schadenfreude

Admittedly I don't know anything about the physics involved with designing an intake manifold, but from a leyman's perspective these custom intake manifolds all seem very 'boxy' when compared to the smoother more rounded shape of the stock intake manifold. Surely the corners and hard edges would create extra drag and turbulence in the airflow? Or is this actually a desired effect?

Seems awfully counterintuitive; would anyone please care to enlighten? Clearly the boxy design has its advantages as shown by the empirical evidence in the link from the OP.

Clown Pedro

Larger plenums and runners flow more air which is great for topend, but tend to hurt low end as all teh velocity is lost. This is what ive read in books and articles. I have never made an intake so i don't know for sure, but that seems to make sense. Im sure if it were easy to fabricate a manifold that was smooth and contoured they would, but the extra fabricating time probably makes it cost prohibitive.

Guns951

The above is very true as seen with ITB setups, the increase in power is all mid/top end

Porschefile

In general, you are correct that more of a square design could have some negative effects. Though, what people are overlooking is if you are making a significant amount more power, you need a significantly larger plenum volume to support the increased amount of airflow. The increase in plenum size can have much more of a beneficial effect than the negative effects of having some square edges. Think of the intake and exhaust sides as trying to force a grape through a straw. The larger it is, the easier it will be. Though on many components it is possible to go too large which could result in negative effects. Going too large on the exhaust diameter (after the turbo) won't really have any negative effect as backpressure is detrimental to turbocharger performance. Exhaust runner diameter is important though and can have the same negative effects if you go too large.

Pauerman brought up an excellent question. If the stock intake outflows the head, how are you going to see much of a gain if any? I cannot say for sure how much of a beneficial effect there will be on our 8v heads as I have yet to get my car running to test it out for myself. I promise I will post as much info as possible once I do. However, looking at this from a scientific standpoint, it is still entirely possible that a modified intake could be made to have beneficial effects and improvement in power. The reason why? Well, again, it all has to do with sizing. The Honda intake link I originally posted is an excellent example of this. It is entirely possible to enlarge components like the plenum, TB, etc while still doing things to maintain airflow velocity. If done right, you can acheive greater airflow volume at lower valve lifts (as a result of lower airflow restriction) which can result in more power "under" the curve as well. While the stock head flow capability might limit your overall power potential quite a bit, if you acheive a greater volume of airflow at lower valve lifts, you can still see improvements in power, torque, etc. I'm racking my brain to try and think of how I can explain this any easier. I mean this is simple high school physics and there's nothing hard to understand about it.

Tommo, sorry but, you are just wrong about the exhaust topic. A larger diameter tube will result in increased liquid/gas flow capability with reduced restriction. Again, forcing a grape through a straw. If you can't understand this, I really just don't know what to say. It's been proven before that on a stock turbo 951, there is a minimal gain from a straight 2.5" to a straight 3", so that's understandable. A bigger turbo is going to produce more substantial gains. The European Car project 951 gained something like 40whp in switching to a 3" on a Kokeln turbo setup. Anyways, enough on this subject.

Driftomagnifico

The exhaust turbine is bigger than 2", the runners feeding the turbo are probably 2" or larger.

They did not run a 2" exhaust pipe. The pipe would choke and reduce engine output GREATLY.



BMW showing a large exhaust pipe.

BTW, torque is a function of volumetric efficiency and mechanical efficiency. Backpressure will ALWAYS reduce torque in a turbocharged engine, this includes manifold, turbine housing, and post turbine backpressures. They directly resist flow and require mechanical pumping to expel the exhaust gas.

Driftomagnifico

They flow very well. I would say they flow almost too well for the typical RPM band you see in a 2.5L.

It is my opinion Porsche designed the DOHC cylinder heads to specifically be used in conjunction with a 3.0L powerplant. This would stand to substantiate the flow volume quite nicely.

Driftomagnifico

It would depend on the manifold design overall. If you have sufficient runner inlet clearance from the angle I don't think you'd see any effect.

95ONE

It is not the CFM that makes the Honda Head flow so efficiently, it is the velocity that has made all the difference. Cramming both into one was their magic.

The major Differences between the Type "R" intake design and the regular VTEC were plenum volume, runner length and size. Larger, shorter, larger... respectively. (older Integra) Shape stayd the same.
Swap a Type "R" Acura RSX intake on a regular VTEC RSX. You gain 5-8 horsepower. Naturally aspirated. Repeatable. Turbocharged versions made more. I'm fairly sure the 951 could gain a little from this.

Hasn't N - E - One done a before and after dyno with their custom intake? (Only a custom intake change and nothing else?) True emperical data here? anyone? anyone? ..... you know it's coming...... Bueller?

Raceboy

968 head is great in any ways but it's big advantage is it's flaw in one way: it's the port size that is good for flow-bench numbers and forced induction but in NA forms the port velocity is too low. VarioCam smoothes it out a bit but in NA form 968 head's true powerband is above 7000rpm.

pole position

You see them in a book and I saw them live and they are 50 mm..........

Duke

I dont' know how this turned into an exhaust discussion. I've kept my way out of this thread but I share opinions with Porschefile and Drifto.

Also, ITB setups have nothing to do with maximum power. Their benefit is throttle response. Contrary to a single TB-manifold which have vacuum in the whole plenum which needs to be filled when you floor it.

Oh, and the Renault F1 turbo engine is a twin turbo V6 (= twin exhausts etc.).
I will end the exhaust discussion by presenting this page: http://www.gurneyflap.com/engine.html
It contains info and pictures of many of the state of the art factory race turbo engines made for F1 etc. Even if you don't know squat about formulas, flow etc. It is quite obvious a big exhaust is preferred by just looking at the pictures.

thingo

I find this particularly interesting as I am putting together a 16v 968 turbo, but it seems as if Porsche were thinking long term with the 16v head, it was meant to remedy the 'deficiences' of the 951, but they changed their mind on strategy at some point.

Duke

Agree, nice findings
The intake ports are too big. But hey, not that bad for a turbo conversion

A.Wayne

Not sure of the conjecture , the housing based on turbo being used , does not appear to bigger than 2 inch , also 2 inch runners for the manifold, i don't think so , it was only 1.5 L making one bank 750 cc , looks more like 1.25 or 1.5 max for the runners , the megatron is more as it is single outlet , this one bigger than 2 inch for sure , as i have seen that engine close up , being single turbo it is larger than the renault twin turbo setup..

A.Wayne

the original 2.5 was run at lemans with a 16 V head , turbo charged .1982