Octane Needed

[Robert S.]

A recent post on gasoline octane ratings got me thinking if I can use a lower octane gasoline than 93. I have a stock 74' L48 engine.
I am thinking with this engine's compression ratio it should be ok to run today's 87 (R+M/2 rated) unleaded gasoline. Any thought or experiences by NCRS members?

[John H.]

Re: Octane Needed

Robert -

Take a look in the owner's manual - it will tell you what octane is recommended for your engine.

[Michael W.]

Re: Octane Needed

Your owner's manual may give you some information that doesn't quite relate to todays world and has confused many new owners. It may state either to use 'unleaded' and/or it may give an actual octane number.

When unleaded gasolines were first being made available to the public in the mid 70s, there was no 'regular' 'midgrade' and 'super' grades of unleaded gas, only one grade which was equal to today's 'regular' or 87 octane. That's what your car was designed to run on although it does no harm to use higher octane.

If the manual does give an actual octane number, be aware that it is probably stated using the 'old' rating system of quoting just the 'RON' vs. the present method of quoting RON+MON/2.

Today's 87 octane gas is roughly equal to the 92 octane gas of the sixties.

[Joe L.]

Re: Octane Needed

robert-----

This is a really easily solved situation and is best solved empirically. Let the tank run down to nearly empty. Then, fill it with 87 octane. If you experience no significant pre-ignition ("pinging"), then your car is fine to use 87 octane.

If, you do experience significant pre-ignition, let the tank run down to about half and then fill it with 93 octane. This will give you the equivalent of about 90 octane in the tank or ABOUT what mid-grade octane is. If you now experience no significant pre-iginition, then you know that you can run mid-grade fuel.

If you experience significant pre-ignition with the above, then you know that you need to run the 92 or 93 octane.

My guess is that you'll be ok with the 87 octane.

[Duke W.]

Re: Octane Needed

Actually I'd recommend just putting a few gallons of 87 in. If it doesn't detonate, let it get to nearly empty again, them repeat. If it doesn't detonate this time 87 is okay.

If it does detonate the first time you can always fill it up with a higher octane.

Beginning in 1971 ALL GM engines were designed to operate on 91 RON unleaded, and today's 87 PON has the same effective resistance to detonation. This includes the 1971 LS-6 and LT-1 engines.

There's absolutely no reason to use higher octane fuel in post-70 C3 engines unless the engine has been rebuilt with higher CR or the spark advance map has been significantly modified.

Duke

[Clem Z.]

Re: Octane Needed

why take a chance for less than $5 difference a fill up

[Joe L.]

Re: Octane Needed

clem------

It depends on how often one has to fill it up. If it's like once or twice a year, no big deal. However, certainly no benefit is being derived from higher octane fuel if the car will run fine on lower octane. Plus, with gas costing $3.39 a gallon (here in California, anyway), who wants to spend a penny more a gallon than they have to?

[Clem Z.]

Re: Octane Needed

joe even once a week it is not a big deal at $5 a fill up difference. also a lot of low priced gasoline's do not have all the additive packages that help keep your engine clean because they are put in at the local distributor and if he is trying to undercut his competeion he could be leaving it out

[Joe L.]

Re: Octane Needed

clem------

I've got the solution for the problem you describe. Go to www.toptiergas.com

If you buy your gas from one of the brands listed on that web-site, you're assured of getting the 1st class additive package no matter what retail outlet you get it from.

However, the best additive packages are much more of a concern for modern cars with fuel injection. With a carbureted engine, I think the additive package is much less of a concern. However, I think it's still wise to use the Top Tier gasoline.

http://www.toptiergas.com

[Duke W.]

Re: Octane Needed

My '88 MBZ 2.6L inline six only has 9.2:1 CR, but it's a "premium fuel" engine and "requires" 91 PON minimum according to Mercedes.

Back when I was a field sales guy and drove it everyday it exhibited a little low speed detonation on hot days with the AC on in low speed traffic when the coolant temp got up to about 100C, which is when the viscous fan clutch tightened. (It also has two electric fans in front of the AC condenser that both engage at low speed controlled by the AC high pressure switch, and they both engage at high speed at 105C coolant temp.)

Now I just drive it during the winter, and even though I have made the spark advance more aggressive (required changing a trim resistor, which is the equivalent of lighter springs in an old Delco single point distributor) I use 87 PON fuel and around town fuel economy is up about 10 percent due to the more aggressive spark advance map.

Since the most efficient operating point of an engine is low revs, high load I tend to short shift, and on warmer days - say above 65-70F - especially if the coolant temp gets more than ten degrees above the normal 80C that it runs at speed, it exhibits a little ping on upshifts, but I can "drive around" this by upshifting at higher revs or feeding the throttle in a little slower on upshifts

Engineering is all about getting more from less, and the ragged edge of detonation is the most efficient operating point at many speed-load conditions, but modern cars with detonation sensors keep them from going beyond the edge. In the case of my pre-detonation sensor cars, I AM the detonation sensor and engine management system.

Duke

[Clem Z.]

Re: Octane Needed

you still have to worry about deposits on the back sides of the intake valves. i bought a used carbed V-6 celebrity with 6000 miles and the car was gutless so i started to look for the problem and i found the back sides of the intake valves so carboned up even with the valves open there was very little opening for the fuel air mixture to pass thru.even using the top tier gas i would still use something like "TECHRON" in the fuel tank every 4/5000 miles to help clean the intake valves