I'm just wondering whats the best MPG anyone has personally gotten from a 225 slant six. I've heard as high as 28 mpg, but I always hear it from
somebody "who knows somebody who knows somebody" kind of deal.

I've noticed so far pretty horrible MPG of about 13 is was I'm averaging. I finally got my motor running solid. I know think I may move on to going with a super six, as well as re-routing my fuel line (mostly for better starting). I also have a MSD 6AL and may consider trying to make that work.

Any suggestions? Low 20's would be great!!!

There was a thread on this board about 1 month ago regarding this topic. Search for it, it had some promising numbers in it.

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1965 Valiant wagon Turbo slant (work in progress)
2000 Chevy 155" cargo van - The Abductor
1970 Newport convertible
1996 Buick Roadmaster Wagon
1966 CruiseAire motor home
1990 Toyota 1 ton box truck TURBO slant (scraped)

hopefully Agressive Ted will chime in. He has a strong fun to drive mill and he's getting impressive MPG figures. He's using a single barrell and we're tuning it good for power and mileage, so far so good! He'll comment if he has the chance

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Juan Ignacio Caino

Please use e-mail button istead of PM'ing. I do log in sometimes but I'll be answering quicker thru e-mail.

Royceflo,

Some history for reference: I had a new 77 Volare with the Super Six and it got 24 mpg local driving and 28 mpg on trips.

It has been a little over 30,000 miles now since I rebuilt my SL6 and I am starting to fine tune it to where it is getting pretty good mileage. First of all Doc helped me put together a plan for a "torque" motor that could get good mileage. I had tried various rear ratios and like the 2.76 the best. I had Randy's Ring and pinion build me a 2.76 gear ratio 8 3/4 Posi since I tore up (3) 7 1/4 rear ends in 5 years. So I wanted a torque motor that would pull the tall gears. The SL6 has a huge bottom end due to the long stroke so I wanted to take advantage of it and with Doc's help we were able to make it work very well.

Juan is teaching me the ropes on tuning carbs and the results have been pretty amazing so far. I am almost up to the same mileage as my 77 Volare. I drive pretty aggressively so the worst it gets is 22.5 mpg, 23 to 24.5 when I back off and drive like I should. The motor has so much low end it's just too tempting sometimes just to have fun with it.

I drive 130 miles a day, stop and go, back roads which are two lane for half of my commute and a little less than 1/2 is freeway. The commutes run from 1.5 hours to 2.5 hours when traffic is bad. So lots of stop and go with occasional cruising at 65 to 70 mph. I do not drive easy, but rather aggressively so my mileage may be better on a trip where I am doing some steady driving.

So this is not the best mileage by far, but it is getting better each week as I modify the 1920 Holley single barrel carb. I plan to keep jetting up until the mileage levels off. As a result the overall power is very impressive for a SL6. I was running a #57 jet and am now up to a #62. I plan to try a#64 and a #66 in the weeks to come. With a #57 the engine showed great potential, but would fall on it's face do to lack of fuel flow. As I jet up the mileage and power has gone up at the same time so I am not pushing it as hard.

It would be fun to just make the swap to the Super Six, but I thought I would explore with Juan's guidance how far the one barrel can be pushed. The key that is making the difference was to unplug the low speed air bleed. Mine was capped with lead so the emulsion circuit was not doing much but dumping raw gas. Now that the emulsion circuit is working throttle response is very sensitive. Power has gone up and mileage has gone up.

Stay tuned! This is fun and I am taking good notes. I am trying to run at least 2 to 3 tanks per change.

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Aggressive Ted

http://cid-32f1e50ddb40a03c.photos.live ... %20Swinger


74 Swinger, 9.5 comp 254/.435 lift cam, 904, ram air, electric fans, 2.5" HP2 & FM70 ex, 1920 Holley#56jet, 2.76 8 3/4 Sure-Grip, 26" tires, 25+MPG

What kind of driving? That isn't too far off if you are doing 15-30 minutes of light to light in the big city traffic snarl....the more you do continuous highway commuting the better that will get...

-D.Idiot

D.idiot, I drive only about 20-25 miles a day to either work or the gym. I would say that even the majority of that is highway, and the other amount of driving is back roads, but there isn't in terms of traffic or lights on my route to work. I have a 66 fury with a twin turbo polysphere getting about the same gas mileage. As far as my thinking goes, that aint right.

Royceflo,

My old 57 Plymouth with the old Poly 318 would get 18 mpg most of the time. Occasionally it would drop down to 16 for true stop and go city driving. On trips it would get 20 mpg. My 68 Plymouth with a 318 would get 21 mpg on a regular basis.

Sounds like your carb may need some attention.

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Aggressive Ted

http://cid-32f1e50ddb40a03c.photos.live ... %20Swinger


74 Swinger, 9.5 comp 254/.435 lift cam, 904, ram air, electric fans, 2.5" HP2 & FM70 ex, 1920 Holley#56jet, 2.76 8 3/4 Sure-Grip, 26" tires, 25+MPG

Valve lash adjustment? When was the lash last checked?

Low cranking compression? Run a compression test and tell us what you find. (you should check the valve lash first)

Slow distributor advance? Search and read-up on distributor recurving. Have you tried cranking more initial timing advance into it?

Bad carburator? What type is it? What is it's history? How do the spark plugs look?

With some work, I bet you can get better mpg numbers.
DD

Adjusted valve lash to .012 and .022 on the intake exhaust last week. I double checked that I did the valves in proper order of e-i-e-i-e-i-i-e-i-e-i-e.

Compressions test was 120psi plus or minus 2 across the block. I have the exact numbers written down in my garage.

The carb is a one barrel 1920, I just rebuilt it last week as well with a GP sorenson kit, adjusted the acc pump lever to the center hole. I have no idea how old the carb is. Also, i've done a full tune up and 3 oil changes to flush all the sludge.

I rerouted the "osac?" valve that was once between the carb and distributor. I have it timed at 2degree below TDC as is. I should read up on the distributor curve as well as check my EGR system

What year is your SL6?

That timing is pretty low. Try at least 5 degrees before TDC if not 10 degrees. With out the smog control equipment (OSAC and EGR) I can run 16 degrees initial. Actually I can run 20 degrees with out pinging but I prefer the feel at 16 degrees.

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Aggressive Ted

http://cid-32f1e50ddb40a03c.photos.live ... %20Swinger


74 Swinger, 9.5 comp 254/.435 lift cam, 904, ram air, electric fans, 2.5" HP2 & FM70 ex, 1920 Holley#56jet, 2.76 8 3/4 Sure-Grip, 26" tires, 25+MPG

Ted, how do you read 16 degrees when the marks only go up to 10?

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

Good info... I would do as A.T. suggested and put more timing advance into it, to see if that helps. Reading-up and asking questions about distributor recurving will be a good next step.

The 1920 carb is "suspect" and may have poor fuel control, even after your rebuilding effort. One thing that drives me nuts is when carb rebuilders drill-out the main jet(s) and do not remark (grind-off) the original jet number.
Does your engine feel responsive to throttle opening (over rich) or is it sluggish to accelerate? (lean or slow distributor advance / to little total timing or faulty vacuum advance)
Have you confirmed that the vacuum advance system is working correctly?
DD

MichaelS,

The easy ways to check is with a dial back timing light.

A poor man's way is to rotate the damper to 10 degrees and file a notch in it at zero, then turn it again to 10 degrees and file another notch and so on.......
Paint the notches with white paint and you can use a regular timing light.
I rotate the the distributor until is reads 16 degrees on the second notch and lock it down.

You can check total timing that way if you file in 4 notches at 10 degrees apart from TDC. I see 52 degrees total at 2500 rpm.

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Aggressive Ted

http://cid-32f1e50ddb40a03c.photos.live ... %20Swinger


74 Swinger, 9.5 comp 254/.435 lift cam, 904, ram air, electric fans, 2.5" HP2 & FM70 ex, 1920 Holley#56jet, 2.76 8 3/4 Sure-Grip, 26" tires, 25+MPG

Ted, My 318 polysphere used to get around 20mpg in my C-body fury II. I get about 10-14 now, but i have two garret T04B turbos running about 10lbs of boost, and it doesnt help much on gas.

At anyrate, I outright replaced the vacuum pot on the distributor today, I was surprised to find that napa got me one same day delivery. We'll see if that does anything to help at all.

I think i've made up my mind to just go ahead and go with a super-six setup. It seems every mechanic, or carb guy around my town has told me that they feel the holley 1920 is garbage. Since I have intake/exhaust manifold from an old pickup I will go and try to find the carb, kick down, and throttle cable for the super-six.

I hear the old carbs of the 2barrel 318s work okay. Any suggestions?
Also, just curious, but would the stock exhaust work with a super-six intake, and if not...why?

Thanks alot

Stock exhaust will work with the Super Six, the difference is in the intake manifold and kick down linkage used for the BBD carb. Choke is different too.

Those turbos have quite an effect on mileage!

I run the newer style 1920 Holley carb and they are really nice carbs and super easy to tune. The Economaster version is especially nice. Mine is a 12R-7610 for 1968 to 1972. I have four 1920's and like the Economaster the best.

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Aggressive Ted

http://cid-32f1e50ddb40a03c.photos.live ... %20Swinger


74 Swinger, 9.5 comp 254/.435 lift cam, 904, ram air, electric fans, 2.5" HP2 & FM70 ex, 1920 Holley#56jet, 2.76 8 3/4 Sure-Grip, 26" tires, 25+MPG