RPM/redline/overspin

[SubinJenkintown]

Hi all and thanks in advance for any response.

 

Question about the RPM redline.

 

Situation: Was driving 05 legacy gt ltd the other day and floored the pedal without realizing that the RPM indicator had gone up to about 7k, right on the redline. Didn't stay that way for over 3 seconds.

 

Question: how damaging could such an occurence be to the engine/turbocharger, etc?

 

Thanks in advance for your response.

[psucaptainkickass]

You fried it. You are never EVER supposed to go to redline. Even though it is there, you are never to reach it. You have seriously degraded the stability of the engine, and more than likely will have to get a new engine block within the next 4k miles. You can try to claim a new block under warranty, but if you read it, it is not covered under the powertrain warranty. In fact, they specifically say that any trip to redline will violate not only the powertrain warranty, but also the 3/36. I am sorry man, but you're screwed.

[John M]

Heh - you didn't even hit the fuel cutoff. After driving cars with a 7k rpm redline (and a 7500 rpm shift point out of 1st ) for 12+ years I find myself hitting the limiter several times a month.

[jerami1981]

if it doesnt stop, you havent gone far enough. the engine will cutoff as a safety precaution when you get too high, i would not however recommend doing this repeatdly every day obviously, but every now and then wont kill you or the car. I have hit it on a few occasions as well when loss of traction has occured.

[Th3Franz]

You fried it. You are never EVER supposed to go to redline. Even though it is there, you are never to reach it. You have seriously degraded the stability of the engine, and more than likely will have to get a new engine block within the next 4k miles. You can try to claim a new block under warranty, but if you read it, it is not covered under the powertrain warranty. In fact, they specifically say that any trip to redline will violate not only the powertrain warranty, but also the 3/36. I am sorry man, but you're screwed.

lol you're such a comedian.

Fuel injected cars have the red line programmed into the ECU, so that when you hit the red line, the RPM does not continue to climb which may cause damage to the engine. Your car is probably fine, just try not to do this on a regular basis.

[psucaptainkickass]

Yeah, I do what I can. OP you're fine...as these fine members have pointed out.

[SubinJenkintown]

Thanks for the responses...was worried as heck when I read the first thread but good to know that things are most likely fine.

 

Can sleep comfortably now.

 

All the best to everyone

[Scotty]

Redlines are set artificially low anyways. Plus if you exceeded the true redline, you would have destroyed the engine.

[psucaptainkickass]

Thanks for the responses...was worried as heck when I read the first thread but good to know that things are most likely fine.

 

Can sleep comfortably now.

 

All the best to everyone

You too! Enjoy the car!

[psucaptainkickass]

I think it just has to do with the stress involved with the engine rotating that fast. I am just there is a limit to the internals of the engine. If it spins to fast it probably generates too much heat and stuff starts to break.

[Th3Franz]

What exactly happens when you exceed the true redline? Everything is timed and turns together, I don't understand why there is a limit...

The valve train has a limit.. you'll tear up your heads in a hurry if you rev the engine too high.

[psucaptainkickass]

I seem to remember reading somewhere that if the revs exceed the speed at which the valve springs can close the valves that would bugger it up...

 

What is usually changed to support higher revving?

 

Are you looking for specifics? Like why can the S2k engine rev to 8-9k? I don't know the specifics, but, things rub inside the engine, and generate heat because of friction. I would figure that they would use materials that can withstand heat better, and have less friction.

[Ric]

I seem to remember reading somewhere that if the revs exceed the speed at which the valve springs can close the valves that would bugger it up...

 

What is usually changed to support higher revving?

The springs obviously!

Stronger springs will close the valves quicker, but cause more wear.

Some manufacturers are trying hydraulic operation, to push the valve down and up.

 

If the valve doesn't close quickly enough, the piston can hit it in some engines. You're talking about major damage if that happens.

[TSiWRX]

Heh - you didn't even hit the fuel cutoff. After driving cars with a 7k rpm redline (and a 7500 rpm shift point out of 1st ) for 12+ years I find myself hitting the limiter several times a month.

 

+1

 

Finally, after installing the TurboXS Knock/Shift light, I'm now no longer a habitual fuel-cut hitter.

[ebpda9]

I seem to remember reading somewhere that if the revs exceed the speed at which the valve springs can close the valves that would bugger it up...

 

What is usually changed to support higher revving?

 

springs. they could not close the valve fast enough, so the piston will hit the valves, or the valve retainers will just fly off. i had my accord to 9500rpm before when the tranny broke on me, and nothing happened to the engine even though i was 1500rpm from where the tach ends.

[KTM 525]

I cant figure why you guys would push the motor anywhere near redline anyway. This motor is so DONE @6K it's not even funny. Shifting beyond this is a complete and total waste. Just my opinion here but show me a dyno sheet that has a hp curve still climbing after 6K on the stock VF40. Most are done by 5500 nevermind even 6K I personally shift at or slightly before 6K when hammering.

[mccorry]

Guys.... at high revs, the valves tend to "float" due to the phenomenon you are talking about. That happens in all combustion engines (pushrod and OHC alike). But... that isn't usually what blows them up. Most over-rev situations fail the motors on the bottom end. The rods will crack, or a rod cap will let go..... that is how most failures occour.

 

It is due to the reciprocating mass and piston speed... along with the loading on the piston, rod, and cranck at high RPMs. This loading will cause the rods to stretch and potentially break.

 

From a BMW website......

Mean Piston Speed

The best known "rule of thumb" is mean piston speed (average speed the piston travels at a fixed rpm). It goes like this:

Mean Piston Speed Result

------------------ ------

Under 3,500 ft/min Good reliability

3,500-4,000 ft/min Stressful, needs good design

Over 4,000 ft/min Very short life

 

Mean piston speed is easy to determine. It's based on the piston stroke and rpm.

 

Cm = 0.167 x L x N

 

Cm = mean piston speed in feet per min L = stroke in inches

N = crankshaft speed in rpm

 

Using the 1988 E28 M5 as an example:

E28 M5, Bore and stroke 3.68 x 3.31 in (93.4 x 84 mm) At 6,900 rpm (factory redline)

 

Cm = 0.167 x 3.31 x 6900 = 3,814 ft/min

 

[mccorry]

Some additional information...

 

From R&T...... http://www.roadandtrack.com/article.asp?section_id=18&article_id=2398

 

Some Examples

 

But back to piston travel and that 2500 ft./min. benchmark. Improvements of metallurgy and design have evidently raised the bar on durability, but piston speeds haven't exactly skyrocketed. Consider the Subaru Legacy GT, a car I enjoy at least in part because it isn't particularly rev-happy. The Subaru flat-4's stroke is 79.0 mm. Thus in one revolution, its piston travels 158 mm or 6.22 in. Hence, at its torque peak of 3600 rpm, the piston speed is 22,394 in./min., or a nice, comfortable 1866 ft./min. In fact, its revs at 2500 ft./min. would be 4823 rpm, equivalent to 116 mph in 5th.

 

As an admittedly extreme road-going example, consider the high-revving Honda S2000. Its inline-4's stroke of 90.7 mm gives piston travel of 181 mm, or 7.14 in., per revolution. Thus, at the engine's torque peak of 6500 rpm, a piston travels 46,421 in. or 3868 ft. in each minute, well beyond the 2500-ft./min. benchmark.

 

And for absolute rev-glee, let's do the same calculation for the Ferrari F1-2000. I choose this particular car because, though F1 bores and strokes are usually highly confidential, Peter Wright offers them in his fabulous Ferrari Formula 1: Under the Skin of the Championship-Winning F1-2000 (reviewed in this column June 2004). The Ferrari has an extremely short stroke of 41.4 mm and revs to 18,000 rpm! Hence, 41.4 mm x 2 is 82.8 mm or 3.26 in.; times 18,000 is 58,677 in. or a mean piston speed of 4890 ft./min.

 

In fact, as an engine's stroke has such a profound effect on this, the really extreme piston speeds aren't in F1, but in NASCAR: At superspeedways, their 358-cu.-in. V-8s are said to be approaching 10,000 rpm. At these revs, a typical 3.5-in. stroke translates into a piston speed of 5833 ft./min., more than twice the Pomeroy dictum.

 

 

Here is a calcuator that you can play with. Based on the R&T numbers above... our motors have a storke length of 3.11 inches.

 

http://www.thedirtforum.com/pistoncalc.htm

 

Using this info.... at 7,000 RPM... our motors have a mean piston speed of 3615 ft/min. This is getting into the stressful area.

 

At 8,000 RPM... the mean speed is 4130.... which is where you could start breaking internals on OEM motors.... it all depends on how well the motor internals are designed.

[TSiWRX]

I cant figure why you guys would push the motor anywhere near redline anyway. This motor is so DONE @6K it's not even funny. Shifting beyond this is a complete and total waste. Just my opinion here but show me a dyno sheet that has a hp curve still climbing after 6K on the stock VF40. Most are done by 5500 nevermind even 6K I personally shift at or slightly before 6K when hammering.

 

I totally know that our snails quit puffing up high.

 

It's just that I literally forget to shift!

 

Too many years in vehicles with higher redlines that actually make power up-high - and too many years of vehicles that were louder, too.

 

I just can't "hear" the LGT going at it - and she revs so smoothly and effortlessly, too.

[Scotty]

The general limitation is the valve springs. Once you approach around 14k-15k rpms, in theory they won't rebound fast enough. F1 engines uses pneumatic valves for that reason. Honda prior to pulling out of CART did a test on their CART engine. They ran the engine that was designed for 13,500 rpm with steel springs at over 22,000 rpm for 500 miles with no problems what so ever. That was higher than what they were testing the F1 engine.

 

Super high RPMs isn't the answer for everything. Besides the associated stress of running high RPM, there is also significant increase in friction. So the more power you gain by going to higher RPM can be offset by the increase friction. That is why the Renault F1 engines were doing quite well compare with everyone else considering that they ran 1,000+ RPM less.

[John M]

I cant figure why you guys would push the motor anywhere near redline anyway. This motor is so DONE @6K it's not even funny. Shifting beyond this is a complete and total waste. Just my opinion here but show me a dyno sheet that has a hp curve still climbing after 6K on the stock VF40. Most are done by 5500 nevermind even 6K I personally shift at or slightly before 6K when hammering.

 

This is in 1st gear only, where the declining hp figures are acceptable since the gearing is so low you still accelerate better than you would by shifting into 2nd sooner. It also puts you in a much better spot when you hit 2nd gear.

 

If I hit the limiter it's usually at the top of 3rd gear when passing someone. I've always shifted by ear and my mind tells me I have more gear left when I actually don't. I've been spoiled by cars that can do anywhere from 110 to 120 in 3rd gear It's taking me time to get back to DSM-like shift points.

 

I did the same thing once in a Green-powered Galant VR4 (probably the first Green ever produced btw). I stuck it in 3rd and passed someone on a 2-lane highway. The Haltech said I shifted just north of 8k rpm It was so smooth and powerful I had no idea I was revving that high! Luckily the engine was built to take it so I didn't do any damage.

[pillboy]

The di-lithium crystals get too hot.

[AWDpower]

My question is, why is the LGT redline 500rpm lower then the STi with the same engine? I know the power drops off with the stock turbo, but many times it's advantagous to stay in the lower gear a little longer, like at an auto-x.

Funny story. My wife asked me if there was anything I could do about our modded 2G Eclipse GSX. I asked her what the problem was. She said she wanted me to raise the rev limiter, at 7500 rpm it was cutting in on her too often! Gotta love women who know how to drive.

[mccorry]

My question is, why is the LGT redline 500rpm lower then the STi with the same engine? I know the power drops off with the stock turbo, but many times it's advantagous to stay in the lower gear a little longer, like at an auto-x.

[/size]

 

The only thing that I can think of is that the cam profiles in our motor don't do much up above 7000. Just a guess......

[PeterJMC]

The STi and the LGT motor are exactly the same except the turbo.

 

The STi has more top end power with the bigger turbo, vf39. The LGTs turbo doesn't provide the same power band as the STi which is why the limit is lower. In the LGT there is no point in getting the engine to 7,500 rpm if it won't produce any power there.