[Diy_efi] Cool runnings.

Patrick Cahill patc at opposition.tv
Wed Dec 18 03:14:11 GMT 2002


Message: 11
Date:	Tue, 17 Dec 2002 18:22:35 -0800 (PST)
From:	Adam Wade <espresso_doppio at yahoo.com>
Subject:	Re: [Diy_efi] Dyno. cooling
To:	diy_efi at diy-efi.org
	Reply-To: diy_efi at diy-efi.org

--- Bernd Felsche <bernie at innovative.iinet.net.au> wrote:
> You're not considering the car as a whole... an
> engine's output power is determined by ambient
> conditions.  Unrepresentative airflow through the
> engine compartment probably means that components
> aren't operating at representative temperatures.

What parts are you concerned about?  We've already agreed that a dyno will
never be exactly the same as real-world operating conditions...  What parts
do you feel would be operating outside of normal parameters given extra
airflow?  Considering that the air moving through the engine compartment
couldn't possibly cool the components to, say, winter ambient temperatures,
there's no way you could end up with below normal operating temperatures on
any component I could think of, unless you were designing a Venus rover or
something.
> Somebody else already mentioned that drawing air
> from underneath the car would be a closer
> approximation to the real road.

The ultimate question is, "Does it make a meaningful difference?"  I'm not
understanding how it could be, when speaking of underhood temps.
> In essence what they need is the
> vehicle/prototype/model moving relative to the
> ground with the air "attached" to the ground; but
> they're only moving air relative to the ground and
> the vehicle.

Unless it's a "rolling road" wind tunnel.  And even that is not PERFECT,
simply very, very close.
> I know that's somewhat esoteric; but keep it in mind
> when you think about how representative of road
> conditions the dynamometer really is...

*grins*  Yes, I keep saying that...


NOW YOUR TALKING!

A rolling road wind tunnel WITH an in floor chassis dyno both calibrated to
run at the same speed!

If you figure out how to build such a beast I've got Adrian Newey and Ross
Brawn's email addresses. I'm sure they would both like to know.

In the mean time, can we get away from the NASA budget end of things and
talk DIY again.

The problem with cooling on a chassis dyno is lack of adequate air flow.
Unless your fan has the surface area to blow over the whole car PLUS about a
meter all around its max. cross section, then it won't cool properly.
Airflow used for cooling is as much about what happens behind, below and on
top of the car as it is in front of it. The motorbike radiators you speak of
are in probably the simplest aerodynamic installation you can get. But place
a car body around the things and life gets complicated. If you are relying
on a single source of airflow at the front of the car then you can double
the air flow required in front of the radiators in order to achieve the
right pressure drop.

Dyno cells also have inherent problems as they are not true to the nature
with which the engine operates in its installed environment. Especially when
it comes to modern machinery which often have positive air box pressure
(read ram effect). And don't discount the effect of inadequate airflow over
an engine to screw up the tune when it comes to heat soak and excessive
temperatures. I could tell the story of the twin turbo Porsche which was
tested in a dyno cell and showed 397HP at the flywheel. From this, the
series organisers calculated it's weight penalty and that was that. Except
that when the engine was installed in the car and the oil was cooled
properly from air flow over the sump and engine, it suddenly produced
approx. 480HP. It seems that there were a couple of temp compensators in the
Motronic that curbed the timing relative to oil temp. On the dyno the oil
ran at 110-120C dependant on load. In the car it ran at 90C the whole time.
The reason for the compensation was that they were relying on oil squirting
on the back of the pistons to help control detonation.

OK, so they were really bending the rules but it still illustrates some of
the effects that can be easily overlooked.

Going back to the genesis of this story. I have found that a good WBO2
(Bosch 0258104002 is a good start), PROPER data logging, an optimised
ignition map and someone who knows what they are doing gets you very close
to the mark. The tricky bit is getting the timing right. However, if it is a
modified street motor, the stock map is a good place to start. It's oly when
you et into big boost or high comp. app's that you have to be real careful.
All of this done at your favourite test track or stretch of road in order to
get the best real world data you can. I mean, even the car companies, with
all of their test rigs, still hit the road to verify what happened in the
lab. And still find plenty of adjustments to be made.


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