Fiat twincam project coming off the stand

[chimpvalet]

No need to doubt the validity of your results, they speak for themselves and the engine is clearly not over-cam'd. Good job!

Steve

[juuhanaa]

Remind us what’s the highest redline rpm you’re comfortable with?

I have cast pistons. I think I could occasionally hit 7800rpm.

I was hoping the bigger cam would lift the torque curve up a bit all along, not just at the very top rpm. The bigger cam has about 10deg more duration at 0.050", only about 0.010 more lift, seat to seat duration just marginally longer than this current cam.

If it were my car, I would be thinking how to lift the power curve in that 4100-5500rpm range without hurting the top.

Love the car / build though.

This!

The intake looked a little short to me, so i did a simple calculus regarding the harmonics, which shows that lengthening the intake would not only increase port volume and reduce frequency, but would also help optimize peak revs your engine sees. Changing your carb "velocity stacks" to longer could be one way doing this.

In addition what could work hand in hand with the above, say your intake cam has 302 seat to seat duration as installed. 256 @0.050". .448 lift? You can try advance the intake while keeping the cranking press under 210psi (98E5 pump gas).

Love the car / build though.

Fiat TC.png

-juhana

[chimpvalet]

The trouble with working on intake lengths on Fiats, including this Spyder, is that they are built with little room to accommodate increases. Note the use of downdraught Webers, rather than the more common sidedraught. Nice car nonetheless!

Cheers

[juuhanaa]

Average lenght and diameter was just an approximate, but i found the lenght was not necessarily maxed out 44IDF is 10,5cm tall, but cant say from the picture how tall the filter is?

Screenshot_20221123-191432.jpg

[Nut124]

Juhana, yes, space is the problem under the hood. The front IDF air cleaner is already just about hitting the hood. I have very short ram pipes or stacks on top to keep clearance to air cleaner top.

Not sure I quite understand your calcs there.

Cam timing: I have thought about advancing the intake cam to about 102-103 from 106. I have always wondered whether overlap in itself adds power or whether overlap is just the result of long duration cams and the desire to keep IVC at below 80 ABDT for mid range torque. Guy croft seemed to believe in 108-110 ICL even for full race cams. I never saw 102 ICL in his book.

Have any test data on this?

[juuhanaa]

I have little first hand experience with this engine, but i have kept my eyes open and i listen what my close friends/team mates tell me. Best folkrace rear engined cars (mainly 133 and 850 Fiats) accelerate from 0-100m in 5sec using 185mm wide street tires.

The class rules limit using one 45mm dual carb, but the plenum manifolds are not seen on the TCs, so guys make them for a two 45mm single barrel. The CID limit is 1650 and they can rev up to 9000 rpm during shifting first gear to second, and i believe the bests are in 180hp range. 1650 versions run both short and 80mm strokes, but there is also smaller 1450cc engines made, so it can be built different.


You could make more top end torque with a longer lenght only, but don't increase CSA or "taper", because your intake is out of tune on higher rpm. Longer "ram pipes" + thick bakelite plate between intake manifold and cylinder head could do it, without changing the look of it or spending much money. And try advance the intake using those nice pulleys on a dyno.



-juhana

[juuhanaa]

Bakelite phenolic resin sheet can be found in pieces up to 20-30mm thick and it is easy to shape. First drill the holes and then fix the sheet against the manifold flange and use a hand router for shaping. Use gaskets on both sides of the bakelite and you may also need a longer studs too, but cake for you.

Screenshot_20221125-124547.jpg

-juhana

[Nut124]

I have little first hand experience with this engine, but i have kept my eyes open and i listen what my close friends/team mates tell me. Best folkrace rear engined cars (mainly 133 and 850 Fiats) accelerate from 0-100m in 5sec using 185mm wide street tires.

The class rules limit using one 45mm dual carb, but the plenum manifolds are not seen on the TCs, so guys make them for a two 45mm single barrel. The CID limit is 1650 and they can rev up to 9000 rpm during shifting first gear to second, and i believe the bests are in 180hp range. 1650 versions run both short and 80mm strokes, but there is also smaller 1450cc engines made, so it can be built different.


-juhana

Is this the kind of racing you are talking about?

https://www.youtube.com/watch?v=raxltjoKPsE

https://www.youtube.com/watch?v=jqmKv0aJvv4

[juuhanaa]

This is what the TC should sound like

https://youtu.be/6uHFODR7RB4

Of course, the power is transmitted to the tires through a box that is designed for a 24 or so horsepower beetle engine.