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Old 10-22-2012, 03:27 PM
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You were told wrong. It will backfire when cold when set like that.

You need fixed timing that does not move with rpm. 36deg BTDC. Then take timing out as boost comes in.
"boost retard". back to 26deg (take 10 deg timing out for 10psi boost@ WOT) (26 to 32 depending on boost and fuel octane)

For your initial dyno testing all you need to do is used fixed locked timing. real simple No computer....Start at 26deg bTDC
and work up Just like I said. Real simple. Get someone to show you how to set the timing at 36deg BTDC
for start up. then when all warmed up and idle mix is set up, reduce the Fixed locked timing to 26degBTDC and start testing. Its just that simple. Roots Blower motors do not run like a N/A motor or a turbo motor.
Or a blownmotor that has direct port EFI. wher fuel is injected blow the blower.

Do it you way and you will find out for yourself. Stick you head near the carb and wack the throttle open.
with only 10deg of timing at idle ha ha ha. Some people have to learn the hard way.

I gave you all the info you need to get it dialed in with the carbs ( the best you are going to achieve with those carbs that are not intended for a roots blower motor.)

Your way will backfire and foul plugs. risk blower rotor damage from blower back fire.
Consider your self warned. pump throttle 1 or 2 times start up with throttle closed, when it startt bring to 2000-2500 rpm and hold while it warms up 36dg BTDC. do not wing the throttle when cold. If it stalls restart with throttle closed.
warm it up. When its warm set idle mix for best idle ( manifold vacuum at idle) 1000 rpm ish at idle 36deg BTDC fixed timing)

Now you will see how nice a blower motor can idle. YA IT NEEDS 36deg BTDC at idle.

reduce the timing to 26deg to test @WOTunder boost. Get the main jetting right then add a bit of timing,while looking for pinging/detonation. (, over heated peppered plugs, huffing valve cover breathers) Too much timing. too lean, lack of octane for boost used.

higher fule octane allows more spark advance @ WOT under boost. AFR under boost should be 10.5:1 to 12:1
12.5:1 is too lean.

14.7:1 at part throttle cruise ( what a 02 sensor/computer ECU will try and trim the AFR to) is too lean for a Roots blower motor. Thats why you ECU contoled roots blower motor will not work like you think.

You must cruise at something richer than 14.7:1 or one of the cylinder will burn up ( too lean)
idle must be what ever AFR the motor needs to idle correcty (12:1+/-)
Not all the cylinder get the same fuel. when fuel is intoducted above the blower on that motor.
correct blower Carbs will work way better than EFI...... save your self the $$$$$ and pain. and disapointment.

When your carbs are all dialed in for best power and you see what the motor needs for timing under boost,,,,then you can map a boost retard curve 's of retard/# of boost (psi) using a simple boost retard box as sold by MSD and holley.

14PSI of boost on pump gas will require a tune that is dead nuts on the money or you will toast that motor.
do not use junk fuel. do not get greedy with the WOT timing under boost @14psi and pump gas.

At part throttle cruise (when the blower is not making boost (vacuum in the intake manifold) and the engien load is light
the motor wil want more than 36deg timing. +10. When manifold vacuum is 0 ZERO ( before boost starts building) timing will want to be 36deg BTDC. Timing mut reduce as boost builts. stating around 4 ---5psi.
both map sensors (manifold pressure and manifold vacuum) read off the intake manifold below the blower.

Again a roots blower motor need a lot of idle timing at idle that does not drop off at low idle rpm
therfore the fixed 36deg BTDC basic timing. Your incorrect curve will cause poor motor warm up, stalling, blower back fire and FOULED plugs.
and generall poor drivability.

Last edited by F-BIRD'88; 10-22-2012 at 03:55 PM.
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