I would like to ask a reasonably direct question if I may.

On the 20v n/a and 16v n/a will a short induction path and removal of the resonator box actually loose power though the lower end of the RPM band?

And if so, will some one care to explain why?

Any engien wants nice cool air with a nice smooth entry path. So if you provide a 20v or 16v with a reliantly short induction path with a cold air feed (specifically the 16v on this occasion) why is it people say they loose power?

The only thing I can thing of is with a longer induction path and a larger volume of air between the filter and throttle body there is a (little) volume of already filtered air before the car starts having to "pull" air though the filter, At high RPM I can see why the shorter the path the better...

Is this all gibberish? Will some one with more technical knowledge with fluid dynamics care to comment?

Second question of the day

Exhaust systems, On a N/A car I understand there is a need for back pressure, something about exhaust gas scavenging with some fairly complex calculations with gas exit speeds, and volumes

With a turbo however you want the gassed getting away from the compressor wheel as quick as possible? so surly we should all be fitting massive down pipes and 4 inch systems for power gains....some one mentioned to me once over spool of the turbo as being one problem.
Is it not as simple as fast gas in, faster gas out?

Sam - great questions and there's huge amounts of information, ranging from relatively simple, to massively technical

However, your comments are broadly correct

On a normally aspirated car, the induction side is "tuned" - I can't remember which way it goes, but lengthening and shortening the inlet tract has a noticeable effect on performance, particularly torque. This is why the VIS was built (Variable Intake System) and also BMW's (in)famous VANOS - they both alter the inlet tract length for better tractability or power

As for the exhaust - yes, an NA needs backpressure, which creates resonances in the exhaust. This is tuned for efficiency (and noise) but serves to assist the filling of the cylinder on the intake stroke and serves to help empty the cylinder on the exhaust stroke

Turbo systems are less complex

On the intake side, its purely a matter of getting as much gas into the cylinder as possible, so its all about making the inlet free-flowing. This in itself is complex, requiring significant knowledge of fluid dynamics and flow characteristics. Simply attacking a head and making the ports bigger will probably do more damage than good.

On the exhaust side, I recall someone saying the best exhaust was no exhaust Probably not too good for the road, but great for performance. Bring your own Nurofen! However, its simply a case of getting the gasses away from the head as quickly as possible - there's no point in trying to create tuned lengths to help with scavenging of the cylinder, as the turbo gets in the way.

I seem to recall that there can be an issue with some turbos - something to do with pressure differential either side of the exhaust turbine. This is what kills the oil seals on standard turbos, but I remember reading it can have an adverse effect on performance too.

There's plenty of mis-information on exhaust sizes, but 3" is accepted to be fine for a modestly-tuned Coupe - quite simply, there's no way that a 300bhp 2-litre Coupe lump could produce enough gas to make a 3" system restrictive.

Have a read of Guy Croft's site - lots of info there

inlet and exhaust runner lengths are as susceptable to tuning improvement in FI as in NA. The lengths to suit FI wouldn't be comparable to NA lengths but optimum still exists. On the inlet side the air behaves in a very similar way, it's just that it's a higher density, the volume the engine "gulps" is very similar. Exhaust gases will still provide pulses to travel back and forth even with a turbo, they'll still travel back to the valve and at certain points in the rpm have the ability to pull gases out of the cylinder. A bigger % improvement is possible than for NA, it's just hardly ever done.

A basic fact of turbo exhausts/downpipes is that every psi reduction seen here is a psi less on the engine side of the turbo (in the manifold) this quite simply gives the turbo a less hard job to do. It's still spinning the same (better potentially lets say) compressing the same amount (capable of more now) of air. Same gasses, less restriction, same intake volume at less boost, closer to the fabled 1:1 intake/exhaust ratio. Achieve that and your really onto something worth tuning lengths for.

I think the induction route on our 20VT's are more clever than we assume.

One example was my test on a RR-session this summer.
I did two runs, first with std airfilterbox (modded with 4"-pipe) and std paperfilter. Then a second run with the top lid of the filterbox lifted (meaning no restriction at all straight into the 3" intake without any filter.)

Surprise surprise - lower boost and lower power! In the region of 10hp if I remember correctly. From 267whp to around 257whp. Quite the opposite of my expectations.

so bottom line is keep the stock airbox?

redd

Inlet tuning is about the inlet manifold design as other have explained, the length of the runners from the plenum to the valve. It’s unrealistic to ever change the design unless you have a full race engine where you may switch to ITB or DCOE carbs etc. What happens before the throttle body is largely irrelevant; you just need as little restriction as possible. Length of the piping etc is not going to matter in the real world.

However, inlet resonators built in-between the air filter and TB do have some effect. I've seen studies done with different size resonators giving minor yet different power curves. They don’t have an effect on peak power but there may be some gain lower down.

I can only guess this has something to do with reversion of air up some of the runners interfering with other cylinders' filling at low rpm, these resonators may have some effect at preventing this.

i think the main thing with the 20vt turbo is the ammount of bends in the pipe work the more bends you can smoothen out the better

Aftermarket Air filters have (in my experience) always gave less torque low down and increased it slightly at the top end.

To the best of my knowledge what happens when you shorten the induction tract, i.e. using a cone filter etc. is that you remove the factory filtration system which incorporates a device or means of smoothing air flow and as a result you hear the induction noise more (the rumble when you put your foot down).

This induction noise is turbulence which effectively chokes up the inlet and gives a dip in torque. Now at higher RPM the vacuum produced by the cylinders is greater and why usually power can be gained as the greater vacuum helps to speed the air and smooth it thus preventing it chocking, the same with a turbo. Although off boost performance can suffer and on boost is improved as restrictions are less.

As Nigel quite rightly said it's very hard to design an intake systems for all conditions. What we get is compromise from the manufacturer. Is it any wonder sticking a 50 quid filter on or messing around with pipe lengths can loose torque. Fiat spend millions in research and development.

The question is a bit more complicated if you have a late 20V N/A VIS engine. The VIS engine has a variable inlet which changes its effective restriction and length to give optimum length under varying situations. It manifests itself as more power than the old 20V engine plus a flat torque curve with peak torque much lower down.