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So you got a Cummins, congrats! But are you getting the best stock performance for what YOU use it for??? Here is some info you may find interesting.. so the HX35 began in 1995, similar spec to the WH1C which was a 94 one year hybring is the H1 series and HX series that honestly I wouldn’t mess with. It’s not really worth building as it’s balanced as an assembly so you can’t drop wheels on or make changes without swapping the turbine also. The center section or bearing housing isn’t that great, the oil paths aren’t as open as the HX. So the HX35 was a 56/60/12 which is the main figures you typically look at in turbo sizing. 56mm inducter (inlet) of the compressor, then 60mm exducer (outlet) on the turbine then finally 12cm which is a air volume measurement. Larger is higher flow with lesser velocity so a slower spool but EGT usually falls as well as response. This can help towing with a loose converter or high rpm with a 16 or 18CM housing you’ll see a EGT drop. So HX35 is a 56/60/12 or 56/82 70/60 HX35W beginning in about 99 is the code for a smaller compressor a 54/78 which is my personal fav turbo for stock or tow applications. The AR of the compressor housing and flow characteristics changed on the W and actually flows only 2lbs less per minute though it’s smaller! That 2lbs you hardly feel BECAUSE the smaller compressor takes less shaft HP to turn to get the same RPM. Essentially to get the same amount of boost it requires a little more rpm but there is a benefit of this, the turbine is now spinning faster allowing exhaust to pass over the blades easier with less restriction as the turbine blades are closer to matching the blade velocity. In turn your drive pressure is a little lower! So that 2lbs a minute compressor loss turns into making the same engine power because it can breath on the exhaust side easier. Your also less likely to bark the W, it will just takes a little more effort because the larger number (exducer) on the compressor is smaller thus less leverage on the shaft during the ‘bark’ which is a pressure fluctuation between the compressor and turbine. now in 2001 due to emissions Cummins changed the turbo to the dreaded HY35...this in my book is the worst stock turbo ever to come on a Cummins along with the HE341 in 2003. This turbo is a 54/58/10. The idea was to improve response but gain some flow in the upper rpm band. The numbers being lower/smaller usually you’d see faster spool right? No! This time they went to an open scroll exhaust housing, issue is open scrolls essentially mix the front and rear 3 cylinder firings together, there is a pulse that assists in spool which is then intermixed in open scrolls causing lag! The pulse can’t hit the turbine direct so it looses velocity and some heat. The HX35 is a twin scroll keeping velocity up and therefore spool. There is more science to it but an open scroll at high boost/rpm will flow a little better vs a twin scroll when compared with the same CM/AR rating. The issue is the turbine dropped from a HX35 70/60 to the HY35 65/58 which is less leverage and less shaft HP also mixed with the lack of cylinder pulse. This was done to offset the open scroll housing but was a fail in my book. So do you have an HY35? 01-02 was factory, easy way to tell is look at the turbine, if there is a V band clamp connecting the center section to the exhaust it’s probably the HY35. Another check is look at the exhaust housing shape, the lump that tappers around the housing will be rounded and narrow instead of a more flat and wide shape. So if you confirmed it’s an HY then search the market for a cheap upgrade AKA HX35W. Changing the turbine side between these turbos make a massive change in response, sound and MPG. The HY can get decent MPG at low rpm but it leave some to be desired. The HX on the other hand can spool as much as 250rpm sooner which makes a big change. Still flows great. Now if your running a larger then 100 over stock injector don’t expect a ton of power gains in the upper rpm band above 2600ish this is more focused on the daily or two rig where response and efficacy matter. EGT, engine load, and drive pressure all drops instantly and in most cases you gain 1-2psi at cruise thanks to the exhaust housing twin scroll and larger turbine wheel making more shaft HP to spin the compressor. The point of this is more usable power under the curve which means a wider and deeper power range available to you and a lower EGT to boot! Now that you know the main point differences, if you want a nice little upgrade trash the HY35 for an HX35W. The HX35 works too but it’s a little more laggy for honestly very little gain. Oh and the HE341 is an HY35 converted for 3rdgen trucks. It’s a 54/58/9, different exhaust housing that flowed better but the best part is the WG design, it flows like mad and exits direct into the down pipe instead of taking 2 sharp 90* turns like the HX meaning way way better flow. The compressor housing changed a little with a 90* outlet and a little larger AR but was quickly scrapped for the HE351 which gained a 60mm inducer compressor. If your good with some fab work a 351CW makes a great upgrade. About another 100hp or so capable of max power thanks to the wastegate and when mulled out you can make some sweet power. It is an open scroll but the large compressor and small turbine with the housing design ended up being very efficient and works pretty well. Now your part throttle cruise EGT maybe up a little, I’ve noticed some MPG drop over the HX35 but if your truck is a worker or you like to race around and have fun, it’s a good turbo to put on for cheap!