Harley-Davidson Evolution Big Twin performance upgrades

Harley Evolution Big Twin Overview

Introduced for the 1984 model year, the 80" Evo Big Twin engine heralded Harley's return to good fortune. Whilst the bottom end is basically a Shovelhead with a lighter crank and stronger connecting rods, the top end is completely different, utilising iron lined aluminium cylinders and a much more efficient cylinder head design. The introduction of the iconic Softail frame and an engineering led approach to quality control procedures, opened up a whole new customer base to Harley-Davidson. Almost overnight, none of us had to be a mechanic or an engineer to keep one running!

Coincidently, Jaguar, during the Egan era, instigated similar quality procedures both within the factory and on their suppliers, and both companies are cited in books of the time, as being role models of how intelligent design and stringent quality control can rebuild an ailing company's reputation.

Although the engine capacity remained as 1340cc (until the Twin Cam replaced the Evo in 1999 for the the Dynas and Tourers, and in 2000 for the Softails), there were numerous changes within the engine itself during this 16 year production run, not all of them beneficial.

The massive market growth during the nineties also saw a similar rise in aftermarket products. As with everything, (unfortunately), the bulk of this is cheap "imported knock offs", but there was also a growth of well designed high performance engine parts.

We have been building Evo engines all through this era and were even sent prototype performance parts to test by the manufacturers that we worked closely with, including Head Quarters, S&S and Crane.

Having owned 6 Evo Big Twins, a Softail, an FXR and 4 FLs, all of which were used as test beds for performance upgrades, (with a combined mileage in excess of quarter of a million miles during this era), we have real world experience of the performance upgrades that we recommend. Needless to say, these experiences, coupled with the continuous R&D that we still continue to do today, and our dyno tuning capability, makes us an exceptional choice for any Evolution engine work. The reliability issues of the cheaper imported crate motors and the current high cost of the better USA built, crate engines, makes one of our high quality, reliable, performance Evolution engine rebuilds, a sensible choice.

Evolution Big Twin Stage 1

Most of these have now been done (often badly), but for those who don't aspire to the higher performance levels, but just want their bike to run well, we offer this.

The best carburettor, in our view, remains the standard CV. Many of these have been incorrectly modified using jet kits that involve drilling the slide, which changes the way that this carb was designed to work. The over fuelling inherent in these modifications is bad for the engine. We return the CV to standard with a stock slide, emulsion tube, and spring, and then use one of our Head Quarters jet kits (all of the parts are made by Keihin) to correctly fuel the engine. With an appropriate exhaust and air cleaner, our Stage 1 kits yield decent performance, no stumbling as the throttle is rolled from low rpm in traffic, and good fuel economy. We do not recommend an aftermarket carb at this level. Put the money towards the next level, a camshaft upgrade, which will give a far bigger bang for the buck. If you already have an S&S Super E or Mikuni HSR 42, we can still work with them. The S&S is very intolerant of open pipes at low rpm, but will not ice up in freezing temperatures as the CV (and the Mikuni, if it's cold enough) will.

Evolution Big Twin bolt in camshaft.

We've lost count of the number of camshafts we've tried in stock Evo motors, but the conclusion remains quite clear. The best bolt in cam for the Evo Big Twin is the Head Quarters 0024. It's as near as we'll ever get to a "one size fits all" solution. This cam is mechanically quiet, gives a steady tick over, rolls on nicely through traffic, has a very strong mid range pull, and hangs on to the power at the top end. It is as equally at home in an FXR as it is in a Glide and is also compression friendly, subject to gearing and weight of the bike. As with all of our cam changes, installation requires the use of a Torrington inner cam bearing and new lifters to ensure long term reliability. (This is true of any cam change, irrespective of manufacturer). Adjustable pushrods are not an absolute requirement, but are recommended to allow finding the "sweet spot" in the lifters.

Head Quarters 0024 camshaft

Head Quarters 0024 camshaft, programmable ignition and CV jet kit, gives excellent driveability and mid range pull in any Harley Evolution Big twin model. This is, arguably, still the best 'bolt in' Evo cam available. It is also mechanically quiet.

At this stage an ignition change is beneficial to maximise the performance gains. This cam makes power to around 5800rpm, and the rev limiter is set at 5200 on the OEM module. Whilst it isn't necessary to rev the bike hard to make this cam work, the revs do pick up very quickly in the lower gears, and even gentle riders can be up against this limiter very often, such is the way this cam performs. The OEM ignition advance curve is also based around emissions, and throttle response is improved with an ignition change. We supply the programmable Head Quarters ignition, which lives in the nose cone, or the Daytona Twin Tec module that replaces the OEM module. Both have proved reliable, and we will make the recommendation based on the year group, model, style, and any future performance aspirations. Both can be set as dual fire or single fire. There isn't really any horsepower gain with single fire, at this level, and it does require another coil, but the engines do seem to run a little sweeter. We have all the software, cables, and experience, to get the best from these excellent systems. We recommend retaining the CV carb at this level.

Evolution Big Twin stroker and big bore kits

Unlike it's Twin Cam successor, we can't make the Evo bigger without pulling the motor and stripping it. We have a number of options here, bigger bore cylinders, a longer stroke crank, or both.

The most common big bore size is the 3 5/8 bore from S&S, which gives 88" with a stock 4 ¼ stroke crank and 96" with a 4 5/8 stroke crank. These cylinders require the crankcases to be bored to accept the larger cylinder spigots, but we won't do this on the 1990-95 cases because of inherent structural weaknesses. It is also our experience (and that of other experienced builders) that this bore increase doesn't really give much of a gain. However, using the 4 5/8 stroke crank with stock cylinders (89") is very worthwhile. The torque increase with the longer stroke is very noticeable, and this is our recommended route. We can do this a number of ways to suit the budget and the application, either with stock heads and a mild cam, or with Head Quarters heads and our all time favourite Evo cam, the HQ-0023. Of all the Evo engines that we've built, the 89", tailored to suit it's application, remains a prime choice. We can also build a 92" version of this (4 ¾" stroke) on stock cylinders, which has blistering acceleration in any of the lighter bikes, for the ultimate "sleeper".

We can also use larger bore cylinders from Axtell and Sputhe, and this bigger bore size does work. The Axtell 3 13/16 bore cylinders are the largest we can use with the stock Evo bolt pattern, although building engines of this type is very expensive and has been eclipsed by the popularity of the larger crate motors. The CV carb works very well, and doesn't really give anything away until we're at the 100hp level.

Forged steel Evolution flywheels

Forged steel Evolution flywheels, H-beam connecting rods, and extended pinion shaft, balanced, assembled and trued here for a custom 100" Big Twin motor.

Custom machined and built Evo Big Twin bottom end

Custom machined and built Evo Big Twin bottom end. This hybrid 100" engine uses an Evolution style lower end and an Indian top end. Mixing and matching components in this way requires considerable experience to ensure reliable results.


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