Lowering Boost Enrichment Threshold - SC Conversion

[sigchuck]

Hi guys, I've posted in the past about my hopes to turbocharge my stock 83 911SC. I'm pretty close to pulling the trigger on dropping the engine and bolting up factory 930 intake/fuel system and exhaust.

I'm planning to limit boost to .5 bar, and my understanding is that is about the point the boost enrichment of the stock 930 WUR kicks in. I'm in the process of having the CIS components gone through and am wondering if it's worthwhile to lower the threshold.

Using a couple online boost compression ratio calculators, it looks like a factory '78 930 would begin enriching around a boosted 10.3:1 CR. Since my engine is 9.3:1 static CR, I'd need the threshold set much lower to achieve enrichment at the same boosted CR as the factory 930 (and lower in general to achieve any enrichment whatsoever).

To get in the same boosted CR neighborhood as the 930 when enrichment begins, I'd need a .3 bar threshold. I'm a little concerned this will result in an extremely rich running engine under boost though.

Any thoughts or real world experience in this regard? Any obvious flaws in my logic around trying to trigger enrichment at the same boosted CR as the 930? Thanks for your help!

[RarlyL8]

Verify you need this with an AFR meter before doing anything. Typically low boost doesn't need much enrichment and OE 930 is very rich at onset.

[Alan L]

As brian says - verify what is going on first.
The 930 dumps a lot on boost - enough to cope with up to 1 bar boost (on a low CR).
The Leask WUR allows you to regulate this - the point it cuts in, and how much it dumps. So you should be able to control it enough. More a matter of finding out what you actually need.
AFR meter essential piece of kit. Best $s you will spend.
Yes the 930 begins a dump around 0.5 bar. You could change this (lower threshold) by changing the spring (softer) that sets this.
Alan

[sigchuck]

Thanks, guys - I definitely plan to have a permanent AFR gauge, so will see what happens under boost with a stock threshold.