octane/CR

[r lane]

I am sure this topic has been discussed adnauseam. Is there a general standard; What octane level will support what CR. I ask for a couple of reasons. I track a 3.0 twin plug MFI car at 12.0-1.0 CR. I have been using 110 octane. Mostly VP fuel but some Sunnoco. I am going through injectors like popcorn. A race buddy suggested dropping to 100 octane as it was without lead. He thinks lead may be causing the issue with my injectors, but I doubt that, but have no hard evidence either way. He felt 100 which is lead free would support my CR. Would love to hear some sound info on this subject. Thanks, Bob

[ant7]

Its a lack of lubrication that wear fuel injectors prematurely, maybe try a fuel additive that has know lubrication properties.
Ant.

Originally Posted by r lane

I am sure this topic has been discussed adnauseam. Is there a general standard; What octane level will support what CR. I ask for a couple of reasons. I track a 3.0 twin plug MFI car at 12.0-1.0 CR. I have been using 110 octane. Mostly VP fuel but some Sunnoco. I am going through injectors like popcorn. A race buddy suggested dropping to 100 octane as it was without lead. He thinks lead may be causing the issue with my injectors, but I doubt that, but have no hard evidence either way. He felt 100 which is lead free would support my CR. Would love to hear some sound info on this subject. Thanks, Bob

[nickelplated5s]

Actually lead in fuel acts as a lubricant. Presumably due to it's low melt point. Personally I run 87 non ethanol and cut 100 low lead into it. Put an AFR on it and check the timing. Someone better will chime in but 12.0 CR?

[Yinkus]

At 12:1 on a 3.0 twin-plug MFI, 100 octane is generally enough—twin-plugging lowers the detonation risk a lot.

[stownsen914]

I wonder if you'd have detonation issues with a 12:1 aircooled race engine on 100 octane, even with twin plugs.

As a point of reference, in a 968 based race engine I run 110 leaded, and haven't had issues with injectors at all.

[ant7]

Exactly, however; since lead has been removed from regular pump gas, an alternative lubricant should be used if premature wear is an issue.
Ant.

Originally Posted by nickelplated5s

Actually lead in fuel acts as a lubricant. Presumably due to it's low melt point. Personally I run 87 non ethanol and cut 100 low lead into it. Put an AFR on it and check the timing. Someone better will chime in but 12.0 CR?

[Rivet]

Klotz or VP top end lube is what I use for a lot of the race vehicles I tend to, leaded or unleaded.

Reed vapor pressure on race fuel is typically higher than pump fuel and will evaporate at a higher rate than pump gas when the fuel is open to atmosphere or left in a vented fuel cell. When the most volatile part of the fuel evaporates it will leave behind lower volatility parts. Those parts can produce solids that will give you injector issues.

Depending on packaging, applications and length of sitting time I sometimes will have a fuel cell vent situation that will allow sealing (but always remember to remove the seal before using the vehicle). This will allow the fuel a better chance of staying stable and true to its original formula.

[11BC2]

Originally Posted by r lane

…I am going through injectors like popcorn….

What is the symptom of the fuel injectors are exhibiting?
When we used to run, E85 fuel in the race car, we would have to drain the tank, and run the engine out of fuel. Then replace it with non-ethanol fuel for storage.

[r lane]

Originally Posted by 11BC2

What is the symptom of the fuel injectors are exhibiting?
When we used to run, E85 fuel in the race car, we would have to drain the tank, and run the engine out of fuel. Then replace it with non-ethanol fuel for storage.

The injectors that are failing will no longer have a conical fog. Squirt off to one side, and any number of iterations. Took a couple apart and unless the valve and spring assembly fail, which I think would be rare, the problem has to be with the little filter canister. It has several layers of fine mesh and a .025'' exit hole. The fuel then hits the spring collett and makes its way past it to the valve head. I am assuming that the pressure effects both the collett and of course the valve as it comes off its seat. I am going to drill through the filter on one of the failed injectors just to see how it effects the spray pattern. I think the small hole in the filter canister is critical regarding its size, so will try not to drill into it. Bob

[stownsen914]

OK, forgot we're talking about MFI, not EFI. Totally different injectors of course. Some of the MFI specialists used to test and *I think* rebuild MFI injectors. No idea what that means exactly, but I'd assume mostly cleaning them. Perhaps Mark or Gus could examine yours and give an assessment.

[nickelplated5s]

Yes, you can rebuild injectors. I went with new based on the rec from the dyno shop my motor is going into. Hopefully this week. 100LL is aviation gas. You can usually buy it at any uncontrolled airport. Additives would help too. Who shaped the space cam for the 3.2?

[917_Langheck]

polyether amine (PEA) - both a cleaner and a lubricant for injectors

[nickelplated5s]

I'm still thinking space cam.

[stownsen914]

Why would a space cam cause clogged MFI injectors?

[Ovis]

Has anybody regularly mixed AV Gas with pump gas? If so, what is your process? The highest octane we can get at the pump here in Alaska is 90. Yet, AV gas is regularly available. Pondering cutting some AV gas into my pump gas, but just want to make sure I won't hurt the motor.

[PeteKz]

You won't hurt the motor running 100% AvGas. It has a small amount of lead in it, which will leave deposits on the plugs. That's one of the reasons plugs had to be cleaned or changed frequently in the olde days of leaded gasoline, and why plugs can go 50-100Kmi now. If you use regular plugs now (rather than platinum or iridium) and change them in 10,000 miles or so, that won't be a problem.

However, the EPA has a timeline for getting lead out of AvGas too, by 2030. It's a subject of contention in the general aviation community. Just be aware that's coming and eventually you will have to find another source of high octane gas.

[nickelplated5s]

Originally Posted by stownsen914

Why would a space cam cause clogged MFI injectors?

Short answer: if the MFI pump wasn't off a 3.2 it won't deliver the right amount of fuel. AKA fuel spray and volume.

Long answer: I don't know of any 3.2 factory MFI motors. In other words the space cam needs to be reshaped for such. It's a long process where craft=art. Mark told me he took 20 times to reshape one for a 3.5. He's got a setup to bench test them. He reshaped mine from a 2.0 to my 2.2. I'm thinking about starting a thread on Early911S pulling together a lot of this. One of the best stories is the system is a 'double blind.' The MFI doesn't know what the air volume is.

Re LL; older piston rings were designed for this as lubricant. Different story for old fuel lines with ethanol fuel.

[nickelplated5s]

Originally Posted by Ovis

Has anybody regularly mixed AV Gas with pump gas? If so, what is your process? The highest octane we can get at the pump here in Alaska is 90. Yet, AV gas is regularly available. Pondering cutting some AV gas into my pump gas, but just want to make sure I won't hurt the motor.

Yes. I buy it 5 gallon containers. I use a battery powered siphon to add, no spillage. Do the math for whatever octane you want. I want the octane boost as there is 87 non ethanol at home. 100 gives me lead and an octane boost. I think about a 10.5 CR.

[ant7]

From my own recollection of messing around with MFI on BMW 2002 motors [many years ago now] which I think is very similar to the systems used by Porsche, the fuel pressure at the injectors on MFI is considerably higher than that of the K Jetronic [used on the 3ltr SC motors] ] and L Jetronic used on the 3.2 litre etc, hence the MFI System is more akin to a diesel set up, again just my ramblings.
Ant.

[H-viken]

I run high comp on my 3L as well. Twinplug, Weber carbs, so not MFI. Run it on 98 octane euro gas (can’t recall the conversion to US octane). Runs great