Whitey Posted October 18, 2006 Report Share Posted October 18, 2006 IA supercharger does not create boost with a "turbine" it is created via centrifugal force with an "impeller" in the vortec style blower, via positive dispacement with a "roots" style blower and and contrary to popular belief a turbocharger is "thermally" driven, not mechanically. A centrifugal supercharger and a turbo create boost the exact same way by using a compressor, the only difference is the supercharger uses a belt and a turbo uses exhaust gas. You can get different compressor trims for a centrifigal charger just like a turbo When a turbocharger is pushed beyond its efficency it becomes mecanically driven via the backpressure on the turbine wheel, however this actually costs power and is why turbochargers are rated in CFM and Trim. This is also why you can command any amount of boost you want, however is will not continue to make power. The exhaust temperatures will simply increase to the point of literally melting the turbine and destroying the turbo. This is why they have a wastegate to bypass the turbine after it's efficiency has been reached (if your wise enough to set it there) Mass flow rate is what drives a turbo. I think you have efficiencies a little backwards. When you go above a compressors efficiency (not the turbine) you start superheating your intake air temps, overspinning turbos, getting crossover and all that other fun stuff. Your compressor efficiency is what your concerned about, not the turbine. Don't get me wrong turbine sizing is still very important but its a gray area with different theories that depend on manifold design and mass flow rate prioritizing. I can run a small turbine, large compressor, give priority to an oversized wastegate and now my engine breathes as if I was running a large turbine housing. Again, your getting into more theory. But I digress, what we are really talking about is if 14:1 is a safe A/F ratio for a banshee engine, it is my contention that is would be unwise because the increase of combustion pressures at certain RPM's would dictate that more fuel be added to lower the combustion temperatures, and more importantly prevent detonation. What it comes down to is a 14.1 afr is safe if your not detonating. From what I have found your going to gain more on tuning your timing more aggressively than leaning out your AFR's as much as possible. If you want more piece of mind than tune it richer to a 13.1. Remember to look at the big picture by checking the plugs, EGT's (if you can) and really see what the engine is doing. You'll be able to tell what is safe and what isn't. If you melted your piston to the wall, thats just your motor telling you to richen it up a little. :beer: Quote Link to comment Share on other sites More sharing options...
csrmel Posted October 18, 2006 Report Share Posted October 18, 2006 2 strokes like the banshee are not supercharged, but they are forced induction motors. at the right rpm the exhaust literally FORCES air and fuel (under pressure) up the exhaust port and into the cylinder. air and fuel that would otherwise get wasted because of the overlap between the transfers and the exhaust. if the 2 stroke had no expansion chambers it would lose huge ammounts of power. no the banshee doesnt have forced induction on the intake and transfer ports. but the exhaust (which is also an intake) has forced induction in every sence of the word. think about that for a moment. we all know unburned fuel and air go out the exhaust during each revolution. the expansion chamber than forces a good chunk of that fuel and air back up into the cylinder right before the piston closes the exhaust port. want to see what a banshee is like without forced induction? take off the expansion chambers and put in their place a pipe like a 4 stroke has. all the sudden the 2 stroke is an inefficent low power piece of steaming crap. id be willing to bet 1/3 to 1/2 of the power of a 2 stroke is made because of the expansion chambers.(depending on the setup, of course). or for a more real world example, look at how shitty a 2 stroke runs when one of the expansion chambers gets a dent in it. the dent doesnt restrict flow in an expansion chamber wider than my fist. it fucks up how the pipe operates. Quote Link to comment Share on other sites More sharing options...
jankab Posted October 18, 2006 Report Share Posted October 18, 2006 A centrifugal supercharger and a turbo create boost the exact same way by using a compressor, the only difference is the supercharger uses a belt and a turbo uses exhaust gas. You can get different compressor trims for a centrifigal charger just like a turbo Mass flow rate is what drives a turbo. I think you have efficiencies a little backwards. When you go above a compressors efficiency (not the turbine) you start superheating your intake air temps, overspinning turbos, getting crossover and all that other fun stuff. Your compressor efficiency is what your concerned about, not the turbine. Don't get me wrong turbine sizing is still very important but its a gray area with different theories that depend on manifold design and mass flow rate prioritizing. I can run a small turbine, large compressor, give priority to an oversized wastegate and now my engine breathes as if I was running a large turbine housing. Again, your getting into more theory. What it comes down to is a 14.1 afr is safe if your not detonating. From what I have found your going to gain more on tuning your timing more aggressively than leaning out your AFR's as much as possible. If you want more piece of mind than tune it richer to a 13.1. Remember to look at the big picture by checking the plugs, EGT's (if you can) and really see what the engine is doing. You'll be able to tell what is safe and what isn't. If you melted your piston to the wall, thats just your motor telling you to richen it up a little. :beer: A 2 stroke is NOT forced induction. The job of the expansion chamber is to create a pressure diff. to hold the charge in while the motor cycles. Quote Link to comment Share on other sites More sharing options...
ojcool Posted October 18, 2006 Author Report Share Posted October 18, 2006 Alot of variance in opinions on this subject. Since it's not raining today I should have some results to share by this evening. Quote Link to comment Share on other sites More sharing options...
dawarriorman Posted October 18, 2006 Report Share Posted October 18, 2006 (edited) I can't find the article now, but on Macdizzys site they have some good information about how the pipe on a 2 stroke works. It does force more air/fuel mixture into the cylinder than would be possible without it. At the time the article was written, a properly tuned 2 stroke 125 bike, when on the pipe, was using about 185cc of air/fuel mixture per stroke. A four stroke 125 motor would use about 121cc of mixture. No, its not the textbook definition of forced induction, and its not supercharged or turbocharged in any way. But when it is on the pipe, it is using more fuel/air mixture than could fit in the cylinder at ambient pressure. In essence its like forced induction. Either way though, I would try and go richer than 14.1:1. Waiting until the pistons melt to decide if that was rich enough is a poor way to tune IMO. Edited October 18, 2006 by dawarriorman Quote Link to comment Share on other sites More sharing options...
dajogejr Posted October 18, 2006 Report Share Posted October 18, 2006 This is a classic thread of opinion...ta-may-to vs. ta-mah-to.... I don't know squat about turbos, superchargers or NOS in relation to a two stroke...but this is a good informative thread.... Keep in mind boys.... In a two stroke, the mixture travels into the crank case and up through the transfers. While a four stroke, the mixutre never enters the crank case (unless your rings are really...REALLY bad)... So, that's where the extra volume comes into play would be my guess, nothing about the cylinder holding more volume....the crankcase is part of the overall volume. That's my take, anyways....and, I'm more than open to suggestion on that. Quote Link to comment Share on other sites More sharing options...
csrmel Posted October 19, 2006 Report Share Posted October 19, 2006 adding to what daojegr just typed, ive read that in the crankcase of a 2 stroke, pressures of up to 6psi can be seen right before the transfers are uncovered. if this is true (and i have no proof that it is) then that sounds alot like forced induction to me. Quote Link to comment Share on other sites More sharing options...
dajogejr Posted October 19, 2006 Report Share Posted October 19, 2006 adding to what daojegr just typed, ive read that in the crankcase of a 2 stroke, pressures of up to 6psi can be seen right before the transfers are uncovered. if this is true (and i have no proof that it is) then that sounds alot like forced induction to me. I think what you read is you leak test the motor at 6PSI.... If that was the case....then a piston moving up and down creating vacuum to get the charge, 2 stroke or 4, would be considered a pressurised charge... Or...fuel injection would be non-naturally aspirated. Sooner or later, we gotta draw the line.... Quote Link to comment Share on other sites More sharing options...
ojcool Posted October 19, 2006 Author Report Share Posted October 19, 2006 I think what you read is you leak test the motor at 6PSI.... If that was the case....then a piston moving up and down creating vacuum to get the charge, 2 stroke or 4, would be considered a pressurised charge... Or...fuel injection would be non-naturally aspirated. Sooner or later, we gotta draw the line.... Well think about it though, when the reed valve closes, and the piston is moving to the bottom of the stroke, there would be a degree of pressure created inside the crankcase, and when the transfers opened it would force the charge to blast in. It would have to because the reed valve isn't going to let the pressure escape. When you think about it a two stroke engine is really a mechanical marvel.. I think we can all agree on that.. Well I ran the bike two more pulls tonight and it did get significantlly richer with the 340's in it.. It's till blowing (inconsistantly I must add) 13-15's. My modifications have not given me the smooth results I had hoped for. The A/f is moving all over the place.. Unfortuantly I don't have what I need to datalog with the RPM vs A/F, I'm having to do it a/f vs time, so it's hard to say where i am in the RPM when it's rich and where it's lean. I guess I'm going to buy what I need to log the rpm as well, and start from there. It does seem to like the 30 pilot though... 1 turn out on the screws and it it's still not loading up in the low revs. I have the bike geared too low as well, I have a 13 43 on it right now cause I was riding trails. Quote Link to comment Share on other sites More sharing options...
duner Posted October 19, 2006 Report Share Posted October 19, 2006 you are supposed to put your sensor in the front of the pipe near the cylinders, drill holes and get some plugs welded, similar to pyros. This is also what I've seen. You measure by how hot the gasses are. Hotter temps = leaner fual mixture. Don't know much about it though. Quote Link to comment Share on other sites More sharing options...
csrmel Posted October 19, 2006 Report Share Posted October 19, 2006 I think what you read is you leak test the motor at 6PSI.... If that was the case....then a piston moving up and down creating vacuum to get the charge, 2 stroke or 4, would be considered a pressurised charge... Or...fuel injection would be non-naturally aspirated. Sooner or later, we gotta draw the line.... yeah i know about leakdown testing but i could have SWORN i read on planet sand some guys plugged in vacuum/pressure gauge into the crankcase of a 2 stroke and the needle bounced between 6psi and a vacuum when the motor was running. its a huge thread. lots of reading. they talk about the crankcase compression ratios and stuff thats just over my head. heres 1 snippet http://www.planetsand.com/ubbthreads/showf...0/fpart/14/vc/1 "As a side note, I did some real rough approximations with a 1.4 CCCR and came up with a case pressure of around 7 psi when the transfers open up. I did that ignoring the effects of the pulses, pipe tuning, ect. Just a simple compression from the piston moving down." Quote Link to comment Share on other sites More sharing options...
ojcool Posted October 21, 2006 Author Report Share Posted October 21, 2006 This is also what I've seen. You measure by how hot the gasses are. Hotter temps = leaner fual mixture. Don't know much about it though. You have been misinformed. Heat has nothing to do with it, other than the preheater in the 02. It measures oxygen content and this has a direct correlation to a/f ratio. Welding bungs into the expansion chamber is NOT going to give an accurate reading. There is raw a/f charge bouncing back and fourth in there. Quote Link to comment Share on other sites More sharing options...
jbooker82 Posted October 21, 2006 Report Share Posted October 21, 2006 adding to what daojegr just typed, ive read that in the crankcase of a 2 stroke, pressures of up to 6psi can be seen right before the transfers are uncovered. if this is true (and i have no proof that it is) then that sounds alot like forced induction to me. But it is forced induction with the exhaust port opean. The 2 stroke pipe reflects a pressure wave back twards the cylinder to help hold the new air fuel chage in. Quote Link to comment Share on other sites More sharing options...
dajogejr Posted October 21, 2006 Report Share Posted October 21, 2006 You have been misinformed. Heat has nothing to do with it, other than the preheater in the 02. It measures oxygen content and this has a direct correlation to a/f ratio. Welding bungs into the expansion chamber is NOT going to give an accurate reading. There is raw a/f charge bouncing back and fourth in there. While there is fuel bouncing back and forth in the chamber, the standards for tuning via EGT are just that. 4 to 6 inched on the pipe from the piston. Like anything, most builders have a temp range they like to run in for gas, and a different for alky...but, that is how it's done. Also....just like a dyno A/F meter and as I've said before. This is a good way to help tune the engine, but should not be replaced for reading plugs and piston crown burn patters.... I'm not going to start WW2 with you Oj...but, even if you get it dead set on a dyno or a/f meter, you still gotta read the plugs from riding at your actual temp, elevation and locale!!! Do a search on EGT or 2 stroke exhaust temp on planetsand.com There is a wealth of information on this subject....many times over. Quote Link to comment Share on other sites More sharing options...
ojcool Posted October 21, 2006 Author Report Share Posted October 21, 2006 While there is fuel bouncing back and forth in the chamber, the standards for tuning via EGT are just that. 4 to 6 inched on the pipe from the piston. Like anything, most builders have a temp range they like to run in for gas, and a different for alky...but, that is how it's done. Also....just like a dyno A/F meter and as I've said before. This is a good way to help tune the engine, but should not be replaced for reading plugs and piston crown burn patters.... I'm not going to start WW2 with you Oj...but, even if you get it dead set on a dyno or a/f meter, you still gotta read the plugs from riding at your actual temp, elevation and locale!!! Do a search on EGT or 2 stroke exhaust temp on planetsand.com There is a wealth of information on this subject....many times over. I'm not tuning via EGT so I don't know what this has to do with anything. EGT has some bearing on what an engine is doing but it is only important to me when tuning with a Turbos. A/F ratio is them most important variable in any tuning application.. Timing affects it, air temp, fuel type, pretty much anything that is going on with an engine will show up in the A/F ratio. There are some "tricks" that I have used in the past to deal with unaccepable EGT's but to actually "tune" with that data only would be dangerous in my humble opinion. But i suppose it would give you a margin of safety, however I don't think it would be ideal for getting the most POWER out of an engine. I can tell you this, since using the A/F meter the bike runs better at every RPM, it's a completly different bike. It makes more power and has more low end torque than it has ever had. I'll look at my plugs the next time I change them, but without a power adder, I don't really see any need to check them other than sheer curiosity. Besides I am using a fuel additive to make my fuel and it discolors the plug so It would be difficult to tell anything from them. I am planning on putting the bikes on the dyno at some time in the near future and we will just find out what the potential for drivability and power is with tuning this way. .. I plan to see what different jets do and where. For example I will see what you actually end up with when you do what is the norm of putting in the richest jet the bike can take and then leaning on it till the drivability comes back, I have a feeling we are leaving alot of power on the table doing it this way. Stay tuned for the results.. Quote Link to comment Share on other sites More sharing options...
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