Which Fiat Twin-Cam for racing: 1608 or 1756?

[helebah]

In setting up a CS Spyder for historic racing I can use a 1608 Dual Carb or 1756 Single Carb engine.

Brakes etc are standard, so maybe I am not after huge HP but good torque at 4 to 7K rpm ?
Which would be the best engine to use ?

In reading these great forums I am thinking the 1608 may in fact flow better due to the dual carbs, or does the extra capacity with the 1756 compensate ?
Currently putting a 38DMS on a standard 1756, to run for the next 6 months while I get something built.

thank you
Graham

[Guy Croft]

Hello Graham

I'm sorry for not being able to reply sooner. I hope this may help:

Standard OE ratings were:
Fiat 124 Sport 1608 on twin 40IDF
110bhp @ 6400
101lbf ft @ 3800
80mm bore x 80mm stroke, 9.8/1 CR
(scource OE Fiat owner's handbook, engine type 125BC000)

Fiat 132 1800 (1756) on single carb
107bhp @ 6000
104lbf ft @ 4200
84mm bore x 79.2mm stroke, 8.9/1 CR
(source OE Fiat workshop manual), engine type 132B1000)

In summary of above, the 1608 produces more power and higher up not only because the twin carbs (slit throttles, intakes) allow it to breathe much better, the 1756 on single carb struggles for air. The 1756 has fractionally more torque, because even though it has a lower CR, it is bigger cubic capacity, and the over-square bore stroke configuration and slightly higher cam lift allows it to develop peak torque higher up. That's my best shot anyhow!

Forgive me if the data does not tally exactly with other sources.

So:

1. On Torque: the 1756 has bigger cubic capacity and because the engines are essentially the same design, so the 1756 is capable - for the right state tune, of developing much more torque than the 1608. A single carb will not hold the 1756 back overmuch in this regard. Things that will help are higher CR, flowed head, race inlet cam, top-quality header and so forth.

2. On Power: here a lot depends on the race-legal level of tune as to whether the 1756 will match a tuned twin-carb 1608 on horsepower. Twin carbs will always give better engine breathing (vol effy - see 'all the power is in the head in GC V/W) and like for like tune, the 1608 will probably give more horsepower than the biggest single-carb setup will on a 1756 (I'm guessing slightly here). But, if the spread of torque on the 1756 is wider, it will be a quicker race car.

Add to this that in terms of rebuilding and tuning things like head gasket, oil pump, water pump, bearings for the 1608 are not as easy to find as for the 1756. And the piston-rod thing with all those press fit units around is a bit of a minefield. The 1800 has full-floating wrist pins and piston commonality with other TC models. The 132 1800 is similar to the 124 1800, go for the 132 if you can find one, bigger (12mm) flywheel bolts and with a bit of luck, the later (more roomy) combustion chamber design that is certainly superior to the 1608 one.

All things considered I'd go for the 1800, and I'd be very interested, if you do, in hearing about your power on that single carb. I don't have much single-carb data around me.

GC

[helebah]

Thanks Guy, exactly the sort of advice I was after. Will let you know how I get on with my "standard" engine on the single carb. I will then start to collect the parts early in new year for the new engine, will contact you then as I am keen to do this once and do it right. The hardest thing I am trying to track down is a certain book, I think one of Bill Caxton's originals would be easier to find (and maybe cheaper !!!!)
thank you
Graham

[Guy Croft]

yes, I'm sorry about the book, I sent one recently to the esteemed Editor of Bentley publishing hoping to tease him, we'll see!!

BTW, he is a member, so here's hoping..

G