366 / 427 help

[Nobighurry]

So I have a 1970 c60 with a 366, 4 speed with 2sp rear. Truck weighs 10800# empty with a gvwr of 24k. I can legally carry around 6.5 tons. Truck only has 34k miles. My problem is it falls flat on hills. I'm in mid-South Carolina, so I'm not talking mountains. Truck runs good, just powerless on hills. I try to avoid full loads just because of the weakness. I can drop from 53 mph to 25 mph on hills. It really isnt safe with that much of a drop.
I have changed it to electronic ignition, new edlebrock 650 carb, plugs,wires etc.
I found a new/remaned 427td but it's going to take $4700 to get it. If I knew the 427 would give me what I need (power wise) I wouldn't have a problem doing it.
I'm hoping someone here has experience with both engines. Any ideas?

[kwmech]

X-tra cubic inches is always better in large trucks. You'll never get to 88 though Joke aside, the 427 should keep you above 25, maybe 35 fully loaded, just keep an eye on the fuel gauge

[Nobighurry]

Yeah, I'm getting 4-5 mpg loaded and not much better empty. I use my truck but am havin b.c. to seriously consider parking it for something more capable and with better fuel consumption.

[kwmech]

For a few more grand there could be diesel conversion considered. A DT466 with an Allison 2500 would bring that truck alive

[Nobighurry]

Originally Posted by kwmech

For a few more grand there could be diesel conversion considered. A DT466 with an Allison 2500 would bring that truck alive

A diesel conversion was my original plan but obviously there's a lot of work there and not to mention the added weight. If I had a higher gvwr, it would be a no brainer, but a diesel could put me down another 800#s in payload. But, would be great for pulling the track loader.

No easy path... very frustrating as I want to keep this old truck doing what it was built for.

[Pvmt-Pndr]

You've done the cheap things aside from trying to bump the timing to see if there is room for improvement, I assume your spark plugs are gaped to .045 and not the .030 for points. Even if there is it's not going to be substantial. My truck is 8500 with a 427 that I just went through and empty on a flat road I'm not impressed (I maybe expecting too much, I also haven't pulled a load with it yet) in comparison to today's gas engine and compared to a diesel option there's 0 comparison. (My opinion).

What I have read a lot of is that a 427 will out pull the 366 in the hills/mountains any day of the week. It becomes more apparent when you do the same mods to did to the 366 to the 427.

Since you have a 4 speed I wonder if a 5 speed or even more gears than that would be helpful to keep that 366 in its peak rpm range. $4,700 seems like a large amount to put into something without guaranteed results.

[lexadmn]

Originally Posted by Nobighurry

So I have a 1970 c60 with a 366, 4 speed with 2sp rear. Truck weighs 10800# empty with a gvwr of 24k. I can legally carry around 6.5 tons. Truck only has 34k miles. My problem is it falls flat on hills. I'm in mid-South Carolina, so I'm not talking mountains. Truck runs good, just powerless on hills. I try to avoid full loads just because of the weakness. I can drop from 53 mph to 25 mph on hills. It really isnt safe with that much of a drop.
I have changed it to electronic ignition, new edlebrock 650 carb, plugs,wires etc.
I found a new/remaned 427td but it's going to take $4700 to get it. If I knew the 427 would give me what I need (power wise) I wouldn't have a problem doing it.
I'm hoping someone here has experience with both engines. Any ideas?

Nobighurry,
I cannot help with comparing engines as I have not driven a 427, but the performance of your truck does sound a bit sluggish.

What I can share with you, is the new carb you installed is not the best for your engine, for initial and maintaining torque. Much too large.

My 1976 C60 weighs 14K empty and has the 366/5-spd, and 2-spd rear with 8.85/6.50 gears.
Pretty gutless when I first got it and performed similar as yours. I live in the Ozarks with a lot of "hills" and it was a struggle to maintain any speed.
I went thru a very similar process as you - updating and repairing/replacing what was needed to help the truck become drivable.

I first performed a compression check and all cylinders were between 110-115.

I replaced the VERY worn distributor with a non-governed / non-vacuum advance electronic along with plugs and wires. Made a noticeable difference.

I replaced the carb as it had an Edelbrock of some flavor on it. The install was a bit of a mess and parts were missing. I ordered and installed a new 600 cfm holley (0-80451).
The C60 did run better, but did not like initial acceleration whether taking off from a stop, or even the initial accelerating at the bottom of a hill. The truck would "stutter" really hard and seemed to have no torque. The engine also seems to be idling rich on fuel.

Frustrated, I knew this was not acceptable, so I sat down at the computer and hit the web - and read, and read....

When I ordered the carb, I did not do any research. No reading, no asking, no calling. I just ordered from a vendor and installed.
I chose a 600 cfm because it was a familiar carb that I had used hundreds of years earlier on hotrods and it was popular then.

What I learned was - I failed to do any research prior to purchasing and bought the incorrect carb for a 366 bigblock.

I did finally go to holley's website and after much reading, and watching one of their videos, I found I needed nothing more than a 425 cfm carb after using one of their formulas.

The formula - cubic inches X maximum rpm, then, divide by 3456 = cfm.
366 (ci) x 4,000 (rpm) = 1,464,000 / 3456 = 423.6
I believed (and still do) I have no reason to go above 4,000 rpm, so that is the formula I used.

I have since bought and installed the holley 465 cfm (0-1848-2) and the performance has made me happy.
The truck will gain or sustain speed going up most hills with a 8,000 pound load.

KC

[C10 - C90 Bill]

If you need pulling power, I would consider a 478 V-6. Which should have been available if your truck was a GMC. The negative would be that parts are getting hard to find for them and they are known to be hard on gas. That said, my '62 478 was amazing. It had torque like a Mack. When grossed out at 32,000 it had no problem. It wouldn't win any race, but it would sure pull. Two other points are Super Tuning and Shifting technique. Not saying you aren't doing it right, but shifting technique is important. I drove more than rode, but there were many times I would ride shotgun and yell out to myself "Shift, Shift"!!! Most of the time I just sat there and didn't say anything. Also again, not saying that you don't, but if you have the gears, be sure to use them all. Super Tuning means getting everything you can out of an engine.

I had a 305 V-6 as well and got an unheard of 15 1/2 MPG Hiway. I also had a 292-6 that performed impressively. Unheard of fuel mileage as well.

No one would agree, but I would also consider a freshly built 292 for your truck. Myself, I wouldn't hesitate.

If I were to go with a Diesel, it would have to be a Detroit, lol.

[C10 - C90 Bill]

lexadmn, you posted 3 minutes before I did, lol, but good advise. I also would try to fix what I had before giving up on it.

[Nobighurry]

Thanks to both of you. I really don't want to give up on the old girl. She started life as a fire pumper and was pretty well taken care of. I've done a lot to it to get it reliable and running well (short of power). I've looked at the 4v53 and 6v53 option, along with the 6bt. All would be nice to have, but again, high expense to convert. I'm thinking the dt466 would be too big.

Unfortunately, if I can't find a reasonable resolution, she may become more of a weekend cruiser than a real worker. which I hate to do.

Truck is very strong off the line, all the way through 3rd and even 4 low, but once you hit 4 hi, it just falls flat. I will look at the carb situation, however, it doesn't feel like too much cfm, it feels like too little to keep the rpms up.

I'll do some digging on it.

thanks

[C10 - C90 Bill]

Also, let us know what kind of Exhaust you are running. I'm going to say that the truck has Duals, but let us know if it doesn't. Also important are Mufflers. Make sure to run Straight-Thru ones. Or as hi-performance as you can stand the sound. Part of the great mileage on my 292 was that I would run the shorter and smaller Straight-Thru GMC V-6 Mufflers, which seem to be no longer available. I have since switched to Cherry Bombs. Sounds great too, lol.

[Nobighurry]

Yes, true duals with flowmaster HP2s that exit just behind the cab, so about as straight and clean as you can get.

So maybe a good example for how it performs on hills would be like being in overdrive when you should be down a gear. I have tried down shifting from 4 to 3 but the rpms just rocket up. I can only turn about 3200 - 3400 and the truck is flat out screaming. I need to keep it right at 3000 but, it is almost impossible. Once it drops to 2700ish, all you can do is try to keep moving forward.

[C10 - C90 Bill]

Do you downshift in order? 4th Hi to 4th Low before going to 3rd Hi?

I'm also thinking Timing, if it's not a Carb problem.

Also, when adjusting the Carb, use a Vacuum Gauge and adjust it to the Highest Vacuum on the Low side (start with the Screws turned more in than out).

Would love to see the truck in action.

[Nobighurry]

Yes, on the down shifting. Sometimes the splitter gets hung up in between (makes clicking sound) and by the time you get it back in gear, you've lost so much momentum, you need to drop a whole gear. Maybe the splitter is my whole problem - as I talk through this...

[kwmech]

Originally Posted by Nobighurry

Yes, on the down shifting. Sometimes the splitter gets hung up in between (makes clicking sound) and by the time you get it back in gear, you've lost so much momentum, you need to drop a whole gear. Maybe the splitter is my whole problem - as I talk through this...

Could be also that previous owners did not know how to properly shift the rear and cracked or broke the anchor fork on the 2 speed section of the diff, which could cause the problems you are experiencing

[Nobighurry]

Thanks for the input. Just loaded it down for a couple runs tomorrow. Going to try the down shifting and see what happens.
Kwmech, I've been considering replacing the splitter just to make sure I'm good there. I totally rewired the truck and the PO had spliced things all over the place. Actually used multi strand speaker wire for the splitter. It was a mess.

[Pvmt-Pndr]

I'm even more curious to find out what is going on with your truck now because my 427 does the same thing as your 366. After I hit 4H/5L it also feels like it falls flat. Keep us posted on your results please.

[Nobighurry]

When I see trucks like this, knowing they hauled probably 15 - 20 ton, there is no reason my truck should be falling flat with a mere 2 ton load. Truth told, it falls flat whether it is 2 tons or 6 tons. Hardly a difference.

[C10 - C90 Bill]

If shifting from 4 Hi to 4 Lo and keeping your foot on the gas and just dump the clutch in and out quick and it doesn't downshift right away, then yes, there is something wrong with it.

[Nobighurry]

Thanks. I'll give that a try. I'm pretty sure I'm letting off the gas when changing the rear.

[Pvmt-Pndr]

Let off when you go from low to high but keep your foot in it when you go from high to low. You need the rpms to allow the gears to mesh.

[C10 - C90 Bill]

Just to add a little extra, when downshifting the Rear from 4 Hi to 4 Lo, hit the button down first, then dump the clutch in and out quick with your foot still on the gas as mentioned. When downshifting from 4 Lo to 3 Hi, pull the button up first, then complete your downshift as normal. The rear will shift itself during the stick shift. Always using the clutch will make things go smoother.