Ask Away! with Jeff Smith: Spark Plugs, Fuel Delivery & Timing Recommendations to Avoid Detonation on a Supercharged SBC

I purchased a 1959 Apache Truck four years ago with a supercharged small block Chevy 350 engine and a Weiand blower. The block is a 1969 model and heads are 1970 492s. My ignition timing is 10 degrees initial with 32 degrees total with 10 degrees added from the vacuum advance on the HEI distributor and I always run 91 and, sometimes, 93 octane pump gas.

The truck runs nice with mild detonation from time to time. The air fuel mixture averages 12.1:1. I have no idea what is the best spark plug for my application and the internet searches are not helping. A long threaded plug versus a short thread (tip length, etc.). Hot versus cold?

In addition to a spark plug recommendation, what would you suggest for initial and max timing?

Also, would my current 600 cfm Edelbrock 1406 be okay or would a Sniper EFI system work better for maximum performance and eliminate the detonation?

If you have a chance, I would love to hear your thoughts.

M.K.

Let’s deal with a spark plug recommendation first. The small block Chevy 492 heads were early 1970s castings with decent intake and exhaust port flow for the time. These heads use a 0.430 inch thread reach spark plug with a 14mm thread and a tapered seat that seals to the head. A standard heat range plug using AC Delco spark plug numbers would probably be a R43TS plug. These plugs also employ an extended reach feature that pushes the nose of the plug farther out into the combustion chamber.

This is a good thing for normally aspirated engines, but it may be an issue with a supercharged engine.

With the blower, your inlet air temperature under boost will be higher and more likely to detonate based on your tuning inputs like air-fuel (A-F) ratio and ignition timing. Because a projected nose spark plug also extends the length of the ground strap, this might be a problem.

What can happen is the longer ground strap can get red hot during combustion and remain hot enough that it can cause the incoming inlet air and fuel to ignite early which can cause pre-ignition. Pre-ignition is early onset of ignition much earlier than the firing spark plug and is usually death to an engine. You didn’t mention which spark plug you are using but likely it’s a projected nose plug since that is what is usually stocked at the auto parts store.

What would be a better choice is a slightly colder plug with a non-projected, or standard center electrode. I’m most familiar with Autolite spark plugs so I will use their numbering system. An Autolite AR23 is a slightly colder heat range than the stock plugs. This plug still employs a projected nose but it is not as deep as other plugs so it should work.

Solving the Detonation Issues

As for the detonation issues, I’m assuming the detonation occurs at wide-open throttle (WOT). Your ignition timing sounds very close. Try pulling the initial timing back to 8 degrees initial which will produce 30 degrees of total timing. If the engine still rattles, you will have to modify your HEI distributor to reduce the total mechanical advance. If you have 10 degrees initial and 32 degrees total, this equals 22 degrees mechanical advance. By limiting mechanical advance to say 18 degrees, this will allow you to retain the 10 degrees of initial timing while reducing the total to 28 degrees. Then you can add more initial timing if the engine does not detonate with 12 degrees initial.

I like the fact that you are using vacuum advance at part-throttle. I think that helps with better throttle response but most blower folks don’t use it because they think it’s bad. That’s not the case at all—so good choice. If the engine detonates at part throttle, then you may have too much vacuum advance. There are adjustable vacuum advance canisters for the HEI that would help in this case.

Choosing The Best Fuel Delivery Method

As you mentioned, the Edelbrock 1406 carburetor you have is a 600 cfm and while it would be fine for a mild, normally aspirated small block, it is a bit too small for a supercharged engine. As a rule, Roots style blowers are very sensitive to inlet restrictions. Any 600 cfm carburetor will tend to restrict the inlet airflow at higher engine speeds when inlet air demand is the highest.

My suggestion would be that a 750 cfm carburetor would offer a chance to make a little more power at the top of the rpm band. But don’t be fooled, this change won’t be worth anywhere near 50 horsepower. More likely this change may bump the power by perhaps 10-15 hp but probably not much more. At part throttle you may not notice a change as the difference in area uncovered by the throttle will be minimal. Unless you are searching for more peak power, this is not a change you need to make right away.

As for a throttle body style fuel injection system, these setups work well but unless you are concerned with part-throttle fuel efficiency, there will not be any power gain to be achieved with an EFI unit. You can see decent performance gains by tuning your existing carburetor and ignition timing.

The more important issues will be to ensure the engine does not detonate under high load.

I’m also assuming that the 12:1 air-fuel ratio is at WOT. The air-fuel ratio at part throttle could be slightly leaner—closer to 13.0 which will allow the slightly colder plugs more of a chance to survive 10,000 miles or more without fouling. A colder plug runs at lower temperatures which can eventually lead to fouling and misfire. This is a balance you might have to play with between experimenting with too cold versus just right heat range plugs. Much will depend upon how you drive. The heavier your right foot—which is likely with a blower—the more you will want a colder heat range plug.

Experimenting With Octane

Another experiment you might try, if pulling the timing does not solve your detonation problems. If you have access to E85 fuel where you live, try mixing 10 to 20 percent E85 with your pump gas. This will have a dramatic affect on octane by adding two to maybe even three numbers, taking 91 octane to as much as 94 octane.

As a mixing example, adding one gallon of E85 to 6.5 gallons of gasoline will produce the equivalent of E20 fuel. The benefit is that the added ethanol offers a much higher octane boost at low mixing levels. After E50 (a 50/50 mix of gasoline and ethanol), the major octane benefits begin to drop off up to E85. However, E50 fuel (mixing 91 premium with E85) can offer dramatic octane benefits up to roughly 95 to 96 octane fuel.

This mixing of E85 fuel and 91 octane premium to E30 will require you to re-jet your carburetor richer to compensate for the oxygenated ethanol fuel. My estimate would be increasing the main jet size by three to four numbers is a good place to start. You mentioned that you are monitoring the air-fuel ratio, so I assume you have a gauge.

When mixing fuel, the air-fuel ratio indicated on your gauge will indicate an erroneous reading because of the mix of ethanol with straight gasoline. These two fuels function under different optimal A-F ratios. I could get into a long description of how to calculate the difference, but there’s an easier way. Most A-F ratio meters will offer readings in either A-F or a measurement called Lambda.

Lambda is the stoichiometric or chemically correct (minimal emissions at part throttle) A-F ratio for all fuels. Lambda for all fuels is 1:1. For straight gasoline Lambda (1.0) is a ratio of 14.7:1. With your current pump gas, the best rich A-F ratio for gasoline at WOT is close to 12.5:1 or slightly richer. If we divide 12.5 by 14.7 we get 0.85, which is a Lambda equivalent of this air-fuel ratio. So with a mix of 20 percent ethanol, you can read the A-F ratio in Lambda for this mixture of ethanol and gasoline at 0.85 Lambda and know that the A-F ratio is slightly rich and safe.

If the Lambda is 0.9 or higher at WOT, that’s getting closer to stoichiometric and way too lean. If anything, slightly richer at 0.82 to 0.83 is certainly safer. At part throttle (something around 0.91 or 0.92 Lambda) would be a good number to begin your A-F experiments.

When mixing fuel, it will be difficult to constantly monitor the mixture of the ethanol and gasoline. There are sensors and gauges that will help with this but these units are expensive. Innovate makes a gauge package using a GM Flow Fuel sensor that will show A-F ratio, ethanol mixture percentage, blower boost level, and fuel temperature all in one gauge. It’s pretty cool and easy to install.

The good thing about mixing fuel is that the fuel’s octane rating goes up while your actual cost per gallon goes down! That’s a win in anybody’s book and much less expensive than adding race gas at $10 per gallon.

Hope this helps get you dialed in on your blower Apache!