Re: engine compartment heat

["hydee" <hydee@underground.prestel.co.uk>]

I'm not a Jag expert. The best answer will come from those who have 
tried one or the other or both who'll know what the drawbacks are.

>I almost hestitate to reply.  All this has been discussed before.

I wasn't around to see it, neither presumably was the original poster 
of the question. So why not discuss it again with different opinions 
from different backgrounds?

> All this air goes in, and out the BOTTOM, lifting the nose of the
> car and causing lousy handling.  In the meantime, damn little of it
> makes it to the upper rear area of the engine compartment, where the
> temperature problems are worst.

 Scoops bring the air in the top. (assuming you put the scoops on the 
top of course.)

> > Scoops will however be better.
> 
> > If the car is moving forwards there 
> > can be an absolute maximum of 1 atmosphere of pressure 'sucking'
> > the air out of the ducts.
> 
> Wow.  Where did you get your science education?  This is so screwed
> up as to be difficult to respond to.

If you can create a 'negative pressure' then you really ought to tell 
the rest of the world. The air around you is at 1 atmosphere. You 
CANNOT get below zero pressure at all. Even in theory. 1 Atmosphere 
minus zero = 1 atmosphere.
A greater than 1 atmosphere pressure drop will occur due to the front 
grill acting like a scoop where the pressure is higher than 1 
atmosphere, i.e. the front of the car. But scoops are bad things so 
perhaps we should block the front of the car to prevent air getting 
in.

I got my 'science' education during a mechanical engineering degree where I was 
taught fluid mechanice by an ex-Rolls Royce jet engine air flow 
expert. ( I got a 2:1) And have furthered my knowledge with PhD research. How 
about you? Go to the library and get a book before you claim to be an 
expert.

> > Scoops on the other hand can give you a much 
> > higher pressure difference and therefore improved airflow.
> 
> Unfortunately, "improved airflow" seems to be the confusing issue. 
> There is no problem at present with the QUANTITY of air flowing
> through the engine compartment; it is more than adequate.  The
> problem is with where it is going........

See above..........

> and how much it's lifting the
> nose of the car on it's way out ....   ........ OUTLETS and improve the car's cooling when parked AND the handling
> at speed.
> 
Well unless I missed something earlier on there was no mention of 
handling being involved, just airflow. (not that I've found any 
handling problems despite regular 100mph+ cruising and attacking 
Scotlands twisty bits every day) Your right here to a limited 
extent.

Having read a few of your posts I'd guess you've done a fair bit of 
work rebuilding Jags. This may make you a good mechanic. It doesn't 
make you an engineer. If you were you'd appreciate little things
like bonnet angle will create a downforce which will counter act, 
and may over-ride, any upward force created by air escaping 
underneath.

Now I don't know if this is the case I've never had the inclination 
to try and find out. I've no idea if Jaguar did, probably not to any 
great extent. How is your handling upset by 20 gals (imp) fuel sloshing about, carrying passengers? 
probably one fat passenger will make a far greater difference than a 
few extra cubic foot of air flow.

Fluid flow is an extremely complex subject not easily understood by 
the layman. Perhaps I should keep my informed opinions to myself in future.
If I find the time (which I doubt) I could try modelling it on the 
finite element analysis fluid flow modeller in the lab then we would 
now for sure.

In summary scoops and outlets will BOTH work. Which you use 
isprobably less important than the correct positioning of them.

Incidently is there anyone on this list who ever worked for Jaguar 
designing their cars who could tell us any design secrets?

---

Follow-Ups:

• Re: engine compartment heat
• From: "Jim Cantrell" <jimc@sysdiv.sdl.USU.edu>

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