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post #62 of (permalink) Old Sep 17th, 19, 6:00 PM
hjdca
Tech Team
Harry
 
Join Date: Jan 2011
Location: Los Angeles
Posts: 310
Re: 11.5 mpg 396?

Quote:
Originally Posted by McHaggis View Post
Get a wideband and start tuning. Donít be afraid to go beyond 14.7:1 at cruise. I ended up about 16.1 but I run an MSD 6AL ll programable with a map sensor so I was able to tune the vacuum advance as well.
10.5:1 355sbc in a 69 Javelin
3.73 gears 3600 rpm 9.5 lock up converter
750 DP Speed Demon on a Victor Jr intake
HR cam 228* 230g on 107* centres, 1.6 rockers
1-3/4Ē headers into 3.5Ē collectors.
TH200R4 trans
This was never setup to get mileage but after lots of tuning with an innovative wideband I get 22mpg highway cruise at 60-70. This is about 2200-2300rpm.

Yes itís a small block but the old set up was with a TH350 and 2.87 gears with a 2200rpm non-lock TC and a 650DP Holley and I typically got 13 before adding the OD trans but the big improvements came form leaning the carb as much as I could and getting the timing curves optimised. Driveability is very good too.
I agree, it can be done, and I did it - with big cam, 750 Holley Ultra HP double pumper , aluminum heads... You need a wideband and a great understanding on how to tune a carb.. The ultra HP was key for me because it has the "air bypass" that allows you to close the butterflys at idle and tune the carb without "tip in" stumbles with a big cam. With a big cam, you also need to run approx. 20 deg. of initial timing at idle and add a vacuum advance which is fully advanced at idle - giving you approx. 36 deg. of timing at idle. To also aid atomization, get a higher degree thermostat ie. 195, and run hotter plugs. All this will allow you to close the butterflys more which is one big key to improving gas mileage. Then, using jets and air bleeds, pump cams, and pump squirters to get cruise around 14.7 afr, 2 barrel acceleration around 13.2 to 1, and WOT at 12.2 AFR. Also, a high energy ignition like a MSD will burn more fuel at idle and allow you to lean out more. Overdrive is also a big plus on the fwy. I doubled my gas mileage on my 396 compared to a stock Holley 750 double pumper.

Harry
1970 Chevelle SS396 (408), Fathom Blue,
Original Block & Forged Crank, KB Pistons, Edelbrock 6045 Heads,Comp Cam 292H, March Serpentine, RPM AirGap,750HP,100hp Nitrous
A/F Wideband,Tremec TKO 600, 12 bolt,
30 spline Moser Axles, Richmond 3:73's, Hotchkis control arms, swaybars, springs, Wilwood 12" rotors D/S, 4 piston calipers

Last edited by hjdca; Sep 17th, 19 at 6:17 PM.
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