How do I model a supercharger rotor?
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That was helpful - I was not understanding how this would work until I saw this animation. The key seems to be that some of the liquid seal is extracted with the gas, and must be replenished - is that correct?
That is cool ... except, how do you control the amount of fuel going into the mixture? Is it a function of how much fuel is fed into the liquid input?
Would a thorough running in on the bench (just the blower) help in this regard? Even going as far as heating the assembly up to engine running temperature. What's the clearance on a full size blower meant to be? Say a 6-71?
Bentwings
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We often took either a new blower or a rest ripped one out of the box and bolted it on the racer. At best it got a warm up of a few minutes then it was run flat out. The big hemi would run 8500 rpm snd that was it. It was valve spring limited even with titanium valves and moving valve train parts. Valve springs lasted 4-4 runs at best. So we were. Instantly ordering them and replacing them. We ran the blower at 50% over drive. It wasn’t well known then that the blowers were good for about 8500 blower rpm . We sacrificed top end speed for the power boost of the line line. High gear was like ho him, look around and see if the competition was close. We kept leaning the fuel and adding timing. Actually probably going down on overdrive might have helped overall but we were going quicker and faster each run by leaning it out and adding timing. There were multi stage lean out valves but we just didn’t get that far. It was important to make a good show and make all your runs match racing. The power of the dollar.in 5 years running we only lost one run due to breakage. We blew up a clutch but the remains stayed locked up enough to make it down the track. Believe me I had my knees in my face on that one. Through a hurclean effort and a borrowed clutch we came back and won the event. Cost wise it was an expensive weekend but the effort got us a return match race that we also won so I suppose it was a wash dollar size. It’s a great memory we hash over every reunion. One of those “play hurt” sporting events.
I did watch the video and got a good handle on how these blowers are developed. I have a set of near new rotors to measure and a perfect case so I’m going to try and model 3dprinted blower. I don’t have s printer but there are lots of shops near me that do. I’m not sure what scale to model to but it’s easy to change that. The screw blower video was fun but sad that the unit didn’t work. It apparently made some cool noise. The real racers run them pretty fast but I think the printed model might not survive.
byron
lohring
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Model engines use a needle valve. You may need to get fancier if anything other than full throttle operation is needed.
Lohring Miller
Bentwings
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The mechanical fuel injection used in top fuel and alcohol racers is far from simple or cheap. Even back in the old days. The fuel pumps are very high pressure and very precise items. Carefully flow tested and mapped. Even nozzle check valves and distribution blocks are tested and log books are kept. The. Percentage of fuel that goes into the blower is noted and manifold temp pressure and weather data enter into the picture. Rules prevent computerization but allow viewing after each run. In the old days we only had spark plugs to monitor the burn conditions. They were replaced after each run so en back then a notable expense. You always talked nice to the suppliers and ran their stickers if you had their sponsorship. They were about $ 5:00 bucks apiece then, much more now. So 24-36 a day counted up. They couldn’t be used in your daily driver or truck either. There was quite a trick to reading spark plugs depending on fuel used and motor tune up. Everything did something to how hot the plug got and what it looked like after a run. We didn’t have nice cameras like today if you took any with your 35mm camera it would be a week before you got them back. Sometimes I wonder how we ever got down the track. It was a bad run if you didn’t burn or wreck at least one piston a run. 1-200 bucks each depending on if you could save them . I cut new grooves in lots of them. Race stories are always fun when bench racing in the shop.
byron
lohring
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There were two supercharged four strokes introduced around 1990 for acrobatic RC airplanes. The first was the YS 120 that used the crankcase as a supercharger. The second was the OS 120 with a roots supercharger. Both used pressurized fuel systems with a metering diaphragm. The YS system prevailed but both are far more complex and heavy than the two stroke OS 60 Hanno Prettner Special they replaced.
The pressure fuel systems all these engines used could be used to regulate fuel flow into another style supercharger.
Lohring Miller
The pressure fuel systems all these engines used could be used to regulate fuel flow into another style supercharger.
Lohring Miller
Bentwings
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I do remember those engines. I don’t think I ever saw a boost pressure posted. Those were not ver big units but very well made. And they did work, probably more as good atomizers than actual pressure. The glow carbs were usually fed from muffler pressure to the fuel tank so fuel dribbled through the needle valve. The velocity of air intake and thrashing in the crankcase provided mixing. Actually back in early we2 days a draw through carb was used sustained negative g loads caused engingines to cut out. It was well known on Spitfires. When the big radials came out with big power something needed to change. I think it was P&W working with Bendix came up with the throttle body fuele injection that ultimately came out on cars with the early efi. These allowed inverted flight and capability I’ve negative g maneuvers. Ed’s entually it was a floatless carb. Another one of the amazing aero progresses the war brought about. It did take the auto industry 20 years to further developed the concept. I had an original ‘57 chev with the Rochester fuel injection. This unit did have a float system but it remained full controlled by a thing called a spill plunger. This allowed a calibrated fuel flow modulated with an intricate vacuum system. At one point I had a copy of about every tech manual available at the time I also had the adjusting and calibrating equipment that not even dealers had.the car also had an original one of some number nascar related 4 speed transmission. Almost all of these one offs became racers. Mine was raced for several years I was able to equal the nation drag race record but just never could get the exceed percentage at the qualifying races to set the record myself. I never should have sold that car. There a still a few in existence but priceless collector items. There are only a few select parts custom made for them these units were made until 1963 on corvette even those are very rare. It was an exotic car in its day the street versions were often removed and the two four barrel carb systems installed when dealers couldn’t service the fuel injection. These units were stuffed under the bench and forgotten about.
Anyway the glow carb is a very simplified variant of today’s racing mechanical fuel injection. I’ve often wondered if a modern O2 sensor and gage could be made up to help Rc gas engine tuning. The oil in the fuel might make for shorter life for the sensor but I trade a sensor for a well tuned engine any day. I’ve used these on all of my streetrods with great success.
byron
Bentwings
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Thanks. I did look this up it apparently had been updated several times. I’ll try and possibly test the free trial version.
I did order the plastic drawing kit more to help me understand the process . I may end be able to plug in some scaled numbers to get some kind of representation.
byron
lohring
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Pipe pressure was used on two strokes. The above engines used either timed crankcase pressure (YS) or gear pump pressure (OS). a regulator diaphragm adjusted the flow. Walbro carbs also use crankcase pressure with a regulator diaphragm.
Lohring Miller
Lohring Miller
Bentwings
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I used perry pumps and regulators including the vibrating pump on my big twin pattern plane. It was a hungry plane with two 7.5 ducted fan motors. I ran lots of nitro as I was into top fuel drag racing. There was always left over or wrong mix fuel. I just used a hydrometer to sort out what I needed. Glow plugs props fuel were my big expenses. I flew the plane a lot so it wore itself out constantly. I had spare motors just like we did for the the race car. I could try in through the pattern twice then I had to land for fuel. Mid air refueling would have been cool. We didn’t have the computer radios but today I think it might be possible with FPV and positioning. I found a pet tracker that is radio frequency activated. They flame one foot location. Night mid air refueling?LOL CONSIDERING RADIOS AND THE HOBBY TODAY ID BE IP FOR ABOUT ANYTHING. I was pretty wild in the old days. I’d do anything sports related. It didn’t matter what sport if it was was a game winner gamble Id roll it. Bases loaded 1 run lead score 3-2 count on league heavy hitter and I threw a knuckle ball for a called third strike. Coach made me bat boy for the next three games.
Bentwings
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When you consider that most yard two strokes can operate in about any position, the Walbo and Tilison carbs are pretty good. Our aerobatic planes can fly about any position near constant. They aren’t totally float-less but there is enough crankcase pressure to force fuel through anyway. If you were to come up with a good draw through supercharger they would probably work fine. A blow through might require some fuel tank pressure. I’ve not seen any thing like this but I’d be game to try it if I could. Someone noted that they had built a centrifugal blower so maybe two staging something like this with with the first stage a draw through on the carb. My question then is you now have a lot of fuel laden air even lightly compressed it could get dangerous. Most of this would be metal so I wouldn’t expect parts flying all over if there was a back fire.it doesn’t take long before the idea of using a ducted fan as the primary stage but scroll design gets complicated fast. I think a neat small planetary gear drive would be nice here. I may look into this after I get this roots blower out of my head. I haven’t seen much that worked on the Briggs motors but it would be a start. Putting a Walboro carb on the intake of a leaf blower then ducting to the Briggs would be my first choice.those blower housings are often plastic so some static spark could set off a spectacular flash.LOL. My son and grand son are like me try anything once. .
Bentwings
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Today I had opportunity to view the near new condition of a 6-71 blower. It indeed is perfect without a single scratch or mark on the rotors or case. I’ve never seen a new one up close so I’ll be able to get real measurements either in person or by various media. It will be nice to model one from exact measurements for a change. We set up two ways to measure the rotor of one using the lathe readouts the other by assembling the rotors and end plates the measure the diametrical clearance in the case knowing the rotor clearance and the case bore. The case is also without any scratches or scuffing. All that has been done is remove the mounting lip from the side. Standard first step in building a Streetrod or race modified blower. Even the intake and exhaust ribs are in marked and intact. Not sure what we are going to do with it other than preserve it. It would be a collector item at some point so for now it will serve as a measurement source. It’s all clean just a little dusty hopefully I’ll be able to model this exactly then make a high helix model. I may 3D print a scale size for testing as the screw blower guy did. If it works ou I may 3D metal print a scale model too. It would have a better chance of surviving a very high speed test .
Bentwings
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I’m planning on some scal 1/4 or there about. I’ll have to see about 3D printing cost. We don’t have CBC machining so carving from bilirubin stock is not possible. I know a couple shops near by but material has become so expensive lately that just raw material would be out ic my budget. I can’t print money like the current politicians can.
byron
byron
Bentwings
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Hi, I thought I might have described hoe the 6-71 and other gmc blowers work in the real world
So here goes I can get a little long winded when I get into one of my favorite bench racing or shop chats
first the common application is on streetrods. These blower are pretty big so they often stick out of the hood a ways. For street use modified carbs are used. Holley makes special modified ones you can buy o
i ws in the middle of a reply and a spam call came in and cut me off so if a reply comes that you don’t understand just delete it. I start over. 6-71 and others like them are pretty big so they stick out of the hood quite a ways. Add carbs and scoop and you get quite an impressive unit. So how does it work? Carbs are normally mounted on top of the blower so they are called draw through. The blower pulls air and fuel through the carb just as normal. At idle the carb acts as normal seeing a vacuum of 15 or more inches. Since only a limited amount can go through there is vacuum in the intake manifold again 15-17 inches. Some will argue this but my cars have had vacuum boost gage on bothe carb and manifold for a long time. When you open the throttle more air/fuel goes in and vacuum drops ubtill you open the throttle far enough then you begin to get boost in the manifold. Now interesting enough there is a huge flow of air through the carb it continues to do it’s job until you exceed its capacity then you begin to see vacuum between the blower and carb. As though the blower isn’t there. You can clearly see this if you have dual vacuum boost gages.I could write a book on what goes on and tuning . I’m not the authority on this but I’ve been very successful puttin 30k miles on my street rods and 200k plus on factory supercharged cars. I. Now tuning. Pulling air and fuel through the car lubricates and cools the blower some. There are inter coolers available that boat guys use these add another’s couple inches to height so not real practical for street use. Blower are great mixture atomizers. You get pretty even mixture in each port and carb calibrated air fuel ratio.I use a gage for this to 0 2 sensor it’s used for accurate tuning adjustment just as modern EFI does. Is just the brain is in my head not an electronic box.
Now lighting the mix is another story . Race cars have used magnetos for many years. Of late there are electronic ignitions that have many features I’ll leave for another day. Roots blower are pretty easy to work with. Now modeling presents another issue. These font scale very well it’s hard to get a small one to pump aid under pressure as you can see in the nice example the gentleman that modeled the screw compressor or blower. Clearances become very critical snd just getting enough air into the unit is hard. This was discovered ver early in the early diesel development. So centrifugal blowers and turbos were developed to force additional air into the gmc blower. This compound supercharging. It could be done in a model but it’s quickly apparent how complicated this could get. Plus you still have machining tolerances to deal with.
we use a highly developed mechanical fuel injection on the race cars. Grossly simplified its similar to glow fuel model enginges carbs. Basically a barrel valve with a threaded needle valve for fuel control fed with light fuel tank pressure to provide fuel at all time not depending on vacuum assist. In modern race applications ther is a high pressure fuel pump and various pressure/volume control devices. A percentage of fuel is injected above the blower rotors to provide sealing and cooling of the mixture the rest is sprayed into the ports as fine mist the ratios determine power and. Oolong effect.this is well beyond discussion here. Nitro and alcohol do all kinds of things in engines. Sometimes very bad thingsLOL. I’ll try and post a couple pictures of my Streetrod willys. I also have a couple pictures of a friends motor and supercharger ver simulate to mine.
byron
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