tjl IROC

I have a stock 350 with a 142 blower and matched 700 holley carb. is there any good manuals out there for tuning blower carbs?

and second has anyone ever used an air/fuel ratio gauge from JC whitney to help tune their carb? part number JW815401 and JW815403?

xpndbl3

iTrader: (5)

the air/fuel guage won't help you, get it dyno or wideband tuned. or take it to the track and jet for best MPH and watch your plugs so you don't go lean

Damon

A blower carb on a roots blower operates almost exactly the same as any other carb with one main difference- the power valve. On a N/A combo the power valve opens as manifold vacuum drops down below a certain threshold- typically around 5-6" of vacuum. On a blower motor, however, what the engine is seeing is that there is already some small amount of boost beginning to form in the intake while there is still 5-6" of vacuum present under the carb (giving you my own personal observations from my mild 142-blown 383 engine).

Obviously, you don't want to be running a lean cruise A/F ratio when the motor just begins to jump into boost. That could lead to engine damage from detonation.

So a blower carb has a separate external line to read vacuum/boost directly from the intake below the blower. That way the power valve does it's job and opens before you start to build boost in the intake manifold- assuring you are never running a lean cruise A/F ratio while under any amount of boost.

On mild 142 roots blower motors with small cams I've found that a boost-referenced carb is not strictly necessary. You can simply install a power valve with a higher vacuum opening point and achieve largely the same results. I run a QJet, which is a little different than a Holley, but my step-up spring is the rough equivalent of an 8" Holley power valve. I know that boost just begins at about 5" vacuum under the carb. And I know that my "power valve" is fully open at about 7-8" of vacuum- hence, no chance that I'll ever be running lean as the motor jumps into boost.

Big blowers and big cams would make this unworkable, but mild cams and small blowers do a good job of holding a very constant relationship between vacuum at the carb and boost in the intake. It's very predicable across almost all RPMs, which is why you can get away with this on these mild combos.

head

iTrader: (2)

Anything from JC Whitney is usualy bad news !!!!!

tjl IROC

Im located in western MD and haven't found anywhere that can dyno so I have to do it myself

Is there any types of sensor that would help?
any manuals?

tjl IROC

anybody

Damon

There are stand-along wideband O2 sensor kits available from various sources. Something that you can monitor your A/F ratio on a little LED screen as you drive/make a WOT run. Requires you install an O2 sensor bung in your exhaust somewhere (header collector). They aren't cheap, but probably worth the money if you're tuning it yourself.