Fuel?

[Guest]

Hi there. I've recently been looking at a 924, and i was wondering what the best alternative to LRP would be? There's nowhere locally, that sells it I've spotted the lead replacement stuff that comes in bottles, that you add to unleaded, and i was wondering if that would be good? Also. Is there a particular kind of fuel that's best suited for use with the bottled stuff?

Would it be worth getting the engine modified to use unleaded? (i'm really new to this so be gemtle )

Thanks in advance John

 

[Guest]

John,
I was worried about the potential loss of leaded fuel at the time of withdrawal and asked my servicing guy what I should do (this was for my 924 CGT at Turbo2) he suggested that I should do nothing and just switch to unleaded - which I did, and have not had any problems. We've just taken the CGT engine apart after several years running on unleaded and have not observed any noticable damage or wear and thus I guess his ascertion that "these engines will run on almost anything" was correct!
Regards
Dave

 

[geoff Ives]

The 924 does not require leaded fuel. The only problem with fuel in this model is related to octain (not to be confused with lead) as standard unleaded is not quite the reccomended octain. HOWEVER my car has done close to 200K miles and at least 100K on standard unleaded. 2 years (20K miles) ago I had to replace the head gasket and have the head skimmed and at that time nothing else needed doing, we didn't event bother grinding the valves. Just use your car and if you are going to give it a heavy thrashing ( I dont know where you could in this country) fill up with super unleaded.

 

[vincematthews]

Yes I'd go along with that . . . basically Porsche = unleaded !
I've been using unleaded in a 2.0L 924 for the last 3 years and no problem.

 

[Guest]

Thanks for the speedy replies. I was up looking at the car yesterday (haven't bought it yet) The guy said he filled it up with LRP so i was a little put off, but i might just fill it with super to be in the safe side

would you recomend runniong it dry before switching to unleaded?

here's the beastie i'm looking at

 

[vincematthews]

I would run it low but not dry. The fuel pump(s) are immersed & use the fuel for internal cooling of the electric motor; I would be wary of getting air in the system! though I think the pump(s) should cut out if the engine is starved of fuel and the revs drop off.
You may want to change the fuel filter (anyway) after changing over.

I bought my car from a trader and didn't know what it had been run on before. I used unleaded straight off with no problems. There used to be a FAQ about it on this forum, but I guess its being updated?

 

[924nutter]

Having just ground the valves in on my turbo, and noted the condition of the seats knowing that previous owners have run it on unleaded, I can confirm that the valve seats are damn hard, I could hardly touch them by hand and had to use my trusty Gunsons valve lapper, and even then they needed extensive application. If the 924 suffers valve seat recession you are running it on valve grinding paste. Ideally 98 octane, and some super unleadeds are only 97. Once in a while it is nice to go somewhere that sells 99 octane 4* and run a tank through, but it is completely unneccesary. If you run the tank dry, you will starve the fuel pumps of the fuel which they rely on for lubrication?

 

[937carrera]

For a standard 924 I suppose 95 unleaded would be OK, but for a turbo car I would suggest at least 97 Octane fuel (BP Ultimate). Optimax is much better, and my CGT road car can certainly tell the difference. As noted above old fashioned 4 star is still available, but at a premium price.

In Germany the equivalent products (Aral Ultimate / V-Power are both 100 RON)

 

[geoff Ives]

That's interesting. The usual reason for using a higher octain petrol is for engines with a higher compression ratio to prevent detonation and pre ignition. Turbo cars have a lower ratio and while you may well be correct I would really like to hear the reasoning as I have just added a 931 to my stable.

 

[937carrera]

The static compression ratio is indeed lower, but the effective compression ratio is much higher.

Puttting it simply, a normally aspirated 924 has a compression ration of 9.5:1, with the air at 1 bar. A s2 931 has a compression ratio of 8.5:1, but it runs at 0.65bar boost. That means 1.65 times atmospheric pressure. 8.5 x 1.65 = 14:1 as an effective compression ratio.

Now the real world is more complicated than that, but the illustration explains why turbo cars give more power. More air plus more fuel = more energy.........and why turbo cars are much more sensitive to fuel and ignition timing.

 

[geoff Ives]

Thank you. I realised that there was something that I was missing.
Geoff.

 

[924nutter]

Detonation can occur in the turbo due to the higher cylinder pressures when the turbo is boosting. As carrera937 says the normally aspirated 924, at 9.3: 1 takes air at 14.7 psi and generates a pre-combustion pressure of approx 137 psi. Air in the turbo is boosted by 0.7 or (0.65 depending on whether it is an S1 170 bhp or an S2 177 bhp) to 14.7+(0.7x 14.5 [1 bar] ) x 8.0, which is approximately 199 psi. When the bang occurs the cylinder pressure, with a higher starting point can rise to a level where the mixture will ignite in another part the combustion chamber, giving rise to two flame fronts. When these two flame front meet they generate that familiar pinking sound. A higher octane value will increase the mixture's resistance to detonation. Incidentally, Geoff, the Carrerra gt generates 0.75 bar boost. Cleverly, whilst the valve timing is identical, Porsche used a clear chill casting for the gt cam and used a different material for the cam followers. The extra boost was achieved by using a different waste-gate and a turbocharger which used the same turbine but had a larger turbine housing.