Nissan "L" Series Engine: Advantages To Non-Crossflow Design?

[Pennyman]

Hey everyone! Just had a brainstorm and thought I'd share it with you.

The "L" series engine in the older nissans have a non-crossflow design (Is there an actual term which describes the design?). From an engineering standpoint, this design isn't as efficient or as user friendly as a crossflow design. This becuase the exhaust is located right below the intake, heating the intake air, and making the gas flow in the exact opposite direction when its expelled from the engine. Not to mention it makes it a pain to work on the exhaust system and put on a header (which I will be doing in several days to my Z, arg!).

I'm an engineering student, and I'm curious if there's any advantages to the non-crosflow design.

In theory, there should be. For example, if you were crowded for space in the engine bay (which the Z isn't) or if the intake/exhaust assembly was on the passenger's side to offset the weight of the driver (which isn't the case on the Z either). Although, I am aware that the engine does lean toward the passenger's side to equalize the weight.

Any insight into the reasoning behind this design? Was it the best Nissan could do for the money? Was it the best the 70's engineers could to? This should be a good discussion topic.

Was covered a little bit in this thread:

http://classiczcars.com/forums/showthread.php?t=7316&highlight=non-crossflow

[Arne]

As you pointed out, most of this was covered in the earlier thread.

And the answer is almost always money. The L-series engine design was cribbed from the Prince engine which was more or less a Mercedes engine built under license. Why spend a pile re-engineering a new solution when a perfectly reasonable one already exists in-house? And lastly, the valvetrain is much simpler to design on a non-crossflow head - all the rockers go the same way.

[26th-Z]

I would like to point out that the intake and exhaust IS on the passenger side. These are right-hand drive cars and the American imports are backwards.

[Pennyman]

I would like to point out that the intake and exhaust IS on the passenger side. These are right-hand drive cars and the American imports are backwards.

Very good! Thanks for mentioning that, I TOTALLY should have caught that myself, hehe.

[260DET]

Cost is often mentioned as the main reason why the L was not a crossflow but I think that there is another possible explanation. If you look at a RB30 head, which is a single cam two valve design, they are fairly simple too. But I'm not a production engineer or whatever so who knows as to any cost differences between the two.

The alternative theory is that the Nissan OHC design was an evolution of the pushrod design common at the time. The L head looks like a pushrod head with some OHC gear tacked on if you get the idea. On the other hand the RB30 head looks like an integrated design, nothing tacked on.

Anyway, its a theory but if you look at the history of car design there are lots of examples of evolution in design rather than big steps forward, identifiable in just a few cars such as the VW beetle and the British Mini

[halz]

The cylinder heads on crossflow engines sometimes tend to be wider than non-crossflow heads. This is especially true of twin cam crossflow heads where valves are often paired at 90 deg to the line of the crankshaft. In L-series engines, the valves are paired parallel to the crankshaft...narrower head, less metal, lighter, cheaper to make....

[Carl Beck]

Hey everyone! Just had a brainstorm and thought I'd share it with you.

The "L" series engine in the older nissans have a non-crossflow design (Is there an actual term which describes the design?). ..snipped.

Hi Pennyman:

"L" series OHC engines have both the intake and exhaust valve off-set in the combustion chamber, to one side of the vertical centerline of the cylinder bore. So the term that describes this design is "L Series." (actually a carry-over from the days of "L Head" designs aka Flat Heads).

In an "I" (eye) series engine, one valve is offset to one side, and another valve is offset to the opposite side of the vertical centerline of the cylinder bore. So the term that describes that design is an "I Series". (also sometimes called a "Y" type).

FWIW,

Carl B.

Carl Beck

Clearwater,FL USA

http://ZHome.com

[civ104]

Another thing to consider was the design of the 2000 series roadster engines. They were actually modified valve in head blocks which were modernized with the addition of an overhead cam which was added through a clever cam drive chain which drove the lower engine counterbalance shaft as well. I know, I had one of these chains snap on me with rather disastrous results. Luckily, it wasn't in today's rare parts era.:classic:

[7240z]

I don't know why you're planning on changing the exhaust layout of the engine. Why mess with a good thing?

[Mr Camouflage]

From an engineering standpoint, this design isn't as efficient or as user friendly as a crossflow design. This becuase the exhaust is located right below the intake, heating the intake air, and making the gas flow in the exact opposite direction when its expelled from the engine.

I think the term crossfow/ no cross flow is a bit confusing to some. People tend to think that the air in an engine flows smoothly from the intake to the exhaust, and that having the air seeming do a 180 and travel back in the same direction would be bad for efficiency, when the air isnt really flowing at all.

When what really is happening is that the air is being sucked into a cylinder, then the vlaves close, end of flow. their is no air flow once the valve is shut, theres no where for it to go. then the pistons compresses the air/fuel mix, the spark plug ignites it causing the mix to explode, which increases the pressure in the closed cylinder, forcing the piston down.

Once the power stroke is done, the air, which is still compressed needs to escape, which it does when the exhaust valve opens, not so much because the piston is forcing it out in the exhaust cycle, but because pressurised gas will move to a lower pressure area to equalise pressure with the surrounding enviroment.

So the air doesnt really flow, it get sealed in a container,compressed and exploded, and allowed to vent to a lower pressure zone through the exhaust.

So I guess what i'm saing is that air flows through an engine, like water flows down a river - if the river was a line of buckets, and you emptied the water from one bucket to the next, and then the next etc...

(Sure theres valve overlap, but i'm not going into that, maybe soemone else can)

[ZSaint]

I see the biggest weakeness of the non-crossflow L-series engine is the existence of exhaust heat. 1600-1800 degrees is directly below the intake. (Have you ever seen an engine at 7500 on the dyno? The headers are glowing!!!) Most people who compete do not seal off this heat. I have seen the dyno work on engines with and without cold air boxes. I believe you lose about 2-3% of power with "hot air". I had a 280Z that I drove in CP and we had a complete air box. It made 7 more hp on the dyno then it did w/o the air box. A person can gain hp on a crossflow engine with a cold airbox. But, with the L6 engine, it is mandatory. Datsun tried to overcome some of this weakness with the heat shield. It solves the radiant heat problem but not the hot air problem. Why do all of the new cars use cold air boxes. HORSEPOWER!