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Old 01-18-2012, 07:55 AM
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mighty demon.BC carb ?

Ok i got a.489 BBC with.8-71 TBS blower got 2x.750 blower carbs also from TBS. Just got motor.fired up i had no vacuum lines hook up to the carbs
these carbs are boost ref power vavles.i found the nipple that pulls vac.through the vavle are these suposed.to be tied together.into the back of the blower manifold?

With no vac lines.hooked.up..shouldnt it run rich.?.which i think it is
would this.make my headders glow.red at.about 2000/2500 rpms?

This is my first blower carbed engine.

And for my trans. Mod valve.run it.to a full vac source off the base plate...did i need.to tee.them together on both carbs to the mod vavle..thanks

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Old 01-18-2012, 04:13 PM
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Power valve vacuum source goes to the intake manifold plenum below the blower.
Preferably with a in line one way check valve to avoid boost pressure acting on the power valve diaphram(s).

Same with the trans vac modulator, to the intake manifold under the blower with a in line check valve. A mechanical (cable acted) trans modulator is another way to work the trans shift pressure. Get one from a GM desiel van w auto trans.

The Rich idle and hot header tubes is mostly caused by a lack of initial spark advance timing at idle. Roots supercharged motors need a lot of timing at idle.

You need to shorten the mechanical advance curve to allow this.
You need a 10DEG or less timing curve.
Fix the timing curve to allow this.

If the blower motor has a fair big cam just lock out the mechanical advance curve and set the locked timing at 28-30 deg BTDC. (street Blown on pump gas, without boost timing retard)

If you are using boost retard, set the locked timing at 36deg BTDC at idle and take 6 to 8deg timing out with the boost retard box, under boost WOT

A roots supercharged motor will idle much cleaner with a generous amount of spark advance at idle. The bigger the camshaft, the more timing it will like at idle. 26deg +++ @ idle

Last edited by F-BIRD'88; 01-18-2012 at 04:21 PM.
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Old 01-18-2012, 04:30 PM
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Thanks.real helpfull

whay about this.eze idle.adjustment ?

I thought like you.said.that.boost pressure.would be pushed back on the.power V.

Or.would it be better to take.the nipple.fittn on the boost ref port .back to the base plate full vac source.would.thay work ?

Ans same.with.trans mod.V. Cause the.carbs.base.plate.or not pressured.by boost

im going to post up so pics.soon from my home pc..not.sure.how.to do them from cell.and can see and type.better.to without all the.peroids
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Old 01-18-2012, 05:55 PM
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ok heres my 68 chevelle i need help on pic 1



pic 2


pic 3


pic 4


pic 5


pic 6
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Old 01-19-2012, 05:42 AM
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No the power valve goes to the intake manifold only.

Never to the base of the carb or any where above the blower.

You do not want vacuum acting on the power valve when the motor is WOT under boost. This would close the power valve @WOT and burn the motor up.

Start with the idle eze closed. Give it all the spark timing it needs at idle.
Make sure both carbs throttles are open correctly at idle, and the same amount.
(idle fuel transfer slot exposure) Flip the carbs over and look and preset them , before install.

The idle eze will only be needed if the cam is huge., if at all.
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Old 01-19-2012, 07:44 AM
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Cam is a comp cam blower grind
255/262 @.050. Lift is .652 on both. On a 114 lash is .020 .
O got.dart.pro1s rect.port 345 cc intake.runners

is.that a big cam.and heads .thanks man u been big help can i call you still dont fully still dont understand pm.me.with.# of.you dont mind thanks Gary
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Old 01-20-2012, 05:58 AM
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ya thats a big cam.
Lock out the distributor curve.

NO boost retard box = 28 to 32deg timing.

with boost retard box 38deg timing, reduce to 28-32 when under boost WOT
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Old 01-21-2012, 12:24 AM
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Okay so my pic #5 the nipple.goes.to the.vacumm andbboost to the.bottom.of the intake.below.the.blower.correct
so when i fired.up my car.wirhout.rhe.ref.nipple.hooked uo the.power valve.so open or.closed.?.. I think.the.boost ref. Carbs.it.dont.get.a.vac.signal.to the.power.valve.so it.would.run rich correct my.plug are.black.
So of.i hookthem up the.nipple.the.the.boost.ref. It will get vac.closing the.power valve.then when it.gets.boost.it.willpush.the.power.valvw.open to get more.fuel correct?
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Old 01-21-2012, 12:32 AM
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In pic.#3.the.big.plig at the rear.of the.cars.am going.to run it to the.vac mod. Valve.correct?

i need.to lock oit the.dost.like.you said ?.is that to much.i thought more.like.22/26.with.pump gas i might.have less boost then planned on .i as.set.up for 12%ud.but.that.was.with.srock iron heads.i got.345.cc.ingake.runners now.TBS.I might.noy see 8psi.bit less pulleys.are.59 /58 upper and.52/54 lower cant rember.but.of i only.see.say 5/6 psi .i will the.swap the.pullehs.and.od.ot.13.5.% i thinllk.

Let me know.if.am.rigth.or not.thanks
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Old 01-21-2012, 08:49 AM
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What is the static compression ratio?
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Old 01-21-2012, 01:09 PM
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Quote:
Originally Posted by cobalt327
What is the static compression ratio?
8.2to 1 compression flat.top srp's blower piston.heads.are.cced.at.122cc .010 in the hole.not.sure on head gasket thickness .042 ? Flat tops have -3cc
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Old 01-22-2012, 06:39 PM
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What should.this.car.run in the quarter mile. Has.3.50 gears with.dlocker 10 stall speed
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Old 01-22-2012, 07:50 PM
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That CR is fine, my concern was that it may have been on the high side for running any real boost. No idea on the times, though. I'd imagine traction will be a problem. What's a "10 stall speed"?
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Old 01-23-2012, 02:13 AM
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The.transmission converter is.a.3800.stall speed.with.3.50 gearrs.with.Detroit locked.for.the.street .hopen for.some.low.10's.at least.got.32x17.50x15 mt et street dot slicks
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Old 01-23-2012, 04:43 AM
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Quote:
Originally Posted by 2fox
The.transmission converter is.a.3800.stall speed.with.3.50 gearrs.with.Detroit locked.for.the.street .hopen for.some.low.10's.at least.got.32x17.50x15 mt et street dot slicks
I may be wrong, but that's an awful lot of stall for a blown BBC, IMHO. But that's really not your question. Best thing I could suggest is get the tune to a safe point and start track testing to dial it in unless you can buy some dyno time.

Quarter mile trap speed and ET along w/the other vehicle specs (mainly weight) is as good an indicator of real world HP as most anything. You have enough motor to run your numbers- the question comes down to chassis set up and traction.
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