Okay, I'm in the mood for a little writing, so I'll give a little explanation about why this head is such a big step forward in Enfield performance.
First, Kevin's head is a "Big Head" casting from Hitchcock's, which is a faithful and quality reproduction of the highly sought-after vintage Enfield "Big Head". Only a small number of these were made, and they were mostly used on the "Fury" high performance model that was designed to compete against the BSA Gold Star on the track. It has a shaft rocker system built into the head, similar to the AVL, and it has a more down-draft port than the regular Bullet head, and larger valves too. So, it started out with a top-grade head with a lot of good performance features already on it. There were 2 port options for this head, small or large, and we requested the small port head so that we had more room to do the porting the way we wanted to.
Then, we had Mondello's do a "high port" job on this head, so that the already "more down-draft" port was made "even more down-draft", so that the approach angle to the valve was straighter for better flow with less turning. The port was enlarged to suit the much higher valve lift that was going to be a part of this project. The new custom valves were made with longer stems to allow them to be lifted higher, and we used our Ace "beehive" ultra-light valve springs and our Ace titanium retainers to cap off the valve gear. This gives us an ultra-light mass with extremely good control over the valve spring harmonics to allow the valve to faithfully follow the cam profile without losing contact or getting "spring surge" or "float". The head also got our other standard Ace features of thermal barrier coatings in the combustion chamber and exhaust port, etc, like we do with all the Fireballs.
The big improvement came with the hi-ratio roller rockers which we had custom built by one of the biggest roller rocker manufacturers for racing in the US. They make roller rockers for just about everything, including NASCAR and NHRA and anything that uses pushrods and rockers. We contacted them and asked for a custom job, and we got it.
The reason why this is such a big deal is that the Bullet has always been a "lift challenged" design since day one. The cams are located fairly close to the crankshaft seal boss in the timing chest, and there just isn't a lot of room for big cam lobes to fit down there. And the standard rockers are a 1:1 ratio, so there is no added lift imparted to the system with the rocker arms. Whatever the cam lobes give, that's all we normally would get. This limits the valve lift to a maximum of maybe around .440" as the largest I have ever heard of for valve lift in any Bullet. Most performance cams for the Bullet don't go over .400" lift, and many(such as our own Ace Magnum Cams) have lift that is less than .400" lift. The stock lift is .3125", and sometimes less if the cams are off-spec(which is common). With the Hi-Ratio Roller Rockers, we can impart a lift multiplier at the rocker, which multiplies the cam lift by the rocker ratio employed(1.45:1 in this case). So, we can take that .352" lift that the Ace Magnum Cams have, and bring that up to about .510" lift with this new addition of these rockers. And the rollers reduces the friction and side thrust on the valve tip to almost zero, so the concerns of more stress and wear on the valves and valve guides are pretty much eliminated. With high lift, this would be a concern with a non-roller type rocker.
Why do we need more lift?
More lift gives more room for the mixture to flow into the cylinder. It's as simple as that.
For the math involved, let's just accept as a "given" the fact that when a valve is lifted to 25% of it's head diameter, then the valve curtain area where the mixture can flow is equal to the area of the whole valve head. In other words, when the valve is lifted to this 25% of its diameter, the valve is basically "out of the way" in terms of being a flow obstacle. We call this ".25D", and it is a basic goal for valve lift in a performance engine. Lifting higher than this .25D is fine, and will give more "area under the curve" and can produce some better flow at higher lifts too. But .25D is the very least where we'd like to go with valve lift. How much higher it can go is related to physical constraints in the head design. We do get higher than .25D with this head.
Let's look at the math.
In the Big Head, they use a larger valve than the normal Bullet does.
It is 1.94". If we take 25% of 1.94", we get .485". So, .485" valve lift would be the .25D figure for lift that we'd be shooting for as a minimum.
No normal Bullet, whether it's a Big Head or not, has ever been able to lift the valve that high. Most never even get anywhere close to that lift height.
A normal Bullet with a 1.75" valve would reach .25D at .438" lift, and only the most radical racing cam that I have ever heard of for a Bullet can barely reach that.
And if the typical performance valve size of 1.84" is used in a performance Bullet engine, the .25D lift height of .460" would never be reached.
So, for all intents and purposes of performance, the Bullet has never really been able to get the lift it needs, and pretty much has never hit the .25D lift height in the whole 50+ years of its existence.
This means we get to use the whole port, and not just part of it, because we can now get the valve out of the way. We're hitting .277D with this .510" lift height that we are getting on this head. No Bullet before(as far as I've ever heard of) has ever lifted the valve more than a half-inch like this one does. This is the kind of lift height that is seen in the racing Norton Manx or the Matchless G50 or other OHC bikes of that vintage racing genre. That's what we are trying to match with our pushrod Bullet. Now we can be on equal, or maybe even better terms, in the way of valve lift, as these other racing bikes that we are trying to chase down with this engine mod.
All of this equals more flow. More flow means more power, as long as it can be exploited with the rest of the design. In this case, it also means rpms. The higher the rpms, the more flow is needed because there is less time for the mixture to get into the cylinder. But we need to breathe well in the high rpms because Torque x RPM/5252 = Horsepower. The more power strokes we can get in one minute means that many more times we can get our Torque figure applied to the crank at that rpm, which means more power. So, our aim is to raise our torque peak higher in the rpm range, so we can hold on to as much torque as possible at these higher rpms, giving more horsepower, and a faster bike.
The valve lift allows more torque to be built, and with our proper porting can give more flow at higher rpms too, and it all adds up to what we need to compete against these legendary vintage racing machines which produce more than 50 hp at rpms of 7000 or higher. We need to be there with them.
Kevin's head that we modded now flows over 240 cubic feet per minute(cfm) at full lift with the bare head(no manifold on it). This equals the intake flow of an LS1 Corvette intake port. That's where we are with this head.
With the manifold on it, it drops to about 231 cfm. The manifold always drops the flow at least a little bit.
231 cfm flow, fully exploited with a full-house racing build, should be able to yield 59 hp at the crank, if everything was done perfect and it got all it could give.
It's the first time in Bullet history that anybody has done what has been done with this head, as far as I have ever heard of. And I've looked pretty hard at most everything that has ever been done to a Bullet. I don't know what Linsdell has in his, and his is very fast, but I don't think he's talking about what he's done to his. Regardless, we plan to be able to do anything his Bullet can do.
So, that's why I'm so excited about it.
Maybe I'm a technical nerd or something, but it gives me goosebumps. Because I know what this can be made to do.
Topic: Big Head Hi-Port Roller Rocker Fireball starts up! (Read 9112 times)