[padgett]

Recently on Engine Masters they compared different octanes of gasoline from 87 E10 to 116 Leaded to E85 and one thing was strange: all had peak power at about 29 BTDC.

Now when I was first doing serious stuff stuff back in the early 70s, an SBC with 4" bore and 11:1 compression on Sunoco 260 wanted 34-38 degrees all in. And was common knowledge was that high octane burned slower (lower flame front propagation rate) so needed more advance than reqular for the same peak power (pressure) point about 8 degrees ATDC.

What this makes me wonder is "what changed ?" Does the E10 make 2021 gasolines all burn at the same (faster) rate ? Is the only reason I can think of where all "octanes" now want the same (lower) advance.

The cast did fall over themselves saying not to use 87 in a warmed over street engine but why not ? Are computer controls overriding the mechanical advance ? YWTK

[johnta1]

I'd say back in the 'Day', octane was an actual additive (Lead formula) that raised the octane.

Now the octane is a rating that shows when the gasoline (or fuel) will spark knock.
Alcohols usually provide oxygen along with a octane rating increase.
Different volatility and other things.

I'd also say that a lot higher octane rating like 117 or so could use higher ignition advance. 89 to 93 octane rating may not be such a gap to require higher advance? (just a degree or 2 at most?)

[Don 79 TA]

ahhhh the ole myth that race gas burns slower......
well maybe on some it does, but like everything performance related, things have come along way
there's quite a few properties one needs to consider in picking fuels
certain fuels will ignite much quicker, and some will aide in cooling
check out VP's site and Sunoco's website, VERY interesting, along with ETS fuels
most important thing is that these fuels will more than likely need some changes to your tunes
especially the oxy fuels
but..... the best thing about them is that they are consistent in quality
and BECAREFUL some of these fuels will do DAMAGE to your components when sitting for long periods
as they say.... so much has changed
as for wanting lower advance, from what ETS told me, i should try taking timing out as these fuels "pop" much quicker
and i can tell you, they sure do

[73LeMans]

My 462 likes 29 degrees, so I wasn't surprised to see such a low advance number. I don't think its the fuel that wants lower timing, I think its the cylinder head.

I've played with timing many times at the track and since going with KRE dports many moons ago, 29 degrees was the sweet spot. Much different than when I ran my 68 #16 castings, that's for sure. I believe the CNC work that can be done to the chambers now, even vs 15 years ago, makes them very efficient at burning the mixture and as a result, don't need the crutch of more advance. I'm thinking that's what we saw on Engine Masters - a very efficient, aluminum headed, fuel injected LS. Now, put a carbureted, iron headed old school engine on there and I think the the results would have been much more interesting.

I've been burning VP C12 (108 octane) much longer than any compression ratio, static or dynamic, demanded it. Since 1997 actually. Iron heads, aluminum heads, 10.5:1, 11.5:1 CR...didn't matter. What I saw was the 16s needed more timing than the aluminum heads, all else being the same. (The iron heads were maxed out for racing without getting into epoxy) The sweet spot for iron was 34 degrees, but I found no change in the car's performance all the way up to 38 running 108. There was no "need" for race fuel, as pump 93 would surely support that level, but I ran it anyway, despite some telling me I was losing power because of the slow burn.

What I can tell you is that, as already mentioned, quality and consistency is excellent across the VP line. (the only one I can speak to) Even better? When I disassembled my 20 year old fuel system to replace in 2017, it all looked brand new. The rubber in the braided lines never broke down and if I didn't tell you, you'd never know how old any of it was by looking. Neven an issue with long term storage and never an issue with gumming anything up.

There are more benefits to race fuel than just power. Since I only burn about 85-90 gallons a year, its worth the cost.

[nUcLeArEnVoY]

I run a NOS AC Spark Plug Division mechanical fuel pump on my '79, so I dare not use any ethanol fuels since they'd eat up the diaphragm. Only REC-90 touches the inside of my tank, but even today's non-ethanol gas is still formulated differently and of higher volatility than the fuels of yesteryear. If my car sits even for a just a few days, I can usually expect 4-6 seconds of cranking on a cold start because of evaporation. (Using the original Qjet.)

[Stuart]

Originally Posted by padgett

What this makes me wonder is "what changed ?" Does the E10 make 2021 gasolines all burn at the same (faster) rate ? Is the only reason I can think of where all "octanes" now want the same (lower) advance.

Today the octane rating on the pump is (R+M)/2:

The (R+M)/2 you see on the label refers to the average of the research octane number (RON) and the motor octane number (MON) ratings. To determine the RON, the fuel is tested under engine idle conditions with a low air temperature and slow engine speed. To determine the MON the fuel is tested under the more stressful conditions of higher air temperature and engine speed.

I think the number on the pump was different back in the early 1970s, but I don't recall exactly how that number was calculated or when things were changed.

[KEN CROCIE]

1970- ron only on the pump

[amcmike]

Did they use a GEN1 SBC, and which heads? Head design can also play a factor in how much advance is needed to make peak power.

[Schurkey]

Flame speed of air/fuel is surprisingly slow, unless there's turbulence in the combustion chamber.

Build an engine with crappy turbulence, you'll have slow flame speed and need extra advance. More energy goes to heating metal, less to making pressure that pushes the piston down.

Get the squish/quench right, you have increased turbulence, and need less timing...and lower octane rating.

Years ago, squish/quench was either a big secret, or just plain un-known outside of labs. Now it's all over the internet. Of course, there's more to turbulence than just squish/quench...but that's a secret.

[Formulas]

How many times in the show did they state that if your engine is being fed proper octane then additional timing would not show gain with higher octane ?,
I didn't count but more than once

And that only if you were running insufficient octane with a timing retard crutch would you see a gain with more octane and timing

They used 87 91 110 116 and E85 ,,, moved timing slightly and AFR. on all to verify And all octanes provided max power at 29 and that fact debunked the fast / slow burn rate in relation to spark timing
E85 was the only appreciable gain at 29 timing still and obviously different AFR. the power claim there was more oxygen in the fuel

Technically higher octane might burn slower to the micro degree but when it comes to spark timing no change in their LS based test mule

nothing burger

.