[67GP428]

My 65' 2+2. Has a freshly rebuilt 421 HO motor. Build to factory spec. With today's fuels what minimum octane fuel should I run? Is running 110 octane to high? I'm planning on buying 10 gallons at the VP Racing gas station. Or do I mix with the 91 octane?

[dhcarguy]

You should be fine mixing 5 gals of 93 with 1 gal of 110. that would give you 95.8 octane which should be fine if engine is a stock rebuild. Try running on 93 alone, it may be fine if timing is correct.
Smarter guys can answer better. (lol)

[dhcarguy]

Originally Posted by dhcarguy

You should be fine mixing 5 gals of 93 with 1 gal of 110. that would give you 95.8 octane which should be fine if engine is a stock rebuild. Try running on 93 alone, it may be fine if timing is correct.
Smarter guys can answer better. (lol)

.

My sister and husband live in Goodyear, Az.

[promptcritical]

No 93 octane here. 91 is the best you do on pump gas. So up the amount of 110 I suppose.

[Mike Davis]

Run 100LL from your local airport mixed with 93 or 91. I run it in my 66 straight and 50/50 mix with non-ethanol 93 in the 67. $3.75 a gallon vs $10. Been doing this for years.

[mchell]

Not sure anyone can tell what fuel you will need to run.

Technically, you need only run the octane necessary to prevent detonation on YOUR engine..... many things such as compression, quench, camshaft, tuning etc etc come into play.

One engine has success running strictly pump gas with moderately high compression when another engine with low compression has trouble running pump gas without detonation..... the choices made along the way affect this greatly

That being said.... if you rarely drive the car and don’t mind chasing around for racing fuel “to be safe”, I understand .

What was your machinists thoughts on fuel requirements?

[72projectbird]

Originally Posted by Mike Davis

Run 100LL from your local airport mixed with 93 or 91. I run it in my 66 straight and 50/50 mix with non-ethanol 93 in the 67. $3.75 a gallon vs $10. Been doing this for years.

100LL is where it's at. Got a small airport about 5 minutes away and they sell it pretty cheap.

[67GP428]

The motor was built by other? Engine was on a engine stand when I bought the car. The seller said it was build to stock spec? The 421 HO was suppose to come with the 068 cam? The way it idles and the low vacuum. I would say its has hotter cam? Car runs and drives Great! Just under hard acceleration its tends to ping. I've messed with the timing. So I feel it could be the fuel? I will know if it makes a difference real soon.

[Cliff R]

Before buying expensive octane fuels or boosters try 92-93 octane E-10 fuel. You'd be surprised how well the ethanol does as an octane enhancer with higher compression. I'm usually able to custom tune most of the older higher compression engines for it with perfect success, provided the own or engine builder didn't get stupid and put some whiz-bang fast ramp short seat timing cam in it on a tight LSA.

To date I've successfully tunes scores of these engines on this new fuel, including Olds W-30' 455's, high compression Pontiac 400'/455's, Stage 1 Buick 455's, and quite a few small and big block Chevy engines from the late 60's including several LS5's that didn't have the compression lowered during the rebuild.

Typically all that is needed is to tighten up the mechanical curve to about 10-11 degrees (20-21 at the crank), shorten up the VA to about 10-14 degrees, and set the initial timing around 8-12 degrees.

Another BIG player in success here is the cooling systems ability to keep engine temps in check and how tight the quench is. The most troubled engines I've had in here had TONS of quench in them, and some even gave troubled with lower compression if/when they teamed that deal up with a pretty small cam on a tight LSA.......FWIW......Cliff

[ta man]

Originally Posted by Cliff R

Before buying expensive octane fuels or boosters try 92-93 octane E-10 fuel. You'd be surprised how well the ethanol does as an octane enhancer with higher compression. I'm usually able to custom tune most of the older higher compression engines for it with perfect success, provided the own or engine builder didn't get stupid and put some whiz-bang fast ramp short seat timing cam in it on a tight LSA.

To date I've successfully tunes scores of these engines on this new fuel, including Olds W-30' 455's, high compression Pontiac 400'/455's, Stage 1 Buick 455's, and quite a few small and big block Chevy engines from the late 60's including several LS5's that didn't have the compression lowered during the rebuild.

Typically all that is needed is to tighten up the mechanical curve to about 10-11 degrees (20-21 at the crank), shorten up the VA to about 10-14 degrees, and set the initial timing around 8-12 degrees.

Another BIG player in success here is the cooling systems ability to keep engine temps in check and how tight the quench is. The most troubled engines I've had in here had TONS of quench in them, and some even gave troubled with lower compression if/when they teamed that deal up with a pretty small cam on a tight LSA.......FWIW......Cliff

How much of a factor does lower engine water temps help? I've ran a 160 stat for years and my engine seems to work ok . Many will say that is too cool..but my oil temps sit at 210.
I can run on a mixture of 87 and 91 with zero issues.

[67GP428]

Thanks Cliff and others for all the information. I'm using a 160 T-Stat and keeps the temperature at bay with my big 4 core Radiator. They do still sell 93 octane here. Just have to locate it. Should have new updates this weekend.

[Ram Air IV Jack]

Originally Posted by Cliff R

Before buying expensive octane fuels or boosters try 92-93 octane E-10 fuel. You'd be surprised how well the ethanol does as an octane enhancer with higher compression. I'm usually able to custom tune most of the older higher compression engines for it with perfect success, provided the own or engine builder didn't get stupid and put some whiz-bang fast ramp short seat timing cam in it on a tight LSA.

To date I've successfully tunes scores of these engines on this new fuel, including Olds W-30' 455's, high compression Pontiac 400'/455's, Stage 1 Buick 455's, and quite a few small and big block Chevy engines from the late 60's including several LS5's that didn't have the compression lowered during the rebuild.

Typically all that is needed is to tighten up the mechanical curve to about 10-11 degrees (20-21 at the crank), shorten up the VA to about 10-14 degrees, and set the initial timing around 8-12 degrees.

Another BIG player in success here is the cooling systems ability to keep engine temps in check and how tight the quench is. The most troubled engines I've had in here had TONS of quench in them, and some even gave troubled with lower compression if/when they teamed that deal up with a pretty small cam on a tight LSA.......FWIW......Cliff

I'll give this a try Cliff. My RAIV stock motor has all the original specs and is set at the 15 degrees initial stock setting. Right now I use a 1:4 mixture of Blue Sunoco at 110 that I purchased last year for $7.40/gal with 91 octane pump gas. However now that the feds are fooling around with the oil industry, I expect that to go up. I'll see if I can find this new E10 locally... Thanks!!!

[70GS455]

If you have E85 available in your area, it's an alternative at 105 octane provided your fuel system is suited to compensate for it. Can also mix with 93

Sent from my SM-T817V using Tapatalk

[Steve C.]

Sunoco fuel finder......

https://www.sunocoracefuels.com/fuel-finder


.

[promptcritical]

Originally Posted by 67GP428

Thanks Cliff and others for all the information. I'm using a 160 T-Stat and keeps the temperature at bay with my big 4 core Radiator. They do still sell 93 octane here. Just have to locate it. Should have new updates this weekend.

If you find 93 at a pump let us know. I'm in Goodyear and haven't seen it, but I haven't really looked. I'm still dialing mine in. It pings a little sometimes from certain conditions on tip-in but seems okay under full throttle.

[steve25]

The main criteria in regards to the octane any motor needs is how much hot cylinder pressure your making and the tune put into the motor.

If you are making 190 psi then 93 octane should be fully workable for you with the right tune and no oil intrusion.
Surly if you only have 180 psi 92 or 93 octane should be a breeze!
Lowering water temps below 165 in hopes of still making near max power with less octane is 200 % counter productive.

If your pinging on tip in then your likely bringing in the timing too fast!

Even a mire 2 degree drop in timing can turn that around without a loss in power at that rpm range since ping cuts into making power even more.

[ta man]

Originally Posted by steve25

The main criteria in regards to the octane any motor needs is how much hot cylinder pressure your making and the tune put into the motor.

If you are making 190 psi then 93 octane should be fully workable for you with the right tune and no oil intrusion.
Surly if you only have 180 psi 92 or 93 octane should be a breeze!
Lowering water temps below 165 in hopes of still making near max power with less octane is 200 % counter productive.

If your pinging on tip in then your likely bringing in the timing too fast!

Even a mire 2 degree drop in timing can turn that around without a loss in power at that rpm range since ping cuts into making power even more.

Design of the combustion chamber will make a huge difference in octane requirements.

[PontiacJim1959]

PRE-IGNITION VS DETONATION

Pre-ignition and detonation are not one and the same.

Pre-ignition is exactly that, ignition previous to the desired moment while detonation is an explosion (same as when you detonate dynamite, B-A-N-G) of the fuel-air mixture charge instead of the normal smooth and progressive (productive) burning of the air/fuel mixture. However, one condition can progress into the other.

The normal and desired combustion process within the cylinder is rapid but it is not instantaneous. The air/fuel mixture burns evenly and smoothly with the flame front advancing at a controlled and measurable rate, about 35 feet per second as combustion begins and increasing to roughly 150 f.p.s. and then slowing down as the combustion process nears completion.

Detonation. If sufficiently heated and compressed, any combustible mixture of gasoline vapor and air will catch fire. If the temperature and pressure of the unburned portion of the air/fuel mixture within the cylinder reaches a critical value, a spontaneous and simultaneous explosion of all the remaining unburned charge occurs. This violent process is called detonation. You very likely have heard it coming from your engine, especially upon acceleration. It's typically an audible sound and it is often referred to it as "pinging" or "knocking". But it can also be masked by the loud sound of your exhaust. Make no mistake, just because you can't hear it doesn't mean it isn't there. It's important to realize that this spontaneous combustion occurs AFTER normal ignition and AFTER some portion of the charge has burned. The engine is unable to convert this explosive energy into useful work and horsepower is lost. Supersonic pressure waves are set into motion that can produce harmful and destructive effects on engine & combustion chamber parts such as damaging rod bearings, pistons, rings, valves, and head gaskets.

Since light detonation cannot usually be detected from the driver's seat, any effective protection from its occurrence must be provided in other ways i.e., prevention. Detonation is caused by either excessive temperature or excessive pressure of the fuel-air charge. Control of these two factors is what constitutes avoidance of detonation. Controlling these two factors can be broken down into two parts; (1) the design of the engine and selection of the engine parts - especially when increasing HP & TQ and (2) proper tuning/servicing/maintenance of the engine. The design of the engine and selection of the parts can include engine cooling design, fuel grade, compression ratio, ignition timing & ignition curve, spark plug selection, and the intake charge temperature due to a turbo or supercharger, all of which are some of the major factors that can promote detonation tendencies. They all influence the temperature and pressure of the fuel-air charge just prior to combustion.

Proper tuning/servicing/maintenance of the engine can be seen as "operational control" of detonation by its owner/driver by adjusting and maintaining the best timing & timing curves for the engine, selecting the proper grade of octane gasoline for the compression used and outside air temperatures & altitudes, avoiding too lean of an air/fuel ratio, maintaining the cooling system in peak condition to keep the engine & air/fuel mixture temperatures down to a "best running" condition.

The most immediate and sure counteraction for detonation is a prompt reduction of engine power by letting off the gas pedal. Then adress the problem and make adjustments to eliminate the detonation.

Pre-ignition.

Pre-ignition is the uncontrolled ignition by an object heated to an incandescence (glowing red hot) state within the combustion chamber of the air/fuel mixture before the normal ignition spark occurs. This premature combustion results in excessive pressure being exerted on the piston during the final portion of its upward travel on the compression stroke which leads to destructive tendencies. The same excess heat/pressure conditions that result in detonation are present when pre-ignition is encountered. But unlike most detonation, pre-ignition will usually be detected by engine roughness, backfiring, and possibly a rapid increase in engine temperatures. Any sustained operation, even for a brief period, in this condition can result in broken, cracked, or burned pistons, a broken cylinder heads, scuffed cylinder walls, and damage to the valves and spark plugs. Pre-ignition can be caused by several possibilities, such as localized hot spots, carbon deposits, machining irregularities, sharp edges & pieces of metal that were not deburred or removed, , valves ground with too sharp an edge, and glowing spark plug electrodes.

Detonation can progress into pre-ignition, and vice versa. If you let detonation continue long enough to cause damage, this damaged area (in the form of broken parts or anything that causes a sharp edge) can become heated to an incandescence state - much like a knife or chisel's edge held against a grinding stone, pretty soon it glows. The same thing can happen depending on how the internal part within the cylinder has broken and now you've created an ignition device inside the cylinder(s) and the whole situation will get worse very rapidly.

Conversely, once the pre-ignition process begins, the rapid rise in combustion chamber temperatures can raise the air/fuel mixture temperature to its critical value, thereby initiating the detonation process.

Inlet Charge Temperature

Higher induction temperatures can contribute towards detonation. Generally, a reduction of the air temperature entering the engine by 25 degrees is said to be the equivalent to a one point increase in fuel octane. Reducing inlet charge temperatures can be done using some form of outside cold air induction as opposed to using the underhood heated air generated by the engine.
The exhaust crossover found on the intake manifold and used to heat under the carburetor is a big offender in inlet temperatures as the 1,000-degree exhaust gasses pass through the manifold. Aluminum manifolds can amplify the heat transfer effect at about five times the rate of cast iron.

Cool It

The effect of coolant system temperatures on detonation is similar to that of inlet temperatures by lowering gas temperature within the cylinder. Reducing engine coolant temperatures can somewhat lower the tendency for detonation and allows more cylinder pressure and thus more power on pump gas. A general rule of thumb is that a 10-degree drop in operating coolant temperature is equivalent to a gain of one octane point.

Lowering a coolants operating temperature can be done by increasing radiator capacity, using a good fan and shroud system, and upgrading/using a high-flow water pump, and clearancing the impeller to the water pump divider. A 180-degree thermostat is generally recommended for a street engine, but a 160-degree thermostat can provide advantages. If an engine has developed heavy rust and scale inside the water jackets or even the radiator, it can create reduce coolant flow and become an effective thermal insulator which makes it tougher to transfer heat out of the engine and into the cooling system.

Ignition

Ignition timing has a pronounced effect on power and how much power can be made without detonating. The optimal ignition point for max power is a function of cylinder pressure versus crank angle in the running engine. But if the engine wants to detonate, then optimal timing may never be achieved. A general rule of thumb in terms of ignition timing settings in relation to detonation tolerance is that 2-degrees of ignition timing is equivalent to an octane point.

In practical terms it pays to be on the conservative side when tuning the ignition curve to minimize the tendency to detonate. Since most engines respond with diminishing returns as optimal timing is approached, lower spark advance can provide a buffer to detonation without a significant reduction in power. For example, if your 400CI makes peak power with 38-degrees total advance, reducing total advance to 34-degress can provide two octane points of cushion for what may be a marginal loss in output.

Spark Plugs

Detonation can be encouraged by the use of inappropriate components. The plugs must dissipate sufficient heat for the application. Performance engines often benefit from a cooler heat range as the power level and cylinder pressure increase.

Fuel Management

Changes in air/fuel ratios have a direct affect on the flame speed, temperature, and the reaction time of the end gasses—all factors in the detonation tolerance. The first thing to consider is what the actual air/fuel ratio is in the cylinder. Rich mixtures do tend to suppress detonation, but at the price of reduced fuel efficiency, and that isn't usually a good trade-off for a street performance application. The factor often overlooked here is the mixture distribution. Detonation will occur in the lean cylinders, so to compensate, the mixture has to be richer overall. A finer range of mixture control and distribution avoids this compromise, leading to improved efficiency, and power at a higher detonation limit.

Squish/Quench

The squish/quench effect on engine efficiency has been well documented and researched since early in the last century. What is this effect, you ask? Squish/quench is achieved by building in a close clearance between a substantial portion of the piston-to-head area at top dead center. Research shows that if the piston rises to within .050 inch or closer to the flat of the head, combustion is improved. Squish occurs as the piston closes the gap in the quench portion of the head as it approaches TDC, rapidly displacing the mixture in this area, creating combustion-promoting turbulence and speeding the burn.

With a tight quench clearance, most of these end gasses are squeezed out near TDC, reducing the chances of auto-ignition (detonation). The temperature of auto-ignition is 1375 (f), so clearly the cylinder head temperature is significantly cooler than the end gas temperature at or near auto-ignition levels. Due to the temperature differential, the thin layer of detonation-prone hot gasses at the extremities of the chamber is actually cooled by the proximity to the head, further diminishing the tendency to detonate. It is from this cooling effect that the term "quench" is derived.

An engine with an effective squish/quench will gain significant detonation resistance, as well as improved torque production. Consider .040-inch to be an effective target for piston to quench-area clearance.

Rod Ratio

Another factor to consider is rod ratio, as higher ratios tend to lessen detonation.

Gearing and Car Weight

With higher gearing (3.08's vs 4.11's) or heavier weight can create and extra load on the engine and it will be more ping prone. The car won't accelerate as fast and the engine will not rev as quick which allows more time for heat to build up in the combustion chamber meaning more likely to have a hot spot and a place for the spark to start at the wrong spot and wrong time. With lower gearing, the piston will accelerate away from TDC faster meaning a slightly faster than normal flame front could still be chasing the piston instead of colliding with it witch causes more pressure and a faster burn that causes more pressure and a faster burn... long story short it burns so fast it is more like an explosion and damages your engine and causes audible knock.

All things above will lead to success with high compression on pump gas; good quench, cam overlap, heat control, timing, and fuel ratio, BUT, what works with one combination may not work with another.

[67GP428]

i will also check the type of spark plugs. I still feel it's the 91 fuel in my car at this time? I'm not even sure if I have harden valve seats? Is Lead additive possible needed also? I have no issues on tip in? Might try to run it with the vacuum advance disconnected.

[72projectbird]

Originally Posted by 67GP428

i will also check the type of spark plugs. I still feel it's the 91 fuel in my car at this time? I'm not even sure if I have harden valve seats? Is Lead additive possible needed also? I have no issues on tip in? Might try to run it with the vacuum advance disconnected.

If it was recently rebuilt I'd figure they installed hardened seats?

I'd start with the simple stuff before going off the deep end with parts changes. A little carb and ignition work might get you where you need to be.