TD performance guide for 998 turbo snowmobiles

Version 1.0 (check back for updates or additions)
High Performance 998 turbo technical bulletin from Turbo Dynamics. This document will explain the verifications and testing you will need to get the max performance and best results out of your high performance 998cc Yamaha turbo based snowmobile.


Rule# 1 Always drain old fuel and put in fresh fuel (if going to pick up fuel at pump put 5 liters in truck/car then the rest in your canisters to get rid of the 87octane left in the line/filter)

Engine performance checks:

Plug gap .020 for trail .017 for boost higher than 25 psi. Best steady RPM for Pump gas tunes is 9000rpm give or take 100 rpm. For race tune at high boost 8400-8600rpm. For professional racers custom settings may be required). TD head gasket upgrade is recommended for power levels of 340HP and up (with head studs).
Best bang for the buck power upgrades are: Tune + Clutch weights (Max spool 16), TD dash reflash / multimap, Then followed by aftermarket exhaust, header, intake and stage 6 kit.

1-Verify all clamps, including intercooler tubes, throttle body assembly, intake box, oil lines, coolant lines and more. It is VERY common so see many loose clamps. The Throttle body ones are especially tricky and hard to spot. removal of the plastic intake is required for the proper inspection of the clamps that connect the throttle body to the engine. Those can pop off but still the sled will work well but not make good power (loss of boost pressure). This is very important to check. The higher the boost the more critical that is. For additional pressure you can shave 1/8 of an inch off the two aluminum tube spacers on the throttle body holding assembly. A boost leak tester is a very good tool to have to verify if there is a boost leak, either from a damaged charge tube, loose clamp or weak BOV. You can use a compressor with an air pressure regulator lowered to 20 psi to test. You can plug the tester at compressor outlet of the turbocharger. You will need a high flow fitting for it to flow enough to test. Motor has to be spun counterclock wise to close the valve and find the spot where it holds pressure. A helper is nice to have to hold the primary clutch when pressurizing the intake.

2-Never ever spin motor clockwise when looking at the clutch. Always spin it counter clock wise. The motor could skip timing on the chain/sprocket if spun clockwise.

3-For yamaha only, retorque factory clutch to 110 ft-lbs, back off bolt and then finish at 60 ft-lbs

4-Make sure there are NO exhaust leaks, especially pre-turbo which kill power and spool up. That means when installing an aftermarket header, before closing everything up, set up the sled so you can start it and verify air leaks before the header gets too hot (hand near the exhaust flange and V band to feel airflow. Also by the sound you can sometimes spot it. It is very common for the V-band not to be installed right and leaking quite a bit...(check for zero gap between the flanges.

5-Any power over 300HP will require a fuel pump upgrade and a 3 bar or 4 Bar map sensor. After upgrading the pump , make sure the Red plastic 90 Deg return fitting at top of tank is drilled (remove restrictor within) to same size as the ID of the fitting, Black return is OK no change needed. When installing a fuel pump make sure there are no leaks between the pressure outlet and the sending unit cap. You can test it visually, be careful not to cause spark and have a lot of airflow when doing this test. Also be sure the suction pucks are sitting flat at the bottom of the tank in the proper angle like factory and that the fuel pump suction filter is also flat on the bottom of the sending unit.
If you install an aftermarket pump it is highly recommended to add an adjustable fuel pressure regulator to be able to monitor fuel pressure. Stock Fuel pressure is 43 psi key on but most header tunes required 47 psi key on. Also recommended is the use of our wideband o2 sensor module to monitor proper operation and success of the install. You can also fine tune the performance better giving you an advantage over the competition. Look into the closed loop option for automatic fuel adjustments via ECU and o2 sensor.

6- Aftermarket Wastegate upgrade (new upgrade for 2020-2021 season):
For the stock turbo, we use an 11 psi Spring and set crack pressure to 11 psi (very important to verify this). Crack pressure is set by using a mityvac pressure tester or similar tester to pressurize the top nipple of the aftermarket wastegate after it has been installed on the turbocharger. Crack pressure is the pressure require to start moving the arm of the WG.
On the big turbo upgrade we use the 14 psi spring for a 12-13psi crack pressure.
We will not send tunes to people who do not test crack pressure. Because if the crack pressure is too high, that will result in an unwanted overboost condition and inconsistent boost control.
Keep in mind the new WG upgrade tunes are a game change in performance ! Quicker spool and high boost level achieved on Max20 and up maps.
After a high performance kit install (tune, muffler and clutch kit)

For additional details on Wastegate install please look further into this guide !


** Big turbo charger and any sled over 30 psi of boost , a 4 bar map sensor upgrade is mandatory **


Chassis Prep for high performance:

- Make sure you have a tight track (must not hang at all when lifted off ground, must be hard to push off the front bogey wheel). It is highly preferable to have a 4 wheel kit upgrade to maintain reliable track tension especially when running high power and or fast speed. Running a loose track will rob lots of top speed (as track baloons from centrifugal forces). Also loose track can cause ratchening of track drivers which will damage them and or the track. Radar running require a VERY tight track as high speed cause expansion.

-Billet track adjusters are highly recommended for the models using the thin cast aluminum adjusters (some factory cast adjusters can crack and loss of track tension can ensue)

-If you get excessive ski lift during acceleration, you can tighten the limiter straps located in the front of the skid. Having the sled lift just an inch off the ground or just light ski pressure provides best acceleration, wheelies aren't fast, they just feel fast. If you raise the limiters too much you will reduce traction and the sled will slow down. A good compromise is best.

-Stud length... for most trail riding we recommend 96 to 144 X 1.45 -1.6" studs on high performance sleds with 1- 1.25" track lug height. 1.6" studs will require tunnel protectors (STM). For Drag racing on snow short distance (660-1000 feet) 1.6 - 1.8" studs are prefered amounts of 144 to 192. For radar running long distance (2000 feet+) 1.35" - 1.45" are best amounts of 96 studs.

-Gearing: Trail 24/50 or 21/41 for up to 123mph GPS or 21/38 for 130MPH 21/37 135mph 22/37 140MPH.

-If you want to make a number put a light guy on it LOL !


Don't bang on the rev limiter or under rev on pump gas (must run 8500rpm+ on pump and not bang limiter at 9600rpm)

Yamaha clutches:
They work well up to about 320HP. They can do more but not as efficiently as aftermarket clutches. We recommend dalton weights. We have a seperate chart for explaining require weights. For the secondary clutch we recommend our clutch kit for better backshift and better RPM control. Heavy duty rollers are recommended for the secondary clutch.

Cat clutches:
They work OK up to 320HP. They are not my prefered clutches, aftermarket clutches are a big upgrade on cats. Again dalton weights are recommended with the tornado clutch kit for secondary.

-STM 3VL (for limited trail and drag racing/radar) work very well. most of our records are with this clutch or the stock yamaha clutches with weights.
-Tapp (For trail and racing use, new clutch this year, very quiet and are known for durability)


TD flasher/ Copitrail tech points:

-Firmware, this is constantly evolving, lots of new features this year, closed loop adjustment fine tuning adjustment for advanced customers. Use the latest release firmware unless specified otherwise (special firmware available for racers and tuners).
-When flashing doing a "DASH" reflash, first unplug first the Wideband O2 module from the harness completely.
Update to latest Gap flasher App on the app story or Copitrail app on the apple store.
If you are going racing and everything is going well with the sled and the app don't change it last minute.


More Turbosmart Wastegate Install instructions and tips for the stock turbo AND big turbo owners

In order to be eligible for Wastegate upgrade tunes, you must be mechanically inclined as this is not as easy as most other mods. Careful attention as to be given to the routing of vacuum lines and exact crack pressure of wastegate. Also you will need more options to confirm everything is working as it should. Namely a TD flasher with datalogging capabilities and closed loop option. These are our flagship tunes so we want them to represent us well as well as stay safe. WG upgrade tunes are an advantage for big setups, with Header and those want to run race fuel and have maximum ECU control over the boost (new boost timer feature)

You will need to remove and drill the factory restrictor/nipple located on the compressor outlet of the factory turbo. And then reinsert the factory nipple into the housing. The smaller nipple will be blocked off and the larger nipple goes to the new dual port solenoid we provide. This will ensure proper boost control and is mandatory. Please follow the diagram for proper the vacuum line routing. pay careful attention to the routing, improper routing can cause a dangerous overboost condition or an underboost condition. New TD ecu tunes will need to be sent to run this special WG upgrade. 



TD wastegate upgrade tunes, technical notes and warning !


Congratulations, you got yourself the meanest, quickest spooling tunes on the market. And now you don’t need a wastegate locker to achieve 30 psi of boost on the race map.

Also you can make full boost on the max spool 20 tune when combined with a header (previously Max 20 with header only achieved 17-18 psi with the stock WG)


A few important warnings with the new WG tunes:

Be careful on the first run, keep an eye on boost and only go for 1 second full throttle and check log for potential overboost.

If the vacuum lines aren’t installed properly it can create a FULL boost situation on all tunes.

Refer to diagram for install.


Max20 might require more octane than without the WG upgrade.

For now we will recommend 96 octane for Max 20 with wastegate upgrade as boost reaches 20-21 psi instead of 17-18 psi


Note that Wastegate tune with BT in them means they are boost timer ready and have higher boost limiters. That means be careful adding boost on the boost timer.

Max 17 and Max 20 can go up to 28 psi before boost cut kicks in. For more boost, use the Race maps but only on race fuel.


The highest boost that we recommend on pure 91 octane is 18 psi. You can try up to 20 psi but it has to be fresh good quality 91 and know that you are close to the limit of the pump fuel.

93-94 octane usually work fine up to 21 psi.


If you have an overboost condition (max 17 runs 20 psi+ or Max 20 runs 22 psi +) call us right away at 18005490246

Engine torque spec with head studs