Octane vs. ethanol
 

Short version: 91 octane non-ethanol or 93 octane 10-15% ethanol?

Wondering which fuel would be more forgiving on a 9.7:1 467, basically a Jim Hand build. 95cc 6x heads, 250cfm. Flat tops, all forged, RAIV cam, Rhodes lifters. 3750 lb T/A, manual, 3.70 rear, 28" tire.

91 non-ethanol has more energy than 93 ethanol blend.

Tune the engine for93 octane 10-15% ethanol fuel and i t will ping with 91 non-ethanol fuel.

Tune the engine for 91 octane non-ethanol fuel and it will be ping-free with 93 octane 10-15% ethanol, but may use a little more fuel.

I ran my old 455 with any octane I wanted and it never grumbled once. It was 9.97 to 1 compression, 6X-4 heads with 91cc chambers, zero decked and Crower RAIV camshaft. I never ran any quicker at the track with 92-93 octane but that's what I typically ran in it. Got stuck once using 89 octane and raced it all weekend at Norwalk at the Tri-Power-Nat's on it and it ran fine.

My last 455 was 11.3 to 1 compression with KRE aluminum heads and Dave's Old Faithful cam on a 112LSA with the ICL at 109.5 degrees. It ran fine on 92-93 octane and really didn't care if you ran 87-89 in it but it would "run-on" after shut-down on 87 octane if it was fully heat soaked and really hot/humid.

Keep in mind when you read what I've done that I have the tune "spot-on" and it's not adding any timing at high RPM's or going lean, etc..........

Street on 87 Octane, no ping and yea will heatsoak too (sometimes) when really really hot.
Strip on 89 Octane (12.2s) and no ping (top signature)
Shot too low with 9.0:1, nicely over-cammed with 112 LSA, with ILC advanced 2-4*
Pretty sure no reversion, which is a big deal for low compression.

428 (4" stroke): take 5* off 0.050" Duration (top sig)
400s (3.75" stroke) take 10* off 0.050" duration (top Sig)

High compression, do same for bottom sig durations.

i would run the higher octane and be sure to tune for the ethanol blend, which will need a richer tune than the 91.

The best we have here is 91 and it's 10% ethanol. The only real gas here without ethanol is 87 octane so that's not much of an option for me.

I tune for the ethanol content (so slightly rich) so the cars are safe whether I find real gas or not, plus it compensates for the occasional crappy tank of gas you might get. Better to be on the richer side of things and forget about it.


for a 9.7:1 455 build with iron heads and the RAIV camshaft, as long as the build is solid and things done properly, should run just fine on 91 octane. I'm running an iron headed 11:1 compression engine on 91 and have been daily driving that for the last 6 years, but many things have to be paid close attention to. It's really dynamic compression at play, and a RAIV camshaft in a 455 does a pretty decent job controlling that. I just wouldn't go too crazy with lean AFR's searching for gas mileage.

I think CA is messing with the E in our gas.About every 3 weeks I take a 750 mile trip to the Sierras.My 2017 Yukon for the last 3 years has been dead on 21 MPG like clock work.My last trip was 22.2 and the one before was 19.1.My car is a Flexfuel car so the computer is set to run any E level.I always run 87.Makes me think that some of the gas is getting a higher load of E than 10%?Tom

Just for giggles, and I'm sure someone here may know the answer.

There are formulas available to plug numbers in and figure your dynamic compression, which is useful for figuring octane requirements.

Off the top of my head, a 9.7:1 455 with the RAIV camshaft will probably have a dynamic compression of somewhere in the mid 7's which is easily safe on pump gas (camshaft install position plays a roll) If it started to get up in the lower 9's for dynamic it starts to become a problem.

Cranking compression can be another clue.

Yep, I suspected that years ago and tried a few of those cheap test kits you could get on ebay. I found that the 10% ethanol pumps were sometimes actually as high as 17% at times..

Which is why I tune things on the rich side anymore because you just never know what you're getting from one tank to the next.

I haven't done the test in years but I'd bet money it still fluctuates.

Yep, Tom when the price of gas hit $5.00+ a gallon here in Ca. I saw on the local news the manufacturers were going to increase the ethanol to help slow the rise in fuel price
I think they just need to increase production and stop making up for the lost revenue from COVID. But that's JMHO

Always best to test; every engine is different.

Put in half a tank of the highest octane you can buy.

Find a level stretch of highway that culminates in a medium hill (up).

At the speed limit, in high gear, push down just hard enough to maintain speed without downshifting; listen for ping.

If no ping, and you wish to try less octane, mix half a tank of the next lowest octane with what is in your tank (should give you an octane that is an average of the two).

Repeat the test until you experience ping, or get to the lowest octane.

As stated there is less energy in ethanol diluted fuel that in real gasoline.

Depending on whose figures you believe, gasoline has 117,600 BTU/gal; ethanol 67,000 BTU/gas.

So E10 would be (.9 times 117600 plus (.1 times 67000).

105840 + 6700 is equal to 112540.

Obviously, the math would be different for different percentages of ethanol.

Jon

they announced a few weeks ago they are raising the ethanol to 15%, wonder if cali & other states label it accurately? gas stations around me in the midwest still label each grade & show the E0, E10 & E15, so at least we know & have a choice.

E15 is cheaper by about .05-.10/gal than E10 around here & a lot cheaper than E0... but it takes more to get the same distance, so any savings on ethanol is a complete wash on MPG.

just a gimmick to make people think they are spending less. plus ive read it takes more than a gallon of gas to produce a gallon of ethanol, if thats true the entire ethanol thing is a scam... just like electric vehicles!

Ethanol has an octane rating of 100 so anything with it will have an overall higher octane level. But, ethanol has 37% lower energy compared to gasoline because of the number of carbon atoms so you will use more ethanol than gasoline to make an equivalent power level.

The next generation of energy will be deuterium and dilithium crystals. ;)

Jon

X2. But remember cranking compression on the same engine will vary. It is less in Denver than it is at sea level.

Stan

I bought a barrel of all gas 92 for 5.50 $ before it shot up.Have 70 gallons of it for bracket 11.5-1 engine.
It was much cheaper than 87 is right now.
You can still get it in Oregon for a decent price.

The little test kits with the graduated syringe or the vile that you mix the fuel sample with water are pretty accurate. The new law just signed last week allows 0-15% ethanol for road use with a sticker on the pump. When the limit was 10%, we used to sample fuel all the time. A class project was for students to bring 9 fuel samples from 3 different gas stations. Then we would test them. This would give us a couple hundred different samples in a typical test. We would use the home test, (water method) and the electronic tester sold through Kent Moore/SPX to GM. Results were all over the place. From 0% alcohol to a high of 13.5% the last time we tested which was a year ago. With the limit now raised to 15%, I wouldn't be the least bit surprised to see 17%-18% out there. Your lawnmower won't like that stuff.

Isn't elevation figured into the equation? It should be.

I'd use a formula that includes it if that's what you're looking for. Or better yet, just figure it at sea level then you'll know it's fine everywhere.