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Username Post: I have previously mentioend here about rebuilding/recurving the distributor        (Topic#375773)
DHMelton 
"3rd Year" Silver Supporting Member
Posts: 427
DHMelton
Loc: Central Arizona
Reg: 04-10-22
09-30-24 06:00 AM - Post#2874307    

The factory (396 with TH400) dizzy has a lazy advance curve, to allow the 80% of people who use regular octane gas instead of high octane gas. Factory spec is something like full centrifugal advance is achieved at about 5000 RPM.

I recurved my cent adv to be all in around 2700 RPM, and changed the vac can to one that has full advance at a somewhat lower manifold vacuum (8-10 in hg).

Anyway, took a 120 mile trip on (mostly) highway, cruising at about 60-70 MPH, (2500-2900 RPM), with the windows up.

Got 17 MPG with the 325/396 rebuilt to as bone stock, and using the original Q-Jet. My previous MPG for the same trip before the dizzy work got me about 14 MPH

On the return trip, I had both side windows open,and ran 75-85 MPH (sometimes reached 90 MPH) most of the way. Mileage dropped to ~12-13 MPG.

I could tell the motor was working harder to overcome the added aero drag as engine temps went from 180-185 on the first leg, to about 215 on the return leg.

Doug



 


Magnetocheck 
"2nd Year" Silver Supporting Member
Posts: 530
Magnetocheck
Age: 70
Loc: Charlotte, NC, USA
Reg: 09-05-22
09-30-24 06:18 AM - Post#2874309    
    In response to DHMelton

interesting Real-world data, Doug. Thanks for posting. I love my Holley but am thinking about switching to the New QJs made by PRW
I realize that it will take a long time to repay the $600 cost in gas, but I enjoy playing around with it.

Bob
'65 Impala SS 396 Convertible
Member, National Impala Association
--It's weird being the same age as old people!


 
DHMelton 
"3rd Year" Silver Supporting Member
Posts: 427
DHMelton
Loc: Central Arizona
Reg: 04-10-22
09-30-24 07:42 AM - Post#2874313    
    In response to Magnetocheck

My go to Q-jet replacement was a Holley 6210 spreadbore 650 CFM double pumper.

It has smaller primaries than a Q-Jet for better off idle throttle response, and the secondary pump shot gave the car more oomph when you floored it. It wasn't as fuel efficient as the Q-jet if you were a throttle stomper though.

The out of the box 6210 has (IIRC) #62 primary jets, its much more fuel effect than a 4150/4160 which comes with about #72 primaries.

You can down jet a 4150 to #62 primaries but you have to modify some orifice sizes in the metering block so you keep a proper AFR at part throttle. (this is explained in Holley Carburetors by Dave Emmanual). Some math is involved.

BTW, I think the vac can is full open at 14" in Hg, 8-10 is where it stats to open IIRC, i will have to go and look what B # vac can I used.



Doug



 
astrojet427 
Member
Posts: 183
astrojet427
Loc: Vancouver, B.C.
Reg: 11-22-05
09-30-24 08:23 AM - Post#2874315    
    In response to DHMelton

Thanks for that real world mileage data. The windows up down numbers especially interesting. Corroborates similar effects with pick ups that have an open box or ones that have a box cover at highway speeds.

I’ve always understood the golden rule with vac advance cans is: the vac can should be fully deployed 2” below what your engine’s idle vacuum is to avoid an unstable idle from erratic vac advance timing. Generally, B22 or B26 cans usually good choices for mild performance engines.

1967 Impala SS. 327-Turbo 400; 12 bolt-3.55 gears.

1972 Corvette Convertible Mille Miglia Red 4 speed.

1963 Cadillac Eldo-Triple Black.


 
bobb 
Super Senior Member
Posts: 6827

Loc: paradise
Reg: 09-05-03
10-02-24 07:38 AM - Post#2874371    
    In response to Magnetocheck

if you like tweeking it get a af ratio monitor. i have an ngk and i like it.

70 L camino, grampa engine, g-force 5 spd, road rage suspension. Pray first before all else fails.


 


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