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Supercharger boost control ?
The Knobs - 22/1/10 at 12:16 AM

How can you limit maximum boost on a Rotrex ? I dont want to change the pulley size but want to raise my rev limit and try and limit boost to a set level. Is it best to fit an inlet restrictor or some sort of manual pressure/bleed valve of some sort ?

matt_gsxr - 22/1/10 at 12:38 AM

Wouldn't it be better to have a restrictive inlet if you want to do this.

I think that would waste less power and generally be gentler to the compressor.

Matt

T66 - 22/1/10 at 01:16 AM

Restricting the inlet ? I would of thought that would be harsher on the compressor rather than easier...

Reverse flow of the gas into the compressor is not good for vane life.

What a compressor produces is down to how much gas it can move up to a limit rpm.

Spin it faster, it moves more gas, so I assume the rotrex will be the same.

If you delve further increasing your rpm, I suspect you will have to establish whether the rotrex is capable of moving enough gas for that rpm you go for.

If it isnt then a larger one will be needed.

My C84 top end is 12600rpm , I think the C94 is 10500rpm.


Have a read here......

http://azbiketech.yuku.com/directory


Interested to see if you can reduce the boost on the Rotrex

flak monkey - 22/1/10 at 07:58 AM

You could fit a bleed valve to do the job. Burton power sell them, amongst others.

Or adjust your fuelling and timing to take the extra boost. I wouldnt expect to see much more boost just raising the rev limit.

BenB - 22/1/10 at 08:26 AM

Fitting a restrictive inlet IMHO would be a bad idea as it would effect flow more than boost. A BOV would limit the upper limits of boost.

Though it does always seem a shame to be spinning the charger faster than needed (IE having higher parasitic loses) then dumping the un-needed boost.

If you wanted boost at lower RPM as well as at high RPM a Rootes blower is the way to go (as long as you like big intercoolers- they're flipping inefficient).

bimbleuk - 22/1/10 at 08:27 AM

Yes you can use an inlet restrictor to limit the maximum airflow. If you see some of the OMEX kits they now use inlet restrictors to both limit air flow and help to increase mid-range torque delivery.

Previously they told me Rotrex didn't condone the use of them claiming it caused premature bearing failure but I believe they don't void the warranty anymore.

It does reduce the overall efficiency of the Rotrex and causes extra work and heat. However it can significantly increase the driveablilty as a benefit.

[Edited on 22-1-10 by bimbleuk]

boggle - 22/1/10 at 09:15 AM

an apexi electronic boost controller?

Brommers - 22/1/10 at 11:15 AM

quote:
Originally posted by boggle
an apexi electronic boost controller?


A boost controller controls the wastegate solenoid which alters the air pressure being fed to the wastegate, thereby allowing the wastegate to be controlled to limit boost.

A supercharger doesn't have a wastegate...

boggle - 22/1/10 at 01:09 PM

and that is an interesting point...

never played with super chargers. only turbos....

MikeRJ - 22/1/10 at 03:26 PM

quote:
Originally posted by BenB
Fitting a restrictive inlet IMHO would be a bad idea as it would effect flow more than boost.


And "boost" is created by having more flow than the engine capacity, so....

BenB - 22/1/10 at 03:37 PM

quote:
Originally posted by MikeRJ
quote:
Originally posted by BenB
Fitting a restrictive inlet IMHO would be a bad idea as it would effect flow more than boost.


And "boost" is created by having more flow than the engine capacity, so....


I know, but a restrictive inlet would effect flow throughout the RPM range whereas you really want to have an unrestricted inlet and controlled boost (seeing as IIRc the plan was to have good boost at lower RPM and limit upper boost). A restrictive inlet will be restricting at low RPM too!

It's a bit like rather than fitting a wastegate to a turbo'd car just making the exhaust out of 1/2" tubing to prevent the turbo spooling up.

But hey ho. Each to their own.

bimbleuk - 22/1/10 at 04:00 PM

quote:
I know, but a restrictive inlet would effect flow throughout the RPM range whereas you really want to have an unrestricted inlet and controlled boost (seeing as IIRc the plan was to have good boost at lower RPM and limit upper boost). A restrictive inlet will be restricting at low RPM too!


Nor strictly true when using a restrictor. It will only restrict once the maximum flow for the orificie size is reached. On the Rotrex you have 50-60mm inlet diameters but in reality when a restrictor is fitted they are usually about 35mm in diameter. On the Honda kits that OMEX do the restrictor of that size will flow up to 280BHP before the power graph flattens off.

[Edited on 22-1-10 by bimbleuk]

MikeRJ - 22/1/10 at 04:56 PM

quote:
Originally posted by BenB

It's a bit like rather than fitting a wastegate to a turbo'd car just making the exhaust out of 1/2" tubing to prevent the turbo spooling up.

But hey ho. Each to their own.


Thats exactly what some early turbo installations used. Not ideal, but inexpensive/reliable wastegates weren't easily available at the time.

A restriction by it's very nature will have more effect at high flow rates (high RPM) than low flow. I'm not suggesting that this is the best solution, just that it is probably a workable one.

I don't particularly like the idea of using any form of boost control on a supercharger, since it will be absorbing more power if it's being spun more quickly than is needed anyway. The correct sized pulley would seem to be the best overall solution.

[Edited on 22/1/10 by MikeRJ]

The Knobs - 22/1/10 at 08:42 PM

I will get it on the rollers when its together and see how it copes. Bottom end should be able to cope as it has now got steel rods and some l/c forged pistons.

Antnicuk - 23/1/10 at 10:11 PM

I looked into this, the best way is to fit a external wastegate into the boost pipe and then connect it up to an electronic boost controller, you then then have variable boost on a supercharger.

very easy to and has been tried in the states and works. We did it several years ago on a supercharged 3 rotor rx7

[Edited on 23-1-10 by Antnicuk]

darren(SA) - 27/1/10 at 08:19 AM

quote:
Originally posted by Antnicuk
I looked into this, the best way is to fit a external wastegate into the boost pipe and then connect it up to an electronic boost controller, you then then have variable boost on a supercharger.

very easy to and has been tried in the states and works. We did it several years ago on a supercharged 3 rotor rx7

[Edited on 23-1-10 by Antnicuk]


+1, use an electronic boost controller as it will hold the wastegate shut preventing any boost leak up until the required boost level.

But with a external wastegate will it not loose boost too quick being on the inlet side? so will it not surge a bit while trying to control the boost?

[Edited on 27/1/10 by darren(SA)]

boggle - 27/1/10 at 01:31 PM

external wastegate is on the outlet side...

Antnicuk - 27/1/10 at 06:15 PM

i think he means the boost inlet side, external wastegates are meant to go between the engine and the turbo on a turbo application but if used on the inlet side work in the same way. They dont open too quickly, as long as you dont get one too big for your application, i would 35- 40 mm would be plenty, they fluctuate as the boost increases so will bleed boost off slowly. They do this with exhaust gasses also, if they are too big they will dump the gasses too quickly and drop boost but if sized correctly they will open enough to control boost but not too much that they cause a drop.

[Edited on 27-1-10 by Antnicuk]