Hi all,

Had a bit of hesitation and what I thought might be slight knock in my '79 (L48 46,000 miles). So, I fed her some 93 octane gas at most recent fill-up and have noticed significantly better performance.

Now, the engine purrs, no hesitation, and any knocks I thought I might've heard are gone. As well, there is more zip when on the accelerator.

To be honest, I was not expecting such dramatic improvement. Which leads me to a couple of questions...
1) Should I have expected this type of improved performance? And...
2) If not, what does the improved performance with higher octane gas suggest?

Just curious as to what others' experiences and thoughts are.

 

I think you may have just gotten lucky to use a gas with a different set of additives and cleaners that are working better with the particular varnishes and deposits in your car's system.

I forgot when, or where, but on one list a petrochemical engineer got on and explained how gasoline is basically the same except for the additives. If you run the same brand for 20,000 miles, eventually these additives will become the next type of deposit you have. His advice was to run one brand for awhile (don't remember how long now) then switch to another brand for the same amount of time, then switch to another.

The logic is that the cleaners in one brand attacks the deposits made by the previous gasoline until eventually, it is clean as it will get and not the new additives will become deposits.

At that point, you need to change brands so the next brand cleans out the previous brand's deposits and starts the cycle over.

Actually, there is less power in 93 octane than 87 octane. The only reason it is needed and makes more power in high performance engines is if the 87 detonates, then you lose power. If you can run the 87 without detonation, then you get more power with it.

Lots of guys used to run 110 and 114 at the track, instead of just filling up with pump gas for their IT race cars, which ran stock compression. They never seemed to be in the lead.

The ones that did usually cut the 110 with 87 or ran 93 and advanced the timing some.

 

Octane rating relates to the the gasoline's ability to resist knock. It's actually HARDER to ignite, so it usually offers no performance increase in an engine that doesn't need it.

An engine may require higher octane fuel if it has higher compression or advanced timing.

1) I don't think a L48 has enough compression to require it, but the compression could increase if the engine has a lot of carbon build up in the cylinder. If excess carbon is your problem, you can run Seafoam or GM Top Engine Cleaner through a vacuum hose. Insert vacuum hose into the bottle with the engine running and shut it off once it sucks the bottle dry. Let it sit overnight and then take it out and run it at highway speeds. NOTE: Your car will smoke like crazy for a bit! Don't worry, it's normal. Change your oil after doing this.

2) Check your ignition timing.

 

my daily driver is a 95 mercedes e320. it requires premium fuel. i have tried mid grade and didnt so much notice a difference in power as much as i did in fuel mileage. runs smoother and gets alot better mpg on premium.

 

Could be that the octane you were running included ethanol in the mix which produces less power per unit of fuel. The 91/93 octanes are usually pure gasoline so you would see an improvement over an ethanol blend if you were not jetted properly (richer) for the ethanol blend.

 

The owners manuel for my 1974 L-48 states to use 91 octane.I use it and the cars runs beautifully....not sure if this would have changed by 1979 GOOD LUCK

 

was towing my 1969 with my 2006 Ridgeline to run the car at NJMS. As per the owner’s manual, I filled the Ridgeline with 93 octane to tow. I used the tank if gas to get there. For the return trip, I used regular to see if there was a difference. I was only half way home and used most of the tank.

My assumption is these computer driven cars, sensed detonation, and retarded the timing. That knocked the bottom end out of the performance and gas mileage.

In the case of your car, unless you were running a lot of advance to the point where the car was a lot of detonation going on, I have to agree with Procrastination Racing that you either had a bad tank before or that brand works better.

I can tell you from my last rebuild, detonation has a miserable affect on pistons. The detonation caused the tops of my forged pistons to chip. This engine has a lower compression ratio.

 

I think this is what you are referring to - I once posted this link on a previous CF thread: http://www.vettenet.org/octane.html

Not everyone found it useful (particularly Mike Ward)

 

If I were to guess it would be one of 2 things.
1. Your motor isn't a stock CR anymore so the higher CR demands higher octane
2. Your timing isn't stock anymore and demands higher octane

OR even both..............

 

Must be it. I probably still remember it from when you posted it on VetteNet.

I pay attention to all that kind of stuff. You never know when you might need it.

Wow, that was long ago.

 

My 76 L48 with true duals, running 14 degrees before top dead center shows no difference between regular and high octane. I've even bought the race gas at 100 octane I think it was and mixed as recommended and could tell no difference with it. The engine compression is only 8.5 to one anyway so it's unlikely that octane alone would do it. Perhaps the additives as suggested here were more responsible for it. I have a 2008 LS2 6.0 liter running 420 hp that is much higher compression and does require 92 octane so I buy it for both then if I put five gallons in a can for home I can use it for either car, the riding mower which "really scoots" now...just kidding but they all work on it. I have been unable to identify any feel or power increase going from 87 to 92 or 93 octane gas with my very tight, well running engine and four speed. I am running a 3:36 factory rear end and without getting into the engine and caming it up, new big valve heads, etc. if I wanted more punch I think I'd consider replacing the rear end with something in the 3.73 range. My other one with the LS2 actually has a 4:10 rear end and it has serious punch even though the vehicle weighs 1200 pounds more.

 

The thing about octane is, all you need is enough. Any more than enough doesn't do you any good. So, from what you're telling us, 87 octane isn't enough. A bone stock L48 should almost be able to run on kerosene, so you may be looking at an engine that has had some mods done to it. Or it may be carbon build-up. You don't need to go out and buy Sea Foam, or any other kind of snake oil, to clean the combustion chambers of your engine. All you need is some plain old water. Take the air cleaner off, while it's running, bring the rpms up a little, like a fast idle, and jam something easily removable in the throttle linkage to keep it there. Take a spray bottle full of water, and spray the water into the primaries of the carburetor. Not too much, you're not trying to drown the engine, just a little bit to get some steam going in the combustion chamber. DO NOT JUST POUR A BUNCH OF WATER DOWN THE CARB THROAT!!! HYDROSTATIC LOCK DOES VERY UGLY THINGS TO INTERNAL COMBUSTION ENGINES!!! Have you ever removed a cylinder head to replace a blown head gasket, and seen 3 dirty combustion chambers, and one squeaky clean? That's from the coolant leaking into the combution chamber while it's running, cleaning the chamber. Carbon build-up may not be your problem, but it doesn't hurt to have nice, clean combustion chambers.


Keep the shiny side up!
Scott

 

An interesting read. Your credentials are solid and I'm not contesting whatever testing you have witnessed/been a party to, but I think more variables may be at work than can be introduced or perhaps were introduced in the testing done.
I would guess that atomization of the fuel and air fuel mixture would heavily influence the deposit outcome (hence part of the difference in PFI vs carburetoration?). As well as things like combustion pressure ratio, turbulence during combustion, and combustion chamber efficiencies would all play an important part in the deposit result.

Sure the quality of the fuel would make a difference but I can't see how using 91 or 93 octane would confirm the quality being higher. I understand that the refinement of higher octane is better and there are fewer contaminates and such in higher octane fuel but how does that equate to fewer deposits in the engine? If the contaminates are kept in solution and not allowed to burn onto surfaces by good combustion chamber turbulence and flow then that should alleviate/mitigate the Problem... if one exists at all.
In the read it refers to deposit levels in percentages only. I would guess this is due to the variation in deposit levels for various engines and combustion chamber designs. That being said it the deposit level at max is only 10 thousandths of on inch on the exhaust valve and that has no discernible effect on the performance of the engine then what is the issue?
I didn't really see anything there that would cause me to want to switch to 91 or 93 octane for a 8.5:1 CR engine that runs without ping on 85 or 87 octane. To each his own.
I had a Suzuki that I rebuilt at 183,000 miles. During it's entire life I ran 85 octane from the same station. On tear down the valves and heads were incredibly clean. Almost like new. I understand that it was a PFI engine and therefor was much more accurate in the fuel delivery, but if I am to believe the reading there should have been some build up right? There was none. Or at least so little that the metal of the valve was visible under any deposit build up. The valves were worn but did not suffer from any deposit problem. Combustion chambers were the same. What dictated the tear down was failure of the cylinder wall and some loss of compression on #1 cylinder.

 

Your '74 requires leaded fuel. Hard to compare leaded to unleaded as the lead increased octane. I believe 91 was about as low as you could get in leaded fuel back in the day.

 

The way octane was calculated back then changed. 91 then is 87 now. No need to run premium in an L-48. Even a 74

 

The 74 can run either leaded or unleaded, it didn't matter as the heads were low compression and had harden seats....That started in 71 when GM lowered compression ratio's to use either leaded or unleaded gas while getting ready for the cat converters for 1975.
75 was the first year that required "unleaded fuel only"

 

I think that there are alot of variables that have to be taken into consideration, such as: where you live; the climate in your area; above, below, or at sea level; the state of tune of your engine; your driving habits; etc..
I know that with the performance vehicles that I have had over the years, have run better with higher octane than with lower. Call it the SOP test or whatever you like, you get to know your vehicles and how they respond in different conditions, and with different fuels.
With our 2003 Vibe, that now has 112,000 miles on it, nothing but 87 and it runs great. But I'll never use that in the 82, or the 74 when it's completed.
Yes, gas is expensive, but I look at it this way, I drive the 82 everyday, want it to perform at its peak, no matter what. I don't want to hear any pings, or feel any hesitations, etc., from using a lower octane fuel.
So if your 79 runs better with higher octane, use it. Or switch different octanes every other tankful, and see how it performs.

 

Hi all! Sorry I've been tied up for a couple of days. I really appreciated the insight!

First, the engine in my '79 is stock - except for the replaced cam and lifters (only slightly hotter than stock). Definitely a low-compression situation, so I would really be surprised if I experience detonation with 87 octane.

I suspect it may be a combination of things... Here in the Cleveland area, we only run the ethanol gas during the cold months (at least advertised). I still suspect I've been running real gas all summer. Second, I bought the 93 gas at a station that has a LOT of traffic - meaning the gas was likely very fresh.

After reading all of the comments above (thanks again, all!) I really do believe this to be a carbon/deposit issue. I've been tempted to do the Seafoam/water steam clean, but have been hesitant. Force feeding water-based material down the gullet of the carb in any way shape or form seems extreme.

That said, the allure of reclaimed performance is strong!

 

Too funny


It certainly doesn't hurt to do a top end clean.
Your engine could have lots of coke, 79 was a long time ago, especially considering the use of leaded gas back then.

Do the water trick first on a hot engine.
Use the 3/8" pcv port on the carb (distributes more evenly) and reduce the hose to a 1/4" or smaller vacuum hose and suck out a bottle filled with water. Run engine at least 1500 rpm.
I've sucked in 1 gal of water out of a milk jug at 1 time with no problems.
This get rid of almost all the heavier coking.

Next get some "Mopar Combustion Chamber Cleaner" from a Chrysler dealer and follow the directions. Use a hot engine and let it sit overnite.
After a brief run hot, change oil and filter.

Seafoam is virtually useless compared to this stuff, one of the biggest reasons, being it is a foam, expands to soak everything, unlike a liquid.
Additionally, it, along with other pro cleaners contains more than just the "naptha" and oil in Seafoam.
GM Top End Cleaner, Motorvac Carbon Clean and a couple others contain other more powerful cleaners, but none work nearly as well as the Mopar stuff.



Be aware also that the pro cleaners require special pressurized equipment (carb or efi) to apply for proper results.

In a lot of cases, you may well increase your compression a bit, because the cleaners can release sticky rings (only if there was not excessive carbon).

With constant use of E10, especially rotating brands, I have yet to see top ends carboned much at all, in healthy well tuned engines.

 

Seadfoam is NOT a foam I don't know where you got that rediculous idea. It is called Seafoam because it was originally marketed for boats, thus the name Seafoam. Dump a can of it in some gas in a clear conatiner if you don't beleive me. AND it works as well or better than everything you mentioned and it is way less money. The Naptha in all of the things you mentioned is what delaminates the carbon anyway. There are different types of Seafoam, one is poured in the tank, the other is blown into the carb under pressure while it is running, does a great job. I have seen SF work wonders in my and others cars and in my extreme high performance bikes for decades. And I have seen E10 cause issues (bad issues) with rubber in fuel lines flaking and clogging everything. Even the small tiny sludge passing the fuel filter and eventually causing injectors to clog. I have seen needles in carbs get oxidation based on the E-10. I hate the stuff thank goodness we have a zillion places to get 100 percent gas.