So I am starting a budget turbo mod sled. I am trying to figure out how to do the math and determine what turbo will work with the engine. The engine is a 2 stroke 670cc ski doo that will be getting rebuilt soon. I've been trying to do my research on turbos but most info I come across for matching turbos to engines is for cars and not for a 2 stroke small engine. any help would be appreciated.

 

You're in luck. There's a few turbo sled heads here. They'll be along shortly, probably out playing in the stuff.

 

What's your turbo budget? A 2 stroke will act like double its displacement, so that and na hp/tq is what I use to start.

 

well that's good to hear! I tried scrolling and searching but didn't find to much on turboed sleds in here. If it wasn't for college classes and work I'd be out playing in the stuff to right now.

My budget is less then 1000 at the moment but I am also not in a major rush to get this done. I know I could simply go buy a kit for 2-3k with a turbo already but I want to build it myself and find my own turbo. I've got a few sitting around that I believe one of might work for my situation. I have seen the na hp/tq but I haven't figured out what it means yet. I obviously know hp is horsepower and tq is torque.

 

na = naturally aspirated. What HP and TQ does it make stock?

 

$1k for the complete build doesn't give you a lot of choices. A ball bearing turbo is the way to go, but the turbo alone would take your budget. With that said, i've built a couple with journal bearing turbos, and they do work quite well, but require more supporting parts, add some weight, and complicate things a bit. A GT2871, Gt3071, or something like a 60 trim T3 (preferably with a stg 2 or larger turbine) are all decent turbos for a otherwise stock 670, and can be found in China copies if really on a budget. There's much more to making a carb'd 2 stroke run than you'd think. Air box design, and the proper fuel system are key.

 

The engine makes about 120-130 horse power but I am unsure on torque.

I know a ball bearing would be ideal as it is internally lubed and what not but there are still companys making and selling journal bearing turbos for sleds such as silber turbos. I know that with a journal bearing I will need to have a oil tank and pump to supply lubrication which in turn would add weight but I'm not worried about it for this reason: The sled this engine came from, I was able to gut everything from it that i might need so I have the oil tank for the 2 stroke oil which happens to be molded with the coolant bottle. That is what I plan to use for the oil to run as the lubricant and then find a pump to supply it to the turbo. I am not afraid to premix my fuel as I am use to it anyways. The sled chassis I am using is a vintage chassis with not much to it and in turn is already rather light weight. I have a turbo picked out already but I need to figure out how to confirm if it is the right sized turbo for the job. I know I will probably need to run an electric fuel pump as well as figuring out the power jets for the carbs. As for the air box design, I've got some pictures and if all else fails I will attempt to find a used one for this project but it is not my concern at the moment as i am taking this one step at a time and the turbo is my first step.

 

A couple comments... most bb turbos still need an oil supply and tank, its only the few oil less, or self contained that don't. Bb turbos require much less oil volume/pressure, that and the fact that they spool quicker are the benefits, but like you said, it's not mandatory. One of the issues you need to work around with most 90's Rotax's and a journal bearing turbo is that the charging system is good for roughly 14 amp at 12 volts, and by the time you add a fuel, and oil pump you're very close to maxing it out (It works for a race setup, i've done it, but without a headlight, and you will need a battery and a rectifier/regulator to charge, and get the pumps started). Unless you want to run 2 oil pumps, your oil tank will need to be mounted under the turbo so it can gravity drain. Most jb turbos of that size will require a min of 30 psi of oil pressure at about 1.8-2 gal/min to be happy. What turbo have you chosen? On my 700 twin Rotax I ran a t3to4e 50 trim with a stg 3 turbine, and it worked well for drag racing, but was on the large side.

 

And this is why I came here to ask my questions. That's a lot I didn't realize or think of to consider. So I would need a battery setup to better supply power to the pumps? What if I had a large oil pump on the return side of the turbo, with a gravity feed to start it, would the pump be able to suck the oil up through the turbo? If that makes sense ha. The turbo I have came off a smaller 4 cylinder diesel but the openings on it in my head appear to be an appropriate size to flow the proper air and has a waste gate on it, but that's were my knowledge of turbos stops. The turbo appears to have been made by perkins but that's as much info I have found on it with the markings on it. I might be able to share a picture of it. I can take measurements on it though it that would help.

 

Measurements, and a pic would be helpful. pull the housings, or measure as close as you can the small diameters of the wheels (the ends you can see when assembled). You need to supply the turbo with pressurized oil, there's no way around that. Beyond that, you have two options... gravity drain it to your oil tank (most common), or use a scavenge pump to pump it back to your tank. You may get a fuel, and oil pump to run on just the charging system, but they draw more amperage at startup than you typically have, so you need a battery, and a rectifier for dc voltage. It's not all that difficult, but you need to understand how the system works, especially when it comes to tuning.

 

I will see about getting a few pictures of it and post them tomorrow. Does this forum have a restriction on how many posts someone may have before they can share a pic? I know some other forums do which is why I ask. Anyways I will also see about taking this turbo apart and getting measurements, shouldn't be hard as it has hardly been used before.

 

This is what happens to a Wiseco forged piston when things go wrong with a turbo 2 stroke.... This didn't make it to the 1/8 mile mark. #ad

 

pics of the turbo, haven't got measurements yet

#ad


#ad


#ad


#ad

 

Its BEAUTIFUL!!!!!!!

 

Sorry for the Delay in these measurements, the mountain powder was calling my name. Here are the outer diameter measurements of the compressor wheel and exhaust wheel. The base of the compressor wheel is 2.145 inches with a top measurement of 1.58 inches. The exhaust wheel is 2.125. Is there anything else I need to get a measurement from? the compressor wheel is a 6 blade wheel with 6 smaller inner blades towards the bottom.

 

That's 40 mm on the compressor inducer, which is quite small. Just for comparison, here is a map for a t3 45 trim which is also a 40 mm. At 120 hp na, you'll need a compressor that will support 240 hp at a pressure ratio of 2 (just an easy ratio to compare) at a minimum, and that puts it off the map on a generic 40 mm map. A 47-50 mm compressor inducer would be much closer to what you're looking for, possibly even a bit larger if you're not real concerned with a little lag. #ad

 

Okay so this is where I start to get lost but I really want to learn more about and understand. I understand the basics of how turbos work obviously but I don't understand how to get the ratios or read the charts. I know the difference between inducers and exducers but I don't know what the 45 trim is when you are referring to the t3 turbo. Does the compressor inducer really help reduce or effect lag? I always thought that was more involved with the exhaust side of the turbo. I guess I will do some more looking around at turbos.

 

I’m also working on gathering parts to boost my 670 next spring. I’m machining my own boost referenced adjustable fuel pressure regulator to return to tank and a basic inline fuel pump and I plan to boost reference the carbs. I’m thinking of using a copper shim head gasket to drop the compression a hair, my turbo is a smidge on the small side (about like a t25) but I like my boost coming on early.

 

I’m using a small aluminum rectangular tank below the turbo as a ‘sump’ for my setup.

 

Quick spool is nice, but beware of back pressure, it'll eat pistons before you know it. The CVT masks a large turbo nicely. What turbo are you running?