Gas octane / grade should i use?

stevesykes · Veteran Member

Gas octane / grade should i use?

4SPEED427 · A Poncho Legend!

If it's the 4 barrel engine I think it's higher compression like around 10.00 to 1 or maybe 10.25 to 1 which would like 91 octane. The 2 barrel version would be fine with 87.

And with that said, I'm sure some others here will say otherwise!

seventy2plus2 · Canadian Poncho Superstar!

Start off with regular fuel. Under hard acceleration, listen to see if the engine pings. If it pings, more octane is required and a move to mid-grade is your next step. Try the hard acceleration again and listen for pinging. If still exists, then step up to premium.

73SC · A Poncho Legend!

I just referred to the 1969 Canadian Pontiac owners manual. Here's what it says:

All 6 cylinder engines ~ Regular

350, 250 HP and

396, 265 HP ~ Regular

All other V8 ~ Premium

So basically its saying all 2bbl V8 engines are Regular and all 4bbl V8 engines are Premium. Now what octane rating was Regular in 1969 and what was Premium ?

davepl · Poncho Master!

Almost... I think you forgot the 255hp 4bbl.

The 255hp 4-bbl is fine, as is the 250hp 2bbl (same motor) on 87.

My 350/300 4-bbl would still ping on all fuel except the 92 at a particular Shell station. Maybe their additive package was different, and I realize that doesn't help anyone, it just shows that they are "marginal" on typical 91/92 octane I think.

-- Edited by davepl on Wednesday 6th of April 2016 12:08:55 PM

stevesykes · Veteran Member

How would i know if mine is the 255 250bhp model?
I know it's a 4 barrel but wouldn't know which bhp.

I've put the cheapest in, not noticed any pings, but wasn't sure if i would hear / feel it as all i can hear is fan noise and air sucktion when the back barrels opens. No judgers or anything. Might try better fuel next time just to see if anything changes, otherwise i'm sticking with the cheap stuff!!!!

Thanks again for the replies.

cdnpont · Canadian Poncho Superstar!

The regular fuel of 1969 was probably better than any premium out there today.

73SC · A Poncho Legend!

stevesykes wrote:
How would i know if mine is the 255 250bhp model?
I know it's a 4 barrel but wouldn't know which bhp.

I've put the cheapest in, not noticed any pings, but wasn't sure if i would hear / feel it as all i can hear is fan noise and air sucktion when the back barrels opens. No judgers or anything. Might try better fuel next time just to see if anything changes, otherwise i'm sticking with the cheap stuff!!!!

Thanks again for the replies.

You would know by turning to the engine page in the Owners Manual.....and you will see that the 255 HP 350 doesn't exist on there !

73SC · A Poncho Legend!

cdnpont wrote:
The regular fuel of 1969 was probably better than any premium out there today.

I believe Regular fuel was 91 Octane in the late 60's and early 70's. Premium would have been up around 100, so you are correct mark.

cdnpont · Canadian Poncho Superstar!

Ah, good old Tetraethel lead.

Pontiacanada · A Poncho Legend!

cdnpont wrote:
Ah, good old Tetraethel lead.

Ya, heavy metal(s)!

Fake68 · Guru

Guys, don't forget old Regular was WAAAAAAAAAAAY better than todays regular. If you are running anything but Shell premium in old cars you need to account for alcohol content in fuel (Only Shell sells 100% premium gasoline now). Personally, I prefer not to have any alcohol in my tank... ESPECIALLY for occasional use or seasonal vehicles.

If you have reasonable compression ratio you can live with regular but try to burn it off and refill with premium for storage.

I pay the extra for Shell premium plus add race fuel to boost octane for my 11:1 engines.
Gary

davepl · Poncho Master!

I have to confess I don't see the 255hp variant in what you posted, but I've certainly seen it somewhere... the shop manual perhaps? Are you sure the LM1 never made it into our cars? I can't say I've ever seen one, but read it somewhere, I just don't know where! Definitely not in the specs on the back of the brochure either.

Maybe it was a Camaro engine and I just got it cross-wired in my head?

-- Edited by davepl on Thursday 7th of April 2016 11:20:00 AM

66 Beau · Poncho Master!

If you do have a 300 HP (041 casting heads) at 10.25:1 compression, you might eek some extra performance by tuning for premium. But then if you ran regular, you might have to de-tune (back off timing or richen up the mixture) or suffer ping. I suspect power gains would be quite moderate, possibly not noticeable to the seat of the pants feel.

I had a 300 HP 350 and ran it on propane (with hardened valve seats). I did have some issues switching back to regular gasoline after tuning the engine for 100+ octane propane. But properly tuned, it ran on regular without problems..

If your engine was originally a 2 bbl (9:1 compression), then regular is a no brainer.

I agree with Fake68. Regular if you're going to burn it. Switch to premium in the fall. Leave no ethanol in your tank for storage. I have had no problems with bad gas and sludged-up carburetors since I started doing this. If your like me and only go through a couple tanks a year, might as well just run premium, regardless. That way you don't get stuck with ethanol in your tank over the winter.

CdnGMfan · Canadian Poncho Superstar!

Factors affecting octane requirements include, but are not limited to, combustion chamber configuration, load, dynamic compression (directly affected by camshaft).

The 300-horse L48 has a 10:25:1 compression and a relatively mild cam. It worked great on 1969 premium, but today they can really be pingers on modern fuel. Some overlap courtesy of the cam can help bleed off some cylinder pressure and lower the octane requirements some.

Many people assume that higher octane means higher performance, but all that higher octane really is is resistance to detonation (harder to ignite). Unless you engine requires the higher octane in order to stave off detonation, you are wasting money for nothing. Having said that, Shell currently omits ethanol from their high octane fuel. Right there is a case for buying Shell's high octane fuel. I run it in my lawn mower and my antique outboards for that reason alone. My daily driver needs it because it has moderately high compression and a turbocharger. Even with a knock sensor that will retard the timing if detonation is sensed, you are better off running high octane on a modern high compression car because retarded timing will hurt response, fuel economy and combustion temp. Try it and see how the car responds. I tried different brands of fuel in my old Corolla GT-S and found that it loved Shell Gold.

The 255-horse LM1 350 of 1969 was literally the 250-horse L65 350 with the addition of a 4-barrel carb over the 2-barrel. Since the 2-barrel 350 was the base V8 on the 1969 Canadian big Pontiacs, the LM1 was not offered. The big 1969 Chevrolets came base with a 327 and therefore offered the LM1 as the next step up, followed by the premium fuel 300-horse L48 (offered on the Pontiacs as well).

stevesykes · Veteran Member

Thanks for the reply above, very helpful. I'll try a higher grade on this months fill up. Run the cheapo stuff and not noticed anything wrong. We don't have shell up here, nor do we have premium 94, so will try her on 91 and see if there is a difference, i think it was 87 i filled with last week?
Thanks again for the replies,

Steve

cdnpont · Canadian Poncho Superstar!

It's all in the timing Steve. Adjust until you notice it begin to ping. It's as simple as that.