1603cc engine I've just taken to the dyno for it's first tuning session
Only a few pulls as initially we had a serious lean issue above 4500rpm due to insufficient fuel delivery, so we changed the fuel pump to one with approx 82 litre/hour capacity at 4psi and this solved the fuel delivery problem , car runs a return line, and I have a "redline" style inline fuel pressure regulator to install...but would prefer a "filter king" style filter/reg assembly.
At the moment running the stock Marelli S135 (non vacuum advance) distributor body with a pertronix optical trigger kit. Static set at 10 degrees, mechanical advance is about 28 degrees from memory, so total advance will be 38 and as per stock is all in at about 3500rpm or so... that's one of the things we are going to change. There's another model of Marelli S135xx distributor that uses a 22 degree mechanical advance plate, and we will try the timing at around 15 degrees static...as the low revs response could be improved a little.
These runs were with 32mm venturis, and given the power curve flattens (but doesn't fall away) from about 5600rpm, I think it could use some more air flow, so next are some 34 or 36 venturis I have to try. We also have the 42/44 style bellmouths to fit which will also help with air entry and improve things. On the dyno we had 20 degree ambient temps, but inlet air was 40 degrees, so there could be a decent gain with a cold air box ... but we just ran out of time to try all these things ... the fuel pump not keeping up and the A/F ratio spiking to the 16's above 4k rpm were more important fixes for this first dyno session. the engine is also only just 1000km old, and I built it quite tight, so as it gets more miles and loosens and beds in, power should increase again...
We started with 125 main jets / 220 air correctors, and it was immediately obvious that the engine was needing more fuel. In went some 135's and things improved but the A/F still leaned out around 6k. In went 145's and the A/F was more acceptable all the way to 7500.
But the A/F graph still looks like hilly terrain above 4500rpm too, so there is some sort of air flow harmonic/pulsing going on... as I think the air speed thru the small 32 venturis is very high. Overall happy with the result, and the engine pulls
very quickly from 4k to 6500rpm in it's current setup, good enough for the owner ( Mark ) to run up another 1000km before we have another series of dyno pulls.
overall quite happy with the results, I think there's another 20hp at the wheels with this engine once sorted.
SteveC