ProCharger Guys, quick question...
 

What does your boost gauge read at 3500 rpm cruise?
Boost or Vacuum?
This is assuming your gauge is in the manifold.
Thanks

I can play with the throttle right around this rpm range and get 0 to 3 psi. :p

In the manifold pressure is about 0, in box before carbs is about 4psi. I think I have to be about 4k before motor starts to be in boost. This last summer I ran it up & recorded all the reading but by the time I got home I had lost the sheet we recorded the info on.

at 3500 rpm cruise, I am reading 2-3 inches Vacuum. I am at "0" around 4000rpm cruise

At 4000 RPM about zero boost. Other variables probably come in to play though. Like what size pulley, which determines turbo charger RPM, and therefor affects boost or vacuuuuuuum relative to engine RPM. Why do you ask?

AT 3500 I READ -2 TO 0 ON EFI 383 TWINS.
STEVE

I read about 0 at 3000 then it climbs up .

3500 RPM reads 0 from the manifold. I do not get boost till 4000 RPM,

Thanks Guys,
it appears then that even though the blower is making boost at cruise, the intake is not seeing it unless you are hammering the throttle blades wide open.
Reason I asked is I'm working on a timing curve for my F.A.S.T. ECU and am trying to confirm what's happening there. I'm thinking I can run 35 degress at 3500 at easy cruise and no boost in the manifold but when the manifold (MAP sensor) reads boost/heavy load at 3500 I can run 30 degrees.
Just doing a little preliminary studying:D
Later,
Dave

Is your system going to have a knock sensor? If so I would say you could run that much timing. You have to remember that just because your motor is not in boost it is still under heavy load. The way I compare it is just think of the motor as a N/A motor. When it goes to "0" vacuum motor is under heavy load. When I had my turbo setup I had the timing set so it would retard when ever motor would see boost.

Thanks TJ,
If I understand you correctly, I think I'm looking at it the same way. The motor as it is now, N/A, I run 36 degrees in full by 3200 rpm so I'm assuming that if I see the same MAP at 3500 rpm as I do now (I'm sure it will be a little different) I should be able to safely run the same timing.
The ECU will read and adjust timing by what the pressure/vacuum is in the manifold so maybe something like -3 @ 3500 rpm will get 35 degrees, 0 @ 3500 will get 32 degrees and +3 will get 30 degrees.
Hmmmm?
What do you think?
Dave

That makes sense, Im interested in that FAST system also. Does that ecu have the knock sensor? You might want to sneak up on the retard slowley so you cant feel it happening. Greg

The turbo motor I had I had a hobbs pressure switch plumbed to the intake manifold. When the intake saw boost the switch would close thus causing my MSD retart to back the timing off. Motor has so much toque at that point I never felt the timming being backed off.

As far as what you are seeing now with your N/A motor will be different after blower if you are using RPM's as your refrence point. You will need to use your boat speed as refrence point. Remember you will be running a larger prop or gears so the motor is going to be loaded at a lower RPM than before. As engine RPM goes up the motor has less time to detonate. I would look at what your timing is now when motor is not under boost at the higher speeds & then back timing down a little from there to keep motor from detonating. I guess when you posted your question you should have asked not only vac/pressure but also speed at that RPM. If I am not mistaken at 3500 rpms I am running about 65 mph, 4k = 75 mph.

Great observation TJ.....bigger prop = more speed = more engine load at same rpm.
Guess I won't know until I'm sitting there with my laptop monitoring the real world readings (A/F, EGT, MAP, etc....)

HPJunkie, I did not upgrade to the knock sensor. I did upgrade to the wide band O2 sensor however.

Thanks,
Dave