Riders who’ve been about for a although nicely remember the days of carburetors. Back then the first factor any person searching for far more Harley-Davidson performance would do was bolt on a larger fuel mixer. Bear in mind these giant SU carbs? Fast-forward. Right now we’re all dealing with fuel-injected Harleys, throttle bodies in location of these carburetors. Is larger nonetheless far better? You bet it is, and the guys at Speed’s Functionality Plus have lots to say on the matter.
“We’re continuously asked about this,” SPP’s Jason Hanson says. “A rider has the free of charge-flowing pipes, the great air filter, maybe some higher-lift cams, and now desires to know if a larger throttle physique makes sense.” The answer is yes, typically. A lot more air and a lot more fuel can make a bigger bang. But, Jason cautions, “When picking that throttle physique it all comes down to what else and how a lot has really been carried out to that motor.”
Going huge here is not usually much better. The pitfalls of more than-carbureting an engine are effectively identified, and the identical holds accurate with throttle bodies and injectors. So what’s proper and what’s incorrect right here?
“Let’s commence off with these mild efficiency engines,” Jason begins. “A TC 88 maybe opened up to 95 inches or an OE 96. A bump up in throttle body size is certainly going to be an improvement.” And for those engines, the SPP techs have identified that a very good range of throttle body size to function with is 48mm up to about 52mm. And 55mm, Jason says, is actually about as huge as you’d want to go with that mild 95-inch motor—and then only if that engine was also mounting some leading-of-the-line cylinder heads capable to take benefit of the added flow, heads like the raised port versions Speed’s likes to use. In most instances sticking with the stock injector size will be just fine in this throttle physique size range.
Remember, all this is going to be custom tuned to the optimum air/fuel ratio, and, normally speaking, those factory stock injectors will be up to delivering the added fuel essential. Injectors, by the way, are obtainable in a quantity of sizes, determined by the amount of fuel they’ll flow. It is measured in grams per second. There’s the stock version, one size up at four.9 grams/second for a 25-% increase, and two sizes up at five.three grams/second. A lot more on this in a minute.
Going up in displacement to the 103-inch engines and the 110s, a 55mm to a 58mm throttle body can be a good pick. If that engine also has those best-of-the-line/high-flowing heads, a 103 can even benefit from a throttle physique all the way up to that 58mm size. Those heads can handle the added flow of that bigger throttle body. And as soon as again, typically speaking, when you’ve moved up to anything bigger than a 55mm throttle body you may possibly want to feel about also bumping up the fuel delivery capacity of the injectors. The critical dimension here, Jason says, is cylinder bore/combustion chamber volume and a large bore coupled with a shorter stroke, such as a 114, seems to respond far more right here, he says, as opposed to a smaller bore with a longer stroke. But it all depends on the particular bike. And for the truly huge engines, the ones up in the 124 and above range? Speed’s has had excellent success with the S&S Throttle Hog 66mm throttle body, coupled with injectors two sizes up. “There’s even a 70mm Throttle Hog offered,” Jason says. “But that is for the genuinely intense cases.”
Needless to say, for very best outcomes with any throttle physique upgrade a larger than stock intake manifold should be integrated in the mix, a manifold with flow capabilities matched to that oversize throttle physique. Instantly choking down any added flow with a stock manifold can noticeably limit the gains.
Bear in thoughts, all the size recommendations right here are just that: general recommendations. Like most efficiency upgrades, choosing the best throttle body boils down to a case-by-case, bike-by-bike deal. And for that it’s best to seek advice from with a pro. Cease by and speak with Jason, Jamie, or any of the Speed’s Performance group subsequent time they’re at a nearby event. Creating and tuning engines on actually thousands of bikes during any provided season, they’ve seen it all. The schedule’s on Speed’s web site the throttle bodies, manifolds, and injectors are on the truck.
Make contact with/Source:
Speed’s Functionality Plus
(605) 695-1401—MN
(605) 695-2272—SD
Speed's Spotlight: Throttle Bodies and Injectors
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