kos11-12

I am suprised by the CR ratio that I hear using 98 Ron pump fuel,
I have a 10.5 CR engine ( twin pluged) very sensitive to 98 RON fuel, and a race engine with 12.2 CR witch I use 100 MON race fuel ....both accurately measured

I experience pinking with another engine (not my 964) at any RPM if lower octane is used ....
But also with old fuel..... (4months)

on the 964 I have an additional map for 95 RON..., but did not found it economical...just as a emergency, in France 95 RON is difficult to find, replaced by 95 E 10....

Geoffrey

My race car has 12.0: compression and runs on 93 octane (R+M/2) street fuel. Properly tuned for fuel, you can have excellent reliability without detonation with the acknowlegement that some performance is given up by using the lower grade fuel.

Stock 964 is about 10.8:1 compression.

NineMeister

Amongst other factors, the octane rating that an engine will tolerate will be down to the combination of static compression ratio, camshaft timing, volumetric efficiency, temperature and combustion speed/stability; unfortunately you cannot simply isolate static compression ratio from this string of variables and expect to form an accurate relationship to RON to deduce what an engine will need.

For instance, build a 3.6 engine and tune it for 95RON. Then change the bore diameter to build a 3.8 litre and the tune relationship is disturbed. Burn speed will slow due to the increse in flame front travel, combustion temperature will change as a result (causing more or less heat to be transferred into the head/piston) and as a result the ignition timing will need to be ammended for optimum torque. It's a similar story with camshaft timing resulting from a cam switch in the same capacity engine. Especially important in respect of combustion is the relationship between the intake valve closing point and TDC - the effective stroke and the resulting dynamic compression ratio of the volume change for that stroke. As you can see, this is not an easy question to answer.

If your 10.5:1 twin plug engine is sensitive to timing on 98RON then you have too fast a burn speed, probably due to a particularly high volumetric efficiency and in-optimal camshaft timing. It will probably be a similar story with your other un-named engine. Sorry I cannot be more specific.

kos11-12

very interesting thanks for replying ....!!

Babalouie

I'd echo what Colin said With my other car, I had a noticeable amount of pinging when I swapped to a smaller cam, and had to compensate by reducing the amount of ignition advance.

kos11-12

Did not realise camshat duration & lift would do a difference...

in my case reverte from 7 to 8 grade plugs and fresh fuel made it run perfectly (as before)... I will try 9 (on a dyno)

alexjc4

So, some colder plugs and fresh fuel cured it?

Did the plugs coming out look like they'd been getting too hot and causing the pre-ignition?

kos11-12

The plugs did not looked good, I didn't run it for long, just not pay attenting, the petrol was way off...4/5 months, I would have probably be find with a normal tuned engine... I don't think you can keep petrol on a highly tuned engine for longer than 3 months, some said you use to be able to ...

on the 964 I never have much fuel in the tank 10/15 L, & feeling right up only before leaving for a long trip, for the 2 race engines, I drain everything after race week ends, and keep the fuel well sealed (air gap, no air vent)...

NineMeister

You raise a good point in that spark plugs can have an influence on the burn in a cylinder, however in the case of the later Porsche engines they would have to be a very long way out of the optimal heat range before they would have any noticeable affect on ignition timing. The issue here is unlikely to be pre-ignition, it is more likely to be detonation after the spark event which is the result of an unstable or overly fast burn rate in the combustion space.

NineMeister

If you think about it logically, compression of the charge in the cylinder can only occur when the intake valve is closed. From this point to TDC is the effective stroke, hence the change of the volume in the cylinder space from IVC to TDC is called the effective compression ratio. However this is only part of the story.

The compression that occurs is actually the combination of this effective compression and the actual pressure in the cylinder space at the point when the intake valve closes, the charge pressure can be above or below atmospheric pressure and is the result of the VE (volumetric efficiency) of the engine. VE benefits from resonant and velocity effects present in the intake manifold, hence a race engine that works exceptionally well with conventional tuning methods might increase the trapped pressure from 1.0 bar atmospheric to 1.25 bar or more, whereas more conventional engines will be in the 0.8-0.9bar region.

A good rule of thumb for IVC (inlet valve closed) compression is around 7.5:1 or slightly more for a race engine, hence under dynamic conditions said race engine's cylinder might see a dynamic compression ratio of 8.0 x 1.25 or around 10.0:1 (assuming that temperature is constant).

Makes you think, doesn't it?

alexjc4

Thanks Colin, nice insight into engine design and configuration considerations.

In practice would you say that, with a road car engine that's already build, if you find the thing's tuned on a knife edge and won't tolerate anything other than super fresh fuel, then the thing to do is just to remap with more safety margin?

NineMeister

Absolutely correct. You do not want to run any engine on a knife edge that you cannot control. Provided nothing on the engine has been changed which can contribute to the problem, if your engine is not tolerant of fuel more than a few weeks old you definitely need to reconsider the mapping of the engine before detonation damage occurs. Sometimes you can achieve this with a simple global ignition trim (eg Motec), other times a visit to a rolling road would be preferred.

Most tuners will set up an engine on a dyno and set the ignition timing for MBT (mean best torque) which usually implies that there is a 1 to 2 degree headroom before the onset of detonation. The safest way to approach MBT is from a retarded state, so for instance if I was using this method to tune a 9m Motec conversion of a 964, I would begin with a previous map from another identical engine with a -2 or -3 degree ignition trim, then once fuelling was set to produce the best power I would then increment the ignition timing until no further gain was seen across the rpm range - which occurs long before det can be heard. I would then revisit the fuel tables to see if there is more to be found before finally experimenting with individual cylinder trims. But even then we are not done, as final tuning would then be carried out during accelerative tests on the road, sometimes requiring different advance curves for the varying acceleration rates of different gears (on the more critical race engines).

HiWind

very insightful thread thanks!

What is typical range for ignition advance on higher compression 3.8 w gt2 or RSR profiled cams?