MotorHead

I have been experimenting with 91 octane and 91/93 mixed 50/50 in my 11:1cr 406ci. I usually run Sunnoco 93 but it seems the 91 is OK as I do not seem to have any detonation/pinging even on real hot days.

Yesterday I put a full a tank of Sunoco 93 back in and I noticed or thought I noticed it was running smoother. So the question is, while it does not ping on 91 does it run rougher, will 2 points in octane smooth the motor out ?
Or is this just my imagination like and it is like putting in 9 dollar spark plugs and you "feel" you have more power ( not saying I have more power with the 93 octane, just runs smoother ) ?

Techno

I'll get beat up for saying this but your engine right now is detuned. Since its running on 91 ok its detuned even more than it needs to be. You could run it on 83 if you wanted to.
There is no pump gas sold that will supply 11.25 or 12.5:1 unless you live in arizona or nevada, forgot which but is 100 octane.
Maybe delta sonic, they used to sell that pearl gas at $4 per gall.

IF this worked Auto makers would build hi comp engines and racers wouldn't buy racing gas, money better spent on engine parts.

Now I know someone is going to claim they run diesal compression on pump gas with normal timing.

MotorHead

Yup you might get beat up, new Z06 is 11:1 CR just like mine

Techno

My bad. The zo6 engine is just like yours but the ls2 is 10.9?
How did you manage it?

big_G

Aluminum heads "I have heard"' let you run about 1 compression point higher than cast iron...less hot-spots in the chamber?

tmoneymoney4

with both of my muscle cars i run 91 all the time. when i run anything lower it seems that the engine continues to run after i turn the ignition off... with the higher octane gas i never have that problem. so in my experience i would say that a higher octane does help the engine run smoother.

VETDRMS

Hey Wayne, My motor is at 10.7:1 and I run 91 octane all the time. It's the only gas I can get here. Being at altitude helps a lot, I live at 3500' and drive over 6000' in some areas.

I ran some 110 octane leaded race gas once and it did run smoother, but at the track it made absolutely no difference in time. I ran the 110, made some passes, switched to 91, and no change.

MotorHead

It is the general consenus around here that 10:1 for iron and 11:1 for aluminum heads will work. That is no guarantee that you will not have any detonation problems but the majority of us running these compression ratios don't have any problems.

One thing that is commonly overlooked is the vacuum advance. This can add timing to you advance at part throttle leading to over 50 degrees total advance in some cases and this is where you are going to run into pinging problems. I don't run a vacuum advance and I have had my timing up to 40 degrees ( by mistake ) when I first put the motor in with no pinging.

Travis, if I only had 91 I would no know the difference, and as you said I don't notice any increase in power, just seems to run smoother

My original thoughts on the topic of octane was that you should run the lowest octane you can if it doesn't ping, and running a higher octane above that was just a waste of money, but I may have to re-think this.

It does seem that my motor runs with no pinging etc. on 91 but it also runs better on SUnoco 94 ( made a mistake in my original post said Sunoco 93) so I guess I will continue with the 94

rponfick

If you are half deaf like I am, you couldn't hear it pinging anyway. So, run it hard until the engine blows, and don't sweat the small stuff. As you say, altitude covers a multitude of sins.

Techno

There is a misconception about octane me thinks. High octane gas burns slower than low octane gas. Ping are explosions. Fuel in an engine burns not explodes. High octane fuel is a "detuning" of the fuel.

If you pump in 93 when you've been running 85 then all your doing is reducing hp so yes the engine does run smoother.
If your compression is high enough to take advantage of the increase in octane then you can increase timing producing more hp. An early high compression shark goes to the track and finds 1970 pump gas, you call it race gas. Now it can retune eliminating the previous detuning that was done. Yet at 11.25:1 With or without aluminum heads these engine do run on 93 octane. low octane for the time.

If you have an aluminum headed for the street high compression engine then it must have 93 octane and it must be timed for this fuel.
If you switch to 91 or 92 then it will ping. If it doesn't then it means your timing is retarded, this is called detuning. If your mixing or using low octane fuel in this high compression engine then your engine is retarded, detuned, making less hp than it would if it had a lower compression ratio to run on this octane. BUT if you increase timing when you add 93 octane then hp goes up. Your optimizing compression and octane requirements.

You cannot make a change on an engine and not change something else. On the C6 it can run low octane fuel and the computer is making changes for this fuel and it also loses hp.
Octane and compression go hand in hand.
Everything is a compromise and compromises must be optimized to make any sense in making them. Running 12.5:1 compression but fueled with 83 octane, whats the point in this extreme? It will run but produce less power than if it had the proper compression ratio.

MotorHead

Some good points there. When building a street motor with a limited budget you must leave yourself a little safety margin in case you can't get the gas you want in some places.

It seems to be generally accepted that older small block chevys operate the best at 34 - 36 deg advance and that is where mine is. So, in my case the compression ratio I picked was due to the specs on the cam and since I will not be advancing the timing any more so right now it runs on either 91 or 94 octane.

It is entirely possible however that it is making less power at 94 octane than at 91 ( reason it seems to run smoother ) something I have to look into

VETDRMS

In regard to timing, I tried the higher octane (110) fuel specifically to test how timing would effect my ET. I ran 110 with timing at 34,36,38, and 40 degrees of total timing.

The result: NOTHING, my ET did not even vary by more than a couple hundredths of a second. Yes, there is a difference, but in anything but an extreme case (12.5:1 on 83) the difference will be minimal.

Simmo

My understanding of octane ratings is: Higher octane fuels resist spontaneous combustion better than lower octane fuels, rather than high octane fuel being slower at burning. VETDRMS motor ran the same mph with 91 and 110.

stingr69

I agree. High octane fuel does not "burn slower". High octane fuel is better described as "more difficult to ignite" so hot spots in the combustion chambers due to carbon buildup or other reasons will not prematurely ignite the fuel. The same can be said about high cylinder pressures causing spontaneous combustion. The high octane reduces the tendancy to spontaneously combust when the pressure gets high.

-Mark.

big_G

Here's some light reading for ya....http://auto.howstuffworks.com/question90.htm

Solid LT1

The one poster is correct, high octane fuels burn at a "slower" rate (although it is still really fast.) The motor should run smoother with higher octane if the timing advance is near the correct level for the engine's optimum power output. I wish we could buy 93 octane in California at the pump but, we have only 91 octane in our "great state". The higher octane should let you run a more agressive spark advance but, there is a potnetial to get better power fom the lower octane fuels and also the potnetial to cause serious motor damage if the timing isn't absolutley correct for the motor's requirements. It is better to play it safe for long engine life like the OEMs in Detroit do.

mareze

I usually put in Sunoco 94. I found a gas station that sells 110 and noticed a difference in the smoothness. And yes the sweet smell of yesteryear, Leaded gas....