SU's on 4.6 needles

[ChrisJC]

I'm sure I've seen the answer on the forum before, but I can't find it.

So, what are the recommended needles for a 4.6 with twin HIF44 SU's? The 3.5 needles lean out at the top end.

Chris.

[kiwicar]

try this it cover it, need to sort the springs first .
http://www.v-8.org.uk/forum/viewtopic.php?t=1875

Mike

[ChrisJC]

I did find that, but your notes sounded like quite some effort on my part to sort it out! I am sure I've seen somebody just say 'you need B??' needles and all will be right.

But it sounds like I need stiffer springs too.

I do have a wideband on-board though, so I can at least see if it's somewhere near......

Chris.

[kiwicar]

Hi with the wideband most of your work is straight forward drive it and log it, but you need to sort the springs first or you will never get it right as the needle will never be in the right place to do it's work. On the good side I think there are only about 5 springs to choose from for the 1.75 SU start with the stiffest as the 1.75 su was never used with anything as big as the 4.6. If you can't get the SU's to work for the full rev range on the 4.6 even with the stiffest springs (i.e they hit the top of their piston travel 3/4 of the way up the rev range) you can try radiusing the front of the piston or radius the ventury step (but only a very very small amount), or a very short stack that is basically a radius on the face of the carb this causes the piston to rise less for a given flow through the carb. Long inlet trumpets tend to make the piston rise more quickly.
Personally I would convert to 2"SU's off a 4.2 jag, they are intended for this size engine.
There used to be alot of information on tuning SU's for BL A and B series engines, I wouldn't flow the carb without reading up on it first, alot of work has been done tuning 1.25 and 1.5" SU's to work as a single carb on 998 and 1275 cc minis, this has alot in common with what you are trying to do with the 1.75s on a 4.6 .
Good luck.
Mike

[RoverP6B]

In my case I am using twin SU HIF6 carbs, yellow graded springs which are 8oz, and BBW needles on the 4.6.

The midrange section of the needles, approx measuring point 4 through to 10 were further carved to suit the fueling requirments.

Of course, all this was done to suit the specifications of my engine.