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Heads up for more Horsepower

Quick Throttle - Number 27 - September 1999
     I am often asked how much more power I will get if I put a set of Quantum 4-valve heads on my Harley. It was a hell of a question, and if I was going to answer it correctly I was going to bolt a set on. That's just what I did, and here's the test, along with a few tips on installation. I was given a Fat Boy with 19,185 original miles on it. The bike came stock except for an S&S Super E carburetor and a set of drag pipes.


Stock Fat Boy as it came to us.
Cylinder_heads_removed_dont_disturb_base_gaskets.jpg (66000 bytes)
Cylinder heads removed, don't disturb base gaskets.
Bring front cylinder to TDC.

     We strapped the Fat Boy to a Super Flow cycle dyno and ran it. The mo­torcycle accelerated from 40 mph to 80 mph-which is approximately 2,550 rpm to 4,575 rpm-and it pro­duced a corrected horsepower of 65.7 and a corrected torque of 75.5 foot/pounds.

      Then the motorcycle was turned over to my team. We randomly picked a set of Quantum 4-valve heads, rock­er boxes, covers, and intake parts off the shelf. We also chose a set of pre-made exhaust pipes that fit this motorcycle. All the pieces are avail­able unchromed as well. The inside of the pipes were sprayed with a heat-resistant paint and set aside for future use. A crane Hi-4 single-fire ignition with a matching crane coil was also selected.

    If you are not a motorcycle techni­cian, I strongly recommend you con­sult the Harley-Davidson shop manual on how to dismantle the top end. Start by removing the negative battery ter­minal, drain the fuel, remove tanks, exhaust, heads, etc.

       NOTE: When installing Quantum 4-valve heads on 1984 and 1985 Evo­lution big twin engines, you must use late style 1986 and up base studs (H-D part number 16387-85) or equiv­alent. Take care to set them all at the proper height.

       If you are careful and do not move or break the seal on the base gaskets, you should not have to change them unless you want to. You can use a head bolt and spacer to hold the cylinder in place while you do the rest of the work.


Set up dial indicator for TDC and new timing marks.jpg (52531 bytes) Set timing on rear cylinder.jpg (60527 bytes) Apply Anti-seize to threads.jpg (53904 bytes)
Set up dial indicator for TDC and new timing marks.
Set timing on rear cylinder.
Apply anti-seize to threads.

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