Speed’s Spotlight | A Replacement For Displacement

A replacement for displacement

A replacement for displacement

Photo: Joe Kress

Swapping out cylinder heads on a Harley, going from the OE stockers to a set of high-flow/high-compression replacements, is generally looked on as a final step in the buildup of a hot-rod bike. Heads, the thinking goes, come after everything else has already been done, after the cams and the big-bore kits and the free-flowing air filters and exhausts. That’s the conventional outlook. But how about screwing on those high-performance heads before some of that’s been done, putting those heads in place instead of a big-bore package, for instance? Can that make any sense performance-wise, dollar wise, or both?
“Absolutely,” says Speed’s Performance Jamie Hanson. “But it all depends on what, exactly, that rider’s looking for. Is it horsepower or torque? And how will that bike be ridden? And what’s been done to it so far? In the right situation, high-performance heads can really add some snap.” So what’s that right situation? Again, it comes down to what you’re looking for, torque or horsepower. Torque, Jamie explains, is what gets you there. Horsepower keeps you there. And for torque the prescription is displacement and cam timing (the big-bore kit route). For horsepower, on the other hand, it’s all about flow (and hi-po heads are all about that). With the prevalence of big, heavy bagger bikes these days it’s not surprising that most often the goal of a hot-rod project is torque, the oomph to get that weight moving. Horsepower to keep things rolling might not top the list. But how much torque is enough, and is a big-bore kit to get it always necessary? The answers here might surprise you.

Let's say we have a 96- or 103-inch engine and it's already set up with a performance air filter, a good exhaust system, and some fairly aggressive but not too wild cams. Would it make sense to put a set of high-performance heads on those engines, or would it be smarter to increase the displacement with a big-bore kit first? “We do a lot of 103s set up like that—stock displacement and pistons and using the MR103/585 cams,” Jamie says. “Most often they’ll have horsepower in the mid-90s, torque above 100.” Plenty respectable in anyone’s book, and a 96-inch motor with a similar setup wouldn’t be too far behind. Adding a pair of S&S Super Stock heads, the 79cc combustion chamber versions the guys at Speed’s prefer, can bring the horsepower up to around 100 or more and bump the torque up a couple notches, as well. All this with the stock barrels and pistons. The smaller combustion chambers in those Super Stock heads will have raised the compression ratio to 10.2:1 on the 103, and on a 96-inch engine they bring things up to 9.8:1. That's a nice little motor, especially with that 585 MR103 cam set. It’ll make for a snappy and still plenty torquey bike.
A boost in compression wouldn’t be all those Super Stock heads brought to the game either. The valves, measuring 1.940 inches for the intakes and 1.575 for the exhausts are a considerable increase over stock, the ports are bigger and better flowing, the valve springs can accept cams up to a giant 0.640 inch lift, the heads can be decked up to 0.060 inch to further increase compression, and when used in conjunction with S&S Easy Start cams, like those MR103s, the stock Harley-Davidson rocker boxes can be retained. It all adds up to a significant performance upgrade and a viable—not to mention dollar-saving—alternative to increased displacement.
With an older 88-inch motor the picture is a little less clear, the argument for performance heads on stock barrels and pistons is less convincing. Compression ratio with those Super Stock heads would only jump up to 9.1:1, hardly awe-inspiring, despite all the other advantages those head offer, the valve sizes, ports, etc. And with that lower-compression ratio it would also be advisable to lower the camshaft choice a bit, down from the 585s to something like 510s. Given all that, with an 88-inch TC if the choice was between a big bore kit or the heads the big bore would probably make more sense and deliver a bigger and better bang for the buck.
For the 96 and the 103 it’s a different story. Heads can make the bike, and it’s worth a discussion with the pros. Give the SPP guys a call, or chat them up at an event nearby. Contrary to that popular slogan, sometimes there is a replacement for displacement.

Speed’s Performance Plus
(605) 695-1401 – MN
(605) 695-2272 – SD

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