Supercharger boost

texpis · Post by **texpis** » Sat Feb 28, 2009 4:50 pm

Hi
I have just installed a supercharger as pic below. I dont know what make it is I think it is a kenny bell twin screw. It was off a 3.5 engine I have installed it on a 4.2. I thought I would get some boost but I dont seem to be getting any, when revving the engine it still has vac about -0.2 bar. I am guessing the supercharger is not big enough what do you think.

al mackenzie · Post by **al mackenzie** » Sat Feb 28, 2009 6:12 pm

I can't make out from the low resolution photo where the blower breathes in or how it feeds the compressed air to the SU carbs. Can you post any better quality pictures? How did you mount the supercharger? What motor is it from?

topcatcustom · Post by **topcatcustom** » Sat Feb 28, 2009 6:47 pm

You will only get full boost when the engine is under load i.e. accelerating hard or pulling up a hill.

TC

texpis · Post by **texpis** » Sun Mar 01, 2009 9:06 am

Hi all
The supercharger is the same size physical as an Autorota SR4 420 or 422 with 2.4 to 1 overdrive puley, can anybody work out if these would be big enough? It was taken off a 3.5 rover v8 and it came with a manifold and is mounted on top with suck through SUs

Mick

ps I have uped the photo quality of the first post

Post by **Rossco** » Sun Mar 01, 2009 12:04 pm

Ok

Yes in theory it's big enough.

The thing that would worry me is the 2.4:2 overdrive. That's a very high overdrive and means that you'd have to set a 5200rpm engine limit.

As to why you're not showing any boost is a mystery.

Unlike turbo's superchargers do not need engine load to produce boost. They produce boost right from the word go. The reason is that a supercharger produces a fixed output of air per rpm, in this case around 2.2 litres per rpm. That figure is as I've said fixed from idle all the way to red line, OK it's not a perfectly linear graph but it's close. So you should see boost on the gauge even at low rpm.

CastleMGBV8 · Post by **CastleMGBV8** » Sun Mar 01, 2009 12:13 pm

Mick,

Just a thought, does your installation have any form of blow off valve that could be stuck partially open or not seating correctly?

If this is the case with a suck through system you could be venting explosive gas into the engine bay, worth checking.

kevin.

stevieturbo · Post by **stevieturbo** » Sun Mar 01, 2009 12:34 pm

Rossco wrote:Ok

Yes in theory it's big enough.

The thing that would worry me is the 2.4:2 overdrive. That's a very high overdrive and means that you'd have to set a 5200rpm engine limit.

As to why you're not showing any boost is a mystery.

Unlike turbo's superchargers do not need engine load to produce boost. They produce boost right from the word go. The reason is that a supercharger produces a fixed output of air per rpm, in this case around 2.2 litres per rpm. That figure is as I've said fixed from idle all the way to red line, OK it's not a perfectly linear graph but it's close. So you should see boost on the gauge even at low rpm.

I disagree.

If measuring boost in the intake manifold ( ie, between TB and engine ) then at low loads you will always see vacuum.

The only way I could see that you could get boost in there with no load, is to go WOT, but that would carry its own dangers/risks.

if you maintain part throttle, to hold a fixed rpm...the engine will still be in vacuum, just as it would when crusing at a fixed rpm.

Eliot · Post by **Eliot** » Sun Mar 01, 2009 3:56 pm

I'm inclined to agree with stevie, but there is a high probability of a plumbing issue. Check vacuum take-of for the brake servo and breather pipes to rocker cover's etc.

Here's how I pressure tested my system:
http://www.mez.co.uk/dcam/DSCF0927.JPG

and had leaks around the carb:
http://www.mez.co.uk/dcam/DSCF0925.JPG

Although its worth noting that I once left the Air Temperature sensor out of the plenum (20mm hole) and still made good boost - it just made a funny noise which alerted me to the leak.

topcatcustom · Post by **topcatcustom** » Sun Mar 01, 2009 4:02 pm

Elliot, what air temp sensor do you have and is it hooked up to a gauge? I am having trouble finding one!

Blown v8 · Post by **Blown v8** » Sun Mar 01, 2009 4:02 pm

Unless you put it on rollers or take it for a drive, you wont see any boost

Eliot · Post by **Eliot** » Sun Mar 01, 2009 4:44 pm

topcatproduction wrote:Elliot, what air temp sensor do you have and is it hooked up to a gauge? I am having trouble finding one!

Standard GM air temp sensor, as I'm running megasquirt.
you can see it in the 2nd photo above.

texpis · Post by **texpis** » Sun Mar 01, 2009 6:44 pm

Thanks for the replies guys The car needs the bonnet modifying to be able to drive it didnt want to do it if the blower was getting ditched. If the general opinion is it is big enough and it will work I will chop the bonnet and give it a spin up the road. Castlemg I dont believe there is a blow off valve fitted. I have a compressor so I will pressure test, what pressure do I use 1Bar?

Mick

Eliot · Post by **Eliot** » Mon Mar 02, 2009 7:08 am

yes 1 bar.
it wont hold pressure like a tank, but if there is a significant leak you will hear it.

texpis · Post by **texpis** » Tue Apr 07, 2009 8:52 pm

Hi Still have an issue with this I daren’t boot it to hard I have just changed the rings, mains and big ends. But I have given it a bit of boot and the most I get is .1 of a bar. The carbs I have on are 1.75 SU could they be too small and causing a restriction? If it is causing a restriction how do I tell? There must be a formulae to work out if enough flow will be allowed though the carbs?

Mick

stevieturbo · Post by **stevieturbo** » Tue Apr 07, 2009 9:23 pm

It is highly unlikely the carbs are posing that much of a restriction.