I'm planning on running a set of 2.25" Magnum manifolds on my 360 in my 75' D100 short bed. My question is, what kind of power will these support before you're basically restricting the motor. I want to run 2.5" dual exhaust with "X" pipe all the way to the back with good free flowing mufflers. I just don't want to deal with headers so this is the route I want to go and want to build my motor to suite. I was hoping a mild stroker would be okay. Anyone have any real world experience or success with Magnum manifolds?

http://www.moparts.org/Tech/Archive/exhaust/9.html

closest I can find

Supercuda, interesting... Those appear to be the smaller Magnum manifolds. So I might be okay with a 350-375HP stroker with the 2.25" Magnums...

Actually, I take that back. Looks like the headers opened up the motors potential where the manifolds were holding it back. Looks like the magnums would be equivalent to the pair of 340 manifolds. So about 325HP or so.

I'm running em on the stock car. No idea on power output but they are shaped well & you have a pair of the early ('92/early '93) large outlet ones. there is a youtube vid on porting em out with acid & the gain was significant.

I don't think its a matter of potential so much as it is an issue of how much are you losing over some decent headers. Even a stock 5.9 magnum shows huge gains (like 20hp)going from even the best manifolds to a TTI header.

You have the best manifolds but even they are not nearly as good at making power as headers. The more powerful the engine the bigger difference it will make.

If your asking what is the most power you can make with those restricting the engine I would say with enough cubes, cam, compression, good heads, carb, intake... you might be able to force 550hp thgough them. Of course that same engine is probably gonna make 650+ with decent headers.

I think 275-300 rwhp is about the limit you're gonna get with the magnum exhaust manifolds. I do know of a guy who did 430 rwhp, but that was with a LOT of spray.

as close of an apples to apples comparison between the 2, a Dakota with tuning and a performance intake, stock exhaust manifolds and a cat back system made 237 rwhp. same combo with mid-length spin-tech headers made 253 hp.

But, beyond that point, when one starts doing heads, cams, strokers, etc. the spin-techs really start to shine, or rather, highlight the restriction of the stock manifolds.

Of course, all this was on modern EFI Dakotas with Magnum heads too. The Magnum outflows most of the regular production LA heads out there.

Yes, that's why I picked them up because they have the most potential. I'll have to see if I can find that video you're talking about. Doubt I'd do that but I'd like to see it.

I hear what you're saying. I think I'm looking at it the wrong way. I need to just pick a HP goal with the magnum manifolds in mind and go that route. I understand headers are better, I just don't want to deal with all the hassles that come with them. Hence the manifolds.

with more thought I ain't sure if it was magnum manis but I had kinda thought that but in any case it was regular iron & the soak time wasn't too long & what impressed me was the cross section area they gained. EDIT lookup "Acid Porting Cast Iron Manifolds" & the vid is 3 minutes & 18 seconds long to confirm you have the right one. they said 16% more volume & I tried to post the address but it would not take.

The tests above were done with the manifolds having 2" pipes but the headers were run through the collectors with no mention of any pipes after the collectors. Bigger down pipes can change the manifold numbers, and restricting the headers after the collectors could change the header numbers as well.
The OP has larger exit manifolds, larger pipes, and is adding a crossover pipe. I don't see an issue with the exhaust causing a problem with a mild stroker. Sure, more power can be had with the headers, but all the header bs comes with them. You should be able to put together a pretty stout street small block with the Magnum heads and the exhaust to are planning. Have fun!

For years we ran LA heads with LA exhaust manifolds and big down pipes in our cast iron exhaust restricted class on the dirt track and we did pretty well in the class. We ran a pretty big cam with those LA manifolds. When we were finally able to use headers, that little LA really ran, but so did everyone else. Gene

Gene that sounds like you might be able to answer a Q I have with that, they just changed the rules from 2.5" ex pipes to 2". how much power might that cost us? Iron manis. big magnum on dr side/68-70 340 HP on pass side (both have ~2&5/16 outlets).

There are two things happening in the exhaust system and they are often confused.
1. Flow vs pressure drop
2. Pulse tuning

It is unfortunate but people get them confused time after time. The major reason full length headers are better than most exhaust manifolds is pulse tuning. A well-designed full length tube header adds torque below the torque peak as well as above.
Exhaust manifolds do not have long enough runners for the pulse tuning to work and therefore cost power.
Exhaust manifolds tend to have short, cramped runners. The high performance manifolds may have smoother bends and make an effort to connect runners together in such a way that interference is minimized. So, they can flow more exhaust. But even if they flow same as a long tube header, the long tube header will make more power.

There isn't any magic number of horsepower that is maximum for a particular exhaust setup. Look at AndyF's work with his 470 stroker. On this particular engine, running stock HP exhaust manifolds, he was able to get quite respectable hp numbers. In his case the MP 0.528 cam seemed to be optimum. You might look at the parameters of that cam as a starting point for your engine.

In the case of exhaust pipes being limited in size the H or X pipe is clearly beneficial. It lets the exhaust flow "see" both exhaust pipes.

For racing if the class allowed headers I'd sure try to use them, even if the rest of the pipe was 2". The benefits of the pulse tuning are too good to pass up.

R.

dogdays is right on, flow has a lot less to do with it than the pulse pull effect from headers. Heck you could run no exhaust after the port at all for the ultimate flow but it still wouldn't make the power that good headers would. This is why when that magazine article years ago compared all the manifold from 273 junk to 340 gold there was barely a difference in power from the best to worst but headers added about 25hp on a basically stock 5.9 magnum.

400 rwhp with stock 340 exhaust manifolds or thats what the calculators say. You just build more motor so you end up at the performance level you want while taking the loss.

I wonder why Jesse Lackman only tested one side of the Magnum manifolds but both sides of the 340 manifolds?

I remember Jesse Lackman did revealling measurements that factory Magnum rocker arm ratio is actually greater than 1.60 ratio.

His test of Singh grooves in quench flats showed only minor gains.

That Lackman dyno test mule 360 Magnum V8 has flat top pistons.

Have never seen a true 1 7/8 to 2 1/8 magnum manifold comparison but there are a couple of awesome dyno comparos of most exhaust options:

http://www.hotrod.com/articles/comparing-headers-and-manifolds/

and the best one
http://www.hotrod.com/articles/mopp-0307-mopar-crate-engine-exhaust/

Wondering what re-doing the second one would show if using a bigger cam. For reference, the magnum manifolds have been compared to the 68-70 340 hipo manifolds but never seen true numbers

" I just don't want to deal with headers so this is the route I want to go and want to build my motor to suite."

Can you clarify this point?

A truck has plenty of room to work in, I regularly see headers in the local classifieds for like 50 or 100 dollars, ground clearance is a non-issue, and matching up exhaust is easier.

I'm not understanding what is putting you off headers?

poorboy, that's great info. I've always wanted a stroker. I don't mind leaving a little HP on the table and not have to deal with header clearance and other issues. Can't wait to see the results.

dogdays, more great info. I didn't know that about pulse tuning. I agree there's no magic number, I think I was just looking at it the wrong way. I'll just see what I get when the combo goes together. I'm sure I'll be happy.

FurryStump, I'd be happy with 400HP at the flywheel, so I should be good lol!

360view, I thought that was weird too using the same side manifolds...Must have been some reason, I just don't know what.

jerseybud, I think that would be a neat comparison!

Grizzly, trucks seem like they have a ton of room, but not in the frame area. Drivers side you're fighting automatic shift linkage, trans kick down linkage and the frame. Manifolds eliminate all that plus I'll just about never have to touch them again. My truck is dropped about 5-6" and running 17's so it does sit low, another reason for more clearance.

http://www.mazda.com/en/innovation/technology/skyactiv/skyactiv-g/

Mazda says a Tr-Y header
with the tube length down to the 2nd Y join
of 600 mm (23.6 inches)
has several benefits

Back in 1994 Mike Leach made for Magnum 5.2/5.9 V8s TriY headers that Mopar Performance put their name on that had a combined tube length to the second Y close to this.

How much does this help?

In 2012 when Mazda could NOT fit this 4-2-1 header into the Mazda3 engine compartment space then available, but had the other SkyActiv changes on the engine,
static compression ratio in the USA sold model was 12 to 1.

In 2013 when they could fit the 4-2-1 header in,
static compression ratio was raised to 13 to 1.

Overseas where gasoline Octane rating of regular grade is higher than the USA AKI of 87, Mazda raises the static compression to 14 to 1

360view, thanks for the info. I'm not debating the benefits of headers. I know they're better. I was just wondering how much HP I should shoot for with my manifolds. Since asking the question I've changed my approach and have decided to build an engine with a HP goal and whatever the manifolds hold back is what it is...