Carbureted 2 stroke turbo old sled

Oct 3, 2020
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Hi, I'm new on the sled scene, I don't have experience but I'm very interested in the twin carbureted 2 stroke sleds.
So the question.

1)I've never seen a 2 stroke carbureted turbo sled. I mean without auxiliary injectors or other electronic stuff. Just the carburetors! It is possible do it?

2)Anyone with some experience can tell me if is possible increase hp on a carbed set up?

3)If yes how can it work if the exhaust and fuel ports are open at the same time : The fresh fuel is not blow out through the exhaust port by the turbo boost pressure?

Many question hope in some interesting answer! Please answer step by step( 1...2...3...) so this thread could be interesting also for people who want know more about it!

Some example, turbo sled picture, hp gain or loss are good!

TWO2
 
Oct 3, 2020
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My question are about the working of the turbo on these engine, the main question is how it can work. These pages can answer just to my frist and second question. But not to the main question. "how it can work''.
 
Oct 1, 2020
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My question are about the working of the turbo on these engine, the main question is how it can work. These pages can answer just to my frist and second question. But not to the main question. "how it can work''.
By ramming extra air into the engine


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Oct 1, 2020
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Do you know how work a 2 stroke engine? I think not. So, before comment look how it work, than you understand isn't very easy turbocharging a 2 stroke engine.
I do actually. I’m not sure what you’re wanting from this thread. It’s a lot more difficult to turbo charge a carbed engine then it is to turbo a efi engine. from what I sounds like, you don’t know what you’re doing


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Speed-Demon

Member
Premium Member
Mar 25, 2020
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Not sure what you’re trying to build or do exactly, but turbos have been used on carbed 2 stroke snowmobiles since the 90’s. Around 97/98 there were a few guys that finally “figured them out” and would put insane high marks on many unclimbable chutes. Easily 200+ hp. As said above, you’re just cramming more air in through the carbs. Don’t over think the port timing too much, but I know it’s crucial to get it right. I think they were mostly Mitsubishi automobile turbos or some type of aero charger if I recall. Most were premix set ups and had custom “fat boy” pipe and couplers to the turbo with a k&n air filter sticking through the hood for fresh air intake. Electric fuel pump with a regulator to deliver enough fuel and some sort of boost controller. Used the factory oil pump to lube the turbo. Custom aluminum air box to seal the carbs under boost with a blow off valve. Real common to see 670 rotax in a Polaris wedge chassis just going anywhere you dared. Coolest one I ever saw was at Yellowstone. I had just got an early release 97 700 rmk with the new for 97 136” 1 3/4” track and nothing at that time could touch it just short of a psi or onion bay (yes I said onion) big bore. This guy casually rolls into lions head on a 96 cat zr440 with a custom tunnel, a whopping 141” challenger track (that was long back then) and made my sled look like a yammy mountain max (which were pathetic at best compared to Polaris and Ski Don't twins). I about s*#t myself. Never saw anything like it. Best running cat I ever saw! Lol
Good luck on your quest for old school boost. They were badass if you could tune em. But like I said, only a few guys figured them out. And I’m sure it wasn’t cheap or easy.
Also, google 670 rotax turbo snowmobile and I’m sure you’ll find the rabbit hole you’re looking for. Maybe even a complete sled. Unless you’re talking 250 dirt bikes, never seen that. Too peaky of a power band I’m sure. But if it were me going that route, I’d be going 350 banshee, just to be cool. Man, what a pain in the ass that would be! Bog, bog, bog whaaa! Bog bog bog bog whaaa!
 

dansvan

Active member
Premium Member
Apr 14, 2011
139
42
28
You run a larger turbo because you have to pressurize the pipe volume as well as the crankcase and cylinder. I ran a 800 ptek carbed turbo with a gt2871. was crazy easy to tune with power jets. pipe boost referenced rising rate fuel pressure reg. There are entire 50 plus page threads on carbd turbos on this site.
 
Oct 3, 2020
46
0
6
Not sure what you’re trying to build or do exactly, but turbos have been used on carbed 2 stroke snowmobiles since the 90’s. Around 97/98 there were a few guys that finally “figured them out” and would put insane high marks on many unclimbable chutes. Easily 200+ hp. As said above, you’re just cramming more air in through the carbs. Don’t over think the port timing too much, but I know it’s crucial to get it right. I think they were mostly Mitsubishi automobile turbos or some type of aero charger if I recall. Most were premix set ups and had custom “fat boy” pipe and couplers to the turbo with a k&n air filter sticking through the hood for fresh air intake. Electric fuel pump with a regulator to deliver enough fuel and some sort of boost controller. Used the factory oil pump to lube the turbo. Custom aluminum air box to seal the carbs under boost with a blow off valve. Real common to see 670 rotax in a Polaris wedge chassis just going anywhere you dared. Coolest one I ever saw was at Yellowstone. I had just got an early release 97 700 rmk with the new for 97 136” 1 3/4” track and nothing at that time could touch it just short of a psi or onion bay (yes I said onion) big bore. This guy casually rolls into lions head on a 96 cat zr440 with a custom tunnel, a whopping 141” challenger track (that was long back then) and made my sled look like a yammy mountain max (which were pathetic at best compared to Polaris and Ski Don't twins). I about s*#t myself. Never saw anything like it. Best running cat I ever saw! Lol
Good luck on your quest for old school boost. They were badass if you could tune em. But like I said, only a few guys figured them out. And I’m sure it wasn’t cheap or easy.
Also, google 670 rotax turbo snowmobile and I’m sure you’ll find the rabbit hole you’re looking for. Maybe even a complete sled. Unless you’re talking 250 dirt bikes, never seen that. Too peaky of a power band I’m sure. But if it were me going that route, I’d be going 350 banshee, just to be cool. Man, what a pain in the ass that would be! Bog, bog, bog whaaa! Bog bog bog bog whaaa!
Thanks for your answer! Finally an interesting post! Yes for the moment I try to search some info about a carbed turbo to do something. For the moment I'm oriented to turboing a little 250cc dirt bike carbureted but it's tremendous search infos about it. My principal problem is exhaust port timing I'm worried about it because I think all the mixture going through that and the power go down than the n/a version. I need understand howpeople solve this problem before start to do something
 
Oct 3, 2020
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You run a larger turbo because you have to pressurize the pipe volume as well as the crankcase and cylinder. I ran a 800 ptek carbed turbo with a gt2871. was crazy easy to tune with power jets. pipe boost referenced rising rate fuel pressure reg. There are entire 50 plus page threads on carbd turbos on this site.
Pressurize? Is this what I mean who can prevent the fuel mixture blow out the exhaust port by the turbo boost?
 

dansvan

Active member
Premium Member
Apr 14, 2011
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THe turbo increases the pressure on both sides of the cylinder equally. The fuel cannot simply blow out the exhaust port because it is under the same pressure increase the intake side is. Want to blow your mind? Pulse style fuel pumps still work on a 2stroke under boost. Its just pressure differential.
 
Oct 3, 2020
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THe turbo increases the pressure on both sides of the cylinder equally. The fuel cannot simply blow out the exhaust port because it is under the same pressure increase the intake side is. Want to blow your mind? Pulse style fuel pumps still work on a 2stroke under boost. Its just pressure differential.
You have understood perfectly what I mean!!!!!!

Trust me are years I try to understand that. But anyway it's just a your opinion or you have some documents or examples to share about what you said?

Thanks for your answer!
 

dansvan

Active member
Premium Member
Apr 14, 2011
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I've run pressure gauges on the exhaust pipe. They are generally 2-3psi higher than the pressure on the intake side. 3psi is a good number to shoot for on a NA sled as well, for long hard pulls. higher pressure is ok for corner to corner racing but too high and it'll squeak a piston and cost power. Higher elevations have smaller exhaust outlets as well. Back to the carb'd turbo... You need to run a pressure referenced 1:1 rising rate fuel pressure regulator and you plumb the pressure port to the PIPE, it raises the fuel pressure ahead of the curve so its not playing catch up on the intake side, as the pressure in the pipe ALWAYS builds faster than the intake side.
 
Oct 3, 2020
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But anyway, about your previous post...the trik to turbocharge a 2 stroke is maintain an equal pressure between the exhaust and the intake?? Or the turbo do it automatically?? Correct me if I'm wrong.
 

dansvan

Active member
Premium Member
Apr 14, 2011
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Yes. If sized correctly it will maintain proper pressures on both sides
 
Oct 3, 2020
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Amazing...In the old 2 stroke turbo sled how can maintain the correct fueling when the boost come up? did they have auxiliary electronic injectors?? In the late '80'90??? Or they use other?
 

dansvan

Active member
Premium Member
Apr 14, 2011
139
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Adjustable power jets placed in the carb bell or entrance. Every engine combo is different In its requirements. Many sleds had to install smaller main jets as the velocity of the air flowing through the carb and the pressurized float bowl caused the mixture to be rich. Start fat and lean it down. There’s a carbd Summit XP turbo for sale now on Seattle CL. Mine was an animal.
 
Oct 3, 2020
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I search info about carbed sled because I want to turboing a small yz250 2 stroke carbed but I don't have find info about it so I search a similar set up to learn something about it.
 
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