The following information for building a high performance Shovelhead engine was discovered in the archives of a Harley-Davidson dealership. The list of modifications has been attributed to the HD Engineering Group responsible for XR-750 development in the late 70's or early 80's. While it is probably not possible to validate this claim, some of the ideas presented in the modification list are very interesting. Many of the recommend modifications are closely aligned with performance work being done in the 90's, not 1970's technology as practiced by many Shovelhead mechanics.
This does lead us to the conclusion that the originator of this document was familiar with advances in air flow technology as practiced by engine developers like Colin Chapman (Lotus Formula 1 racing, Cosworth Engineering) in the 60's. It is an interesting footnote that the concept of high velocity port design did not make it to mainstream Harley-Davidson performance engine modifications until the mid to late 90's. This Shovelhead modification document distinctly shows "D" shaped ports, optimized for high velocity over the more popular "bigger is better" philosophy that was used then, and unfortunately is still used today.
At least one Shovel engine has been built using many of the ideas present here, with excellent results. As a favor to the Shovelhead fans that frequent the Nightrider Performance Site, the original text and drawings are being presented. Changes used in the current Shovelhead engine build are also noted.
With modification like this, it is quite possible for a Shovelhead engine to run with the EVO and Twin Cam engines. We hope you find the information helpful.
| Area to be Modified | Original Modification Description | Current parts |
|---|---|---|
| Compression Ratio | 9.5:1 | 9.5:1 Pistons by JE, Wiseco or Keith Black |
| Cylinder | Stock bore to be shortened. See figure 2. | 3 5/8" bore standard length |
| Squish Band Clearance | .040" to .050". See figure 1 | .040" |
| Piston | To be machined per figure 1 and figure 4. Piston to cylinder clearance to be set at .002" |
Keith Black Pistons Clearance .002" to .003" |
| Camshaft | Sig Erson #274 HLH
Valve lift .525" @ .053" This cam is not available. |
V-Thunder SHV-4020 |
| Ignition Timing | 36 to 38 degrees BTDC | With Crane HI-4 ignition module. |
| Valve Springs | Spring load at full lift to be 335 lbs. | STD Heads |
| Valves | Manley thin stem FL valves Valve Head Diameters 1 15/16" intake 1 3/4" exhaust Valve Stem Diameters 5/16" intake 11/32" exhaust |
STD Heads |
| Valve Guides | Use XR-750 intake valve guide with seal Use Branch or Manley exhaust valve guide. No seal. |
STD Heads |
| Valve Spring Collar | Use Branch upper and lower. Lower collar must be modified for valve stem seal clearance. See figure 8 | STD Heads |
| Cylinder Head | Machine squish band per figure 3. Intake port to be welded in per figure 6. To compensate for reduced port height, intake port must be widened around the valve guide area per figure 5. |
STD Heads |
| Air Filter | Modify existing air cleaner filter back per figure 9. | |
| Exhaust System | Use 1977 collector muffler. Header pipes to be interconnected 15" from muffler with a 3/4" I.D. tube. | 2-1 Exhaust system like SuperTrapp |
| Carburetor | Modify the standard Keihin carburetor by boring
to 40mm (1.575"). Use 185 main jet and a 100 idle jet. Use main fuel
nozzle per figure 0.
If the float bowl has been for accelerator pump bypass, the bypass must be closed off. |
Keihin 40mm CV or S&S Super E |
| Other modifications | Exhaust pipes and intake manifold will have to be modified as a result of the lower cylinder heads. The cylinder head bolts may have to be shortened by 1/8" to prevent them from bottoming. |