Greetings everybody...

It has been a LONG time since I posted. All is well and the highboy is still running. However what with the unpredictable price of gasoline I am having some thoughts about my engine.

I currently have a 429 4BBL and a C-6 auto in a lifted 77 Highboy. I am running stock 4.11 gears and 35x13.00 tires. Needless to say ....it is a gas hog. It gets about 7-8 mpg most the time loaded or not...up hill or down with or with out a tail wind. TONS of power but hard on the wallet. Seems like filling the tank is like taking a stack of $20's and then

I am seriously contemplating swapping the 429 in favor of a 300 straight 6. Now this poses some questions. I know that the common swaps are the "other" way round...large engines in place of the 6 banger. So I am assuming the following things:

The radiator from the v-8 will be big enough for sure I would imagine

The 77 vintage auto trans should bolt up to the 6 banger right?

The engine stands and mounts will be wrong. I will need different (taller or rather more inboard stands) mounts?

I will need a complete parts truck engine to get the fan, power steering pump / brackets, alternator and mounts.

Will there be issues with grafting this power steering box to the 6 cylinder or should I just use the power steering with with the doner engine and adapt the hoses? Keep in mind that I have power assist with the shuttle valve and hydraulic ram.

I can use flex hoses to universally plumb the radiator...or just get molded hoses for a 77 with a straight six?

I am guessing that the straight six should double my mileage to about 14-16...I hope hope hope

Can I get that kind of mileage with a stock exhaust and carb or will I need to run a header and aluminum intake / aftermarket carb

What vintage engine would a guy want to get..? Early like 69-72 ish? I thought for some reason they were higher HP / torque. But I really want to keep my electronic ignition which poses another issue of engine wiring and duraspark box. I will need a new spark box?

If I do this swap, what would you guys assume the value of the 429 would be? Would it be of interest to rebuilders or should I just try to craigslist it for a few hundred bucks?

Thanks guys for any help you could offer here.

Cheers
Mac.

 

The 300 uses a "small block" bellhousing pattern and will not mate to your current transmission. Personally I don't think the swap is worth it. Sure, the mileage will improve some, but it's not enough in my opinion to go through all this work, and you might be unhappy with the drop in power. Don't get me wrong, the 300 is an excellent motor, but if all you're really looking for is better gas mileage, your time and money is much better invested in picking up a compact pickup or an econobox as a daily driver. Just my 2 cents...welcome back.

 

I think that the earlier 300s do have more HP. I looked on Wikipedia or something one time looking for stuff about the 300s. I think I remember that the older 300s the HP was around 170. mine factory was rated at 120.

 

Thanks for the reply...

I did go get a more economy 4x4. I bought a 4cylinder Jeep Wrangler. It gets good mileage (22-24) but it is just not as nice a ride as my Highboy. I miss driving the beast daily. I hadn't thought about the trans to bell housing issue. Yeah...that would be a bummer having to get into a trans swap too.

Thanks
Mac.

 

I too am thinking about doing the same "reverse'" swap for the same reason. The 300 six will be adequate for 99% of what needs to get done, and the other 1% we shouldn't be doing anyway. I will miss the thunder of the 460 for sure though.

For me though, I'm only going to do it when the 460 doesn't run no more. I never put a lot of miles on my trucks, because trucks are inherent gas hogs, so I would NEVER recover the cost of the conversion. (Another consideration for me is that the swap to the 460 was not done very well in the first place, so, I'm going to be reengineering any kind of swap anyway.)

And for you, I would suggest you do it only if you're gonna drive the truck for many years and for many more miles. The changeover price probably wont be covered for many miles unless the milage miraculously jumps to the low 20s with the six. And my dads 90 fuelie F150 2WD 4.9l with a 5spd only gets around 18mpg. Ours will be worse. (My truck is very similar to yours)

If you are gonna drive it for many more miles, then do the swap as soon as possible, to get the cost recovery off the books and into real money savings.

I'll take what I posted in green back. If the price of the conversion were $150, not an unreasonable estimation, the miles needed to recover the cost would be approximately 750! at $1.80 a gallon. One caveat; The math was done using your worst milage of 7mpg to your high hope of 16. The best case scenario.

Not many. Seems worth it to me.

 

So using the numbers from your dad's 90...it looks like the 6 banger carb engine will be making 12 to 14 mpg max in our "monster trucks"...

Probably not worth it then. I may look at trying to eeeek out any and all extra mileage I can from the 429. I may have to look at the timing gears to see what is up there. I don't know if they are retarded or not. I may have to look at an aluminum intake and electric fan. The combination of those things might gain me a mile or two. Getting it close to 10 or so. It would be nice to get 200 miles outta the tank of fuel. I don't know that the Edelbrock carb I am using will make great mileage. I may have get a Holley 750. The Edelbrock I have is a 600 cfm and it seems like the air flow signal from the big engine is enough to open the air flapper in it so it is flowing the secondaries too much.

Cheers
Mac.