Thursday, May 4, 2017

Inside the Patriot Missile

So, as with almost anything one might endeavor to write about, this one has a “back story” of sorts.  A number of years ago, I had an on-line discussion with the legendary drag racer Granddaddy Joe Smith.  In the course of that conversation, Joe told of one of the many engine configurations with which he competed in Top Fuel, that being a Shovel featuring one injector on each head.  The thing that caught my attention, was the way in which Joe achieved the individual intake tracts.  Ingeniously, he used two rear heads, turning one 180 degrees and mounting it in the front position.  What I found especially compelling of this set up was the how few special parts are required.  The cylinder base stud pattern on a Shovelhead is “square.”  That means rather than having custom built cylinders in order to turn the head around, one can simply turn the whole cylinder and head assembly 180 degrees.  Another neat little part about this method is that since the intake and exhaust ports remain in their original orientation with respect to the cam it does not require the use of a specially ground camshaft as some other conversions might.  One other note concerning this conversion would have to include a credit to Turk Dale, another legendary drag racer who also campaigned a Shovelhead in this same configuration.

Now, seeking to put into practice a principal presented to me by an early mentor in the field of Harley mechanics, the second thought I had (after, “I really want to build one of those!”) was, “who can I find to pay me to build one of those.”   That’s right, the principle is, “let someone else pay you to experiment.”  I even came up with cheaper way to build such a motor: start with an Iron Sportster.  However, after several years of the idea rattling around in my head, it became clear that I would find no one in the market for such an exotic piece, at least no one willing to spend more than nothing to make it happen.  But once my good friend Kevin “Teach” Baas let it slip that he had a Sportster motor that was destined to be a project for the students in Kennedy High School’s Chopper Class, the old light bulb started to glow.

If I couldn’t make any money building a cool conversion such as I envisioned, the next best scenario would be to donate my labor to a worthy cause, in what would eventually come to be the vintage style drag bike known as the “Patriot Missile.”

Kevin has been teaching a class designed to get his high school students interested in what we used to call “the industrial arts” at Kennedy High School in Bloomington, Minnesota.  To make the idea of learning how to weld and fabricate more attractive, Teach came up with the idea of building choppers as a focus of the class.  It has been very successful, to the point where the idea has caught on at various schools around the nation.  After all, imitation is the greatest form of flattery.  And I guess that makes this engine build a form of flattery aimed towards Granddaddy Joe and Turk.

In case any of you are wondering how well this conversion, pioneered by Turk and Granddaddy Joe on Shovelheads, applies to an Iron Head, the answer is, “just fine, thank you.”  There is only one caveat, and it actually makes the whole process even one step simpler.  Since the cylinder base pattern on a Sportster is NOT square, you can’t reverse a cylinder, but the beauty of it is that you don’t need to because on an Iron XL the headbolt pattern is square! 
Since this engine build was destined for the drag strip, I took the liberty of machining a goodly portion of the cooling fins off both the heads and the cylinders, however I did spend the extra time to mock everything up first with the fins intact to confirm that the conversion would also be viable on a flat tracker or even a street bike. If you chose to build your own version of this, you will also find that it will entail some trimming of fins for pushrod tube clearance and such, but that is a fairly easy hurdle to clear.  During the process, I even machined spark plug wrench clearance into the stock front rocker box, which because it now resides on a reversed rear head, has its spark plug on the pushrod side.  Using a front rocker box in its original orientation on a reversed rear head is not quite a bolt on.  Some of the rocker box mounting holes wind up being about “a 1/4 of a hole off” which is easily remedied with a long end mill of the correct diameter.  However, the two mounting holes closest to the center of the head do keep the rockers in proper relation to the valve tips.  Since this was to be a drag bike, before I was through with the rocker boxes, I split them, lightened and polished the rockers, and added grease zerks to the shafts to provide lubrication.

The iron heads were where most of the time was spent, and indeed the area that requires the most modification for the conversion.  Rather than simply welding an extension onto each intake port, I chose to build separate “manifolds.”  The extra length required to provide carb clearance would have made it very difficult to do a good job of porting, so fabricating a couple extra flanges to make the extensions removable was time well spent.  Since porting work is one of my specialties, this project was a good opportunity to push the envelope a bit for Iron heads.  This began with a 2.060” intake valve, a full 1/8” larger than the largest valve Harley ever put in a Sportster.  To compliment this larger valve, the port diameter was also increased from its stock 1.565” to 1.680”.  The large intake valve resulted in the need to cut down a 1-3/4” exhaust valve to a more “valve to valve clearance” friendly 1.650”.  The use of hotter cams would have necessitated either an even smaller exhaust valve or perhaps a 2 inch intake.  If this was to be something other than a drag bike, I would have needed to convert the heads (or at least the one in the front position) to external drain lines, but because of its intended purpose that step could be ignored here.  As previously mentioned I fabricated steel flanges to weld onto the cast iron heads, but then switched to aluminum for the actual intake port extensions.  During initial mock up, I found that a straight port extension would not provide carb clearance without making them considerably longer than a theoretically correct tuned length called for.  Since a curved port extension would be difficult to come up with, I chose to put a 5 degree angle on each end of the extension, knowing that was a small enough change of direction to cause little loss in air flow.  The old round slide Mikunis, being narrower, would have been a better choice here, but I was fresh out of them, and have been for a couple decades.

As for the flow figures, the heads came up to 267 CFM at .600” lift (at 28" test pressure) on the intakes, and though they were still gaining at that lift, I didn’t bother checking them past that point since we were stuck with the .400 lift “P” cams anyway.  Curiosity may have gotten the better of me, but checking them at higher lifts would have necessitated shorting the tops of the guides.  The 267 figure comes out to be about 45 CFM more than a stock head of similar vintage, and even the .400” lift figures showed nearly a 40 CFM gain.

A bit of further explanation may be in order here before continuing with the engine description.  The Patriot Missile project, despite its name, did not have the luxury of being funded by the U.S.  government.  Most of the parts were either donated, “scrounged” or purchased by Teach.  The motor had reportedly been recently rebuilt before it was donated, but to stock specs.  The pistons showed this to be the case, being new +.040 JCC reproductions sporting stock compression ratio.  Stock Sportster “P” cams inhabited the cam chest.  Since high compression pistons and hot cams were not in the non-existent budget, I decided to do what drag racers did in the days before specialty parts were readily available and success depended less on cubic money
The cams would have to stay, but there was one thing I could do about the pistons.  After all, electricity, argon and filler rod all add up to peanuts compared to custom made high compression slugs.  After a goodly amount of time welding, grinding, machining and measuring, the domes came in at 60.7cc, which put the final compression ratio at just over 11:1.  Not surprisingly, the extra material in the dome added a fair amount of weight.  To avoid the necessity of splitting the cases to re-balance the flywheels, a like amount had to come out of the bottom of the piston domes, along with some other strategic areas of each piston.  Leaving out the middle ring also saved a little weight, while decreasing frictional losses at the same time.

In that same vein, selection of carbs was based on two criteria; price and availability.  Several years ago, I purchased a box full of old OEM butterfly Keihin carbs that another shop had designated as parts carbs.  That fulfilled both points of the criteria.  The Keihins were in hand and for all intent and purposes they were free.  Once a matched pair of them were separated from the herd, all that remained was to clean them up and find a way to get some air through them.  Never having bored one of these before, I decided to creep up on the maximum size possible, and found the 38mm carbs could be taken out to 41mm, though that was certainly pushing the limits. 

At this point in time, I have to say that I am quite pleased with the results, especially considering the extremely low cash investment.  Given the dead stock lower end, I don’t expect to see record setting performance, but I do expect to see a smile on Teach’s face when he takes it down the track at the Meltdown Drags in July, ….and I hope that it puts a smile on the faces of a couple of legendary drag racers who provided the inspiration!