How Much Octane Is Enough For Good Engine Performance?

[GPayment]

How much Octane do you really need to achieve good engine performance with compression ratio of 9.5:1?
After about 8 years I finally invested the time and $ to bring my very mild mid 70's 440 to a better compression by using 413 closed chamber engine heads,thin gaskets and larger exhaust valves. This "recipe" from older Mopar guys produces a full point of compression and more torque and H.P but now the question is how much octane do I need to maximize performance?Do octane Do octane boosters really work, and which are best?
Regards
GP

 

[CBODY67]

I went the other way, I put 906s on my '67 Newport 383 factory 4bbl car. With the timing set to stock levels, I could still get trace rattle at WOT on 91 Pump Octane Super Unleaded fuels of the middle 1980s. No clatter otherwise, with the stock distributor and advance curve. When I later put the orig Moper Perf electronic ign kit on it, I noticed no difference in normal dreiving with the quicker advance curve.

When I was in college, I discovered the Library had several years worth of S.A.E. Transactions in an obscure section. When I found them, it was "gold" to me! In one "transaction", it was by the Chrysler engine designers for the B/RB engine. In there, they discussed that the engine, in 1957 times, was optimized for 97 Research Octane fuels. That was "Premium" or "Ethyl" fuel, back then.

AS "Pump Octane" is the average of "Research Octane" and "Motor Octane" numbers, that can mean that 93 Pump Octane just reaches the 97 Research Octane level. At sea level. As altitudes increase, available fuel octane decreases by a number or two.

So, start with Super Unleaded at stock settings and go from there. Might have to back things back a few degrees, with just "trace rattle at WOT. Which would mean everything is quiet at normal driving throttle levels.

Enjoy!
CBODY67

 

[Big_John]

How much Octane do you really need to achieve good engine performance with compression ratio of 9.5:1?
After about 8 years I finally invested the time and $ to bring my very mild mid 70's 440 to a better compression by using 413 closed chamber engine heads,thin gaskets and larger exhaust valves. This "recipe" from older Mopar guys produces a full point of compression and more torque and H.P but now the question is how much octane do I need to maximize performance?Do octane Do octane boosters really work, and which are best?
Regards
GP

I think the question that you have to ask yourself is if you want to do things like add boosters and if you will get enough added performance to justify the extra cost and effort.

IMHO, I'm not a big fan of octane boosters. It's questionable if they work and if they do, they tend to be some nasty chemical mix you don't want to get on your skin or breathe the fumes. That said, I haven't done a lot of research on the subject lately.

I can get ethanol free gas pretty easily here since I'm in an area where a lot of folks have boats in the summer and snow mobiles in the winter. I have 4 gas stations within a few miles that sell it. The issue is it's gone from 92 to 91 and now to 90 octane and it's more $$ (of course). The 413 in my 300L pings a little when going uphill, but seems to be OK the rest of the time.

 

[Snotty]

At 9.5:1, 91 Premium will serve you well.

 

[CBODY67]

Octane boosters? Lots of money for little benefit, usually. Most do not do much for the higher octanes of fuel. Never have, never will. Even if starting with 87 PON, just not consistent enough to trust. Best to buy the higher octane fuel and be done with it.

Back in the 1980s, there were some serious octane boosters, for serious racers who wanted to push the envelope. ONE brand, on the back of the quart bottle, mentioned to also up-size the main jets about 4 sizes with their product. If not, engine damage could result. After a few people tried it, none went back to it.

You can do BETTER by using fuels and such that will keep the combustion chamber clean in the first place. Or use some fuel cleaner additive with PEA in it to do it too. Gumout has several, as do some other brands, so read the bottle labels.

CBODY67

 

[57fury440]

I use this. Without it I get some pinging. I have 11.2 compression. With it, no pinging at any RPM.

 

[Justin Plant]

I sold stuff like this for 12 years working at Advance Auto. I started reading up on what was what and how these chemicals worked. Octane boosters, as a rule, will raise the octane whatever their number says. Naturally. However, you put a decimal point in front of the advertised number. "103 octane" booster is actually .103 octane booster. It cannot raise it to 103, it doesn't know where you start from 87, 89? It's basically pointless. But, the bottles look pretty!

 

[57fury440]

I've used VP products for years including their race gas. They will give you a good idea of what you can expect based on the octane gas you start with. As a rule of thumb, any octane booster that is any good will come in a metal container.

 

[Boydsdodge]

I use the highest octane available at the pumps, then tune the engine for that fuel including a recurve of the distributor. No additives.

 

[wyrmrider]

do you have any quench what is your piston top to chamber flat dimension if 0ver .050 quench is pretty much gone\
for the 67 motor did you have any quench before going to 906? \
same quench problem
and neither of you have hard exhaust seats which can be a big deal if towing of motorhome or truck
how are your guides and viton stem seals? center two exhaust valves run hot
how was your burn pattern on your piston tops when you pulled your heads
do it again measure deck clearance at all 4 corners
ps late aftermarket heads have better spark plug locations and chambers

 

[496 polara]

It all depends on your cylinder pressure and then the rate of advance and total timing. If you are at a true 9.-1 then it may do fine on 89 in the winter but may want premium in the hotter months to keep from detonation. I would start with 89 and see how it does.

 

[Jengel451]

Assuming flat top pistons and closed chamber heads. If you're at a true static 9.5:1 then you should be able to run 91 octane with a re-curve. Really comes down to cam, jetting and timing at various throttle position and loads. There is no blanket answer though.

Octane boosters are mostly a quick band aid for marginal gains at best.

Mid 70's 440's also is vague, if you're just slapping 413 heads on a used 440 block you should measure how far down in the hole the pistons are, I really doubt you'd get 9.5:1 if you throw them on an RV lump, you might be getting into the high 8:1's but again we're missing a LOT of info

 

[57fury440]

It depends on how you use your car. Mine is not a daily driver and I tune it for maximum performance. When you tune it to keep it from detonating you are leaving horsepower on the table. That is the right way to do it for a car that you use daily. If you have higher compression, higher octane gasoline will let you keep the performance. If it didn't matter, there would be no race gas. Those little plastic bottles of octane boosters do not really work. VP race gas and a few others make products that work, but they are not cheap.

 

[fury fan]

How much Octane do you really need to achieve good engine performance with compression ratio of 9.5:1?
After about 8 years I finally invested the time and $ to bring my very mild mid 70's 440 to a better compression by using 413 closed chamber engine heads,thin gaskets and larger exhaust valves.

I have a '74 440 from a motorhome, the pistons are .150" down in the hole.
With my compression calculator (which I've spot-checked vs one from a racecar website) it's 7.9:1.

Mid-70s engines are commonly rated at ~8.2:1, but folks that have inspected & measured actual engines find that number to be inaccurate when the calculations are performed. High 7s are what such engines result for CR for mid-70s engines.

The cc-ratings of cyl heads, as published by the factory, are also off from what cyl head folks measure. I.E. the '80cc' open chamber heads usually measure at 90cc. The closed-chamber heads like the -516s you are using are usually 80cc.

-->> My calculator says a 440 with .150 down-hole, .018" compressed gasket (the steel shim type), and 80cc heads will give about 8.5:1.

Keep in mind - the factory always used a steel-shim head gasket, even in teh mid-late 70s - so you aren't gaining anything there.
You didn't mention pistons, so I assume you have original low-CR pistons.
You are only gaining from the lower CC closed-chamber heads, and the smaller runners that help velocity.
So - nowhere near 9.5:1.
And - if using the OEM cam the larger exh valve probably isn't worth much? (the HP 413s and 66 440 TNT hit 360-365 hp with that smaller valve)

If I've made incorrect assumptions about what has been done to your engine, clarify it and I'll revisit these CR calcs.

But regardless of all that - the octane requirement is largely due to your ign timing curve, as some others have mentioned.
The curve is one of the most overlooked factors in engine performance.

 

[fury fan]

Also - what psi do your cylinders have during a compression test?
I have a near-stock, small-cam '69 440 with all cylinders between 165 and 175 psi at a recent cold compression check. (and 'cold' gives the worst psi readings)
It runs fine on 87 and it's quite spunky, too.