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Location: idaho springs, co
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rpm vs. valve damage

I got a little rambunscious with my right foot today and got my car up to about 6500rpm. Is there a rule of thumb about when damage occurs. I have stock valve springs and a mild Scat cam.

I have no rev limiter and a 10.5lb flywheel, so it is pretty easy to zip past the redline.

thanks all
Old 03-26-2002, 04:47 PM
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There is no hard and fast rule. Running at high RPMs will probably cause wear to happen a bit faster regardless of anything else. But that's just normal-type wear, nothing to really worry about.

Bad Things happening would generally be on the order of the rod bolts failing (lots of noise, no power, throw the whole motor away and start over), or the pistons "kissing" the valves (varies from no real damage to bent valves to valve heads imbedded in the top of the piston) to the flywheel bolts failing (throw the crank away and start over). And there are other possible failure modes as well.

If your motor isn't making Ugly Noises now, it should be fine. But I wouldn't zip it up to 6500 RPM very often myself! With a stock valve train and rods and rod bolts and so on, I'd stick with the stock red-line. The aggressive cam will make power further up, but it increases the chances of the valves "floating" due to the steeper ramps on the cam lobes.

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Old 03-27-2002, 07:46 AM
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that is a big whew..... It still runs great.

Can the rev limiter rotor from a stock FI dizzy be used on a 050?

That is the thing, I really dig the power from 4k ->, I think that the only thing that is going to keep me safe in autoxs, is a rev limiter.
Old 03-27-2002, 08:12 AM
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You can install a rev-limiter onto a 050..... I ran one for years. The necessary part was from a 911 dizzy. Sorry I sold it about 3 years ago and no longer have the reference. I honestly don't know what a stock FI does for a revlimiter and don't know if it is similiar or different.....

my set up involved a part that would slide upward on the dizzy shaft through CF, and interupt the signal at certain RPM.... I believe this was a stock 911 dizzy part, utilized in the manner that the factory did on that car. You could effectively bend the springs and change the redline cutoff, by effecting the resistence.
hope this helps.
brant
Old 03-27-2002, 08:30 AM
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There are rev limiting distributor rotors you can get.

Not sure who carries them these days though. Mebbe Pelican? I think Ronnie at www.type2parts.com used to have them as well, but that was a while back.

Looking for a few myself so keep us posted when you find them! Granted for a VW bus application so limit at 4500RPM
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Old 03-27-2002, 11:31 AM
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The stock dizzy will accept a rev limiter rotor which Pelican sells. I installed one on my car because I kept reving it up to over 6500. I do have a CW crank and the motor goes up there pretty easy.
It is nice when autoxing to have it limit at 5800 rpms. Thats when you know you better shift or back off to keep in the power band. Plus then you don't have to keep one eye on the tach either.
Geoff
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Old 03-27-2002, 11:36 AM
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Yeah, I will probably get one. My motor flies by the redline with no problem. I like to drive it high in the rpm range b/c of the snappy power it has up there so the sound of 5000 rpm sounds just dandy to me as I am used to it. I did notice a change in sound when I got up to 6500 though. It sounded unhappy.

I will try to throw the rotor from the FI dizzy on there too, just to see if it works. I imagine that if it runs at all, it will work.
Old 03-27-2002, 02:28 PM
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We install a MB valve spring that is alot better than something that was designed over 30 years ago. This lets the motor rev a little higher that redline, if you have to, without the valves hitting the top of the piston. You have got to understand that the rev limiters only take care of the electrical portion of the rev limit. You can still rev higher than the rev limiter mechanically. If you check out the incar footage of the Burn Victim on my web site(March 23/24/2002)you will see that I made a second to first shift while the chassis was loaded and moved the shift gate just a little to make that mistake. The motor probably went to ten grand, but because we use a special spring, I was able to finish the race. I should of won that race, but I had to start from the back of the pack because I failed to quailify.(bad fuel pump) Started last, finnished third overall. Chech out the video, it is a half hour long, and you can see all my near misses and near crashes and me blowing off a bunch of 911s, 944 turbos and a bunch of my little cousins(914/4s). OTTO
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Old 03-28-2002, 07:39 AM
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There are a variety of rev-limiting rotors available from Bosch. The springs in the rotor have different rates, so they limit to different RPMs.

Otto's point about the limiter not helping on a missed shift is very much worth remembering. Nothing can help limit the RPMs when you accidentally downshift when you meant to upshift. (It happens in racing more than you'd think!) So the best you can do is try to make sure you're getting into the gear you think you are.

--DD
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Old 03-28-2002, 07:51 AM
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I would have to agree that improving the shifting in my car has been one of the most important upgrades for autoxing.
I finally went and installed the bronze shift bushing and shifting is a breeze.
Last weekend, I was competing against several other 914's in GTU class and I had the same times. They all had R tires, more hp than me but they were complaining that the course had too much shifting in it. I had no problems at all but I did shift alot (2nd to 3rd then back again in several places). The rev limiter helped too because I could feel when to shift to 3rd instead of riding 2nd.
So Otto, what are these MB valve springs?? I assume they are tighter sprung than stock? How much more? What do you do for the pushrods?
Geoff
Old 03-28-2002, 09:21 AM
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Same price as stock, just better overall structure and metal using 21st. century technology, and you don't have to change any pushrods. Leave everything stock, just when we do heads the spring height is set-up. OTTO
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Old 03-28-2002, 10:39 AM
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I take engines to 7K on the dyno all the time, even with tight decks...

Stock pushrods are normally insufficiently lengthed for performance camshafts, we seldom use them.
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Old 03-28-2002, 07:18 PM
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