Let's Talk Miles per Gallon (Or Gallons per Mile)!

[Dodgeboy]

1974 Challenger, 383/727, 4.30gears with 29.5inch tall tires (345/55R15). I get 6 mpg. 750 holley dp (bad tune/jetting, the pro's screwed up tuning it and I just assumed it was good....lol.....now trying a holley 850 track warrior, still playing with the jets. Cam is a Comp cam CRB 306S-10, lift is .555 I&E, duration @ .050 is 260, 3500 stall. 516 heads with bigger valves, fully ported. No idea what comp. Ratio is. Machine shop lost my blueprint.

Timing is set @ 18° intial with total 39°. Mechanical dizzy. Jacobs electronic ignition system

 

[Dodgeboy]

I'm hoping to get closer to 10mpg once the 850 holley is tuned.

 

[DetMatt1]

I'm hoping to get closer to 10mpg once the 850 holley is tuned.

That seems like a lot of carb, no?

 

[todd383]

Interesting, It gotta be the cam,
Can you recall the specs on the cam?

Comp solid Duane Porter design, 580ish lift and [email protected] 114 LSA

 

[Dodgeboy]

That seems like a lot of carb, no?

The Cam calls for a 800-850cfm carb. Orginally the shop got me a demon 850 (back in 2000) but the carb had defects so they used the "shop 750dp" to dyno the 383. Final carb jetting was 82 square with no pv (no vacuum @ idle). Then they got me a new 750dp, labelled the box 82 square, and sent me on my way. Years later i realized they had set the valve lash wrong and now i have 5" vacuum @ idle and a huge traction issue (lol). Peak hp (417hp) @ 5700rpm with wrong valve lash. The cam should be good up to 6800rpm and i wanna rev it up there. I've seen lots of similiar engine combos and i'm convinced my 383 has more hp in there somewhere. Hoping the bigger carb will help me find them. I still have the 750 incase the 850 proves to be too big. Also when i pulled the 750 apart to clean it I noticed it had the factory jets in it, but i'll try the 850 before i re-tune the 750

 

[Cudafor36]

My "old" motor the 340 with a 450 lift/duration cam, ported J heads, and a comp series 650 carb got 9-10mpg. Thats with a 727 tranny. Now I get 18.5mpg with my 392 and a silver sports four speed auto.

 

[DetMatt1]

The Cam calls for a 800-850cfm carb. Orginally the shop got me a demon 850 (back in 2000) but the carb had defects so they used the "shop 750dp" to dyno the 383. Final carb jetting was 82 square with no pv (no vacuum @ idle). Then they got me a new 750dp, labelled the box 82 square, and sent me on my way. Years later i realized they had set the valve lash wrong and now i have 5" vacuum @ idle and a huge traction issue (lol). Peak hp (417hp) @ 5700rpm with wrong valve lash. The cam should be good up to 6800rpm and i wanna rev it up there. I've seen lots of similiar engine combos and i'm convinced my 383 has more hp in there somewhere. Hoping the bigger carb will help me find them. I still have the 750 incase the 850 proves to be too big. Also when i pulled the 750 apart to clean it I noticed it had the factory jets in it, but i'll try the 850 before i re-tune the 750

I’m no expert but you may be giving up throttle response off the line in order to run 6800 rpm and why do you want to run 6800 rpm? Are you racing this car?

 

[Dodgeboy]

I've never been a fan of keeping the rpm's down. One of my reasons for getting a 383 was the fact that they can rev high (7000rpm) with just a valve spring change. Honestly I don't race the car but eventually I would like too.

 

[wedg2go]

Quick story.

My eailier mentioning six miles per gallon finds me at the pump quite often. Which is really not a big thing, but is interesting. Every now and then I will be met with a kind comment of "Nice Car" and of course it turns into a short friendly conversation while hogging the pumps (nobody has complained yet). I enjoy this time, even if it cost me 30 something a pop. I also enjoy leaving the station giving a little show and lightly "Running through the Gears" knowing they're watching. It my only time, it seems, that I ever have that opportunity to crack open the outboards since coming to a complete stop is a rare event around here.

 

[Mopars & Missiles]

Hook up your vacuum advance so you have about 55* total at cruise, I think you'll gain a little mpg doing that. Not much, but some and every little bit helps!