High Octane?

[Jason Porter]

Originally posted by lapuwali
If you feel you need the extra insurance of higher octane gas, fine.

I'm definitely not whistle blowing. But I do feel better knowing that I'm running premium.

Especially with this utter crap we have for gas in Cali!

[AaronM]

Originally posted by lapuwali
If the engine in your car is running hotter than normal, then there's some other problem you're just papering over with higher octane gas. If you can't drive around on a 90dF day w/o pinging, there's certainly something wrong. Assuming an otherwise stock engine, of course.

How are you measuring "hotter", btw? Hotter than what? Do you actually get ping on hot days with lower octane gas? I'm not trying to be a hardass here, but you're handwaving trying to justify doing something you want to do, not something that necessarily NEEDS to be done. If you feel you need the extra insurance of higher octane gas, fine.

Frankly, if CHT is so high that you need to run 93 octane in any stock Type IV other than a '70-'71 1.7, then you need to fix the underlying problem, as you're NOT lowering the already too high CHT by running higher octane gas, you're just preventing some of the aftereffects of the higher CHT (ping), and preventing the CHT from spiralling out of control. However, the high CHT is still cooking your heads, and shortening their lifespan.

I'm just paranoid, that's all. The car only has the original oil temp guage (you know, the little toy guage mounted above the fuel guage that tells you nothing unless it pegs out) and I would rather have the extra headroom that higher octane provides. I have a tendency to worry about things that are not going to happen, just ask my long-suffering mechanic. *laughs*

As for "hotter", I've been told many times that the '75 and '76 cars are tuned to run hotter than earlier models because of emissions concerns. I've never had any issues with it approaching overheating.

Bottom line is that given the relative infrequency with which I drive the 914 it's worth the $5/month during the summer just to put my mind at ease. It's not going to do any damage to run high octane, so I do it for peace of mind.

I've never had the car ping and I've never had an issue with overheating. I just would rather be overcautious since it's not a lot of money. It's the same reason I use Mobil 1 instead of Castrol dino oil. On a car that is a weekend driver, I am willing to be a little spendy because it's still a small amount of money in an overall sense.

So, to sum up, never had a problem on the tanks I ran 87 in, I just want the extra peace of mind and I'm willing to pay for it since there's no downside potential for it.

Aaron

[smooth_eddy]

Well there sure seems to be a variety of thoughts at the pump. My 75 2.0 engine does run fine on the cheap stuff from Chevron, but I do have a rough idle. I will try a better grade and see if that helps any......Eddy

[silver2.0]

Wow, there are alot of opinoins. Since everyone else is sharing there's here goes mine. If your not pinging your ok, and just run the recommended octane. If you have raised your cr, as I have slightly go ahead and run a better grade. I choose to run 89. I think that the temp/ping thing is a little mixed up though. For example, my 2000 chevy truck never pings under it's own load. But If I really load it down in 100 degree weather, and try to pull a hill, it will ping on cheap gas, and loses major power. I think the extra heat may encourage pinging, as will increased loads. WHen I run the good gas in it makes more power when towing, and doesn't ping. Now before anyone says my knock sensor is bad, I already replaced it!

[lapuwali]

Higher temperatures CAN increase the likelyhood of knock, as can a great many other factors: increased compression ratio, advanced ignition timing, spark plug heat range, the exact shape of the combustion chamber and everything in it (including valves and the plug), carbon build-up, reduced cooling, increased load.

Knock is essentially the one thing that limits a spark-ignition engine in making power. If it weren't for knock, all spark ignition engines would have compression ratios in the 17-20:1 ratio, and there'd be little difference between spark ignition and compression ignition engines (Diesels) in efficiency. 150bhp/liter specific power would be commonplace instead of limited to bike and full race engines.

Any spark-ignition engine is designed to run on a particular octane. In poor countries, most cars have low compression ratios (below 8:1 is common), as typical pump gas there is quite poor, in the 75-85 octane range. Octane in rich countries is limited primarily by economics, as well. The technology is available to make even unleaded gas with 100 octane or more, but it costs more. The 91-93 available in the US is now considered the happy medium between price and efficiency. Once fuel efficiency really becomes a concern again, engines will start to be designed to require higher octane fuels to improve that efficiency, and there will be pressure to increase octane at the pumps. However, the fact that hybrids exist shows there is enormous resistance to increasing octane, since just doing that (and increasing CR) would substantially increase fuel efficiency and reduce SOME emissions (HC and CO, but NOx would rise).

In the early 1970s, gas typically had higher octane that we have today. Even cars with points-fired ignitions and carbs had 10:1 compression ratios in 1968, when the Type 4 was first appearing. Throughout most of the 70s and 80s, CR dropped as just doing that lowered emissions, and engine temps rose due to increased load from air pumps and EGR valves. Octane fell, too, partly because of the removal of lead (an octane booster), partly because it wasn't needed anymore. By the 90s, when computer controls matured, CR rose again, but octane did not. Indeed, in California, premium has fallen in octane over the past few years. Such are the politics and economics of oil. Thus, more and more newer cars require premium gas, whereas in 1980 almost no new car required more than about 85 octane.

This is why '73 and '74 914s have lower octane requirements than the '70-'71 models, and the '75-'76 models are lower still.

[Jake Raby]

CHT is not impacted by octane..( I can show you on the dyno or any one of my cars).. The lower octane has a lower threshold for knock and does damage if detonation occurs. You can see the impact that octane has on the engine by monitoring the EGT and the threshold of detonation with varying fuel grades.

Many people do not understand that the same engine that runs good on 87 octane with lower CR, may run WORSE on 93!! When going up in octane the viscosity of the fuel changes and a jetting change is needed. The higher octane matched with lower CR almost always runs rich without a vast amount of timing to help burn it all BEFORE the exhaust valve opens..... thats why it impacts EGT so much..

Its ALL in the combo, and fuel is part of the combo that must be considered when designing the engine.

[AaronM]

Originally posted by Jake Raby
CHT is not impacted by octane..( I can show you on the dyno or any one of my cars).. The lower octane has a lower threshold for knock and does damage if detonation occurs. You can see the impact that octane has on the engine by monitoring the EGT and the threshold of detonation with varying fuel grades.

Many people do not understand that the same engine that runs good on 87 octane with lower CR, may run WORSE on 93!! When going up in octane the viscosity of the fuel changes and a jetting change is needed. The higher octane matched with lower CR almost always runs rich without a vast amount of timing to help burn it all BEFORE the exhaust valve opens..... thats why it impacts EGT so much..

Its ALL in the combo, and fuel is part of the combo that must be considered when designing the engine.

You're assuming carburation when you talk about a jetting change, correct?

Aaron

[Jake Raby]

Yes... Or a change to the enrichment of the AFM if you have stock FI.. Either way its the same difference.The higher the octane the fuel the thicker it is. (to pass through jets or injectors)

[widebody911]

My 911 runs much better on 100 or 110 than it does on 91. I was stuck having to use 87 one time, and it ran like crap.

OTOH, mine has all those whiz-bang electronics to take advantage of the octane

My VW bus doesn't seem to car what I put in it.

I think the only way to get more bang out of you buck with higher octane and not bumping up your CR is to advance your timing.