Missed shift - how bad is the damage likely to be?
 

My wife accidentaly caught 2nd instead of 4th on an intended upshift from 3rd gear exiting turn 9 at Thunderhill this past weekend. The resulting over rev did some serious damage, it seems. The car wouldn't really start after she limped it into the pits. We pushed it onto the trailer and dropped it at our mechanic's shop.

What are the best and worst scenarios for such an incident? I'm assuming most of the valve train is toast, but what else can be damaged? The pistons? Can valve to piston contact mess with the rods or the bottom end?

I'm sure our mechanic will give us the full prognosis in a couple days when he tears it down, but I'm curious as to what we're in for...

Thanks,

Dean

You're going to need to take it apart no matter what, so my prediction is not going to mean a whole lot at the end of the day.

You just need to keep taking things apart until you don't find anything more damaged. At a minimum your looking at a top end (heads, P&C) for one cylinder. But I'd also have that rod checked for straightness and crack tested, so that will need to come out too. The other thing is to pull the sump and check carefully for any debris there. If there is, you'll need to pull the whole bottom end apart to get that cleaned out as well as the oil system.

But you won't know until you start taking things apart. I'd budget for a full rebuild and then be pleasently surprised if it doesn't need to be done.

Good Luck...

As with anything...it depends!

You may have broken rockers (the best news) or may need to get at the bottom end (the worst news)...you won't really know until it's opened up.

My guess, (based on last months similar experience) some new valves at a minimum, and a whole bunch of stuff that 'should' be done when you rebuild the top end. If you ahve case leaks or any bearing wear now would be the timne to address all that also...

I hope I am wrong, but this will probably be rather expensive. On the other hand, if your car is otherwise in good shape, you will be ready for the 100,000+ miles. :)

Buy Waynes book ASAP, it will pay for itself immediately and give you some confidence in deciding what to have done to your engine.

This shift has been done before by many others. You're not alone. Hopefully, the damage will be restricted to the top end (rockers, valves), but pistons are a distinct possibility.

Chris Streit managed to document his mis-shift experience.

Sherwood
http://members.rennlist.org/911pcars

[ForestGump]Mechanical overrevs are like a box of chocolates; you never know what you're gonna get...[/ForestGump]

Wishful thinking, but not likely with the shape of the 3.2 Motronic pistons...

-Wayne

Thanks for the input guys, although it is not all good news :(

The shop is tearing down the engine today, so I should have more info. Good idea RE Wayne's book - it pays to be informed!

Dean

And the result was/is....?

They pulled the valve covers and... bad news, no broken rockers. So, I guess that means the valves are probably pretty badly damaged.

We won't have the engine completely apart until mon/tues, so I still don't know the full extent of the damage. It doesn't look good, though.

I'll update the thread as soon as the engine is apart...

Dean

I went to see my poor engine last night, and the damage was not as bad as I thought it might be...

All six exhaust valves are bent to some degree. All six, of course, made contact with the pistons. The pistons do not seem scarred badly at all. My mechanic was pretty certain that we could reuse the pistons and cylinders.

So, the heads are being sent off for new valve guides, and I'll obviously replace all the valves. The head studs, too, are going to be replaced.

My mechanic thinks it would be a good idea for us to "do the rods" because a) the pistons DID take a shot from the exhaust valves and b) the engine has 174k miles on it.

We use the car mostly for track work, but that is likely to only be the case for another year, then it'll go back to a street car once we have a proper race car. Everything is being rebuilt bone stock.

Thoughts? Is it worthwhile to take the rods out and put in new rod bearings?

Thanks,

Dean

If you think you can avoid having another missed shift, go with stock valve springs. If you're like me, missed shifts at the track with a 915 transmission are going to happen. A set of "racing" valve springs will reduce the chance of valve float. I've missed quite a few shifts with my motor. :rolleyes: I'm convinced that my AASCO valve springs have paid for themselves.
-Chris

Inspect and measure and them. My guess, based on my own recent experien$e, and the relative strengths of rods vs valves, would be that the rods are still A1. Was there a prize for the closest prediction? If so I could email my shipping address. :)

Thanks, guys. Is there any downside to installing racing valve springs other than having to buy them? I mean, do stiffer springs here sap off any appreciable horsepower or anything like that?

PBH - I'd be happy to send you the "most bent" valve to add to your collection. Oh wait, it's Chris who has the collection given his tendancy to miss shifts... :)

Dean

Since the rod bearing are easy enough to replace, I would do it. Rod bearings are pretty cheap, especially since the engine is apart..

Chris

No shortage of bent valves in my shop, thanks for the offer though. SmileWavy

Surprisingly, I don't think I've never bent a valve from missing a shift. One of the downsides of having a smooth shifting 915 is that you can go into just about any gear at any speed...

Stiffer valve springs are going to cause a little more friction and valve train wear.
-Chris

"Surprising" is right! You should be buying lottery tickets for every draw. I've missed 2 shifts (going for 4th and hit 2nd) and creamed valves each time!

My valve springs have saved my valves in some cases I'm sure. I've been able to push the clutch in pretty quick other times.
-Chris
P.S. I've never lost on the lottery. :D

Here's how I shift from 3rd to 4th and I've never miss-shifted. But then there are those that say there are two types of pilots: Ones that have done a wheels up and those that are about to! Touch wood. <pats head>

When doing this shift a racing instructor told me to rotate your wrist 180 degrees so that your thumb is pointing downward and your palm is facing the passenger door. Then it is natural to pull away from your body because all the pressure is on your palm which is facing the passenger door and the rear seat. Very difficult to reach 2nd from that position. Try it 60 times a day for 7 days and you'll never shift any other way because you'll develop the muscle memory to do it automatically.

Happy shifting!

Tristan

Tristan,

Thanks for the tip! I tried your method on the way to work today and it works great. Like you said - make it second nature then you never have to think about it again.

I'm anxious to try it on the 911 since the effort for shifting in that car is much higher. I'm coaching my wife on using this technique now, too.

Thanks again,

Dean

The grip postion described seems a little awkward; rotating the wrist 90º CC everytime you upshift. I think one comfortable hand position on the knob for all gears is most efficient.

Although repetition helps the process of shifting, you still have to pay attention (however little it takes). I think it's better to have a mechanism where the shifting motion takes you to the desired gear without a conscious effort to get it right. When you're trying to drive quickly, there are many things to think about; where you are, where you're going, where others are, how fast you're going, when to accelerate or not, when to brake or not, observe engine rpm, etc. And there are mechanical operations to perform; modulate brakes, steering input, heel-and-toe, match revs and throttle input, etc. Maybe my mind doesn't work that fast and probably why I'm not that quick.

That's why I developed this mechanism.

http://www.seinesystems.com/Images/GateShftBeauty-4.jpg

It allows the driver to concentrate less on shifting technique and more on other important aspects of vehicle control. F1 drivers have the same things to think about and perform, but wheel-to-wheel racing at 200 mph, decisions must be made in split seconds while still driving at 10-10ths. Thus the use of paddle shifters to allow them to concentrate more on driving than shifting and matching revs.

Sherwood
www.seinesystems.com
http://members.rennlist.org/911pcars

Of course you have a vested interest! Belive me there is no 'awkwardness' in the rotation because as you reach the top of third you rotate 45 degrees left and then as you pull for 4th the hand naturally slips down on the left of the shifter.

Here's what's even better about this method: now that your hand is in this 'awkard' position all you do is push away from your body with your palm, towards the right hand top corner for 5th. Now what is simpler and more confident than that? You absolutely know you're going to get 5th instead of 3rd.

Tristan

Get the good valve springs. If you have a little extra wear, or hp loss, so what!! Its a whole lot cheeper than new valves and pistons. If you are a good driver you will not miss the hp or additional wear.

PS
The shift gate mentioned is an excellent idea. Cheep insurance.

"Here's what's even better about this method: now that your hand is in this 'awkard' position all you do is push away from your body with your palm, towards the right hand top corner for 5th. Now what is simpler and more confident than that? You absolutely know you're going to get 5th instead of 3rd."

Tristan,
Upshifting from 2nd to 5th is usually not the problem nor are there dire consequences as a result. It's coming down from 5th to 4th where one can select 2nd instead. At speed, that's around 8500 rpm you're asking the engine to do. Several guys here have done this and experienced the damage that followed, and they're all good drivers too. It's not a matter of knowing how to shift the 915; it's trying to concentrate on several things at the same time with an inherently unforgiving shift setup. Mental errors are inevitable no matter the level of competition. The goal is to minimize the possibility as much as possible.

It seems you have this under control; others might find this mechanism helpful. Sorry for the commercial push, but I think it's cheap insurance.

Sherwood

OK Sherwood, I'll bite. How much?

I think you misread Tristan's last post. He advocated this rotated hand method to go from 3rd to 4th to take away any muscular tendancy to pull towards 2nd. This is the shift that cost us a top-end.

Then, in his second post, he said that it is also appropriate for the 4th to 5th upshift so that you don't accidentally get 3rd - this is also a "risky" upshift that could have the same consequences. No one ever said anything about 2nd to 5th...

That being said, I don't disagree with you, Sherwood, that mechanical assistance and safeguards are also useful. However, what Tristan proposed already feels natural to me after only doing it for a day in my commuter car. And, it makes perfect sense that it turns your natural muscle movements in the right direction for the next gear.

I'll take all the safeguards I can get - mechanical and "natural".

Dean

PBH,
Wayne carries them. Go to the following link, then click on the words, "Gate Shift" in the left column.

http://www.pelicanparts.com/catalog/shopcart/911M/por_911M_pedals_main.htm

Pelican's description doesn't mention it, but the Gate Shift kit also works with 901-5 speed gearboxes using a 915 shift housing.

Sherwood
product info