478 Diesel vs Gas?
 

It's a way long term project (behind many others), but I have a 67 c20 with a 250 inline 6 cylinder. But, I have always had an interest in the GMC v6 engines. Out of the blue, I came across someone selling a 478 gas out of a uni. The last line in his as said also have a diesel. Long story short, I ended up coming home with both a 478 gas and diesel, both out of unis. I always thought the toroflows were interesting as well, just never thought I'd find one reasonable.

The question is, which one would you put in the truck? I think it'll be roughly the same amount of work to make either work. The gas would be really fun just because they are so torquey. But the diesel would be great for fuel mileage (and it smells better).

There is a lot of great information on this site for both the engines and tips on the swap. I figured there would be some good opinions on this too.

Thanks!

Re: 478 Diesel vs Gas?
 

great find:thumbsup:

Re: 478 Diesel vs Gas?
 

Looking at the 478, it appears to have a smaller carburetor as well as manifold of a 305 or 351C engine, what year is it? It may actually be a non Magnum version with smaller port heads. Power wise you will definitely be better off with the gas 478, as far as the Diesel version myself I would only do a conversion to one if it were a twin turbocharged unit that GMC used in some specific applications. I have done the 478M conversion on my 68 GMC K2500 and can help you with any questions regarding this conversion.

Re: 478 Diesel vs Gas?
 

It's got the rear exit industrial exhaust manifolds too.

Re: 478 Diesel vs Gas?
 

Out of a New Idea No.702 Uni System may explain those style exhaust manifolds:thumbsup:

Re: 478 Diesel vs Gas?
 

For reliability, I would go with the Gas.

For uniqueness, the Diesel.

But Toro-Flows, as much as I like Diesels, were problematic.

Re: 478 Diesel vs Gas?
 

Not to mention Toro-Flow parts are pretty much extinct these days. I would consider running the Toro-Flow, but I would want to tear it down and check it out before installing it. And that itself is a problem because even Toro-Flow gaskets are hard to find.

Re: 478 Diesel vs Gas?
 

Toroflow head gaskets are a big problem to find. I have an ACAD drawing of the gasket which I sent to a water jet cut shop. I found the soft copper needed to make a copper gasket. The shop quoted about $30 to cut two gaskets if I supplied the copper sheets. We have one that needs a gasket but we have not gotten around to having them made. We liked the motor, got great fuel mileage. I have a 478 gas in a Massey Ferguson 1100 farm tractor. What really helped it was to install an MSD ignition system. You can choose an MSD 6 or 6AL box. The MSD 6 does not have a rev limiter where the MSD6AL has a place to plug in a little chip that limits the maximum rpm. Various maximum rpm chips are available. Starts better and does not foul the plugs. But then we always open up the carb main jets a bit and I stretch the power jet rod spring a bit to make it come in sooner or the motors do not run right.This is because of the alcohol mix fuel we get nowadays. One has to put more of it in the air/fuel mix to get the same performance of the old good gas these trucks are designed to run on. I quit using the alcohol blend fuel in any older vehicles and farm equipment and even small engines. I buy the 90 octane alcohol free fuel. Yes it is more pricey but we had so many fuel system components destroyed by the alcohol causing time consuming and costly repairs that we consider it well worth it. And then if alcohol blend sits in a fuel tank too long it will phase separate and you will have to drain it out and put in new. The stuff is total crap in my view!! Thanks EPA!!! That is true for the bio diesel now also. It can be destructive.

Re: 478 Diesel vs Gas?
 

Sorry for the delay guys. I appreciate all the responses!

Bigblockv6, is yours the blue 68 I’ve seen on youtube with the 478 in it? The high speed run makes it sound almost Detroit diesely. That’s a nice truck!
I don’t know what year these engines are. I assumed the intake setup would be different on the unis. The uni is meant to run at a constant high speed. I am sure they tuned the engine to run there for economy and power, and wouldn’t need the range of speed or power like a truck would. It would definitely be another expense to using the gas engine vs the diesel.

I have read about over speed problem on the diesel (which the uni wouldn’t have had). I also heard there were injection pump problems, but it looks like it’s an odd ball version P pump, so hopefully that isn’t a huge deal. I also read about head gaskets, but somewhere I saw that if you kept checking torque on them, they were likely fine. Although, massey 478, I like your solution! If I do end up going with the diesel, I’d love to talk with you about having a set cut for mine as well.

massey478, did they use the 478 in Massey tractors, or is that your own creation? I also hobby farm on the side and thought whatever didn’t go in the truck might be fun to put into a tractor. My other thought was if I found someone who wanted one or the other, that would make my decision easier as well.

Thanks again!

Re: 478 Diesel vs Gas?
 

let me put my .02 in here. If I had both gas and diesel 478's side by side, I would go with the gas engine, hands down. If for nothing but the fact that the TORO-FLOW crankshaft MUST be rehardened if you resize the shaft to an undersize set of bearings. The Am. Bsh. fuel inj. pump can be problematic to repair due to availability of repair parts, and the general need to run only SAE 40 series III oil in the engine(DDA 2-cycle eng. oil specs). Keep the gas engine and sell the TORO-FLOW to someone for parts.

Re: 478 Diesel vs Gas?
 

I have some updating since it’s been a while. I just came across someone selling a 305 out of a pickup for $250. It’s stuck, but thought it might be useful for my swap. The bellhousing in particular was of interest to me since mine are out of unis with a sae housing set up for hydrostats.

I found that you need a 16” flywheel to fit in the truck bellhousing. Then, I noticed my flywheels are different sizes between the gas and diesel. The gas one is a 16” flywheel. Then, I went looking for the stamping on the gas and see it’s a 401, not a 478. I think this might be good in that it doesn’t require an externally balanced flywheel and I could swap the whole setup over from the 305 to the 401 without looking for a balanced flywheel. Still a big power upgrade from my current 250. But, not a 478.

I know in my reading that the 478 gas requires a balanced flywheel. Does anyone know if the 478 toroflow does as well?

Thanks!

Re: 478 Diesel vs Gas?
 

Good question! The 478 Toroflow had a balance shaft in the left block skirt and a large harmonic balancer, but I am not sure about the flywheel. I will check some old shop manuals I have.

Re: 478 Diesel vs Gas?
 

I know in my reading that the 478 gas requires a balanced flywheel. Does anyone know if the 478 toroflow does as well?

Thanks![/QUOTE]

A shop that I worked at did a conversion like the one you're talking about. Basically started by someone else giving up on the project. My boss told me he took over the conversion in the early 70's, the ToroFlow was replaced with a brand new 401M in a School bus. When he received it there was the ToroFlow flywheel attached to the 401M and the engine could not run above idle so he had to get the correct flywheel for the 401M. So yes there is balancing on the Diesel version flywheel, Does your 401M have the 6 bolt pattern crank flange or 8 bolt pattern? If you have the 6 bolt pattern the 305 flywheel will work. When I did my 478M conversion I had a custom flywheel built from Tilton Engineering, the basically made me a zero balance flywheel so when the bottom end was balanced considerable weight was added to the flywheel.

Re: 478 Diesel vs Gas?
 

Thank you for the replies. I didn’t know what was behind the hydrostat adapters on the flywheels, so I took them off yesterday. The cast center of the hydrostat adapters have balance drill holes in both of them, but I assume that is for the coupler itself.

The 401 has a 6 bolt flywheel, the toroflow has an 8 bolt. I didn’t take the flywheels off yet. I assume if there was any balancing work on them to match the engine, it would be on the back side, so I can’t tell for sure yet. Attached are some pictures in case anyone else gets curious about uni engines as well.

As much as I’d like to use the diesel, it sounds like the 401 would be a pretty easy swap if I buy that stuck 305 for the bellhousing/ flywheel/ clutch assembly.

Another question, you referred to the 401 as a 401M. I don’t have an M stamped on my block. Were all 401 engines magnums, and they just didn’t stamp them?

Re: 478 Diesel vs Gas?
 

Your 401 then is a 1960-65 if there is no M stamped. Magnum engines were introduced in 1966. Only replacement 401 engines for older trucks may have not been Magnums after that. You should be good on the 401 conversion provided you have a 305 flywheel and bellhousing from a pickup truck.