Some anecdotal notes on the 300 HD manifold

When David/1986F150six told me of his dual carb Clifford intake purchase, it was brought up once again that he would like to find the HD manifold to go with it. I have a trip to Verde valley planned and will try my favorite junkyards again, but I’ve been checking for several years without luck. The other alternative is a Chinese reproduction, but it is over $300 and some years back I read that there were complaints of it cracking... so I thought I’d wing it out there on Facebook and see who all had purchased it and what their experience was regarding quality.

One gentleman said he maintains a fleet of 25 F600’s for an airline. He said about half of them are running reproduction manifolds and they have been fine. I asked him the reason for replacement and he said some of the OEM ones cracked, others had the engine sent out for rebuilding and came back with pickup exhaust set ups. It is noteworthy, though, that OEM ones crack too. He said the quality of the repro units has also improved over the past 10 years.

Another gentleman mentioned that when this manifold is used with an aftermarket intake, there is a heat passage to the intake log that must be blocked. This passage is square on one side and round on the other making it difficult to plug with a threaded plug. It is prone to leaking if not welded, and welding almost always results in warp. When installed the warp creates stress that results in cracking (even when machined flat before installation). He said others have tried to bolt a plate over it, but the heat warps the plate and creates exhaust leaks. He opted for the split EFI with his aftermarket intake as he didn’t want to risk ruining the HD. His suggestion, if attempted, would be to bolt it to a spare cylinder head and weld it a little at a time. The other problem he cited is the head pipe flange and clamp are not available. He said he has a write up on how to make one, but I could not find it. I asked for a link, and I will try to share that here. Another member recommended having a V-band welded on.

Another member mentioned he tried headers with a 1bbl and had a weak bottom end power wise. He put the log exhaust back. When he upgraded to a 4bbl he said the top end was definitely choked, so he went back to headers. He had pretty even power throughout. But his gas mileage suffered worse than usual in the winter from the lack of heat control.

Several mentioned that the split manifold was a much easier install, with the Y pipe readily available off the shelf, while the HD requires a good deal of thought, measurement and fabrication on the exhaust. Some vehicles don’t have room for the EFI manifolds. HD is definitely a boon for those mounting a turbo.

That’s a basic summary of the responses so far. I will add anything else that comes up. Unfortunately the Facebook group is closed, so I can’t just share the link. Hopefully this will be useful to David and anyone else contemplating the HD exhaust upgrade.

All the factory manifolds be it the stock truck, the HD or the EFI manifolds they are all cast iron and from what I have heard over my years if you want to weld cast iron you need to heat it first.

I do like the "bolt it to a head" to try and keep it from warping but you would need to heat both the head & manifold as a unit because as soon as you bolt the hot manifold to the cold head it acts as a heat sink and pulls the heat from the manifold.

Remind me / us why use the HD over the stock truck manifold - pro / con as I don't remember now.
Wonder if anyone has done flow testing on the 3 types of manifolds and if so what did they come up with?
Dave ----

The Exit is much larger on the HD, and there is no butterfly. It is designed to allow the I-6 to run WOT all day as a work truck. Here is a thread on it, post #5 compares the exit surface area between LD, HD and EFI. The HD and EFI are extremely close but folks still claim the HD flows better than the math would suggest:

https://fordsix.com/viewtopic.php?t=56939

Don't the EFI manifolds split the exhaust into two sets of three cylinders, just like the Clifford intake does?  Seems like that would be a good way to go.

Gary Lewis wrote

Don't the EFI manifolds split the exhaust into two sets of three cylinders, just like the Clifford intake does?  Seems like that would be a good way to go.

It does... but I don’t think the Clifford intake is separated inside. I think it’s open all the way through. Also the EFI exhaust is a Y and joins together (at least in its stock form). I don’t know if it makes a big difference or not, but that’s the configuration.

The Clifford isn't separated, but that will only smooth things out a bit in the flow.  And, were I to build a 300 I'd want dual exhausts and not use the Y.

Inline 6 engines sound extremely flatulent when run with true dual exhaust. I have a inline and it has a single exhaust, with a y-pipe just after my Thrush glass-pack. It has the symmetrical look of dual exhaust with out the flatulent sound that I have heard on many six cylinder engines with true dual exhaust. In fact, it sounds more like a turbo style muffler than a glass-pack muffler.

So as promised I looked into the link regarding how to make a head pipe flange to fit the HD manifold since none are available commercially. It’s another closed group, so I will summarize. The member who did it is Kyle McCormick.

He used a 2.5” flanged down pipe, Jegs part number 30002. The pipe fits the outlet, but the flange bolt holes don’t line up. So he ordered a 2.5” split flange from Autozone. The brand is Nickson, part number 17333. He trimmed the gasket from the 30002 downpipe and used it. He sent some pictures via private message of the exhaust which he said required some creativity to route around the starter. He said the welds are very rough but it works nicely.









Note: his truck is a 1968, but ours would be similar.

Great detective work, Jonathan! Thanks for the effort to record.

Yes, thanks!