I've heard overrevving thrown around in reference to stock valve springs and hot southern girls,

but what is the potential ramifications? besides contacting the valves contacting the pistons...

I find it best to just avoid doing it. It's worked for me all these years.

pistons and valves can get intimate due to float.... due to overwhelming the valvesprings im guessing

just is painful, break rockers, bend pushrods.... break stuff....

The inertia of the valves opening becomes more than the valve spring can compensate for, so your valve keeps opening even though the cam is no longer pushing it open and the piston hits it. Bang smack, broken engine. Now you need new heads, new pistons, and potentially more.

The amount of damage depends on how tight the engine is to start with and the components that were used. More critical is how much you over rev it and for how long.

Example: If you are racing along in 3rd gear and hit red line and then try to shift into 4th but go into 2nd instead. . . . If you let out the clutch even part way for a fraction of a second - you just cost yourself a couple of thousand dollars. Don't ask me how I know.

Isn't there some way of ... preventing this... besides being extremely careful ?

QUOTE (Hammy @ Sep 30 2005, 12:19 AM)

Isn't there some way of ... preventing this... besides being extremely careful ?

yepp, don't rev your engine over 2k. never ever! that way, a accidental downshift won't kill it ...

Andy

QUOTE (Hammy @ Sep 29 2005, 11:19 PM)

Isn't there some way of ... preventing this... besides being extremely careful ?

Revv limiters work... Cut the spark, engine can't fire and rev higher. I think on some MSDs you can program the rpm limit to what you want.

Preventing valve float... go with beefier vavle springs, double-springs, don't rev it so high.

Another way is to know what gear you're in and where the next one is

Also, lighten your valve train.

ok, "don't rev it so high" ... Stupid question, but where should one be revving to? In my 1.7 I would normally shift at 4200 or around there, as power seemed to drop out a little past 4000...

You should rev to peak torque. Anything above that is wasted. Stock components are fine up to 5500 rpms.

QUOTE (McMark @ Sep 30 2005, 04:03 AM)

You should rev to peak torque. Anything above that is wasted. Stock components are fine up to 5500 rpms.

peak HP.

broadly speaking, peak torque is where the cam starts working; peak HP it where it stops.

this is for performance driving - on the street, it's a bit extreme. but it you look at any dyno chart - such as the one in your owner's manual - you'll see that peak torque is almost always matched to the dip in specific consumption (fuel per HP). that means it's the most efficient rpm.

QUOTE

Revv limiters work...

they do, and will protect you from a stuck throttle, but they are not magic. not for nothing do racers call the 5->2 downshift the "money shift." (although i have more often experienced the 2->5 upshift...)

get a GOOD shifter and keep all your linkage bits in good shape. if you're really working at it, there is probably enough flex in the stock rubber engine and transmission mounts to make some shifts problematic, especially with tailshifters and their l-o-n-g shift rod...

QUOTE (VegasRacer @ Sep 29 2005, 11:17 PM)

The amount of damage depends on how tight the engine is to start with and the components that were used. More critical is how much you over rev it and for how long. Example: If you are racing along in 3rd gear and hit red line and then try to shift into 4th but go into 2nd instead. . . . If you let out the clutch even part way for a fraction of a second - you just cost yourself a couple of thousand dollars. Don't ask me how I know.

Yep, I did this last year and I chipped all the rings well, the compression rings anyway. I toasted the oil control rings.....

Several grand later I have it almost fixed.

QUOTE (Brando @ Sep 30 2005, 12:34 AM)

QUOTE (Hammy @ Sep 29 2005, 11:19 PM) Isn't there some way of ... preventing this... besides being extremely careful  ? Revv limiters work... Cut the spark, engine can't fire and rev higher. I think on some MSDs you can program the rpm limit to what you want.

Rev limiters only work as the motor is accelerating...

most over rev situations occur when the person is shifting and chooses the wrong gear. A rev limiter does not offer any protection for this senario.

another common over rev (but not initially fatal) situation is when on a race track and decelerating for a corner.
Frequently a person will brake, double clutch, down shift, and turn. Often they actually meant to go from 3rd to 2nd but the motor is still "engine braking" the car, and the driver is so caught up in the moment that they don't notice.

This is the PRIMARY reason to add a shift light on a race car. The shift light is really not necessary to tell you when to upshift, but it can be a learning tool to teach you when NOT to down shift.

No matter what, shifter setup is the king. If you can make the gear you are aiming for, no worry.

I have missed 1 shift (that actually gated) in 5 years, and it was a 5-4 oopsie into 2, where I let the clutch out about 2% before I yanked the stick out of second.

YiPES!

I do have a problem with missing 4th if I palm the shifter, but no gear, no foul..



M

Brant's spot on.

Autometer do a nifty shift light costs <$100, after my 1st AX i bought one as immediatly after my runs i realised i had no clue how high my revs we're going. Basically no time to check the gauges during a run.
The shift light is invaluable as an indicator to when I should change. Mind i don't always shift when it suggests, if the amount of distance to the next braking point is only a few feet. A change up & change down would waste more time. With my gearbox probably screw the entire run

rev limiters cant defy the laws of physics.... or a accidental downshift....

they stop you from over revving when you miss a shift (get on the gas before gear is selected....etc)

You guys that collided internal engine parts- was this a stock displacement motor? My throttle stuck open in a 4-5 shift and revs hit 6K i bet, but no damage that I can tell, and it happened 6 months ago. I thought that as long as the rods were not longer and the heads not shaved, there was no interference. Where are the voices of experience? Jake? Dave? Will they all (1.7, 1.8, 2.0) have catestrophic over rev events?
Inquiring minds wanna know!

QUOTE (SGB @ Sep 30 2005, 12:37 PM)

My throttle stuck open in a 4-5 shift and revs hit 6K i bet, but no damage...

6000 is only 500 over redline; probably not a cause for concern unless you somehow got some really weak valve springs. (in my case, 6000 is 1500 under redline...)

there are subtle reasone why some of the cam, induction, injection, and exhaust restrictions exist - it can save the engine from itself my limiting its freerun speed. take away all those limitations and you're on your own. however - there are rev-limiting rotors available for stock distributors and IMO they're not a bad idea...

assuming nothing is broken, it's pretty hard to make valves and pistons in stock engines collide until the springs get really tired - which should occur when the rest of the engine is really tired, which helps limit the occurrance, and helps you establish TBO :-) ...

anything you do to reduce the amount of elbow room in the combustion chamber will decrease your safety margin.

Scott,

I guess I'm lucky because I'm not in the "you guys" catagory yet...

but any engine... even stock engines can blow up.
I agree with Rich that 500rpm is still within a pretty safe zone. But try missing a 3-4 up shift and letting the clutch out in 2nd... 8000rpm will probably be outside of that safe zone on a stock engine.

So any engine can blow up on a missed shift...
it just depends upon if you let the clutch out, how quick you catch your mistake, the state of engine build, and tolerances, etc...

Just the other day I missed a shift on my stock 2L, probably revving to around 6500. I didn't notice anything right away and have had no problems since.

If I had done damage, it should be evident right away, correct?

It's possible to ever so slightly bend a valve so that you lose compression on that cylinder. But you'd still notice a rougher than normal idle and somewhat decreased power. A momentary 6500 shouldn't hurt. You really get into problems when you maintain an over redline RPM. When you're racing you can't really ease out the clutch like you do on the street, so missed shifts are much more problematic.