Plenty of people get confused over what changes octane rating demand. I've done an extensive literature search over the past 15 years, and have homed in on 3 rules, and about 14 specific factors. Since there are over 9 basic kinds of cylinder head in use in modern vehicles, I've included notes on them
The priority drive for most cost effective power and economy from a given engine is very simple. You rate the 15 aspects of the engine, and improve those you can afford by priority.
The octane ratio demand is a simple result of these 14 factors listed below, and I'd draw your attention to the 9 types of cylinder head in common useage. These have certain characteristics which you must be aware of.
Rule 1:
Optimise Ignition For the Combination first. Any change to an engine spec changes ignition timing demand. Asside from the 15 factors, there are some fixed matters of
1)weight,
2)gearing and transmission type,
3)stall ratio,
4)altitude ,
5)peak engine temperature due to engine condition changes
6) Emission requirements.
I make the assumtion that these factors are a given for any Ford I6. Unless it has had an engine or transmission swap.
These alter the ideal advance curve. The advanced curve, more than all of the 15 items listed below will influence the detonation level most. You can see that automakers must walk a fine line between a minefield to arrive at the advance curve, compression ratio, and recommended octane rating.
Use the Mean Best Torque philosphy for spark timing. Advance it to detonation, then back off timing to safe level before incipient detnoation. Ramp up intial advance as aggressivley as possible before detonation sets in. It's better to have aggressive spark advance, and then pull back on total advance at wide open throttle. Best economy in part throttle situatiuons happens when an engine is running a high level of advance.Any time an engine is suddenly loaded, peak advance must drop back very quickly to avoid detonation.
Rule 2:
Use the right stratergy for the right combustion chamber.
There are many chambers. I haven’t listed the huge range of vintage flat head and inlet over exhast combos. Each time the compression is increased, some engines have huge costs or bad factors which hurt fuel consumption. Bad heads for high compresion on low octane gas are ones which have large surface area. The last engines configeratiosn, from 5 to 9, show immense improvement when the heads are decked. The engines from 1 to 4 show poor benefits.
1.Steep included angle Mopar Hemi’s, Jag XK 6, Alfa Romeo
2.Steep Wedge Porcupine Chevies.
3.Steep Reverse Wedge , the Ford's Pinto 2000 and Lima OHC 2300. 4.Shallow Compound Vertex Hemi (Escort CVH)
5.Shallow included Pontiac’s, Holden’s, Oldsmobile, Mopar Wedge, SBC Chev, Ford’s ohv non Clevo FE's,
6. Shallow porcupine 335/385 Martel Fords, Geelong built Aussie X-flow 4.1's
7.Flat bathtub Weslake A-series Austin/Mini I4's and the , there are only positives.
8.Flat Heron head Kent Pushrod, XKE 12, Y-block.
9.Cosworth narrow Angle Pentrooof (Sierra, Toyota 4AGE, Ford BDA, etc)
Rule 3:
The main inluences in terms of detonation resistance in order for the Ford I6, are:-
1) Advance curve not optimised. Engines on the edge of detonation, like old 351C HO's can cope with todays 93 octane fuels if the spark advance is wound back, but if its stock, it won't run without 98 octane. Old engines high compression engines, like early 70's cast iron log head 250's, can't cope with 87 and need to run 93 just to avoid knock. The ignition is the first key to fixing the problem before an engine build can be contemplated. Electonic ignition systems can be more failure prone than mechanical or vacuum units, but the best, most adjustable ignition system will allow lower octane fuel to be used.
2) Intake Heating due to heat stove or air heating. A reducing in fuel octane from 93 to less than 83 is possible if the intake charge is cooler than the factory intended.
3) Fuel atomisation due to carb, fuel injection design. Carbs are better than throttle body injection (TBI), but not as well metered. TBI or port EFI systems can be metered to run 22:1 air fuel ratios, while carbs have problems getting over 17:1. Becasue of this, a TBI or EFI engines have an extra failsafe device. A knock sensor or air flow/air pressure sensor to ensure it never lean detonates. Carby engines will therefore run closer to detoantion more than a modern EFI or TBI engine.
4) Fuel Distribution due to poor intake design. Old log head engines knock becasue of lean outer cylinders, and too rich inner cylinder air fuel mixes. Any time you improve the efficency of the fuel distribution, you can turn up the advance.
5) Piston Deck Impinging , Flat or Recessed. If impinging, may help or hinder detonation, if well below, it will expose a heated anular edge of the cylinder block, and cause realy detonation. In most cases, a zero deck gives no problems, only benifits. A border line car, which has issues running on 87, may be able to run 93 octane without problems if the deck is raised.
6) Head Material and microtexture (iron or alloy). Alloy can reduce octane demand by 5 (mon+Ron)/2 numbers. Your iron head 5.0 may not like 87 octance, but can hack 95 with no changes bar having alloy heads added. Polishing (removing the fine 'as cast' microtexture) of the intake ports and combustion chamber can cause fuel to drop out, creating a need for high octane fuel. In short, a backyard port and plish can cause increased octane demand.
7) Back pressure and exhast header discharge. Open headers and low back pressure changes the spark advance requirements. Restricted headers and high back pressure casues a differnt advance curve. Most exhasts give 6 to 8 psi back pressure. Reducing the pressure to 4 psi can improve peak power, but ruin low end torque and economy. The right exhast will create the right results for day to day application. Exhast gas can help low part throttle fuel economy if the ignition curve isn't changed.
8) Flame Travel due to plug placement (its most often reduced with hi-po pistons). Some angled plug heads create more power on the same advance curve. The need for high octane is reduced when the plug allows gas to travel to the exhast valve. In pre 1990's engiones, many engines have the plug pointing in the wrong direction becasue of production engineering and engine bay issues.
9) Head Gasket Material and Thickness in relation to Piston Deck. The composite gaskets are thicker, and less prone to heating the fire ring than fully steel items.
10) Valve Shrouding . Each time a head is cut, valve shrouding can be redcued. Some shrouded valves improve mixture motion. Small chanber heads can be made to work very well if the gas flowing with a flow bench is determined.
11) Effective Compression (cam related octane demand) is too high for the other 14 factors . Doesn't always go up with increased compression, as the whole reason for it is usually to raise opening duration to increase effective compression. When a racer bolts on a 320 degree cam, he can loose a huge amount of effective compression, which reduces the octane demand. Of recent note is the Cold Cranking compression test where any engine that exhibits more than 190 psi at the cold cranking level must have the cam timing altered to reduce the compression.
12) Sharp edges, loose tollerances between cylinders. Fixed by blueprinting or having blue printed to 0.1 c/r or C/R balanced on all cylinders accept the detonation prone ones. (If they are reduced by half a point, you can go up another 0.5 points on the other cylinders)
13) Piston Dome Masking (often, the dome is relief cut to suit flame travel and valve opening).
14) Quench is often increased as at the 10 to 14:1 level, people go to alloy heads with closed chambers or welded iron or alloy production heads.
15) Inertial Ramming . Port on port carburation, efi, and vee engines have better thermodynamic properties than single barrel in-line engines. Any time you add mulitiple Weber Carbs or injection, you increase cylinder filling, and the ingniton curve changes. It requires less advance, but also reduces part throttle openiongs to a very low level, whcih is why some miltiple carb and efi engines can be very economical at part throttle.
*I've added humidity to the list of detonation influences. Its right up there with piston short fall as a detonation factor. The addition of water or alcohol at wide open throttle only can allow the part throttle advance to be very high, and allow very lean mixtures to be burnt at part throttle.
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