What type of fuel to use?

[his15153]

Hello,

I just bought a 1969 Chevy Stepside. It has a 350 engine, but the engine and transmission have been rebuilt fairly recently by the previous owner. I know that the octane of fuel to use varies depending on the compression ratio of the engine. My question is, is there a simple way of figuring that out? For now I have been using premium fuel to be on the safe side.

Thanks

[truckster]

In all likelyhood, unless the previous owner told you he built a high-compression engine, you're fine with regular fuel.

[Stocker]

Welcome from Norcal. Hard to answer your question without more info. Does the engine seem to run / idle / perform / sound pretty much like a stock engine? If so, you should be fine with 87 octane regular gas. That's what these trucks were originally designed for.

Try this: run most of the gas out of it, then put in a few gallons of regular. If it pings, retard the timing a bit to see if that eliminates the ping (spark knock). If the engine still runs well with the timing change, you're good. If not (or if it still pings), you might need to stay with premium fuel. Since you only put in a few gallons of regular, there's plenty of room in the tank for enough premium to set things right again.

This is just a basic starting point, there are other factors that can contribute to spark knock AKA detonation.

[Tinkermc]

I would have to agree with Stocker, it is the simple way to find out.

[Sixteen Tons]

I can only say that as soon as I started running premium in my 250 I6, my engine ran better overall. No dieseling, smoother idle and dare I say, I think I'm getting better milage. Way I see it, 50-60 bucks in gas is 50-60 bucks in gas, might as well be the highest octane I can get.

[his15153]

Thanks for the advice. I just filled up the tank the other day, but the once I use it up I will try to try this.

[Mister-B]

Originally Posted by Sixteen Tons

I can only say that as soon as I started running premium in my 250 I6, my engine ran better overall. No dieseling, smoother idle and dare I say, I think I'm getting better milage. Way I see it, 50-60 bucks in gas is 50-60 bucks in gas, might as well be the highest octane I can get.

You're getting about a gallon of fuel less than you could for the same money, each time you fill up.

Sounds like yours runs better on it, so you either need a tune up, or you're stuck using it. Mine runs better on regular
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[Dnrice]

My PO told me to use high octane and I ignored him. Ran cheap stuff because I am cheap and it ran like crap. It would try to turn key off and it would try to start again. I am using high octane and it runs like a champ but it is expensive because I only get about 8 miles to the gallon.

[Rufton]

I had one engine that would knock going uphill at roughly 2000rpm w/ reg but didn't knock w/ premium. I switched to premium when playing around town and WOT. I still ran regular on highway trips and mild driving.

[mrein3]

I bought a new car this past February. The book says to use regular unleaded. I experimented with Regular, "Silver" - the BP mid-grade, and "Gold" - the BP high-octane.
The Silver cost 2% more than regular but got 6% more miles per gallon. I don't have the numbers in front of me but the Gold was more money than Silver - of course - but didn't return that much more mpgs so I run Silver.

My truck I was an idiot. When I built my 383 I put flat-top pistons in which of course raises the compression ratio. The gas that was in the tank (regular) when I did the swap made it ping like crazy. Ever since that first VERY short drive with the 383 I've been filling up with 91 octane.

If your truck doesn't ping on regular, run most of a tank of regular through it. Record your mpgs. Do it again for the mid then high octane. Record $'s spent so you can compare the cost increase verses mpg increase. You may find like I did that even though a higher grade of gas costs more, it may be worth it.

In this economy you can't be leaving ANY money on the table.

One more thing - and I don't work for BP or any other gas company - a couple years ago I discovered that I consistantly got 3% better mpg with BP when compared to "Holiday" which is a local chain up in this area. If you put the miles on a car like I do it doesn't take too many fill ups to figure out stuff like that. Again, its worth figuring this stuff out.

[volksworld]

keep in mind the definition of "regular"....when I was in high school (1971) cheap regular was 94 octane and "hi-test" was 101...

[special-K]

I just run regular so I don't waste my money. If a vehicle pings on that I run hi-test. I don't think I've ever bought a drop of mid-range. Always wondered why they even bother,other than a gimmick. Anything I've owned in 40+ years that didn't require hi-test ran just fine on regular.

The way I was always told,90 octane is the threshold you need to be above with higher compression (10:1 or more). Sure,more is better. I don't recall regular ever being into the 90 octane range. Sunoco 260 was 93 and that was the hot lick around here 40 years ago.

[Koffer]

Originally Posted by volksworld

keep in mind the definition of "regular"....when I was in high school (1971) cheap regular was 94 octane and "hi-test" was 101...

It also had lead in it

[Stocker]

^ And there are different ways of measuring octane -- Research, Method, and the current system which adds them and divides by two (R+M/2), resulting in the current Pump Octane rating.