High or low octane gas, is there a difference?

[Spline64]

Hello,

Hoping you can all chime in on this one. Want more opinions than what I have read and heard so far.

Have a '73 454 with mild cam (no duration specs sorry), headers, edelbrock performer 2.0 intake and a edelbrock 1405 600cfm carb. Otherwise a stock motor.

Engine had about 10,000 miles on it after rebuild when I put it in my truck. I have been running 91 octane in it because I basically didn't know better. Thought that I would get better performance that way but I guess that isn't quite true.
I'm at 4,750 ft. elevation here and it was proposed to me that I'm basically wasting my money, 87 would perform the exact same due to the lack of oxygen. I had ran a tank of 87 before and thought I noticed a loss of power and rougher idle, but that could have been just me. I did not notice any pinging at all. Timing seems to like to be 10 deg. advanced (initial)

I'm going to fill up with 87 this weekend and really pay attention to performance, smooth idle, ect.

So does octane actually help with horsepower? Or should I go with a lower octane and tune the carb accordingly?

Thanks,
Pat

[johnwaynejeep]

I can't offer opinion on whether your specific engine combination will do better with 87 or 91 octane.

I do know that every motor I've ever had produces more power with the 91 octane over the 87 octane. In my 99 S-10 w/4.3L V6 it was a noticeable difference in torque getting off the line. In my 2007 Wrangler w/3.8L V6 I notice a large gain in torque when using higher octane gas, as well as better fuel economy. If I use 87 in my Wrangler I can hear the engine deisel when I take off in 1st or 2nd gear, the only way to stop it is to mash on the gas pedal while starting off. With 91 octane, I can ease into the gear with minimal gas and it will not deisel at anything over 500rpms.

The basic idea is, higher octane produces a better/hotter burn and therefore more power.

[SkinnyG]

If an engine was built to run high-octane gas, then you need to run it.

If the engine has been tuned for high-octane gas, then you probably should run high-octane. However, it has been my experience that tuning the engine to run optimally on the lowest octane gas it can run, brings the smoothest operation and best fuel economy. Ignition timing is the big player here.

Contrary to popular belief, high-octane gas does not produce a hotter burn, it is merely more resistant to pre-ignition. Some say it burns slower, some say that is no longer the case.

More resistance to pre-ignition allows you to advance the timing and gain more power.

[johnwaynejeep]

Originally Posted by SkinnyG

Contrary to popular belief, high-octane gas does not produce a hotter burn, it is merely more resistant to pre-ignition. Some say it burns slower, some say that is no longer the case.

Thanks for the correction Skinny . I'll keep this in mind...pre-ignition, not heat.

[Dead Parrot]

For non-computerized engines, run the lowest octane that doesn't result in ping.

Computerized engines often have a knock sensor and will retard the timing and/or enrich the mixture to eliminate the knock if you run an octane lower then the engine was designed to use. Thus the lower performance and worse mileage.

I have ran the 84 octane stuff you used to get at 9000' in my 350 -260hp. Even back at the about 1000' OKC is at, didn't have any pinging.

Don't know if it is still true, but higher octane gas used to have a slightly lower energy content per gallon compared to lower octane but that was out weighed by the better efficiencies a high compression engine delivers when used with the proper rated fuel.

[Just call me Sean]

Fuel injected vehicles are programmed to run on 87, unless it's a Corvette or something like that.

[meatwagon83]

depends on you compression ratio really, in a 73 454 with its 3-1 compression i would say anything more than 87 is wasting money, put 87 in an engine with 11-1 and it will destroy itself in short order!

(btw the 3-1 is an exaggeration, smog 454s came in with about 7.75-1)

[donut]

Try it, if it pings then it won't cut it. Don't know about tuning for altitude. Was the compression upped during the re-build?

[BigBlocksRule]

Some computer controlled engines will actually advance timing with higher octane gas, but only a little bit, a degree or two. My '97 Tahoe has 10.1:1 compression, I run the lowest priced gas I can find. I ran the high $ stuff for a while, dropped to junk gas at the encouragement of my engine builder. Runs no better or worse, maybe faster since my wallet is lighter. I can beat on this thing all I want and it won't ping.

Octane is a rating that tells a fuel's resistance to detonation. That's all. Race gas actually burns slower than pump piss. I've seen cars at the strip that were built for pump gas actually slow down with race gas. There's the same amount of BTU's in 87 as 93, just less impurities in the expensive stuff.

[trixter99]

Run what ever fuel doesn't make it ping,don't over analyze it.

[Malo83]

My computer smog master runs it's best on 89

[KQQL IT]

My 406 pings and diesels on 87
Tolerates 89 no pinging and the occasional diesel.
And does none of it on 91
9.6:1 with a hydraulic roller cam and dart heads

[INSIDIOUS '86]

Compression, squish, mixture quality and heat play a factor in determining octane requirements. An engine with a tight squish has good quench qualities is more resistance to pre detonation. Iron heads retain more heat in the chamber which makes hot spots more prone to happening where as aluminum don't retain thier heat as much. This actually makes them less efficiant but that quality is overrun by a substantial increase in compression tolerance which let's you run higher compression for an increase in power. Say going from 9.5:1 to 11.5:1. Every point in compression gain is usually worth 10hp/ftlbs TRQ.

High output 305s from the 80s were known to be intolerant to low a crane gas. This was due usually to the factories higher compression and poor quench along with high running temps. The deck hight usually out the piston .024-.028 in the bore and the head gaskets were .028 thick. If the deck hight was at .025 like normal a .028 head gasket would yield a nasty .053 squish. Proper squish is in the .039-.043 area. Gm used to run .015 steel shims on everything which ran quench at a .040. You notice most head gaskets are .039-.041 range. That is on purpose because they are for rebuilds with the block being decked.

Notice compression increased with the vortec motors? They gave a better mix and the quench pads were more efficiant. Hence tolerating lower octane levels.

As for running better with higher or lower octane levels. There's a reason for that. We know that rich mix fuel burns faster and lean burns slow. This is where timing comes into play and vacuum advance. When your going down the hwy your engine is only making 40-60hp with a light load so you don't need a rich mixture like you do on heavy acceleration. So with rich burning fast and lean burning slow the vac advance comes into play on the hwy your lean fuel mix with high vacuum so the vac Adance is at full position with a lot of timing. When you floor it vacuum drops and timing drops off for the richer faster burning fuel mix. Higher octane fuel requires compression to get the flame front to travel as fast as lower octane. High octane is slower burning and harder to ignite vs lower octane.

So in a nutshell if you feel a difference in power with octane it really only means that your timing for the most part isn't ideal for your fuel type.

And factory ignitions are set really conservative on advance curve and initial timing which doesn't help things along with a vacuum advance mechanism that adds A LOT of timing.

[Spline64]

Thank you all for your replies

Well I did run a half of a tank of 87 so far and have noticed a hell of a difference....in sound.
What I used to hear occasionally and honestly thought was due to the exhaust system, sounded like a soft diesel sound. Like what you would expect when you hear a diesel engine idle (again soft, barely audible) or at low RPM except that it usually happens to me at third to half throttle, only under load. At idle or any revs in park, you will never hear it. This has become far more pronounced since the switch to lower octane. I fear this baby has been pinging the entire time. I just haven't been able to hear it do to the loud exhaust and the fact the pinging sound was not happening as loudly as it is now.
I have since retarded the initial timing down to 4 deg. BTC (from 10 deg.). Took it out for a test and the "pinging" noise has become less, and you have to go at least half throttle before you hear it. I have not changed the fuel or added any octane booster to it yet.
Guess I need to check compression as suggested, maybe not the 9 : 1 that I thought it was?
For now, going to up the octane to see if the "pinging" noise goes away, now that I know what I am looking for.
I had no idea what octane did until now and how so many variables can effect the mix your engine actually needs.
And I reallly.....really hope I didn't mess this engine up.....

Thanks,
Pat

[INSIDIOUS '86]

Your engine could be filled with a lot of carbon deposites that can cause hot spots and raise compression. 4* initial timing is pretty low. 10*-12* initial is about normal for stock-mild sbc and bbc engines. You can also try running a 180* thermostat instead of the factory 190* which will help curb detonation some since your in a pretty warm part of the country

This article here is worth reading through as well
http://www.crankshaftcoalition.com/w...EI_distributor

[rich weyand]

What plugs are you running? You could switch to one or two step colder plugs. Note heat range has nothing to do with the spark. A colder plug has more contact surface between the tip and the threads, so that heat conducts out of the plug to the head faster, while a hotter plug has less. If the plug is running hot enough to be glowing when the charge compresses, it can aggravate detonation problems. If it's running too cold, it can carbon up.

You shouldn't be detonating with stock heads at 4750 feet and 10 degrees base timing!

You didn't by chance put hotter plugs in thinking that hotter meant higher performance, did you? Or running super-platinum or ultra-gonzo or something? A little net poking around makes me think standard AC Delco plugs in the 43 heat range is right. Whatcha got in there?

[BigBlocksRule]

If it's a stock rebuild, you're not even at 8:1 compression. Unless your chambers are full of carbon, you ain't detonating from compression, particularly at 4000 feet above sea level.
Easy way to clean chambers uses water and a spray bottle. Run it at around 1K rpm's, then spray the water down the carb in a mist. As long as you're not pouring it in, you'll be fine. I spray until it slows the engine down noticeably, then stop for a bit. The water expands when heated in the combustion chamber and knocks the carbon off of the pistons and chambers. If carbon is a problem, you'll see the black soot coming out of the tailpipe.

[Spline64]

Thanks again all!

Rich,
Running Autolite 26's gapped to .035 as that is what was in there when I got it. It has always ran strong so never thought about plugs.

Sid,
I agree and have tried different timing values. 10 Deg. has always been the "sweet spot" for performance and just a good idle. Maybe I'm not hearing "pinging" at all but it concerns me that it only happens in gear (under load). I can rev till the cows come home or hold at steady rpm's in park and never hear a peep.
I thought 4 Deg. was too low but now scared that I may be damaging the engine. Better safe than sorry while I figure this out I guess was my motivation to retard it. Exhaust is quite loud on this truck. I just wish I could hear it better, but sitting on top of the engine while my son drives is not an option at this point.

BigBlocks,
The engine has about 11K on it now. Do you believe that a carbon build up that bad could be produced in that short amount of time? I can't vouch for the first 10K only the last 1K. I'll try it to see if I see the black soot just in case as I don't know about the first 10K (hell, if it was even 10k).

The motor has always been strong. I bought it to pull my 3K lb. trailer and it does so wonderfully up a 6-7% grade. That just reminded me, while doing that at 10 Deg. initial, when the secondaries open up, I'll I hear is the sound of an engine working correctly. No clanking or knocking at all.

[rich weyand]

Originally Posted by Spline64

Running Autolite 26's gapped to .035 as that is what was in there when I got it. It has always ran strong so never thought about plugs.

Try a set of Autolite 24s and see what you get. The 26s are pretty hot. The flat track boys around here all run the 24s in their BBCs. They cross reference to a R43TS in AC Delco, which is what the bulk of net posters seem to be running in BBCs. Gap at .035 is fine.

[SkinnyG]

What ~total~ timing are you getting?

[Spline64]

Sorry for late reply...

Actually don't know the total timing. Guess I need to purchase one of those "dial" timing lights and get it up to 3500 rpm. Can't troubleshoot without the right tools! May have an issue with the vacuum advance?

Attached is an example of plug wear after changing to 87 octane from 91 and changing initial timing to 4 deg. from 10 deg. BTC on one tank of gas (20 gallons). Sampled 3 plugs and all look the same. First pic is with a flash, to the naked eye, they all look like second picture. Engine is not burning oil at all. I wish I had an example from running the 91 octane but all I can tell you is it had alot more (fluffy) black carbon deposits.

I'm thinking ok right?

[rich weyand]

Switch to Autolite 24s. Your plugs are too hot. They're like $2 a piece.

[Rich84]

So a R43ts plug is a cooler plug??

I am using them and my plugs look like those too.

[Dead Parrot]

Here is a link to a plug color chart. http://1.bp.blogspot.com/-BpC43BNUt1...plug_chart.jpg

There are many versions of these. The last two OK plugs toward the bottom are a bit lean/hot for my tastes but it should give you a good starting point.

Your best bet is to install the plug specified by the engine data plate, owner's manual or engine builder and adjusting the fuel mix to obtain the proper plug color. You can mask a too rich or lean fuel mix by installing hotter or colder plugs but the real problem remains.

The black fluffy carbon deposits the OP mentions are likely caused by a too rich fuel mix. A lot of short trip driving can cause that as well. One irony is that a rich mixture is less prone to detonation but can cause carbon build up which increases the chances of detonation.

[rich weyand]

Originally Posted by Rich84

So a R43ts plug is a cooler plug??

I am using them and my plugs look like those too.

Yeah. R43 ( ~Autolite 24) is colder than R45 ( ~Autolite 26). If they look the same, then you are not running too cold: they would carbon up.

Most BBC recommendations are R43 series. Running R45 series plugs (as recommended for SBCs) or Autolite 26s can cause detonation. The plug runs hot enough ignite the charge without a spark when the charge gets compressed.