intake for a clean induction of fresh air. The chaotic shit in a 2 stroke is a fucking nightmare to understand and accurately model....this is hardcore stuff for CFD modeling. Then you throw in more sonic chaos with a tuned pipe....the whole engine from the airbox to the tip of the exhaust becomes a pipe organ.....For as crazy simple as the engine is, it's hell to really quantitatively understand....

I "four stroke" the shit out of my DT250 riding it around town.
Yep, light load exhaust residual fraction makes misfiring guaranteed....what's cool is the statistical firing of the event that follows the misfired event...it's damn good because most of the residual fraction is gone and replaced with mixture that didn't ignite last time....after a misfire event, the combustion is quite good! As well it oughta be, eh? You miss one, you're gonna do DAMN well on the next attempt...
 

Master-Cylinder

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Just last week, I was pondering replacing the original stainless steel reeds in my RD350 with fiberglass ones. When a stainless one breaks off, it gives the engine a bad case of Torque Ingestion. Not so much with fiber reeds.
 

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Just last week, I was pondering replacing the original stainless steel reeds in my RD350 with fiberglass ones. When a stainless one breaks off, it gives the engine a bad case of Torque Ingestion. Not so much with fiber reeds.
The fibers do wear out faster though but, without the damage, it's worth having to change them more often.
 

whipit1K

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Four strokes are fine, just make sure you break it down and accurately measure the piston and bore every 50 hours...this is critical with the huge bore and very short piston....it doesn't take much clearance to cock the piston in the bore...BOOM!
I have never, ever done this. I ride and race primarily 450cc four strokes and have accumulated as many as 300 hours on original OEM pistons (KX450F). why would I do such a thing? because I got tired of slapping top ends in bikes that did not need them. when you disassemble a bike and the 100 hour piston looks new and the rings measure as fine for the fifth or sixth time, you just say fuck it lets just run it.

DONT suck dirt. DO change the oil. DONT hold it in the rev limiter to warm it up like a fourteen year old, etc. It truly is just stupid young know nothings who give four strokes a reputation
 
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BilletMan

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Risk versus return...I've got real stories of real friends blowing up their 4 stroke dirt bikes because they stretched the maintenance interval. Not punk kids, highly educated professional folks who ride MX on weekends....your opinion is yours and works for you. These things are very complex engines, the bikes cost WAY too much, require more/expensive maintenance. I've owned and ridden the shit out of both....I sold my 4 stroke and very much enjoy riding my 2-stroke...if I overshoot maintenance, no biggie, it won't explode, it'll just wind down in performance...
 
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