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

dtfastbear · 10-21-2003, 04:43 AM

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

jluetjen · jluetjen's Garage

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...

Glenn from Denv · 10-21-2003, 06:16 AM

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.

Porsche_monkey · 10-21-2003, 07:00 AM

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.

911pcars · 10-21-2003, 12:21 PM

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

dtw · 10-21-2003, 12:51 PM

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

Wayne 962 · Wayne 962's Garage

Quote:

Originally posted by Glenn from Denv
You may have broken rockers (the best news)

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

-Wayne

dtfastbear · 10-22-2003, 08:09 AM

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

Porsche_monkey · 10-24-2003, 05:21 AM

And the result was/is....?

dtfastbear · 10-24-2003, 06:41 AM

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

dtfastbear · 10-30-2003, 07:11 AM

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

ChrisBennet · ChrisBennet's Garage

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.

I'm convinced that my AASCO valve springs have paid for themselves.
-Chris

Porsche_monkey · 10-30-2003, 07:31 AM

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.

dtfastbear · 10-30-2003, 07:40 AM

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

rcwaldo · 10-30-2003, 07:44 AM

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

Porsche_monkey · 10-30-2003, 07:55 AM

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

ChrisBennet · ChrisBennet's Garage

Quote:

Originally posted by dtfastbear
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

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

Glenn from Denv · 10-30-2003, 10:50 AM

"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!

ChrisBennet · ChrisBennet's Garage

Quote:

Originally posted by Glenn from Denv
"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.

Ho Hum 74 · Ho Hum 74's Garage

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.

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