Thoughts on the Palio 1.6 motor

[vignesh]

Hi,

Was glossing over the manual of the Palio 1.6 recently and something struck me. The timings for the 1.6 motor as per the manual is like

Intake Opens 0deg BTDC
Intake Closes 34deg ABDC
Exhaust Opens 24deg BBDC
Exhaust Closes 0deg ATDC

So effective valve overlap is Intake Open + Exhaust Close = 0 + 0 = 0degrees.

From whatever I've read and know, a certain amount of overlap is actually desired to gain decent low/mid/top end performance. The overlap will work well in conjunction with a tuned exhaust. I've also read that this kind of zero valve overlap is ideal for turbo applications. Going by this, can we assume that the cam is the biggest bottleneck in this motor?

Your thoughts,
Vignesh

[SunnyBoi]

The timings are right - Below is a photo from the Indian Palio Workshop Manual.

As per my understanding, since the valve overlap is zero, free flow exhausts will not matter on the Palio 1.6. Perhaps the de-cat/free flow muffler helps a bit.

[Ravveendrra B]

WOW! Interesting topic. Thanks for starting this thread.

The figures are right.

You are looking at only TDC (opening for Intake and closing for Exhaust). What about the overlap in the the BDC (bottom dead centre)? The intake closes 34 deg ABDC (after bottom dead centre) while the exhaust opens 24 deg BBDC (before bottom dead centre). Is that not an overlap? Is the 58 degrees an overlap when both the exhaust and the intake are open?

Incidentally, I have an FFE on my 1.6 motor http://myfiatworld.com/forum/f23/making-ffe-234.html, and it did make a difference.

[SunnyBoi]

WOW! Interesting topic. Thanks for starting this thread.

The figures are right.

You are looking at only TDC (opening for Intake and closing for Exhaust). What about the overlap in the the BDC (bottom dead centre)? The intake closes 34 deg ABDC (after bottom dead centre) while the exhaust opens 24 deg BBDC (before bottom dead centre). Is that not an overlap? Is the 58 degrees an overlap when both the exhaust and the intake are open?

Incidentally, I have an FFE on my 1.6 motor http://myfiatworld.com/forum/f23/making-ffe-234.html, and it did make a difference.

Four Stroke Engines have a 720* cycle and as per the manual

Start - 0 - Inlet opens
214 (180+34) - Inlet closes
214 to 360 - all valves closed, compression happens
360 - bang
516 (360+180-24) - Exhaust opens
720/0 - Exhaust closes

Hence there is no overlap whatsoever on the 1.6.

There is 6* of overlap for the 1.2 though. FFE would work for it

[Ravveendrra B]

Hmm, thanks. I did forget about the fact that the inlet and outlet open in different cycles.

Coming to FFE's and the way they work. To my mind they are more effective when there is no overlap.

If we see how the FFE's work - (a) An FFE removes restrictions in the flow of exhaust gas by smoothing out bends, removing constrictions and using wider passages (pipes). If an exhaust did only that, it would be a simple FFE. If one goes in for a 'tuned FFE', the FFE does something else as well - (b) it generates a vacuum just behind the exhaust valve(s) at the time they open. This is done using the fact that exhaust gases come out in pulses or waves. The high pressure of the discharged gas is immediately followed by a period of low pressure. There is a reverse (resonance) wave that travels backwards from the tip (or collector) to the cylinder head. If the exhaust is tuned i.e. the width and the length of the tubes are measured accurately to ensure that the resonance wave reaches the top of the valve just in time to match the opening of the valve, the wave sucks out the exhaust gases. This will help the engine breathe at a particular RPM range for which the FFE is tuned.

In case there is an 'overlap' esp. a large overlap there is a danger that the low pressure created by an FFE will also suck some of the 'charge' (fresh air + fuel mixture) being let in by the open intake valves robbing some power.

One misconception many people have is that 'backpressure' is important. This is true for 2 stroke engines as both inlet and exhaust are open at the same time and there is high risk of the charge escaping with the exhaust, robbing the engine of compression and fuel. For this reason, two stroke engines have exhaust systems accurately tuned to time the backpressure to cut off the evacuation from the chamber at a point so that no 'charge' escapes. Backpressure might also be relevant to 4 stroke engines with large overlaps (as in old sidevalve engines), but definitely not in engines with nil or small overlaps in valve timing.

One fact to be kept in mind is that an FFE (more so one with just 4 primaries one collector and a tail pipe) will work best at a particular RPM not throughout i.e. it will be peaky. It can be tuned for either low range (or low to mid), these typically use narrower & longer pipes, or a mid-range punch, or a high rpm (mid to high) the exhausts for high range use wider and shorter pipes. A low range FFE will choke the engine at high RPM, a high range FFE might be rough at idling and also seem sluggish at low revs. To get a better spread and ensure that the engine is not too peaky one uses a 'Tri-Y' system using 4 primaries, two secondaries and three collectors, 2 primaries join one secondary and then the secondaries join and vent through the tail pipe. This evens out the gains in power. For the Palio 1.6 (and most 4 cylinder engines) cylinders 1 & 4 are joined together while 2 & 3 are joined together i.e. the alternate cylinders from the firing order are joined together.

Cheers,

[prabuddhadg]

I agree about the side valve engines. My father had a Morris 8, and I used to drive that a lot back in Calcutta, in the 80's and 90's. Once the exhaust manifold broke off. The engine note was a raucous din, but the pick up which was already quite placid, actually went further down.

Nothing further to add. Please continue this interesting discussion.

[Dr.Palio]

To start with Fiat have given just ordinary cams. Jack of all Master of none. Little bit of valve overlap is needed for good low end and need to taper off at high rpms. Thats the reason Vtec has 2 sets of inlet cams. Also In the Fiat twin cam tuning guide, they have mentioned advancing the ex-valve timing has good benefit if "Advanced" and poor performance when retarded.
Now the practical issue: After few thousands of running in the new belt, slowly the belt stretches. Fiat engine runs clockwise direction viewed from driver wheel side. After the idler bearing, Inlet cam is connected to the belt and then the Ex-cam. The inertia forces makes the cam little retard and if the mech didnt dial the Cylinder 1 at TDC before setting the cams the effect of reduced pickup will be shown after few thousand Kms. To avoid this I can suggest pull few degrees 1~1.5 deg before TDC then lock the crank then set the cams. The slack will be compensated by retarding the crank slightly. This is just my theory..I will do this during my cam belt change

[vignesh]

At first glance it does seem like the job of the exhaust design in a 4 stroke motor just needs to suck out all of the burnt gases effectively. While that is true and is required to make the motor efficient, if there is a valve overlap, then the vacuum created by the exhaust pulses would also suck in fresh charge via the inlet port during the overlap duration. Now comes the interesting part. If all of the charge gets sucked out along with the burnt cases out of the exhaust, then the motor is not running efficiently. If the exhaust design incorporated to provide a certain amount of negative pulses travelling back to the ex-port at certain rpms, then the charge would not be lost. This means we'd be maintaining the purity of the mixture in the cylinder as well as stuffing a little bit more charge into the cylinder and therefore make a bit more power at certain rpm ranges. Of course this would also mean the fuelling needs to be adjusted and the ignition to certain extent.

Hope that clears the bit about the need for certain amount of valve overlap and how a tuned exhaust can use that to good advantage. But yeah, as with everything more overlap != more gains. Over scavenging will make the motor run leaner and will eventually end up burning the valves.

@Ravveendrra - You are correct about the word "back pressure". The right term to describe this would be "scavenging effect". Nice experiment there with your exhaust fabrication. I guess if you were running an aftermarket cam to provide a bit of overlap and a tuned exhaust, the effect would have been even more noticeable.