This is probably a dumb question but....

[Will.iam]

12 hours ago, cliffy said:

OK School time:-)

What color was 91/96 octane fuel when it was available?

Bonus question

What was different about the engines approved for 91/96 as opposed to those with the 100/130 restriction?

 

Different piston compression ratios?

[RobertGary1]

Sadly only aviation gas is legal at the time. 

[Pinecone]

The old fuel was 91/96.  It was blue and replaced by 100LL.  If a manual specs 100LL, it would not mention 91/96, as it no longer existed.

But those engines that were originally specd for 91/96 should be able to use 94UL AVGAS.  It just needs to be approved.

Yes, car gas and AVGAS octanes are different.  AVGAS octane is similar to car MON.  The number on the pump is AKI, which is an average of MON and RON.  For quality car fuels, you can expect the MON to be no more than 5 points under the AKI.  So 93 car gas is somewhere around 88 AVGAS

 

[cliffy]

Red off road fuel is monitored by some states doing dip tests on trucks on the highway looking for red fuel on the road of course RED avgas was 80/87

Brown avgas was pretty well gone by the early 60s. 

Used to pump red green blue purple jet a1 and jp4 in my early career Saw brown one time as I recall

Bonus question still hangs out there

 

[BobbyH]

21 hours ago, cliffy said:

Bonus question

What was different about the engines approved for 91/96 as opposed to those with the 100/130 restriction?

OK I'll bite:  The higher compression (often turbocharged) engines will detonate (knocking) with lower octane fuels.  The higher octane fuels help keep the full pressure of the combustion during the power stroke of the cylinder.  A lower compression ratio engine doesn't need the higher octane to keep detonation down and can use the lower octane fuels. 

We all like the performance we get out of the higher compression engines, but to keep them from self destructing they need the higher octane fuels. 

[cliffy]

Is there any horsepower or RPM difference between the 2 engines?  No. So the compression ratio should be the same  One could always check the Lycoming  TC to verify

[Fly Boomer]

1 hour ago, BobbyH said:

The higher compression (often turbocharged) engines will detonate (knocking) with lower octane fuels.

The factory-turbocharged Continentals have a lower compression ratio than N/A.  Generally, they are:

360
  7.5 turbocharged and alternative fuel
  8.5 naturally aspirated
470
  8.6 most engines (naturally aspirated)
  7.0 for a couple of engines (also naturally aspirated)
520
  7.5 turbocharged
  8.5 naturally aspirated
550
  7.5 turbocharged
  8.5 naturally aspirated
 

[cliffy]

OK we,re gettig way off course here

The octane references were made regarding the previously mentioned

O-360-A1D  180HP engine being certified for 91/96 octane fuel by certian serisl numbers and others for 100 octane. No mention of any other model engines in the bonus question

Just the 180HP engine. Why the 2 different octzne ratings and why only certain S/Ns?

[bcg]

2 minutes ago, cliffy said:

OK we,re gettig way off course here

The octane references were made regarding the previously mentioned

O-360-A1D  180HP engine being certified for 91/96 octane fuel by certian serisl numbers and others for 100 octane. No mention of any other model engines in the bonus question

Just the 180HP engine. Why the 2 different octzne ratings and why only certain S/Ns?

That's OK, I think we answered the question in the first few posts so now we can wander. 

[Pinecone]

As I stated before, later engines were only certified for 100/130 or 100LL as 91/96 no longer existed.