.png/:/rs=h:200,cg:true,m/qt=q:95)
.png/:/rs=h:104,cg:true,m/qt=q:95)
Turbo Kit Development Timeline
Revisions 1-3, February to July '22
We tested many, many changes during this timeframe, especially with regards to the timing map and curve. Blew 2 headgaskets figuring that out; suffice to say, getting the timing correct with a turbocharger is the most important thing with regards to reliability, and the only real way to figure that out is to test, tune and repeat. We were trying to use just the Dynatek 2000i during this stage which wasn't working out too well. The hobbs switch curve provided by Dynatek sucks (has too much midrange timing, and isn't even a curve, is just flat); you can run curve #4 and retard the base timing, but then the bike runs too hot, has crappy economy, and generally only runs good on boost.
We also tested all of the HSR carbs, 42/45/48, and did tons of jetting changes; different needles, accelerator pump nozzles, float heights, different air filters, mechanical methanol injection (didn't work). It took a ton of tinkering, but we got the bike to the point where the driveability is pretty close to stock. I.E. pop the choke, start it up stone cold, let it idle for 20 seconds, put the choke in and take off. The adjustability of the Mikuni HSR allows us to get the jetting very, very dialed in. AFR's during cruise are in the high 13's, low 14's, dip down into the low 12's at moderate throttle positions and 2-4psi of boost, and are well into the mid 11's under full boost.
.JPG/:/cr=t:0%25,l:0%25,w:100%25,h:100%25/rs=w:400,cg:true)
Revisions 4-7, Still using T28 Turbo
August '22 to February of '23
7,500 Street miles at this point, countless Wide-Open-Throttle pulls. I did a 1-4 gear pull every time I got on the freeway both ways, so at least 2 pulls a day for 12 months, sometimes 10 on an actual proper "ride". Racing people, stoplight fun, random stop-sign runs, a TON of turbo-4 cylinder car kills (STi's, Evo's,GTi's etc.). I rode/ride the bike hard.
This bike was, and still is, my DD. So much data, feedback, and knowledge gained at this point, I knew that I had to go back to the drawing board 1 last time, and re-make the kit with what I'd learned riding the bike every day for a year. I'd also built 3 more draw-thru turbo bikes while riding this bike every day, and dialing in completely different bikes (XJ650 inline four, Twin Turbo Virago) with different cylinders/turbo setups...you learn a few things.
Summary of these revisions; Tested multiple merge collector designs, tried an external wastegate (better boost control). Wasn't happy with the power and spool of this setup, finally found a machine shop willing to convert a smaller turbo to carbon seal, which was the impetus for re-designing the setup from scratch.
Version 2.0 - Clean sheet re-design - Revision 1
Quick Spooling Turbo (midrange monster!), Custom Ignition System
This new setup is very close to the final form of our Turbo Kit. There are some changes that still need to be made, for example, moving it slightly to work w/forward controls, but this is pretty much it. Double the plenum volume, a turbo that's close to half the size of the original, a far superior, completely custom ignition box with the option for 2-step launch control, outputs for a shift light/fuel pump, inputs for a quickshifter and more. It's so much more fun to ride with the faster spooling turbo.
We're currently testing the new setup, making small tweaks here and there. We're going to be comparing different size plenums, and the comparison video will be over on Patreon. We'll be comparing the current plenum to one that's 40% smaller, to see how the bike responds. A direct, back to back comparison.
Version 2.0, Revision 2
In this Revision; Billet Wheel Turbo, MAP sensor, Reduced Plenum
This is the Final Revision of the DoctorMotorcycle Draw-Thru Turbo kit for 1986-2003 Harley Sportster's. In total, there were 7 revisions on the initial setup and 2 on the clean sheet re-design. We took everything we learned in our street testing of the old kit and applied it to the new kit. This Final revision is just a few minor tweaks to get that last 10% out of the setup, and increase longevity.
We did a back-to-back plenum size comparison with a large plenum and a smaller plenum. We also switched to a MAP sensor controlled ignition, which required about a dozen or so ignition tuning tweaks. We have to be careful to not get too carried away with timing, but also not leave too much power on the table. The ignition map also needs to be conservative enough that it can be ran in the scorching heat of Arizona Summers and not cause any trouble.
There's also some small tweaks we've made with regards to part-throttle timing that have picked us up 5mpg when compared to running a basic hobbs switch map. This is because we have full control of the timing at all Manifold pressures, which allows us to run much more ignition advance at idle, part throttle and rolling into boost. This also has the side benefit of making the bike run cooler, especially around town where there's a lot of idling at stoplights. This is a massive step up in quality, reliability, and especially performance, both under boost and out of boost. The most noticeable change with the MAP controlled ignition is out of boost. It's just so much more snappy and sensitive to small throttle inputs.
Version 2.0, Final Revision (currently available)
This is the final iteration of the turbo setup, and is the result of over 3 years of riding, thrashing and tuning. We've had many ups and downs along the way, and almost gave up on carb's a few times, but we persevered.
On this final iteration, we wanted to see if we could squeeze more midrange power out of the setup by making a few changes to the plenum. One of those changes was to increase the length of the intake runner - it's now 2" longer than the prior plenum. Another thing we did was incorporate a velocity stack. This is hard to do on a draw-thru because the plenum has to be designed in such a way that the fuel can't puddle up. Ultimately, we made it work, and this is incorporated in our turbo setups from here on out.
The last thing we wanted to do is make the plenum look a bit nicer. The 3D printed plenum cover was cool, but we are aware of the stigma around using plastic parts, and don't want to cheapen the look and feel of the kit, so we switched to a CNC billet aluminum ring with a metal grille, which looks the business. We couldn't help but feel this is the cherry on top, and a great way to conclude the development of this kit.
We're currently offering these installed, tuned and ready to ride for $4,000 out-the-door, just drop your bike off here in Phoenix, and we'll do the rest! Use the contact form on the main page to get in touch!