Mixture changes after exhaust manifold change

[gungehead]

Hi,
I am about to fit a new 2 inch stainless steel exhaust system, including tubular manifolds.
Will the standard needles (I think I have the slightly richer, c1 needles at the moment) still give correct air fuel mixture? If not, what ones would you suggest?
It is a fairly standard 3.8 with k & n air filter element and 123 tune dizzy.

Thanks for your help.
Dave

[mike020150]

It's difficult to be definitive about the need to change needles just for an exhaust manifold change on its own. These days, every garage has emissions test equipment so, assuming the car doesn't display obvious signs of flat spots after changing the manifold, why not ask a friendly garage for a quick emissions test or find someone with a domestic tester, like the Gunson? Better still, have before and after tests done and you will be able to see some impartial numbers relevant to the manifold change. Mike.

[gungehead]

Hi Mike,
Thanks for you response.

I have now fitted the exhaust and adjusted the mixture at idle, but that just sets the mixture at idle. When driving, the needle meters fuel at various rates depending on the profile of the needle, the vacuum, throttle position, spring rate and dashpot oil viscosity (I am sure I have missed a few factors).

My enquiry was really - has anyone done this mod and found that a needle change has improved performance?

Thanks

Dave