Examples of pressure would help to clarify the explanations.
2 stroke turbo - how it work
- Thread starter Two2
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Clarke673
Somewhere between too dumb to quit and flat earth
Can we take any a point as an example to see what could be expected as intake vs exhaust pressure?
The answer is no. Back pressure is a measure of resistance to flow. That requires you know all system flow characteristics. The compressor map plots mass flow across pressure ratios and over lays compressor efficiency. That allows you to understand whether the boost you are making is efficient or if you are just generating heat.
However, none of that tells you what EMAP is nor does it tell you shaft speed. To generate a pressure expectation, you must understand what limits hp potential...it is turbine mass flow. The turbine AR, wheel size, and mass flow potential dictate hp via backpressure (EMAP). Like I posted earlier...the lower EMAP is relative the cylinder discharge pressure, the more mass flow across the engine, the greater the power output. The compressor wheel only feeds the motor. Bigger compressor wheels won't make more power if the turbine is limiting flow capacity. Bigger compressor wheels help if and only if they can reduce compressor charge temps post turbo. But, that is at the cost of turbo lag, which is also caused by bigger turbine wheels. Thus, there is always a trade off.
Lastly, 300 hp and up require more than 25 lb of mass flow. A common rule of thumb for turbocharged applications is roughly a multiple of 10 when applying the fuel requirements based on brake specific fuel consumption (BSFC) for gasoline fuels. However, 2 strokes require more air than that because they only sample a portion of the mass flow moving through them. For a 2S class engine, you would need in the neighborhood of 35 lb/min mass flow to get to 300 hp. That is a real number, no BS corrections to inflate dyno numbers. That air mass estimate is based on the fuel energy required to make 300 hp using a proper stoich relation on boost. A four stroke can make 300 hp with less because no fuel or air mass is wasted in the pipe. A 4S can get there with around 30 lb/min compressor flow and a properly sized AR housing and turbine wheel.
Circling back to the prior question, to generate an estimate, you have to know all system characteristics and how they dynamically interact with each other. Otherwise, you are making a ton of assumptions you have no idea about. If you have a system, yes you can get there using rules of thumb you pull off the existing system because you know how the system dynamically operates. Your best hope for an answer is to ask someone who has measured all of these components on a sled in operation. Most kit builders should be able to answer your question. Bryce Kendrick from force turbos has measured the data points you want to know about on running sleds.
However, none of that tells you what EMAP is nor does it tell you shaft speed. To generate a pressure expectation, you must understand what limits hp potential...it is turbine mass flow. The turbine AR, wheel size, and mass flow potential dictate hp via backpressure (EMAP). Like I posted earlier...the lower EMAP is relative the cylinder discharge pressure, the more mass flow across the engine, the greater the power output. The compressor wheel only feeds the motor. Bigger compressor wheels won't make more power if the turbine is limiting flow capacity. Bigger compressor wheels help if and only if they can reduce compressor charge temps post turbo. But, that is at the cost of turbo lag, which is also caused by bigger turbine wheels. Thus, there is always a trade off.
Lastly, 300 hp and up require more than 25 lb of mass flow. A common rule of thumb for turbocharged applications is roughly a multiple of 10 when applying the fuel requirements based on brake specific fuel consumption (BSFC) for gasoline fuels. However, 2 strokes require more air than that because they only sample a portion of the mass flow moving through them. For a 2S class engine, you would need in the neighborhood of 35 lb/min mass flow to get to 300 hp. That is a real number, no BS corrections to inflate dyno numbers. That air mass estimate is based on the fuel energy required to make 300 hp using a proper stoich relation on boost. A four stroke can make 300 hp with less because no fuel or air mass is wasted in the pipe. A 4S can get there with around 30 lb/min compressor flow and a properly sized AR housing and turbine wheel.
Circling back to the prior question, to generate an estimate, you have to know all system characteristics and how they dynamically interact with each other. Otherwise, you are making a ton of assumptions you have no idea about. If you have a system, yes you can get there using rules of thumb you pull off the existing system because you know how the system dynamically operates. Your best hope for an answer is to ask someone who has measured all of these components on a sled in operation. Most kit builders should be able to answer your question. Bryce Kendrick from force turbos has measured the data points you want to know about on running sleds.
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Clarke673
Somewhere between too dumb to quit and flat earth
Yeah i was running with my EBC set to 22 lbs and on strait 110 and that map i posted is one i stole from a dude who happens to be runing the same -ish turbo on a 4 stroke Toyota truck/ supra swap. My fueling goals are to supply enough fuel for 300 hp @ around 27-30 sfm. Im running a water to air now and going to try pump gas out this season.
Xp, you obviously have a good grasp of whats going on. Would you say that in a 2 stroke , that the individual pulses could be used to better / more effectively drive a split turbine housing if A. someone could get some twin pipes built just for that purpose and B. out perform whatever the sled would have done with a stock pipe/ basic 28frame.
Daag, you follow Gale Banks? The Boost Gauge is Dead to Me! - YouTube - check out that video anyone who is interested in trying to understand. Maybe its just me but i feel like i know less the more i know! End of the day he who says he thinks he can and he who says he can't both usually get what they deserve. O.p. - Start up a build thread and i'll try and offer my opinions/ help as i can. You can build an old school turbo sled with new school tech for sure! I had one of those planned but decided my time was better spent starting with a good chassis and trying to figure out how to run it off of ONE throttle body PRE turbo.
The joke in there is that this "good" chassis has been totaled atleast twice on paper lol i can get you actual drive vs + map pressure data soon! However id caution useing the data (or anything i post for that matter) to replicate a system running Blow through. Id love to do a carb turbo sled some day and i believe you can run them off of pipe pressure like an RC ntro car but i have never seen it done in person so ??
Your question is a loaded question. It is theoretically possible if the single pipe system is so sub optimal such that splitting the pulse improves flow. In reality, the factory engineers are good at their jobs, so the effort won't be worth it. You implicitly assume the person who builds twin pipes did it correctly for a twin scroll turbo setup. I am leaving many things out. In short, it is not worth the hassel to package all that under the hood. People would do it now if there were easy gains to be had.
Again, you won't make 300 hp on a 2S unless your turbo is capable of efficiently generating 35 lb mass flow through the compressors and out the turbine. But, who cares what the number is. Don't fixate on that. If you like how it feels then that is all that matters. 22 psi on 110 seems unrealistic. I know guys who made 18 psi on 2S sleds, but they had to run 112 to 114 to get there. You must have really low compression. But, that is irrelevant as you pointed out. All that matters is mass flow. Guage pressure is a measure of resistance to flow.
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Would there be any downfalls running the boost map of the pipe pressure instead of MAP? Lets make EPP as short term for that "sensor". Pipe pressure is related to boost pressure, but the variable is the deltaP. Would it be an advantage to increase fueling when deltaP is rising? Will the ekstra fuel helps cool the combustion that is now heated by the higher back pressure (and back feed of exhaust), or would it just make the combustion even worse since there`s not airmass enough to burn the added fuel?
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Clarke673
Somewhere between too dumb to quit and flat earth
you can easily check pipe pressure. there are some "theoretical sciences" Bernoulies effect is a very important one. the high back pressure is a misnomer. There is inherent backpressure, or as i like to call it "bleed pressure" in all 2 stroke apps. Single pipes around 2.5-4 pounds, twins sometimes as much as 5 psi. There is a lot going on here and the heat created and held in the pipe is also a direct driver of the turbo. Im more interested in useing the sonic waves to drive the turbo more effectivly. The way of looking at "heat" as a driver or energy source in and of itself does work! Thats how diesel turbo tech went for years! DELTA P? sounds like a ****ty movie staring FAT Steven Segal !
The Drive pressure in the pipe vs the pressure going into the engine ( COLD OR HOT SIDE POST COMPRESSER Pree TURBINE) is not a direct comparison. What i mean is you can NOT theorize that 10 psi "boost" will mean 14 psi "drive" pressure. As, 1- variables such as IAT, intercooler efficancy, RPM, ECT will all be CONSTANT variables.
2. Because pipes actually INCREASE in size when they are hot. If you want to explain to me how to calculate this GOOD F'n luck lol
3. As the whole system increases in temperature its parascitic losses will all increase
The Drive pressure in the pipe vs the pressure going into the engine ( COLD OR HOT SIDE POST COMPRESSER Pree TURBINE) is not a direct comparison. What i mean is you can NOT theorize that 10 psi "boost" will mean 14 psi "drive" pressure. As, 1- variables such as IAT, intercooler efficancy, RPM, ECT will all be CONSTANT variables.
2. Because pipes actually INCREASE in size when they are hot. If you want to explain to me how to calculate this GOOD F'n luck lol
3. As the whole system increases in temperature its parascitic losses will all increase
Clarke_67, thank you for the link to Gale Banks seminar. It did clear-up many questions. It also helped explain how some pipe builders can make them reach over 7 psi backpressure NA for drag races. I can't vouch for that measurement, but at least I have a better idea how to work it out with proper tools.
What I find interesting is how the OEMs use the intake pressure and pipe temp to fine tune the power band, and Ski-Doo adds the pipe pressure for the turbo. It is reasonable to assume they can extrapolate a lot of information from these three sensors to make the turbo work.
On a NA single pipe sled that was setup to measure the vacuum/pressure with a digital gauge for each throttle body, it was interesting to see the gauges start in the negative/vacuum and reach zero under full power with a pipe pressure of ~4 psi at SL (Sea Level). With this particular NA sled it had a 0 to 4psi ratio.
Since the turbo and exhaust on a Ski-Doo turbo is a one size fits all elevations, there is no way to change the inherent restrictions. And since it only has three sensors (IMAP, EMAP and EGT), I can only imagine through testing that Ski-Doo found a relationship between the three as the boost increases. So for example if the IMAP reached 1 psi @ 1000 feet elevation, what could be expected with EMAP? Would it keep it near 4 psi?
Xpturbo600 explained that <boost increases IMAP and power is gained so long as the increase in EMAP, also called drive pressure, does not increase in proportion to boost.> Let's say the turbo increases IMAP to 2psi @ 2000 feet elevation to compensate for loss in air density. Would the target EMAP still be 4 psi?
What I find interesting is how the OEMs use the intake pressure and pipe temp to fine tune the power band, and Ski-Doo adds the pipe pressure for the turbo. It is reasonable to assume they can extrapolate a lot of information from these three sensors to make the turbo work.
On a NA single pipe sled that was setup to measure the vacuum/pressure with a digital gauge for each throttle body, it was interesting to see the gauges start in the negative/vacuum and reach zero under full power with a pipe pressure of ~4 psi at SL (Sea Level). With this particular NA sled it had a 0 to 4psi ratio.
Since the turbo and exhaust on a Ski-Doo turbo is a one size fits all elevations, there is no way to change the inherent restrictions. And since it only has three sensors (IMAP, EMAP and EGT), I can only imagine through testing that Ski-Doo found a relationship between the three as the boost increases. So for example if the IMAP reached 1 psi @ 1000 feet elevation, what could be expected with EMAP? Would it keep it near 4 psi?
Xpturbo600 explained that <boost increases IMAP and power is gained so long as the increase in EMAP, also called drive pressure, does not increase in proportion to boost.> Let's say the turbo increases IMAP to 2psi @ 2000 feet elevation to compensate for loss in air density. Would the target EMAP still be 4 psi?
This is the first time Ihave been on this forum in years! There is some good information on this thread and guys leading you in the correct direction. I just wanted to add that we use to have 3 points of connection on the system for boost reference. Pipe pressure, Intake pressure (between the carbs and reeds) then air box. You can tune a sled very differently off each reference point.
Just try each one and see what you like. Study out what you need and find a way to over come the problem with the pressures that exist. That is what makes this old technology fun. Not everything was measured and the effects of those measurement could be manipulated. New tech or the new Bypass turbo systems are a whole new world with strategies that we never dreamed of in the past.
FYI, you can run 2 stroke sleds up to approx. 25 psi of pipe back pressure. It just get to a point where the horsepower gain per lbs of injested air is no longer worth it. But is sure is fun until you hit that wall.
The uncharted territory is start looking at specific gravity of air/ fuel mixture in the pipe and turn the pipe into a afterburner where the pipes diverging cone becomes a vacuum. Then we can 10x the air and still gain HP. Two stroke turbos are still bad ass.
Dream big!
only pic i have on this phone of my turbo triple
Have you, or anybody else spec'd out the size of turbo being used on the Skidoo factory turbo?
The compressor side looks a bit smaller than a typical Garrett 2860 we've used for many years.
I'm curious if there is a similar Garrett in size? I'm guessing it would probably be in the 25xx frame size.
I have to say this is all very intriguing stuff to read through and I did have to make another coffee and sit down on the couch to read through it thoroughly and my head does hurt slightly…
I think I’ll just get back to building houses like I know
I think I’ll just get back to building houses like I know
I have not. The wheel size is not a proper factor to compare. You would need to see the compressor map to determine whether the mass flow is similar at different pressure ratios. Only skidoo has that map. Furthermore, new wheel designs often outflow older designs like that used in the 2860. Again, you need the compressor map max mass flow rate to compare them.
Turbine wheels are what make power because they choke flow out of the system, creating exhaust manifold absolute pressure. The more exhaust pressure (EMAP), the less power the system will make because the pressure differential across the engine is less.
Skidoo has likely paired a larger turbine wheel with the Doo compressor wheel relative to that used in the 2860. I don't have time to examine the doo turbo, so I won't speculate much. However, the factory obviously did a good job with it.
Why do you want to know how the turbo compares?
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