silent running

Welcome to the first of many rational discussions that I hope to get started. I do enjoy logging in to SN to see what's going on, but - no offence intended - too often I see threads that are just full of bull****, scaremongering, claim and counterclaim, blind loyalty to one company/fuel/tuner/oil/etc. Just a general unwillingness to discuss things properly and give considered, scientific, measured responses. There also seems to be a tendency to 'shut down' anyone who doesn't conform to the majority SN beliefs. There's too much faith in the experts (with or without quote marks!) and not enough in Joe Average who might not make their living at tuning and servicing Scoobs, but has been around driving/modding/engineering/racing enough to know what's what.

So in this thread, I want to hear from people with genuine personal experience and results in this or who are willing to try stuff out - whether they have their own commercial interest in it say as a tuner, or whether they are just an individual who wants to learn or contribute. If you can't disagree with someone politely or accept that someone else might have a different point of view, then don't post. Even as I write this I'm thinking someone is probably going to reply to this saying "it's already been done" or "this has already been decided on SN" or "what the hell do you know, I'm a tuner and I know better than you". I was gobsmacked just a few days ago to find myself being slagged off on a different thread just because I had dared to question the SN 'experts'! The poster couldn't understand why I wouldn't just blindly accept the SN consensus without hard data to back it up. IMHO the point was eventually conceded to me but the fact remains that to me, 'BECAUSE' isn't a good enough answer...I want to find out WHY - and if you have knowledge and results that you found out for yourself, I want to be able to benefit from it, and vice versa.

If you're up for this kind of meaningful discussion, then post here. First topic will be this: WHAT'S GOING ON WITH BONNET SCOOP AIRFLOW AT HIGH SPEED? There seem to be a lot of reports of blow-ups at high speeds on the motorway i.e. well over 100mph, possibly attributed to a slowing down or stalling of airflow through the TMIC, which would cause a corresponding spike in charge temp just at the time when you are flat out in 5th and the engine's under full load. So, who knows anything about this?

TonyBurns

iTrader: (3)

The later STi's dont suffer from this as they have a splitter and more aerodynamic scoop, BUT at higher sustained speeds, there isnt enough air getting onto the intercooler (on a classic only approximately 1/3 of the intercooler has air hitting it) and temps rise, then you can get det which is what helps kill the engine hence the reason classics aint good for sustained high speed driving, though there were a few bits you could do with an aftermarket splitter which did improve the cooling

Tony

noturbo

iv never heard of that happenin before is that common? i gues a FMIC would solve the problem tho

silent running

Cheers Tony. So you're talking about the STis with the BIG scoop (AFAIK there are only basically two sizes, normal or big) - that they DON'T have this problem at all? Or that they still DO, but at higher speed, which is what we're focussing on really?

For what it's worth, on my classic WRX I fitted a tilt kit and splitter to try and do the best I could..tilt kit because it was so cheap and easy to do, it's a crime not to, and I couldn't see any downside to it at all. And the splitter because it made sense.

Anyway, I've just been out for an airflow test run at my local airfield. The setup is this - Magnehelic air pressure gauge (0-25mm of water) attached to dash with strong masking tape, a length of silicone vacuum hose plumbed into gauge at one end and open at the other.

First test was with the open end at right angles to airflow, just inside the middle of the scoop but not touching the I/C at all. Once beyond about 20mph, as speed increased I got a fairly progressive increase in air pressure until I maxed out the gauge at over 25mm, this was at 80-90mph, averaged over a number of runs in different directions. Taken up to 120mph in one run and not a sign of any lessening of air pressure INTO the scoop - the needle was hard against its maximum stop until I dropped back below 80.

Of course air pressure IN to the scoop is only half the story. Without a lower air pressure underneath it, air flow stalls. So, I reattached my open hose end to float just a couple of cm BELOW the I/C. This time under the same conditions, I got the same progressive increase in air pressure, but on a shallower 'gradient' if you can see what I mean...the air pressure wasn't building as fast UNDER the I/C as it had been ON TOP of the I/C. However, once over around 70mph it started to get going, and by 80mph, had hit the maximum stop -over 25mm of pressure. Up to 110mph it would not budge, and again only dropped slightly back once it got back to 70-80mph.

Now obviously my experiment has been hampered by a number of factors, 1, my gauge doesn't read quite high enough, 2 I've not noted down every single road speed with its associated air pressure both over and under the I/C. 3. I couldn't take it beyond around 120mph.

But I think it does show a couple of very interesting things. Firstly - there's absolutely no problem with airflow going into the I/C at high speed, it keeps getting more and more, and certainly doesn't seem to drop back in any way. The aerodynamics must be fairly unsurprising in this respect - the faster you go, the more air pressure you get across the rear half of the bonnet. The second point which is the more important really is this: engine bay air pressure is too high directly under the I/C once you get up to speed!

In other words, it doesn't matter how good a TMIC you use, the fundamental problem is with the location, not the I/C itself. It's not even really a problem with the bonnet scoop as that is DOING its job as far as my results show. What ISN'T working is the extraction under the I/C or lack of it.

Obviously better or bigger designs may have an effect, but will never solve this problem of stalled air through the I/C at high speed. 25mm pressure on one side of the I/C versus 25mm of pressure on the other = no airflow = no chargecooling beyond what the I/C can handle as a passive 'heatsink'. Perhaps this is why a bigger TMIC seems to work better as it is simply a larger heatsink.

I will try and get two long pipes hooked up to my gauge and do simultaneous measurements both above and below the I/C - I should be able to get it to simply show the pressure difference at each speed.

Any comments?

silent running

There do seem to be a lot of reports of engine blowups either during or just after a high speed run. Perhaps the Japs were right to run them with 112mph limiters LOL

TonyBurns

iTrader: (3)

The new age STi's also have a rubber seal over the intercooler to "direct" the air better, compared to my classic the STi puts considerably more air onto its intercooler and for some unknown reason, i dont seem to get as many dead flies splattered in the scoop

Tony

silent running

What's this rubber seal like? Mine has a rubber seal, I thought all classics did? Have you got an undertray under the engine bay?

bugeyeandy

Excellent topic mate, I have a laptop based data logging system I can use on the newage.
I'll hook it up when I get chance and see what it comes up with as i'm pretty sure it has inlet temp sensor as one of it's output and it records road speed.

The software I use is EcuExplorer - just connects to the OBDII port in the car.

Andy

silent running

Cool. I would be very interested to see how that works out. Can you measure temp before and after the IC, i.e. 2 channel? Where's your inlet temp sensor sited?

BTW I'm gonna see if I can think of some way to shroud the underneath of the TMIC and duct it out under the car. A bit of experimentation should show where there's a low pressure area, then with a decent sized duct it should be possible to get a hoover effect going on LOL. I wonder if even a pair of small rad fans under there might do the job, pulling air through and triggered by charge temp or road speed?

bugeyeandy

I don't think so, pretty sure the inlet temperature is measured inside the inlet manifold.
This will be a good test though because if the airflow is stalling at the intercooler the charge temps should rapidly increase.
Hopefully get it done tonight when the local test track is quiet.
Andy

360ste

iTrader: (1)

Looks like a excellent thread lets hope you know who does not come on talking about cocklinks.

nathanb

iTrader: (1)

pslewis is strangely absent from this thread!

tweenierob

You have a few things to consider, firstly air pressure does not equal airflow.
Secondly you have the gearbox tie bar in the way of any ducting along with the intercooler supply pipe/pipes.

Thoughts?

Rob

The Chief

I will vouch for Classics with standard scoops not being able to sustain a high speed run ahem' ok would it be beneficial to add an STi 8 Scoop to my classic????

Turbotits

iTrader: (2)

Think you'll find its measuerd at the maf,Which aint a lot of good for inlet temp measuring for obvious reasons.

bugeyeandy

Damn, that's not so clever then

R19KET

I think you'll find that when people quote "High speed", they are talking of something somewhat higher than 110-120mph.

I think you also need to take into account what happens when at "high speed" the bonnet starts to lift, and what is causing it.

Is it caused by drag on the scoop, and, or high pressure under the bonnet, or even low pressure above ? I don't know, I've not tested it.

Maybe you should try the "lots of bits of wool stuck over the bonnet", so you can see, how speed effects flow, and pressure.


Mark.

frayz

iTrader: (7)

Great thread

Mafs dont measure air temp, they measure mass.

Hence MAF= Mass Air Flow

ZEN Performance

New age cars have a temperature sensor in the MAF sensor too. It is used for a variety of correction maps, to give a overall better control for different ambient conditions, rather than to catch high temperatures at the throttle.

Turbotits

iTrader: (2)

As i thought

95blackWRX

the engines blowing up i have been told is to do with the full pump. When u get to high speeds well over hundred the fuel pressure is not high enough. causing the car to not get enough fuel then in turn cuts the air which leads to a blow up

bugeyeandy

Turbotits was right unfortunately. Been for a short blast but regardless of speed the inlet temp recorded only varied between 14-18 deg C. Ambient temp was 14 deg C.

If anyone wants a look at the log file i've uploaded it.

http://www.aoxp41.dsl.pipex.com/Bugeye1.csv

(Right Click and Save As - Open with MS Excel or other spreadsheet)

Will have a play with excel later and come up with some graphs anyway.

Speed output is in km/h.

911

I'm not sure that I qualify to comment but:

To have flow, there must be an inlet and outlet.
If at speed on the TMIC Impreza the under body pressure is greater than the 'scoop pressure' there will be little/no flow.
The competition for the under body outlet is shared in some proportion by the Scoop and the Cooling, all fighting to get past the blunt shape of the engine/box.

At 'very high speed' ie AndyF speeds, I can only wonder what pressures are present at the gearbox/floor pan and the post rad zone and the post TMIC zone, let alone the bonnet surface/ Scoop zone?

Strikes me that if the underbody is greater pressure than the others then air flow will stall or that one zone will overcome the other.
Given that the rad is right in a very exposed position, will the flow in that zone cut-off the scoop area and the TMIC cooling will diminish/stop/be minimised?

I worked in the Motorhome and caravan industires on heating systems which had to work and combust efficiently to ISO standards while on the move; we found pressure zones moved all over the place around a vehicle at different speeds as aerodynamics took their toll AND the shape of components too. Our heater designs had the flue of the heat exchanger below the vehile floor pan, some through the roof, some even fan assisted flues.

Finally:

I am sure a FMIC is better than a TMIC for my reasons above.
When AndyF mapped my engine for hill climing (low speed racing) he mapped a high fuel flow at 7000+ rpm for the 'long' times the engine runs at that speed and to the limiter. Can't help but think engines let go due to fuel 'starvation' at high engine rpm as opposed to vehicle speed?

Graham.
(I hope this contribution to this excellent discussion is Bull **** free...and perhaps should be repeated on 22b)

Playsatan

iTrader: (2)

So to help the efficiency of a TMIC you could lower your car or fit a splitter?

ZEN Performance

My personal feeling on the TMIC, is that it's just too small to keep cool with sustained load, despite the airflow that it sees at high speed.

silent running

Yes it's true that air pressure is not the same as airflow but when you're comparing air pressure on the 'IN' side of the I/C with air pressure on the 'OUT' side, you get a pretty good indicator of airflow. The actual flow rate past the I/C fins can't be calculated with any accuracy, but the presence or absence of flow can be inferred. As I mentioned before, equal air pressure on both sides of the I/C suggests zero airflow from one side to the other. More air pressure on the IN side than the OUT makes it a pretty good chance that you've got air flowing the right way. The greater the pressure differential, the more airflow.

Second point about the gearbox tie bar is a good one; I'd not thought about that yet. Obviously it's a real spanner in the works and would mean either the shrouding on the underside of the I/C would have to be almost a shallow tray rather than a box, with exhaust ducting leading out from the sides rather than straight down from the middle. I wonder if rather than keeping the under-shroud very tightly sealed around the I/C it could be a looser affair or even not have a shroud at all and simply run a couple of big bore hoses up there, something like 100mm kitchen extractor hose or brake ducting hose. If an area of substantially lower pressure could be found under the car to feed these hoses to, the 'vacuum' effect might be enough to make noticeable improvements to the pressure differential across the I/C fins.

silent running

So what speed ARE we talking about? The figure I'd heard from a Scooby specialist was that the airflow seemed to stall at around 120mph. I've never had mine fast enough to see the bonnet lift effect, although I've heard of this from other people as well. Can you describe what actually happens and at what speed?

What I do know is that the majority of cars have a broadly similar air pressure distribution around them - areas of high pressure are where you'd expect it across the front bumper/slam panel/radiator grille/headlights as well as the scuttle panel/base of the windscreen area. This is why almost invariably cabin ventilation in road cars is taken from the scuttle panel. Any are where you have a spoiler will also have a high pressure area in front of it, but I'm talking about 80's style rubber ducktails here, I don't know if the effect is the same on the tiny BMW M5-style bootlip spoilers.

Low pressure areas aren't quite so obvious, but AFAIK they are the front half of the bonnet, part of the roof, the rear window on a saloon or the tailgate on a hatch if it's steep enough. The back bumper is about the lowest pressure area you'll get. Anywhere you get a lot of turbulence or air flow detaching you'll get a lot pressure area. The longer and shallower you can get the rear end, the less low-pressure/lift you'll get there. Anyone who's an aero expert, please feel free to correct me if I'm wrong!

What I am not sure about at all is flow UNDER the car. I'd expect this to vary a lot more from one model to another, depending on the layout of the engine bay, exhaust system, drivetrain etc. Anyone got a picture of a Scooby engine undertray? I don't have one on my WRX, but I would definitely like one to try out.

tath

I don't really see why you'd want to try and prop up a design that's inadequate for high speed use with bits of washing up liquid bottles and sticky back plastic...

FMIC? It's not expensive, it doesn't increase lag by more than a fraction (if it bothers you turn your inlet around), and i can actually sit on the Autobahn at 100+ without a spastically clenching ring.

silent running

Graham - I think you're dead right in everything you've said there. I'd imagine there is a hell of a lot more inflow to the engine bay through the upper and lower rad grilles than through the scoop, and as you say, it has to exit SOMEWHERE. With a flat wide engine sitting fairly low down in the front of the engine bay along with a big gearbox at the rear there's not a great deal of room there - in fact each side of the gearbox is about the only clear exit space.

A couple of thoughts - where does the airflow through the main rad end up? Does it tumble around in the engine bay and then 'fall out' under the car? Or does it get forced straight down under the engine block?

silent running

Because I think the actual IC is probably OK for the job but is let down by the engine bay airflow. A few bits of aluminium or plastic sheet, some ducting hose and gaffer tape and a lot of discussion on here cost virtually nothing. A front mount will cost me several hundred, plus I don't like cutting up my bumper or using induction kits unless I absolutely need to. If you know of an FMIC that is a direct fit with no cutting, loss of foglights and retains the standard airbox then I'd love to hear about it! Plus if it comes in black, not silver, all the better.