'83 928s which octane rating to use
 

Just bought an 83 928s. previous owner said to use premium fuel.... Manual says to use 91 octane, but then says "87 on fuel pumps in USA" - Since I'm in Northern California, I'm not sure what to put in my car.... any help is appreciated

i've got an '83 also. It runs great on 87 for 3 yrs.....in general, I put 93 in my 44 turbo and 968 because owner manual recommends it. i've never tried higher octane in 928 and it might make a difference...i also run some techron fuel injection cleaner thru the gas once a yr.....its supposed to be worth it...

Low octane is fine on the Original car and you can burn higher..depends on what you've done to the engine. Also, use copper spark plugs not the 4 plat.

For CA smog tests burn low octane; you get a most complete burn due to the flash point. It will help keep the HC's lower on the tests, particularly if you have higher mileage.

All US 928s from 1977 to 1984 are designed to run on regular grade fuel (87). Since mine is a 1989 I run premium as recommended....Stevil welcome to 928 ownership...

OK. So there are a couple different ways of measuring octane. One is RON which stands for "Research Octane Number" and another is MON which stands for "Motor Octane Number." Here in the US (and Canada) we use a formula of R+M/2 which is the average number of Ron and MON. This is called the AKI rating or Anti Knock Index.

Ingeneral the difference between RON and MON is about 10 points on the scale, as in 95 RON is about 85 MON. R+M/2 of this would equal 90.

Our 928's being from Germany are rated in RON. This means to get the correct octane rating minus 4 or 5 from the RON sticker on the filler door and that is the number you buy here in North America.

RON: 99 = AKI: 95
RON: 95 = AKI: 91
RON: 91 = AKI: 87

Regular vs Premium Fuel -
 

Hey Stevil, When fueling up my 1985 32 valver it would ping a bit on Regular fuel if I accelerated too rapidly or stomp down on a steep grade to pass. I think part of my problem is that the compression is a little higher than normal from a slight build up of carbon on the bottom of each head. The good part was that when I scoped the cylinders during the initial retro-furbish, I was lucky to find no cylinder wall scoring.

One of the many threats generated during a cylinder ping is that the harmonics can dislodge a small particle of this built-up hardened carbon. If it lodges between the piston and cylinder wall, it could score a vertical line along the piston ring sealing surface. Eventually this can cause oil consumption, blow-by and ultimately a loss of compression. So the main reason that I use Premium fuel in the 85 is to try and avoid the ping.

My 84 16 valver seems to digest Regular with no problem, even when pushed hard.

Good Luck, Michael :)

Thanks for the info guys. This is the first Porsche I've ever owned, so any info you can send my way is much appreciated;)

With the compression and cams I think you'll be fine with 87 octane.

I alway use the good stuff, its just better.

Did the previous owner replace the fuel lines? If not, do it very, very soon.

944/944Turbo/928 all very susceptible: they become brittle from heat soak, have at least 39 psig on them, and are now running 10% ethanol.

Just one example, 944.
V8 has the disadvantage of having a deep V that will hold a lot of fuel prior to ignition.
http://www.youtube.com/watch?v=6tUEHhY49WU&NR=1

Takes a slightly special hose, not just any fuel injection hose will work, but the job is not expensive, nor difficult! Be Safe.

83 will have 8 little 50mm hoses on injectors, plus 1 feed hose near passenger side exhaust manifold and I think 4 hoses under the air cleaner of various lengths.

There is no need to increase the octane unless the engine needs it. On that low of a compression the engine might actually get better economy and performance as compared to higher octane. There's the same amount of energy per gallon...just harder to start the flame on the higher octane fuels. Depends on a lot of things, but I generally run lower octane unless I know I need the higher stuff.