Max compression on single carb fast road engine.

[Abarthnorway - Remi L]

Hi all!


I am in the process of parking my 124 CC 1800 coupe for the winter.... and its time for building a better engine.
I have a good 131 2l bottom end with high comp pistons of the 105tc/124 sport type. This will to my knowledge give approx 10.5 (+ -) : 1 CR.... (not measured on this engine)

Engine specs:

C&B mild fast road inlet 10.3 lift/1.8tdc @ 110`/ std exhaust cam
Single (double barrel) 34 mm Weber carb - venturies bored to 28/28 mm
Slightly ported Abarth 43.5mm valve head
Ritmo 125 TC inlet manifold
Exhaust manifold standard Fiat 124 4-2-1

Question is whether the pistons will produce a too high compression 10.5:1 for a single carb unit with slightly uprated inlet cam?

Knowledge of any mods or "oddities" regarding the fitment of 2 liter 131 into a 124 1800 sport are greatly appreciated:-)
Seems quite straightforward, but there are usually quite a few details making "simple conversions" into learning sessions anyway... at least for me.


Best regards

Remi Lovhoiden

[Guy Croft]

No particular concerns about putting the 2 liter in a 124 1800 though of course it is taller by 10.8mm.

As for CR I don't personally think it's worth running higher than 11/1 on the 8V TC in any event because the slow burn rate defeats the point of going higher (big intruder dome) and with your intended 10.5/1 on single carb, the issue isn't the carb - it's whether it detonates or not. Controlling that with a single carb is not as easy as a twin-carb setup because the mixture distribution and quality (atomisation) isn't anywhere near as good. Maintaining the engine and inlet charge temperature low and gasflowing the head/inlet manifold definitely helps.

The highest I have built a TC on single carb was a 'kind-of experimental' big bore 131 1585cc unit - Ford V6 Essex Weber 40DFAV carb with GC 3A cams at 13.7/1. The torque was just under 126lbf ft - about as much as we thought we'd get, and the power was (I think) over 148bhp, for as long as it lasted - the highly modified combustion chamber kept cracking. The top-end fuelling was pretty awful, we figured the airspeed thru the carb was too high and there were choking (trans-sonic) effects that leaned it out whatever the owner did with it on dyno test. Featured p110 GC book..


GC

edited by GC 10th Oct 08

[Abarthnorway - Remi L]

Hi!

As always - Thanks Guy!

Best regards

Remi Lovhoiden