Fiat Coupé 20V Turbo cylinder head - question about porting?

Road-race engines and ancillaries - general discussion
TomLouwrier
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Re: Fiat Coupé 20V Turbo cylinder head - question about port

Post by TomLouwrier »

hi guys,

I can see the value, or rather: necessity, of twin sparks in a head with 2 very large, wide angle, valves and a big intruding dome. The plug sits off centre in the chamber, the dome is just in the way of the flame front and the chamber takes the shape of an orange peel, not a hemi-sphere or 'ice scoop'. This gives you a very slow and uneven burn. This is why those second plugs were introduced in the first place.
But that is all so very fifties, maybe sixties.

In a nice 16v setup as pictured here the plug is right there in the middle, the valves are close angled so no big dome is needed, and the chamber is compact and uncluttered.

Now why would you put 2 plugs in there? Emissions? If so, why develop 2 heads for basically one family of engines: single and twin spark? Just marketing?

regards
Tom
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WhizzMan
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Re: Fiat Coupé 20V Turbo cylinder head - question about port

Post by WhizzMan »

Effectively, with all tuning done, you can still get a faster, controllable burn from two plugs, one in the center, one on the side in this engine design. I believe that with the same cam profile, the twin spark solution would give a max of 5 bhp extra on some rpms and never less than a single spark. This was on a 2.0 engine with about 150 BHP, so there is a 3 percent "gain". Less chance on misfires and better emissions would also be an argument to put the extra plug in. I wonder if there was a TS configuration in the 156 BTCC/WTCC engine, or if they went for a single spark solution and made the valves larger? I know they used the 1.8 16V TS engine as a base, flipped the head, gave it an 86mm square bore/stroke, but other than that, I have no data. Would anyone be able to come up with more details?

Marketing would be a big part, indeed, but the technical benefit was still there when they designed and tested the engines. The most interesting development after this was the use of the second spark plug location to put a fuel injector in in the JTS 2.0 engine. They claimed a lot more power, but in practice, they were struggling to meet the 16V TS engine performance. It seems that deleting the second spark plug in favor of direct injection did not give them any more effectiveness in the engine at full throttle and the lack of the second ignition source was giving them trouble to keep up full throttle performance.

I have a sacrificial 16V TS head put away to do experimentation on. It may be about time to pull it out, do a whole lot of measurements, and take lots of pictures.
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TomLouwrier
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Re: Fiat Coupé 20V Turbo cylinder head - question about port

Post by TomLouwrier »

I'd be interested in that, for sake of Science and the Holy Quest for Knowlegde ;-)

Still not convinced of the true advantage of just one offset plug though. I'll make some illustrations later today to help discussion along.

The 'easiest' way of finding out whether the second plug was put in there by an engineer or a marketeer is to deactivate it. You may have to retard the ignition a couple of degrees though, there are slight differences here between the single plug Fiat and the twin spark Alfa brothers. Then again, the Alfa got cams with longer duration and slightly more lift. (I intend to get me some of those for the 1.8 Coupe, but that will force me on the road to reprogramming the ECU. And that's a Hitachi, not even a Bosch of Marelli. Hmmmmm.)
Yet more marketing here: an Alfa should always make more power than an -essentially identical- engine of the same size in a Fiat. Understandable, but a shame if you're into Fiats...

We really should start a new thread on this, don't you think?

regards
Tom
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TomLouwrier
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Re: Fiat Coupé 20V Turbo cylinder head - question about port

Post by TomLouwrier »

OK, here are some pictures to illustrate things.
First is the original Alfa Nord cc, a classic 2 valve, wide angled, hemi-design. You see the plug is well in the way of fitting bigger valves. Moving it outwards would mean a very long distance for the flame front from the plug all the way to the other side of the chamber.
Alfa Nord single plug.JPG
Alfa Nord single plug.JPG (156.03 KiB) Viewed 9393 times
So they made it twin spark, like this. You can increase valve size now. As far as I know the angle between the valves was closed up a bit as well, in order to get a more compact chamber.
This was (and still is) a common way to go with large bore 2V engines, even as modification on existing engines. Alfa, BMW, Chrysler, Guzzi, Harley, VW, Lycoming... you name it and someone put in a second plug. It gives you faster burn, less detonation and less ignition advance, so there you go on your way to a higher state of tune.
Alfa Nord twin plug.JPG
Alfa Nord twin plug.JPG (187.28 KiB) Viewed 9393 times
On to the 4V chamber, or 16V as it would be called on a 4-pot engine. A pent roof design, straight textbook stuff.

This I understand.
16v single spark.jpg
16v single spark.jpg (83.86 KiB) Viewed 9393 times
This would make sense as well.
16v twin spark - logical.jpg
16v twin spark - logical.jpg (85.17 KiB) Viewed 9393 times
And just maybe I'd even see some point in this.
16v triple spark.jpg
16v triple spark.jpg (84.89 KiB) Viewed 9393 times
But this I don't get, at least not as a means to make a more powerful engine.
Alfa 16v TS.jpg
Alfa 16v TS.jpg (82.12 KiB) Viewed 9393 times
Now if the second plug fires at a completely different timing it may serve to clean up emissions. Is that all there's to it? Tune the engine for power on the main plug and use the secondary to do some 'afterburning' of an otherwise incomplete combustion?

Ideas? Opinions? Facts??

regards
Tom
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WhizzMan
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Re: Fiat Coupé 20V Turbo cylinder head - question about port

Post by WhizzMan »

On phase1 and phase2 16V twin spark engines, they used waste spark. Effectively, they sparked 4 plugs at the same time, the ones in 1/4 and 180 crank degrees later 2/3. 4 coils, 2 plugs per coil, one compressed cylinder and one "empty" cylinder per coil. 2 coils got their spark command exactly the same time.

On phase3, they abandoned waste spark and were forced to get coils that used a much higher voltage to get both plugs to spark at the same time. If they got a misfire on one plug, they automatically wouldn't be able to spark the other one as well. To this day, I have failed to see the benefit of this setup, but there seems to be one.

Both plugs fired at the exact same time in both configurations.

I can think of a good reason not to go for the two plugs on the outside. You'd need to radically modify the head design in terms of coolant galleries and possibly the inlet and outlet port as well. Probably, putting the extra plug in this way, would give some benefit over the Fiat design, but would only require minimal design change to the Fiat part? Maybe they experimented (in computer simulation) with 2 outside plugs and came to the conclusion that it was too much trouble for the gain, compared to one central plug and one outboard?

I remember one of the design engineers being asked about the extra plug. He stated that even tho it looks like an afterthought, it does in fact give up 5 BHP more and cleaner emissions. I got that figure from the story that a design engineer said something along those lines. It's hearsay that I have no direct source for it to quote, so if someone can come with facts or a direct source for information on this, I'd love to hear/read that.
I have a spare set of cams for the 1.8/2.0 Alfa laying around.
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TomLouwrier
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Re: Fiat Coupé 20V Turbo cylinder head - question about port

Post by TomLouwrier »

hi Homme,

Manufacturing reasons. Might be, let's look at the castings. The upper face is different, due to the hole/space for the second plug. The lower face is different too, since the Fiat head has no boss for the second plug in the chamber where the Alfa head does. This means both halves of the mould are different. Without doubt the sand cores used for the cooling galleries are different too to reflect the upper and lower half of the piece.
So I would say these heads are really different parts as casts. The machining and assembly would be done on the same line of course, with 95% of operations identical.

You say the plugs in one cylinder always fire at the same time in the 16v's (that's the modular engines; wikipedia mentions there was a timing difference in the 8v Nord block to avoid the flame fronts meeting exactly over the middle of the piston). That got me thinking.
The secondary plug must be pretty far outboard in order not to hinder the ports and valves. In fact it's quite close to the edge of the chamber. Now if one would fit both plugs outboard, the distance for both the flame fronts to travel to the middle of the chamber would be nearly half the total chamber size (or cylinder bore). For a single centrally placed plug this would be the same, but travelling outwards. No big difference in total combustion speed, so no gains.

I suppose it is possible that the expanding burning gases from the secondary plug 'blow' the burning gases from the central plug aside, towards the end that has no plug. This would give a shorter burn. Optimize ignition and cams for that and you may indeed increase power. 5HP on a total of about 145 for the 1.8TS is 3%. Not bad for 'just an extra plug'. The corresponding 1.8 Fiat gives 130hp, but as mentioned had cams with less lift and different timing so not that good for comparison here.
If that is what gives these TS heads an advantage, then 3 plugs would even be better in theory. But you would of course gain less (diminishing returns) at a certain cost. And all these plugs could lead to cooling issues; not enough water flowing around the chamber. So 2 plugs would do the trick technically and financially.
And you can stick a label 'Twin Spark' on the back as a bonus ;-)

regards
Tom
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FERRARIST
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Re: Fiat Coupé 20V Turbo cylinder head - question about port

Post by FERRARIST »

Really don't believe that 2 sparks per cyl is big gain.......
Few months ago my wife's Smart 4 Two needs engine rebuild, i removed the 3 sparks to check compression level, and was shocked - when i turn the engine on, it starts - huge misfiring, but turning......there were another 3 sparks on the bottom side of the engine.....
WhizzMan
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Re: Fiat Coupé 20V Turbo cylinder head - question about port

Post by WhizzMan »

Are you sure you removed the spark plugs and not the injectors? Smart fortwo aren't twin spark so you must have pulled something else. Even if you got a twin spark engine running with half of the plugs pulled (hard to believe) that doesn't mean there is no power gain in putting two plugs per cylinder in. "It still runs" is something different than "we gain X percent with putting in more spark plugs".

Wikipedia is wrong about the ignition timing for the 8V twin spark production engines. The 155 and 164 phase two had DIS and the ECU did a waste spark trick, just like the 16V. The early 164 and the 75 Had two distributor caps so in theory, it would be possible to separate the spark timing. However, there is only one table and one electronic circuit to trigger both spark "amplifier modules" so the sparks really are timed simultaneously. Probably, people that think that the position of the distributor arm under the cap influences timing, came up with the myth that they had to spark at different times. This myth is even repeated for the 16V engine...

Race twin spark engines as used in the sixties and seventies GTA cars used a single 8 plug distributor cap with a distributor arm that had two contact circuits. They had either two mechanical points ignition systems, electronic equivalents, or some solution to trigger both at the same time. With two separated timing systems, timing them exactly the same would prove difficult I suppose. I have no exact details on how the racing teams solved this, so it could be they ran on slightly different timing per plug, intentional or not.
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FERRARIST
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Re: Fiat Coupé 20V Turbo cylinder head - question about port

Post by FERRARIST »

WhizzMan wrote:Are you sure you removed the spark plugs and not the injectors? Smart fortwo aren't twin spark so you must have pulled something else.
That's my wife's Smart 4 Two head, and as you can see there is 2 holes for the sp. plugs - and i repeat - engine turns for 5-6 sec roughly before i turn it off......have no explanation, just tell you the facts......
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WhizzMan
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Re: Fiat Coupé 20V Turbo cylinder head - question about port

Post by WhizzMan »

I stand corrected. The first version (450) had twin spark indeed. I was looking at pictures of the newer 451 model.
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kaci
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Re: Fiat Coupé 20V Turbo cylinder head - question about port

Post by kaci »

Hello Mr. Croft.
Will continue the theme in Fiat Coupe Forum?
I know you're busy, but many people waiting to read something from you.
Regards,
Krastyo
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