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Blowers and superchargers.

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  • Blowers and superchargers.

    Got some friends who are big into vintage car racing with things like Alfa's and Alvis's.

    Blowers for these are very, very hard to source and consequently very expensive. The VMCC governing body is quite lenient over what or what can or cannot be fitted and basically it has to look period.

    Two ways to do this, start from scratch and make the lot, case will have to be patterned and cast anyway to get the period look, off the shelf gears and the rest made to suit.
    Second way is to get a modern Merc Kompressor off Ebay <>£250, gut it and make a new case round it.

    Stumbling block for me at the moment is finding something out on the design of the rotors. Seen the Wiki pages with the diagram but also read that this isn't the mathematically correct shape and this is borne out from some pictures I have seen of the rotor shape where the head is a semi circle.

    Soooooooooooo any links please to any papers on two lobe design so I can explore all options ?

    Sir John , Earl of Bligeport & Sudspumpwater. MBE [ Motor Bike Engineer ] Nottingham England.

  • #2
    Rootes type blowers?


    • #3
      I'm not going to be any help with a direct link for information but I can suggest a few search terms. Try GMC 6-71 Superchargers or for modern ultra high performance maybe "Screw Compressor" I could be wrong but I believe these were banned over here for drag racing. I never did hear exactly why. That would be a damn interesting project. Wish I could be more help.



      • #4
        Check your PM's.


        • #5
          Hi John

          These may help.



          • #6
            I know these guys do it all - maybe they can help. There's some technical info that you can download too.


            "To invent you need a good imagination and a pile of junk" Thomas Edison

            Better to have tools you don't need than to need tools you don't have

            73's KB3BFR


            • #7
              John -

              The vintage cars probably used Roots type blowers, which are noisy, not very efficient and don't really compress air - they just shift large volumes very quickly. If the rules allow, a modern screw compressor should be more efficient and they actually do compress air. You could make the housing look like a vintage blower so that may be an ideal solution.

              The rotor profiles are quite complex and it is the profiles that make them work, so concentrating on getting them right is essential. As well as the geometry of the rotors, there is the equally complicated problem of how to make them. Specialised milling and grinding machines are used for this. The clearances (rotor to rotor and to housing) need to be of the order of a few microns. This means that metrology for these parts is also quite complex and specialised machines may be used (I was involved peripherally with this in the distant past). Given this, you might be better off (as you suggest) finding a reasonably common set of rotors that you can source second hand and building the rest around that.

              Holroyds (in Milnrow) are the place to go for screw compressor rotor making and inspection machinery.
              Just having a look at their web site may be helpful.

              If you use Holroyd as a search term with screw compressors, you will find lots of papers that should help.

              Here are a few to get started:


              Note that the papers do contain some generalised rotor drawings.

              There are patents in this area you may need to be careful of if you are going into production!.


              • #8
                Got a couple of books on this sort of stuff, Ill dig them out and drop them round.
                how urgent?

                Just south of Sudspumpwater UK


                • #9
                  I'm gonna watch this! Crazy tight opposing tolerances are exciting!

                  I'd look for a modern unit and modify the housing to make it look how you want.

                  Housing are easy to work on or reproduce.

                  Last edited by vpt; 09-20-2011, 08:07 AM.


                  • #10
                    Originally posted by vpt
                    I'm gonna watch this! Crazy tight opposing tolerances are exciting!
                    how ABOUT them backlash specs..............

                    Keep eye on ball.
                    Hashim Khan


                    • #11
                      Originally posted by J Tiers
                      how ABOUT them backlash specs..............

                      There is at least a 1/1000 of a degree leeway there, should be easy to work out.


                      • #12
                        Originally posted by J Tiers
                        how ABOUT them backlash specs..............
                        Not sure how backlash is accounted for on the straight cut gears in VPT's post, perhaps he can shed more light on this.

                        I've done several GM Rootes type blowers in the past and they use a very novel approach.
                        Rotor lobe to housing, and rotor lobe to lobe clearance varies depending on where of course it's measured, but does go down to about .003". Been a while but I think there are about 8 or 10 critical measurement points to take, and this is on a medium speed, 2100 rpm 2 cycle Detroit Diesel. Mind you they are overdriven as Detroits do like LOTS of air.

                        Essentially rotor gear backlash is held within .0005-.0025". The rotors are then timed to one another to establish running clearance by selecting shims that go behind one of the helical gears to move it in or out relative to the other gear.
                        Home, down in the valley behind the Red Angus
                        Bad Decisions Make Good Stories‚Äč

                        Location: British Columbia


                        • #13
                          The statement that Roots type blowers "don't really compress air" is incorrect. Of course they do, and modern dragsters use them to compress atmospheric pressure air to 50 psig air in the intake manifold of the engine.


                          • #14
                            General replies,
                            Sorry should have said Rootes Blower, I know these are not the best things but they were original and do improver the performance and with the Vintage guys it's all about looks more that wring the last ounce out of them.

                            Rob, thanks for that last link, some good information there.

                            Wilmac, thanks for the links but screw compressors are definitely out doe to the complexity of manufacture.

                            BWS still digesting that link.

                            Dave, Small planes, be interested if it had geometrical design in it, no rush, it's an over winter project.

                            Most of the links I have found are just advertising or mumbo jumbo on amounts of air moved.
                            I'm more interested in basic design.

                            Sir John , Earl of Bligeport & Sudspumpwater. MBE [ Motor Bike Engineer ] Nottingham England.


                            • #15
                              There is no change in the volume of air as it passes through a true Roots blower, and therefore the air is not compressed in the blower. In comparison, a screw type compressor or a vane type compressor DOES internally compress the air that passes through it. The fact that the Roots blower does not compress air internally does not mean that it is incapable of delivering a positive pressure ratio from atmospheric to inlet manifold.

                              There is a significant difference in the performance of screw and Roots type. You need to choose based on the requirements. A dragster doesn't need to be thermodynamically efficient.