Via3d

Several questions on gas for the stock high compression SBO's 10.25:1 or higher...
What gas and/or additives have folks used with success?
(no hardened seats or SS valves)

I am getting a HC SBO for a street cruiser .... Should I use a higher octane? ethanol free?

Is anyone using additives?

I typically use 91 octane with no ethanol for a late 70's 403. Small noticeable difference, and idles better cold.

Would it be even better to use a higher octane like 100 ? There are few 100 octane pumps around my area.

I was understanding Higher octanes are better efficiency, and also higher compression is better for efficiency too... so spending a little more in gas may offset some of the cost.

Charlie Jones

For 10.25 compression , the minimum octane I would recommend would be 91 octane .
Ethanol free is preferred if the carb or fuel pump still has non ethanol resistant components .

oldcutlass

I would try the 91 and see if you can run your desired timing, if not then move up to 93. I doubt you need 100 octane.

Fun71

That 403 is 8.5:1 compression at best and likely would run fine on 87 octane fuel.

Higher octane fuel is not more efficient on its own. It has a greater resistance to spontaneous combustion (pre-ignition) for use in high compression engines, which is where the increased efficiency come in. Using it in a a low compression engine does nothing but drain your wallet faster.

Via3d

Yes concur... in the 403 I was using non-ethanol. Ran better noticeably (maybe just the octane level higher than 87 I was using) .
and agreed Charlie, I felt safer with no ethanol gas being the stock carb, hoses, etc, since ethanol deteriorates those older items

Ethanol has higher temp for igniting right? burns clean.... So is using ethanol better?

Thanks old cutlass,

Summary All, IF ethanol is o.k. or better to run then I will just use and pay the cheaper pump gas 93 rather than than ethanol free blend. Otherwise stick to the 91 non-ethaol if it runs well as as oldscutlass suggests.

So is the main concern soft valves?
and pre-ignition banging deteriorating the non-hardened valves?
In this regard, does a Lead additive or octane boost relieve this concern of excessive premature wear? I prefer to stay away from Lead additive if possible.


Apologies for the kazillion questions... learning experience thanks for all the quick responses.

Tedd Thompson

I think the harden valve thing is a exaggerated problem. I have been running a SBC (as late as of yesterday) with 9 1/2 compression for 20 years on stock fulie heads with no problem. If you had some killer valve springs and freshly ground valve seats maybe but on a stock upper end that has had some leaded gas run through it I haven't seen any problems.

On the other hand if I were to rebuild the heads I most certainly would give it a three angle valve job, the cost is the same..... Tedd

Via3d

OK, I references the AMA specs for 66 & 67 SBO. The high compression motors were spec'd to use 99 octane.

Hmmmmm,,,,, How best to achieve? Am I correct to assume meeting this spec of using 99 octane, would be better if there truly is an issue due to using no Lead and the valves are not hardened???

joe_padavano

You need to understand the difference in RON vs. MON octane ratings. 99 is RON. MON is a different octane rating standard that is typically 8-12 points lower. Today's gas uses an Anti Knock Index that is really (R+M)/2. 99 octane RON is closer to 93 octane AKI. More to the point, the actual octane rating needed by your particular engine depends on the CR, carbon buildup, ignition timing, cam profile, and other factors. The factory octane ratings assumed a blueprinted engine (specified CR with no carbon), stock timing, and stock cam profile. Olds engines came from the factory with larger than blueprint combustion chambers, to allow carbon buildup and head resurfacing without exceeding the blueprint CR specs.

Via3d

Thanks Joe !! I knew there was some more science behind the 'Olds madness. LOL

SO, thanks for the brief intro to gas octane differences and terminology. What is AKI?

I live 18-20min. from Road Atlanta Speedway, can get 100 octane... or higher, ethanol or no ethanol.


What is the opinion on gas to use for :

motor = '67 330ci High compression 10.25:1 stock motor.
cam = stock cam is 278 @ 433 & 282 @ 433
Distrib. = HEI from my '79 403ci unit on it (put new stock points in I suppose)
Carb = 800cfm 4bbl Q-Jet the '79 17095253 (rebuild w/. Cliff R. hp kit)
intake manifold = #16 stock iron intake from my '79 .403ci (more on this later post)
exhaust = going 2.5" dual exhaust
exhaust = likely headers too ceramic coated (outside only)
timing = TBD
Spark Plug and gap TBD (another post)
Gear ratio = 3.23

joe_padavano

Via3d

O.k. I read up on the RON/MON values for octane ratings.
Interesting read...

--->>> So are the AMA spec ratings RON back in the 60's for octane ratings ??

oldcutlass

Charlie Jones

Yes , Research Octane Numbers were the standard back in the sixties
As noted , today's gasoline ratings are an average of Research and Motor ratings . so they are 8 to 12 points lower .

Via3d

DOH!

Charlie Jones, The pic that oldscutlass posted indicates that cutrent AKI was introduced 80 yrs ago. Which means it may not have been RON only in the 60's.

I did see that Pontiac in late 70's indicated (research octane) when stating its rated octane fuel for a particular car/engine for MVMA specs. But other mfg's did not distinguish this.

I am leaning towards using lead alternative/additive and run highest octane at the pump in Georgia and see if that works but it still has 10% ethanol.

I just wish I had more definitive data on the 99 octane specification form late 60's. And when the current AKI standard was implemented, still researching.

Fun71

All I have available here in Arizona is 10% ethanol fuel and my car has been fine with it for the past 20 years.

Charlie Jones

You are overthinking this .
Yes , both methods have been around for over 80 years . However , the Research number was higher. So it was the one used by the oil companies in their advertising . And auto makers in their recommendations .
About the same time that leaded premium gas disappeared , the "new " octane ratings came to be . Can't remember the exact year , but it was between 1975 to 1980 .

I had a '63 Dynamic 88 with 10.25 compression as a daily driver between '97 and '02 . I put 40,000 miles on it ,
I used 93 octane unleaded with ethanol in it . And Mobil 1 oil .
I never had any problems with it .

Via3d

Thanks all...

I will start with 93 with ethanol and alternatively try 91 octane no ethanol. If it runs well either of these, if I am still hesitant on the hardened set myth or curse I can always use some Motor Medic lead alternative

oldcutlass

The lead alternative is not really needed. If your really want to use a lead additive, Octane Supreme is the best. See link below:

http://www.wildbillscorvette.com/OctaneSupreme01.htm

olds 307 and 403

What motor is that carb off, I assume a 403. They are calibrated lean but should be passable on a high compression 330. The lead additive doesn't seem necessary unless it helps with spark knock, valve seat wear isn't an issue. I would try 34 to 36 total and add a much vacuum advance as it will tolerate. I ran a 9.6 to 1 Olds 350 with around .065" quench and a 204/214 280/290 .448/.472 114 LSA Performer cam with a Holley reman 800 cfm Qjet then put my 403 carb on. It stumbled and pinged at part throttle no matter where I set the timing on 91. I sent the 403 carb to Everyday Performance and that made a huge difference, like I said the idle and off idle are very lean. I had to cut back the vacuum advance to 10 degrees and slow down the advance curve. Most HEI's come in very slow that will help. Also buy an adjustable vacuum advance canister. Also the very cold NGK 5670-8 plugs also helped. Good luck.

Via3d

yes Qjet '79 .403 17059253.
Cliff R suggested change to 44 primary rods and new piston springs. This should help calibrate/allow more fuel from the original lean condition
I anticipate my manifold has more volume too. for another post......

going with the stock R44SZ ('z' for HEI') and I settled on starting at .045 gap...

I think I will make a post re: overall 330 tune and adjustments, etc.

Timing I have no idea yet and will be hands on.

olds 307 and 403

You are on the right track. Get this https://www.google.ca/url?sa=t&source=web&rct=j&url=https://www.summitracing.com/parts/acc-31035&ved=2ahUKEwiT1aabhc_eAhWI7oMKHZrjCxAQFjACegQ ICBAB&usg=AOvVaw1KEaFZ0TM81Jt002mheOSm
Or this https://www.google.ca/url?sa=t&sourc...e8rhCBYEs6E15P
I have used both with success and it will reduce the vacuum advance from probably 30 degrees to 20 degrees before you even adjust it. The mechanical advance curves can vary a lot. I find most are slow, even with lighter springs don't come in before 3000, which help tuning with high compression.

Via3d

interesting, I haven't considered vacuum advance units yet. I will look into these