[AVL Power!]

A lot of people underestimate these motors Tom. Wait till you guys start setting records, things will change real quick


-Sanket

[ace.cafe]

DanB, I am looking forward to videos and reports from Scottie too, and I'm sure that he will oblige!

Scottie, and others, regarding the performance increases involved with this porting and rocker system, it is fairly straightforward to do some relative estimates about it, based on air flow.

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.

[ace.cafe]

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.
------------------------------------------------

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

------------------------------------------------------

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.
 

[High On Octane]

Excellent explanation of how these things work Ace!  I would say that you have definitely built these heads to the absolute MAXIMUM potential.  I absolutely cannot wait to bolt these babies on! 

Just to put things into perspective:  The work that has been done to these heads is the exact same work that is done to 3000hp top fuel dragsters, just on a much smaller displacement motor.  Usually just being able to get a 15-20% increase in power is a nice accomplishment.  In full race trim, this motor has the potential of producing 80bhp.  That is 100% increase over stock.  Absolutely IMPOSSIBLE to achieve with bolt on parts alone.

[ace.cafe]

It's about 1.875 hp per cubic inch.
Or about 114 hp per liter.
Hard to do much more than that with a small bore long stroke aircooled pushrod twin from the 1950s.

Even Ducati only manages 92 hp/liter from their aircooled 2-valve twins, and those are pretty much the current state of the art in todays aircooled twins. Maybe their race versions are up around what we have, but they have overhead cams and much bigger bores and shorter strokes than we do, which makes a big difference.

The power potential of this Enfield twin platform is pretty potent.
With a supercharger it would probably do 3hp per cubic inch which would hit Scottie's goal for that blown version(125hp).

[High On Octane]

Just a hypothetical thought for further down the road.......

I started thinking about this yesterday.  I don't know the clearances off the top of my head so this would take further investigating to actually make possible.  BUT, If I were to custom machine a 1 piece cylinder for my twin that would accept 79.75mm pistons and the original 90mm stroke, that would make my bike just cunt hair over 899cc............

Thoughts? 

[ace.cafe]

Just a hypothetical thought for further down the road.......

I started thinking about this yesterday.  I don't know the clearances off the top of my head so this would take further investigating to actually make possible.  BUT, If I were to custom machine a 1 piece cylinder for my twin that would accept 79.75mm pistons and the original 90mm stroke, that would make my bike just cunt hair over 899cc............

Thoughts? 

I think it comes down to crank spacing between rods.
You can mod the barrels, and also make bigger holes in the top of the crankcase for bores, but the rod spacing will probably be the determining factor.

[motorman2whel]

WOW Scott your going to have one wild ride ! , That's Very Cool .

[High On Octane]

OK  Tom, so I when I go to install these heads and the pair of Mikunis, what do you recommend for initial settings, assuming I have the .312 cams good for .452 of lift no base gaskets unmodified cylinder height?  To refresh your memory, here is a pic of the current motor with base gaskets at TDC.



I've never had the cylinders off so I have no idea what pistons these are or what it has for rods.  I do know that the pistons are marked .040 and aside from spewing oil everywhere she runs like a raped ape.  I beat the hell out of it and she never knocks, whines or complains.  Mechanically I haven't had any issues other than the oil return ports getting clogged with silicone.  And remember that the 1st time I replaced the head gaskets the cylinder had a brand new cross hatch that had even been seated yet, so for the first 1500 miles I treated her like a new motor and broke it in properly.  I'll do a compression test sometime in the near future and see what it is at now, I do know it pings on regular fuel, not sure if that is timing or fuel related.

Scottie J

[ace.cafe]

You have to do a baseline test of the rig for piston-to-valve clearance first.
I have no idea what those cams have for lift at/around TDC. So, you must put some soft modeling clay wrapped in Saran wrap, squeeze the clay into disc shapes approx .100" thick, and place them on the piston crown on the valve reliefs. Put the head on with a used gasket, and tighten it down, and set it all up to run.
Then rotate the engine SLOWLY by hand for 2 full rotations, OR UNTIL YOU FEEL ANY BINDING OR RESISTANCE LIKE A VALVE HITTING A PISTON. If you feel something like that, then stop immediately and back it off the other way. You want to do it slowly and carefully enough so that if it does touch the valve, you aren't pushing it hard enough to bend anything. After you get your 2 revolutions, take off the head and look at the clay, and see if there are any indentions in it, and how deep those indentions are, by measuring the thickness of the clay at the thinnest point of the indention left on top of the piston.
Once you get the first measurement, you can decide if you want to use more or less gasket thickness under the barrel. You should have a minimum of .060" clearance from each valve to the piston crown, for safe margin.
Then, once you know that nothing is going to be hitting, you can do a compression test to see where you are.

[High On Octane]

Ok.  I should be able to reuse the piston rings, shouldn't I?

[ace.cafe]

Ok.  I should be able to reuse the piston rings, shouldn't I?

I would expect so, unless there is some reason to suspect some problem.

[High On Octane]

I just found these.  What do you think?

http://www.classic-motorcycles.co.uk/cgi-bin/ecom.cgi?Command=ShowProduct&db_pid=4903

[ace.cafe]

I just found these.  What do you think?

http://www.classic-motorcycles.co.uk/cgi-bin/ecom.cgi?Command=ShowProduct&db_pid=4903

In a word, NO.
Make custom pistons. There is nothing worth a crap off the shelf.
You can get a minimum order of 4 from any piston company, and you have 2 twin cylinder engines. It all adds up just right.

[ERC]

I like the way it says "picture not actual one" nice.  ERC