V6 Fi Vs 318

-
Valveguy

Valveguy

Well-Known Member
Joined
Jun 22, 2008
Messages
110
Reaction score
4
Location
Nebraska
I have a 74 duster 225 slant six. I put on headers and a 4brl intake and 390 cfm carb. I can only achieve 17 mpg at best driving the car 55 on a flat road. Would I be better off, money wise and mileage
wise, trading up to a 318 or turning my slant six into fuel injection? Thanks!!!Tom
 
WOW 17 after just a header and intake... and a 4bbl Not suprized. Stock engine and some bolt on goddies and you were looking for what..... 35 mpg
Maybe a geo metro is what you are looking for...

Need to get into the engine a bit more before you can make the power and the economy you want but if the 318 is the bandaid you need.... hurry up and swich it out8) 25 MPG on my 4bbl /6..
 
How are you getting 25 mpg on your slant six? I can't get anywhere close to that. What are you using for a carb? Tom
 
17 mpg is all its getting? what gear is in the car? my dart with a basically stock 74 360 in it and a 2.94 gear got 17 mpg on the way to carlisle last year and that was with a way too fat 780 holley on it..

had a little 64 valiant with a slant and three on the tree that got 26 mpg
 
Yes, I know, it doesn't make any sense! I'm pretty sure the car has 2:76's. I had a 74 duster, (318) as a kid and without working too hard, I could get 25 mpg! Is there anyone out there that can give me a place to start as far as getting to the bottom of this problem or should I just give up and think about a 318 or ????? I have no idea. Thanks to everyone for the feedback. Tom

Wicked/Six, What would be a good strategy to get more efficiency out of my 225? Thanks! Tom
 
I have never even gotten 17 mpg's out of a /6, ever.
 
Holy cow, if the worn out /6 in my 65 got 17, I'd try to find out what was wrong. I consistently got 22. That was with 3.23 gears.

Seriously, make sure your vacuum and mechanical advance are working properly, your float level is correct, and everything is in good condition. Don't drive like an idiot, and you'll be fine.
 
I have a holley 600 vac secondary, I had to do alot to it to make it run at the elevation i'm at. I would check your plugs and see if they are black or really dark, the carb may be running fat(rich). Have you done any work to the engine other than the intake and header?
Also what type of driving do you do a majority of the time, in town stop and go or mostly highway. With those gears your in town mialage will be down but should get better than 17 on the freeway. I run 4.10's around town and at the track and 3.23's when i take long trips on the freeway, Average 25 on the freway and 22.5 intown.
If you get into the engine mill the head to bump compression this will give you more power and increase your mialage as well, to an extent don't think a 12-1 comp lumpy cam race motor is going to get great mialage! Just get everything adjusted like valves and timing and get the carb just right and you should see some improvment in power and economy.

TF
 
TGKELLY2 said:
Yes, I know, it doesn't make any sense! I'm pretty sure the car has 2:76's. I had a 74 duster, (318) as a kid and without working too hard, I could get 25 mpg! Is there anyone out there that can give me a place to start as far as getting to the bottom of this problem or should I just give up and think about a 318 or ????? I have no idea. Thanks to everyone for the feedback. Tom

Wicked/Six, What would be a good strategy to get more efficiency out of my 225? Thanks! Tom
Click to expand...



First of all 55 mph is not cruising fast enough for 2.76 gears with a slant.My slants sweet spot with my 2.93 is around 65-70m.p.h about 2500-2700 rpm.My stock 88000 mile slant will pull about 25 m.p.g,and I get about 15-16 in town with the 904.You need to adjust your shifts to happen later.I usually shift it manually around town,keeps up better with traffic,and gets slightly better mileage.

Just pretend you have 3.55's and buzz the engine up higher,trust me it can take it and it will like it.
 
Also,what size tires do you run? Running bigger heavier tires are not only unnessary on a stock slant but kill performance/mileage.Lightweight mags with a 195-205-215 tread width, about 25.5-26" tall tire would be optimal.Keep them aired up,but remember air expands up to 5 lbs when it heats up,so keep that in mind when filling them.Nitrogen on the other hand does not expand any where near as much as that,so you can get away with a bit more cold pressure.

The slant really needs more gears. Because of the long stroke and limited factory induction/cam design you are not going to see much more than 4500 rpm with a stock slant.A 5 speed with a close gear spread and overdrive would be perfect.You could run your 3.73's or 4.11's and still be able to cruise up on the highway at 70-80 mph easy.The manual tranny is usually lighter and uses less power from the engine to operate,both good things for performance/economy.
 
With headers, 390 cfm 4-bbl and 2.76 gears out back...I'm able to get 22-23 MPG consistently. Most of my driving entails 65 MPH highway speeds to and from school at about 2600-2700 RPM. 17 seems pretty low. My slant likes the higher RPM's as well so I usually shift it myself. I also agree with Grinder, wheels/tires can wreak havoc with performance. I put wheels on my Dart that were 2 inches wider, and immediately the 1/4 mile times went down 4-5 hundredths.
 
Oh my gosh! I never expected such great help and thank you to all who submitted this helpful info!! I'll try it and see what happens. And, to answer some questions and to fill you in with a little more of the cars history.
74 Duster, original 225 engine, automatic transmission. The only changes I have made are the Clifford 4 barrel aluminum intake, headers & valve cover. 390 cfm Holley carb with no real alterations thus far. I had a tune-up, new plugs and have only put a few hundred miles on since. Recently, I have tried premium fuel but it doesn't seem to matter. 215/65/15 tires on the rear and 205/65/15 on front. I too have thought that there is something screwy with the bad gas mileage but all the mechanics pretty much just shrug there shoulders. Honestly, with the computer type repairs they are doing on cars now days and the fact that these guys were born after the car was made so I don't expect too much from them. Not to discredit them, they do their jobs well but they don't see much on a regular basis to compare it to. Also, I drive the car extremely easy trying to get better mpg and also knowing that it has the weak 7/14 rear that I don't want to replace. I was also guessing on the 2:76 ring & pinion gears, I'm sure it is stock for a 74 slant 6. Thanks again to all!!! Tom
 
Answering A Couple Other Questions: I Only Do Highway Driving, Very Little Town Driving, And Everything Possible Has Been Changed To Royal Purple, Transmission, Differential & Engine. I Also Keep My Tires Full. I Will First Try Moving The Car A Little Faster To See What Happens There. Thanks, Tom
 
I think you should keep the slant 6 and try tinkering with it to get the best fuel economy. The slant should get better mileage than 17. A 318 won't do much better. I can only manage an average of around 12mpg with my mild 318.
 
Humor me, take an appropriate wrench with you and head out into the country to some quiet section road. Advance the timing a tad and go drive a mile, floor it, cruise it, stand on it half way etc... Did it ping bad? If not, advance some more and try again. You should be able to get a fair bit more than the factory setting with decent gas. This is an age old technique called "timing by ear" and it may help you. (Down the road we can try "timing by stopwatch" to really dial in your dist).

Once your ignition timing is closer to optimal, try going down 4 numbers on the primaries.
 
Ok, cool, I'll do it. My car is in a garage accross town right now but maybe as soon as this weekend I'll do exactly what you are saying. I'm looking forward to it already, thanks C130 Chief! (C130 as in, the big awesome army haulin' machine?) Tom
 
I used to get 19-20 mpg around town and 22-23 mpg on the hiway with my \6 with 390 4bbl, headers and cam. It was an A833OD tranny but I had 3.55 gears.

Slant6.jpg
 
The Clifford 4 bbl and that 390 is just plain TOO much for a stock slant.Look at the size of the plenum/runners on the intake they are HUGE. Since you already have the 4 bbl,and headers its time to put some money into the head and cam.Port/Polish the head with a valve job(big valves not necessary,but will help) and go with a new erson cam with whatever specs you would like...and bam,you will get a huge increase in performance and economy(if you can keep your foot out of it) If you dont go over 55,I would reccomend some gearing too. some 3.23's or 3.55's would be perfect for what you do.




Also you need to work on your own cars,mechanics will screw you like an all night glory hole if you give them half the chance.
 
Hey Grinder,
Its good that someone finally said what I have sort of been thinking. In retrospect, I am disappointed that I didn't go the Super6 rout. The person at Clifford (of course) told me that his set-up was going to be better for what I wanted (economy and some power when I need it). Don't misunderstand me, I'm not going to bash Clifford because they have been very good at answering questions and getting parts out to me. And on the Mechanics part of it, you are 100% right. I need to be done with them and dive into thing more myself. Thank you for this info, I'll put all of this into the mix and go from there. Thanks!!!
 
TGKELLY2 said:
Hey Grinder,
Its good that someone finally said what I have sort of been thinking. In retrospect, I am disappointed that I didn't go the Super6 rout. The person at Clifford (of course) told me that his set-up was going to be better for what I wanted (economy and some power when I need it). Don't misunderstand me, I'm not going to bash Clifford because they have been very good at answering questions and getting parts out to me. And on the Mechanics part of it, you are 100% right. I need to be done with them and dive into thing more myself. Thank you for this info, I'll put all of this into the mix and go from there. Thanks!!!
Click to expand...



Any time.If you want a step up from the stock super six I have an offy 2x1 bbl. intake I would trade you for the clifford.
 
Ok, I'll take note of the offer and if the oppertunity comes, I'll be in touch. Thanks!!!
 
The mileage on my slant dropped to the 17-18 range for some reason and I spent quite a bit of time finding the problem. The coil had slipped in it's bracket and was resting on the fuel line. Over time it rubbed a small hole in the fuel line and the leak wasn't visible unless the engine was running. Replaced the fuel line from the fuel pump to the carb and I am back up to 20 around town and 25 on the highway. If your fuel filter looks like it is empty after sitting overnight you may want to check into that. It's a cheap fix.
 
Grinder said:
The Clifford 4 bbl and that 390 is just plain TOO much for a stock slant.Look at the size of the plenum/runners on the intake they are HUGE. Since you already have the 4 bbl,and headers its time to put some money into the head and cam.Port/Polish the head with a valve job(big valves not necessary,but will help) and go with a new erson cam with whatever specs you would like...and bam,you will get a huge increase in performance and economy(if you can keep your foot out of it) If you dont go over 55,I would reccomend some gearing too. some 3.23's or 3.55's would be perfect for what you do.




Also you need to work on your own cars,mechanics will screw you like an all night glory hole if you give them half the chance.
Click to expand...

For a stock headed slant, the 390 is too big. But if you do some head work (angle job, porting, etc.) and add a mild cam...I have found that the 390 is a nice piece. But then again, I bracket race my Dart almost every weekend...So a strictly street slant would probably work better with the super six.
 
Man, do the work yourself. You'll be glad you decided to rely on your own intuition instead of a young mechanic trained to deal almost entirely with new computer-controlled turdboxes. I understand there are young mechanics out there who were lucky and got exposed to old-school vehicles but they are getting harder and harder to find. Besides, getting your timing and carb dialed in aren't that hard at all and once you get around to disassembling your engine some more you'll feel like a badass once it's all back together and running better than it did before.
 
You must log in or register to reply here.
-

Back
Top