...so I'm reviewing the pros and cons of running small block magnum exhaust manifolds in an A-body, and I am researching this very situation for my build now, and wanted to pose my ponderings, and to ask if there have been any fresh solutions to the three big issues.

I'm building a 71 Demon (based on a 73 Duster chassis). It will have a nice warm Magnum 318 with OE heads, an 833od 4-speed. It will have power steering, and hydroboost brakes.

As I understand from the wealth of info I've found...drivers side magnum exhaust manifolds have some significant interference issues with 1) a power steering gearbox, 2) the steering column, and 3) the z-bar.

Seems running manual steering resolves the gearbox interference problem, but some grinding is still necessary for the column.

Seems using a standard set of 340 manifolds will resolve all the issues, tho there will be a large disparity between the 340 and magnum exhaust port sizes.

Q1 - Would running 340 manifolds on a magnum head induce legit high-rpm flow problems...or is that not a 'thing'?

Q2 - Is there a proven rack and pinion steering conversion for the A-body's? I looked around and haven't yet found something that is a bolt-in and proven solution. I'm not opposed to going with an aftermarket K-frame and rack n pinion steering if that resolves the steering problems, but I don't want to do a full aftermarket front suspension setup in order to get r&p steering.

Q3 - Will using a hydraulic clutch resolve the 4-speed linkage problem?

...or just pony up the $ for 340 manifolds and fugetaboutit...

Thanks,
- Sam

I have run several sets of magnum mannifolds on A-bodies with no issue a couple minutes with a grinder wouldn't fix. All of them I can recall were automatics and at least one I am sure has power steering. They are cheaper and flow better especially if you find the 618 and 619 castings from 92-93. The brake booster is zero issue.

I'd love to believe a set of magnum manifolds would work on a magnum engine in an A-body but until your reply the interwebs have said it's a hard 'no'. Do you have pics of your install(s) and other tech on how you did it, because it would be a game-changer to find out how it can be done. Bottom line is I'd like to avoid having to grind anything, but still more intel is better than less.

This post has pretty detailed intel on all of the interference issues, and is where I got a lot of my initial intel - https://www.forabodiesonly.com/mopa...-an-a-body.109176/page-4#post-1974340834

...I'll probably just grab a set of 340 manifolds and just keep things simple...

I remember asking here on Moparts in 2004 if Magnum manifolds would work on an A body and at that time the speculation was the Driver side would be a bear. True Hi Po 340 manifolds are expensive. If they rival the cost of the TTI small block A body headers I would go with the TTI units. Here are the TTI shorty headers.
https://www.ttiexhaust.com/TTi34025.htm

You can run block hugger headers for the LA motors but you need to modify the port opening. Look up jeep magnum manifolds used in the ZJ's.

The common magnum truck manifolds for sure will not fit the driver side power steering A BODY.

The a body is designed to go up and over the power steering box, the magnums just crash right into the box.

Not even close.

Jeep starters are on the opposite side, if you use the drivers side one it will dump right directly on the starter.

...one of the other ponderings I'm pondering is just how much power would be 'prevented' if I simply ran 318 manifolds on my magnum 318...I mean really - how bad could it really be. I figure 318 manifolds are to a 273 what 340 manifolds are to a 318...at least an LA 318...but magnum exhaust ports are really quite a bit smaller compared to LA ports, and I'm just wondering if I'd ever notice any seat-of-the-pants difference running 318 manifolds on my mag 318 (with a nice 340 street cam), od 4-speed, and 3.23 rear gear. The car is being built to be a fun street/highway cruiser - not a track car - just a nice fun driver. Even if there was a power limitation...I'm not sure I'd ever notice.

I want the car to feel like my original one was, and it had a basic 318 with 360 heads, 4-bbl cam and intake/carb, 3.23 gears, and od 4-speed. It was so perfect - fun to drive but not so powerful as to get myself in trouble. I'm wondering how much power that LA 318 had compared to a stock magnum 318 (and OE heads) with a similar cam.

I remember a magazine article years ago they tested a 300 hp 360 magnum crate engine (factory 360 magnum with slightly smaller cam and M1 dual plane intake) and all the manifolds from the 273 318 340 and magnum all made right around 300 HP with maybe 10 HP from the best to the worst and the shorty headers were slightly better with the long tubes being the best by quite a bit (around 320 HP). Surely someone knows about the article... I would run whatever I have laying around unless I am building significantly more HP than that.

My interest in Magnum manifolds did not concern wide open throttle peak horsepower.

I was more concerned about all 8 cylinders exhausting about the same
so that on the part throttle intake stroke
all 8 cylinders would have nearly the same “balanced” air to fuel ratio.

My fuzzy memory is that the 1992 press release about the then new 5.2 Magnum V8 specifically said “new balanced flow exhaust manifolds.”

Then in 1994 Chrysler “choked up” to a smaller size the exhaust manifold exit diameters on both the 5.2/5.9 v8s.
My guess is that this was done to create more flow into the EGR valve to meet tougher NOx emissions limitations from 60% throttle to 90% throttle opening.
Remember that back in those days the “Congressman John Dingell passing power loophole” was still in the EPA law and EGR could be turned off above 90% open throttle.

When Dulcich did his many dyno horsepower tests on Magnum and other manifolds 0n a MP Crate V8 he “went the extra mile” and tested the two unique driver/passenger Magnum designs which are not symmetrical. Dulcich found one side to flow slightly better. This makes sense because the rearmost #7 and #8 cylinders seem to be “more challenged” in how they merge into the flow.

found his nonMagnum comparison, but not the other one....

https://www.motortrend.com/how-to/mopp-0307-mopar-crate-engine-exhaust/

really good info - thanks

I'm really beginning to think I'd be juuuust fine with a set of 318 A-body exhaust manifolds on my warm Magnum 318...and I happen to have a set still bolted to the LA 318 in my core '73 Duster...

Good discussions

So to recap, I'm building a fun 71 Demon. Dropping in a nice warm magnum 318, with a 4bbl and nice '340' cam, and an A-833od 4-speed, and 3.23 gears - just a nice clean-running hot street machine that cruises perfectly on the highway and idles with a bit of lope - probably a 5500 rpm limit. Looking at the Whiplash cams from Hughes. Could care less about the engine making 'maximum power' - all I care about is the engine making nice power without sacrificing streetability and don't want it to be overly smelly - no 108* cams, and no headers - want things OE-looking and OE-sealing. That said, here's some additional thoughts on suitable exhaust manifolds.

To me I basically have three options: 1) '02 Magnum manifolds, 2) 340 manifolds, and 3) 318 manifolds.

The magnum manifolds have significant physical interference issues with factory A-body power steering, steering columns (all), and z-bar linkage. I am probably going to run a hydraulic clutch setup which I think will cure the z-bar problem, but unless I can find a nice aftermarket K-frame setup with power rack and pinion steering I am not seriously considering magnum manifolds.

340 Manifolds would flow more than enough...but I don't really think I will 'need' 340 manifolds....and for darn sure I don't want to pay the huge $$$ for them. Maybe.

So...why not 318 manifolds. Check out the port sizes in the below pic. The 318 manifold ports match the magnum head exhaust ports perfectly - almost exact. That alone indicates to me that 318 manifolds 'match' magnum 318 head ports, and that has to count for something. 340 manifold ports are HUGE compared to magnum ports...which IMO also probably means the huge ports and internals are likely 'unnecessary'. I already have a set of 318 manifolds...and so I am seriously considering running them.

The car I'm cloning had a warm 318 with 360 heads, and I think I had 340 manifolds on it - I honestly don't remember, but they were either 318 or 340 manifolds, but definitely wasn't headers. The thing ran great. Assuming the same cam profiles, I'm inclined to think an LA 318 w/ 360 heads and 340 manifolds would run nearly identically as a similar magnum 318 with LA 318 manifolds, or at least so closely the same that I'd never notice the difference...at least not at/under 5000 rpm.

Meanwhile, I'm pulling the 318 manifolds off today and will measure the outlet port sizes, and I'll measure the 318 exhaust port sizes for comparison to the magnum ports. Hopefully the outlets are at least the same as the 1-7/8" magnum outlets. So, I offer these idea to the congregation for y'alls thoughts.

couple interesting thoughts from others in another parallel thread on For A bodies Only - the bottom line being there probably isn't much difference unless yer really racing or pushing an engine harrrrd -

https://www.forabodiesonly.com/mopa...-an-a-body.109176/page-4#post-1974344910

Sounds like the bottom line is there likely isn't much difference between regular and "HP" manifolds, at least not so much that anyone would really notice, with the exception being real racing where an engine is really being pushed hard and/or money is involved. That is a really interesting point, and it's one that is difficult to describe because, lets face it, most conversations about muscle cars revolve around 'power!' but not much around just a nice warm driver, and that is what I'm building - fun enough to enjoy the 'V8 thing', but not powerful enough to get busted all the time for chirping tires - just a nice 'reminder' of the musclecar era without all the anxiety .

I've also noticed it is getting difficult to find a machine shop to discuss a 'stock' engine rebuild - most seem to want to go for maximum power (for sure when considering cams), but fortunately one of my local machine shops takes the time to listen and understands that some folks just want a nice stock-ish rebuild. As far as cams, I can say that the cam I got from Comp for my 360 is juuust right - it's right on the edge of loping and should have real good midrange improvement - it's their #20-745-9; 212-218 (264/270), .480/.480, 114* LSA, with matching springs. With my otherwise stock mag 360 it seems to work really well for my 'truck' application. With the Demon tho I want a big more cam - I want that nice musclecar lope but I don't want the big overlap and have a bunch of unburnt fuel dripping out the exhaust pipes, so for my 318 I'll be looking at something with a higher LSA like the one in the 360, and like the cam above if the exhaust duration is increased a bit I'll be that much further towards not 'needing' the HP manifolds.

What is "nice power"?

...while a very subjective question it is a valid one. In my case, 'nice power' will be whatever the original LA 318 had here at 7000 ft altitude: it had 360 heads, a "4bbl" cam with a nice mild lope, and I don't know if it had 318 or 340 manifolds...but it was just a nice warm 318 and sprinted happily to around ~5500 rpm - nothing fancy. I've no idea what the hp was but it was 'plenty'. I can only presume a similar magnum 318 with a similar cam will run just as well, and that will be plenty for me.

A magnum with similar cam will make quite a bit more power, it will get more valve lift, the heads flow better, lot more compression and at 7000 foot elevation that compression is everything.

your thinking too hard about this, run the 318 manifolds or magnum manifolds whatever you can get to fit and it will be fine. I would spend an hour getting the 318 manifolds opened up at the port so they don't make an edge that restricts flow though.

...at 54 yrs old and now with a young family...for me, thinking in depth about my coming projects is just my way of exercising my brain in-between the precious little time and resources I have available to actually buy parts and build them, so it gives me a chance to vet my ideas and fine tune my plans, including whether or not to buy a set of 340 manifolds...which thanks to all of these great discussions I'm now convinced I will not need to do. I also wouldn't be surprised if there are others pondering the same question, and if they read this they too will be reassured their 318 manifolds will be fine for the j.o.b. Plus, now I can put that (340 manifold) money towards a nice 318 cam...but that's another topic

...but I will ensure the ports match as best I'm able - good 'put

Like so many others, this has been a really good and in depth discussion about exhaust manifolds for the small block LA and Magnum engines, and I appreciate everyone's patience and sharing of intel .

- Sam

I know someone running 10.4’s through 70 340 exhaust manifolds. I’m running mid 11’s at 117 mph through 68-69 340 exhaust manifolds. Point being you can get much more out of them then people think.

as referenced from the link shared above (thanks for that), this graph pretty much sums it all up - the bottom line being there isn't a huge difference between 318 manifolds and headers...at least not enough that I personally will care. This is the results tally from the Motor Trend LA 360 crate motor exhaust manifold comparison. Yep I'll use my nice, simple, quiet, good-fitting 318 manifolds I already have