Just wanted to post some technical mumbo jumbo regarding the heads, posted on the Enfield forum by my head builder, Tom Lyons.

These heads flow 30 cfm more at peak lift, than the stock heads do. That's for each head.

There are estimation formulae which are used to make some useful predictions about power from air flow, and we use those to help guide us in our work. For a high performance build with high compression and all the proper tuning methods for making highest power, the estimation is .256 times cfm. Since this is just about 25%, we can use an easy rule-of-thumb and say that there is about one hp for every 4 cfm of peak airflow, in a racing grade engine. This is an estimate method, and it can be a little more or less in the real world, but it gives a useful directional indication of what the head will probably be able to do.

In Scottie's heads, our work has provided 160 cfm peak flow per head. This is an easy one to figure, because it comes out evenly when divided by 4.

160/4 is 40 hp. That's at the crank.

And there are two heads involved, at 40 hp each, so that's a total output estimate of 80 hp at the crank, from this engine in normally aspirated(carb or EFI) full race tune.

Now, there are other ways to look at this, based on factory performance levels or street use.

We can look at this engine, and see that it peaks at 130 cfm in factory form. And we see Scotty saying that the bike is rated at about 40hp from the factory in its 1958 form when new. That's 20 hp per cylinder. So, we can divide 130 cfm by the factory 20 hp per cylinder, and we find that this factory engine requires 6.5 cfm of peak airflow to make one hp, instead of 4. This is due to a lower stage of performance that the factory street machines often reflect.

So, to extrapolate the basic street tuned expectations with standard compression and similar tuning as a factory bike might be, we can divide our 160 cfm by 6.5 and get 24.6 hp per cylinder, or a total estimate of 49.2 hp, just by putting on this head, not including any higher revving of the engine or anything else. Same rpm limits as stock. That's nearly 25% increase, essentially all from increased torque.

But hp comes from torque x rpm, so we also made the heads to allow higher revving for that purpose. In fact, we made it to rev about 30% higher than stock. So, take that 50 hp, and times it by 1.3 and we get 65 hp if it goes up to 8450 rpm. That's probably more than a street bike is going to be able to do. So, let's look at a more reasonable rpm max for the street, at like 7200 rpm, and multiply by 1.11, and this gives us 55.5 hp for a reasonable expectation of street power at reasonable street max rpms for a street built motor with a bottom end that can handle 7200 rpm. Approximately 40% power increase at that stage of tune.

So, depending on how you want to build this engine, I'd estimate the power range to fall between 50-80 hp at the crank. The more "racy" you build the engine, the more power it will be capable of. The breathing is there for it. It all comes down to the level of the rest of the build, and the rpm that you wind it up to. These heads can go over 8000 rpms. That's where the max power levels will be. But, do we move enough air?

Let's see.

8250 rpm has 4125 intake cycles per minute.

4125 intake cycles of 350cc each equals 1443.75 liters per minute.

We want to have availability of 125% volumetric efficiency if we can get it, so 1443.75 x 1.25 =1804.6875 liters per minute is needed.

There's 28.3 liters per cubic foot of air. So, we divide 1804.6875 by 28.3, and we get 63.76 cfm.

However, we only get about 2/3 of a revolution on the intake cycle to get that air in, we have to multiply that by 1.6 to get that same amount in in that shorter time period. So then we get 102 cfm needed to feed this engine at 8250 rpm at 125% volumetric efficiency.

Okay, so we have 160 cfm, but it doesn't flow 160 the whole time, because we have the valves getting open, and then closing down. So, we can look at the average flow rate over the whole lift cycle which is typically about 2/3 of the peak flow rate. I don't have the chart in front of me right now.

So, 66% of 160 is 105 cfm estimated average flow out of this head.

We need essentially 102 cfm to feed it, and we are averaging about 105 cfm over the lift cycle, which gives us just a little more than we need, just in case.

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Okay, now let's look at the port size.

Our 1.25" port gives a minimum cross sectional area of 1.227 square inches.

A very well ported head that flows exceptionally well can utilize a mach index of up to 0.6 of the speed of sound before choking. That's about the limit. We strive to size our ports to reach our mach limit at the approximate maximum rpm that we plan to reach, in normally aspirated(N/A) trim.

So, let's see how we did.

When we input the Super Meteor engine specs into the mach index calculator, along with the .040" overbore size of the piston, at 8250 rpm, and .500" lift, and a 1.227 sq. in. port size, here's what we get.

**Intake Port Mach Index**

Your bore size is 2.8 inches with a stroke of 3.54 inches

and with a valve diameter of 1.5 inches and cross sectional area of 1.227.

Running a valve lift of .500 inches at 8250 RPM,

Your intake port velocity is 307.22 fps

Your intake valve mach index is 0.59

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I think that's looking good to me.

So, we can feed the engine the right amount of air to hit 8250 rpm at 125% volumetric efficiency, which is plenty of available air.

And, we can reach the target rpm of 8250 in N/A trim, just a hair below the choke speed, so that we get the fastest moving air possible at the time that we need it.

Everyone following that?

You ain't getting that kind of work off anybody else doing these engines.