Which cylinder naturally runs lean?

[nelson8708]

I went and had my 460cc injectors flow tested and i have one injector and is rich by 3-4%. I plan on put that injector on what ever cylinder runs the leanest. I know on the honda k20a2 motors the number 3 cylinder runs lean. So my question is what cylinder on the ka runs the leanest?

p.s. i have a ka24e but i am guessing it is the same on both probably....my guess is cylinder 4.

Thanks,-Nelson

[480sx]

Its a toss up between 3 and 4. Heres a few ways to think about it.

The old rule of thumb is that cylinder three runs leaner, for whatever reason. Cylinder three will always be your hotter cylinder, because its further away from the water intake for the heads. It runs hotter than 4 because 4 has 3 sides of the cyl exposed to air, where as 3 only has two. So naturally, three would be a good choice for your higher output injector.

The other way to think about it is that cylinder four is the furthest away from the hot coolant discharge that runs through the intake manifold. This would mean that the number 4 runner will be the coolest on the intake mani, and deliver the coolest air(in theory).

With all that being said, why didnt you just get them to balance the injector flow?

[nelson8708]

i can get them flow tested for free and just wanted to make sure that there wasnt a specific one that ran lean. I figured four would be the leanest because it is at the back of the intake manifold and when the air rushes in you would have slightly higher pressure at the back of the intake.

[480sx]

No, the pressure stays the same across the manifold. Air is lazy, it doesnt really like to build up extra pressure anywhere if it can help it.

I would just toss it in cyl three for sure. Its the cyl that always blows, runs hottest, and is generally a pita.

[WDRacing]

Agreed, I've popped many motors do to detonation alone, always the #5 and #6 on the RB20. I'm not to sure which one went first, I wanna say 5 had more damage.

So I'd put injector with a tad more flow in the #4.

WD

[C-Kwik]

No, the pressure stays the same across the manifold. Air is lazy, it doesnt really like to build up extra pressure anywhere if it can help it.

I would just toss it in cyl three for sure. Its the cyl that always blows, runs hottest, and is generally a pita.

Problem is you are not dealing with non-moving air. Air has mass and thus, momentum. As a result, some cylinders may get more air then others. And since injector duty cycle will be the same for each cylinder for a given overall airflow, it will not compensate each injector.

That said, I have heard a couple of theories on the rear cylinders being more susceptible to detonation. 1st, the coolant in a motor typically moves through the engine front front to rear. So as the coolant is heated as it moves through the engine, it becomes less effective at removing heat. Thus, the rear cylinders run hotter. The 2nd theory I've heard is that in a typical manifold design, the main stream of air through it is towards the rear of the manifold. As a result, the momentum of air is may cause a slightly higher pressure in the rear of the manifold than the front which may feed more air to the rear cylinders. This would in turn create a slightly leaner condition in those cylinders.

Both seem somewhat logical and empirically, many (such as WD) have observed detonation has a tendency to occur in rear cylinders first (I've seen this in a Supra myself). As such, I'd play it safe and run the highest flowing injectors towards the rear.

[nelson8708]

So its really a coin toss between cyl 3 and 4. My question is does running a richer afr help prevent detonation? That sounds more like a timing issue. I do agree with 480sx that it seems a lot of ka's seem to blow cyl 3 before the others, but i am not sure if running lean was the problem.

[C-Kwik]

My question is does running a richer afr help prevent detonation?

Richer mixtures are cooler. This is because as fuel (or any liquid) atomizes, it absorbs the surrounding heat as it converts from a liquid to a gas. Ever wonder why the little compressed air dusters get colder as you use it more? Same principle.

As detonation is primarily a function of heat, cooling the mixture can reduce the liklihood that detontation might occur.

That said, running a slightly richer mixture will generally reduce power output to a very small extent. And way too rich may cause some fouling.