Need Overdrive Trans - Any such Beast???

[bad 68]

Ok, I'll chime in and you all can tell me i'm full of crap, but, I can get 16 strait hiway driving at 60. I only get about 13-14 mixed hiway and town driving. About 10 mpg off road driving. About 7 trail riding. I calculate hiway numbers by actual State hiway distance markers not my spedo.

Here's my combo:

351W GT40 iron heads
9.6 comp.
A331 motorsport cam
long tube headers with dual 2.5" exhaust, Edelbrock RPM mufflers
Edel Performer intake
MSD 6al ignition, Vacume advance distributor. 10 degrees base timing
Holley TA 670 with 60 mains, 6.0 powervalve and Black secondary spring
NV3550
4.10 gears
285-75-16 BFG KM2s
Hard top
3.5 SP lift, no body lift
about 2100 RPM at 60
This engine is torquey at 2000 RPM. really starts to pull at 2800 RPM.

 

[Idaho Slim]

OK, have been keeping track of numbers etc., took a long road trip for a Predator hunt this weekend, and had a couple legs that was easy to maintain some constant speeds, this is what I experienced; edited the list to compare;


32.5 inch diamter BFG's;

60 mph = 2600 rpm = 14.2 mpg ..... w/35's: 60 mph = 2450 rpm & 14.6 mpg

65 mph = 2800 rpm = 13.4 mpg ..... w/35's: 65 mph = 2650 rpm & 13.5 mpg

70 mph = 3000 rpm = 12.1 mpg ..... w/35's: 70 mph = 2850 rpm & 11.6 mpg

The readings were at OPTIMUM conditions, it was disappointing to only drop 150 rpm, but I think it is a positive trend, note that the higher rpm at 70 the mileage dropped, that is a no-brainer, the strong motor easily tuirns the larger tires at that speed, but as a hit to the mileage (Gotta feed the Beast ;D ) But the trend at lower rpm's is very encouraging, exactly what I was hoping. The engine runs so much easier at lower R's (duh) that I defientley will look to swap in an overdrive this spring. Too damn cold to mess with it now.

I knew that when combined with the bigger 35's using the combo of the stronger motor that mileage will suffer under hard use and any times of hard acceleration and getting frisky, which it does, but I had a great combined mielage of 12 mpg for a 190 miles of wheeling thru 2-3 feet of snow and hwy travel, with a off road/ hwy split of 30/70. To be continued this spring .............

oh, forgot ............ having this much power also has it's drawback's, while scrabbling up a hill covered with 2 foot of snow , I snapped a front axle when the aggressive BFG found frozen ground, ugh.

 

[Idaho Slim]

Bad 68, I see we have a similar combo, I am having issues with my TA starting in extrme cold, and the powervalve keeps blowing out, any advice?? I am seriously thinking of dumping this carb, Love/Hate relationship.

 

[broncnaz]

Your biggest overall issue is that going with a OD tranny will drop the RPM's way lower(more like 700 RPM) than the current 150rpm due to tire height differance. if lower RPM's is all it took you'd get 20mpg just putting around the hills. For the money involved with a OD and axle gearing swap I think your going to be dissapointed as I doubt you'll really see any differance from what you have now. getting almost 15mpg on the hiway with a 351 is pretty good i dont see you getting much more.

You really shouldnt be blowing power valves on your carb they have a check valve to help prevent that. But if you are blowing them then that tell me its back fires through the intake. Which means to much timing probably also why its hard to start.

 

[Idaho Slim]

You really shouldnt be blowing power valves on your carb they have a check valve to help prevent that. But if you are blowing them then that tell me its back fires through the intake. Which means to much timing probably also why its hard to start.

hmmmmm, that is exactly what is going on. Backfires when it is cold (under 5 degrees) and "fart kicks' but does not start. I know the more timing the harder it is to turn the motor when it is hot, same hold true when super cold???

So generally speaking, is this Holley Truck Avenger really the ticket for an "off road" carb?? It is one of the first models out when I bought it, what 8 - 10 years ago? Are the newer ones changed / different?

I am going to knock the timing back (16 now, will try 10) this afternoon, thanx.

 

[broncnaz]

Yeah 16 degree initial is a bit high. Some people can run it though. Depends on how the engine is built and elevation. I can usually only run about 12. and yes more timing makes cold starting harder as well. I think most of the newer holley carbs come with blow out protection. Your should have it. I havent personally runa TA carb I've had very good luck with just standard holley's although I usally do modify them a little kinda similar to TA's but I was doing it way before TA's came along all they really have thats different is check valves in the metering block.

 

[Bronk]

12 degrees is the intial timing there is a adjustment allen screw in the vacume advance that be adjusted i think it's used to get the knock out at cruising speed but i've never had to play with it

 

[Idaho Slim]

Update

Haven't been real happy with this carburator's performance, so had it really gone over. Fuel was in the oil causing excessively burning of oil (1 qt. every 200 milles) for this new of a motor (< 10,000 miles.). Found it was WAAYY over jetted in the secondary's from the day i bought it, adjusted the floats and another overall rebuild. Anyone with a Holley TruckAvenger may want to check their jets, huge 88 secondary jets for a 351 motor and a 670cc carb. Also re-checked timing @ 12 degree's, the results; an increase of a solid 1.5 mpg hwy, 15.2 @ 65. Might not have to install the 5 speed O/D after all ........ but maybe will anyways cuz it would be soooo kewl. ;D;D

 

[Annapolis76EB]

I think 16 is possible. My rebuilt 96 Explorer 5.0 motor with stock 91 Mustang cam and Edelbrock carb is getting 15.8 and I am fighting a miss still. I think when I finally get this thing tuned right I will crack 16. I am running a C4 with 33's.

 

[OklahomaBroncoB]

My '73 has a 351 with Edelbrock 4bbl and original C4 and I average around 14.7 in town. pretty new to all of this but I have heard of hydrogen kits that can increase mpg by 65%, if I do this I will post pics and kit info.

 

[lowbush]

Pfft. In 35 years, 9 pickups, 3 full size Broncos and 11 EBs..professional engine builder of both stock and HP engines for a wide range of applications..offroad racer for 7 years (one of the 3 racers an EB) yadda yadda..;-)

Pfft. I'm gonna pull at least low 20's out of mine and still keep a few ponies under the hood. I have a secret that will rattle your teeth out. Should be a minor improvement over the 347 Stroker in there now.