needle profiling
Turbo Mini Forums

I've been thinking at the last day of the year...

Needle profiling is very hard. I thing most people agree. The hardest thing is to know wich part of the needle you have to polish. There are a few guidelines, but I want to know exactly wich part to polish.

If I would know the difference between the pressure in the dashpot and the pressure below, I should be able to calculate where the piston is at a certain difference. The piston is rising because of this pressure difference.
If I take a few measuring points as described in the book "how to build and power tune SU carberettors" by Des Hammill (and explaint in another post, use search) and record the pressure difference with that points I think I should be able to draw a straight line through those point. Now I can go for a drive and look at the lambda meter and the pressure meter together to find out wich part of the needle I must profile.

Does this make any sense? I think it's just basic science, but correct me if I'm wrong.

Regards,
Remko

Here is a guide done by a member

http://www.turbominis.co.uk/forums/index.php?p=vt&tid=159683

Sounds OK but how to record the pressure difference? under boost that may be a problem. maybe the blocked pipe that comes from the float chamber will give the pressure below but the pressure above? modify a dashpot piston with a takeoff?

Then estimating the piston lift? are we sure its linear? depends on the spring I guess.

I found that if you are not afraid of ruining a few needles the positions become apparent after a couple of tries, just take a guess and see what happens, you learn!!

Some people have been playing with installing a lightweight linear pot to measure the piston lift but on the turbo, the cap needs to be sealed..still would be worth looking into. I think I'll go do that!

website:- http://www.terryhunt.co.uk

The problem is that you are dealing with very low pressure differentials. Less than 0.2 psi.

The piston rises because of the lower pressure caused by the air flow over the bridge, rather than manifold vacuum and this is just below atmsopheric pressure.

The piston rises against the spring which is rated in just a few ounces. If you then calculate the piston area, you will see what sort of pressure you are dealing with.

Saul Bellow - "A great deal of intelligence can be invested in ignorance when the need for illusion is deep."
Stephen Hawking - "The greatest enemy of knowledge is not ignorance, it is the illusion of knowledge."

Whilst I just drove down fedex to get a part I had an Idea..


get an old damper piston and drill a hole in it. insert an alloy wire (light as possible) and fashon a pointer. get a clear plastic test tube kinda thing and glue it upside down on the damper it so it covers the lot and maintains the seal. As the piston rises it pushes the wire up and down and you can see the position of the piston..

Maybe something like a washer added to the piston shaft will be needed to give the wire a chance to rest on the piston..

I think I'll try it....

Edited by Turbo Tel on 31st Dec, 2007.

website:- http://www.terryhunt.co.uk

The pressure above is the easy part. Take an old carb, pull out the little pipe for the vacuum hose. Drill a 3mm hole in the top of the dashpot and put the little pipe in it. The pipe is 3,2mm, so it won't come out.

Estimating is not what I intend to do. See Colas for the guide already on this forum. As you can see in that topic you should be able to see the first 8 or 9 point on the needle by this guide. Compare this to your pressure difference (dashpot and vaccuum pipe) at this points and you will be able to draw a line. When you use 3 or 4 points, you should be able to tell if it's lineair or exponential.

I'm awair of the little pressure difference. That is not a real problem I think. Just need to have accurate measuring system.
@turbo tel: this has noting to do with boost, the pressure DIFFERENCE will stay the same. The pressure in the dashpot will rise too.

Many factors may influence, eg leakage past the piston. may be minimal but even so... But so would adding the weight of a piece of wire as a pointer! but either way it would not be far off.. and better than nowt.

Which vaccum take off are you referring to? The pressure above the dashpot piston comes from inside the throat of the carb via the 2 holes in the piston Your Idea of drilling into the dashpot would be fine.
The pressure below the dashpot piston comes from inside the plenum via the 2 top holes through the flange (Item 5 in this pic http://www.zparts.com/zptech/articles/mal_...arb_111601b.htm ) the single lower hole through the flange feeds the float chamber and would probably be a good place to look at that pressure (item 4 http://www.turbominis.co.uk/forums/index.php?p=vt&tid=6298 )

I'm thinking of trying that wire pointer thing, maybe even attaching it to a linear pot or even better an optical position encoder, you could plot resistance against position, even go the whole hog and display the station number with LED's or a display..

Keep us posted!!

Edited by Turbo Tel on 1st Jan, 2008.

website:- http://www.terryhunt.co.uk