Simple boost reference mod straight from Holley

[502 JET]

502Jet:
I'm following this thread and trying to learn here. Isn't vacuum, vacuum? How can it be false? If it has a value, then can't it be used like any other vacuum?
Isn't this what Warp Factor is referring to??
In certain situations you will not get a power valve numerically small enough to open because the blower is pulling vaccume (thats why I say false vaccume) greater than the pv rating holding it shut.When in the intake is under boost/positive pressure and it would open the pv if the pv was manifold referenced.
There is no rite way or you must do this way or that way.You tune an engine to suit your needs and how you want it to operate or you pay someone to do it for you and you run it how it was set up.Manifold referencing is just another way of tuning an engine.You can also enlarge the two passages under the pv to get more fuel (can run a smaller jet) so your part throttle is running clean but you get the added fuel when the pv opens.

[Warp Factor]

Have you ever loaded up the plugs with this set up? If so, how did it happen?
Yes, I had to replace 2 plugs that weren't firing (over the last 6 years, aside from normal plug changes). Both times, I had been running in rough water, causing fuel to spill from the float bowl vents into the carb throat.
There is no rite way or you must do this way or that way.You tune an engine to suit your needs and how you want it to operate or you pay someone to do it for you and you run it how it was set up.Manifold referencing is just another way of tuning an engine.
I agree.
If your current setup works OK, why mess with it?
If it's not, it's easy to hook up a vacuum gage to the manifold and see what the vacuum is under various conditions, such as idle and cruise. Check the same thing at the carb base.
If you don't know the rating of your power valves, remove them and look, it should be stamped on the part. A 3.5, for instance, means the power valve opens when vacuum drops below 3.5.
Let's say you show 5" vacuum at the manifold at a slow cruise, and your power valve is a 3.5. The power valve wil be closed, and you should be OK running boost referenced.
Let's say the manifold vacuum is 2" at cruise, and 3" at idle. The power valve will be open under these conditions, and may give problems with plug fouling.
If you reference from the base of the carb, be sure your vacuum drops far enough at high boost to open the power valves.
This is still guesswork though. The real way to know what your A/F is doing is to set it up using a wide-band A/F meter. This can only be done if you're using unleaded gas, and is difficult to do with water injected exhaust.
Second best is probably setting it up on the dyno, where you can actually measure air and fuel going into the motor, but this won't give you load conditions under actual operating conditions when installed in the boat.
I've gotten a little spoiled by modern fuel injection systems on cars. They'll read the A/F ratio, and automatically adjust. Or you can type the amount of fuel you want into a spread sheet, and that's what it will do. Timing can be handled the same way, using the spread sheet or knock sensors. These things can be varied according to coolant temperature and manifold air temperature.
At some point, I'll probably ask "Unchained" to help me set something like this up on my boat, since he's the person I know with the most experience with this, and is using something like this on his twin turbo boat. Prices on components have come down to the point where it wouldn't cost much more than a couple of good carbs. Cold starting, driveability and fuel economy are much better than can be achieved with carbs, not to mention emissions, if that should matter to you. Electronic controls will usually allow more power too.
Maybe he'd do a thread on this if there was sufficient interest.
:rollside:

[502 JET]

As I was typing my last post I was thinking F- it: GO EFI as Warp as mentioned.
If you buy two carbs and pay someone to modify them your pretty close to the efi system.I went to a local well known carb shop to get gaskets and asked them what it would cost for them to do my carbs.They charge $550. for each carb.They look real pretty and in most cases work great but for that much money I would invest in the efi system before I paid them $1100. for carb work.Tuning with an efi must be nice you dont have to get any gas on you like changing jets.