Starting a high performance Harley engine can be a difficult matter and it has been for several decades. Now, however, the problem has become more than merely difficult.
Back in the Shovelhead days, the main difficulty was getting enough power out of the starter to turn a high-compression engine. Some aftermarket suppliers like S&S even adjusted the design of their cams to reduce cranking pressure enough that the stock Harley starter system could rotate 98-inch strokers. Things generally didn't break, though. The hard-cranking problem persisted with the early Evo models until Harley-Davidson improved the power output of their electric starting systems.
However, starter parts began to break in engines with high cranking pressure and some poorly designed aftermarket ignition systems. I'll explain.
First, a definition: cranking pressure is the pressure -- usually in PSI (pounds per square inch) -- an engine develops during a normal compression test. T measure this pressure, a technician (mechanic) removes the spark plugs and threads a compression test gauge into one of the spark plug holes. He/she then opens the throttle fully and cranks the engine over until the pressure gauge stops climbing. This normally requires five to seven compression strokes. The tech then records the pressure and repeats the procedure for the other cylinder.
The higher the cranking pressure, the more power required to rotate the engine. Stock cranking pressures for the Evo and Twin Cam are around 150 psi and 160 psi respectively. A high-performance Evo can exceed 190 psi and the Twin Cam 200. Such pressures, multiplied by larger bore sizes, increase the load on the starter system even more than the 20 percent increase from the higher cranking pressures alone.
Some early aftermarket ignitions (Crane Hi4, Dyna 2000, CompuFire Elite and maybe others) could fire too soon causing a very powerful "kick-back" and starting system damage. Here's how that happens: Electronic ignitions turn off if the system voltage drops below a certain value. These earlier ignitions energize the coil as soon as they are turned on. If they turn off due to sagging voltage during cranking, the coil fires no matter where the piston is. Starter system damage due to this too-early ignition is inevitable.
Harley solved the problem by making the ignition wait until the engine proved it could turn over. Harley electronic ignitions do not energize the coils until they have received at least one and I believe two signals from the timing sensor indicating the engine is rotating and isn't likely to stall part-way through a compression stroke. As far as I know, all aftermarket ignitions now follow a similar scheme.
So, unless you have one of these earlier aftermarket ignitions, you are unlikely to break starter parts because of high compression and a low battery.
So, why is starter damage a more severe problem now than it was a decade ago? Twin Cam engines, because of their better combustion chamber shape, can run well at higher cranking pressures than could the Evo. They are also bigger, with some examples exceeding 120 cubic inches. Thankfully there are no current ignition problems similar to the ones that plagued the earlier Evo engine builders.
What we have now is Dieseling. Yep, compression ignition. And, in this case, at the wrong time. The starter ring gear you just shredded may be shards in the bottom of your Harley's primary cover because of premature ignition due to a too-high cranking pressure. And, are you ready? The culprit could be your tappets.
Let's go back to cranking pressure for a moment. The two important factors in determining cranking pressure are: compression ratio and cam timing. Everything else being equal, higher compression ratios result in higher cranking pressures. Everything else being equal, the earlier the intake valve closes, the higher the cranking pressure. Neither means much without the other. The main reason that high compression ratios work so well with those "big" cams is that those big cams close the intake valve later and a high compression ratio is necessary to get the cranking pressure up.
All Harley-Davidson air-cooled V-twins use hydraulic tappets to maintain correct valve lash adjustment and to keep things quiet. An Evo or Twin Cam cylinder/head assembly can grow more than 0.040 inch between room and running temperature. The hydraulic tappet lengthens (pumps up) to accommodate this growth. When the engine cools and shrinks, oil within the tappet chamber bleeds out and the tappet shortens itself.
Stock and most aftermarket hydraulic tappets have a total hydraulic travel of about 0.200 inch. Normal pushrod lengths, whether solid or adjustable, place the tappets in the middle of their travel at room temperature. Thus, the tappet has about 0.100 inch of reserve travel for growth. It is this 0.100 inch of available travel that is causing much of the broken starter trouble. Anytime one of these engines stops at least one valve is open. Which one and how far depends upon the position of its cam lobe. The force placed on a tappet by an open valve is rather large. It can range from just below 200 pounds to over 700, depending on the position of the cam and the valve spring design.
If your high-cranking-pressure, heavy-valve-spring engine happens to stop turning with an intake valve open 0.150 inch or so, it "bleeds down" and bottoms the tappet. This can happen in just a few minutes. As a result of this tappet bleed-down, the intake valve is going to close much sooner than it is supposed to, and the already high cranking pressure is going to escalate dramatically.
If you start this engine while it is still hot yet after the tappet has bled-down, the cranking pressure and chamber temperature might become so high that the air/fuel mixture ignites without waiting for the spark plug to fire. Your engine has just become a diesel.
Because this combustion happens much too soon (in crankshaft degrees), the engine backs-up and hammers the various parts in the starting system. One Harley owner I spoke with tested this theory by removing the spark plug leads and then engaging the starter - the engine fired, backed-up and broke a tooth off the ring gear.
The Cure:
There are two things you can do to prevent dieseling and damaging your expensive, powerful engine: fit compression releases, and limit the hydraulic travel of your engine's tappets.
Compression Releases:
Compression releases have become a standard part of high compression, large displacement hop-ups during the past several years. Their purpose is to reduce cranking pressure so the starter can turn the engine with enough authority to get it going. Compression release installation is a machining operation and usually requires removing the heads. I have not heard of a single starter breakage problem when compression releases are used.
Operation is simple: the rider pushes both compression release knobs down and starts the engine normally. When each cylinder fires, its compression release valve closes and the engine runs normally.
Limited Travel Tappets:
If you limit the travel of your engine's tappets, the distance they can be pumped-down is also limited. If they do not pump down very far, the chances of extreme cranking pressures and dieseling is much smaller. Tappet modification or replacement does not require any machining or head removal.
There are two ways to limit a tappet's hydraulic travel: First, you can fit travel-reduction spacers under the hydraulic unit in the tappet body. S&S makes these for standard Harley tappets. Secondly, you can replace the stock tappets with special limited-travel tappets. JIMS makes such tappets for Evo, Twin Cam and Sportster engines. The S&S travel limiters reduce total hydraulic travel to about 0.065" and the JIMS Hydrosolid tappets have 0.050" hydraulic travel.
The common adjustment procedure is to bottom these tappets at room temperature. So, a hot engine with its intake valve open can only pump the tappet down about 0.040 or so inches. Upon starting, the cranking pressure should then not become so high that dieseling can occur.
Summation:
Many perfectly capable Harley mechanics are spending countless hours to solve their customers' broken starter problems, and their customers are investing many thousands of dollars for more powerful starter motors, stronger ring and pinion gears, Bendix units and even the occasional tranny main shaft. All these things are good and useful, but they may not be entirely necessary.
The big problem is not in the strength and power of these parts, it is the very powerful, explosive back firing caused by dieseling. If performance engine builders fit compression releases and limited-travel tappets to all high-cranking-pressure engines and if the riders faithfully use the releases then there shall be very few blown starters.
The dieseling phenomenon is not obvious except in retrospect and not many engine builders know about it. If you know anyone who could make use of this information, pass it on.
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