Strange noise after switching to SSI exhaust

[aldente295]

Hi all,

I'm trying to chase down a noise that I'm hearing in my '80 911SC.

The sound has only started since I switched from stock exhaust to SSI's and a Monty 2-in 1-out.

It only occurs when revving above 5K in gear (i.e. driving). It doesn't occur when you rev the engine out of gear.

The sound is a kind of metallic rattle..?, almost as if the engine is misfiring (?). It's definitely 'in time' with the engine so I'm going to rule out something physically loose causing it.

The best example I've been able to get of the noise is this video on Youtube here.

In the vid you can see the tach (as well as my frustration as I get passed by everyone on the track) and if you jump to about 1:10 (down the main straight) and also around 1:37 you can hear a sort of rhythmic noise which is the issue.

Since that time, I've done a few things to try and chase down the issue:

- Adjusted the valves and checked for broken head studs (none) and broken valve springs (none). The studs were replaced about 30K ago by the PO.

- Adjusted mixture, timing and idle

- Replaced the spark plugs, distributor cap, rotor, spark plug wires (These were all due for a change anyway)

- Replaced the original Bosch CDI unit and coil with an MSD 'Streetfire' and high vibration coil (When I described the noise to my wrench he thought it could be a faulty CDI, and since they were both 33 yrs old I figured for the money it was worth trying. At worst I now have a spare CDI in case the MSD ever dies)

The car now runs and sounds much better below 5K (pulls strong, sounds great) but I still haven't solved the actual problem!


- Has anyone experienced similar issues when swapping over to SSIs/non stock exhaust?

- Could this be some sort of pre-ignition issue? Could it be pinging I'm hearing? (I'm using 98 Octane...)

- Could this still be blow by, even though there are no broken head studs?

- What is this noise??? :-)

[budge96]

I think your chain tensioners are shot I would be careful starting the engine no less taking it
out on a track!
It is just short of jumping time be careful or valves will hit pistons wich will destroy everything you have there ...nice helmet ! Bert

[aldente295]

Thanks for the reply Bert. I've done a bit of reading about tensioner and the sound is usually described as "chain across a trash can lid" or a marble in a can. This isn't the sound I'm hearing, and it's also only at high rpm under load - no noise at other times.

Any other thoughts guys?

[mreid]

Free flowing exhaust often exposes noises that were present before, but in audible. Do you have hydraulic tensioners? If not, install them before you race again, but that's not what it sounds like to me. Is it down on power over 5000 rpm? If not, it sounds like a resonance in the exhaust. Perhaps a loose heat exchanger or muffler baffle.

[DUK]

Chain tensioners

[DaddyGlenn]

I love a good puzzle....

Couple of questions... does the power drop off like a switch when you hit 5K rpm? Or does the power gradually drop off, getting worse as you get further above 5K?

I'm thinking a fuel starvation problem that may cause a lean situation and pre-ignition under high rpm and full throttle.

I too would recommend keeping it off the track until we resolve this.

[Jesset100]

Also, make sure your rev limiter is adjusted to the correct limit.

[Ronnie's.930]

Obviously, it is difficult to diagnose sounds via the internet, but to me, it sounds like extreme detonation, and if so, I would image that you have some damage at this point (such as broken rings, etc). You really need a wideband AFR meter on there to get a better idea, but I would imagine that a leakdown test is in order at this point, as well.

[RSTarga]

I can't believe you kept driving it with that racket.
Drain the oil,look for metal, do a leak down , it sounds like rebuild time.

[aldente295]

Thanks for the responses so far.

Mark, I do have the Carrera tensioners already. And it is down on power over 5k.

Glenn, it seems to drop off gradually rather than a sudden drop. But I don't really experiment with it much, it scares me a bit! I too think it sounds like a fuel related issue. It hasn't been back on the track since and won't be til it's resolved.

Jesset100, the msd has a rev limiter dial on it, set to 6600. Behavior hasn't changed since moving from the Bosch cdi.


Ron, yeah I suspect my next step is to take it back to the original mech who did the exhaust and take him out for a test drive, then do some diagnostics.

RSTarga, believe it or not with the helmet on I couldn't hear the noise! Oils been out, no chunky bits. Also drives perfectly at lower rpm.

[blpetry]

The rhythmic nature of the sounds leads me to believe that it is tied to one cylinder. 5000 RPM / 60 seconds / 6 cylinders / 2 strokes per spark = 6.9 "bangs" per second per cylinder. Listening to your video and tapping it out, sure sounds like it happens every time the plug fires on one of the cylinders. That said, since the new item in the equation is the SSI, I'd get under the car with a flashlight and look for leakage where the new exhaust mounts to the heads. Hopefully that's it, because the next things to check would be a leak at the head/cylinder interface or a maybe a leaking valve or who knows.

[aldente295]

Interesting... I like the theory. Actually I hate it because of what it may represent, but you know what I mean! I'll check the exhaust seals but am not hopeful. Would a leakdown test confirm your theory do you think?

[DaddyGlenn]

Originally Posted by aldente295

Interesting... I like the theory. Actually I hate it because of what it may represent, but you know what I mean! I'll check the exhaust seals but am not hopeful. Would a leakdown test confirm your theory do you think?

A leakdown will tell you the condition of the seal of the rings and valves and head to cylinder seal.

[DaddyGlenn]

Just remembered an easy test you could do.

Get it warmed up and run it to where you hear the sound. Then while you are still hearing the sound, simultaneously press the clutch, lift off the gas pedal and turn the ignition off. Take it out of gear and slow to a stop with the engine off. Pull the spark plugs and take a look at them.

This should give you and indication of the fuel mixture when the sound is happening.

[gsxrken]

Really puzzling and I'm curious to learn what it is. Chains would probably make the noise just by revving in the driveway. They don't know the car is under load.
Such a noise would seem to be even easier to hear with a stock exhaust, which kind of rules out the SSI change. And while some exhaust leaks can sound like hells bells, they don't just leak over 5k RPM.
Is it in any gear or just 4/5? I'd lean toward a fueling issue too. Compression test, AFR O2, and good luck

[Sway Bar]

Have you ruled out the exhaust? Try your old one if it fits?

Reason I suggest it is that I recently tried from a friend a very free flowing M&K on my MFI...sounded awesome, revved great with no issues but under power at 4200rpm I thought I spun something. Worst mechanical resonance I've ever heard that sounds very similar to yours. Fortunately he had another more tame M&K that we quickly swapped out and no issues. (he's gonna have a hard time getting it back)

Anyway, was a quick check without playing with mixtures etc etc.

[aldente295]

An update.

Had my wrench perform leakdown and compression tests; 5% and 180 on all cylinders (ie no problems found).

He's scratching his head at the moment as well. He's ruled out chain tensioners, broken rings, blowby, bent valves, etc etc. As far as he's concerned the engine 'should' pull with great power, but doesn't....

Next step I suppose is to put it on the dyno and get some mixture readings under load. Unfortunately the only place I can get this done is only open Monday to Friday and is out of town. It's also the place that I took it to for a test drive/listen, and the head mechanic there declared it was blowby, and I'd need a full engine rebuild that would be upwards of $20K. Oh, and remarkably he just happened to have a replacement engine available, under the bench, ready to go for a mere 9... ahem.

With Christmas coming this may need to wait til the new year. But I'm pretty relieved with the leakdown numbers. Will try and do some more analysis and will post back with anything new I come up with.

[willcall]

I'm certainly no expert but if you are having power loss it could be the O2 sensor. If they read incorrect levels of mixture they can retard the power and no amount of reving will bring the engine to life, so to speak. May mean the O2 sensor needs to be moved further into the exhaust flow or further away by using a spacer bolt.

[mreid]

Can you bypass or swap out the MSD? There are multiple examples on several other forums of MSD boxes randomly dropping cylinders at higher rpms. In fact, on ffcars.com many wont even use the MSD box any more. It seems to be worse in the boxes that have the rpm limiter function.

[Horrido]

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