Hello i am putting a B&M 144 blower on my stock 8,5:1 350, planning on running about 6psi boost. I want to use my holley 750vac nr 3310-11 i will boost reference the powervalves but do i need to change the jetting ? (carb is stock) if thats the case what to do with the secondary side since it has a metering plate ? I am going to use a hei distributor starting with 28 total timing, should i use the vacuum advance ?

I run a QJet on top of mine, and there is no way to boost-referece the power piston in a QJet. Works just fine. In mild blower applications with a mild cam that pulls plenty of vacuum at idle and light throttle you don't absolutely need to boost reference it.

Vacuum advance... definitely for a street driven combo. Helps considerably with part throttle performance and economy. I used to run mine straight off the carb (which worked fine) but this summer switched to running it off the vacuum port on the lower intake manifold. I was initially concerned about whether the vacuum canister would tolerate the strain when the manifold went into boost, but that doesn't seem to be a problem. It's held up just fine so far.

I'd start your tuning with the stock jetting. It will probably be plenty close. If you're the nervous type, jet up 4 to start with (that's about where I ended up vs. when I used to run my carb on a previous N/A combination). On the secondary side of your 3310 you can either buy different metering plates or convert it to a secondary metering block with the usual replacable jets (Holley sells a kit to do this). I have a X-reference list of metering plates and their equivalent jet numbers if you want to email me for it.

Thanks for your answer, what do you think about 28 total timing ? when using the vacuum advance should i be worried about it will raise the ignition to mutch that i get detonation on crusing. I will start with the stock jetting, din´t use this carb on the engine before the blower , i think it will be fine,

28* should be a good starting point. That's how much I run on my 383 blower combo (8* initial with 20* centrifugal all in by about 3000 = 28* total).

I run 16* of vacuum advance on top of that. Vacuum advance goes away when you get near WOT and the the engine won't jump into boost until you're near WOT anyway (it's one of the reasons why I switched the vacuum line to the intake manifold so there is NO possibility of even the slightest vacuum advance creeping in under boost). Without boost present at idle/part throttle you're basically running a low compression engine, which can tolerate quite a bit of advance.

Ok then i will run my vacuum advance from the manifold, just have to check how mutch my centrifugal an vacuum advance adds. When i have tuned the primary of the carb is there a gudeline like if i have 74 jets in the primary i should have 78 in the scecondary ?

No, not really. Obviously, secondary jetting only comes into play when you are near WOT, especially on a vacuum secondary carb that doesn't open the secondaries until there is sufficient airflow through the primaries. And the overall calibration is just different between the two sides of the carb, even if they use the same size jet.

The (stock #21) secondary metering plate in a 3310 is about equivalent to a #75 jet, FYI. And the number stamped on the metering plate has NOTHING to do with it's equivalent jet size- smaller number's aren't leaner, bigger numbers aren't richer. How they line up with their equivalent jet size is all over the map. For example, go up to a #22 plate and you're actually going down 10 jet sizes to a #65 jet size equivalent.

Not sure how well this will display on this BBS, but here's the cross-reference between plate number and equivalent jet size. First number is the plate, second number is the jet size equivalent.

3/56
4/59
5/63
6/69
7/53
8/64
9/64
10/65
11/72
12/73
13/63
14/85
15/82
16/64
17/69
18/63
19/69
20/69
21/75
22/65
23/64
24/72
25/77
26/79
27/79
28/65
29/75
30/64
31/74
32/59
33/64
34/53
35/69
36/64
37/69
38/71
39/69
40/59
41/69
42/94
43/71
44/74
45/79
46/75
47/78
50/79
53/65
54/82
55/78

Ok i checked my carb it had 70 jets in the front an a number 6 plate = 69 in the back, fells a little small, what do you guys think ?

IMO, I think you're better off setting up on the carb a bit on the rich side, conservative on total advance and get it on a dyno to set max timing and A/F ratio set so it's safe to cruise around. Detonation can kill a boosted car pretty quickly. Too lean and too much advance....

Sorry it had a nr 21 plate = 75 in the back, but i put a metering block in there with #83 jets instead, and i think i will go with 74 or 75 in the primary