I have read an awful lot of data either conflicting or confusing ( for me ) concerning the later slant exhaust manifolds.

I have seen first hand Super six engines in the salvage yard pilfered for parts only to leave the exhaust manifold behind cause the guy removing the parts said that all later manifolds are the same.

I have collected a few manifolds now and am doing some comparisons. Maybe you can see differences with the picture of the outlet diam.

These manifolds pictured above have exhaust outlet diameters in sizes ( from top to bottom )

1 15/16
2 3/16
1 15/16/2ish Closer to 2 depending on where measuring instrument is held, all of these manifolds have a certain degree of out of round
And lastly the one that is still attached to the head I just removed which reads in at 1 15/16

What gives with these variations of sizes? I have read that Super six had a larger exhaust outlet ( 2 1/4 ) I have yet to see this manifold and I have measured a few Super set-ups.

If there is a 2 1/4 outlet manifold than are the intake exhaust ports a larger size as well? I would assume not since there was not a special Super six head.

Can a smaller diam exhaust manifold ( one of the 1 15/16 jobs ) be opened up at the outlet end to the 2 1/4 size and then blended back up into the manifold.

I dont see why not but I have to question if the effort would be worth the outcome....evidently it must be if there truly is a special Super six manifold.

I ask these questions also cause the one manifold I have that measure 2 3/16 ish is in very poor shape, it looks like someone tried cutting off the studs with a heatwrench and scathed the collector mating surface pretty badly in two areas

Also this manifold is pretty badly warped which can be seen with the straightedge.

I have another manifold that is alot straighter that I would prefer to use.

According to Docs manifold instruction sheet 1/8 of a difference between first and last runner is about the maximum amount for a user. I am pushing those limits on my best manifold.

There is quite a bit of meat there Id like to know why I couldnt split the differences with my de-burring bits and make some of these manifolds more use-able. Can someone explain to me why this would not work?

i'm also curious. while not a lot of difference, I would think the bigger openings would flow better, esp with a 4bbl setup.

any thoughts about the new Dorman castings available? re: outlet size and quality? http://www.ebay.com/itm/Exhaust-Manifol ... 9a&vxp=mtr

_________________
1970 Barracuda 225 /6, 3spd on the floor, Blue/Blue

I think the difference is not in the manifold, but in the pipe used with Super Six cars. My opinion is that the /6 exhaust manifold has a very good design and would be considered "high performance" if the same design were used with a V8 manifold. They are almost "mini headers".

But there are clearly differences in outlet diam, ( and no its not carbon build-up on the walls) see post above

They aren't that good, in fact Mopar's HP manifolds for the Max Wedge engines would be exactly as described, but the manifold is better than the "other" brands log exhaust log manifolds found on their 6 cylinders.

Just as an FYI...I used the same engine in two different tests with the same drive train but changing the exhaust configuration netted a loss of 1.25 seconds in 1/4 mile by going from individual 2" dutra duals, to a single late stock manifold with the heat riser removed and the opening hogged out to 2 1/8" coupled to a 2 1/4" single pipe with a low restriction V-8 muffler. (16.53 to 17.78 )

Feather Duster and Super six cars got a 2 1/4" header pipe right off the manifold, unlike everything else, which was a noodle pipe.
It looks like more work than that, the throat looks like it's been worked with a die grinder, if it was a hot wrench, the edges would be slagged hard and not as uniform.

You can do the same thing as well, and use a long shank cylinder burr to open the outlet, or get a 2"+ cylinder hone and go to town on it.

Because it would not be truly uniform, I have used manifolds that were out of tolerances, but it required either a) a Remflex gasket if it wasn't too bad.... or b) bolt the stack together and take to the machine shop and have them put it on their table, it will be "flat" across all of the runners and ready to install when you pick it up later in the week (and you could get away with using the stockish gasket found over the counter).

-D.Idiot

I may have confused you, here is a better picture The red arrows at each end of the manifolds show that the manifold runners in the middle are lower cause the ends of the manifolds have warped a bit.

All of my manifolds have done this to some degree.

Can I or why cant I make the outlets ( marked in yellow ) a bit larger to match the outer runners. Im assuming that an exhaust port in the manifold that is a bit larger than the port leaving the head would not be an issue.

I am assuming that if this manifold were bolted to the head and with the two outer runners being curled a bit as they are that the middle runners may not line up with the middle exhaust ports in the head possibly causing a restriction.

Better yet why cant I bolt the intake and exhaust manifold together and then run my straightedge end to end finding again the two outer exhaust manifold ports and cleaning out everything in between whether it be intake manifold or exhaust manifold?











Ok so as far as the manifolds are concerned I guess I was confused, all the manifolds outlet ports are the same sizes ( the deviations I am seeing may be due to casting or ambitious previous owner ) and it was actually the collector of the manifold that was enlarged on the S.S cars and trucks.

Doc said in an article he had written ...........Check any used manifold for warpage, use the gasket as your "inspection tool". If the end exhaust ports are more then 1/8" higher then the intake port next to it, best find another manifold............

Ok my end exhaust ports are 1/8 higher ( on some of my manifolds ) . Why suggest to find another manifold? It would be easy enough to dremmel out the ports so that they would still match the exiting port on the head.


Just to clarify further, is there an issue with opening up all the exhaust ports just a bit so that as the manifold slides there is no chance of there being a step or ledge that the gases would have to go around as they exit the head at any given time during the heat cycle.

I understand that the manifold prob. moves/slides a very minute amount but I wouldnt be surprised that it was enough to make this ledge at some point ( especially with a warped exhaust manifold ) since the exhaust manifold exit openings ( in the manifold) do not appear any larger than the head exhaust ports.

I have not read any threads where opening up these manifold ports was common practice or recommended so that is why I am being cautious.

Have you done this with your own in the past?

EDIT: I know there are ( at least ) two different ways of matching the intake exhaust manifold ports to the head, one is called gasket matching and the other is port matching.

Not certain yet which one of these two I should be going for( maybe both ) ......port matching has me confused cause again why wouldnt we just open up intake ports at the head slightly so there would be no chance of a ledge incoming fuel would stumble on.

And

Exhaust ports ( on the manifold ) a bit larger as well, then the exhaust port on the head so that again no chance of fuel stumble as the exhaust manifold slides.

I hope this makes sense.

To re-cap hog out exhaust manifold ports a bit
Hog out head intake ports a bit
No chance of ledge

Dosent really answer the question but a bit more info http://www.slantsix.org/forum/viewtopic ... ed6e9bde8f

A.T. PMed me about this and I explained my reason why I have not given much info. about porting factory manifolds, it's because I never spend much time doing that. Here are my reasons:

A factory 1 bbl intake has way more port volume and passage flow then the 1 bbl carb. The port opening is not a restriction.

For best fuel vaporization and suspension, it is nice to have the intake manifold ports / runners smaller then the head port and valve pocket, that keeps the volocity up thru the runners.

Having a "step" on the floor of the intake, where it meets a larger intake port at the head, helps get liquid fuel that is running along the manifold floor, back into suspension. The mis-match acts like a "ski jump".

With factory exhaust manifolds, spending more then 30 minutes on one is a waste of my time. Reach-in with a burr and knock-out any "rough stuff", then break the sharp edges at the port openings. Spend more of your time on resurfacing and doing a careful installation.

We all know that the factory exhaust manifolds are limited by the simple fact that all 6 exhaust 'pulses' have to make their way out one, 2 inch opening. We also know that it is highly likely that the factory exhaust manifold will warp, crack and break sometime during the life of the car so keep this in mind as you spend time on it.

If I were changing a manifold set (or gasket) I would look at installing the lighter aluminum intake manifold or at least use a 'large hot-spot' cast iron intake. For MPG, make sure thr heat valve is working.

Install the manifold set as high on the head as possible and use a large exhaust pipe (2 1/4 works well)