patsmx5

iTrader: (16)

I'm puting a 1 Liter Lysholm twin screw supercharger on my miata. It has the ports for an oil feed and return line and it will have it's own oil cooler. I will also be running the blower unthrottled. The TB will be in the stock location and the blower inlet will be attached to a 4" pipe going to an airfilter. There will be pop off vavles and BOV's on the crossover tube. I haven't worked out all the details on how to vent the boost but that's not the problem at hand right now.

In general, throttled roots type blowers have cooler outlet temps when not in boost because these are external compression blowers. Since typically the blowers are throtled, when there is no boost there is no compression and the heat associated with it. However, when going into boost, outlet temps climb and are usually higher than the outlet temps of any other form of FI.

In general, throttled twin screw superchargers have higher oulet temperatures when not in boost as these are internal compression superchargers. However, when making boost, these blowers are more efficient than roots type blowers and have cooler oulet temperatures.

However, I will be running a twin screw unthrottled. From what I have gathered, the general consensus is the blower should run cooler as it's always pulling in cool air.

The blower is off of a 95 millenia S. On the millenia, the blower had a 1.5:1 drive ratio. With a 7K redline, that would put the blower loafing at 10.5K.

Here's my concerns. I'm told that on a SC motor you can rev the **** of them and the torque curve dosn't drop off like on a NA motor. Say, if you turn the motor 6K and it makes 200hp, turn 7K and make 230, turn 8K and it will probably still make 230 or better. Most dyno plot's of MP 62'd miata's I've seen support this way of thinking.

I don't know what RPM my supercharger is rated at, as the information seems to be hidden and even Lysholm can't tell me the specs except that it's a 1 Liter twin screw. Most new lysholms that are between 1.2L -1.8L of displacement are rated at 16K. Im assuming mine will turn 16K.

I'm assuming that when you exceed the rated RPM of a supercharger, the additional load and work that is being done results in more heat than the blower is designed to operate with, and this leads to failures. I don't think the blower will simply explode the first time it see's 20K. For reference, Big Gulp spun a MP62 to 23K using water injection pre supercharger. The heat of vaporization kept the blower cool and allowed it to keep making power above it's rated RPM without destroying itself. Again, it's my assumption that the additional heat is the main factor in failures when the max rpm is exceeded.

So, here's the ten dollar question: How fast could I spin this blower utilizing a seperate oil cooler and water injection pre blower to aid in cooling? It dosn't have to last forever, but it can't just fall apart and fail the first time it spins really high. I can get these blowers for 28 bucks at a local yard. Assume it needs to last 1 year. I'm thinking of a 2.5:1 drive ratio.

Here's the thirteen dollar question: How fast should I spin the motor when making boost? I'm wanting to make the maximum amount of HP with this blower. I have done some extensive porting and unshrouding so the head does flow prety well and I would assume the cams are now the limmiting factor not the head itself. I'm guessing somewhere around 7.5K on the engine. Again, this motor isn't going to last forever. I probably won't have the car making boost until the first of 08, and the motor will be rebuilt in the summer of 08.

With a 2.5:1 drive ratio and a maximum of 7.5K RPM's on the engine, that would put max blower RPM at 18.75K. If I can push it harder, I will, but I'm really uncertain about the numbers.

Well, let's hear it. Are all of my assumptions right? How fast can this blower turn with all the right cooling? How fast can a supercharged engine turn before it stops making useable power? I need the answers so I can decide on the crank pulley diameter and have it built.

EDIT: I should have put this in General.

magnamx-5

iTrader: (7)

Your reasoning is pretty sound to me. If it is not to much trouble i would start out at a lower blower speed say 10K rpm with the raises rev limiter to test the durability of your parts and engineering. Jumping of the moving truck at 18.75K rpm is kind of abit drastic for stuff you engineered and built your self. But if you are ever confident in your products and abilities the reasoning behind your post is pretty right on. FYI to make it easier to pull your crank bolt should you need to do so cut down a 21 mm socket so you can clear the sway bar easier.

patsmx5

iTrader: (16)

Yea, the pulley will have a hole in the center as you suggested. I understand you reasoning behind testing the system before really going all out. However, I'm going to go all out probably. I'm hoping to hit 300 whp on boost without the nitrous. I'm hoping for some more feedback from other members. The blower has a 4" pulley. I know the crank pulley will be at least an 8". I want more like a 10", as that would give the 2.5:1 ratio.

Markp

Yes, they run very cool in this configuration. BTDT. I used a 6" crank pulley with a 2.67" blower pulley.

I would attach the 7500 RPM run at 251 RWHP to confirm your thinking, but yes this is true. I was running a .89 liter/rev at a 2.25:1 ratio if I remember correctly. It made 200 ft/lbs of torque (roughly) everywhere. I would attach the dyno chart but Phillip is too cheap to let me post graphics anymore.

That is a pretty safe bet.

Partially, there are limitations to the gearhead as well. You should be ok to about 17k RPM intermittently.

Don't water inject pre-blower, it really isn't needed and will ruin a twin screw as it relies on VERY close tolerances to work properly. A 2.5:1 ratio is very agressive. I would not run more than that and I would not inject pre-blower on a unthrottled twin-screw... it runs so damn cool you can put your hand on the blower right after a boosted dyno run. It's pretty amazing actually.

Most of your assumptions are right, the trick is keeping the lobes meshing nicely at high RPM, there is an RPM where the lobes will fail to mesh, why this happens I am unsure but from what I understand it does and can happen.

The most critical part of a twin screw install is preventing case flex in the blower. This is what I have seen kill them. Additionally the sealed bearing are rated to a maximum RPM as well, I believe it is these bearings that are the real limiting factor to how fast you can spin the blower safely.

A 2.25:1 pulley with a stock '99 motor should put you above 250 RWHP, as I was running a blower that was 10% smaller. (That number was uncorrected at 3500' asl.)

Mark

patsmx5

iTrader: (16)

Well, Im gussing your right about injecting water pre blower as the blower does have very close tolerances and they are internal compression, and adding an incompressible liquid could cause damage it. Your saying that in realiity, heat isn't going to cause a blower to fail prematurly. It's really case flex and the gearhead.

I've also heard about case flex being the #1 killer of twin screw superchargers. I wonder if there's any way to strengthen the case up from the outside? Just a thought...
EDIT: Perhaps artificial aging would increase the cases strength. Can't remember how hot you have to get alluminum to do that and those temps could be detremental to the seals and whatnot. It would be worth looking into. I'll ask my machine shop teacher about it when I get him to make my pulley. It could be done if I rebuilt the blower, but I'm told rebuilding a twin screw is practically impossible without the proper tooling, equipment, precise measuring devices, knowhow, etc. I still wonder if it could be done.

About sealed bearings. My blower does have oil feed and return lines. Do you think it still has sealed bearings? I'm guesisng there are 2 in the front between the gearhead and the screws, and 2 more in the rear. I'm pretty sure the ones in the rear are sealed, but unsure about the ones in the front. I wonder how the oiling system works?

hustler

iTrader: (6)

www.miatasupercharge.net

magnamx-5

iTrader: (7)

My gues is internal compression eventualy prevents the meshing i haven't any idea as to when this happens though. Good info mark BTW if you have photobucket etc you can hyperlink or send it to me and i can post it for you.

patsmx5

iTrader: (16)

I'm thinking I'm going to order Bells' book Supercharged. Seems like I've got some learning to do and I can either do it now or do it when I break something.

Markp

The oiling system is strictly for the gear head. I have proven the case flex issue on the OpCon units and I assume that this would be the same for all of them since they use an aluminum case. The problem occurs when the two rotors grow from heat expansion and there is asymmetric pressure on the case. This causes either the case to contact the rotors or rotor to rotor contact depending on where the flexing occurs. It is easy to avoid simply by tightening down the blower and hand spinning the rotors. If you force required to spin the rotors increases you are placing a load on the supercharger that may (will) cause it to fail in the future from either a bearing failure or contact of the rotors to the other rotor or the housing.

Don't worry about the strength of the case, it's strong enough, you just have to be careful about causing deflection (even a few thousandths can cause a problem when you have less than .013" clearance between rotors to begin with.) Finally, since it will be unthrottled you should not have a heat problem as long as you bypass enough air.

Mark

patsmx5

iTrader: (16)

Hey mark, am I correct that I could not tear down the blower and replace the bearings and seals? I'm sure I could take it apart, and can probably source the bearings and seals, but I'm unsure if I could reassemble it properly. I'm assuming there is more to it than just bolting it together as you have mentioned the close tolerances between the rotors themselves and their relation to the case.

Also, what do you mean by "you should not have a heat problem as long as you bypass enough air."
The blower does have a bypass valve on it, but I wasn't sure if I was going to use it or not. The vavle recirculates the air. I would assume that would make the blower run hotter if it was pulling in air that's already been compressed and then decompresed as opposed to pulling in frest air.

I think I will order a copy of Corky's book "Supercharged" tomorrow. Thanks for all your help.

Markp

I would have a machine shop press and replace the bearings. It's not a simple task for us mere mortals but the average machine shop has a good press to get the bearings in and out. Disassembly is not exactly difficult.

Bolting it together is fairly straight forward, but still I would put it on the machine shop to get it right.

You want to bypass air post blower (noisier) to keep cool air circulating through the blower. That seems to work best. Just my .02c

Mark

patsmx5

iTrader: (16)

Thanks. I got a 20 ton press and 2 years of machine shop experience from high school. I'll consult a machine shop but I might just give this a try, as I'm fairly certain the blower I have leaks oil into the discharge. If it's pretty straight forward and all I've got to do is press new bearings in, new seals, and carefully reassemble, I can probably do that. I would definatel feel better about spinning it over 16K if it were new. Thanks.

Markp

You may need to throw the thing into the freezer. Then pull it out, the AL will heat up faster and make the bearings easier to press, they are a real bitch.

Mark

patsmx5

iTrader: (16)

Yea, that's a good idea. I've done that assembling electric motors for the lathes in our machine shop. Remember Katrina? Our shop had 2' of water in it. It didn't ruin all of our machines, but it got all the motors wet and the electrical on most of them. I cleaned and replaced the bearings in 13 3 phase motors. We would freeze the bearings to make it easier to press them in so we didn't break the ends of the motor, as they were really old motors and if we broke one it would be hard to source a replacement cap.

Now this is what's weird. I've ALWAYS been told you can not absolutely not rebuild a blower. Only the manufacturers have all the right tools, know how, etc. to properly do it. Always been told I couldn't do it. Of course, a lot of people said I couldn't rebuild an automatic transmission either and I did that over the christmas holidays. I suppose you just have to take your time and make sure you really understand what's important and make sure everything goes back exactly as it was.

Oh, and for fixing those 13 lathes, I've got a 16X60 Clausing Gearhead Lathe sitting in my shop. Just don't have 3 phase to run it.

Newbsauce

iTrader: (2)

Just a quick note: Most of the questions you can't get answers to.. just call Corky. He usually is very helpful.

patsmx5

iTrader: (16)

Well, I just ordered his book "Supercharged". Hope that will answer some of my questions and maybe I'll learn a thing or two

I sent him a PM on m.net a while back but he didn't respond. I'm assuming he only responds to emial. I might try contacting him again. I posted this topic on M.net and was hoping he would chime in and toss his two cents in. If he dosn't I'll call him or emial him.

patsmx5

iTrader: (16)

I searched some mazda millenia forums and it turns out that a couple people have rebuilt these superchargers succesfully. It turns out that these blowers are quite notorious for oil leaking from the gearhead to the compressor. There is a bearing at the gearhead that's lubed by the oil, and then there's a gap, and then a oil control seal. Between that oil control seal and the bearing, there is a vacuum port that is supposed to remove any oil that gets past the bearing. Most agree that that port becomes clogged up, and oil builds up and leaks past the seal.

Those who have tore down there blowers and seen all of this said the vacuum port that was suposed to remove oil was completely pluged up with sludge. Sure enough, I checked and those ports are pluged up on my blower too.

That's probably an oversimplification, but basically they agree that it's a poor design and extended intervals between oil changes that causes oil to leak into the compressor. All that tore down there blowers said the bearings and seals were fine, but those ports become clogged and cause the blower to leak oil.

I've got some part numbers to order the bearings, but the seals seem to be almost impossible to source. Some people carfully removed the bearings and seals, cleaned the blower up, and reinstalled everything and so far so good. I'm planning on changing the bearings and hopefully the seals too. Some had to measure all the dimesions of the old seals and have custom seals made.

It seems that extended oil changes are what causes the failure, as the blower ran off the engine's oil system in the millenia. I'm thinking I may just build a seperate oil system for the supercharger. That way, It can have it's own fresh supply of clean, cool, synthetic oil at all times. That would also help protect the gear head and the front bearings that are cooled by the oil. I would probably run the best oil money can buy.

Those that tore them down said it was a nightmare. All the bearings are glued into the blower. The bore they are pressed into has a grove cut into it with a hole going out to outside of the blower. After assemlying the blower, glue was injected into the holes, and then a metal ball was pressed into the hole, seal the unit. Turns out baking the blower in a oven at 260*C kills the glue and makes dissasembly easier, and dosn't harm the case, bearings, or seals.

Bottom line is the blower can be rebuilt for less than 100 dollars with new bearings and seals if you supply the labor. That's what I'm planning on doing. It should be intersting.

Markp

That's awesome that you can rebuild them so cheaply especially if you have a source of $25 blowers.

Mark

patsmx5

iTrader: (16)

Ok, I was a little off in my last post. The order is bearing, seal, then the area where the vacuum port is. The seals leak a little when new, as the screws inside turn 30K RPM's with the snout only turning 10K. What oil seaps past the seals runs down and puddles in this little camber with a vacuum port attached at the bottom, where the oil is then sucked out. The millenia had a vacuum pump so that there is always a source of vacuum to these areas, even when the motor is making boost. Heat and dirty oil causes the seals to leak, and the dirty oil plugs up the vacumm ports. This combination means lots of oil bypassing the seal and none of it being removed. That means all that oil goes into the compressor.

So, that means I'm going to need a vacuum pump too. Hopefully, a small electirc one can be had to do the job. Looks like this blower is going to have it's own electric oil pump, oil cooler, oil tank, and even it's own electric vacuum pump. When I rebuild the blower, I'll clean those ports out perfectly and perhaps even enlarge them or just anything I can to make sure they don't become pluged again. I'll surely keep the oil changed in the blower to reduce the chances of failures.

Mark, you had mentioned case flex is the cause of failures. I'm trying to determine the weak links of the blower so I can correct them and extended it's RPM range to around 17K to possible 19K. I'm not sure if case flex would be a problem at those RPM's or not.

I do believe the bearings won't like those RPM's, as the bearings turn faster than the snout pulley. One screw turns 2X the speed of the snout, and the other turns around 3X the speed of the snout. Those numbers are approximations. The bearings by the gearcase are lubricated and cooled with oil from the gearcase, but I'm unsure about the ones in the rear, by the intake. I believe they also recieve oil, but I may be wrong and they may be sealed bearings.

magnamx-5

iTrader: (7)

Hmm why not just run lines like us turbo guys do and do a gravity drain? It is not like you are going to run rotella oil in the car or something FWIW Amsoil from Soflarick is a good deal.