Intro:

This is a very interesting project undertaken by one of the members of our Discord channel. He’s kindly done a bit of a write-up with how he did it etc and given permission to publish it here.

So this is reposted more or less as-is, in his own words. Hopefully you find it useful and interesting – I have certainly been following the progress closely!

This is a work in progress so there will be updates added as it develops:

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Turboing a R125

There has been a lot of talk on the internet about turbos on 125s. Often the discussion follows by people doubting if it’s possible. There aren’t any turbo r125 on the internet apart from an r15 that only made 24 hp. There also was one guy in my comments who said that he saw a turboed wr155 pushing 30-40 hp but I haven’t seen any proof of it. 

Please note: This blog is written at the same time that I am doing the project. This means that at the moment this isn’t a perfect guide to follow if you want to turbo your r125, but it should give a good general idea of what needs to be done and how much work is involved. At the end of the blog there are my recent changes/mistakes I have made and what I’m doing to the bike at the moment. 

The idea of turboing an r125 first came to my mind last winter after I got a seized project r125 for dirt cheap. At the same time, I came across a turbo Honda monkey project, and it got me wondering if turboing an r125 was possible. I spent at least a month researching everything I could find about turbos until I couldn’t think of anything more to research about.

Going into the project I didn’t even know if it would work and actually build boost, so I planned everything to maximize the engines’ ability to build boost. This ended up complicating it more than necessary but was vital for experimenting. 

The choice of the turbo is important. I chose the rhb31 vz21, because it’s the smallest mass-produced turbo. The turbo for sure is small enough and on a short straight exhaust pipe I tested the bike on, I already got to 1 bar of boost. This makes me wonder if it would work on a stock displacement engine. I have started looking into slightly bigger alternatives and a few I have come upon that could work on a 180+ cc build are the td025 and kp31 or kp35. Turbos for small diesel engines that are around 1.2l could work. Experimenting with different turbos is something I’d be interested in doing. Maxpeedingrods is in my opinion the best place to buy a turbo, and they are pretty cheap. 

The budget of the project is 1200+ atm and will probably still increase as it’s not finished. Even if you don’t like buying Chinese items, AliExpress is your best friend if you don’t want your budget to be 2x or 3x from my budget. Tho, you are free to buy whatever you want.   

One of the most important things for the project and keeping the budget low is planning everything from the beginning and not from the halfway. I know a guy who has oil draining problems on his turbo pit bike project but can’t make a good fix because he fitted the turbo too close to the engine and physically doesn’t have enough space. 

Turboing the yzf-r125 

I’m going to use a of words and concepts that are related to turbos and 4 stroke tuning and explaining them is beyond the scope of this document, so I’m going to have to assume the reader is at least somewhat familiar with turbo system basics. I will describe how I did everything but also what I’d change or keep in mind. 

After doing research I decided on how I’d plan everything. The most important thing for me was maximizing the engine’s ability to build boost. To achieve that I had to make the exhaust between the engine and the turbo as short and straight as possible. The turbo is spooled by the engine’s exhaust gases. A short exhaust keeps the exhaust velocity and temperature as high as possible which transmits more energy to the turbo and maximizes the turbo’s ability to build boost. The second goal of the project was to keep all the fairings and fit everything under them. I succeeded in this fairly well.  

Engine upgrades: 

These are the engine upgrades I think are necessary: 

• 180+ cc kit 

• 22/25 head 

• Strengthened crankshaft (connection rod) 

• Racing oil pump 

• 6-spring clutch 

• Copper head gasket 

• Improved cooling (better waterpump or bigger radiator) 

With a turbo the engine overheats quickly after a few pulls. Especially on hot summer days. Therefore, increased cooling is necessary if you want to do more than a few pulls with it. A camshaft is optional as it builds boost with the stock one already, but a camshaft with moderate lift and a sharp shape is the best. The overlap of the valves should be minimal. A 200 cc cylinder might be better than a 180 cc as it would build boost a little earlier. Also, a bigger cylinder gives more freedom with the exhaust, and I think it would make the turbo placement easier. This is because the extra exhaust gas from the bigger cylinder compensates for the energy loss of a longer exhaust or one with a bend/bends. With a 180 cc cylinder the bikes starts building boost at around 7-8 k rpm and takes off at 9-10 rpm. General knowledge of the engine should be good and therefore installing these engine upgrades should be straightforward.

One IMPORTANT thing to do apart from the upgrades is lowering the compression ratio. This can be done by adding a spacer below the cylinder or thicker/more gaskets. The Malossi/Athena 180 cc kit is especially good for turbos because it has a dished piston which reduces the compression ratio as is. A flat and especially a domed piston needs a bigger spacer/gaskets to lower the compression ratio. The engine might work with the stock throttle body if all the extra hoses are removed/blocked. I used a 32 mm Koso throttle body in my build, and I calculated that it shouldn’t restrict the airflow.  

Turbo oil feed/return 

The engine has an oil pressure check bolt in the cylinder head that is suitable for a turbo oil feed line. It has m6 threads and you can make a hose fitting from a regular m6 bolt by drilling a 3 mm hole in the middle or use a designated hose fitting (m6 to 8mm hose fitting).

An oil cooler should be mounted either before or after the turbo to cool down the engine oil. In my case I mounted an oil cooler before the turbo. The oil cooler is quite far from the turbo, but the engine’s racing oil pump still has enough power to push oil to the turbo. The oil feed is the reason why a racing oil pump is necessary.

Because of the main goal of making the exhaust as short as possible, the oil return became quite complicated. The oil return on a turbo works by gravity. In this case the oil return flange and the engines oil fill hole are on the same level so the oil return couldn’t work by gravity. Because of this I had to fit an oil catch can and an oil pump to pump the oil back to the engine.

This is something I would change if I’d do everything again. It’s better to have the turbo mounted on the right side of the engine to let the oil drain work by gravity straight from the turbo to the oil fill hole or create a new oil return hole. An extra oil catch can and a pump takes up extra space and electricity.  

The turbo has an m8 oil feed, so I had to use a few adapters (from AliExpress). M8–>AN4–>AN6. AN-fittings are great, and I recommend them. Do your research on normal and PTFE-hoses and decide which you will use yourself. I started with PTFE-hoses in the oil feed and return but at the moment I only have them in the oil feed because the connections became very complicated on the oil return/catch can. Normal hoses seem to work ok. Tho don’t use the cheapest you can find but use reinforced fuel hoses instead. 

Fuel System:

Upgrading the fuel system is crucial to a turbocharged vehicle. For example, the r125 has a stock fuel pressure of 2,5 bar. Standard atmospheric pressure is 1 bar and it works against the fuel pressure. That means that the true fuel pressure is 2,5-1=1,5 bar. Now let’s add an additional 1 bar of boost. 2,5-(1+1) = 0,5. As you can see, the fuel pressure is lowered to only 0,5 bar and the fuel injector’s efficiency is drastically reduced. The second problem is the changing pressure in the manifold. Sometimes it’s 1 bar when there is no boost and 2 bar on peak boost. Even if we raise the fuel pressure to 3,5 bar, the actual fuel pressure would still vary between 1.5 and 2,5 bar. Because of this an external fuel pressure regulator (fpr) with boost compensation is needed.

The fpr works by a valve that a spring regulates. When adjusting the fpr and compressing the spring, the fuel pressure required to open the valve increases. Between the fuel chamber and the spring is a plate that opens the valve. The spring is in a second chamber which isn’t filled with fuel. The boost compensation works by letting air from the manifold enter the springs chamber. When there is 1 bar of boost in the manifold (2 bar total) the same pressure is in the spring’s chamber. The additional air pressure works with the spring and increases to total pressure needed to open the valve.

This system increases the fuel pressure at the same rate as the pressure in the manifold increases. This means that when the fuel pressure is set to 3 bar, it stays at 3 bars despite the amount of air pressure in the manifold.

The stable fuel pressure helps with tuning and allows a smaller injector to be used. 

The fuel system was something I had a lot of trouble with. Originally the idea was to use the stock fuel pump and increase the fuel pressure. The first idea was to block the stock fuel pressure regulator (fpr) which is inside the fuel tank and fit an aftermarket external fpr.

I spent some time making a plug for the stock fpr. After managing to make one I found out that the fuel pump wasn’t powerful enough and began struggling when pushed to a higher fuel pressure than stock. The next idea that ended up working was to use a second pump in line with the stock pump.

On this system the stock pump feeds the second pump. The second pump is much more powerful and is good up to 7 bar. The second pump cavitates sometimes so I’d consider making a system with only one high power pump if I’d start over.

This is because two pumps drain the battery quickly. After the pump there is a t-fitting. One goes straight to the injector and the other branches to the fpr. The fuel pump pumps a lot of extra fuel, and the injector doesn’t use all of it. That’s why a fuel return is needed. The stock fpr is inside the tank and it releases the fuel back already inside the tank. An external fpr needs a return line and a place to return to the tank.

I drilled a hole and installed a hose fitting just under the fuel cap. I chose the position because it never gets under the fuel surface and reduces the chance of leaking drastically. Also, if it leaks, it wouldn’t go into unwanted places because of the overspill function that the bike comes with.  

I have set the fuel pressure to around 3-4 bar and I use the boost compensation. For the injector I use a 180 cc injector and with the increased fuel pressure it seems to be big enough. Also, everything after the fuel tank is E85 compatible because I might try switching over at some point. I don’t know about the compatibility of everything inside the tank to E85. 

Also, the new fuel pump needs wiring. I used a relay and wired the activation wires to the stock fuel pumps wires (positive before the pump and negative after) Tho you should have a good understanding about wiring when starting the project, so I won’t go deeper into wiring. 

Exhaust:

When starting the project, I wasn’t sure if it would build boost, so I made the exhaust to maximize it. I just had a short 27 mm ID pipe between the engine and the turbo. I made all the flanges myself. After the turbo I made a traditional exhaust with a silencer. The silencer is basically a straight pipe so that it doesn’t restrict the exhaust gasses flow. You can also make a side exit after the turbo. Making the exhaust is pretty straightforward and doesn’t need anything special. You should be able to find good instructions online. 

Tuning:

For the project I got a development ECU from Tuneboss. The ECU is great, and it got the bike working through the boost too. The app makes managing the bike very easy. My only problem with it is connected to turbocharging. The Tuneboss app only has a RPMxTPS tuning option which works but isn’t optimal.

The optimal would be RPMxMAP which uses the Manifold Air Pressure sensor instead of Throttle Position Sensor. The problem of using TPS for tuning is that you can only have a fixed value for the same RPM regardless of if you have boost.

With 1 bar of boost, you need 2x the fuel when compared to without boost. This means that you can only tune it to one of the two and it won’t work for the other.

Also, the bike doesn’t always build the boost at the same RPMxTPS rate, so the mixture isn’t always spot on.

Using MAP, you can tune it accurately regardless of the amount of boost at a certain RPM. When using the MAP, the TPS becomes basically useless because the throttles position affects the manifold air pressure which makes them basically connected.

This is why I’d consider also Microsquirt and Speeduino for turbocharged applications. In my opinion the Tuneboss is excellent for naturally aspirated builds, and it offers everything you need for around the same price or cheaper as the previous options and doesn’t require modification of the loom or soldering. Basically, in this case it works on the Tuneboss but isn’t perfect.

*[Note from NorthCust: Please note that this setup is not using the wideband input of the Tuneboss – which is something it can do, and would adjust fuelling on the fly]. Tuning by MAP would be better but the StayTuned would allow some additional flexibility in the fuelling. There is *some* fuel adjustment by MAP in that changes in MAP do adjust the fuelling, but not the the extent of being able to rely on it entirely as a primary tuning input]

Intake pipework:

Intake side pipes. The pipes between the turbo and the throttle body should be airtight and you should avoid tight bends and other restrictions. In my build I used reinforced silicone hoses and 3D printed flanges. Note that regular filament doesn’t withstand the strain and you need to use a nylon carbon mixture (if I remember correctly). I also made a headlight delete and replaced it with a ram air intake in the place of the headlight. 

Extra:

I also added an extra coolant temperature sensor before the radiator and a boost gauge to see the boost it builds 

The future of the project:

My next step will probably be changing my 22/25 head with the intake valves leaking a little to a cnc-machined 22/25 head. I’ll also be testing with different camshafts.

If I find the turbo restrictive I might try increasing the boost pressure a little to 20 psi (1.3 bar) or experiment with a bigger turbo.

Also in the future depending on personal conditions I might be doing a 300-400 cc turbo build. A 330 cc build would be quite straightforward, but a 400 cc would need already some more work so for a turbo project a 330 would be better

Recent changes/mistakes 

I tested the engine with a higher rise camshaft and apparently the valve springs were too weak, and the valves ended up slightly hitting the piston. Though I think the valves had already hit the piston before, so it didn’t change much. Still I changed the head to a 23/21 head with perfect valves to be sure of keeping all the boost pressure in the cylinder. 

After testing it seems like in my setup with the oil cooler between the engine and turbo, the turbo didn’t get enough oil. I decided to have a separate oil system for the turbo. I have the oil catch can full of oil and the scavenge pump pumps oil from the catch can straight to the turbo. Right now, I don’t have an oil cooler but will try to mount one in the future. 

I’m also testing using only an external pump and using e85 as fuel. These topics will get updates in the future 

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Questions?:

If you have any questions, feel free email them to turboR125@northcust.co.uk or join the Northcust Discord server where you can contact me too (user: @TurboYzf-r125) 

Also please check out my videos on TikTok and below:

https://www.tiktok.com/@turbo_r125

Video 1 – Intro

Video 2 – Exhaust

Video 3 – 3d-printing intake component

Video 4 – Short test ride – Proof of life!

Turbo flutter

Sadly the video part of this clip didn’t work but the audio does, and you can hear the turbo fluttering.