[64speed]

Just like the headline says. 468 with Edelbrock 87cc heads. Zero Deck, 1.25.1 compression. Crower 60919 Cam. I run 93 octane now but there is a 70 cent difference right now between premium and midgrade and I drive it enough to where its a consideration. What say the panel?

[Jim Moshier]

You might try a mix of premium and regular and premium at 50/50 60/40 but using the midgrade pump it's never a consistent blend, If you noticed as there are no Mid grade tanks just a blend of premium and regular and I'm pretty sure there is way less premium than regular fuel being mixed at the pump. Not really an answer but just more questions to think about.
Jim

[Bruce Meyer]

With 1.25:1 compression ratio I am surprised it even runs. LOL.Im guessing you meant to say 11.25:1. IMO No. Don't run 89

[64speed]

I meant 10.25.1 sorry

[Formulas]

If confident nothing in the tune is whacky and it runs coolbi think it should work, sneak up on it i suppose wouldnt hurt like when your down a 1/4 tank fill up with the 89 see how it works

[mgarblik]

Originally Posted by 64speed

I meant 10.25.1 sorry

I just wrote a long, detailed reply based on your stated 12.25:1 compression ratio! Fortunately, I was able to delete it. I run a carbureted 87 CC E-head 468 on 90-91 Octane pump gas with no issues at all. The fuel is called "recreational fuel" in this area of the country. Zero alcohol. Been running it 2 years on the street and put hours of dyno time on it. Everything looks great. Running 89 octane is slightly more risky because if it contains alcohol the engine will run a little leaner. I would listen to the engine as carefully as you can especially at light load around 1500-2500 RPM's. That's when the engine is most likely to detonate, especially hot. You will be saving about $12.00-15.00 a tank of gas. I guess you have to decide if the savings is worth possible engine damage if there is light detonation you can't hear. You could run a tank of the fuel and pull the plugs and inspect the porcelain very carefully with a magnifying glass. If you see any sparkly specks, that's tiny bits of piston ring, and piston material. Change plugs and go back to premium fuel only. If you catch it early, the damage won't be terminal.

[steve25]

Why don't you first find out what your cranking compression is across all 8 cylinders and report back.

You should have this info for your records anyway for your notes and since this is a new motor, as it can help with trouble shooting down the road should a need arise .

[kingbuzzo]

[Stan Weiss]

Originally Posted by 64speed

Just like the headline says. 468 with Edelbrock 87cc heads. Zero Deck, 1.25.1 compression. Crower 60919 Cam. I run 93 octane now but there is a 70 cent difference right now between premium and midgrade and I drive it enough to where its a consideration. What say the panel?

What ICL is 60919 int at? Was Steve ask for cranking compression numbers would really help.

Stan

[b-man]

Why risk it?

Is your tune on point or kinda on point?

Keep putting premium in it and forget about putting low octane cheaper fuel in it, the potential savings in fuel now may very well bite you in the ass later.

[72projectbird]

Throw a tank of 89 in it and see what it does. If it doesn't like it, it'll let you know. You won't kill it with a single tank of fuel it doesn't like.

On the other hand, I feel like if an extra 70 cents a gallon is an issue... ya know.

[grivera]

I believe 64 has said he nearly daily drives this car so definitely worth finding out if he’s throwing money away. My bird has a similar build but my miles per year are so few that I use 93 at every fill.

[turbo69bird]

I have a 12.5 to 1 motor o could run it on 87 if I wanted to take enough timing out and richen it like crazy and putt putt around so it’s kind of. A loaded question.
Yes you can but what will you trade off to be able to do it.

There’s alot of variables but 10.5 isn’t that high and w Pontiac’s it’s usually not accurate. It’s usually lower than calculated.
A plug change can also make a difference. Sometimes you just hav to play with it and see but DONT push it. Start FAT and LAZY. And tune back into it .

[locomotivebreath]

I’ve been curious about the same thing except that a station around the corner put in a 90 octane ethanol free . I buy it for my small equipment but sure like to run pure gas in the GTO . Not sure is 90 will cut it at 10.1 with iron heads

[JLMounce]

My previous engine combo was a 462 with KRE heads around 10.2:1 compression with a smallish roller cam that had Around 63 degrees of overlap. I was able to run this combination on 87 octane which is the mid-grade we have in Colorado. I never heard any audible detonation in this combo, but the plugs did show some speckling for a little while. Based on that I switched to 91.

I would think your combination with the 60919 that has more overlap should be able to do this on 89 in the same circumstances. You may need to dial back the total timing a hair. I'd run some 89 through it and check the plugs after a tank for any signs of detonation. If they look okay, you're probably fine to continue. If there some speckling on the electrode and porcelain, you'll need to either detune it further, or go back to premium.

That said, what's an engine build cost? I just had mine done and the machining and build was around 6K with parts. It's probably 4K with a basic rebuild assuming you just need bearings and pistons. If each tank of 89 octane saves you around $15.00 and you fill up once per week, it would take you 266.66 weeks to recoup the cost of an engine build. That's 5.12 years. It's up to you how much risk you're willing to take there.

[Verdoro 68]

Originally Posted by JLMounce

That said, what's an engine build cost? I just had mine done and the machining and build was around 6K with parts. It's probably 4K with a basic rebuild assuming you just need bearings and pistons. If each tank of 89 octane saves you around $15.00 and you fill up once per week, it would take you 266.66 weeks to recoup the cost of an engine build. That's 22 years. It's up to you how much risk you're willing to take there.

This is the way I look at it. Maybe you could run 89, but what are you really saving in the scheme of things?

[JLMounce]

Originally Posted by Verdoro 68

This is the way I look at it. Maybe you could run 89, but what are you really saving in the scheme of things?

I don't know what I was doing there, but the math on the years is wrong. It's a bit over 5 years. So not as bad, but again, the point stands that the savings here may not be worth it based on the chance you're taking.

[steve25]

Seems to me that in another post here in the street section we spent days explaining why vacuum advance helps so much with part throttle fuel mileage while allowing a few less degrees of total mechanical timing to keep a motor out of detonation when it might have too much cylinder pressure and or too little octane

Of course this can be done via electronics if you have such a set up.

[Formulajones]

Yes, probably the most misunderstood part of timing and why people disconnect it.
It's only a benefit, no reason not to use it. I have them working on nearly 800hp engines here on the street with pump gas, and a big reason why these big stroker engines nearly 600 cubes get 15-16 mpg. So for sure it's something that can be used on milder 500hp engines.

As far as what pump gas to use. There are a lot of variables. It might or might not work ok. Chances are you may have to detune a bit but I'm not there tuning that combo so can't really answer the question with certainty.

My own stuff, I've been driving various classics daily for all of my driving life, most all of them with hot street engines using pump gas and I've always used the high test for decades. When I lived in Ohio I was always buying 94 Sunoco and even 25 years ago paying over $4 a gallon which was crazy at the time. Especially when prior to that it was $1.50. I never swayed to the cheaper stuff and just kept using it. Difficult for a teenager with no money and $4 was minimum wage lol. But I made it work.
These days in Arizona 91 is the best we have and it's nearly $4 a gallon decades later. Still using it daily since I've been here almost 20 years. Took a while to find what each car required to make it work after a steady diet of 94. Trying to drop them to 87 or 89 would be another long experiment that I really dont have the time or patients for nor do I even care. The cost difference just isn't worth it to me. Im so used to paying for premium pump fuel (all my life) I really don't know anything else, meaning the budget has adjusted to it and have no reason to change.

I'm not saying dont try it, I just have a hard time justifying it.

[Jay S]

I have helped with quite a few pump gas 455s and 60919s either side of 10:1 compression with both iron and Aluminum heads. With a decently tuned 455, 10.25 SCR, 60919, bigger rocker ratios, 300 plus cfm E heads, single plane intake, and headers, 89 octane should work fine most of the year.

You will probably want higher grades (or a least a mix) in it when idling in traffic with A/C, and the engine temp is higher than 200.

It is best start with the best pump gas octane you have available and work your way down, don’t switch to lower octanes with full tanks. Add the lower octane to the tank when you have a 1/2 full tank (or more) of the higher octane left in the tank, then see how it runs, and look at a plug before you switch to the lower grade entirely.