Adam M

|But mat, not all heat is acrried away by air flow, primary heat loss is through radiation. Heat flows due to a temperature gradient. If the outlet of the turbo is 160 degrees, there will be a massive gradient even with very little air flow, so much heat will radiate away from the intercooler. It is also thermally very efficient since aluminium is a great conductor and the surface area of the fins is enormous. Consider a radiator with no wind in the room, if the water didnt lose heat, then heating wouldn't work, you cannot stop convection except by using a vacuum, and you have to actively reduce heat loss through radiation by employing a reflective heat shield.

Air flow through the grille will ensure underbonnet temps are way kess than 160 degrees so you will always have a temperature gradient.

I still am confused as to what the problem is with the airflow coming out of the scoop at high speeds. The heat would rather leave the intercooler out of the top due to convection anyway so what is wrong with the scoop working in reverse.

Since there is talk of low pressure sucking air out through the scoop and therefore increasing drag, why doesnt someone (pete) have a modified scoop made with the top section effectively cut off. This would allow the low pressure to suck air out of the engine bay but without the car looking stupid. Similar to the bulge on the evo bonnet but more in keeping with impreza styling. It would allow heat to rise out of its own accord at low speed and suck it out at higher speeds. I have no idea what effect this would have when using a top mount, but with a front mount I am sure some advantage would be yielded.

GavinP

Corky Bell's book "Maximum Boost" has some interesting figures in it:

He claims that to calculate intake airflow into an engine, it approximately 1.5 x BHP. e.g. 300bhp = 450CFM

He also states that at 60mph, a front-mounted intercooler of 12" square (300mm x 300mm) will have 5280 CFM flow through it.

I wonder how much air will flow through a scoop and through a top mounted IC at the same speed ?

Thanks

Gavin

Pete Croney

Adam

Good point, but you can only suck at 1 bar and even then you need a complete vacuum. What we are talking about is a low pressure area and it certainly isn't low enough to form clouds on a damp day.

I suspect that the reversed airflow, out into the low pressure area, is negligable and way short of what is required to cool the charge when running at high boost and revs.

Radiant heat loss will also be negligable. Especially on an STi, as the intercooler is silver, the worst colour possible for radiating heat.

R19KET

Pete,

I assume you were referring to my car ?

Just to clarify a few details. My engine let go to a head gasket failing. I was actually running a Motec M48pro, which dataloged the water temp' rising to 118 deg'C this didn't show on the dash gauge. There was also emulsion in one of the heads (2/4) and this took out no.1, and no.3 big end barings.

Cause: Well I'd say constantly pushing the boost levels(1.5bar held, 1.7bar OB) but I always knew it was only a matter of time.

My intake temps never went above 45 deg'C, and det' was not an issue.

As for the intake temp issue, I'm very dubious. Having done several 5th gear, 7500rpm + runs, my intake temps were never a concern.

If the theory is correct, and some of us have just been lucky, I'm AMAZED that any of the earlier cars are still running. Given their very low scoop, and tiny IC, I'd have thought they'd only be a distant memory......Unless of course, all pre '97 owners drive/drove like nanny's.

Mark.

Pete Croney

Hi Mark

Thanks for clarifying that. We have spoken for a while and I heard you threw a bearing, nothing about head gasket failure.

The older cars run much lower boost, especially at high revs and this would produce much less heat.

Its only a theory, but we have still yet to see a FMIC car having lost a bearing. The idea was started when I looked at what Subaru had done with the bonnet shape on the 01.

Cosie Convert

Pete

I can think of a few other reasons you may not had a bearing failure on a FMIC equipt car.

The first one is that I would guess the ratio of front mount cars to top mount cars to be about 100 : 1 ? therefore the failure ratio (all else being equal) will be similar.

Secondly, someone who goes to the extent of a FMIC install is pretty serious about "proper" tuning and not just cranking the boost up. So probably has a safer set up.

Finally, most of the failures were on STi's.....with that tantilisingly high red line !!

just my 2p

cc

RB5SCOTT

20.27,

you still at work Pete

R19KET

Pete,

Sure, the ealier stock cars run less boost, but there are plenty of tuned ones, Bobs springs to mind, We won't count Pats

There have been plenty of engines blow with FMIC's too, most I know of in AUS. FMIC's are pretty new to the UK, so time will tell.

I have spoken to numerous engine builders (Grp N,A, WRC) and none of them can give me a specific reason to why the big ends have a tendency to let go. Each have had the problem, but have no consistant reason, or solution, even though most use uprated barings.

Mark.

Iain P

Do we know if any of the failed engines had oil coolers fitted?

mutant_matt

Chaps,

In case you haven't seen it, we now have two discussions going on about this. The other thread can be found here....

http://www.scoobynet.co.uk/bbs/threa...D=65110&Page=2

and also for anyone wanting to familiarise themselves with the original Big End/No3 thread.....

http://bbs.scoobynet.co.uk/Forum9/HTML/002503.html

Matt

[Edited by mutant_matt - 1/18/2002 8:27:28 AM]

Adam M

I think Cossie converts point is a good one. adn I would not have thought the TMIC:FMIC ratio is as low as 100:1 so it is definitely worth considering.

INterestingly I wrote to some jap tuners and people in the jap market and they were surprised we had a specific problem with number 3 big end. They had just as many if not more no. 4 deaths but didnt seem to reveal a specific solution.

I still like the original theories relating to backing off after a high speed run that Bob was toying with. This seemed to be the most common factor. How that relates to recent oil changes I dont know but it seemed plausable.

I would be interested to know if prodrive would ever release any secrets relating to their experience. Since they use the same blocks, surely they are constrained to use similar bearings, and oil ways.

Is it possible the standard bearings simply arent up to the job once the power is raised, and the distance from the flywheel determines the load they experience potentially explaining why nos 3 and 4 seem to let go first?

clutching at straws btu since it was never resolved why not?