Gas mileage - where is the logik?

[BlueBull]

I had my 1967 camaro convertibel last summer with a 327 engine 4 bbl. The fuel consumption was about 12 miles per gallon. Because I wanted more torque and power I built a 406 csb, alu heads, edelbrock performer RPM intake, holley 750 vac and a new MSD streetfire HEI distributor.

The intake and carburator was transfered from the old engine. The TH350 transmission was rebuilt and is now equiped with a shift kit.

To the point - gas mileage is almost exactly the same as before. OK, I have a much stronger engine now but still unable to get the logik behind this. Engine runs about 2500 rpm at 60mph and cars weight 3550 lb. The engine is still very recently assembled and maybe this may improve?

Many years ago I had several Mustangs, a 390 manual and a 427 automatic. Non of them was close to this figures. The 427 1967 mustang ran about 20 miles per gallon and the 390 1969 fastback even better.

Any ideas to impove this is very much appriciated. Very often its possible to improve economy as performance.
Thanks

[gearheads78]

Holley 750, 406, TH350, gas mileage.....

Lets play a game. Which one does not belong?

You are never going to get great mileage but lockup converter, overdrive, and fuel injection or at the least a properly tuned Quadrajet will get you better.

What heads, compression and cam are you running?

[6'9"Witha69]

The 427 1967 mustang ran about 20 miles per gallon and the 390 1969 fastback even better.

MPG or GPM?!

That is not a very believeable statement IMHO. What gears and are you talking all Highway MPG?

And yes, I've owned Mustangs and still have a 390 powered FERD.

[Damn True]

Logic perhaps?

[BlueBull]

Hmm, yes logic..

OK, it sounds stupid I know. The car is not built for fuel economy and really I don't care. I would with pleasure select additional 50-100 hp and sacrifice economy even more.

Anyhow, I will make a test with a new holley 650 vac today and see if any change can be had. Spark plug reading looks good but the exhaust outlets are really dark. After one hours driving the rear bumper has a thin black layer from the emissions.

I really should appriciate if you can say if 12 miles per gallon (highway miles) is OK related to the cars configuration (approx 3:1 on the rear). What's yours figures??
Thanks

[jasonsnova]

ditch the th350 go with a t56 or ad a gear vender to the th350, or get a 700r4 auto,
last year my car was 355/th350 4000 stall converter w/411's and a holley 750dp it got 8mpg on a good day...usually less, i swapped in a t56 and i nailed down just shy of 20mpg , if i get rid of the holley it will probally do better...

[Blown353]

A properly tuned carb and ignition will work wonders.

The streetfire distributors in my experience have a too-slow vacuum advance canister that never pulls full advance on a car with a "decent" cam. Without enough advance at light load you'll have a hard time running "lean burn" on the freeway. At freeway conditions & light load around 50-51 degrees of advance is usually the magic number. You'll want to change to a canister that gives somewhere around 14-16 degrees of advance that is in by about 1.5-2 in/Hg less than your in-gear idle vacuum.

Also, most aftermarket carbs, especially double-pumper holleys but also to a lesser extent the VS carbs, are setup to have the mains come in too early and are thus way too-rich under light load cruising conditions to achieve good mileage. You'll need a wideband and plenty of time to tweak it; to "clean up" the cruise & light load mixtures on a Holley, you typically have to tweak the high speed air bleeds on the primary sides to bring the mains in later, drop down on the primary jets and tweak the IFR's and idle air bleeds to tailor the cruise mixture to the leaner side (15:1 or leaner), and then open up the power valve channel restrictors to compensate for the now leaner primary jets to maintain a safe mixture under heavier load. Sometimes when you go down this route you'll get surging AFR's at cruise-- which means you now have to dip into the emulsion bleeds too.

Your goal will be to achieve a 15:1 (or leaner) mixture at light throttle cruise while still maintaining idle quality and richer mixtures under heavy load. You can substantially increase mileage, but it will take you time.

However, if you have a "sloppy" converter then you probably won't see much, if any, gains. It just kills the efficiency of the entire drivetrain.

[BlueBull]

Thanks guys.
I will make some tests on ignition settings and carb fine tuning (perhaps replace the Holley). The TH350 is recently rebuilt and the converter is a new B&M Holeshot 2000. I'm concerned the converter stall RPM is so close to the 2500 rpm highway rev. Perhaps I have much "slip" at cruising??

What about the X or H pipe on the exhaust? Do you think it can have any impact on gas mileage because I don't have any connection between the tubes today.

The cam I have is a Comp Cam XE274H. 230°/236°@0,050, ,490/,490, LSA 110°, opinion on this??

[pdq67]

The year I DD my 406, M-20, 3.31 posi- and 26.5" tall tires in my '67 1st Gen car, the cam is still a melling CRV-1 or -2(?).. The 292/230, 109/107, .480" lift jobber and running on the hwy at 55 mph b/c WE HAD TO!!, it got right at 15 mpg!My car new stock w/ my 350SS engine only got right at 17 running like 65 to 70 mph or so before the speed limit dropped.My old junk301 w/ the Duntov in it got like 16 mpg.All had a Q-Jet on them, duals, but no headers.pdq67

[greygoose006]

i would say that the th350 is you main culprit of poor economy.
i second the notion towards a t-56, unless the car is built strictly for drag racing.

2500 rpms on the highway is too much.
i would look to get down to more like 1600-1200. i wouldnt go lower than 1200 though.
with a .5:1 OD in the t-56, and 3.73 - 4.11 gears, you should be set.
after that, tune the carb to suit you, but i would do the transmission thing first.

if you MUST keep the th350, go with a much lower stall converter.
your engine has enough torque that it doesnt need that high a stall unless it has drag slicks.
go with the lowest stall you can find, and if possible, find an accesory OD unit.
gear vendors is good.
if it were me, i would look for a lower OD though