Update 6-7-2026 Below

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.

Close-up of a motorcycle engine with a turbocharger and air filter.

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 2/3, 2024 to 2025

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 3.0, New Turbo, Fine Tuning, Late '25 to Present day

From 2024 to 2026, we had local Sporster rider's giving us feedback with our GT15 turbo setup. We found that in general, people were less concerned about spool and more worried about mid to top end power and overall speed. They didn't like that the kit signed off at 6,500, and we also were dealing with rideability issues that were tripping people up. Cold-start fuel puddling was the thing that was the biggest issue, and it seemed clear that this wasn't going to work on a mass scale - it was obvious, as just about everyone had the same feedback with the setup; I want more power and better rideability.

On top of all this, we're also dealing with all of our input costs skyrocketing. Argon has gone up 50% since 2022, diesel is $6/gallon so shipping is also going way up, insurance is rapidly rising etc. This setup isn't economically viable unless it's around the 4k mark. These bike's are just so old now that anything higher and people just go the NA route. So what do we do?

We used PSiclone as a test bed for our T25 based kit. We've applied everything we've learned in the past to this kit, and tested out new ideas as well. This turbo is physically much larger, so it had to be moved out and forward. The exhaust manifold is now equal length, to help balance out the cylinder's and lower backpressure even more. This also has the side benefit of making the air fuel ratio more similar between cylinders, which means less detonation, heat, and the ability to run more boost with more timing. Win/Win. It's a little more fab and fitup, but well worth it. It's superior in every way compared to the old setup.

To address the driveability issues, we're currently making about a half dozen changes to the coldside.

We're shrinking the plenum as much as possible; more plenum volume makes the bike spool faster and feel more punchy in the midrange, but it also exacerbates cold start and driveability issues. We're seeing just how little plenum volume we can maintain and still keep 99% of the power without all of the cons.
We're switching intake manifold's and shrinking down the plenum outlet to increase velocity.
We switched to a flange mount on the front of the turbo instead of a silicone hose. It adds time and more fabrication, but the mount is so much more sturdy and cleaner looking that it's worth it. We're just going to eat the cost, it is worth it in quality alone. No more worrying about kicking the carb off!
Jetting changes, Removing Thunderjet - We really don't want people to have the ability to easily alter the jetting at WOT. We also don't want random people to walk up the the bike and turn the knob on the thunderjet and unknowingly richen or lean out the bike, and have it run weird. We had people do that when we ran manual boost controllers, and it's why we don't run them anymore. Thunderjets also require the use of a check valve, which can either clog or just straight up break is it ages, neither of which is a desirable outcome.
We're trying a few more small tricks to get the driveability better that we aren't going to share publicly, but will have a big impact on making the bike run better when cold. It will never be as good as an NA bike, but it will be far better than before.

We're wanting to do one final local kit before going into full on kit production for 91-03 Sportster's - if you have one with mid controls and want to have one installed and tuned for $3,850 OTD, hit me up via the contact form below, and let's build a monster!