Well, not exactly irrelevant, but the area between the inlet and outlet is still very important.
Case in point. Stock Mazda rotary exhaust manifold had 2" inlets, and a 2" outlet. Aftermarket headers have the same, but since the manifold resembled a 4" square tube with 2 holes in the side, and one at the end, the header allowed for 15-30 "Free" HP gain from an otherwise stock engine, even though it still utilized the stock exhaust pipe and muffler.
This particular exhaust manifold was designed to help lower the sound output, at the cost of flow. Adding the header resolved the flow issue, but tended to require an additional muffler be placed along the way. Which the header manufacturer addressed by making a header/presilencer combo that bolted directly in place of the stock manifold and all 3 cats, even retaining the factory hanger locations. $350 for these 2 items netted a 20-30 HP increase, and up to 10% better MPG, but forget passing any form of emission test.

 

You're right about the gain over EFI manifolds, but for those of us who live in country where it gets really cold, having a heated intake manifold is more important yet. The HD manifold is designed to work with the stock intake (not perfectly) better than the EFI can. I know, I know, you can rig up a water heated intake, but they're much less effective than good old exhaust heat at preventing carb icing.

BTW- over at the fordsix site they were discussing 3 vs. 4 bolt flanges- they are referring to the bolts that hold the intake and exhaust together. Lazy JW has a 4 bolt HD manifold.

 

Regarding the useability of this HD manifold say on and 80's 300 6?

From what I can read I can't find the complete answer to whether or not this currently produced manifold From Stevens will or will NOT bolt up to my OEM intake on my 86. Even the guy at Stevens wasn't sure. I have found that some of the Stevens manifolds have the 3 bolt config for the intake side which is needed.

I thought I would try to resurrect this old thread instead of starting a new one.

Anyway, if anyone has successfully used a Stevens 300 HD in a retrofit I'd like to know how that went, and also what if any exhaust flange parts were used...

Thanks for any feedback

 

The Stevens HD will work fine, it is 3 bolt where it attaches to the intake just like stock. You will have to have the exhaust changed, thats it.

 

I appreciate the input. Thanks...

 

I wonder, has anyone used the HD exhaust with an aftermarket 4bbl intake? I wonder if you could setup a stock exhaust heat system with that instead of the coolant plate.

Sam

 

"...I was thinking the same thing... Even with the .3 more exit hole I'd rather have the split manifolds. Just seems like they would perform better..."

Mike (an administrator) on ford6 ran bench tests, dynamo etc on the small sixes (lrg log head, small,2V, 1v, cross flow, each w/ & w/o port'n polish, etc). Has some nice comparison data between them all laid out in a chart he's made a 'sticky' of. Wonder if any 1 on the 300 forum over there compared the efi & HD for the 300? I hear the HD has more grunt. I understand from the literature it's "the scavenging effect" that really counts...

 

just saw one of these babies on ebay just yesterday...price seemed decent for a new piece. just a heads up fellas!

 

Hey FTE; Newbie - first reply here! I'm going to have to resurrect this thread for a 2nd time. I have a 79 f150 inline-6 and I would like to go with the Offenhauser intake and dig up an HD manifold, or purchase new if necessary.

After reading through various threads including this one here on FTE and fordsix's thread here I contacted Steven's about this HD exhaust manifold for the 300 and I was a little surprised by his response. He did confirm my initial question regarding the three hole flange for the intake/exhaust connection but he advised this manifold just "isn't supposed to work for my '79", "It's only going to work for the HD trucks through '74". He was a little exacerbated about it; apparently they get more than 50% of these manifolds returned to them because they do not fit the customer's engine. Who knows what the details were to each return but I assumed some modification was necessary as I know the application for this manifold goes through the '74 model year but seeing this has been done before by users on this forum I knew it worked somehow (and it's likely it worked on post-74 300s though I don't think I've seen the year of the engine specifically mentioned in all the threads I've viewed). Nevertheless, I would ask what kind of mod performed or what issue would need to be resolved to fit this manifold and this representative (I apologize, I didn't get his name) simply responded that this manifold just wouldn't work.

Anyway, my point is, what mods are necessary (besides changing my exhaust pipe connection) to fit this HD manifold to my block/intake? Grinding? Drilling? Unique Gaskets? In my searching, I've seen this set-up work, I just haven't come across fitment details... specifically for aftermarket intakes such as the Offy. The fact that he has a plus-50 percent return rate at Steven's tells me there may be something more going on than just fabbing a new exhaust pipe connection... but I could be wrong. Any replies would be much-appreciated!

-Mike

 

Hi veritas, welcome to the forums! That's some really interesting information. I'd never heard of issues before.

However, now that I think about it, I've had a few discussions with people with late 70s trucks who've had fitment issues with some of the exhaust stuff that is brainless and easy on the 80+ models. The constantly used Walker Y-Pipe that goes onto the EFI manifolds was a piece of cake for me, but a late 70s owner I talked to had to modify and work around some really tight spaces.

I wonder if the HD manifold just points it down too close to the frame on the late 70s models.

 

Thanks AB, those are some valid points to consider. The problem with that rep at Steven's is the fact that he didn't point to specific applications (whether they were mid-late '70's or '80s fords). I think he just reacted with my inquiry for my particular truck with a broad-brush statement that ordering a part not specifically called-out for my '79 is just asking for issues (which is true, in a sense but it's not the point). Who knows, maybe it does work for my vehicle and all the returns he experienced were from '80s applications; I just have no idea and he obviously wasn't in the mood to brainstorm about it. Unfortunately I don't have any concrete info to go on as all the times I've seen/heard this manifold installed, I didn't know the engine/truck's mfg. date.

So the search continues. I'll keep digging for something and just start PMing various persons on different forums that I know have worked with this issues in some way. Ideally, I'll find a manifold used or in a salvage yard which would provide a low-risk, (possibly) high-reward situation. Thanks AB, for jumping on a reply so soon! If I come across new info for this manifold, I'll mark this thread to remind myself to re-post.

 

U tried fordsix site? the 300 forum? I've used their help a good deal but for the falcon engines (which - this IS one but the manif. is for use w/that motor in 50,000 lb and larger trucks).
Is this one to hand off to Numbers Dummy? No, I guess not cuz it's an after mrkt piece...

 

Thanks chrlsful, I'm now on the fordsix site but could you direct me to the 300 forum you mentioned? I couldn't track that one down. I've tried to contact a few members on fordsix that seemed to have been in the thick of this mod but I don't believe I'm given that access until I post more. Anyway, an update; I did end up finding an HD exhaust and intake assembly at a local salvage yard that came out of an f-500 so things are looking up. The only downside is that their asking price for the assembly is as much as a new HD manifold so I still need to head to that yard and get a good look at the assembly to confirm whether it's worth the price or not. I'll update as I get further in the know!

 

FORDSIX PERFORMANCE • Index page

 

Happy 4th!!!

500 truck - Yes! that's the right one (5 & 700 'sizes').
H3 has provided a link that may B it, ck it out.
EDIT:
240-300 "Big Block" Six Performance
The Big Block of inline Ford sixes
Yup das it!. I C '250 Restorer" has a final post on a topic there. May B even PM him (" 'Chad' sent me ") he has written mag. articles on our 300 engines.
Like our guys here (I call this the 300 forum too) very helpful. I am learning there are at least 2 of these from reading this thread, AND very! similar widda fins & everything. I'm going to suggest pic 1st to ID the differences.
Good luck w/the research, due diligence 1st, then purchase. I like ur style...

BTW: there may have been a change on there of 5 posts to access more on ford6 (believe it's always been 25 to post a pic, or may B that's here?).