Hi everyone

I've got two questions regarding the timing setting on my 1977 L48 (no emissions and true dual exhaust):

1) Here in Belgium the standard fuel is rated at 95 ('EURO 95'). The premium fuel is rated at 98 (SUPER 98).
I read that my L48 was made to run on 88... So will the 95 fuel allow me to set the timing for better performance, or won't the octane rating have any effect on the timing?

2) What would be the best timing to run an engine as cool as possible? Retarding or advancing?
A friend of mine said: "The more horsepower you create, the hotter the engine will get." I'm confused... Is this correct?

 

Read this thread and after you read it email lars for his timing papers.
https://www.corvetteforum.com/forums...stock-l48.html

 

You can reset timing for better performance with the stock L-48 engine...and still run on 'regular' fuel. My distributor has been re-curved and idle timing ends up at 12* BTDC. Runs fine on regular fuel. Not sure you will get ANY benefit out of running premium fuel in a stock L-48 engine. It will just cost more....

 

It is the more horsepower you use that causes heat.......my 500hp 406 runs nice and cool at 175-180 in Texas heat cruising around....if I were to try and drive this car flatout for 5 miles it would need a better cooling system.......
Both excessive retard and advance can both cause high EGT's........you set timing where the engine is most efficient. It is not a question of "advance or retard" it is where it is at now and where to put it to make it happy.
You use the lowest octane requirement your engine will allow to make the most power.

Jebby

 

So many mistakenly seem to think higher octane fuel is a better quality fuel though I just commented about this in an MPG thread in the general section the other day. I think a lot of folks are throwing their money away on higher octane fuel with no benefit.

 

not only that, it produces less power two fold.
1. because the high octane fuel is not burning as completely it is therefore exiting via the exahaust valve prior to complete combustion.
2. Since the timing is not optimized ( advanced enough) peak pressure during combustion is reached after 15* ATDC.

 

Do you have any technical documentation to support that statement?

 

Not anything I could easily find,scan in and post up. But google is your friend and you can find info on the subject if you want to.
Once you throw variable valve timing, variable ignition timing, port fuel injection and knock sensors in the mix the rules change.
It's not an often discussed subject anymore, if it ever was, due to the technology of the modern engine.
It's not like it's a big factor in HP and torque, but if every little bit adds up to a big bit then it's part of the whole power production picture.

https://nasaspeed.news/tech/engine/o...es-more-power/

 

I've seen that link before. I agree with the first note in the comment section. The test is meaningless. I've looked before, and there are countless posts and articles on the internet written by shadetree mechanics/racers, and car magazine articles written by guys with journalism degrees repeating the higher-octane/slower-burn-rate relation, but I have never seen any serious engineering/chemical-engineering sites/books state that octane and burn rates are connected. The closest statement I have ever been able to obtain from a credible source was a conversation with a fuel supplier representative at the PRI show several years ago. He stated that each fuel's burn rate is somewhat different (due to the components/additives), but there is no straight relationship between octane number and the burn rate.

If you have better luck than I have of finding a credible source regarding this subject, I would enjoy and appreciate you sharing the link.

Thanks.

 

In order for a fuel to receive a higher octane rating it has to be more detonation resistant by definition, correct?

more resistant to spontaneous combustion through heat and pressure. More resistant to igniting too easily you could say?

 

Yes. But none of that describes the (normal combustion/non-detonation) burn RATE.

 

Hot Rod’s “Engine Masters’ has a GREAT video on comparing octane ratings to hp. Same engine on a dyno starting with regular and going up to race fuel. As long as your engine doesn’t pre-detonate the power is the same. Optimizing timing made a significant improvement. They did point out how pre-detonation is much less likely to occur on the dyno than if the engine being used hard and getting hotter.

 

Was the EM engine 11:1 scr? Probably not. But if you increase the octane, you can change the timing and thus gain power for higher compression engines. The calorific value of gas (the amount of energy it contains) does not significantly change by adjusting octane level. But higher octane can allow for a better timing curve to optimize engine power on high compression engines. Put in 87 octane and it will ping so bad the timing will need to be retarded, thereby losing power because the curve is no longer optimal. That's what happens with newer high compression engines that run on 87 octane vice the recommended rating of 91 or 93. The engine detects knock using 87 and will retard the timing. Under the exact same load condition with 91 octane, there won't be any knock and hence the timing isn't retarded. Yes, octane rating will effect an engine's potential maximum power given high cr's.

 

You pose an interesting observation.
And are correct in that I can find no specific evidence scientifically supporting a faster burn rate other than it's simply stated that higher octane is more resistant to ignition and has a slower burn rate and that lower octane is more volatile and burns easier, hence faster. I would like to find a graph of some sort showing the faster rate of burn and the slower rate of burning of various octanes. Have not found that yet.

The energy contained in various octanes is identical if they are all pure gasoline vs the ethanol enhanced octane rated fuels which in fact do contain less energy due to the lower energy density of ethanol.

As resdoggie points out higher octane allows for us to take advantage of earlier ignition netting higher pressures to produce more power. However with diminishing returns once you have ignited the fuel so early that peak cylinder pressure is reached too soon (prior to approx 15 ATDC) thus creating more negative work.
So although higher octane allows for higher cylinder pressures and earlier ignition it cannot be lit too early despite it's ability to function under those higher pressures without detonation.
Thus a lower octane fuel may be of at least financial benefit if peak cylinder pressure is low enough due to air density, CR or cam dynamics.

I'm gonna try to find a test or graph that does in fact show the burn rates if I can. I would like to see some hard evidence of that as well now that you bring it up.

 

As long as your engine isn't detonating, use the lowest octane you can. This creates the most HP. Lower octane fuel has more power, as they said. They have to lower the fuel power for high compression engine's.

 

I love that show..

 

Your engine is probably close to 8:1 compression if it’s stock, you will be fine using the lower octane fuel. I swapped 64cc heads on my l48 with a HE268 cam and recurved distributor and it ran on US 87 octane. I think I calculated mine at 9:1 compression with 64cc heads. Your 95 octane is like our 91.

 

I have 64cc iron heads and flat top pistons in my 355 wit 10.15:1 compression and run 89 octane with no issues.

 

some information for you to ponder. Not a graph or test just scientific information.

gasoline is comprised of linear chain compounds and branched chain compounds.
there is a greater ratio of linear chain compounds in lower octane fuel vs higher octane fuels.

linear chain compounds have a higher combustion speed than branched chain compounds.

https://link.springer.com/article/10...430-013-0094-y

Octane number (in this case RON) follows several general trends. Normal paraffins have the lowest octane ratings, and the longer the hydrocarbon chain, the lower the octane value. Branching has a tendency to increase octane number, and the more extensive the branching, the greater the improvement in octane rating. The octane number of an olefin is better than that of its corresponding paraffin, and an internal double bond results in a greater increase than a terminal double bond. Aromatics are by far superior to their aliphatic counterparts.

https://www.sciencedirect.com/topics.../octane-number

Interesting info I thought. And what I could find on the web for free. Quite a lot of reading on sciencedirect.com if a guy wants to spend some time reading it all.

 

Running advanced timing without detonation is going to affected by more than just CR of the engine. If you do not have sufficient quench action, run too hot, too lean, too low of gearing, low dynamic CR, etc you will run into detonation despite your low CR if the octane rating is too low for the cylinder pressures achieved.

to answer the OPs question, yes you can likely advance the timing to something more optimal with a higher octane fuel with the goal of achieving higher cylinder pressure for more power.

combustion chamber turbulence is very important for avoiding detonation. The stock L-48 has very little (none perhaps) quench therefore very little turbulence, and will run into detonation much sooner at lower cylinder pressures than an engine with good quench action.