Fiat Coupe 16v Turbo with Tipo 16v cams

[Saint]

Hi all

I am currently in the process of building a 16v Turbo Fiat coupe into a fast road/club day car - most of my advice to date has come from the Fiat coupe car club forum (FCCUK)

I have a fairly long list of bolt on mods (intercooler/exhaust/evo cars chip etc) but very little engine component changes apart from an Integrale evo turbo unit and tubular manifold.

I am looking to change cams and have come across a set of 16v Tipo 5 door cams which are much higher lift than the standard coupe turbo cams (10.7mm vs 8.??mm) while swapping the inlet is a common modification for the coupe and gives an extra 10-15BHP the exhaust cam swap is less common. Normally the exhaust cam is swapped for a coupe inlet cam which has been machined but as I am swapping a full set of cams with another owner this is less of an option for me.

Are there any problems with running both the Tipo 16v (5dr) cams in a turbo configuration (ie too much overlap etc)

Thanks for any advice

[Guy Croft]

Just change the inlet cam, OK? Leave the turbo exhaust cam alone.

GC

[Saint]

GC

Can I just ask what the issue is with changing both? would be interested to know.

Thanks

[Guy Croft]

On a normally aspirated engine, lift at top dead centre of ex and inlet valves is used to purge the cylinder of ex gas at the end of the ex stroke, by allowing incoming charge to flow over the top of the piston carrying ex residuals with it; at the same time, at higher engine speeds, this valve 'overlapping' generates high intake mixture speed and momentum, encouraging good cylinder filling. This is sometimes loosely called 'ram-charging'.

The turbocharged engine in this particular case - when it has manifold pressure - has a excellent scavenging across the valves at tdc on the overlap, even with modest lift at tdc on the ex, and so just doesn't need the exhaust valve lifted higher than standard. This holds true even if the inlet cam is changed for an OE normally aspirated version. It has excellent filling too because of the manifold pressure without the need for ram-charging.

Would more overlap help the off-boost response? No, because the engine speed is too low.

That's the short story, based on using an n/a inlet cam. Change to lifts higher than that and things get a lot more complicated.

GC

[Saint]

Thanks GC

So basically I will be forcing unburnt fuel through the overlap of the cams (moreso with N/A cams) - this is more than is necessary as the standard exhaust will have enough overlap to do this anyway - hence I am just wasting pressure/fuel

Think I get it now - thanks for the advice

[Alexis]

Hi Saint

Glad to see you found your way over here too :)

most of my advice to date has come from the Fiat coupe car club forum (FCCUK)

Shouldn't that be, most of the advice has come from me? :) But that's the answer i couldn't give when yourself and JMA asked me why not to use the 2 x tipo cams because i didn't know if what i was telling you would be correct

[pietch]

Interesting discussion...

Just wondering what affect an n/a inlet cam on a 16v turbo will have on low down power and turbo boost pressure build up...

I am concerned that the n/a steals power from the bottom to feed the top and that this results in slower turbo spool up.

Can anyone confirm my suspicions?

[Guy Croft]

I will answer but - first could you please explain exactly you mean by 'spool up'?

Thanks,

GC

[pietch]

Hello Guy,

'Turbo spool' in my opinion would be the speed that the turbocharger is accelerated from exhaust gases to produce positive boost pressure.

I found this on a Google search, not sure of the source: …œA side benefit of reducing overlap is going to be a smoother idle. Increasing blowdown this way may also spool the turbo faster.‚

Look forward to your feed back as always.

[nabilhpe]

If it helps, I did a camshaft swap on a Fiat Ritmo that has a turbocharged 2.0 16v Lancia engine:

First we tried the Tipo n/a intake cam while keeping the std turbo ex cam. There was a significant increase in top end power, and using the T3/T4 60trim turbo with a stage 3 and .63 a/r turbine we were hitting full boost at about 3500RPM

Then we tried the n/a ex cam as well, full boost was now higher up at 4000RPM, and top end power was more too, the engine pulling easily to 7000RPM.

I hope this is useful,

Nabil

[Guy Croft]

I'll remark on what the search said first:

…œA side benefit of reducing overlap is going to be a smoother idle. Increasing blowdown this way may also spool the turbo faster.‚

As a general rule yes reducing overlap - the period for which both inlet and ex valves are held open around tdc (top dead centre), and their lifts during that time will result in a 'smoother' idle relative to an engine with more overlap. However it is surprising how much overlap an engine can tolerate and still give a smooth idle. By smooth idle I mean no misfiring and vibration/rocking due to reverse flow of ex gas into the intake at that low speed (intake momentum is of course low), and good pickup when the throttle is opened - something often linked to 'smooth idle'. However it is a very generalised thing.

Insofar as the second part of the quote is concerned, it's either wrong or perhaps just badly written. Blow-down is of course reduced by reduced overlap. 'Blowing-down' (a term first coined during the early days of supercharged units long before the turbocharger was ever used on car engines). To 'blow down' you have to 'overlap' (an odd verb, I know, but one worth remembering) as I have described - and that overlapping blows fresh charge over the top the piston, purging ex residuals form the cylinder. I studied this quite intensively at University, many years ago of course, but I can remember very well that the power gain from doing this is substantial - albeit that fuel is lost down the ex pipe. So not attractive to a manufacturer working under rigid (and laudable) emissions constraints.

The fact is however, that blowing down can only be achieved when there is sufficient inlet charge (fuel/air) 'column inertia (momentum) to to force remaining gases out of the cylinder. Why? Because during the overlap phase the cylinder (or at least the clearance volume above the piston) is at atmospheric pressure and if the charge in the inlet tract is not above atmospheric it won't go in. And if there is overpressure in the ex tract - as there commonly is on a turbocharged engine at low speeds (simply because of the flow restriction of the slowly spinning turbine) you are not going to get any purging at all.

To get into a good blow-down state you need to run run the engine faster (normally aspirated units) or be on boost (turbocharged units). The supercharged engine of course has the best of everything since its boost (inlet manifold pressure) is directly crank-speed dependent.

To refer now to your original question about using a normally aspirated inlet cam in a turbocharged engine. The increase in lift at tdc is very modest - and remembering that my suggestion was to change the inlet cam only and retain the std turbo ex cam, this is not going to have any noticeable effect on idle - and pickup from idle. The reality is that we have not increased the overlap event to any real extent because the ex phase is unaltered.

However the peak lift and lift integral (area under the cam curve) is quite a bit more - and thus the amount of charge induced during the intake phase is going to be far greater. This will impart more energy to the turbine and make a huge difference to your 'spool' up under all conditions, at the same time giving the turbocharger the capability of generating higher intake boost - or more intake mass flow. The overall effect will be more torque everywhere and the rpm at which peak power is developed will be a bit higher to because the cylinder is filling more effectively - in other words the engine is 'breathing' better.

This does presuppose that the ex valve, port and manifold can cope with the increased massflow of ex gas, without getting into a serious pumping loss or backpressure problem which I can tell you on the 8v and 16v TC units it certainly can, either in standard trim or ported/modified.

I hope this sheds some useful light on this.

GC

[pietch]

Nabil that¢ž¢s for sharing your experience with the cam setup in a 16v Turbo.

Guy, what an explanation, I had to print it off and read it three to four times before it all sunk in, a very pleasurable and informative read! Thanks so much!

So in layman¢ž¢s terms more overlap equates to more blowdown which causes a loss of boost build up at low RPM but once overcome (by coming on boost) promotes much more efficient cylinder filling with a slight loss of fuel during the overlap (blowdown).

For a performance enhancement replacing just the turbo inlet cam for n/a equivalent (rather then switching both cams to n/a) will allow faster turbo spool, less overlap and less fuel loss. Of course in this case there will be less blow down which will also equate to slightly less power once on boost.

I suppose it all boils down to what sort of engine characteristics you require to match your driving style and gearbox ratios.

I currently use the Tipo inlet cam with my old turbo inlet cam as the exhaust, this does not have quite as much lift as running two Tipo cams but from the above I understand that switching my exhaust cam back to standard will improve my turbo spool.

I wonder if there are any other proven methods to reduce turbo spool?

Thanks again, I am sure this forum post will help many!

[pietch]

Guy; I will be changing my cam setup to include the Tipo N/A inlet with standard turbo exhaust cam.

I recall reading on another forum that advancing the Tipo cams 1 degree was the best setup, but also remember being told that retarding them 5 degrees would give much improved turbo spool and improve idle.

What I need to know is can I advance or retard the cam using the standard pully, i.e. moving them a few teeth? I don't want to spend out on venier pullys at this stage.

Or would a standard cam install be the best method without any retard or advance?

Thanks

[Guy Croft]

No - you must not move the belt one tooth either way on either cam, that is a huge increment.

You must use adjustable camwheels for that.

GC

[pietch]

Guy is it worth really advancing or retarding the tipo inlet cam then?

If so is advancing or retarding better for less turbo lag... and technically why is this?

Thanks so much.