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Fuels - Pinging*

FROM: Darren Fritzsch.
Glenn (and anyone else with pre '86 models) are you running super and are you getting any pinging - mine pings around 5500 (so I back off). Have you ever tried avgas?

*NB: These comments were first published back in the year 2000 - Webmaster.

Jim Snee - 15th November 2000

Sharksters, I was intrigued by the discussion re;- Ping(k)ing (no, not the capital of China), so I e-mailed the Shell technical people asking what sort of regime we should be using on our pre-unleaded motors. I got a 'phone call from one of their Techos the following day which i thought was pretty prompt, the gist of which was as follows;

These motors run at high cylinder pressures due to high compression ratios and high volumetric efficiency (good breathing). This demands a high octane rated fuel to resist pre-ignition. He told me that the octane rating between different models varies a bit, but "typically"(78-82 928 & 928S) was specified as min 95RON. In this regard PULP(95), and Optimax(98), are OK, but Super is now typically 91-93 Octane.

The problem with Optimax and PULP is that they do not protect the valve seat, and he quoted a Porsche manual which recommended one tank in three to be a leaded fuel. (I seem to recall someone quoted that in the discussion) to prevent what he called "valve-seat-recession" which he described to me as a high-temperature micro-seizure (like scuffing) wear mechanism between the valve disc and seat.

A solution to this would be the addition of an alternative compound to protect the valve seat. He was very wary of suggesting the use of "upper-cylinder-lubricants"as he says most of these consist of high molecular weight (thick) petroleum products (oils) which do nothing for the valve seat. He did recommend a product called "Valvemaster"which is available at Shell service stations, although it is not a Shell product. Apparently this uses a Phosphorus compound to do the job. The "lead-free-super" that Shell market apparently uses Calcium compounds to do this job, but does not have a high enough RON.

One other thing that he told me was that the lead also acted as a catalyst in the combustion process, and the result of using unleaded fuels is an increased likelihood of carbon build up in the combustion spaces. Apparently Optimax is designed to combat this by having naturally detergent properties which improve the combustion process and reduce build up in the cylinder head. To help defeat carbon build up he also suggested that we take a good look at the plugs we use, and consider using a higher temperature plug, or a Platinum tip plug, particularly those of us who do a lot of City driving. He also suggested it would be good for the engines health to take them for a good high speed run at least once a week to get the engine up to its designed operating temperatures and burn off any build up.

The prescription seems to be...

  1. Use Optimax fuel to get good Octane rating and prevent pre-ignition.
  2. Use valvemaster or something similar to prevent valve seat recession.
  3. Take the baby for a good fang once a week to prevent carbon build up (woo-hoo!)

I wonder if '3' would be a valid defence against the revenue-raisers?

He gave me a number to call for technical queries and said "any time" 1800 805 000

Jim Snee

Glenn Evans

If it's not pinging more than that (which would indicate over-advanced ignition timing; I don't expect that YOUR car would suffer from coked up heads!!), check that your full throttle enrichment line to the control pressure regulator has vacuum. Mine doesn't and the mixture obviously has been leaned to compensate (I have fiddled with the mixture myself).

Recently it has been detonating at full throttle (at middle to high rpm) when it is above normal operating temperature (like when I take off after clearing a traffic jam). I discovered that there was no vacuum only a couple of weeks ago. I haven't had time to get my hands on it yet. The control pressure regulator is the thingy in front of and below the oil filler which has two solid fuel lines, a vacuum line (perhaps was rubber once; now feels like as hard as a rock!!) and an electrical wire (actually a pair inside the outer insulation) connected to it. The vacuum line pushes on from above, then curves over sharply to go down the side of the regulator and connect near the throttle body on the right hand side.

To get access under the inlet manifold, Len observed Bob Whyms remove the inlet plenum chamber ("spider body") by:

  1. Removing the air cleaner (remove inlet ducts (spider legs!), top of air cleaner, filter element, hose (push fit) on side of air cleaner, hose (screw clamp) underneath right hand air cleaner inlet, two bolts (13mm) inside air cleaner AND the (bolt with locknut) at the rear of the plenum chamber (this presses down on the front section of the air cleaner to keep it sealed against the sensor plate housing);
  2. Loosening and sliding back the eight rubber sleeves which join the plenum chamber to the intake runners; and 
  3. Loosening the two big clamps around the big rubber sleeve underneath the plenum chamber, which connects it to the throttle body.

Len was kind enough to divulge this and saved me from removing a couple of inlet runners instead. Thanks again, Len. I'm champing at the bit to get this done, and change all the rubber lines while I'm in there! BTW, Bob Whyms says that it is quite normal to have oil in your plenum chamber (extremely glad Len remembered to tell me that, before I had a heart attack!).

Glenn Evans, '80 928 Petrolblaumetallic 'S' look, 5 speed

Stuart Greaves

Hi Charles, (Falzon) Long time no chat.

Very interested in your comments about 'stale' PULP. I'm sure my 16V motor is less sensitive and susceptible, although there's a characteristic higher frequency 'whizzing' sound under WOT beyond 4k rpm, which sounds more like a heat shield rattle or possibly even an induction noise. I've never detected that awful 'ball bearings in the heads rattle' in my 928, even with ordinary unleaded (I don't make a habit of that!).

To protect against the odd "bad batch" of fuel I carry a bottle of Super 104+ octane booster (and spark plug killer). Solves all pinging problems for me instantly. Kills sparks within a half dozen >tanks if used constantly. ...

I've heard that the 104+ is so good it can even stop pinging in the car next to yours at the lights if he sits beside you long enough. (Unless of course it's a WRX-STI in which case Subaru say you drive it gently or you're stuffed, basically). Do you reckon that's true?

Regards, Stuart Greaves

Eddy Sain

Hi Guys,
For what it's worth I'll share what I have found out about fuels ( knowledgeable friends in the oil and gas industry who found out for me and from talking to the "cocktail mixers" when I have been on sites) and what I know ( from observations).

Firstly don't be too sure of the ROM or Octane rating that you think you are getting as it can vary from batch to batch and the octane rating has been steadily decreasing on all all fuels over the years. Considerations for what fuel to use in my mind is to use the fuel with the highest ( hopefully) octane rating our vehicles have a variety of compression ratios from 10:1 (9:1 in the US ) for 81 & 82 models to 10.4:1 ( (9.4:1 US) in the 80 to 85 S's and the S4 varied between 10:1 and finally 10.4:1 for the GTS so all the base parameters are not equal. What may work in your car may not work in another due to different compression timing settings vacuum etc. the comparison is an excellent starting point but consider some of the variables before you really get worried.

In my 85S I always used leaded fuel and never had any pinging problems except for the odd batch of stale fuel in the country. One of the reasons I bought the 85 was that it was the last of the leaded engines as I am still a believer that leaded fuel ( real leaded fuel not what we get now) is good for an engine but not the environment. I am lead to believe ( pardon the pun ) that some brands of leaded fuels no longer use any lead but can not confirm this.

In my 91 S4 I use ultra but I will hunt out and try the Optimax as if I use normal unleaded the vehicle performance is down considerably and it does knock (had to use it when there was no premium in the country never again ).

A trick which I have been told and it does work is to mix leaded with unleaded premium as this increases the octane rating (something to do with the chemistry) I use this in my truck ( 11.1 to compression 460 cub V8) if i can't get Avgas. It is mixed approx. 50% to 50% and does work better than each fuel individually, but not as good as Avgas. Remember though if you are putting real leaded fuel in a car equipped with catalytic converters you will poison the converters.
Avgas I find the best but a bit dear if you haven't got a good source and you don't don't need to run a full amount in the truck anything over 30% is good and you can't feel or see a difference to a complete fill (I'll check this when I get the dyno in).

A word of caution I wonder what the Avgas problems currently happening to aircraft does to cars? You can mix your own additives to increase octane and my chemical , process, and operations engineer mates have given me some custom graphs to blend Toluene, Xylene etc. into the fuel and it does work but sometimes hard to get and if you use too much it can "melt" plastic lines and fittings.

I also tried some special NASCAR fuel a couple of times 130 octane high lead it was great but I can't get it any more and WOW could I advance the timing. All in all try different brands of fuel as some do vary ( a lot come from the same tank) always use service stations with high volume so you don't get stale fuel, fill after they have filled so you don't get muck and water in the fuel from the dregs (hard to know but it has happened to me) and try the 50:50 in your unleaded engines it works.

On coke deposits a very old trick ( not sure how to do it on the 928 past the hot wire Mass air meter) is to slowly pour water into the intake ( done it lots of time in a carb engine) while running a few revs and this will clean out the carbon deposits ( ever seen a cylinder with a water leak from a head gasket always clean) this trick has saved a lot of work for friends who had pre ignition or worst a bit of carbon on a valve face an old trick but it works. I do it but try it at your own peril and if you are concerned DON'T.

Regards, Eddy Sain

Glenn, my curiosity got the better of me so I went out (into the sun, and returned a little crispier than when I started) and had a look at this pressure regulator thingo.

It is exactly where you described, so my compliments on an accurate description, perhaps a sideline in authoring repair manuals would be a profitable experience?

The rubber is not too bad, but has been repaired by (someone else) cutting away the offending pieces and slipping a smaller section of hosing inside the remaining hose. I really have my doubts about the intelligence of some people who possess the inclination to pick up a screwdriver. Given that dodgy brothers have already had a go, I figure that I will have to look at the other side (the vacuum source side) to see what awful things are contained at that end. No doubt tee pieces from an aquarium set up and lots of silicone ? Needless to say, I will re-run this line as a definite priority... ...later on.

After all that I haven't checked the vacuum yet, I just wanted to ensure I could find the thing. Regards, Darren Fritzsch

Glenn Evans

Darren, I simply pulled the vacuum line off mine when the engine was running (after making sure that there was no dirt around it to be sucked in). If there is vacuum (and your control pressure regulator is working properly, you will only enrich the mixture (not a problem). You should be able to feel the hose sucking if you put your finger over its end. I then attached a vacuum gauge and revved the engine, and got absolutely no response from the gauge!!

Fortunately, mine doesn't ping in normal driving, so attacking the hoses can wait until the weekend.

Glenn Evans, '80 928 Petrolblaumetallic 'S' look 5 speed

Darren & other leaded users

I live in Sydney and use Shell Premium My 928 performs well, plenty of revs and no pinging All pre 86 928's need 98 RON Shell PULP while less than 98 RON is higher by half a point than other premium brands (will use Optimax when available) Previously the lead in the super was needed to lubricate the valve seats and raise the octane rating to suit high compression engines in other cars.

As the 928 has alloy heads with hardened valve seats, there is no need for lead, or to worry about valve wear.

John Wagland '82 928S Man. Rosewood Metallic

Glenn Evans

I agree with John. I used PULP regularly, though never for more than a couple of successive fills, in the early days of having my car.

Do you remember that Shell was the first to introduce high octane unleaded, and beat all the other oil companies to the market by heaps (certainly months, if not a year or more)? I knew someone senior in Shell's Melbourne office at the time; he said that Shell could easily produce unleaded of over 100 octane (RON), whereas the other Australian refineries could not, and had great difficulty eventually producing 96 RON (premium unleaded) economically. Shell's super "Half Lead" contains only 0.3 g/litre of lead (I think), about half the concentration of older supers, and still much less than super which is not produced by Shell.

(Tip: Shell supplies BP in Sydney, so super users will cut down on lead emissions if you use either of these.)

I do not know whether Shell's ability to produce higher octane unleaded is due to better refinery equipment or whether it uses additives, or whatever.

The petroleum distillation process produces a variety of products of different volatilities. The most economical approach is to produce a lower octane petrol in greater quantity and add an octane booster. Tetra ethyl lead is used for super because it is relatively cheap. To produce a higher octane petrol, to which an octane booster will not be added, more volatile aromatic compounds (such as toluene and xylene) must be captured with the petrol, which reduces the yield (the overall quantity of petrol produced per barrel of crude oil).

Swings and roundabouts: you get rid of lead but aromatics are carcinogenic.

AVGAS these days is 100 octane low lead (0.3 g/l lead, if I remember correctly). This is equivalent to the old 100/115 octane (lean and rich octane ratings) AVGAS (green??). I am pretty sure that AVGAS octane ratings are MON (Motor Octane Number) rather than RON (forget what the R stands for), as used here for auto petrol, so the RON for AVGAS would be several points higher. The RON rating indicates the tendency to detonate at low revs; the MON test is conducted at higher revs, and produces a lower number. Just to be difficult, the US uses (RON+MON)/2 for automobile gasoline.

I'm dredging all this up from years ago, so don't quote me.

Glenn Evans '80 928 Petrolblaumetallic 'S' look 5 speed

Bart Shephard

That other well known, Bruce Buchanon, strongly recommends PULP over leaded for all 928's. His reason: leaded is often dirty (still stored in older tanks?) and from his experience, clogs the fuel filter much faster. - Bart

Eddy Sain

Glenn, excellent information and right on!!!!

Some refiners are making "leaded" with no lead ar all just additives and a real struggle to get the right octane rating or even close.

You can of course buy some Tetra Ethyl Lead and it is not that cheap but you only use a very small amount the problem is that it is a contact poison and absorbs through the skin.

I got some from a refinery a few years ago when I commissioned their Hydrogen compressors for making petrol out of Naphtha but blending cocktails is a pain and doesn't work if you want the catalytic converters to operate.

Anyway my compliments on your excellent presentation of the information you should be a petroleum engineer.

Regards Eddy

Glenn Evans

Yeah, my Chilton's 928 manual vaporized just before I described the control pressure regulator to you. I have the part number written on a piece of paper which is in the manual, and I was going to quote it so you could be sure that you identify the thing correctly. The 'S' CPR looks the same in magazine photos as mine does, so it's probably the same. From what I've read, our K-Jets are capable of delivering a lot more fuel than our engines require, so I doubt that those in the 928 and 928S would be much different. Perhaps the metering funnel is different? Do you know, Eddy?

I still can't find the Chilton's. At least I can't blame the kids for this.

Glenn '80 928 Petrolblaumetallic 'S' look 5 speed


Eddy Sain

These are the compression ratios according to the Porsche Tech book

1981, 82 with Engine 28.09/10/15/16, 09 being R.oW manual, 10 R.oW auto,
15 USA+ Japan man, 16 USA+ Japan auto is 10:1 (with USA 9.0:1)

1980, 81, 82, 83 M28.11/12/19/20 11 being R.oW manual, 12 R.oW auto , 19 USA+ Japan man, 20 USA+ Japan auto is 10:1 (with USA 9.3:1)

1984, M28.21/22/19/20 21 being R.oW manual, 22 R.oW auto , 19 USA+ Japan man, 20 USA+ Japan auto is 10.4:1 (with USA 9.3:1)

Most S4 are 10:1 and the GTS 10.4:1

Hope this helps.

With regard to PULP in cars that can run leaded yes the PULP has higher octane rating than Leaded (which has been steadily decreasing in this regard over the years) but irrespective of the hardened valves and seats I would still personally prefer to use leaded to lubricate the valves which was a secondary function of the lead in the fuel (assuming the fuel band you use actually contains lead and as I am reliably informed some "leaded" fuels do not).

An alternative is to use PULP or any other fuel that does not cause pinging and use an upper cylinder lubricant. Always use the highest octane fuel you can get as in real hot weather or poor conditions it will give you a better chance to avoid pinging if your engine situation is marginal.

Regards Eddy Sain

Glenn Evans

Petrol octane ratings (RON) are:

  • 97 for super

  • 96 for premium unleaded

  • 92 for regular unleaded.

Only the pre-1980 928 (4.5 litre engine, compression ratio 8.5:1) can run unleaded. All later rest-of-the-world models, including the 4.5 litre 928 from model year 1980 on, have compression ratios of 10.0:1 or more, and were designed to run on 98 RON.

In my enthusiasm for writing "War and Peace" last night, I forgot to address a couple of the points others had made in their posts.

Charles said that the octane rating of premuim unleaded (PULP) can drop rapidly in storage. True; this is because aromatic compounds in the petrol, which increase its octane rating, are more volatile than the rest of the petrol, and will vaporise away. Super does not suffer from this; to the same extent, anyway.

As far as I know, all petrol marketed as 'super' contains lead. It takes more crude oil to make petrol to a given octane rating without using a lead additive, and costs the oil company more to produce.

I have read (in the paper, or did Darren tell us??) that WA has banned leaded petrol altogether. Super, or whatever it is now called, is PULP with an additive to provide the upper cylinder lubrication which lead previously did, and which heads without hardened valve seats relied upon.

I do not know what the octane rating is; PULP's 96 or the old super's 97 RON. Do you know about this, Darren? I expect that the other states will follow suit.

Eddy wondered what the contaminated Mobil AVGAS does to cars. I have heard that Mobil proposes to "clean" the contaminated AVGAS and mix it with super for automobile use. I ask: why only super? My guess: nearly all unleaded cars are fuel injected, and I suspect that Mobil thinks that the engines which run super will overwhelmingly be carburetted and less susceptible to contamination because they do not have fine tolerance metering systems and injectors. Our fuel filters are much finer than those used for carburetted engines to protect the fuel injection system. I presume, but don't know, that other fuel injected cars would have similarly fine fuel filters.

Mind you, aircraft (even those with carburetted engines) have very fine filters also, and the contamination damage apparently hasn't been confined to the filters and tanks. Even if the fuel filter did capture all the gunk, I'd rather not have to fork out for another filter prematurely, especially if the indication that it is clogged is the fuel pump dying after trying to do the equivalent of pushing s*** uphill!

Glenn '80 928 Petrolblaumetallic 'S' look 5 speed

Glenn Evans

Eddy, you said that some crude is almost usable as diesel. The crude from Barrow Island (west of Karratha, WA) could be used as engine oil, and was used in some West Australian Petroleum vehicles up there back in the '60s. As I recall from what my Dad told me, the guys up there reckoned that it would be OK, and did it; I don't know whether its effects on the engines were studied, or whether the practice continued.

I hadn't previously thought much about how much a supposedly standard product (auto petrol) can vary, although I know about the huge variation in crude oil.

What this comes down to is that a heavy crude (like that from the Arabian Gulf) can produce a lower quantity of volatile products, like petrol, from a given amount of crude, and can produce more heavy products, like tar.

I am not familiar with the details of oil refining, so I don't know how accurately the different compounds in crude can be separated (how pure each product is). It seems to me that we, as consumers, should be asking more questions. This landsharks thread is a start!

Glenn '80 928 Petrolblaumetallic 'S' look 5 speed

Please note: these comments are the writer's personal opinions. I take no responsibility nor give any kind of warranty as to the accuracy of the statements included on these pages. The information in Tech Tips is based on the extensive experience of Porsche specialists and owners who drive, service and even race their 928. Although this information is generally considered 'best practice' niether 928 Australia, Landsharkoz or the Webmaster will accept any liability for such information, or, for any use to which it is put. If you have doubts about a specific point, or if you have any questions, you should consult your Porsche specialist or post a question on the Landshark mailing list. - Leonard Zech, Webmaster.

928 Australia 2011