More hot air

["Graham Hilder" <graham.hilder@domain.elided>]

Hullo again all,

Lots of responses, thankyou everyone, to my query about how much HP =
difference might really be expected in a standard Alfa motor, when =
changing from hot to cold air intake. As I've mentioned, I don't doubt the =
theoretical rationale in the slightest: it's clear enough that colder air =
should give more power in theory. But my question was, approximately HOW =
much more, in actual practice?

We started with Rich's original theoretical figures, estimating 13 per =
cent, using a simulation written for a 2 litre motor.   There were posts =
from Greg and Keith, reinforcing the theoretical rationale, in that theory =
states that the power from an engine is ROUGHLY proportional to the =
reciprocal of the ABSOLUTE intake air temperature. =20

There was a post from Scott about intercoolers, suggesting that they were =
effective, and one from Keith demonstrating this specifically, in that =
when Saab introduced intercooler on the 900, they went from 145 to 175 HP =
(albeit with an extra 8 valves).  OK then.... intercoolers work, and =
appear to give massive demonstrable power gains when introduced (I'll be =
generous and assume NOTHING else relevant was changed). I don't have the =
knowledge to assess how applicable their magnitude of results (20 per cent =
for the Saab) are to a normally aspirated Alfa motor.

The post from John gave the closest practical experience of the actual =
situation I'm interested in...... whereby switching his Simca's air =
cleaner input from the winter to the summer setting made a huge difference =
which he could really feel.=20

And there's my own practical experience whereby restoring the jammed air =
intake selectors on certain of my Alfas back to their proper cold rammed =
air input positions hasn't made any noticeable difference on road or =
track.=20

So.... what have we got from all this?  A large power gain predicted by =
theory, and obtained by turbo intercoolers in practice. Carburretted =
aeroplanes suffering takeoff power loss on hot days or when carb heat is =
on.... but to what extent, I don't know. John's practical experience of =
massive increase on his normally aspirated Simca motor, but my practical =
experience of undetectable change on samples of my normally aspirated Alfa =
motors. From this, there's no doubt that in theory and practice, switching =
to cold air will give an improvement.  Only the amount seems in dispute. =
For my normally aspirated near-standard Alfa motors, theory suggests the =
vicinity of 10 per cent (ref Rich's, Greg's and Keith's posts).  Righto =
then...... I'm more than pleased to accept that in practice I should be =
able to pick up around 10 per cent more power from my Alfas (Suds or =
Nords) by correcting them to have their proper cold air inputs. So, the =
question now is: should I expect to notice this percentage change when =
driving the cars? Aprroximately how much increase is typically needed =
before one actually notices it?  It depends on how you drive, of course, =
so to set the scene, you can take it for certain that all our Alfas are =
driven with the passion expected, and all the power available gets called =
upon rather often (and there's lots of it..... these aren't clapped-out =
old bombs). As another measure, considering that in those of our Alfas =
which see the track we typically lap our home circuit in 98 seconds, how =
much difference on the stopwatch might be expected from a 10 per cent =
power increase?  I certainly don't have the experience to guess how much =
more power you might need to reduce lap time by say 1 per cent (one =
second), but I guess there are plenty out there in Digest land who do. I'd =
be interested to hear any suggestions on this.=20

Regards again,
Graham Hilder,
N.Z.
(Alfasud, GTV, Alfetta GTV)

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