Boost Control on the 20B Supercharged Project

[antnicuk]

I know I know, its 2oclock in the bloody morning and i get a brain wave on how to sort my cousins car out.

If you are not bothered about how to control boost on a supercharged 20B engine, i suggest that you skip this threat as it may get pretty dull.

At just gone midnight this morning one of my cats starts chasing a mouse around the bedroom that it has bought in. After spending 1/2 an hour catching the bloody thing i can\'t sleep (the mouse not the cat). While i\'m laying there trying to count page 3 girls so i can get some sleep i suddenly figured out how to control the boost the 20B turbo charged project that my cousin and i have been working on for a couple of years. Well at least i think i have.

We are using a centrifugal type supercharger which looks like the compressor housing of a turbo but is driven from the crank.  This type of supercharger doesnt produce a constant level of boost like the traditional types but increases the boost pressure as your revs increase.   This causes a problem when you have fitted it to an engine that not many people know a lot about.  The reason being is that not only do you have to get the pulley size correct to ensure that you run the correct amount of boost without over spinning the unit, you also need the correct spec unit to ensure that you have enough boost at low RPM without having too much at high RPM.

At the moment the unit we currently have fitted is too small (1000cfm) and doesnt create any boost until 3k rpm, but at 7k its still only producing 8 psi.  We are currently awaiting a much larger unit (1850cfm).  By changing the pulley size we can ensure (hopefully) that we get a lot more boost early in the rev range but our dilema was how to control the boost when it reaches our desired maximum. There is the obvious Pop off valve method but this is a little crude and would just dump the boost to atmosphere, I know there are some kind of recirculation valves you can buy but its very difficult to find someone who has enough knowledge of superchargers to assist us.  

While contemplating the future of the feisty but somewhat mawled field mouse that i recently ejected from my bedroom much to the relief of my wife, I suddenly thought that a wastegate normally used in a large turbo conversion, may do the job.  If it were to be plumbed into the charge pipe with the dump pipe fed back into the front of the supercharger inlet, it would open at its usual preset boost level using the usual vacume pipe from the plenum to the underside of the diaphram and vent the boost thus keeping it at a constant level.  Thats the theory, but not only does this give us some way of stopping the car running too much boost, with a conventional boost controller, we could, within the perameters of the S/C, raise and lower the boost.

Does anybody have any ideas if this would work?

[antnicuk]

this is the engine bay as it looks now but will change soon as when we get the new unit we will fit a charge cooler instead of the front FMIC.

[antnicuk]

the car FYI

[kev-yorks]

Should work i would think  not sure if your going to get a steady boost or if the pressure would fluctuate though. Another option  would be a butterfly valve like your normal throttle control plumbed into the pipework someware.To restrict flow the advantages there are its going to be a lot smaller you can control either elecrically or mechanicly and you can physicly put a stop on the linkage as a failsafe boos control measure.
Kevin

[chrispy]

no help whatsoever but it looks bloody nice

[BlitzBoy]

Kev\'s Idea of the butterfly valve is a good one coupled with a pop off valve as a safety net if it boost spikes to an unsafe level.

On the old z31 300zx single turbo I used to own boost control was just done with a pop off valve, If wanted to increase boost levels I would just change the spring in the valve for a harder one.

[rx loony]

i have a better idea dump the supercharger and get turbos on there :cool

[antnicuk]

why............
supercharger doesnt block up the exhaust so keeps it nice and freeflowing which a rotary likes,  the type we have gives a similar boost pattern to a turbo except you can set it up to come in nice and early.  You need a huge turbo for a 20 b, but then you get lag or if not laggy its too small at the top end.  Hopefully the supercharger will resolve both these issues.  It will take a little power to turn it but the 20B isnt short of a few FTLBs of torque.  

We will see how it goes but first impressions are good. Its managed 400 bhp at the fly at 8psi on a stock tired engine.

One other thing to remember, which a lot of you big single turbo boys know about, when you pop your engine,  you quite often take the turbo with it.......

[rx loony]

like u said a supercharger uses power to create power,the constant feul consumtion and its far easyer to get more power out of a turbo charger,fair play there are ups and downs on both sides but the turbo wins in my book.My mate supercharged his saxo vts and had no end of problem,so he changed to a turbo and has never looked back plus he has more power

[kev-yorks]

Put a GT42 on it boosts from 3000 no lag and loads of power.
Kevin

[bobster]

I have 2 friends both running roots chargers their MX5\'s and the only way to regulate the boost is with the pulley selection...one thing you do have to do with them is move the throttle body to the inlet side of the charger and this is what actually regulates the air through the charger.

The theory behind running an external wastegate as a means of dumping the excess boost back into the inlet track is actually very good. I don\'t know how it would affect the boost pattern in terms of stability... it could get very spikey. Having said that external wastegates seem to far more acurate for controliing boost in bigger snails so I don\'t see why not.....having just said that, it goes to show how efficient they are at dumping excess exhaust gas.... I\'m confused now :evillaugh

Just my thoughts