So I started a thread a month or so ago about replacing an older 4776-2 600cfm double pumper on my 64. It seems some kind of side argument started in that thread and it got locked. Anyway, so for the record I thought I would document what I bought and how it went.

So I got this :

http://www.summitracing.com/parts/hl...783c/overview/

Bolted it on and have done some setup and driving, here are the comments :

1) Still need to adjust the choke idle bump higher, but cold starting is way easier with the electric choke as expected

2) I probably should have used a dyno run back to back, but it doesn't feel as quick. One thing for sure is the throttle response is not even close to the DP, there is no instant surge of power as before. I guess this is also expected given the differences in the carbs.

3) Both car and driver no longer smell like a gas station after a drive, which was another thing I was trying to achieve with the change over.

4) Need to look at the gas mileage closer, but everything so far points to a win there.

5) Carb adjustment out of the box was minimal, factory settings very close to perfect for idle mix and float level.

So I guess I got what I was expecting pretty much, but I kind of miss the aggressive throttle of the DP even if it cost a quarter every time I hit the throttle. Of course I haven't even started looking at changing jets or secondary springs.... that's a slippery slope !

 

Pretty sure you can 'punch it up' some more. Others with more Holley experience can chime in.

I can tell you that upping the power valve from the factory 6.5 to what my car 'called for' at 10.5 helped a lot...

 

Yep, I am sure there will be plenty of good advice here ! I think my first move will be to disassemble the old carb and take a look at what power valve and jets were in use there, while I think the car was running rich before, at least that will give me a reference point because it was making good power at those fueling levels.

 

Better is to hook up 6 feet of vacuum line to the intake and crack the hood and run the gage to inside the car, watch the vacuum gage in cruise and get a PV 2" less than vacuum at cruise.

 

From what I understand, you want the power valve about 2" below idle vacuum. If I used a 10.5 in my BB it wouldn't idle right or would surge because the power valve would be getting uncovered at idle. So in my case removing the original and installing a 6.5 was perfect.

 

2" below cruise vacuum, not idle.

 

2" below cruise vacuum will have the PV open almost all the time; it doesn't take much throttle plate opening to result in a major drop in manifold vacuum. If that's what you like, that's fine, but it's certainly not the optimum solution.

 

What would you consider a better target delta?

 

.....but a lot of folks have to do this if the carb is a little small for their application anyway
however, i have torn apart original holleys for the L72 mills that had a really high# p/v from the factory......

 

About half of idle vacuum; that's why 95% of VS Holleys come with #65 PV's (6.5" Hg. opening); as the camshafts get more radical, the PV number goes down.

 

My Chevelle has 21" at idle hence the 10.5.
I tried an 8.5 and the car had a bog when gas was applied.

 

The perceived difference in acceleration punch might be in part the difference in the secondary diapram spring. Your old one might interchange on the new one or there are kits available.

 

The power valve has NOTHING to do with mixture at idle! Whether it's open or closed, it has NO effect since there is no flow upwards in the main wells to the boosters at idle.

The power valve only opens and enrichens the mixture when vacuum drops below its set point provided that the main circuit is flowing. Most corvettes will cruise at 16 to 19" Hg of vacuum on the transition circuit; below 15" of vacuum, the main circuit does most of the work, but at ~6 or 7", the power valve circuit adds additional fuel through the PVCRs to the main circuit for best WOT mixture.

 

Guidance differs on the power valve selection. Some say it should be 2" below idle vacuum. Others say it should be 1/2 idle vacuum.

As another poster noted 2" below light cruise vacuum is one other way to 'dial it in'. I tried various methods and each one still pointed me to the 10.5. The key is to have it activate on acceleration but not have it so light that it is constantly engaged.

This is straight from the Holley web site in their carb tuning instructional guide:

"An accurate vacuum gauge, such as Holley P/N
26-501, should be used when determining the correct power
valve to use. A competition or race engine which has a long
duration high overlap camshaft will have low manifold vacuum
at idle speeds. If the vehicle has a manual transmission, take
the vacuum reading with the engine thoroughly warmed up and
at idle. If the vehicle is equipped with an automatic transmission,
take the vacuum reading with the engine thoroughly
warmed up and idling in gear. In either case, the power valve
selected should have a vacuum opening point about 2" Hg
below the intake manifold vacuum reading taken."

 

That must be old information or a misprint - in either case, it's incorrect; a power valve that's rated at 2" below idle vacuum would be open almost all the time. The following comes directly from the Holley site as well:

"In either case, the power valve
selected should be 1/2 the intake manifold vacuum reading
taken. EXAMPLE: 13” Hg vacuum reading divided by 2 = 6.5
power valve. If your reading divided by 2 lands on an even
number you should select the next lowest power valve. EXAMPLE:
8” Hg vacuum reading divided by 2 = 4 power valve.
Since there is no #4 power valve you should use a 3.5."

See http://www.holley.com/data/TechServi...wer_valves.pdf