I've been on here for years now and since i've gotten the X seen a few mentions of getting upgraded springs and such to be able to get the car to more safely run 8000-8500 RPM. Me... being someone who is admittedly not very educated in the area of the technical side of engines, was curious to how people know what makes people able to tell if the engine will in fact handle this? Is there some sort of math trick, or is it just from R&D that other places have done? I'm not really thinking myself about doing it, but I have wondered how people do figure these things out.

The only thing I know is I had a friend who was quite a good mechanic and told me that how fast an engine could spin had a lot to do with how balanced the engine was. If it's perfectly balanced then it can spin much faster...

so... let's discuss! lol

 

Any engine produced by Mitsubishi in the past decade is more or less "perfectly balanced." Especially the 4B11 which lost a balance shaft and thus had to become even more perfectly balanced in order to stave off any unwanted 2nd order vibrations...

Balance is of utmost importance because of the acceleration present on an engine turning at 8000+ rpm. Any imbalance that is present starts to wreak havoc on your bearings and crankshaft, almost exponentially with engine rotational speed.

Beyond that, you need to be concerned with piston/rod velocity and acceleration which is dictated by RPM and stroke. Too much piston velocity, and you're approaching a very risky situation whereby components in the rotating assembly will start to give in (wrist pins, rod bolts, rods themselves, piston crowns, crankshaft pins, etc). Engine builders toss around some rough ranges that are acceptable, but in the 4 cylinder engine realm, those ranges seem to be much higher. Anything under 4500 ft/min is considered "safe" for a "built" engine.

With a stroke of 86 mm, our 4B11's can likely safely rev to 10,000+ rpm even though piston velocity and acceleration would be considered "unsafe" at that engine speed according to conventional wisdom. I figure somebody will build a 12,000 rpm 4B11T soon enough. And with how these engines love to make top-end power, it would be quite a build.

But for as far as you're concerned with, you can safely rev to 8500 if you have uprated valve springs and stronger rod bolts installed.

 

I heard of someone on one of these forums getting to 9000 rpm on the stock valvetrain. They say mitsubishi did an awesome job with the valvetrain.

 

I plan on installing uprated valve springs when cams go in, what is involved with replacing the Rod bolts? Can this be done from below with the oil pan off with reasonable success?

 

I've also heard that Xtremeboost grenaded one at ~8200 or so but there are many factors determining what is "safe". Since my tires are about .1" shorter I was forced to set mine to 8300 in order to get the 119 mph trap speeds and so far so good although it only takes once to blow it up. In general I avoid bouncing the rev limiter at full load or over-revving the engine from a mis shift as that's just asking for trouble. I've stuck with an 8000 rev limit for Road racing and daily driving which is plenty of RPM for the turbo I have at the moment.

From what I've read this seems to be the trend:

7900-8000 Mostly Safe for Stock valvetrain

8300-8500 Safe with valvesprings and/or rod bolts (prob both if you rev high often)

9000+ Fully rebuilt and balanced engine with better components

 

Yes but ideally you would have determined each bolt's optimal stretch torque beforehand...

If they are typical ARP 2000 bolts, you just torque them to 47-ish ft-lb and call it a day, and hope nothing breaks

But really, I'd never want to do a slap job on rod bolts. The best way is with the block on a stand, and with a rod bolt stretch gauge in your hand... I just found my rod bolt stretch gauge on my counter the other day. Thought I lost it lol; hope I don't need it any time soon with the 4B11T (crosses fingers)

But in all reality, you'd be fine to install some ARP rod bolts from below with a very, very good torque wrench, very constant angles with which to apply torque, very clean rod bolt holes, and very clean rod bolts (and ARP moly lube).

 

No one is really going to be able to tell you how high is too high, but just because people are turning 9000 and not tearing **** up doesn't mean that is good. You could put a motor on a Spin Tron for about $800 (plus setup of course) a day and find out what rpm the valve starts getting out of control with various springs. I turned 9000 in mine trying to stay in 4th and driving out the back door and it sure didn't feel happy.

I can't imagine that Mitsu is sitting there balancing rotating assemblied within a couple grams, but who knows for sure.

It is always safe to over spring the motor. When you start pushing the stock springs hard, they will fatigue very fast.

 

If you are going through all that trouble, just get some good rods with good bolts. ARP 2000s are mediocre bolts anyway.

 

Thanks for the info guys!

 

Better than stock. And more than good enough for 99% of all people who want to make 600-700 WHP with their 4B11T.

Now strictly speaking, what is better for rod cap clamping; studs or cap screws? Every aftermarket rod I've ever seen has come with cap screws, but that's how they're designed for the most part.

 

i'm still wondering about HiBoost's question of if it's possible to change the rod bolts by pulling the oil pan or not?

 

I answered yes, but I wouldn't recommend it unless you're really, really, anally careful. I gave the details about what you need to be **** about above...

 

I lol'ed at that

 

At BMW there was a recall for con rod bolts on the e46 m3. We used to just drop the pan, use a torque wrench and a stretch gauge depending on the production date. Easy job, easy money...

 

Oh, well that's interesting.

Thing is, there might not be room on that on the 4B11. I can barely manage to use a rod bolt stretch gauge on other engines when they're out of the car! I usually just keep track of the bolts and rods and tighten them in a vice, torque until I get the desired stretch, and make notes. Gets it pretty spot on typically. I know it can be argued that that is completely pointless and counterproductive, but meh