Turbo vs non turbo manifolds

[DefeXion]

I have a question.

Why do the non turbo engines have 6 pipes joining into a chamber, where the turbo has them joining onto a rather large straight pipe? More importantly why is this advantageous for each type of engine?

[Nick Mann]

?

I don't understand the question.

I assume you are talking about the inlet manifold. Have you got a picture of the n/a V6 for comparison?

[DefeXion]

Yeah, working on inlet for now, exhaust can wait a bit longer. Here are some diagrams.

My uneducated guess is that because the NA engine is effectively sucking the air in on the induction stroke, it needs a manifold that lets each cylinder take an even amount of air. Whereas because the turbo forces air in, basically the valves open and the compressed air forces its way into the cylinder. Thus you don't need so much tuning on the manifold, just as long as it's under pressure.

Now, basic racing rule - more air = more fuel, and to get more air you need less restriction on airflow so the induction stroke can do it's job. So of course, I'm interested in making a few changes here and there to get more airflow. Recently I fitted a VR4 airbox so I could use the bigger VR4 air duct and that worked really well on my low down torque. Now I'm eyeing up this manifold thinking hmmm.

I'm concerned about the different manifold designs though, which is why I'm asking this question. If I used a turbo manifold on a non turbo legnum, would the cylinders furthest from the throttle body get less air because the first cylinder gets use of the incoming air before the others?

[Beastlee]

Looking at the comparison I'd say the turbo manifold has better airflow.

[DefeXion]

sorry, i redited my post with a more detailed explanation.
So you think the VR4 manifold would work well in an STR?

[Nick Mann]

I understand now!

But I don't know enough about airflow to answer your question.

[Turbo_Steve]

Assuming the STR is normally aspirated (I don't actually know!) then I wouldn't put the turbo manifold on: there will be a significant difference between the two:


A turbo manifold is usually designed to stabilise the pressurised air within it. This means more volume. As the air is pressurised, it travels quickly past the throttle butterfly when opened, and fills the manifold quickly. It will be shaped to reduce the impact of the valves snapping open and closed which can affect the body of air inside the manifold. It's like a stabilising resevoir.

An NA manifold needs to ensure the distance between each cylinder and the throttle butterfly is the same. There's no issues with standing waves in the airflow, as it's always a vaccum. It won't serve as a resevoir, as it would only store -ve pressure, and will encourage run-on.
A larger inlet manifold has the potential to destroy your throttle response on an NA car.

Inlet manifolds are complicated beasts, especially on NA cars, and I would be disinclined to start changing it. The size of each runnel from the centre to the port is important, and will be tuned for the specific engine model in your car. Assuming your NA unit has different sized ports and a different compression ratio, the pipes on the turbo manifold will be of different dimensions, which will either move your peak torque to a different part of the rev range, or possibly slow the airflow completely costing you power.

It was popular at one point to put the SR20DET manifolds onto an SR20NA engine, until peoples dyno graphs started coming back in: most of those guys found that the bigger manifold was only effective if you had wilder cams and were raising your rev limit significantly.

[Louis]

Remember the VR4 manifold is force fed by the turbos, so the normally aspirated manifold may actually flow better!

[Nick Mann]

To be honest, if the VR4 manifold was better for the V6, then why didn't Mitsy just fit it? It's already in their parts bin, and one component on the shelves is easier than two. They use common components where they can, look at other similar age Mitsubishis and you will find the same climate control panel, for example.

[Beastlee]

Yes but you have to take in manufacturing costs too, there may be differences that are not immediately obvious on the VR4 manifold that make it more expensive (Wall thickness etc.)

[Gly]

*cough* http://www.clubvr4.com/forum/showthread.php?t=16997&highlight=intake+manifold

[bradc]

Intake manifold design for an NA car is indeed very trickey, it's a bit of a black art really, you need to worry about what rpm's you are going to get resonance from other cylinders to suck the air down and various other things.

I'd say the best plan is to just leave it alone, if you're really worried maybe bore it out a little bit and get it polished on the inside.

[Johnny_Cashed]

Off topic, why does it look so much like a hand?

[Turbo_Steve]

Brad, polishing isn't always a good idea...it's out of vogue at the moment :) If you do consider getting the metalwork done, there would be more benefits in attacking the head.

[DefeXion]

Thanks for all the information (especialy steve's post). I learnt heaps.

I was really wondering about the manifold design anyway, so I can say I know a little about manifolds now :).

Just out of interest, did you ever try bolting the turbo manifold on carsten?

[bradc]

Johnny, again it comes down to trying to get an even amount of airflow into all cylinders. If you look at the centre plenum you can see that mitsi tried to get it as circular as possible in the confines they had. I bet if they could have had the throttle enter from the back of the car they would have.

[Gly]

it will bolt on.
but you need to bore out the TB flange,

and after having a look at the designs, i thought it safer not to mess with.

[DefeXion]

so we still don't know what would happen. Oh well. I'm satisfied with the NA design now anyone since I understand why it's like that.

[matsondawson]

I'd be willing to be the na plenum allows the air to build up erm velocity or um flow... such that when one valve closes and the other one opens the air is already trying to flow into the engine.

If you put the turbo one on you'd lose this, the air would have to be 100% sucked into the engine from a large volume thus acheiving no shared flow.
Hence you'd lose power, probably heaps of power.

Now since the turbo is presurised air, the size of each erm tube :) is probably close to the size of a single cylinder i.e. 2.5L/6 = 420ish cc thus holding enough charge for one stroke of the engine. With the big air resovour thing designed to absorve the air bouncing back from the valve closing? Might even smooth out the transition from throttle open to throttle closed.

ask dr. stupid :)

[matsondawson]

Some na plenums have those tuned flappy butteryfly things in them, does your car have this? It sort of provides back pressure at a certain rpm from the air bouncing off the butterfly, like super charging. I think the tiburon has it, probably heaps of cars have it.

[Turbo_Steve]

Indeed, some even fit venturi blades in the cylinder runnels (Ferrari tried this for a while).

Of course, another option would be to forget the whole inlet tract and go for throttle bodies with tuning trumpets into a large carbon fibre airbox. It works really well, but will require a complete ECU change, and cost £1000's!