Ask Away! with Jeff Smith: Talking HP, Boost & Compression in an E85 427 c.i.d. Engine

I have a question. I am building a 427 cubic inch Dart motor with LS3 heads and a Kenne Bell 4.2L blower. I am looking for maximum horsepower out of this motor. What compression would you recommend? The blower is capable of 32 pounds of boost and it will run on E85. Thanks. – T.B.

Wow, T.B., there’s a lot to discuss here.

First, that Kenne Bell supercharger is a great blower because it is a screw supercharger instead of a Roots design. This might not seem like a big deal, but the screw supercharger is a true compressor while a Roots design is merely an air mover. This means that the screw compressor tends to not heat the air as much for a given amount of boost pressure. That’s a good thing.

Right off, you say that blower is capable of 32 psi, but realistically even with E85—which is an outstanding fuel for supercharged engines—I would recommend keeping the boost numbers very conservative, at least at first until you gain some tuning experience with this package.

E85 pump fuel offers an anti-knock index (AKI) rating of roughly 100 octane, which is far better than any pump gasoline. This might encourage you to raise the static compression ratio, which will make more power. If this was a normally aspirated engine, you could crank the compression up to 13:1 or perhaps more. But with a supercharger, you’ll be better off keeping the compression ratio, conservatively, no more than 10:1. This makes the engine much more responsive at part throttle when the engine isn’t under boost.

If you haven’t assembled the engine yet, this will probably require using a dished piston. The best design is called a reverse dome piston, which uses a flat portion for the quench area and then a deeper dish to match the combustion chamber area. If the engine is built with a tight piston-to-head clearance (around 0.040-inch) this will create mixture motion in the chamber and improve combustion efficiency. This means you don’t have to run as much boost to make the same power.

As an example, JE Pistons offers a 427 inverted dome piston with a 12cc dish that according to JE’s catalog would produce a 10.3:1 compression ratio with a 70cc combustion chamber volume.

While you mention that this blower has the potential to make 32 psi, frankly, that’s roughly twice what is realistic.

Let’s do some math—we promise to keep it simple.

Our atmospheric pressure here on earth at sea level is equivalent to 14.7 psi. So if we use a supercharger to create 15 psi of boost, we’ve effectively doubled the pressure feeding the engine. This means that if we add 15 psi of boost, it’s roughly equivalent to doubling the size of the engine.

So if you’ve built a 42-cid LS Dart motor with the big LS3 ports, this should make somewhere in the neighborhood of 630 hp if we don’t spin it real high and keep the rpm somewhere around 6,500.

Now if we add a blower with 15 psi, that’s roughly equal to 1,260 hp. The power will actually be slightly lower than that because we have to subtract the roughly 40 to 50 hp it takes to spin the supercharger at high engine speeds. That means conservatively this engine could make 1,200 hp.

As we mentioned, E85 is a great fuel for this engine and we heartily recommend using it. However, as an oxygen-bearing fuel, it offers 23 percent less energy per gallon of fuel than straight gasoline. This means we will need to burn at least 25 percent more fuel than gasoline. We have to do a little more math to size the injectors and fuel pump properly.

First off, let’s just assume for a moment that you’ll run this engine on gasoline. Our friends at Aeromotive who make fuel pumps and fuel delivery systems deal with this stuff every day.

For a normally aspirated LS engine there is a reference called brake specific fuel consumption, or BSFC. This fractional number represents the pounds of fuel needed to burn to make one horsepower for one hour.

For a normally aspirated LS engine, a number around 0.450 is typical if not slightly conservative. So if we multiply 1,200 hp by 0.450—we get 540 pounds of fuel per hour. This is the amount of fuel that the fuel pump and eight fuel injectors must deliver.

Gasoline weighs 6.2 pounds per gallon, so we need a pump that can deliver a minimum of 87 gallons per hour or just about 1.45 gallons per minute at 58 psi of EFI fuel pressure. That’s a big demand. Aeromotive just released a brushless A1000 pump that can produce 2.0 gallons per minute which would be more than enough.

But wait, that math was based on gasoline. With E85, you will need to deliver more fuel.

The BSFC number Aeromotive recommends is 0.85 to 0.95 for supercharged engines running E85.

So if we multiply 1,200 hp by 0.90 BSFC, we get 1,080 pounds of E85 per hour. E85 has a higher density than gasoline so it weighs 6.6 pounds per gallon. When we divide 1,080 by 6.6 pounds per gallon, we get 164 gallons of fuel per hour or 2.7 gallons of fuel per minute—again at 58 psi.

Think about that—nearly three gallons of fuel per minute and we have to do that with high pressure. Aeromotive also has a brushless Eliminator pump rated at 3.0 gallons per minute at 70 psi.

That pressure rating is critical because the fuel injectors are going to be located underneath the supercharger. That means we need to boost reference the fuel pressure at 1:1 for every psi of boost pressure.

So let’s assume 15 psi of boost pressure with a base fuel pressure of 58 psi—which equals 73 psi of line pressure.

This is important because the pump has to be able to meet that demand at maximum power. Also, your fuel injectors must be able to flow sufficient fuel to feed that engine. Let’s look at that.

Let’s assume we will use eight injectors and they have to feed 1,200 hp. For E85 as we already calculated, that will demand 1,080 pounds of fuel per hour. Injectors are rated in either cc’s per minute (cc/min) or pounds per hour (lbs/hr). We’ll use lbs/hr because we already have the fuel flow in pounds.

At 1,080 pounds of fuel divided by eight injectors, this means each injector has to be able the flow a minimum of 135 lbs/hr of fuel at peak horsepower.

That’s playing with fire since that offers zero safety margin, so it’s likely this number will be closer to 155 lbs/hr at 1,200 hp in order to not max out the injectors and potentially run lean and hurt the engine.

There are injectors that large and rated to work with E85, but they aren’t cheap and will likely be low impedance injectors which demand a high-end EFI system to control them.

The simpler and less-expensive EFI systems will not control low-impedance injectors. Higher-end EFI systems like Holley’s Dominator, John Meaney’s Big Stuff 3, or Motec can drive up to 16 low impedance injectors, which might be the best way to go here.

We’ve just touched the wave tops of some of the technical issues you will face building this engine and installing all that power in a car.

Notice that we didn’t touch on the transmission, driveshaft, rear axle, and monster tires you’ll need to hook up all that power.

This could easily entail an investment of $30,000 to $50,000. Or, you could pull your power goal back to around 750–800 hp. It will cost a lot less and still be tons of fun.

Good luck!