Ported Inlet and Exhaust manifolds

[jNk]

I am gathering information for my cars possible tuning and the inlet and exhaust manifolds porting seems to be a recurring topic at several forums.
Unfortunately I have not found any one yet which lists the pros and cons regarding this upgrade; just the usual opinions like "better airflow" or "better spool" and so on.

Does anyone here have experience with them or know some answers for the questions bellow ?

- Is it more of a Stage3 (300bhp+) or higher upgrade or even a Stage2 (~260bhp) car can benefit greatly from it ?
- How is the Torque and BHP curve affected by this ?
- How is the Turbo Spool affected by this ?

[Steve Webb]

There are a few things you need to take into account with when dealing with turbo cars.
First thing I would do is make sure that the manifold ports are matched with the ports on the head. Make sure that they are the same size and shape, ie there is no step or lip between the head and the manifold.

Making sure all the casting imperfections are removed is a good place to start it will give you much more return on effort than trying to smooth the inlet runners.
Perhaps a bigger influence on how the motor performs would be the plenum size. If you think about the plenum as a reservoir for boost pressure, then a larger plenum will present more stable boost pressure to each cylinder, however as the reservoir is larger it will take slightly (very slightly) longer time to fill. There is actually a formula for plenum size, but I can't seem to find it right now. The shape of the plenum also has a big influence, but this is usually compromised due to packaging reasons.

When it comes to the exhaust side of things, because of the turbo, you want to get the exhaust gases to the turbo as quickly as possible and with the exhaust pulses arriving at equally spaced intervals.
This means, matching the port size obviously, then keeping the runner diameters small to keep exhaust speed high, but not small enough to create a restriction. To make sure the exhaust pulses arrive at equally spaced intervals you should aim to keep the runner lengths as equal as possible.
Do this and you will be making sure the turbo spins up as fast as possible. After the turbo you just want the least amount of backpressure, which means as large a diameter as you can fit under the car.

All of the above will help smooth out torque and power curves, not just at full throttle but at all driving styles. The changes will make all engines better, but obviously as you modify an engine, other parts of the engine will become the restriction to how much power it makes.

[jNk]

I am mostly aware of these theories, nevertheless thank you for the detailed technical answer ...however I still have question about their application

- I'm not messing around with the plenum size (maybe only with the the Kappa 16v inlet manifold if it gives me low and medium rpm advantages, but if its only better at 5k+ rpm then im not interested) and keep the stock with porting
- I know the tubular and 1-4, 2-3 split exhaust manifold is better than our stock log one but I wont spend 2k EUR to a Supersprint one and i'm not aiming for 350bhp+ so will keep the log with porting
- Already got a 2.5" open system from the resonator, do I need to upgrade even the elbow(after the turbo) and the remaining stock part of the exhaust ?

[Steve Webb]

The engine in the Q4 was tuned for mid-range torque rather than top end power (as in the Integrale) and I guess one of the ways they did that was the inlet with the long U shaped runners.
Now, in my opinion, when the Alfa marketing people sat around a table and decided what the Q4 was going to be, they got it wrong, the performance was far too close to that of the V6, and the body styling was far too tame. The Q4 should have looked something like the Ti-Z and it should have delivered its power like the Integrale.
One way to try and rectify this is to use the Integrale/Kappa manifold, yes you might lose some midrange but I can live with that.

Do Supersprint manifolds really cost that much now??

As for the exhaust question, one thing the Q4 does have, is a Pre-cat in the downpipe, this is a restriction right after the Turbo, if you modify an Evo 2 item, you can get rid of the restriction. In the UK I can get away with running my Q4 without Cats of any sort, and will run it with a straight through exhaust all the way to the rear silencer. I've got no scentific tests, but the car feels much more responsive like that.

[jNk]

A like the torquey character of the Q4 which is somewhat more refined for everyday driving than the top-end Integrale, at least for me. Moving this power delivery way higher up in the rpm range seems useless when im mostly using my car in town or in short routes.
If the Kappa manifold adds only high rpm (how high btw?) advantage but the inlet/exhaust porting can help on low/mid-range then I might think about it.

Yes, its pricey indeed: ALFA ROMEO 155 2.0 Q4 Integrale - Manifold Stainless steel 310S - €1,648.00 + VAT / €2,010.56 VAT included

That small pre-cat is still there at my car, where can I find a 'grale Evo 2 exhaust ?

[Steve Webb]

The front pipe can be had here there are others out there, but they are much more expensive.

[AlfaJack]

I got the downpipe with flexi made for £190. It cuts out the pre-cat and only uses one flexi so tidies up the whole system nicely. It was a bodge job before of cut pipes and caps where I had taken out one of the flexi sections.

[Steve Webb]

Sounds good Jack, especially when you look at Zagatos website, they are asking £480 for the Integrale one!!!!!

[arguti]

In respect of exhaust manifold design, i remember reading a few threads by Guy Croft on his forum and on the fiat Coupe forum where he (exhaustively!!) tested the factory exhaust manifold against several of the aftermarket stainless ones and demonstrated that despite its compromised design, the factory one flowed better than some of the others - he also went on to point out that on turbo cars, the exhaust manifold design is absolutely critical not only from a power perspective but also the risks of destroying your engine fro ma poorly designed one. i think he only recommends one after market manifold for the Integrale and that was either £1,500 or £2,500 - cant remember which!

[Steve Webb]

Exhaust manifolds are a contentious subject. Some people will argue for days about what size primaries you need, and the shape they need to be.
When you are at F1 levels of performance I guess this is the case. However if you get the chance, take a look at Engine Masters on YouTube, they test all sorts of performance theories and come back with interesting results. While most of the work they do is on big V8s it still does raise questions about some widely accepted performance mods.