Fiat Coupe 16v Turbo with Tipo 16v cams

[Guy Croft]

Advancing and retarding the cam timing is something the skilled operator will do on a bench dyno and he will log the characteristics that spring from it, carefully recalibrating the fuelling to optimise. Of course it can be modelled on top software too, if you have it, which most won't.

Then and only then will he make a reasoned judgement on which settings give the best torque response for the intended use.

In order to do this the valve lift must have been mapped out lift by degree in the region of closest valve to piston proximity, usually the most critical bit is inlet TDC to 20 deg after. If the inlet FL is 'advanced' ie: brought forward to increase lift at TDC and this is not done there is more than a sporting chance of a valve clobbering a piston.

FWIW I think engine building carries enough risk without doing that. Yes I've done that cam_swing_on_the_dyno thing and very nerve racking it is. I always suggest folks use OE timing on turbo units, eg: 16v Integrale 103.5 inlet, 105 ex.

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[SteveNZ]

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.

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I did this simply because I thought the timing mark was incorrect. The cam timing mark was almost half a tooth out. I tried it both ways, this is my result. Although the boost level is different you can see how different the power and torque curves are and how much longer it takes to come on boost.

RPM scale is incorrect.

[Guy Croft]

Very good illustrative post, Steve, thanks!

Any chance of a legend or some notes please to say which is which/what the cam timing difference was?

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[pietch]

SteveNZ; your image is great! Can you explain it a little better?

It sounds to me like your saying your exhaust cam was a tooth out, thats 16 degrees right? Was that advance or retard?

Hope to hear some answers soon!

[Mats]

Interesting. You didn't perhaps do a run with the "slow boost" cam setting at the lower boost?

[SteveNZ]

The lines with the same colour are from the same run. The colours (red and blue) have a match to some information at the bottom. The engine is SOHC so only 1 cam to adjust. The red and blue lines indicate 1 cam tooth difference in timing. I cannot remember which way.

The red line was from 2005. I had a feeling something was not quite right. Top end power was not as good as I expected and it had a lot of boost creep on the track. Boost would creep about 2psi.

I decided the factory timing marks would be closer (slightly) if the cam was aligned 1 tooth round from where it was. The difference was dramatic. Top end power was significantly improved. There did not seem to be any adverse affects, no more lag. It also cured the boost creep. The boost would actually start to drop slightly, rather than rise. I decided this must be the "correct" cam tming. I had the car on the dyno and recieved the blue line.

Im guessing the red line was 1 tooth advanced. Maybe someone can comment on that.

Interesting. You didn't perhaps do a run with the "slow boost" cam setting at the lower boost?

Yes I did, but not at hand. There were other changes made though. Power was the same to 1 kw .

[Guy Croft]

The red and blue lines indicate 1 cam tooth difference in timing. I cannot remember which way.

Fair enough, and thanks for the additional info. Here's my take:

1. If you advanced the cam (just typically, setting inlet valve lift earlier than the standard-ish full_lift_at 110 crank deg after tdc, at 108 deg), then that would increase the inlet lift at tdc by an amount 'x' but it would also, being an SOHC unit, reduce the ex lift at tdc by a commensurate amount. (In short, because the crank rotates clockwise the inlet will be 'more open' at tdc, the ex will be 'more closed'..)

The inlet event would finish earlier so there would be reduced filling beyond bottom-centre. The inlet valve would be further open at tdc but the ex valve would tend to give less effective purging on the overlap. I say 'tend' because obviously, at higher engine speeds when the boost is on and the inlet tract momentum is high the incoming inlet air/fuel might outweigh the theoretical loss of overlap in-ex.

The ex would open earlier in the power stroke and this would give a bigger initial pulse and better scavenging, noting of course that this is why some full race ex cams open at about 90deg after tdc on the power stroke - it does work, but again mainly at higher engine speeds. But, as I said above, it will close earlier too.

2. If you retarded the inlet cam, making it open later, say 112 instead of 110 deg, the inlet lift at tdc will be less, the ex will be more. Same rule re overlap applies as above except to add that the ex flow on the SOHC head is very effective anyway and giving it more lift at tdc for a given inlet and ex peak lift configuration might be a waste of time per-se.

The inlet event would be extended beyond bottom-center and thus filling would be enhanced. The ex event would be delayed, with the valve opened later in the power stroke - thus extracting more useful work from that cycle, something well-proven to be effective on blown motors - and called 'extending the power stroke'.

Of course I have no clear idea which was the case with this motor but I would just add something else: There are 42 teeth on a camwheel in this case. So the pitch is 8.57 cam degrees, which is 17.1 crank degrees. That is a massive increment and so a 'tooth out' on cam timing can easily lead to bending a valve.

On the SOHC both inlet and ex valves have the same proximity to the piston unlike the TC where the inlet is already closer to the piston than the ex even before lift starts. So on the SOHC you have an equal chance of bending either whichever way you turn the cam to reset the timing, whereas on the TC if you advance the inlet cam - open the valve more at tdc you are sure to clobber a piston.

I hope I have got my synopsis correct, and I hope it's useful to an extent. Perhaps, Steve, after reading it may shed some light on which way you moved the timing.

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[nabilhpe]

Hello Guys,

Guy, do you think there is a gain to be noticed for using a bigger throttle body say from an Alfa Romeo 6cylinder on the 2.0 16v turbo engine? assuming we are using :
- N/A intake cam on the fuel side, and turbo intake cam on the exhaust side
- Bigger turbocharger than stock,basically T3/T4 60trim, .60 a/r compressor
and .63 a/r turbine with a stage3 turbine wheel.
- a much better front mount intercooler
- 1bar turbo boost
- 2.75" exhaust pipe going under the passenger door, with no silencer (drag
race car)
- piggyback electronic box to fine tune the air fuel mixture

Thank you
Nabil

[Guy Croft]

Not at that low boost level, definitely not.

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[nabilhpe]

Not at that low boost level, definitely not.

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at what boost levels you think it should make a difference? what about if we fabricated individual throttle bodies with custom air box, same style as the one used on BMW M-powered engines, we tried this once on the M102 engine taken from the BMW 745 turbo from the 80s, you used a set of ITBs from an M5 engine and it made a difference in performance, boost was +1.5bar, turbo was a T70.

[Guy Croft]

That is a subject for a different topic, but I want some good photographs.

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[nabilhpe]

That is a subject for a different topic, but I want some good photographs.

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photographs of the BMW engine?