Strange rattle from 930 under full boost?

[Emission]

1986 930 - 120,000 miles

Under full boost (K-27 - 1 bar) as the engine pulls to redline, there is a strange 'rattle-like' noise coming from the engine. Could it be the engine 'knocking' (I am burning Chevron 91 octane)? Is this normal?

The noise is only apparent when the car exceeds approx .7 bar of boost (on it's way to 1 bar).

The engine runs fine, idles perfectly, and is smooth. Nothing seems out of the ordinary other than the noise.

[1bar]

This sounds like pre-ignition. VERY bad if so - you can destroy your top end, or burn through your pistons very quickly if you're experiencing this problem. Bruce Anderson's book describes the problem. The cure is to dial down the boost, reduce the charge air temperature (bigger intercooler), increase the fuel delivered (lower air/fuel ration, or run it richer), and / or run higher octane fuel.

Is there any way you can pump a gallon or two of race fuel per tankful? Can you at least find 92 octane fuel from one of your local gas stations?

Don't ignore it. Find someone who really knows what they're listening to, or at least someone who has an air/fuel meter that you can check out your mixture at high boost with. This usually means a dynamometer setup. It's not cheap, but you won't like paying for a new engine.

[Emission]

There is a local 'Unocal 76' that sells racing fuel (100+ octane) for $5.00 a gallon or so. I figure if I put $25.00 bucks in (5 gallons) with half a tank of 91, I will have a tank of 95+ octane or so. I will try it tomorrow and see if the sound goes away.

I have only had the car two months. How do I dial-down the boost? Is that a new chip?

[1bar]

I'm sure there are others that have more experience than I do with a modified 930 like yours. The boost is controlled by a spring and diaphragm in the wastegate. It opens a valve and lets exhaust bypass the turbine when the intake rail pressure reaches a point determined by the boost spring. 0.7bar was the stock spring setpoint (at least for my car). A 1bar spring allows much higher boost. This is simple vacuum-controlled mechanics, no electronics involved (yet).

If pre-ignition is what's happening, you should replace the 1bar spring with a 0.7bar or a 0.5bar spring, and add an electronic boost controller. You get an accurate boost gauge for your trouble, and you can set the boost anywhere from the spring setting (even a fancy electronic contoller can't go lower than that) up to 1 or 1.2bar.

In the winter, when it's cold out and raining (poor mans water injection ) you can probably get away with 1bar or slightly higher without problems. In the summer at high ambient temperatures and especially after the motor and intake comes up to temperature ("heat soak" as it is called), you need to keep the boost to a lower level.

Bruce Anderson's book also has a section on mixing gasoline to adjust the octane level. It might be helpful - it is for me!

Good luck!

[Emission]

I have Bruce's book in front of me. What page(s)?

I have noticed that the noise is more likely to occur once the car has been 'heat soaked' (the return trip from the store, for example). Today, it was hot (90 or so), and I noticed it even more.

I don't NEED the full bar, so I will look into the .7 bar spring. In the winter, I will just swap the other spring out!

[CamB]

Steve Weiner at REnnsport also has a table on his website:

http://www.rennsportsystems.com/~porsche/2a.html

Cam

[scphelps1]

I am getting this same 'rattle' you describe at what seems to be the same time; while boost is building to 1 bar. I never thought of it as being detonation. It sounds like it is coming from the left bottom rear, like it is actually coming from the turbo/wastegate area. I still have the original turbo, and thought this might be the issue. I have been thinking of going to the K27-7200 and this 'rattle' was starting to help me jump off the fence on that decision.

[cstreit]

91 Octane is pretty low for 1-bar... I would think about at least 93...

[Emission]

Unfortunately, in Southern California, the refineries are only making 91 these days.

Last year, the last of the major stations stopped selling 92 octane. At the time I didn't care. Now, with the Turbo, the issue frustrates me.

I think a reduction in boost is the only safe approach.

[JohnC]

Don't mean to spoil the party here but you 930 guys hearing "rattling" should check to see if you're head studs are all in one piece.

Also, hows the cylinder sealing at the junction between the head and cylinder? any oily leakage?

[john walker's workshop]

i put an adjustable boost on my stock 87 930, and at 1 bar, there was preignition, so i had to turn it down to .8 where it wouldn't ping. it's more a distinct change in the exhaust tone, than a rattle. like from a throaty roar that changes to a higher pitched raspy sound. usually the first thing to go is the top compression ring. they can break into many little pieces. make sure the ignition timing is not too far advanced also.

[930fan]

Originally posted by 1bar
In the winter, when it's cold out and raining (poor mans water injection ) you can probably get away with 1bar or slightly higher without problems

Actually, cold weather means that air becomes more dense and packed with oxygen. This means that you have to richen your mixtures to prevent a lean condition from causing detonation. If you have an accurate boost gauge, you will notice that with the same w/g spring, you will produce more boost when it's cold than when it's hot.

In my experience, running 1 bar is risky without fueling mods. What JWW said has happened to me

[MoreGAS]

Gents- that rattling at full boost is destructive causing your rings to break, ring lands to break, and at extreme driver insensativity- breaking or holing a piston. The only safe remedy(and practical) is to put in a .8 bAR spring, or add a supplemental fuel injector driver to (however efficiently) add more fuel at full boost conditions say above 4500 RPM as you get toward peak tqe then past to peak HP. I have a wide band a/f ratio meter we use to test with on the dyno or road tests, and with no fuel enrichment, 1 BAR on pump gas is ragged edge lean when everything is just right! Add in a poor condition fuel pump, or poor fuel, or a near the lean edge of spec. warm-up regulator(whichh also controls the mixture under boost) and the engine is going too have a major failure.
I have rebuilt quite a few tbo motors that were suffering from lack of fuel and it is expensive to replace Ps and C's, so remedy this ASAP.

Regards,
Kevin Roush
GAS Motorsport
Performance Porsche
Upland, Ca
909 985 2909

[Emission]

OK, I went to Unocal and dropped 5 gallons of RACING 100 OCTANE ($4.259 a gallon!) into my tank on top of 1/2 tank of 91 Octane. I drove 10 minutes to get the new 'mixture' into the system... then floored it in third up a long hill....and....

Perfect. No ping - just awesome acceleration!

Darn. This is good and bad.

Good - I know what the problem was.

Bad - To solve it I will now be dropping my boost back to normal levels again (.7 bar or so). There goes 30+ horsepower. Hopefully, there is no permanent engine damage.

BTW, the engine runs soooooo sweet with the high octane diet. It is too bad it is so darn expensive!

[scphelps1]

I thought the earlier 930's provided enough fuel without enrichment to supply up to around 400 hp or so. The only thing that has been done to mine is the 1 bar spring. It already had the euro headers.

Funny you mentioned the WUR if it is set toward lean. I am having cold start trouble and I am thinking this is the culprit. I have to pull some things off tonight to see if I can figure out what is wrong. I have symptoms similar to others that have posted recently. I have to give it gas or else the idle is so low it will stumble and stall. Once it is warm, it starts, idles and runs fine.

I also have to add a CO tester to my list of things to get. What do you set the CO to on a 930? Do you set it once the engine is warm? What about adjusting the WUR?

Thanks,
Clayton