Stock L6 exh manifolds, Skinny vs Fat ???

[Guest Al 260Z]

Which flows better ???

 

There are two distinct styles of stock L6 exhaust manifolds, skinny and fat. Evidently Nissan engineers couldn't make up their minds and they switched back and forth. Here's a summary of 4 different manifolds my bud and I lined up yesterday:

 

73 240: Fat type with air injection ports, casting #N33.

74 260: Skinny type with air injection ports, no casting no, weight 14lbs.

77 280: Fat type w/o air injection ports, casting #P71, weight 26 lbs.

82? 280ZX: Skinny type w/o air injection ports, w/ oxy sensor port, #F54.

 

All have port for EGR tubing. All have separation between cyl 123 and 456 groups.

 

Close inspection shows sloppy casting with respect to port matching for either square or round exhaust port heads. Any of the above exhaust manifolds could be used on either a square or a round port but all would require about the same amount of work with a grinder to achieve good port matching according to OEM gasket pattern.

 

The fat type have a bulbous pocket the cylinders discharge into. The skinny type look like they maintain exhaust gas velocity better. but may have some choke points in the flow path.

 

ANYONE FLOW, DYNO, OR STREET TEST THE TWO STYLES AGAINST EACH OTHER ?

 

Pending expert input, I'm favoring the skinny style. 12 lbs of weight is a lot. Also I tend towards the "velocity is good" theory. Current pipe dream is to port-match, extrude hone, coupled to 2-1/2" mandrel exhaust.

 

Later,

Al

[DAW]

Al, you might be interested in another one to include: LD28. Square port "skinny" type. I don't know if there is any advantage over the L28 "skinny" manifold but it may have better port match since I think it a true square port manifol (nice on a P90 na engine). Stay away from the "fat" emission manifolds (except for a rowboat anchor). DAW

[Guest Al 260Z]

... fat style exhaust manifolds may be less susceptible to warpage.

 

Jet-coat or similar treatment inside and out is part of my dream setup (after extrude-hone). Ceramic coating should substantially reduce heat transfer rate and reduce temperature differentials throughout the manifold (probable leading cause of warpage).

 

Later,

Al

[WHP]

I'm facing the same delema. got all those manifolds on my bench now. Dyno data would sure help! There is a dyno test I think its on Zcar.com with the N42 manifold on a stock L-24. It was 4 hp less than expensive headers. I'm using a modified E31 head on built L-28. I have a new set of motorsport headers but they port match really bad and look poorly made inside too, pure junk. The L-24 used the skinny manifold and a long twin downpipe I'm guessing for scavanging, but I'm skeptical if it will flow well enough for the larger hot L-28? I settled on a P71 "fat" round port manifold as it looks like it will flow better after port matching. The port squish for bolt clearance is less so the passages are larger. BTW, it has the same size outlets for the downtubes as the 240. I'm thinking of using the long 240 downtubes welded to the larger conector flange and add 2.5" exhaust on where the downtubes converge with a fabricated conection to make a good transition. I'm going to live with the extra 14# and not worry about exhaust leaks. Port matching is half done, boring......

Will

[z-ya]

Great to see some discussion about using the stock exhasut manifold. When I rebuild the engine in my 280Z, I'd like to ditch the header, but I don't want to loose any HP in the process. My plan is to build a mild 3.0L with around 9:1 CR. I want to improive low end torque, and make it a good street driver that get's decent gas mileage (it get's 28MPG now, and it has over 175k miles on it!).

 

For the exhasut, I was thinking about using the stock 78' manifold, and have a custom 2.5" downpipe made. Then I was thinking about using a glass pack along the driveshaft, along with a quiet turbo or other muffler at the rear. The goal is to have a free flowing exhast, that is relatively quiet.

 

Comments, suggestions??

 

Pete

[Guest Al 260Z]

There hasn't been any clear answers on this matter here or at Zcar.

 

This is what I think: Skinny probably flows a little better and may produce better mid-range torque, similar to having runners on the intakes. While 260 is often put down as the "smog car", it is generally accepted that the N36 intakes of the late 260 flow better than earlier versions (valid?). So it could be argued that Nissan engineers were paying attention to flow characteristics when they chose the skinny style for the 260. Also, apparently the early 80 ZX L6 engines posted the best stock performance values of all the L6s, this with skinny type exhaust.

 

I'm pursueing both options. Just finished port-matching a 280Z fat style stock exhaust manifold, and will take it and down-pipe collector to get ceramic coating at powder coat place this week. These will mate up to 2-1/2" mandrel-bent exhaust with resonator and sport muffler. This was said to be fairly quiet, and I'm keeping my fingers crossed about that. After I get this beast on, then I'm gonna try to dial in my carb jets to match new cam and exhaust. Maybe also put on a 280 style hood and adapt a ram air scoop to the carb-side hood vent...

 

Also have a new 260 skinny style stock exhaust manifold. Did a bunch of port-matching on it this weekend and intend to eventually get 'er extrude-honed and jet-coated. If I get custom collector to match to the same 2-1/2" exhaust above, then I'll be able to do bonna-fide comparison test -- maybe even on dyno. But that'll probably have to wait at least a year due to too many projects and too little $$$ and time.

 

The ZX exhaust manifold would be way cool to use with a wide-band oxy-sensor.

 

Current set-up is 260 stock exh manifold and downpipe, expanded to 2-1/4" pipe (not mandrel-bent) to Flowmaster muffler. This provided large performance boost over stock 2" pipe, but it is too loud. If the 2-1/2" system had not turned up, I would be looking into resonator for the center section of this pipe.

 

I wonder if a high-flow cat converter wouldn't provide the same noise reduction as a resonator? Sooner or later my conscience is gonna make me clean up my Z's smog. Wide-band oxy sensor sure would be nice for do-it-yourself tuning... Also considering exhaust gas temperature sensing... Both would be really cool...

 

Beware different downpipe gasket patterns and different geometries between Z exhaust manifolds. Best to assume that if you can swap manifolds you will need to modify the downpipe/collector.

 

Later,

Al

[David K]

The Nissan manifold is designed like a header. When i went headers on my old black z, it didnt make any difference in the world, except create a hell of a lot of noise. Waste of $.

[Joe-Z]

what about a P30 manifold?

Thats whats on my 71''

[johnc]

Keith Thomas (katman) is the one to ask about this. He tested a ported N42 exhaust manifold on Sunbelt's engine dyno a few years ago. Way back in the mid-1990s I watched various stock exhaust manifolds and headers tested on an ITS/BSP 2.8L engine but I don't remember any of the casting numbers. Headers worked better anyway and that was the point of the test way back then.

[DAW]

It's not just about mass flow of gas i.e., how much you force into one end of the manifold and how well it comes out the other. The "best" flowing manifold may not always be the best performing manifold throughout the usable rpm and load range. Take an L28ETT manifold and run a pipe right off the turbo flange with an adapter and it will probably outflow all of the manifolds, but that doesn't mean it will provide the best performance on a n/a engine. The reason the "skinny" manifolds offer a performance advantage over the "fat" is because of the scavenging effect. The best skinny manifolds are true 3s-into-2 design. That is, they are two 3cyl manifolds cast together. Not just any three cylinders either, they are grouped by ignition interval to optimize the "suction" effect produced by high speed gases flowing past a port that is just opening so that not only are exhaust gases expelled more efficiently, but the intake charge is actually pulled in to some extent by this action. Velocity of exhaust gas flow is the important factor with this effect. A tri-Y design provides the optimum scavenging for low to mid range, coupling cyls 1&6, 2&5, 3&4, but that makes for a lot of pipes and junctions and will become limiting at high rpm. The best ones from the 3s-into-2 aspect are the early 240Z manifolds. Some later 240Z manifolds/downpipes do not adequately maintain separation of the 3 port segments from each other and they are more like 6/1 manifolds. The problem with them is that they are for 2.4L volume of exhaust flow and might prove restrictive for 2.8L and larger. The "fat" manifolds are just huge chambers designed to concentrate exhaust heat so that unburned hydrocarbons are broken down, and this at the expense of scavenging effect. The manifold for the P79 head is good from the volume aspect and it has a 3-into-2 design. One drawback is that the O2 sensor locates between the two 3-cyl segments so that it samples from both and this allows some communication between the two segments but this may be a minor compromise and worth it for the sensor. The down pipe(s) should not join right together like the 280-280ZX pipes do, rather they should have a length of run like the 240Z (except with larger pipes) if nothing else, the 240 junction should be eliminated and a 2-pipe collector with 2.5" or 2.75" collector fitted. You might still be able to purchase a downpipe from MSA that is made that way. I think that may be what's now used on their "twice-pipes" setup. Alternatively, you could take two 240Z systems and try to fabricate dual secondaries (maybe a balance tube where you eliminate the junctions) to run on back into a single dual inlet muffler. Remember, as the gas flows through it will cool and reduce the volume and larger pipes may not be as critical toward the rear of the car as near the engine. As to headers, make sure the primaries are equal length or don't bother. Early Cyclone headers are good for an L24 due to the small dia tubes, NISMO headers are the best I've seen for L28 or larger and have well-designed collectors/junctions. Sorry if this is to exhaustive a discourse but I'd rather do it this way than write four posts equal to the same volume. DAW

[Guest Al 260Z]

I knew there had to be some folks out there with detailed knowledge on this funny business !

 

Per DAW it sounds like the earlier 240 exhaust manifolds were also skinny type. '73 is fat, but that was smog territory. Funny that the notorious smog 260 should be skinny style. Both the early and the late 260s I've owned had skinny style exh mans.

 

Thanks, DAW, for explanation of fat-style chambers for hydrocarbon reduction. That reminds me of the "after-burner" chambers required on the exhaust of Mazda rotary engines due to incomplete combustion in the tips of the Wankel combustion chambers.

 

Interesting point about maintaining the separation of the downpipes. I guess this must help separate the exhaust pulses from the two groups from interfering with each other and reducing the scavanging effect. Fortunately my new setup is this style.

 

While fat clearly isn't the best choice due to reduced scavanging effect and excess weight, I'm going ahead with my conversion, since all the pieces are in hand and additional costs are minimal. I figure that a port-matched fat style with mandrel-bent 2-1/2 exhaust probably flows a lot better than what I got in place now.

 

Later,

Al

[DAW]

For full-out high rpm, that would be the way to go (or L28ET manifold), but streetability might suffer. DAW

[WHP]

When I look at the internal size of the 240 "skinny" manifold I am afraid it will be restrictrive on a modded 2.8. However, the interesting thing is the exit holes where the downtubes attach are the same size as my ZX boat anchor. Should I look into opening the ZX manifold up to the next larger size tube?

 

Will

[z-ya]

I was thinking of enlarging the two output ports too, but then you will need to make a gasket. Also, what is the diameter of the two manifold output ports? Somewhere like an inch and a half or so?

 

 

So if you calculate the cross sectional area given each tube is around 1.5" in diameter, you get a total area for the two tubes of: 11.1 square inches.

 

For a 2.25" exhaust pipe, the area of the cross section is: 12.49 square inches.

 

So it's cross sectional area is somewhere between a 2" and 2.25" pipe.

 

Probably just port matching the pipe to the manifold and gasket will have more of an effect than enlarging the holes. Maybe not. If you are running a 2.5" exhaust, enlarging them may make a difference.

 

Pete

[Guest Al 260Z]

Z-YA, best check your math. A=(pi*d^2)/4.

 

If nominal dimension of pipe is ID (which I don't think is the case) then:

1x 1.5" ::> 1.77 sq.in. 2x 1.5"::> 3.53 sq.in.

1x 2.25" ::> 3.97 sq.in.

1x 2.5" ::> 4.91 sq.in.

1x 3.0" ::> 7.07 sq.in.

 

I noticed that on my custom downpipe, the two downtubes are larger than on the stock collector. Instead of sticking up into the manifold, they are flush with flange surface and their ID are almost perfect match for OEM manifold/downpipe gasket hole. (I just dropped 'em off for ceramic powdercoating so I can't easily measure 'em till next week.) I have in front of me an OEM 260 manifold/downpipe gasket and the bores are 1.7"+:

1x 1.7" ::> 2.27 sq.in. / 2x 1.7" ::> 4.54 sq.in.

So there must be some room to maneuver in terms of downpipes depending on what stock tubing sizes are available. I'll check at home tonite whether 260 gasket and 280 gasket have same size holes or not.

 

On my port-matched fat manifold fit with oversize downpipes, I did quite a bit of improvement on the manifold contours at the downpipe outlets. Got rid of the nasty ridge which the downpipe tubes are supposed to butt against and smoothed out the contours quite a lot. I used a carbide burr in a drill motor which worked great.

 

Best sit down for price of custom downpipe. I asked this morning at custom exhaust shop with steady stream of ricer cars lined up. (They've got a reputation for good but pricey work.) The man there said it probably run $500-$600 to duplicate the twin-tube downpipe with mandrel bends and collector that I showed him. I hate to think what he'd want for stainless steel. Thankfully, he only charged me $30 to open up the bottom tube which had been flattened by a close encounter with a speed bump.

 

Later,

Al

[DAW]

I went out and rummaged through a bunch of manifolds and my vote, for use on a rectangular port head, is the W25 off an LD28 (yeah, you heard it right). Similar to the E30 from an early 240Z except it does not have any air-injector tubes/bosses to restrict flow, the I.D. of the runners appears a bit larger (2.8L vs 2.4L), and the outlets are larger. The W25 definitely provides a better mounting flange for f.i. intake manifolds than does the E30. While you're at it, check out the intake manifold from the LD28. It would need injector bungs, and it's tall, but looks like it could have some significant flow plus low-end torque (long runners). DAW

[WHP]

Acccording to the manifolds I have in hand the twin exit holes are the same 240/260/280/280zx, although the 3 stud bolds change size and location. I would get NISMO headers except I'm on a tight budget as this is my daily driver. I don't want to divert funds from my other car, a 302Z project. I'm going to open up the twin holes, contour the inside of my port matched P79 and make an oversized twin downtube with crush bends. I think it will not be so bad since the bends are not that sharp. It will connect to a 2.5" mandrel exhaust.

 

Will (high performance projects are always compromises, usually $ related)

[DAW]

The P79 is fine. It has large I.D. runners, larger dia. outlet studs than the early manifolds, and better flanges for attachment with the f.i. manifolds, and O2 sensor to boot. It's an art making hp on a budget...more like a sport for me. The downpipe I had referred to I had purchased from Impact Parts in Glen Wild, NY many years ago. It wasn't expensive and it fit perfectly but I don't know if that company even exists today. If it does, maybe they could figure out where they were getting those pipes they sold...it's the type of thing worth a group buy. DAW

[z-ya]

Al,

 

Thanks for the correction, it was a long day. The formula I remember is A = pi*R^2 . I was doing (pi*R)^2, and the multiplying by two because there are two ports.

 

So now that I have my brain screwed back on right, it looks like area of the two 1.7" holes are somewhere between a 2.25" pipe and a 2.5" pipe. So if you are running a 2.25" exhast, there is no sense enlarging the manifold exit ports. Even for a 2.5" pipe, it may not neccessary. Definitely port matching is a good idea.

 

Pete

[Guest Al 260Z]

Double holes on the downpipe flange gasket are identical size and spacing on the '74 260 and '77 280 OEM gaskets. They both measure a hair over 1.7" diameter. Only the stud pattern changes from skinny studs to fat ones.

 

The limit on how big you can make the downpipes is the clearance required for the stud nuts. Be sure to leave enough room to get a box end or socket on. These suckers are a bitch when they get frozen in place.

 

OEM studs and nuts are highly recommended. For the early skinny style studs, they come in a high-alloy heat treated version (gun-metal grey finish). You know you're getting extra-special hardware as they are several bucks apiece. Oddly, the new studs for the 280 did not appear to be high-alloy type, being gold-colored cad-plated.

 

If you're gonna make a custom down-pipe, best be sure to pre-select which manifold you're gonna use. There are some significant variations in geometry between various styles - especially between fat and skinny style manifolds.

 

Later,

Al