Thoughts on octane...

[richpet]

So, with my performance builds, I have just always used premium fuel, often the clear/non-ethanol. But, I wonder if I am wasting money unnecessarily.

Current motor is aluminum headed (which usually means one can run more compression safely, or lower octane), Crane 268 cammed, .040 over block. Static compression is about 9.5-1 give or take.

Around here, clear premium (91/92 octane) runs $7 per gallon.
Standard (10% eth) premium (91 oct) is $5.25.

I'm wondering if at my somewhat conservative level I could run mid-grade (89 oct, 10% eth) fuel safely. It's about $4.90.

I know, a motor is a LOT more expensive than fuel.

Thinking this:

At half a tank fill with mid-grade. Drive. No knock? Fill with mid-grade again at about half a tank. Drive. No knock? Switch fully to mid-grade. Of course, include a few full throttle hits while listening for issues.

In summer because of the heat I run premium anyway and probably still will. Maybe.

Anyway, this seems a logical approach. I drive the car 5-8k a year (I didn't build it to look at) so, what are your thoughts?

Plugs look good, but I am running one range hotter than 'stock/recommended ' for the heads. I can run a cooler plug. So easy to change them...

At this point I'm spending $450-550 a month on fuel. If I can knock that down...

Thoughts?

I can always throw a can of octane booster in the car so if I get knocking I could add it, drive gently for a while, and move up a fuel grade. I know it's not a huge difference, but it feels psychologically better to see a smaller number on the pump.

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[Chuck W]

Doesn't this car have an aftermarket FI system on it?

You could always tune it for mid-grade (If there is any difference).

[richpet]

It does, and the 347 I'm building will too.
I can always tell it to enrich the mixture of course. I'm just wondering if the current tune is being fed almost 'too safe' a fuel rating.
With no knock sensor I have to rely on my ears. I just think my combo doesn't necessarily need what I feed it. It just seems that the Al heads offset my slightly higher compression and cam profile.
I guess this is more curiosity than anything. I will be doing the lower octane testing I think.

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[bwguardian]

Maybe your signature is not updated...you aforementioned a 264 cam and your signature shows a 268 cam. Maybe it's a different car given it also shows 9.5:1 though you state 9.7:1. So what's the LSA of the cam you have in the engine? I don't see a problem you could run lower octane fuel with your combo, but you may have to take some timing out of it. Speaking of which, how much timing are you running in the motor? Lug the motor driving down the road and give it some juice to see if she wants to ping...

[richpet]

Oops! Totally my confusion on that. Sigh...
The signature is 100% correct. I said 9.7 for some stupid reason. And misnamed the cam. I had just spent some time on the bronco site reading about someone's build and listed his cam I think.

Corrected in my original.

I have lugged it a bit and done that with zero knock/ping.

I have 12° initial. Not sure on total, but I have the adjuster on the dizzy most of the way advanced (adjusted with a little hex wrench inside the vac advance).

LSA is 110° IIRC.

I think I'm just overthinking again. Typical. Think I will gradually lower it (octane) and monitor.

Appreciate it!


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[mmb617]

Allow me to jump in here with some info that may or may not be relevant. My car has a 408 with aluminum heads, and by my calculation right at 10.5:1 CR. It has a Lunati cam with 110* LSA and 242/252 duration. I run 13* initial timing and I don't remember exactly but I think it is 30* total timing so that would be 17* of advance, all in by 3k rpm.

Anyhow even when I was racing and going WOT I never had any problem running pump premium gas which around me is 93 octane and is "up to 10%" ethanol. The current price for that fuel here is $4.49.

[richpet]

Our premium is 91 and 10%.
And is over $5

Big picture is that the cost difference is actually pretty minimal. I think it's more psychological.

But, I get in my head...

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[bwguardian]

Oops! Totally my confusion on that. Sigh...
The signature is 100% correct. I said 9.7 for some stupid reason. And misnamed the cam. I had just spent some time on the bronco site reading about someone's build and listed his cam I think.

Corrected in my original.

I have lugged it a bit and done that with zero knock/ping.

I have 12° initial. Not sure on total, but I have the adjuster on the dizzy most of the way advanced (adjusted with a little hex wrench inside the vac advance).

LSA is 110° IIRC.

I think I'm just overthinking again. Typical. Think I will gradually lower it (octane) and monitor.

Appreciate it!


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You'll be fine on the plain ole regular issue. My 85 has GT40p heads which the combustion chambers CC'd right around 59cc. This puts me at about 9.4:1 CR. I run the E303 cam which has 110* LSA. I have about 14* initial timing and can't recall total, but it's all in early. I have no problems running regular ole 87 octane...

[bwguardian]

Oops! Totally my confusion on that. Sigh...
The signature is 100% correct. I said 9.7 for some stupid reason. And misnamed the cam. I had just spent some time on the bronco site reading about someone's build and listed his cam I think.

Corrected in my original.

I have lugged it a bit and done that with zero knock/ping.

I have 12° initial. Not sure on total, but I have the adjuster on the dizzy most of the way advanced (adjusted with a little hex wrench inside the vac advance).

LSA is 110° IIRC.

I think I'm just overthinking again. Typical. Think I will gradually lower it (octane) and monitor.

Appreciate it!


Sent from my Pixel 6 Pro using Tapatalk

You'll be fine on the plain ole regular issue. My 85 has GT40p heads which the combustion chambers CC'd right around 59cc. This puts me at about 9.4:1 CR. I run the E303 cam which has 110* LSA. I have about 14* initial timing and can't recall total, but it's all in early. I have no problems running regular ole 87 octane with a slightly modified stock 4180c carburetor...

[Tigger]

I am a little north from you. I live in Beaverton and work in Canby. I run premium in both my Foxbodies. Both have pretty much stock motors. In Canby there is a VP gas station where you can get non ethanol premium. I have been on vacation for the last 3 weeks so I do not know the current price but before I went on vacation it was $5.49 per gallon. Costco was $4.95 for premium. If the delta is 50-60 cents per gallon, I typically pull the trigger and go to VP and get the non ethanol stuff. I get 1-2MPG more and feel the cars run better. When the delta is higher than that, I stick with Costco premium. Costco 92 octane and top tier. I probably could run normal regular but for $4-5 per tank I like the piece of mind that I am getting the best fuel possible so I tend to go premium. Best part of all we can pump our own gas now. Been like 70+ years that there has been no self serve fueling in Oregon.

[richpet]

I've driven maybe 50-70 miles a day this week and filled/topped up with mid-grade (89oct and 10% eth) each time. No knocking at all whether higher rpm or lugging under throttle.

When the temps hit 90° I tend to run clear premium because I have had the eth fuel (premium) boil in the line and cause a vapor lock issue. No biggie - let it cool and then start it and drive off. But inconvenient.

So at this point I think I will run the mid-grade (89 oct/10% eth) in all but the heat of high summer.

So current tank is close to all mid-grade and so far so good. Plan to drive it tomorrow and top it off the same way.

Last cars and coffee of the season Saturday in P-Hill. 8-10 am for any locals ...

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[qikgts]

You could always "blend" higher octane fuel yourself by adding some from nozzle a and some from nozzle b... I'm sure there are good affordable octane boosters out there too if you're just looking for two octane numbers increase in a tankful.