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optimum cruise rpm? https://www.slantsix.org/forum/viewtopic.php?t=28623	Page 1 of 2

Author:	tophat [ Sun Apr 27, 2008 4:57 pm ]
Post subject:	optimum cruise rpm?
I am wondering what optimum cruise rpm would be for a 225 with about 9.2:1 compression, super six with a bbd 2bbl, mopar electronic ignition, 2.25" exhaust, otherwise it is stock. The car is a 63 Dart. Right now I turn 2000 rpm at 55 mph and get 19mpg, or 2400 rpm at 65 mph and get about 17 mpg. My main goal it to get better fuel economy. TopHat

Author:	70valiant [ Sun Apr 27, 2008 5:07 pm ]
Post subject:
something doesn't sound right there. I get 17.8mpg cruising at 70 mph turning 3100rpm. When I had 2.76's in the rear, a BBD, headers, and a direct drive fan I would get 20-21mpg at 70mph at around 2600rpm. Perhaps your vacuum advance is not set correctly?

Author:	emsvitil [ Sun Apr 27, 2008 5:19 pm ]
Post subject:
Optimum cruise rpm is theoretically at peak torque rpm

Author:	tophat [ Sun Apr 27, 2008 5:33 pm ]
Post subject:
I have tested my vacuum advance and the can holds vacuum, I think it is a 9R? can. It is adjustable, and adjusted for max advance at 15", at cruise I am getting 15" to 17" of port vacuum at cruise, and 13" to 15" of manifold vacuum. Theoretically, what would be peak torque on this engine? As best I can tell the cam is a stock 1968. The head is port matched and the exhaust runners are mildly polished. TopHat

Author:	DusterIdiot [ Sun Apr 27, 2008 5:49 pm ]
Post subject:	yep, but...
Quote: I have tested my vacuum advance and the can holds vacuum, I think it is a 9R? can. It is adjustable, and adjusted for max advance at 15 I would pull the distributor and confirm (the vacc. can advance arm is visible without disassembly...)... mech governors are stamped 9R,11R,13R,15L... and vac.. cans are varied, but common OEM versions are 6.5,8.5,10,11... Your rpms and cruise Vacc. readings look correct, everything is very 'subjective'... I would take a hand pump and a dial back timing light and pump down the vacc. can to 15" and turn the idle screw in to about 2300 and see what your advance is mech+vacc+initial... then drop out the vacc. reading and see what mech + vacc. is... best 'mileage' is gained in the high 40 to no more than 52 degrees of advance at your readings (without pinging, or signs of detonation).... (if your vacc. advance is limiting you to only 17 degrees of advance during cruise and the total is about 47, you could swap in one of the other cans VC-239? and see if you can dial in a few more degrees and get another mile or two out of it...and/or you may need to swap out the light spring for a lighter one and get the mech advance to limit out a bit sooner to get that extra 1-2 degrees you need, but not have the weight stops bounce off the end oof the governor slots...) Good luck, -d.Idiot

Author:	CrAlt [ Sun Apr 27, 2008 5:56 pm ]
Post subject:
high 40's? Wow.. i think im only in the high 30's.. Whats your initial timing at?

Author:	Dart270 [ Sun Apr 27, 2008 7:16 pm ]
Post subject:
Vac advance can make a big difference. Another easy way to check is just unplug vac adv and check full mech advance - should be around 30. Then, plug vac adv into a straight manifold vacuum port and check total advance at cruise RPM - say 2400. As DI said, you should be around 45-52 deg for best economy. Lou

Author:	CrAlt [ Sun Apr 27, 2008 7:37 pm ]
Post subject:
wow.. maybe theres some millage left in my old dog..

Author:	Aggressive Ted [ Sun Apr 27, 2008 8:18 pm ]
Post subject:
Tophat, You can get a lot better mileage. Mold old 77 Volare with the super six got 24 in town and 28 on trips. I am assuming your BBD has been rebuilt properly. My current car still has the 1920 Holley which is not running all that great at the moment however, combined city, back roads and freeway I am getting 22.5 mpg give or take .1 over the last 10 tanks, 300 gallons. Here is my set up: Distributor re-curved with a red primary spring and extra long looped secondary spring from a 400 distributor. 340 distributors are good to rob springs from too. Using a 15R governor with a 11R V8 vacuum advance screwed in 4 turns, (VC-208 11R can $10.26 at www.rockauto.com). Blue Streak cap, MO-3000 Echlin rotor, Chrome Blaster 2 coil and 8mm Blue Summit wires, NGK UR4/6630 plugs gapped at .040. Timing set at 16 degrees initial, 16 degrees mechanical and 20 degrees with the vacuum advance for a 52 degree total at 2500 rpm. For cruise at 50 mph I am 2000 rpm, at 60 mph I am 2500 rpm. On the freeway I am between 65 and 70 mph. I have about the same compression as your running, reground my cam to .435 lift, but advanced it 4 degrees, valves set at .012 and .022. The torque hit is right a 2000 rpm. If I am on wet pavement and hit the gas quickly it goes sideways in a heart beat. On dry pavement launches are nice and straight. Rear tires are P235 60's.

Author:	DusterIdiot [ Sun Apr 27, 2008 9:49 pm ]
Post subject:	Yep...
Quote: wow.. maybe theres some milage left in my old dog.. Yep... redial in your timing, then retune the carb to match the new advance... and see what happens.... -D.Idiot

Author:	tophat [ Tue Apr 29, 2008 7:30 pm ]
Post subject:
I pulled my distributor again tonight and changed some things around. I had my lightest fly weights in it, 56.2 and 56.3 grams, yellow light spring and a 7 coil heavy spring with a 0.28" loop. I found out mu vacuum can is a 6.5X and my governor looks like a 9L. I put in my heaviest weights, 57.9 and 58.1 grams, yellow light spring and 5 coil heavy spring with a 0.41" loop. I am going to run this for a day or two and see what happens. Would you guys suggest a change from a 2.76 rear gear which has me turning about 2600 rpm at 70. to a 3.23 gear which would have me turning about 3000 at 70. I know the 3.23 would make the car more drivable and "fun to drive" but I don't want to kill mileage with it. I have a 3.23 gear which is why I am thinking about changing to that. Thanks for the help so far. TopHat

Author:	DusterIdiot [ Wed Apr 30, 2008 7:23 am ]
Post subject:	Change this...
Quote: I found out mu vacuum can is a 6.5X and This is what's killing some of your mileage... 13 degrees of advance at vacc... (so say you set it up at 12BTDC and the 'stock' springs you have only alow you to have about 10 degrees of advance at 2800 rpm... you are getting 12+10+13 = 35 degrees of advance at highway speeds... great for drag racing not so good on the gas....) Rear gearing shouldn't matter too much as long as you aren't peeling onions off the line every stop light, it's the springs and the vacc. can that's doing the sparky shuffle... Try this: Get one of the 'long' vacc. cans (Standard VC-208 or 239) and dial that guy in to get about 20-22 degrees of advance under cruise vacc. Next get the GM Mr.Gasket Spring kit and replace your light spring with one of the silver springs in the kit... If all goes well, you should have 'good' street manners and good mileage.... -D.Idiot

Author:	tophat [ Wed Apr 30, 2008 8:05 pm ]
Post subject:
Yes DI my vac can is limiting me to about 10 degrees, I have about 32 degrees total advance right now. My long loop spring seems to not really do anything as the weights hit the ends governor slots before slack is taken up in the long loop. The yellow spring seems to be doing all the work. I seem to have the centrifugal advance all in by 1800 rpm or so. On 87 pump gas it seems to ping badly at anything but the lightest of throttle. Could my plugs be to hot? Are the NGK ZFR5N plugs good for detonation resistance? Would this be a goos candidate for water/alcohol injection to help cure detonation? TopHat

Author:	DusterIdiot [ Wed Apr 30, 2008 9:00 pm ]
Post subject:	Ummm...
Quote: I seem to have the centrifugal advance all in by 1800 rpm or so. On 87 pump gas it seems to ping badly at anything but the lightest of throttle. Could my plugs be to hot? Are the NGK ZFR5N plugs good for detonation resistance? Man, you must have some really light springs, is the advance really all in by 1800? (I mean you set the distributor at TDC, and with the 9R governor and no vacc. adv. you are at 18 BTDC on your timing light at 1800 rpm?)... If so, that's a bit too quick...you need to change the big spring to a shorter loop...then change the vacc. can to make up for some of that... The ZFRN plugs act a bit hotter in certain cases and don't supress detonation (if you're running higher compression, you may need to go back to the stock plug/'known' cooler plug and work up from there...)... Quote: Would this be a good candidate for water/alcohol injection to help cure detonation? Only if you're trying to heavy compression on the street, it'll be a bandaid for something else that needs more attention and fixed before going to something like that... -D.Idiot

Author:	Aggressive Ted [ Wed Apr 30, 2008 10:37 pm ]
Post subject:
Tophat, Save that distributor for a motor like Brennan's. Try and find a 77 or 78 Volare distributor with a 15L govenor to go with that Super Six set up. They were designed to work together. Your motor should be putting out about 200 foot pounds of torque at 2000 rpm which I find perfect for the 2.76 gear ratio. My motor has plenty of punch at that point and will not ping. DI is giving you good advice. I have set mine up to run water injection, but I only use it once in a while, maybe every two months.

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