Moparts
Where is my dang MPG?
Posted By: Secret Chimp
Where is my dang MPG? - 06/25/12 05:28 PM
I'm still searching for decent MPG on my 318. I've asked this question more than once on this board but I'm back again to see if there are different thoughts after my most recent change.
My most recent update was going to a clutch fan to a fixed fan - some improvement, but not the 19+ mpg people are getting with less mild setups than what I have.
Here's my setup:
1967 Coronet station wagon (about 3,800 with me and gasoline)
318/727
Split dual exhausts with Flowmasters of some kind ahead of the rear axle on the stock manifolds
2.93 rear
Summit 6900 cam
Edelbrock Performer intake
MSD 6AL ignition on older Mopar Performance distributor
625 cfm Carter AVS 4966S leaned down to a rod change away from a light lean miss at cruise on a 4 hole spacer
Here's my distributor curve/setup:
15 initial
15 degrees vacuum at 18inHg
19 degrees mechanical all in by 3100
(this was set up by FBO)
The absolute best I've gotten out of this car is around 14.5 mpg all highway at 60-65 mph and around 2100 rpm.
What else can I try? I'm out of ideas short of just trying a different carburetor ($$$ with unknown benefits, unless some western WA Mopar people have something I could borrow). I fiddled with a strip kit but I can't lean the carb any more than it is already. Choke comes off all the way.
Posted By: stumpy
Re: Where is my dang MPG? - 06/25/12 06:19 PM
14-15 mpg should be about the best you are going to get with that heavy a car.
Posted By: JohnRR
Re: Where is my dang MPG? - 06/25/12 06:25 PM
Thermoquad ...
I got a 70 coronet 4d myself and get around 22 mpg highway. Mine is a complete bone stock 318, 2 bbl carb,mopar electronic ignition and 2.76 gears. The timing on mine is set at 10 degrees initial, never bothered to check my advance or anything. Next week I'm putting in a 318 magnum with a 4bbl carb and headers for fun. Will prolly say goodbye to my mpg as well
Edit: if it makes any difference my 318 is 8:1 compression and open chamber heads.
Posted By: tcat446
Re: Where is my dang MPG? - 06/25/12 06:42 PM
I calculated mine yesterday at about 7mpg. 1973 318 4 bbl. I have the larger cc heads on mine lowering the compression with a trailer tow cam in it and 3:91 gears. I know mine has a few issues with the valves leaking. I would do a leak down test to determine if the engine is up to snuff. Have been told that if compression is down you mileage will suck. Not sure but have been told that. Worth a check. One other thing to try is do a compression test to see if you have a few cylinders low.
Posted By: Secret Chimp
Re: Where is my dang MPG? - 06/25/12 06:50 PM
Quote:
14-15 mpg should be about the best you are going to get with that heavy a car.
I'd expect that much around town, but I don't need much throttle to keep it going at 60 and we have relatively flat highways here.
I'm wondering if I got a couple of bum carbs. Another fellow posted here about significant mileage differences between his lean-as-possible Thermoquad and an AFB that got another 3-4 mpg over it.
Posted By: Secret Chimp
Re: Where is my dang MPG? - 06/25/12 06:51 PM
Quote:
I calculated mine yesterday at about 7mpg. 1973 318 4 bbl. I have the larger cc heads on mine lowering the compression with a trailer tow cam in it and 3:91 gears. I know mine has a few issues with the valves leaking. I would do a leak down test to determine if the engine is up to snuff. Have been told that if compression is down you mileage will suck. Not sure but have been told that. Worth a check. One other thing to try is do a compression test to see if you have a few cylinders low.
I ought to, it runs strong and doesn't blow smoke when I run it up but it's the one major aspect of the engine I haven't checked.
Posted By: Secret Chimp
Re: Where is my dang MPG? - 06/25/12 07:04 PM
Quote:
Thermoquad ...
I've been curious about those things for awhile but it seems easy to get into a money pit with them. Where would I want to start, especially for a non-emissions car?
Posted By: tcat446
Re: Where is my dang MPG? - 06/25/12 07:08 PM
Quote:
Quote:
I calculated mine yesterday at about 7mpg. 1973 318 4 bbl. I have the larger cc heads on mine lowering the compression with a trailer tow cam in it and 3:91 gears. I know mine has a few issues with the valves leaking. I would do a leak down test to determine if the engine is up to snuff. Have been told that if compression is down you mileage will suck. Not sure but have been told that. Worth a check. One other thing to try is do a compression test to see if you have a few cylinders low.
I ought to, it runs strong and doesn't blow smoke when I run it up but it's the one major aspect of the engine I haven't checked.
MIne runs strong too. No smoke that I have ever seen either. Worth a check.
Well... I plan on trying one with my 318 magnum. I got one with no emission stuff off a 1973 440 that was in a motor home. Got a rebuild kit for it at oreillys for a reasonable price.
Posted By: Black_Bee
Re: Where is my dang MPG? - 06/25/12 09:00 PM
I daily drove a '67 Coronet 2door HT for a couple of summers. 318, auto, 3.23.
It was a stock (except as noted) 100,000+ mile 1970 318 with a 600 Edelbrock carb on a LD4B intake. Pertronix ignition, single exhaust through a Dynomax muffler that dumped at the axle.
Anyway, I consistently got between 17 and 19 MPG. I drove pretty much 100% on the highway going between 55 and 60 MPH. I tried all kinds of driving styles and carb/ignition tuning to bring the numbers up, but that was it... the weather made more of a difference than anything else.
***EDIT.. Sorry, I was looking at my fuel logs, and that 17-19 was my imperial MPG calculations (I'm in Canada)... I actually was getting 14-16 US MPG.
Posted By: Fury Fan
Re: Where is my dang MPG? - 06/25/12 09:35 PM
FWIW, my 68 Fury with stock 318 would get a consistent 17 MPG with ~80% highway driving. Starting from 110,000 miles for the next 20-30,000 miles too.
Had elec ign and an MSD-5, good coil and wires, but no fancy carb or dizzy tweaks.
Posted By: furious70
Re: Where is my dang MPG? - 06/25/12 09:44 PM
did u degree the cam when you built the engine?
Posted By: Dodgem
Re: Where is my dang MPG? - 06/25/12 10:10 PM
So your running 34 deg mechanical total and 15 more vacuum? for 49 deg at light cruise??
Posted By: Secret Chimp
Re: Where is my dang MPG? - 06/25/12 11:49 PM
Quote:
So your running 34 deg mechanical total and 15 more vacuum? for 49 deg at light cruise??
No, I'm not cruising on my full mechanical, it comes in at 3100.
Would excessive timing be masked by a rich mix though? Maybe that lean miss was the right mix with too much vacuum advance.
Posted By: Supercuda
Re: Where is my dang MPG? - 06/26/12 12:08 AM
Sounds to me to be a timing issue. 3100 rpm for max mechanical advance is way too high of an rpm for your combo.
Posted By: Dodgem
Re: Where is my dang MPG? - 06/26/12 12:27 AM
Quote:
Quote:
So your running 34 deg mechanical total and 15 more vacuum? for 49 deg at light cruise??
No, I'm not cruising on my full mechanical, it comes in at 3100.
Would excessive timing be masked by a rich mix though? Maybe that lean miss was the right mix with too much vacuum advance.
Well that is where you gas mileage is you probably set it up By FBO's instructions he's wrong period.
Posted By: Crazy68Dart
Re: Where is my dang MPG? - 06/26/12 01:48 AM
Need to see what your timing is... get as much advance as the car will handle at light cruise. If mechanical is not all in until 3100 and you have 15 initial and 15 at cruise (assuming you have that vacuum you posted at your cruise RPM) then you likely only have 30-34 degrees of advance which is not going to help your MPG at all.
What is your cruise RPM? What do your plugs look like?
Posted By: dogdays
Re: Where is my dang MPG? - 06/26/12 02:00 AM
TIMING FIRST!
Disconnect the vacuum line and plug it. Check total timing at 3000 rpm, should be about 35 - 36 on a smallblock. Then the vacuum advance will be on top of that. That's a start.
R.
Posted By: furious70
Re: Where is my dang MPG? - 06/26/12 05:07 PM
Quote:
TIMING FIRST!
Disconnect the vacuum line and plug it. Check total timing at 3000 rpm, should be about 35 - 36 on a smallblock. Then the vacuum advance will be on top of that. That's a start.
R.
with that mild cam I would think full mech timing could come in closer to 2400-2600rpm
Posted By: Secret Chimp
Re: Where is my dang MPG? - 06/26/12 05:15 PM
FBO set my diz at 3100 for full mechanical advance, despite informing everything above (more or less) about the engine. Should I just throw in a Mr Gasket 925B set to drop the max advance RPM? It pulls all over but it doesn't really "come alive" until close to 3000 rpm, even if it doesn't improve mileage it feels like I'm leaving a lot of low-end power on the table.
Posted By: dogdays
Re: Where is my dang MPG? - 06/26/12 05:57 PM
A mild cam will be less tolerant of a fast advance curve unless the engine won't detonate ever.
SC, your previous posts are confusing and I was trying to shed a little light on the subject. So, have you set total advance to 36 degrees without vacuum at 3100?
R.
Posted By: Secret Chimp
Re: Where is my dang MPG? - 06/26/12 06:26 PM
Yes, as near as I can get it (I'm using a dial-back with a small scale on the dial so I might only be at 34). That's what the recommended setup for me was via FBO.
Posted By: VoodooCLD
Re: Where is my dang MPG? - 06/26/12 08:56 PM
Heh, i get 19 MPG with my 4 cylinder 2 wheel drive 95 toyota tacoma.
Good luck with the 318.
Posted By: patrick
Re: Where is my dang MPG? - 06/26/12 09:04 PM
Quote:
Yes, as near as I can get it (I'm using a dial-back with a small scale on the dial so I might only be at 34). That's what the recommended setup for me was via FBO.
and are you using vacuum advance to ported vaccuum on the carb?
Posted By: Secret Chimp
Re: Where is my dang MPG? - 06/26/12 09:28 PM
No, I'm using manifold vacuum. Let's not start that debate. It doesn't matter what source you use as soon as you're off idle, at that point vacuum is vacuum so it's not affecting my cruise timing, period.
Posted By: Crazy68Dart
Re: Where is my dang MPG? - 06/26/12 11:54 PM
Quote:
No, I'm using manifold vacuum. Let's not start that debate. It doesn't matter what source you use as soon as you're off idle, at that point vacuum is vacuum so it's not affecting my cruise timing, period.
Correct, plus manifold vacuum is going to help clean up the idle.
Posted By: hooziewhatsit
Re: Where is my dang MPG? - 06/27/12 01:06 AM
What's your cruising RPM? And how much vacuum do you have there?
Posted By: cornucopia
Re: Where is my dang MPG? - 06/27/12 01:10 AM
my STOCK 1968 coronet with 318/auto and 2.76 got 20 MPG average on the highway. I had a 318 duster with OD-833 and 3.23 and pulled down 25 MPG on a 600 mile trip one time.
Posted By: cornucopia
Re: Where is my dang MPG? - 06/27/12 01:14 AM
Quote:
Heh, i get 19 MPG with my 4 cylinder 2 wheel drive 95 toyota tacoma.
Good luck with the 318.
WHAT? something wrong with your Toyota! 
I have a 2wd '96 tacoma 4 cyl and I get 25-28 on the highway...
Posted By: Secret Chimp
Re: Where is my dang MPG? - 06/27/12 03:04 AM
Quote:
What's your cruising RPM? And how much vacuum do you have there?
2000-2100, 19-20 inches.
Posted By: fox
Re: Where is my dang MPG? - 06/27/12 01:57 PM
If you understand carbs, you need to lean the off idle circuits.
It is about to the point of running on the mains but fuel is still flowing in the off-idle system.
I'd suggest finding the idle air bleeds and open them about .003 and try that.
Posted By: SomeCarGuy
Re: Where is my dang MPG? - 06/27/12 02:06 PM
Might the carb be too lean?
Posted By: Secret Chimp
Re: Where is my dang MPG? - 06/27/12 05:43 PM
Quote:
Might the carb be too lean?
Maybe? It was too lean to run right on light throttle when I used .089 jets (same as stock, just Edelbrock pieces instead of the OEs) and rods with a .068 cruise step. Right now I'm using the original .066 cruise step rods and a .089 jet, which is how the carb came set up. I haven't tried going richer on it.
Posted By: Secret Chimp
Re: Where is my dang MPG? - 06/27/12 05:45 PM
Quote:
If you understand carbs, you need to lean the off idle circuits.
It is about to the point of running on the mains but fuel is still flowing in the off-idle system.
I'd suggest finding the idle air bleeds and open them about .003 and try that.
I don't, but I'm trying. As I understand it though, AVSes only have fuel orifices you can drill out to enrichen the idle circuit, not air bleeds. The idle mixture screws in front are all I have to mess with as far as I know (and I have them turned in a quarter down from an unhappy lean idle right now).
Posted By: Secret Chimp
Re: Where is my dang MPG? - 06/29/12 06:55 AM
Here is a random thought I had... tell me if I'm nuts.
I know it's bad to have too much of the idle transfer slots exposed on your idle setting.
What if I don't have enough of the transition slot exposed at my current setting - that is, what if the throttle opening I need to cruise on isn't enough to completely get on the main circuit and is still including the idle circuit too much?
I tried leaning down my idle to the point where I had to crack my idle screw open more yesterday and it still drove just fine, no stumble. I set it back to a vacuum-based idle again, but ought I try to lean down the idle circuit to the point where I need more of a minimum throttle opening and possibly get the idle circuit out of the equation, or at least run it leaner so when it is included it uses far less fuel? Or is this kookoo?
Posted By: hooziewhatsit
Re: Where is my dang MPG? - 06/29/12 07:32 AM
Was there a particular reason why Don set up your dist to not have full advance at your cruising RPMs?
Everything I've read indicates for a street car that's the thing to do, and could definitely be related to your MPG woes...
Posted By: Secret Chimp
Re: Where is my dang MPG? - 06/29/12 03:57 PM
I have no idea. Which spring combo should I try? Some say 925bs bring it all in by under 1500... too quick I'm assuming.
I also need to stop late night drunken ruminations about this car ;P
Posted By: Rug_Trucker
Re: Where is my dang MPG? - 06/29/12 04:24 PM
