Building Better Heads for H-D Big Twin Engines

Building Better Heads for H-D Big Twin Engines

The Harley-Davidson cylinder head aftermarket ranges from mildly modified stock castings to all-out racing billets.

The Harley-Davidson cylinder head aftermarket is broad and deep with offerings from mildly modified stock castings to all-out racing billets. We see the whole range of this market on a daily basis here at GMS. The most popular sector of this still growing market is the Twin Cam 88/96 based engines with the EVO still holding its own. I was told by a factory rep visiting the local dealer that there have been more Twin Cam engines produced than all other models combined since the birth of the company. This market should be around for long time to come.

There is a new engine starting to show up in the performance market in the form of the Milwaukee Eight, which is a four valve push rod design, but the M8 is new and not yet entrenched in the performance world. What I will do for now is discuss some options for making the more popular two valve V-Twins breathe and accelerate. The manufacturers mentioned here can easily be found on the net as they are all well-known in the V-Twin market. We will also show you just how big and bad the V-Twin has gotten in full competition form.

By far the most common cylinder head mod we do for TC and EVO customers is a simple, but very effective valve and blend job. We use a 1.900˝ intake valve and a 1.610˝ exhaust valve with a proven seat angle package and blend the chamber and bowls to wake up the port. This is a very good “bang for the buck” upgrade for any cam and/or big bore project. Just these simple changes can produce an additional 15 hp and 15-20 torque on these builds. See my February 2017 article in Engine Builder for more info and a dyno sheet on this head.

Next in line is a 2.00˝ valve modified head still using the same 1.610˝ exhaust valve. On these we remove the factory guides to allow a full port operation. Oversized intake seats are used to support the intake valve diameter. The heads are then reassembled with performance guides and stainless valves held in place with a spring and retainer package that matches the cam choice. Kibblewhite and Goodson’s AV&V lines are good one-stop sources for most street performance components.

Although we do custom work for this market, sometimes starting with an aftermarket CNC-ported head, we do not offer any CNC finished heads. We prefer to do each job that comes in on a one-by-one basis to ensure that each client gets exactly what they are looking for. That does not mean that there are not some fine CNC options out there that will produce your customers’ desired result. You just have to be sure that the heads will do what the supplier says they will do and sometimes the only way is to try them. Also do your homework to see if any other builders or riders have had experience with the product you are considering.

Harley’s Screamin Eagle line has had some offerings over the last few years that may also satisfy your cylinder head needs. These options range from stock heads that are milled for additional compression to their Hurricane competition heads. Also popular and easy to find are 110 CVO (Custom Vehicle Operations) heads. These will accept 2.100˝ and 1.650˝ valves without changing the seats making a serious but economic option for that budget build. Delkron, S&S Cycle, T-Man Performance, and R&R Cycle all have purpose designed heads for the twin cam with S&S providing EVO, EVO Sportster Shovelhead and Panhead replacements.

Keep in mind that there are varying bolt patterns available to support oversized bores in aftermarket cases and cylinders from most all manufacturers. One thing to mention here is that Axtell Sales, Hyperformance, and Trock all still do custom cylinders for one off combos. This is good to know when you’re swapping heads around on large bores.

Now lets talk about the big stuff! S&S cycle has a cylinder head option for any, and I mean ANY appetite. The standard S&S Super Stock head is a fine bolt on replacement for most any street performance build and it gets even more interesting from there.

The B2 and variant B3 heads are dominant on the street and record breakers in the AMRA and land speed racing realms.

S&S B2 and B3 (B2 variant)

The designer of a performance cylinder head must consider engine height just as they would in the automotive realm. The engine and components must fit within the chassis and under the fuel tank and/or bodywork of an unmodified motorcycle just as fitting an engine under an automotive hood. (Remember Chrysler’s Slant Six?) Getting into a serious power plant will require raising the backbone of the frame to make room for tall straight ports and the long valves that they must use. Couple this with the fact that these heads are mostly placed on tall, big bore, long stroke engines with longer connecting rods (Hood scoops or just take the hood off for you car guys).

An S&S B2 head is almost 4˝ tall and 6.150˝ tall with the S&S rocker box on top. The S&S rocker box is a bit shorter that the OEM to allow a taller head to fit several stock chassis combos. The B2 comes CNC ported with plenty of room to enlarge for big cubes. The valve size is 2.200˝ on the intake and 1.800˝ on the exhaust. The intake seats will allow 2.300˝ diameter valves to be cut in with valve to valve still adequate for the big cam grinds. It will support bore sizes from 4.125˝ to 4.500˝. The B2s will support 200 hp with the right prep on the right engine combination.

A variation on the B2 is the B3. This head is basically a shortened version of the B2 to allow the S&S T143 engine to fit in a stock FLH chassis as well as Dyna models. The large bore of the T143 also let S&S use the 2.300˝ intake valve to help feed this big bad mill. The valves are shorter and it appears to me that the ports are slightly lower to get the package short enough. This head still does an excellent job on its home power plant with crate 143s making around 165 hp (rear wheel) and modified versions reaching into the 190 hp range. Testament to this series head is that the B2 head also holds the Hot Street and Dresser records (eight total) in the American Motorcycle Racing Association (drag racing).

The S&S billet Pro Stock Head has more in common with automotive mountain motor heads than anything offered on a factory H-D.

S&S Cast Pro Stock

Back when S&S first stepped into the NHRA Pro Stock class this head led the charge. It’s much taller, 4.650˝ alone and 6.760˝ with the S&S Billet Rocker Box. Jesel also makes a very nice rocker arm system for this head. The valves are 2.450˝ intake and 1.900˝ exhaust as delivered with room for more. The ports are high and much straighter than the B2 and are also rotated for easier accessibility when it comes to intake manifold and exhaust fabricating. They will support from 4.500˝ to 4.800˝ bores with 5˝ bores sometimes being used. These heads will support a 160 cubic inch v-twin at 8,000 rpm and produce 250 to 275 rear wheel hp. These are the heads that hold the records in Outlaw Street and Pro Gas (AMRA) as well as several land speed marks. You won’t see many of these around but when you do, they are bolted to a serious machine.

S&S Billet Pro Stock for Buell and Victory

This is a clean sheet design aimed directly at NHRA pro stock. Where the cast Pro Stock still uses the stock EVO valve train architecture, this beauty is all different. It resembles a GM Pro Stock car head in valve train and combustion chamber design and is carved from a solid block of aluminum. The total head is 8.700˝ tall with rocker covers in place. The intake valves are in the 2.700˝ range and the exhausts are 2˝. The bore size on this purpose built rocket is 5.125˝ and swings a 3.875˝ stroke for a 160-inch total displacement. Up until last year, this all billet 60-degree engine used a conventional knife and fork connecting rod and multi-piece crank shaft/flywheel assembly with in-line cylinders. The mill is now being produced with a one piece forged crank/flywheel and side by side automotive style rods. These heads will feed this engine enough to churn out 325 to 375 rear wheel hp @ 10,000-plus rpm! In NHRA- legal trim, these heads can be seen helping to produce elapsed times in the 6.70s and speeds in the high 190s on the Buell and Victory entries.

When it comes to rocker arms and the related components, the forged replacement roller rockers from S&S cycle and R&R cycle are great upgrades. We’ve run this basic valve train with these parts to 8,500 rpm and 1,000 lbs open spring pressure. We also have our own 4140 steel rocker supports that replace the S&S aluminum ones as well as pushrods made for us by Smith Brothers. As I mentioned before, Jesel has an excellent valve train system that can be done on several different designs of castings as well. The valves for most all of the street performance heads are stainless made by many of the same manufacturers that make automotive valves. When it comes to racing Titanium is the natural choice. Victory 1 Performance offers a true one piece forged valve that we use exclusively as well as forged titanium retainers and locks.

Other Things to Consider

Although we do quite a bit of testing on our Saenz 680 flow bench, I’ve not said anything here about flow numbers. The reason for that is both simple and complicated. Almost all of the published flow figures available do not tell us anything but max numbers. Some ads or web sites don’t even mention the depression that the numbers were obtained at. None of the ads for cylinder heads ever say what the minimum cross sectional area is. Most of us being engine guys knows that an open barn door will move tremendous amounts of atmosphere but will not make horsepower. The same goes for drawing air through heads. The head must be correctly sized for the job with the straightest possible path to the back of the valve from the beginning of the intake tract.  I feel that providing approximate power figures and rpm range is a good indicator of potential.

Even if max hp numbers are compared when discussing heads or any other components, you must take into account the torque curve and how the engine actually performs in it’s intended role. A high peak hp reading wont do much good if the engine has lazy torque in the middle ranges. This is especially true in the street world. As I’ve stated before, don’t get caught up in a dyno or flow bench war with other builders when that is all they are advertising. The street performance build must perform well at all throttle positions in all of the usable rpm ranges to satisfy the client.

Also common engine building experience is important when putting a good combo together. Don’t use excess compression to make mid range torque when the heads are too big or the overall package is mismatched. Make sure you know how your customer rides and the intended use of the proposed build. When building street or touring/cruising applications, we like to stay to the conservative side which has proved to be a win with our customer base.

When it comes to serious performance or racing, we first look at the rpm range the engine will live in and the goals the client is expecting. Then we discuss service or maintenance , budget, expected life of the engine, cost, and where responsibilities lie. Finally, I can’t say enough about learning to spot the difference between performance use and abuse. Educate your customers to properly care for their new investment and you will have a head up on your competition.

This article courtesy of Engine Builder.

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