I'm planning to turbocharge my Suzuki DR650 single cylinder 4-valve 9.5:1 compression motorcycle. I plan to use the IHI RHB31 waste-gated turbo and blow through the stock carb. This is for sane road use so 8 psi is likely the highest boost I would aim for, running pump gas and no injection. Questions:

1) What mods will this stock sidedraft CV carb need to get things up and running? I assume the main jet should go up 2 sizes and the needle raised a notch.

2) The engine has an exhaust gas temperature gauge which has been great for tuning after previous exhaust mods. Is an EGT useful for tuning a turbo or should I switch to broadband O2 sensor?

3) Any hardware suggestions for fuel pump and boost-referenced fuel pressure control?

4) I see flow reducer valves to control oil flow to the turbocharger. How to adjust these?

5) I read that for single cylinder turbo engine, an intake plenum 4 to 5 times the engine displacement is needed. True?

6) Is the 9.5:1 compression likely to be a problem? If the piston were machined to reduce compression to, say, 8.5:1 to allow more boost, would this reduce detonation without reducing power ?

7) I read that for a 4-valve head, you can assume volumetric efficiency of 95%. True?

 

1: You will have to tune it after you get it boosted. A wideband is a good idea. I would put the carb in a box, and figure out how you are going to raise your fuel pressure to account for boost.

2: It could be, I think wideband will be more straight forward, especially for tuning things like tip-in

3: What type of pump does this run stock? Is it one of those little pulse driven diaphragms?

4: Not necessary for a journal bearing turbo. You will need whatever oil pressure you can get to it, depending on how this engine is oiled you may need an auxiliary pump.

5: No. May be beneficial to upsize the plenum, but it isn't a strict requirement. Boxing the carb may help with that a little, all you are trying to do is ensure you have enough pressurized air charge to fill the cylinder.

6: It could, it's hard to say without actually running it, with a carb you may be fine as you will get a good temp drop. It also depends on how much boost you are looking to run in it, and what type of fuel you have available. Cylinder and piston design effect the way everything jives, altitude also makes a difference. Low boost probably fine.

7: Volumetric efficiency depends on more than just the valve count in the cylinder head. You could be at 80% in some areas and 120% in others. Cam, head design, intake design etc. all play a part in the number at a given rpm.

 

Thanks.

1) I had assumed the installation would need extensive tuning to get it running sweet. I just don't want to burn exhaust valves the first time I leave the driveway. I was thinking that with no-boost part throttle, the carb would be running off the needle and little change would be needed. But once boost came on, the carb would be running off the main jet so it would need to be bigger. I read somewhere that jets dispense fuel according to air mass flow (which, for a given throat velocity, increases with boost pressure) so a blow-through setup is relatively insensitive to boost pressure compared with a draw-through.

Why make a box? As long as the carb is vented to the plenum, doesn't that accomplish the same thing?

2) I like the robustness of the EGT, but it takes about 5 seconds for the temp to stabilize. This gets a bit exciting at full throttle. What is "tip-in"?

3) No pump. It is gravity feed, in which case boost will blow bubbles in the gas tank. Is there a compact, low-flow EFI fuel pump that would do the job?

4) Good access to oil from the oil cooler lines.

6) I saw on another post (a DRZ400 which is a similar single cylinder) the guy used a 0.010" base plate under the cylinder to reduce compression. I suppose this has the same effect as a thicker head gasket ?

 

You need to do a lot more research. You can’t run a gravity fed system with a blow through carb. In order for their to be flow, you need to pressurize the bowl. As you add manifold pressure the pressure in the bowl must rise 1:1 as well. The easiest method by far is to add another adjustable jet in the inlet. Then use a needle valve on that to add or remove fuel according to what your WB02 says.

This guy is great! He goes through all the details on his channel to make a blow through carb. He’s built a TON of blow through JY bikes.

https://www.youtube.com/@DoctorMotorcycle

 

"gravity feed" was a reply to Ben's question:

" What type of pump does this run stock? Is it one of those little pulse driven diaphragms?" which, in turn, was a response to my original post "Any hardware suggestions for fuel pump and boost-referenced fuel pressure control?"

Do you have suggestions for pump/regulator hardware?

 

I do, look at that guys channel and copy what he does verbatim.