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Missed shifts... when is it an issues? Physics of it...

Read lots of posts on here about missed shifts....

Obviously if you bend valves, you did something bad. Need to redo the heads, rod bolts,rebuild, etc...

But what if you have no immediate damage? Leak down normal, car runs fine... then, whats the thought process... Hard to know what peak rpm was unless you have a datalogger...

Was doing some mental math for the fun of it...

If you are trying to up shift from 3-4, and hit 3-2 instead...

Say red line is 6500, and you usually shift below that....

You take your foot off the gas, RPM starts to decay immediately... By the time it slips into gear you have decayed from 6500 to 5500 or so...

Now, you pick the wrong gear...

If your cars speed stays exactly the same, you have RPM going up quite a bit. Could jump 33% or so. So, in theory, you are going from 5500 to 7500+

But, in reality, the motor immediately starts slowing the car down... So if you were going 100mph, you are decaying to 70mph or so... So you never get the full 33% RPM increase...

So, your motor doesnt spin up as much? No?

Long question made short... if you guys miss a shift yet the motor and valves seem fine... do you do a rebuild? And with a 5-2 I suspect the answer is different than a 3-2 (rather than a 3-4). 5-2 is a HUGE ratio difference. 3-2, not so much.

How do you assess the motor if there are no apparent issues? You can't reach rod bolts without a complete teardown...
Old 07-13-2017, 08:12 AM
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you will know if you did damage. no reason to rebuild just in case.
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Old 07-13-2017, 11:14 AM
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Originally Posted by OffCamber00 View Post
you will know if you did damage. no reason to rebuild just in case.
That what I figure as that makes sense... short of rod knock what does one monitor? Oil pressure?

Seems like most folks who have something like this occur just keep on trucking...

It seems like there are many factors that prevent severe overevs on a miss... if you are off the throttle, at redline you lose spark, and the motor becomes a huge brake...

To (hopefully) prevent any issues on the track, I actually artificially set my rev limiter to kick in 1000rpm below redline... So, if one is upshifting and downshifts by mistake, you have an extra 1000rpm buffer ...

Of course, you lose some peak HP.
Old 07-13-2017, 12:03 PM
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Quote:
Originally Posted by bpu699 View Post
I actually artificially set my rev limiter to kick in 1000rpm below redline... So, if one is upshifting and downshifts by mistake, you have an extra 1000rpm buffer ...

Of course, you lose some peak HP.
Jeez...why even bother putting the car on the track???
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Old 07-13-2017, 12:49 PM
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Jeez...why even bother putting the car on the track???
Driving a car with 300hp on the track rather than 350, you still learn...

All of these threads on here about people blowing motors and missing shifts makes me a wee bit paranoid...
Old 07-13-2017, 01:14 PM
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Basically, if there are no new noises and no warning lights, just figure "if it ain't broke don't fix it". I don't think there's a lot of cumulative damage that is done by a single overrev...

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Old 07-13-2017, 01:30 PM
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Which gearbox?
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Old 07-13-2017, 01:31 PM
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Setting your rev limiter lower than redline will do nothing to help a missed shift if it's a big enough miss.

My engine's built so the rockers are the weak link. When I miss a shift and hit 8400 rpm, the rockers break and that helps prevent bent valves. I think I've missed about 5 shifts in the last 10 years that did damage. In 4 cases only rocker replacement, a 60 minute fix and I usually didn't miss a session (thanks to the JWE guy's doing a quick fix). Once I missed big enough to bend a valve. I think 3 of the "misses" were due to an issue with the shifter linkage that we eventually figured out and fixed.

If you miss a shift and the car runs fine, no reason to worry. Just be thankful you got lucky and keep driving. Done it more than once.
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Old 07-13-2017, 01:44 PM
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forged to cast??

Didn't Porsche, at one point, step down from a forged rocker set up to a cast rocker, so that in the event of an over-rev or missed shift, the rocker would bend/break rather than the valve?
Hopefully preventing catastrophic valve/engine failure.
chris
Old 07-14-2017, 07:05 AM
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I've heard it referred to as a "mechanical fuse". I don't know if it was an intentional design, but it does seem to work decently for that.

--DD
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Old 07-14-2017, 07:17 AM
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I've seen "Money $hifts" dealt with in the two extremes possible and both scenarios worked out well.

1. Replaced broken rockers, changed oil and filter, then just go forward.
-or-
2. Shop guy said, "could of did something" , complete motor rebuild w/upgrades

Somewhere between is a comfort zone you can live with
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Old 07-14-2017, 08:52 AM
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Quote:
Didn't Porsche, at one point, step down from a forged rocker set up to a cast rocker, so that in the event of an over-rev or missed shift, the rocker would bend/break rather than the valve?
Hopefully preventing catastrophic valve/engine failure.
I can't speak to what Porsche has done, but Jerry Woods designed and built my engine components so the rockers would break before the valve springs would allow the valves to float and hit the piston. Cheap and easy to replace rockers. My engine is designed to rev to 8000k without a problem, it simply stops making much power at about 7200 and I have the rev limiter set to 7400. With my gearing, shifting at 7000-7200 is best.

When I miss a shift (e.g. 3-2 instead of 3-4) I usually hit ~8100 rpm. My worst was at Laguna a year or so ago when I went 2-1 instead of 2-3 and hit 8500 or so. That was a problem.

My engine also has small screens installed so a piece of the rocker can't get sucked into the internal oiling area. That happened once and meant the engine had to be removed and spun around on the stand to get the damn piece to fall out. It won't happen again.
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Old 07-14-2017, 02:43 PM
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Tom,

My engine stops making power at 6700...it's levels out until about 7100 and then it tails off after that. At least that is what it feels like as I have not it on an engine or chassis dyno.

Are you going to be at Thill in August?
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Old 07-14-2017, 07:49 PM
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Mine peaks at ~6400 at 318 hp and then at 7200 it's 280 (also 280 on the upslope at ~5600 rpm). The flapper opens up at 6700 (when it really starting to lose power) and power increases again to 7200 rpm). I see the same relative results on the engine dyno and the chassis dyno.

I hope to be at Thill. I may have to go to the UK for work, but I'm hoping to get out of it. I haven't driven the car since March and I want some practice before Road America. You should come to Miller in mid-Sept. That's a hoot, even if it is a high HP track.
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Old 07-15-2017, 11:44 AM
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Well depending on how bad it could stretch the rods, overstress the rod bolts, or damage the rod bearings, etc.. This is pretty extreme and 7500 RPM probably wouldn't do this.

Thing is, with these, or even 'slightly' bent valves the damage might not be immediately apparent and only show up after a number of cycles.

I would suggest watching and listening the engine carefully over the next few events, change the oil and look for bearing debris, etc..
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Old 07-16-2017, 08:18 AM
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[QUOTE=bpu699;9660743

If you are trying to up shift from 3-4, and hit 3-2 instead...
[/QUOTE]


Install a spring to center the shifter in the 3-4 plane. Then you have to be a gorilla to miss the shift...

I never missed a shift before, not even close, but my gearbox guy put in one the last time I had it apart. Now I miss 2nd from time to time and get 4th, but that's ok.

With the above I assume 915 box...
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Old 07-17-2017, 05:58 AM
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Well, even if you have a spring (and even a G50), **** happens. If the shifter linkage comes a bit loose, the spring means nothing. And in the middle of a very close race, it can happen, even if you are not a gorilla.
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Old 07-17-2017, 02:07 PM
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I asked which gearbox because sometimes the gear breaks before the engine on a 915. This is especially true if re-geared with a short 2nd and you make a 5-2 shift. Almost never happens on a G50.
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Old 07-17-2017, 02:50 PM
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Quote:
Originally Posted by Tom W View Post
Well, even if you have a spring (and even a G50), **** happens. If the shifter linkage comes a bit loose, the spring means nothing. And in the middle of a very close race, it can happen, even if you are not a gorilla.
You put the spring in the box, like in a G50.
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Old 07-17-2017, 10:51 PM
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Thanks for the clarification Matt. I wasn't aware that was a possibility on a 915. I've always had a G50.
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Old 07-18-2017, 10:26 AM
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