Hello everyone. I'm new here to the Slantsix forums and /6 engines in general. Been around on a number of Mopar forums for years but just recently got into A-bodies and the /6. recently picked up a '72 Valiant with the 225 1bbl engine. The front end is crunched up and I'm currently in the process of fixing that. Anyways, on to what I'm thinking.

I have always have been a performance orientated guy running BB's and big C or B body cars. However, this time I'm going for a max MPG setup with the Valiant. I would like to see 30mpg on the highway if possible.

I'm thinking a nice long rod 225 with compression that I can still run 87 octane. I would do the EFI setup from a mid 80's GM product (per the "articles" here), a T5 trans from an early 80's V6 Ford(?), electronic ign., headers, cam(?), and 8.25 or 8.75 rear with 2.45 or 2.74 open diff.

Does this sound about right? I'm concerned with the engine rpm not being high enough on the highway. Here is what I calculated...

.068 OD + 26" tire + 2.74 gear = 1800 rpm
.068 OD + 26" tire + 2.45 gear + 1600 rpm

Will the little /6 be able to handle this? What cam in your opinion would be best for this setup? Any other input or critiques are more than welcome. Thanks.

There are some things here that will probably not work for you.

You wont get 30 mpg with a box like a 72 Valiant. The coeficient of drag is way against you. My wifes Eagle Talon with a CD of .28 gets 30mpg. Your Valaint with a CD of .45 wont.

Your final drive ratio is too high for overdrive. I have a .70OD and used to have 3.23 gears and 24" tires. I now have 3.55 ratio and get better mileage. Slant like to be at about 2000-2200 at 60 mph for maximum efficientcy.

Your best cam for mileage is a stock 72 cam degreed properly.

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Yeah....Im the one who destroyed this rare, vintage automobile.....

I was able to get my brother's 1983 Dodge shorty van equipped with a 225 and an automatic transmission to get 20-21 MPG on the highway. That is VERY good considering the weigh and aerodynamics of the van.

You can read about what I did to get it there in THIS THREAD

Thanks for the input. I will be lowering the ride height somewhat to help with the aerodynamics. I have read of others getting 24-25 mpg without OD and I figure I should be able to get as much or better. Maybe not 30 mpg better but possibly close to it.

BTW, I should have mentioned that my cruise speed is figured for 75 mph since I live in AZ.

The A833OD high gear is .73:1. The factory put this in front of 3.21 gears in many cases giving an overall high gear ratio of 2.34:1. With a 25.5" tire like a 195/75R14 that's right about 2300 RPM at 75 MPH. If you turn a 225 much slower than this it will be very lazy in high gear. This combo will be slow, but livable.

If you get the compression up a bit, dial in the carburetor and have a good exhaust system mid 20 MPGs should be no problem, but 30 is going to be tough. The GM fuel injection system is actually a handicap. Factory EFI systems are meant to keep a stead 14.7:1 air/fuel ratio so the catalytic converter works properly. Leaner is better for economy. If you want EFI use something programmable like Megasquirt.

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Joshua

Don't forget you can improve the drag coefficient of the vehicle a smidge. Underbelly pan, wheel skirts, masking the grille, etc. Don't forget to run skinny tires too and get tires that have low rolling resistance. Synthetic fluids also help.

Thanks for giving me more food for thought. I will be reading a lot and asking a question or two. Thanks again!

IIRC, can't the GM system be tricked with the temp sensor in to running a richer or leaner mixture?

I done know if the GM system can be tricked. BUT i do know alot of folks here have experience on the Megasquirt system. Go to the efi section a look thru the posts there.
Frank

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Scrapple: Because a mind is a terrible thing to waste.
73 Duster - Race Car
66 Dart Wagon - DD
178" FED
82 D150
All Slant powered

Lose weight!, Take out anything that isn't needed. Lose the power steering and AC if you have them. Electric fan will help to, the stock fan is a drag.

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Grandpa's 1974 Dart Custom 4 door 225 auto, Aluminum Radiator, 1920 Holley.

I have a 5 speed early A with a slantsix. I hav played with aero on it and my previous 69 Dart. The land of powdersnow allows a lot of "wind tunnel " experiments ( the snow coming across the car will reveal a lot)

I also drive a Buick Roadmaster wagon that in fine tune will get me 20 mpg...4500 lbs CD .34 and an Eagle Talon 3000lbs CD .28 mpg 29-30

I once got 27 mpg in my Valaint with 5 people on board 1000 miles across the Canadian Prairies with a tailwind in the rain. The wind noise in the car at 80 was unreal. Stock bbs

My son coaxed the same car to a steady 27 mpg over a winter commuting by himself at 45 mph in 5th gear. Stock BBD 3.23 ratio

We built the motor (Comp 264 9.0cr) no change im mpg

I rebuilt the motor (erson 280/270 9.5cr) 3.55 ratio ,no change in mpg

Its about aero and terrain.

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Yeah....Im the one who destroyed this rare, vintage automobile.....

Aero is nontrivial. My '64 Dart has gotten 24-26 MPG with a racy motor at 75 MPH average, and my '68 Dart will not go over 25 MPG even with a stock motor with multi-point EFI and OD. I think the '68 4dr back glass and shape is a killer, but haven't tried to mess with that yet. Duster or Demon is probably best.

I would also not go below about 2.1 final drive (OD * axle ratio), but I've seen good results with 2.2 - 2.5 final drive. T5 is definitely a help. 9:1 comp is where I'd aim.

Lou

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Home of Slant6-powered fun machines since 1988

Sandy in BC were do you get the drag coeficients from? do you know what a '74 Dart 4 door is? Not good I'm sure. I've been contemplating closing off some of the grill area for a test.

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Grandpa's 1974 Dart Custom 4 door 225 auto, Aluminum Radiator, 1920 Holley.

Early A s are much more Aero than the late As.

My 69ht used to pile up snow on the back window and rear valance > I added spoilers to keep the snow clear and believe it made some mileage difference....but not lots. I used station wagon rear window spoiler froman Aspen on the back edge of the trunk and an Eagle wagon one at the roof line.

I once left my Valaint parked facing upwind in a ski hill hostel parking lot. We had a huge dump overnight with wind. The Valaint looked like a cuda in the morning from the snow buildup on the trunk. I suspect early Barracudas have a pretty Aero rear ...especially compared to later As.

My highway/rear view mirror/powder snow studies leave me to believe the best Aero move is Bonneville/Moon discs. The turbulence from my wheel wells through the Ansen Sprints is considerable.

The headlight rings on early As need some study.

The drag coeficients I got from "some site" for the Talon....Impalass.com for the Rudemaster and by extrapolation for the A. I just used the one I found for a 64 Fairlane

A chin spoiler would be good.

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Yeah....Im the one who destroyed this rare, vintage automobile.....

Sandy,

I agree that the early Barracuda is likely the most slippery A-body extant. I think the '66 with the flush grill would be the best of the bunch. Chin spoilers are good as are rear spoilers on the rearmost edge of the trunk lid. They keep more positive pressure on the back window to better push the car along. Full wheel covers do help especially when the wheel is close to the outside of the body.

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Joshua