Fiat 2 litre Engine rebuild/modification

[dkermode]

Hello Guy,

Thanks for the opportunity to share my experience and request advice.

I have a 1979 2l Fiat Spider. I am in the early stages of rebuilding and modifying the engine. I will list the present components and then throw out a few questions.

- 10:1 cast pistons
- rally cams (IIIA)
- 40 IDF's (37mm choke) with waffle manifold
- triple valve springs
- aluminum flywheel (10lbs - custom made with steel insert)
- 4-2-1 header (12" primaries)
- 43 mm intake valves and 37.5 mm exhaust valves
- knife-edged/balanced crankshaft
- lightened connecting rods

Concern #1

It seems from reading your book "Modifying and tuning Fiat/Lancia Twin-Cam Engines" and various other resources that the 37.5mm exhaust valve is a little large given a 43mm intake. Generally I have found that there is a greater difference between the two. Perhaps this is no real concern. I would assume this would have the effect of increasing or moving the peak horse power point into a slightly higher RPM range at perhaps a small expense in terms of lower-end torque.

In general, does a larger valve essentially have the same effect of a higher lift cam - more flow with the same duration? Does that have the effect of moving the peak horse power higher into the RPM range with the same cam?

Concern #2

From reading the same sources as above, it appears that the header I have, with 12" primaries, may be a bit on the short side. How significant is this concern in terms of street operation? I am wondering about the extent of the interference that may occur. Could it take a few horse power off the top end or could it have a fairly dramatic effect which could really mess this up?

Concern #3 (last)

Current wisdom (again same source) suggests that the cast pistons have a RPM limit of 7200 RPM. Is this a sustained RPM limit or a momentary limit? With the cams and big valve could I be getting close to building a engine with a peak power beyond what the mechanical internals (pistons) can deal with?


Any comments from anyone are very welcome and appreciated.

thank you!

doug

[Guy Croft]

Doug, hi

sorry to be pitifully slow answering.

In your first concern you wrote:

It seems .... that the 37.5mm exhaust valve is a little large given a 43mm intake. Generally I have found that there is a greater difference between the two. Perhaps this is no real concern. I would assume this would have the effect of increasing or moving the peak horse power point into a slightly higher RPM range at perhaps a small expense in terms of lower-end torque.

The 37.5mm ex valve - if the seat work is done effectively, will offer a marginally superior cross-flow effect on the scavenge phase - and thus tend to enhance the high rpm torque (and, it follows, the power too). I say tend because I don't have precise data so to an extent I am citing 'theory'. The gain will depend on realtive curtain are, see GC V/W 'all the power is in the head'.
Will it hurt the lower end torque at the same time, yes it could. But this is true of all engines, building for high cross scavenge adds on at the top end of the power band, subtracts at the bottom.

Would it also reduce pumping loss - the power needed to pump the exhaust gas out of the cylinder once the cylinder pressure has dropped to atmospheric? Well yes I suppose, depending on the effectiveness of the header itself, although the ex port is such a high flow thing in the TC that a valve of that size is not necessarily a pre-requisite. I have run countless 43.5/36 valves and got exactly the results I expected.

You also asked:

In general, does a larger valve essentially have the same effect as a higher lift cam - more flow with the same duration? Does that have the effect of moving the peak horse power higher into the RPM range with the same cam?

Not exactly. A bigger inlet valve gives more flow pretty well throughout the lift range, but the flow may peak at a level lower than the head is capable of producing due to the intrusion of the valve into the flow - if the cam lift is insufficient. The cam should generally be chosen to match the inlet flow, ie: no need to 'overlift' but try not to 'underlift'. What I mean in more detail is that if the valve flows well up to 11mm don't use a 10mm lift cam if you want to win.
What a bigger inlet valve will do (up to a point, shrouding can be an issue) is give better breathing and thus more torque across the whole rpm range, though not necessarily yeilding an increase in peak torque. The latter is finite quantity, I've never had more than 156 lbf ft from a 2 liter 8v; it;s a function of volumetric efficiency (highest at peak torque) and atmospheric pressure. And exact cubic capacity, eg: a 2048cc 2 liter TC will peak at 156lbf ft, a 2.1 liter about 160lbf ft.

You make a good point regarding what you say here:
does a larger valve essentially have the same effect as a higher lift cam - more flow with the same duration?

A bigger inlet valve offers an improvement in the overlap phase, but not quite as good as a cam with more lift around tdc. Not quite as good because although the curtain area is a bit bigger the valve is still intruding into the airstream to a great extent, whereas with a different cam with more lift at tdc the intrusion is physically reduced and the flow gain is much greater.

In my experience bigger valves of either inlet or exhaust move the peak power up only very marginally, 100-300rpm, not more. Likewise porting work on its own.
To get an engine to develop high top-end torque/power you have to have a greater extent of in/ex valve lift on overlap, and well, going back to what I said earlier, more cam lift too.
With insufficient overlapping the cross-scavenge won't work - and - with insufficient inlet lift throughout the valve lift cycle the cylinder filling is going to be pretty poor.


I hope this makes sense anbd is helpful in your understanding, some of these questions are not easy to answer in what little time I get to do it. I'll try and come back to your other points another day.

GC

[Guy Croft]

Hi Doug,

you wrote:

Concern #2; From reading the same sources as above, it appears that the header I have, with 12" primaries, may be a bit on the short side. How significant is this concern in terms of street operation? I am wondering about the extent of the interference that may occur. Could it take a few horse power off the top end or could it have a fairly dramatic effect which could really mess this up?

12" is way too short for a 2 liter engine running anything other than say, standard cams.

Your setup being:
- 10:1 cast pistons
- rally cams (IIIA)
- 40 IDF's (37mm choke) with waffle manifold,

your primaries need to be I reckon, if you get them in the vehicle, between 23 and 25" long, matched for length, with overall length incl matched secondaries from head to end of last collector before tailpipe 32". 1.5" bore primary, 1.75" secondary, 2" bore tailpipe, single large capacity glassfibre packed oval muffler, with 2 - 2.5" perforated sleeve. Say 18" long x 8" wide x 4" height.

Yes, I indicated 17-19" primaries in my book, and that's OK, but lately, advanced software analysis has firmly shown that much longer primary short secondary is best on that motor. The rampipe has an unexpectedly significant influence too, being considered in analytical (pressure wave) terms, part of the whole in/ex tract length.

That's as close as I can give you, and yes I'd expect chronic interference at idle and pickup with the length you cited with 3A cams.

GC

[dkermode]

Guy,

Thanks very much for the information you have provided. It is certainly of value. I am interested to find out what you think of the merits light-weight forged pistons for the street. Certainly more costly, however, I guess, if nothing else, there is peace of mind at higher RPM with the forged pistons.

Thanks again.

doug

[Guy Croft]

Cast pistons are always heavier than forged, given a half-decent design of the latter anyhow - it is easy to produce and over-heavy forged item. Most often folks order direct from the usual names and ask for a copy of a street piston, that's what they get. To get lightness (accurately) needs a detailed design not just a spec sheet - and close co-operation with the manufacturer.
So, predisposing that good forged ones will be way lighter than production OE spec pistons, how much of an issue, really, is lightness?

Well, being technical, the answer to that turns on the engine speed you're likely use - whether routinely - or by acccident when you over-rev it. Too much stress (compound of many factors) and too many working cycles - aluminium (and titanium) components will fracture.

I became aware of the risks of cast piston fatigue failures a long time ago when I had some break up in two other engines, a GpA Skoda and open-class SOHC Fiat 1300. They failed because the pistons suffered fatigue - too many cycles and too high stress. In both cases I determined the point of propagation of the first crack (characteristic beach marks from fatigue fracture) as being the region around the lower ring groove.

Now I never had a cast piston break up in a TC, but that said - after the SOHC failure I was very strict about rpm limits. I'm not going to dwell on fine detail here (hours run, stresses) here because no-one keeps records on that kind of thing, I'm just going to tell you my current recommended limits. Yes, in my manual the limits may be different; one moves on, times/views change. These are limits that I have run/observed for sustained periods in race and road use, in some cases over periods of years.

2 liter (90mm stroke) 7200
1800 (79.2mm stroke) 7500
1600 (71.5mm stroke) 8000

The ONLY thing I am going to add is that if you are competing in ANY form of motorsport or want to run higher (and let's face it a well sorted TC is going to fly past any of those rev limits) go forged.

GC

[vcg]

Hello!

I have been through this last summer when I rebuild my 131 Racing engine. My number one concern would be that 40IDF with Crommodora Waffle manifold is a very serious limitation factor for this engine. 37 venturies are almost no venturies, I am pretty sure the 34 is the max these carbs work with without modifying them, ie enlaging the carb tops to make them 40mm like the barrels (DRLAs are like that and work with 36 chokes).

40IDFs are good for a 1600cc engine. Stock high performing engines had 44IDFs (124 Abarth Rallye 1800cc, Escort RS2000). Definetely avoid the waffle unless you have access to someone like Guy who can open it up proffessionally, and my recomendation is go for the Alquati one that will even enable you for a block mounted Digiplex from a Lancia!

Keep Strong,

[Guy Croft]

Good point Vassilis,

and quite right too, I must admit I did not notice the choke size. I've run 37mm chokes on 44IDF but I would not do so on 40's.

GC